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THERMODYNAMIC ANALYSIS OF AN INTEGRATED PHOTOVOLTAIC
SYSTEM FOR HYDROGEN AND METHANOL PRODUCTION
By
Payam Esmaili
A Thesis Submitted in Partial Fulfillment
of the Requirements for the Degree of
Master of Applied Science in Mechanical Engineering
Faculty of Engineering and Applied Science
University of Ontario Institute of Technology
June 2012
Payam Esmaili, 2012
i
Abstract
A solar based integrated system for hydrogen and methanol production is investigated.
Energy and exergy analyses of a hydrogen production plant, thermodynamic assessment
of methanol synthesis plant, and exergy analysis of the integrated solar based system for
hydrogen and methanol production are performed. The analysis of hydrogen production
is found to be essential in order to investigate for further design parameters for methanol
synthesis procedure. The present analysis shows the effects of temperature and current
density on hydrogen production. Thermodynamic parameters of the methanol synthesis
plant, such as temperature and pressure, appear to be an important role in methanol
production. Based on the methods of physical domain of the system, the optimum
temperature of methanol synthesis is obtained for the final design of the methanol plant.
It is concluded that increasing pressure improves the methanol synthesis process;
however, methanol conversion takes place at 493 K. The energy and exergy efficiencies
of the system are reduced by 30% if the electrolyser operates at 300 K. The efficiencies
of the system are also highly dependent on the solar intensity. The system efficiencies
can be tripled if the intensity of solar radiation is increased to 600 W/m2 instead of 250
W/m2.
Keywords: Phovoltaics, Solar Energy, Hydrogen, Storage, Methanol Synthesis,
Thermodynamics, Energy, Exergy, Efficiency.
ii
Dedication
In dedication of my loving parents
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Acknowledgments
There are a number of people who should be acknowledged for the help they have given
me during the production of this thesis. First and foremost, I would gratefully like to
thank my co-supervisors, Dr. Ibrahim Dincer and Dr. Greg F. Naterer, for providing me
the unique opportunity to work in this research area, their expert guidance and
mentorship, and their encouragement and support at all levels. Financial support of the
Ontario Research Excellence Fund is gratefully acknowledged.
To the other professors and post-doctoral researchers who have helped along the
way, I am grateful. Most notably, I thank Dr. Liliana N. Trevani; her wealth of
knowledge has been a constant source from which I have drawn my ideas.
I would also like to thank Pouria Ahmadi, Mehdi Hosseini and Halil Hamut who
have been there since the beginning, for their companionship and support; it would have
been much less productive not having their friendship during the many late nights
studying in the ACE building.
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Table of Contents
Abstract ............................................................................................................................... i
Acknowledgments ............................................................................................................ iii
Table of Contents ............................................................................................................. iv
List of Figure Captions .................................................................................................. vii
List of Tables ................................................................................................................. xii
Nomenclature ................................................................................................................. xiii
Chapter 1: Introduction ................................................................................................... 1
1.1 Motivation and Objectives ................................................................................... 3
Chapter 2: Literature Review .......................................................................................... 5
Chapter 3: Background .................................................................................................. 13
3.1. Methanol Synthesis Process ................................................................................... 13
3.1.1. Chemistry of Methanol Synthesis ....................................................................... 15
3.1.2. Conversion of Syngas to Methanol ..................................................................... 16
3.1.3. Carbon Monoxide Hydrogenation as Primary Reaction for Synthesis of Methanol
....................................................................................................................................... 17
3.1.4. Carbon Dioxide Hydrogenation as Major Reaction for Methanol Synthesis...... 18
3.1.5. Chemical Reactions under Critical Syngas Conditions....................................... 19
3.1.5.1. Carbon monoxide-Free Syngas Feed Conditions ......................................... 19
3.1.5.2. Carbon dioxide-Free Syngas Feed Conditions ............................................. 20
v
3.1.6. Active Procedure of Methanol Synthesis Catalyst .............................................. 23
3.1.7. Properties of Methanol ........................................................................................ 24
3.2. Carbon Capture and Storage .................................................................................. 25
3.3. Hydrogen Production ............................................................................................. 29
3.3.1. Steam Methane Reforming.................................................................................. 31
3.3.2. Gasification ......................................................................................................... 32
3.3.3. Hydrogen from Water ......................................................................................... 34
3.3.3.1. Electrolysis ................................................................................................... 34
3.4. Photovoltaic Panels ................................................................................................ 39
Chapter 4: Systems Description .................................................................................... 44
4.1. PV Panels ............................................................................................................... 44
4.2. PEM Electrolyser ................................................................................................... 45
4.3. Methanol Synthesis Reactor ................................................................................... 47
Chapter 5: Energy and Exergy Analyses ...................................................................... 50
5.1. General Equations .................................................................................................. 50
5.2. Exergy of Flows and Streams................................................................................. 53
5.2.1. Reference Environment ................................................................................... 53
5.2.2. Exergy of Material Flow.................................................................................. 54
5.2.3. Exergy of Work ............................................................................................... 55
5.2.4. Exergy of Thermal Energy .............................................................................. 55
vi
5.3. Overall Efficiencies ................................................................................................ 56
5.4. Procedures for Energy and Exergy Analyses ......................................................... 56
5.5. Electrochemistry Analysis of PEM Electrolyser .................................................... 57
5.6. Ohmic Overpotential .............................................................................................. 58
5.7. Activation Overpotential ........................................................................................ 59
5.8. Exergy and Energy Analyses of Electrolyser ......................................................... 61
5.9. Methanol Production Analysis ............................................................................... 61
5.10. Solar Energy Analyses ......................................................................................... 64
5.11. Energy and Exergy Efficiencies of the Integrated System ................................... 65
Chapter 6: Results and Discussion ................................................................................ 68
6. 1. PEM Electrolyser Analyses ................................................................................... 68
6.2. Methanol Synthesis ................................................................................................ 77
6. 3. Integrated System of Solar Based Methanol and Hydrogen Production ............... 94
9. 4. Energy and Exergy Efficiencies ............................................................................ 96
Chapter 7: Conclusions and Recommendations for Future Studies ........................ 102
7.1: Conclusions .......................................................................................................... 102
7.2. Recommendations for Future Studies .................................................................. 103
References ...................................................................................................................... 105
vii
List of Figure Captions
Figure 3.1: Schematic of overall mechanism synthesis. Subscripts (a) indicate adsorbed
species……………………………………………………………………………………24
Figure 3.2: Principles of three main CO2 capture options…………………………….....27
Figure 3.3 Chemical structures of three amines………………………………………….28
Figure 3.3: Hydrogen production method………………………………………………..31
Figure 3.4: Basic solar cell construction…………………………………………………40
Figure 3.5: Photovoltaic cells, modules and arrays……………………………………...41
Figure 3.6: Mechanisms of typical off-grid PV system………………………………….42
Figure 4.1: Schematic of solar based methanol and hydrogen production………...…….46
Figure 4.2: Schematic of the methanol synthesizer………………………………..…….47
Figure 6.1: Overpotentials related to platinum electrodes and ohmic losses………...…..69
Figure 6.2: Total cell potential versus current density at different temperatures……..…70
Figure 6.3: Comparison between thermal energy demand and heat production caused by
irreversible overpotential losses (T = 300 K)……………………………………………71
Figure 6.4: Comparison between thermal energy demand and heat production caused by
irreversible overpotential losses (T = 300 K)………………………………………...….71
Figure 6.5: Change of cell potential when temperature is changing for a constant amount
of current density…………………………….……………………………………….….72
Figure 6.6: Change of energy efficiency at different temperatures……..…...…………..73
Figure 6.7: Effect of membrane thickness on PEM electrolyser plant performance…….73
Figure 6.8: Effect of electrode exchange current density on electrolyser efficiencies…..74
Figure 6.9: Effect of electrode exchange current density on cell potential……………...75
viii
Figure 6.10: Effect of temperature on electrical energy inputs………………………….76
Figure 6.11(a): Energy and exergy (a) rates and (b) efficiencies versus current
densities…………………………………………………………………………………..76
Figure 6.11(b): Energy and exergy (a) rates and (b) efficiencies versus current
densities…………………………………………………………………………….…….77
Figure 6.12: Temperature dependence of equilibrium constants for principle
reactions……………………………………………………………………….…………78
Figure 6.13: The methanol yield in different temperatures and pressures
……………………………….………………………...……………......79
Figure 6.14: Dependence of equilibrium conversion degree of RWGS reaction on
temperature in different pressures
…………………..80
Figure 6.15 Dependence of equilibrium mole fraction of CO on temperature in different
pressures
…………………………………………….80
Figure 6.16: Dependence of mole fraction of CO2 on temperature in different pressures
……………………………………………………….81
Figure 6.17: Dependence of equilibrium mole fraction of H2 on temperature in different
pressures
…………………………....……………….82
Figure 6.18: Dependence of equilibrium mole fraction of methanol on temperature in
different pressures
…………………………………..83
Figure 6.19: Dependence of equilibrium mole fraction of water on temperature in
different pressures
…………………………………..83
Figure 6.20: Dependence of equilibrium conversion degree of methanol reaction on
temperature in different pressures
…………..………84
ix
Figure 6.21: Dependence of equilibrium conversion degree of WGS reaction on
temperature in different pressures
…………….…….85
Figure 6.22: Dependence of equilibrium conversion mole fraction of CO on temperature
in different pressures
……………..…………….85
Figure 6.23: Dependence of equilibrium mole fraction of CO2 on temperature in different
pressures
……………………….………………86
Figure 6.24: Dependence of equilibrium mole fraction of H2 on temperature in different
pressures
………………………….……………86
Figure 6.25: Dependence of equilibrium mole fraction of methanol on temperature in
different pressures
……………………………...87
Figure 6.26: Dependence of equilibrium mole fraction of water on temperature in
different pressures
…………………………..….88
Figure 6.27: Effect of initial mole fraction of CO2 on equilibrium conversion degree of
methanol
……………………………………………..89
Figure 6.28: Effect of initial mole fraction of CO2 on equilibrium conversion degree of
RWGS reaction
……………..……………………...91
Figure 6.29: Effect of initial mole fraction of CO2 on equilibrium conversion degree of
methanol
……………………………….…………..90
Figure 6.30: Effect of initial mole fraction of CO2 on equilibrium conversion degree of
WGS reaction
………………...……………………90
Figure 6.31: Effect of initial mole fraction of CO on equilibrium conversion degree of
methanol
………………………………………...91
x
Figure 6.32: Effect of initial mole fraction of CO on equilibrium conversion degree of
WGS reaction
……………….…….......………...91
Figure 6.33: Effect of initial mole fraction of H2 on equilibrium conversion degree of
methanol
………………………………………...92
Figure 6.34: Effect of initial mole fraction of H2 on equilibrium conversion degree of
WGS reaction
…………………………...………92
Figure 6.35: Dependence of methanol concentration in liquid products at equilibrium on
initial composition of synthesis gas and temperature (P = 5 MPa): (A:
) (B:
) (C:
) (D:
)………………………………...93
Figure 6.36: Schematic of solar based methanol and hydrogen production (designed for
0.86 CO2 mol/s)……………….………...…………………………………………….....95
Figure 6.37: Schematic of the methanol plant (designed for 0.86 CO2 mol/s) …...…......95
Figure 6.38: Exergy destruction in hydrogen, methanol and solar energy plants …….....96
Figure 6.39: Exergy destruction in the system and related efficiency versus ambient
temperature.……………………………………………………………………………...97
Figure 6.40: Energy efficiency of the system versus solar intensity at different
electrolyser temperatures………………………………………………………….……98
Figure 6.41: Exergy efficiency of the system versus solar intensity at different system
capacities…………………………………………………………………………..…..…99
Figure 6.42: Dependence of the energy efficiency of the system on electrolyser
temperature for different system capacities ………..……………………..………..100
xi
Figure 6.43: Dependence of the exergy efficiency of the system on electrolyser
temperature for different system capacities………………………………………….…100
xii
List of Tables
Table 5.1: Solar PV parameters………………………………………………………….65
Table 6.1: Design parameters for the PEM electrolyser…………………...…………….68
xiii
Nomenclature
A Surface (m2)
D Membrane thickness (m)
Energy rate (kW)
Exergy rate (kW)
ex Specific exergy (kJ/kg)
F Faraday constant (C/mol)
h Specific enthalpy (kJ/kg)
I Incident solar intensity (W/m2)
J Current density (kA/m2)
Jo Exchange current density (A/m2)
Joref
Pre-exponential factor (A/m2)
KE Kinetic energy (kJ)
Kx Equilibrium constant (mol/s)
LHV Lower heating value (kJ/mol)
N Number of moles
P Pressure (MPa)
Heat flow rate (kJ)
R Gas constant (J/molK)
RPEM Proton exchange membrane resistance (Ω)
s Specific entropy (kJ/kgK)
S Entropy (kJ/K)
xiv
T Temperature (K)
U Internal energy (kJ)
V Cell potential (V)
V0 Reversible potential (V)
Work rate (kW)
x Molar fraction
z Number of electrodes
Greek Letters
α Balance factor
β Packing factor
η Overpotential (V)
λ Water content
λ (x) Water content at location x in membrane
ξ Equilibrium conversion degree
σ Ionic conductivity in (s/m)
τ Transitivity
Ψ Efficiency
Subscripts
0 Reference environment
a Anode
act Activation
c Cathode
D Destruction
xv
en Energy
ex Exergy
g glass
MeOH Methanol
ohm Ohmic
PEM Proton exchange membrane
sc Solar cell
T Total
Acronyms
AC Alternating current
CCS Carbon capturing and storage
DC Direct current
DMFC Direct methanol fuel cell
FC Fuel cell
GHG Greenhouse gases
HHV Higher heating value
LHV Lower heating value
MSW Municipal solid waste
PEM Proton exchange membrane
PFSA Perfluorosulfonic
PV Photovoltaic
RWGS Reverse water gas shift
SMR Steam methane reforming
xvi
SOEC Solid oxide electrolysis cell
WGS Water gas shift
YSZ Zirconium-oxide based ceramic
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Chapter 1: Introduction
The main source of energy in today’s industry is based on fossil fuels. Carbon dioxide,
which is one of the main causes of global warming, is released from combustion of fossil
fuels (Hasegawa and Yokoyama, 2010). As a consequence of the greenhouse gas
emissions (GHG), the overall average temperature and sea levels have increased by 0.4 K
and 15 cm over the twentieth century, respectively. The results of GHG emissions on
climate change are now one of the most important challenges facing humanity. It is stated
that 40% of all species will be in danger of extinction if no actions for GHGs are taken in
a timely manner (Abu-Zahra et al., 2007).
Fossil fuels play a key role in today’s energy policies. More than 75% of global
energy demand is supplied by fossil fuels. The increase in the atmospheric carbon dioxide
concentration is directly related to fossil fuel use (Huntley and Redalie, 2006). Three
methods can achieve a significant decrease in carbon dioxide emissions:
Enhancement of energy efficiency of equipment;
Investigation of renewable energy sources ;
Improvement of carbon capture and storage (CCS) technologies and
transformation of carbon dioxide to other valuable materials, such as methanol
(Huntley and Redalie 2006).
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The first technique will not resolve the emission issues completely. The strategy
of switching to renewable energy sources is not easy to achieve in the short term. The last
option is more probable (Wee, 2009). Converting carbon dioxide to other valuable
materials can be applied on any source of flue gas at existing industry emitters. The
production of methanol from exhaust carbon dioxide can be one method to recycle
carbon dioxide, which can help reduce the amount of carbon dioxide in the atmosphere.
The investigation of carbon dioxide as a carbon source for chemical production
and fuel synthesis has received much attention. The recovery of exhaust carbon dioxide
could contribute significantly to reducing climate change (Huntley and Redalie, 2006).
Carbon dioxide can be used as a feedstock in the production of many chemicals.
Although methanol synthesis technology is completely mature and widely
available since 1923, the catalytic synthesis of methanol has attracted extensive attention
from industry, academia, and government. Over the years, many scholars have tried to
use efficient catalysts that would support the synthesis reaction. Due to the vast volume
of methanol needs in a wide diversity of industrial parts, the scope of commercial
production sectors has been increasing. The environmental limitations on the process
have also played an essential role in the production of methanol.
Solar and wind energy are exceedingly preferred as renewable energy sources,
since they can be easily transformed to electricity by wind turbines and solar cells.
Nevertheless, the electricity production from these renewable sources suffers from
unstable and fluctuating behaviour. In other words, the overall required and supplied
solar and wind electricity do not provide a good match, which requires the utilization of
energy storage options.
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In this thesis, a solar based integrated system for hydrogen and methanol
production is investigated. In order to produce methanol, a suitable ratio of hydrogen
must be produced. In this system, hydrogen is supplied from a conventional PEM water
electrolyser. PEM electrolysis is chosen because of its higher efficiency compared to an
alkaline electrolyser. The overpotentials related to an electrolyser are studied
individually. In order to evaluate the system, energy and exergy analyses are investigated.
The operating conditions such as temperature and pressure are studied parametrically in
order to give a better understanding of the system.
1.1 Motivation and Objectives
Methanol synthesis is an important topic for research. Methanol synthesis contains
several models and applied problems in a diversity of topics including: traditional
thermodynamics, condensed phase thermodynamics, reaction mechanisms, reactor
design, reactor modeling, reactor alignment, catalyst scheme, catalyst life management,
pore diffusion and external mass transfer, unreacted feed stream recycling, separation,
energy integration, process integration, process economics, environmental engineering
and cost accounting.
Although methanol synthesis technology has been studied since the 1920s, there
is a lack of studies related to investigation of a renewable and sustainable energy source
in order to decrease carbon dioxide emission. In this thesis, a new design of a methanol
production plant, which is upgraded with solar based hydrogen production, is studied and
analyzed thermodynamically in order to provide a promising way to reduce carbon
dioxide emissions.
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A solar based integrated system for hydrogen and methanol production is
investigated. The objectives include the following tasks:
energy and exergy analyses of hydrogen production plant;
thermodynamic analysis of methanol synthesis process;
energy and exergy analyses of the integrated solar based system; and
parametric study of the integrated system with respect to design factors.
The first objective of the thesis has been studied previously (Ni et al., 2008).
However in the present study, the analysis of hydrogen production is essential in order to
investigate the design parameters; such as temperature and pressure, for methanol
synthesis. This analysis shows the effects of temperature and current density of
electrolyser on hydrogen production. The second objective of this thesis is
thermodynamic analysis of the methanol synthesis process. The effects of temperature
and pressure on the methanol production are studied for the final integrated system
design. Energy and exergy analyses are investigated in order to evaluate the system
performance. Exergy analysis is a method that uses the conservation of mass and energy
principles together with the second law of thermodynamics for the analysis, design, and
improvement of energy and other parameters of systems. The exergy method is a useful
method for improving energy-resource use, as it enables the locations, types and
magnitudes of losses to be identified and efficiencies to be determined. Exergy analysis
of the integrated system is performed based on the optimum point of hydrogen
production, as well as the available extracted carbon dioxide from the plant. The final
designed methanol plant is modeled and associated efficiencies are studied. Energy and
exergy analyses predict the conditions of the system in higher carbon dioxide production.
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Chapter 2: Literature Review
In this chapter the background of methanol synthesis will be presented. Yoshihara and
Campbell (1996) studied the kinetics of simultaneous methanol synthesis and reverse
water–gas shift from CO2/H2 and CO2/CO/H2 mixtures. The analysis was conducted at
low conversions over a clean Cu (110) single-crystal surface at pressures of 0.51 MPa.
They reported a conversion of 8 × 103 methanol molecules per second per Cu surface
atom with the absence of CO. This study was conducted at 530 K; in which the activation
energy was 67 × 17 kJ/mol. They reported a CO production at a rate of 5 molecules per
second per Cu surface atom; and activation energy of 78 × 14 kJ/mol. When compared
with previous rates of Cu (100) and polycrystalline copper foil, these rates were found to
be higher in both methanol synthesis and CO production. The surface of the catalyst was
covered after the reaction by nearly a full narrow layer of adsorbed formate. However,
the other species like carbon or oxygen were not observed. Moreover, the addition of CO
in to the feed caused an increase methanol production rate. It should be noted that Cu
(100) and (110) are both crystal structures of copper; however, the direction vectors in
their cubic unit cells are different.
Li et al. (1997) used Cu-Zn-Al and Cu-Zn-Al-Mn catalysts for methanol
synthesis. These catalysts were arranged by a high speed collision co-precipitation
method. The methanol yields reported were 28.4 and 33.8 mol/l.cat.h over the ternary and
quaternary catalysts, respectively. The results showed that Mn allows diffusion of copper
6
in the catalysts, avoiding the copper particles from sintering. The diffuse reflectance
spectra advised that the formate might be the intermediate in the methanol synthesis.
Fisher and Bell (1998) studied the relations of CO and H2/CO with Cu/SiO2,
ZrO2/SiO2, and Cu/ZrO2/SiO2 in infrared spectroscopy with the purpose of understanding
the nature of the species involved in methanol synthesis, the kinetics of the formation and
consumption of these species. For Cu/SiO2, they showed that carbonate species adsorbed
on Cu sites and methoxide (CH3O-) species adsorbed on Cu and Silica site. These results
were observed at 523 K, a total pressure of 0.65 MPa and a rate of H2/CO=3. The
formations of carboxylate, bicarbonate, and formate species on zirconia were observed
when carbon monoxide adsorption occurred on either ZrO2/SiO2 or Cu/ZrO2/SiO2. In the
existence of hydrogen, formate species were hydrogenated to methoxide species adsorbed
on ZrO2. The existence of Cu greatly accelerates the rate of formate hydrogenation to
methoxide species. In this process, methylenebisoxy species were observed as
intermediates. Cu also supported the reductive elimination of methoxide species as
methanol. Therefore, methanol synthesis over Cu/ZrO2/SiO2 was intended to occur on
ZrO2; with the main role of Cu, adsorption of H2 had occurred. The overflow of atomic
hydrogen onto ZrO2 provides the source of hydrogen required to hydrogenate the carbon-
containing species. Overflow of absorbed CO from Cu to zirconia enabled formate
creation on zirconia at lower temperatures than with the lack of Cu. The reductive
elimination of methoxide is performed to the slow stage in methanol creation from CO
hydrogenation. The lower rate of methanol synthesis over Cu/ZrO2/SiO2 from CO as
compared to CO2 hydrogenation was credited to the absence of H2O formation in the
former reaction. The improved rate of methanol synthesis from carbon monoxide over
7
Cu/ZrO2/SiO2 as compared to Cu/SiO2 was due to the lower energy-barrier (formate)
pathway available on Cu/ZrO2/SiO2.
Nerlov and Chorkendorff (1999) studied the catalytic activity of Cu (100) and
Ni/Cu(100) regarding methanol synthesis from different combinations, which contained
carbon dioxide, carbon monoxide, and hydrogen. The reactions occurred in a combined
ultra-high vacuum /high pressure cell device; with conditions of reactor pressure of 0.15
MPa and temperature of 543 K. It was observed that the charge of CO to a reaction
combination containing carbon dioxide and hydrogen does not cause an increase in the
rate of methanol production. The investigation showed that the role of carbon monoxide
in the industrial methanol process relates to the alteration in reduction potential of the
synthesis gases. Furthermore, for the Ni/Cu (100) surface, it was observed that Ni does
not stimulate the rate of methanol formation from mixtures containing carbon dioxide and
hydrogen. On the other hand, the charge of carbon dioxide to the reaction mixture causes
an important rise in the rate of methanol production. It was found that the charge of
carbon monoxide to the synthesis gas creates exclusion of Ni to the surface, whereas this
was not the case for a reaction involving carbon dioxide and hydrogen. It was suggested
that carbon monoxide serves as an organizer in the system.
Jung and Bell (2000) showed that the synthesis of methanol from both carbon
dioxide and carbon monoxide, associated with Cu/ZrO2 catalyst, involved the overflow of
hydrogen molecules formed on Cu to the surface of ZrO2. The atomic hydrogen then
participated in the hydrogenation of carbon-covering species (such as: HCOO-Zr and
HCO3-Zr) to methanol. Based on their analysis, it was determined that the rate of
8
hydrogen overflow from Cu was more than an order of scale faster than the rate of
methanol production.
Tsubaki et al. (2001) developed a new system to synthesize methanol from
hydrogen and carbon monoxide. This system worked at high pressure and low
temperature. Carbon dioxide as well as water were involved in the system which was
copper-based. Alcohol, as a catalytic liquid medium, with the support of copper-based
solid catalyst, reformed the reaction path to a low-temperature direction.
Velardi and Barresi (2002) studied the feasibility of the methanol-synthesis
process in unsteady-state conditions. They used three catalytic fixed bed reactors with
periodic alteration of the inlet situation, in order to investigate benefits and limitations in
comparison with the former proposed reactor. The influences of the chief operating
limitations (such as initial temperature, switching time, initial flow rate) were studied.
Mignard et al. (2002) studied the feasibility of methanol production from flue gas
carbon dioxide with the aim of reducing carbon dioxide emission levels and increasing
the level of renewable energy. It was showed that important benefits accumulate from
successful development of a methanol process and that it may facilitate the absorption of
increasing levels of carbon dioxide.
Carnes and Klabunde (2003) studied several nanoparticle metal oxides which
were organized for the catalytic hydrogenation of carbon dioxide. These catalysts
include: ZnO, CuO, NiO, and a binary system CuO/ZnO. The catalysts were prepared
through sol–gel synthesis and were found to have high surface areas and small crystallite
sizes. The catalytic production of methanol was studied at various temperatures in a flow
reactor. The percent conversion and turnover numbers were calculated for each sample.
9
It was found that the nanoparticles ZnO, CuO/ZnO and NiO were much more active
catalysts than the commercially available materials. The nanocrystalline CuO sample was
found to rapidly reduce to Cu, where it lost all activity. The results suggested that the
catalytic process was efficient for several nanoparticle metal oxide formulations,
however, copper metal is not active, but small copper particles in a CuO/ZnO matrix are
a very active combination.
Kordabadi and Jahanmiri (2005) proposed a method through which an
optimization of methanol synthesis can be achieved to improve total production. A
mathematical heterogeneous model of the reactor was investigated in order to increase
the reactor performance. This study was performed for both steady state and dynamic
conditions. Genetic algorithms were carried out as authoritative methods for optimization
of the problems. Initially, an ideal temperature profile for the reactor was calculated. In
order to maximize methanol production, a stepwise method was monitored to develop an
optimum two-stage cooling shell. This optimization method provided a 2.9% additional
methanol yield in 4 years, which is the catalyst lifetime.
Golindo and Badr (2006) compared different methods of methanol synthesis from
different sources of CO2. They showed that methanol production from biomass is cheaper
and more efficient than from CO2 of flue gases.
Gallucci and Basile (2007) proposed a theoretical method for carbon dioxide
conversion to synthetic methanol from both a traditional reactor and a membrane reactor.
The aim of this method was to study the prospect of increasing carbon dioxide conversion
into methanol, regarding a traditional reactor. A mathematical model is investigated in
order to simulate a traditional chemical reactor. When comparing experimental with
10
modeled data, the validation of the simulation was studied. Moreover, the model was
carried out in order to predict the performance of carbon dioxide conversion and
methanol selectivity. The results illustrated that it is possible to achieve both higher
carbon dioxide conversion and methanol selectivity regarding a traditional reactor
performance at the same experimental conditions.
Toyir et al. (2009) studied methanol synthesis from carbon dioxide and hydrogen
in order to achieve to carbon dioxide mitigation. Highly efficient Cu/ZnO based material
catalysts were investigated. They categorized the metal oxides contained in Cu/ZnO-
based catalysts into two classifications. Al2O3 or ZrO2 enhanced the diffusion of copper
elements in the catalyst. Ga2O3 or Cr2O3 enhanced the chemical potential per unit copper
surface area of the catalyst.
The durability of Cu/ZnO-based catalysts had been enhanced by adding a small
quantity of silica to the catalysts. The additional Silica caused the catalysts to be
repressed in the crystallization of ZnO which was controlled in the catalysts. The
catalysts were very active and exceedingly stable in methanol synthesis from carbon
dioxide and hydrogen.
An et al. (2009) developed an extremely active Cu/Zn/Al/Zr fibred catalyst for
methanol synthesis from carbon dioxide hydrogenation. Several factors had an influence
on the performance of the catalyst, such as the reactor temperature, reactor pressure and
space velocity, were analysed. In order to model the methanol synthesiser, the kinetic
factors in Graaf's kinetic model were used. A carbon monoxide circulation was
considered and consequently, a higher methanol production was obtained. According to
the simulation, the space time yield of methanol improved and there was no carbon
11
monoxide in the production stream, which proved a decent potential application for
industry.
Rihko-Struckmann et al. (2010) simulated a low-pressure methanol production
process with CO2 and H2. They designed a methanol reactor cascade with a recycle loop
to increase the methanol production. They used a Cu/ZnO2/Al2O3 catalyst as well. The
exergy efficiency was reported as 83.1%.
Nieskens et al. (2011) described experimental results which were achieved from a
fixed bed reactor by means of a CoMoS catalyst. Only carbon dioxide and hydrogen were
used as the feed of the reactor. In order to use CoMoS catalysts, they proposed that
carbon dioxide and hydrogen can be used. The molar ratio of hydrogen to carbon dioxide
was 3. A slight temperature increase did not disturb the conversion, because of a
thermodynamic equilibrium constraint. Although the hypothesis was proven, a method in
which carbon dioxide and hydrogen were changed into alcohols is economically feasible
only if the hydrogen can be achieved in a cost effective manner.
Farsi and Jahanmiri (2011) performed on an investigation and optimization of
methanol production in a dual-membrane reactor. They assumed that a connector reaction
and parting in a membrane reactor increases the reactor efficiency and decreases the
distillation cost in stages. In this arrangement, a methanol reactor is sustained by Pd/Ag
membrane tubes for hydrogen saturation and alumina–silica compound membrane tubes
for water vapour elimination from the reaction sector. A Pd/Ag membrane was used for
water-shift reaction and methane steam reforming process. A steady state one-
dimensional mathematical model was established to calculate the efficiency of the
12
pattern. The results of the conventional reactor were compared with existing industrial
plant data. The main benefits of the improved dual membrane reactor are given as
advanced CO2 conversion;
disabling the limitations which are enforced by thermodynamic equilibrium; and
enhancement of the methanol synthesis rate and its purity.
A genetic algorithm was employed as an evolutionary method to maximize the
methanol production as the target function. This configuration predicted a 13.2%
improvement in methanol production.
Although methanol synthesis technology has been studied since the 1920s, there is a
lack of studies related to the investigation of renewable and sustainable energy sources in
order to decrease carbon dioxide emissions. In the following chapters, a new design of a
methanol production plant, which is upgraded with solar based hydrogen production, will
be studied and analyzed thermodynamically in order to provide a more promising way to
reduce carbon dioxide emissions.
13
Chapter 3: Background
In this chapter, the background of methanol synthesis is presented. The overall process of
methanol synthesis is discussed and different procedures are explained in order to provide
an explanation of the technology. Other related processes of the integrated systems, such
as hydrogen production, solar energy and CCS, which are investigated in this thesis, are
separately discussed as well.
3.1. Methanol Synthesis Process
Although methanol synthesis technology (over different catalysts) is completely mature
and widely available since 1923, the catalytic synthesis of methanol has attracted
extensive attention from industry, academia, and government. Over the years, many
researchers have tried to use efficient catalysts that would support the synthesis reaction.
Due to the vast volume of methanol loads in a wide diversity of industrial parts, the scope
of commercial production sectors has been ever increasing. The environmental
limitations on the process have also played an essential role in the production of methanol
(Lee et al., 2007).
Methanol is poisonous and lethal if taken internally by humans or animals.
Methanol is more poisonous than ethanol. Although the methanol molecule comprises
only one carbon atom and has a low molecular weight of 32 (gr/mol) which is almost the
same as oxygen, its synthesis is relatively difficult. Methanol has a high octane value of
14
105 and burns easily. Methanol is a solvent as well as chemical reactant in a number of
important chemical syntheses. Recently, methanol has become a widely held choice for
the development of fuel cell technologies, particularly direct methanol fuel cells which
are called DMFC (Larminie and Dicks, 2003).
The methanol economy is a theoretical future economy in which methanol fuel
would have switched fossil fuels as the energy means for the transportation sector. It
compromises an alternative to the hydrogen and the ethanol economy. Many opinions are
suggested for choosing the methanol economy instead of the hydrogen economy, due to:
energy generation cost;
simplicity of conversion processes;
sustained dependency on fossil fuel sources;
volumetric power density;
safety related with the fuel in several aspects of synthesis;
delivery; and
storage.
DMFCs are widely established to power portable electronics. They can have an
identical feasible power source in many applications (Larminie and Dicks, 2003).
According to these criteria, methanol in DMFCs can be a reliable method for electronics
and other domestic applications.
Methanol synthesis is treated as an active topic for research. Methanol synthesis
contains an excessive arrangement of model and applied problems in a diversity of topics
including:
reaction mechanisms;
15
reactor design;
reactor modeling;
reactor alignment;
catalyst scheme;
catalyst life management;
pore diffusion and external mass transfer;
recycling of unreacted feed stream;
separation;
process integration;
process economics;
environmental engineering; and
cost accounting.
In this chapter, a description of methanol chemistry and synthesis technology is
discussed with importance placed on its significance as an alternative fuel and
petrochemical feedstock.
3.1.1. Chemistry of Methanol Synthesis
The catalytic production of methanol has been widely available since the 1920s. The first
commercial factory for the production of methanol from syngas (mixture of carbon
monoxide and water) was developed by BASF (Lee et al., 2007). A major adjustment
was a switch from high-pressure to low-pressure synthesis. Both high-pressure and low-
pressure procedures used heterogeneous catalytic conversion to methanol from synthesis
gas normally initiated from natural gas, or otherwise, from coal. The quality and structure
of synthesis gas varied widely, depending on the procedure of conversion as well as the
16
style and quality of the feedstock. Thus, a diversity of commercial procedure designs
replicated and included these differences. Consequently, it is crucial that the chemistry of
synthesis gas conversion is fully clarified in the synthesis of methanol.
3.1.2. Conversion of Syngas to Methanol
Synthesis gas is a combination that includes hydrogen, carbon monoxide, and carbon
dioxide as major components. Synthesis gas is called syngas. Syngas is normally formed
through:
Steam reforming of natural gas;
Gasification (partial) oxidation of coal;
Gasification of biomass;
Gasification of municipal solid wastes (MSWs); and
Coke oven gas.
Methanol synthesis from syngas is normally accompanied over a heterogeneous
catalyst system (Cu/ZnO/Al2O3). It should be noted that Cu/ZnO/Al2O3 is the reduced
form of CuO/ZnO/Al2O3. The main stoichiometric reactions taken place in this chemical
conversion are:
CO2 + 3H2 CH3OH + H2O (3.1)
CO + 2H2 CH3OH (3.2)
CO + H2O CO2 + H2 (3.3)
According to the methanol synthesis reactions, two of these three reaction
equations are independent, based on stoichiometric rules. Stoichiometric independence
can be confirmed by one of the following methods:
17
Gauss removal mathematical procedure;
by deriving third equivalence balance from a linear arrangement of the other two
balances.
In this particular problem, a linear combination of any two stoichiometric
equations would yield a third equation. Therefore, this problem can be solved by
separation of the system with only two independent stoichiometric reactions. If the
stoichiometry and material balances are the only problems, there is little difference
among the options chosen as the principal reactions (Cybulski, 1994, Lee et al., 1989,
Chinchen et al., 1990, Lee et al., 1990).
3.1.3. Carbon Monoxide Hydrogenation as Primary Reaction for
Synthesis of Methanol
The primary reactions for carbon monoxide hydrogeneration are given as
CO2 + H2 CO + H2O (3.4)
CO + 2H2 CH3OH (3.5)
Based on this method, methanol is principally produced via direct hydrogenation
of carbon monoxide. The second reaction is called a reverse water gas shift reaction
(RWGS). Therefore, the route of the WGS reaction is determined from material balances,
not from chemical thermodynamic considerations. Experimental data involving standard
syngas mixtures that contain 2 to 10% carbon dioxide express a reduction in carbon
dioxide concentration in the reactor sewage stream. The water gas reaction occurs in the
route of reducing carbon dioxide concentration in the reverse direction. This is reliable
only when the main reaction is assumed to be hydrogenation of carbon monoxide.
18
It should also be noted that the first reaction of methanol production is
exothermic, while the second reaction (RWGS) is endothermic. According to this
application, through reduction of carbon dioxide in the RWGS reaction, more carbon
monoxide is created to enhance the synthesis of methanol. The role of carbon dioxide in
the total synthesis is important in the WGS reaction. A lack of carbon dioxide in the feed
supply can be harmful to the total synthesis, by quickly deactivating the catalysts and
directly lowering methanol productivity by the procedure. Normally, 3 to 5% of carbon
dioxide exists in the syngas combination for the vapour-phase synthesis of methanol,
although this value is higher, 6 to 9%, for the liquid-phase synthesis (Lee et al, 2007,
Cybulski, 1994).
3.1.4. Carbon Dioxide Hydrogenation as Major Reaction for Methanol
Synthesis
In this section, the main chemical reactions in the synthesis of methanol are:
CO2 + 3H2 CH3OH + H2O (3.6)
CO + H2O CO2 + H2 (3.7)
The synthesis of methanol ensues primarily through direct hydrogenation of
carbon dioxide. The WGS reaction occurs in the frontward route overwhelming carbon
monoxide to create the principal reactants of carbon dioxide and hydrogen, consequently
enhancing the methanol production. Numerous authors studied a variety of reaction
experiments in order to clarify the reaction and chemical pathways, including isotope
labeling and kinetic studies concerning a lack of one of the syngas elements (Lee et al.,
2007, Cybulski, 1994).
19
3.1.5. Chemical Reactions under Critical Syngas Conditions
Various conditions will be considered in the above-mentioned mechanistic processes
(Lee et al., 1990). These conditions are carbon monoxide-free syngas feed conditions and
carbon dioxide-free syngas feed conditions. The consequences of using CO and CO2-free
feed gas will be discussed below.
3.1.5.1. Carbon monoxide-Free Syngas Feed Conditions
If the feed syngas has a lack of carbon monoxide, experimental date shows that the
methanol production is very low and gradually declining. This reaction occurrence can be
described based on the carbon dioxide hydrogenation reaction as follows:
CO2 + 3H2 CH3OH + H2O (3.8)
CO2 + H2 CO + H2O (3.9)
According to a lack of carbon monoxide, the WGS reaction occurs in the reverse
direction; in the direction that would produce more carbon monoxide. Therefore, the
main reactant, which is carbon dioxide, is required by both reactions. Assuming that the
reverse water gas shift reaction is a quicker reaction than the methanol synthesis reaction,
the methanol production level would decrease. Moreover, both the reverse water gas shift
reaction and the methanol synthesis reaction yield water, with concentration growth in
the system that unfavourably influences the conversion of carbon dioxide to methanol by
forcing the chemical system closer to the equilibrium condition. Furthermore, large
amount of water can deactivate the catalysts (Sawant et al., 1989). Consequently,
methanol productivity declines.
20
The same can also be described by the carbon monoxide hydrogenation
mechanism as
CO + 3H2 CH3OH (3.10)
CO2 + H2 CO + H2O (3.11)
Due to the absence of carbon monoxide, the water gas shift reaction occurs in the
reverse direction. According to the carbon monoxide hydrogenation, carbon monoxide is
the critical reactant for methanol production; nevertheless, the foundation of this
component is pending the RWGS reaction, since there is no other source of carbon
monoxide in the feed. Thus, the reaction is restricted by the absence of the essential
reactant. Both procedures can clarify the condition. Consequently, the experiments
accomplished under these circumstances do not require to establish which of the two
procedures over the Cu/ZnO/Al2O3 catalyst is the correct one for the synthesis of
methanol.
3.1.5.2. Carbon dioxide-Free Syngas Feed Conditions
If methanol production is accomplished over the Cu/ZnO/Al2O3 catalyst system using the
carbon dioxide-free syngas, methanol production is also considerably lower than the
general syngas feed conditions, and it quickly declines even further (Lee et al. 1990).
According to the carbon dioxide hydrogenation process, the following
stoichiometric equations can be expressed as:
CO2 + 3H2 CH3OH + H2O (3.12)
CO + H2O CO2 + H2 (3.13)
According to the lack of carbon dioxide in the feed syngas, the water gas shift
reaction occurs in the forward direction, causing carbon dioxide production, which is an
21
important reactant for the methanol synthesis reaction. Due to the inaccessibility and
restricted supply of carbon dioxide, the main reaction of methanol synthesis does not
progress appropriately, resulting in low methanol productivity. Moreover, the absence of
carbon dioxide causes the production of carbon which is generated catalytically in the gas
phase:
2 CO(g) CO2(g) + C(s) (3.14)
Consequently, this reaction occurs on heterogeneous sides and includes carbon
deposition. This reaction may lead to catalyst deactivation through polluting by carbon
deposition. A quick decrease of methanol productivity would be expected. Nevertheless,
the situations causing carbon deposition may be different between vapour-phase and
liquid-phase synthesis. Accordingly, carbon dioxide is an important feature of the syngas
mixture for the steadiness of catalytic activity. The lack of carbon dioxide in the feed
syngas may lead to permanent damage to the catalyst.
If carbon monoxide hydrogenation is considered as the mechanism, the reaction
of carbon dioxide-free syngas would be denoted by:
CO + 2H2 CH3OH (3.15)
CO + H2O CO2 + H2 (3.16)
Carbon monoxide is required by both reactions. The water gas shift reaction is
faster under these conditions and continues in the forward direction while there is some
water in the system. Since the carbon monoxide is available in the system, this by itself
would not justify the low methanol productivity.
Thus, there are different essential issues in low pressure methanol synthesis
technologies. These issues can be expressed as follows (Lee et al., 1989):
22
The existence of carbon dioxide in the feed syngas combination is necessary.
Many designs and procedures may set this carbon dioxide level differently.
Nevertheless, there is a minimum value for the procedure to be functional. If
carbon dioxide is lacking or deficient in the system, the potential catalyst failure is
greatly promoted.
The existence of carbon monoxide in the syngas feed alignment is also very
essential. A lack of carbon monoxide in the syngas feed not only results in low
methanol efficiency, but also in a reduction in productivity.
There is an ideal value for the temperature of the methanol synthesis reaction
from the viewpoint of an optimal transformation of syngas in addition to the
dynamic reaction rate. The speed of reaction is improved with an increase in the
temperature by succeeding the Arrhenius-type of temperature dependency,
although the equilibrium conversion is thermodynamically not preferred with a
rise in the reaction temperature. There is a limit for the temperature at which the
procedure can be activated. This maximum is frequently directed by the
temperature tolerance of catalyst elements, especially the copper module of the
catalyst. This temperature is about 553–573 K. Above this temperature, the
catalyst could be subjected to damage.
Although the sensitivity of total methanol production to the carbon dioxide
difference is not as significant as that of a main reactant of general chemical reaction
systems, the function of carbon dioxide is diminished by the existence of a very active
WGS reaction. On the other hand, the absence of carbon dioxide in the feed syngas
would cause permanent damage to the catalyst (Cybulski, 1994). This can happen when
23
the amount of carbon dioxide increases and with the absence of CO2, forming of C–C
bonds, and wax formation is possible.
3.1.6. Active Procedure of Methanol Synthesis Catalyst
The methanol synthesis uses a technique in which both CuO and ZnO are advanced onto
an absorbent structure of support, usually alumina, Al2O3. The formula of this catalyst is
most commonly stated as CuO/ZnO/Al2O3. This form of catalyst is an oxidized form that
is stable upon exposure to air or other oxidizing environments. All catalysts of this type
are transported in oxidized form for protection and storage. Thus, this catalyst must be
reduced before it is used in a hydrogenation reaction such as methanol synthesis (Sawant
et al., 1987). Otherwise, hydrogen as one of the reactants in the hydrogenation system
could be totally spent in the reaction of this oxidized form of catalyst. Through this
exothermic procedure, sintering of catalyst could occur and create permanent damage.
The following processes occur during methanol synthesis over CuO/ZnO/Al2O3
catalyst (J.M. Thomas and W.J. Thomas, 1997):
1. Hydrogen is adsorbed on the partially oxidised copper component of the catalyst.
2. Carbon dioxide is adsorbed on the partially reduced ZnO component of the
catalyst, mainly as CO2-.
3. Hydrogen and carbon dioxide interact at the Cu/ZnO boundary to create another
species on the copper component of the catalyst. This compound is consequently
hydrogenated via methoxy to methanol, leaving partially oxidised copper.
4. The function of the CO is to retain the copper in a more highly reduced (more
active) state that can be achieved by hydrogen reduction alone.
24
CO2
CO2 (a)
H
H(a)
H(a)
H(a)
H(a)
H(a)
H(a)
H(a)
H2 CO(a)O(a)
H3CO(a)
H(a)
CH3OH H2O
CO2
CO
HCO2(a)
ZINC OXIDE
COPPER
Figure 3.1: Schematic of overall methanol synthesis. Subscripts (a) indicate adsorbed
species (modified from J.M. Thomas and W.J. Thomas, 1997).
Figure 3.1 shows the overall mechanism of the reaction. The zero oxidation state
of copper, Cu0, is playing an important role in the reaction process. It should be noted
that this Cu0 is close to the oxide surface site. Atomic hydrogen is formed at the Cu0 site,
however, HCO2, which is a formate, is made near the boundary at the oxide site. The Cu0
is formed by reduction of the CuI/ZnO solid solution with the CO, CO2 and H2 in the feed
gas (J.M. Thomas and W.J. Thomas, 1997).
3.1.7. Properties of Methanol
Methanol is also known by the following names: Methyl alcohol, Carbinol, Methyl
hydroxide, Methylol, Monohydroxymethan, Wood alcohol, Colonial spirit, Columbian
spirit, Hydroxymethane, Wood naphtha.
25
The density of methanol at room temperature (298 ) is 791.8 . The heat
of formation for methanol in a gas condition ( ) is –201 ± 0.2 , while that
of liquid methanol ( ) is –239.5 ± 0.2 . The heat capacity of methanol
gas at room temperature is 44 , while the value for liquid methanol is
81 ± 1 . The Henry’s law constant for methanol in water at 298.15
(room temperature) is 210 ± 10 .
The critical temperature and pressure of methanol are 512 ± 2.2 K and 81 ± 1.0 kPa.
The standard boiling and melting temperatures are 336.8 ± 1.3 and 175.0 ± 2.0 ,
respectively. The triple point of methanol is 174.5 ± 1.5 . The flash point of methanol is
284.5 K. The enthalpy of vaporization of methanol at room temperature is
37.73 (Majer and Svooda, 1985). Methanol can be processed as a powerful
solvent for a diversity of modern procedures. The hydroxyl assembly in its molecular
arrangement has unique properties that are generally not achievable with carbon dioxide.
Its critical point is higher than CO2, however lower than H2O.
3.2. Carbon Capture and Storage
Carbon capture and storage (CCS) technology is presently considered to be practically
achievable at commercial scales. In addition to current injection of carbon dioxide (CO2)
into reservoirs in the oil and gas industry, up to 20 practically achievable, electricity
generation projects, combining carbon capture and storage technology, producing more
than 350 MW, have been proposed around the world. The European Union organization
of carbon capture and storage has shown progress in progress up to 15 new technologies
by 2020. For instance, in the United Kingdom and the United States, a competition is
26
taking place for the best method of carbon capture from small to large industrial scales.
The winner will be supported by governments to start the industrialization of carbon
capture technology (Gibbins and Chalmers, 2008).
Carbon capture and storage (CCS) technologies are capable of achieving a major
reduction in CO2 emissions in the short term. It is recognized that the difficulties
accompanying the transport and storage of large quantities of CO2 are not insignificant
(MacDowell et al., 2010). In order to address the problem of carbon dioxide emissions,
many other problems such as environmental impact and carbon dioxide transfer are
involved. There are several technology choices which are compatible with CCS activity.
However, few have obtained any degree of recognition from an industrial viewpoint.
Three technologies are commonly accepted as viable for commercial organizations,
namely post-combustion CO2 capture using amine solvents, oxyfuel combustion, and
calcium looping technologies.
In amine based carbon dioxide capture, the carbon dioxide gas flow is connected
with a liquid phase amine solution. The amine solvent reacts reversibly with CO2 and
creates water and salts. In post-combustion carbon dioxide capture based on amine
absorption, carbon dioxide is detached by a chemical absorption procedure that includes a
flue gas stream to an aqueous amine solution. Carbon dioxide responds to the amines to
form a soluble carbonate salt. This reaction is reversible and the CO2 can be released by
heating the solution with the carbonate salt in a separate stripping column (Figueroa et
al., 2008).
In oxyfuel combustion technology, the fuel is consumed in a combination of
nearly pure oxygen (normally more than 90% purity) and carbon dioxide. The main
27
benefit of this technology is that it yields a flue gas which is mainly carbon dioxide and
water vapour. It is relatively easy to remove water vapour from the mixture, which will
result in a pure carbon dioxide stream which can then to be compressed, transported and
stored.
In the solid looping process, calcium oxide and carbon dioxide are reacted to
create calcium carbonate (CaCO3). This technology is similar to amine-based absorption
technology in which the carbon dioxide capture occurs in one unit, which is called
carbonator (Figueroa et al., 2008).
Gasification or
partial oxidation
shift + CO2
separation
Power and Heat
Power and Heat
Power and Heat
N2, O2, H2O
Air
Fuel
Fuel
AirO2
Air
Air separation
CO2 separation
H2
N2, O2, H2O
Air separationAir
Fuel
Recycle
CO2 dehydration
compression
transport and
storage
Post-Combustion Capture
Pre-Combustion Capture
Oxyfuel (O2/CO2 Recycle Combustion) Capture
Figure 3.2: Principles of three main CO2 capture options (modified from Jordal et al.,
2004).
Post-combustion capture includes the elimination of carbon dioxide from the flue
gas which is formed by combustion. Power plants currently use air, which is nearly 75%
28
nitrogen, for combustion. This generates a flue gas at atmospheric pressure and typically
a carbon dioxide attentiveness of less than 16%. Therefore, the thermodynamic
motivating power for carbon dioxide capture from flue gas is low (carbon dioxide partial
pressure is normally less than 0.14 kPa). This creates a major challenge for the
improvement of cost effective advanced capture procedures. Despite this problem, post-
combustion carbon capture has the highest short-term feasibility for decreasing
greenhouse gas emissions. This technology can be added to existing power plants that
make 66% of carbon dioxide releases (Gibbins and Chalmers, 2008).
Amines react with carbon dioxide to create water solution compounds. According
to this compound formation, amines have a capability of capturing carbon dioxide from
flows with a low carbon dioxide partial pressure; however, the capacity of the absorption
is limited by the equilibrium. Although amines have been utilized for several years,
especially in the elimination of acid gases from natural gas, there is still scope for
improving the efficiency. Amines are used in three forms: primary (AMP), secondary
(DEA) and tertiary (MDEA), each with its advantages and disadvantages as a carbon
dioxide solvent. Their chemical structures are shown in Figure 3.1. In addition to options
for the amine, additional substances can be utilized to improve the system performance.
Design alterations are possible to reduce capital expenses and increase energy efficiency
(Gibbins and Chalmers, 2008).
CN
H
H CH3
OH
CH3
CH2
AMP
H N
CH2 CH2
CH2 CH2
OH
OH
H3 C N
CH2 CH2 OH
CH2 CH2 OH
DEA MDEA
Figure 3.3 Chemical structures of three amines (adapted from Wang et al., 2004)
29
3.3. Hydrogen Production
Hydrogen is necessary for transforming petroleum into various synthetic materials in
industrial applications, such as polymers, chemicals, and pharmaceutical raw materials.
Presently, hydrogen is receiving much attention in the media for new applications
involving renewable energy and clean energy technologies.
Hydrogen has been considered as a future energy carrier. Hydrogen releases
significant energy during combustion. By using a fuel cell, it is relatively easy to convert
hydrogen to electricity, without any CO2 emissions. Hydrogen can be produced via
biological processes, photo-electrochemical, thermochemical among other methods.
Currently, the main technology for direct production of hydrogen is steam
reforming of natural gas. Steam methane reforming (SMR) is one of these methods, in
which hydrogen is produced from methane in natural gas. This system uses high-
temperature steam as well. About 95% of the hydrogen used in the United States is from
SMR. Another method is called partial oxidation. In this method, hydrogen is produced
by burning methane in air, after which ―synthesis gas‖ is produced, which reacts with
water to produce more hydrogen. Renewable electrolysis is another method to produce
hydrogen. In this method, an electric current is used to split water into its components:
hydrogen and oxygen. The required electricity can be produced by wind, solar, or
hydroelectric power, which are renewable energy technologies. Also, nuclear energy can
be used with water electrolysis to produce hydrogen. In water electrolysis, increasing the
temperature of the water will result in less electricity to split water into hydrogen and
oxygen, which reduces the total energy required.
30
Gasification is another method to produce hydrogen, where coal or biomass is
converted into gaseous components under specific conditions, such as high pressure
steam. Synthesis gas is reacted with steam to produce more hydrogen. It is more efficient
to produce hydrogen from coal or natural gas reforming than the other method from
which coal and natural gas are burned to produce electricity, followed by used to produce
hydrogen. Pre-carbon capture technology is a common hydrogen production loops in this
case.
High temperature water electrolysis is another method to produce hydrogen, also
called a solid oxide steam electrolysis method. In this method, steam is supplied and
electricity is applied to split water molecules.
Hydrogen is used in the following process (Goyal et al., 2006):
ammonia manufacturing, petroleum refining and petrochemicals production;
hydrogenation of unsaturated oils of soybeans, fish, cottonseed, corn, peanut and
coconut;
hydrogenation of inedible oils and greases for manufacturing soaps and animal
feed;
production of float glass;
oxy-hydrogen cutting of glass;
energy storage technology;
electronics industry;
production and processing of silicon;
an alloying element in various amorphous materials;
a fuel for rocket propulsion;
31
chemical feedstock for production of certain chemicals; and
using in fuel cells as a fuel.
Therefore, hydrogen production from a sustainable and renewable source of
energy is important. In the following section, different methods of hydrogen production
will be discussed separately.
3.3.1. Steam Methane Reforming
Hydrogen is promising carrier of energy because it can be burned to produce energy very
similar to fossil fuels, or converted to electricity energy in a fuel cell without any
emissions of carbon dioxide. In steam methane reforming, three main steps are
considered (Midilli et al., 2007).
Coal Natural
Gas
Fossil Fuels Nuclear
Gasification
of coal
Steam
reforming
Partial
oxidation
Plasma arc
process
Thermo –
chemical Electrolysis
Renewables
H2
Photo –
chemical
Photo –
biological
Photo–
electrolysis Gasification
of biomass
Solar Wind Biomass
Figure 3.4: Hydrogen production methods (modified from Jaber, 2009).
32
Methane is reformed at an specific temperature and pressure to produce a syngas
(a mixture of H2 and CO).
Steam is supplied to endorse the second step. In this step the main reaction takes
place: CH4 + H2O ⇆ CO + 3H2.
The third step is defined as purification, in which the other elements of CO2, N2
and CO will be removed.
Steam reforming of natural gas or syngas is another name of steam methane
reforming (SMR). This method is the most common method of producing commercial
bulk hydrogen in addition to hydrogen used in the industrial synthesis of ammonia.
3.3.2. Gasification
Gasification is a process where carbon monoxide, hydrogen and carbon dioxide, are
produced from organic or fossil based carbonaceous materials. This process occurs by
reacting the material at high temperatures (usually more than 1073 K), without
combustion. This process is controlled by adjusting the amount of oxygen or steam. A
gas mixture will be produced which is syngas or producer gas and itself a fuel. Syngas is
also known as synthesis gas or synthetic gas. Although the resultant energy from
gasification of biomass and combustion of syngas is considered as renewable energy, the
gasification of fossil fuel in which the required energy is coming from materials such as
plastic is not considered to be renewable energy (Dincer, 2011).
Since syngas can be combusted at higher temperatures, using syngas is more
efficient than direct combustion of the original fuel. Syngas can be used directly in
combustion engines to produce methanol and hydrogen, or processed through the
33
Fischer-Tropsch procedure into synthetic fuel. Gasification may also be initiated with a
material which otherwise be disposed as recyclable waste. In addition, the high-
temperature process refines ash elements such as chloride and potassium, allowing clean
gas production from otherwise problematic fuels. Gasification of fossil fuels is widely
used on industrial scales to produce energy.
Different types of gasifiers are currently available for commercial use: counter-
current fixed bed, co-current fixed bed, fluidized bed, entrained flow, plasma, and free
radical. A counter-current fixed bed (so-called up draft) gasifier consists of a fixed bed of
carbonaceous fuel (such as coal or biomass) through which the source of oxygen flows in
a counter-current alignment. This source of oxygen is also called a "gasification agent.‖
In the process of gasification of biomass, it is required to force air into the reactor by
means of a blower or fan. This generates very high gasification temperatures, up to 1273
K. A fine and hot char bed is created beyond the gasification zone, and when the gas is
forced through this bed, all complex hydrocarbons are cracked down into simpler
components of hydrogen and carbon monoxide (Wall et al., 2008).
A co-current fixed bed (so-called down draft) gasifier works similar to the
counter-current type, but the gasification agent gas (steam, oxygen and/or air) flows in a
co-current alignment with the fuel. Another name of this gasifier is a down-draft gasifier
because the agent gas flows downwards. Heat is needed for the upper part of the bed; it
can be applied by combusting small amounts of the fuel or an external heat source. To
increase the energy efficiency, the thermal energy of outlet gas will be transferred to the
gasification agent in the inlet flow. In this method, tar levels are minor compared with
counter-current flow, because all tars lead through a hot bed of char (Wall et al, 2008).
34
A fluidized bed reactor is another reactor for gasification. The fluidizing material
is typically air; however, oxygen and steam can also be used. The ash is left dry or as
heavy accumulates solid that defluidize. The whole process occurs in low temperature
conditions (Cohce, 2011).
In an entrained-flow gasifier, which is another type of gasifier, the fine coal and
the oxidant (air or oxygen) and/or steam are fed into the top of the gasifier. This results
in coal components to be surrounded or entrained by a gasification agent. The entrained-
flow gasifier operates at very high temperatures in order to melt coal ash into inactive
slag (Cohce, 2011).
A plasma gasifier transforms the organic waste into a fuel gas. It should be noted
that organic waste, which originates from plant or animal sources, still has all the
chemical and thermal energy from the waste. The gasifier transforms the inorganic waste
into a sluggish vitrified gas. In a plasma gasifier a high-voltage current is applied to an
incinerator, which makes a high-temperature arc.
3.3.3. Hydrogen from Water
Hydrogen from water became common and available in the market since the 1890s
(Norbeck et al, 1996). Water splitting has three general categories: electrolysis,
thermolysis, and photoelectrolysis.
3.3.3.1. Electrolysis
Water splitting in its simplest form uses an electrical current through two electrodes to
split water molecules into hydrogen and oxygen. Low temperature electrolysers have
system efficiencies of 53–75% (Turner et al., 2008). It involves the exchange of electrical
energy to chemical energy in the form of hydrogen, with production of oxygen as a useful
35
by-product. The most common electrolysis technology is alkaline based. However,
proton exchange membrane (PEM) electrolysis and solid oxide electrolysis cells (SOEC)
units are developing faster (Norbeck et al., 1996, Pattersson et al., 2006, Grigoriev et al.,
2006). SOEC electrolysers are the most electrically efficient; however, they are not
developed as mature as the other technologies. Many challenges are involved in SOEC
technology such as: corrosion, seals, thermal cycling, and chrome migration. PEM and
alkaline electrolysers do not have these SOEC challenges. PEM is more efficient than an
alkaline electrolyser. However, alkaline systems are the most developed and economic
technology. According to their low efficiency, the energy costs are relatively high. High
pressure electrolysers have recently been developed (Janssen et al., 2004). By using high
pressure electrolyser units, there is no need for using hydrogen compressors. It should be
noted that hydrogen from electrolysis is generally more expensive than fossil fuel
sources. On the other hand, if an electrolyser’s required energy is derived from a non-
renewable source, it is more efficient to produce hydrogen than natural gas reforming
(Koroneos, 2004, Bradley, 2000).
3.3.3.2. Alkaline Electrolyser
A common water electrolysis unit contains an anode (positive terminal), a cathode
(negative terminal), source of power, and an electrolyte (usually KOH solution). A direct
current (DC) is applied to supply the electricity and make electrons move from the
negative terminal of the power source to the cathode. At the cathode, hydrogen is
produced. In order to remain the electrical charge in balance, hydroxide ions pass through
the electrolyte solution from the cathode to anode. In order to increase the conductivity of
the solution, electrolytes are used. The electrolyte is a substance having free ions that
36
make the substance electrically conductive. The most common electrolyte in which ions
are in a solution, however liquid electrolytes and solid electrolytes are also promising
(Krumpett, 1999). Potassium hydroxide (KOH) is the most common substance used in
water electrolysis. Potassium hydroxide protects the solution from any erosion which can
be caused by acidic electrolytes (Bellows, 1999). Nickel is a common electrode material,
not only because of its high activity, but also its availability (Joensen and Rostrup-
Nielsen, 2002). During the process of water electrolysis, which is enhanced with an
electrolyte, hydrogen ions flow up to the cathode, while hydroxide ions flow up to the
anode. By means of a separation duct, gas receivers gather hydrogen and oxygen, which
are produced and exit from the cathode and the anode, respectively. The half reactions on
the cathode and anode, respectively, can be written as
Cathode: 2H+ + 2e
- H2 (3.15)
Anode: 2OH- ½ O2 + H2O + 2e
- (3.16)
The overall chemical reaction of the water electrolysis can be written as
H2O H2 + ½ O2 (3.17)
Water electrolysis works reliably for small scales of hydrogen production by using a
renewable source of electricity, it is considered sustainable.
3.3.3.3. Proton Exchange Membrane Electrolyser
PEM electrolysers were developed based on the latest advances in PEM fuel cell
technology (National Academy of Science, 2004). The materials of electrodes of PEM
electrolysers are typically platinum, iridium, ruthenium, and rhodium. The material of the
membrane is usually Nafion, which not only splits the two-half cells, but also performs as
a gas separator (Turner et al., 2008, Turner et al., 2008). In PEM electrolysers, water is
37
split at the anode to protons and oxygen. The protons are transferred via the membrane to
the cathode. In the cathode, protons are converted into the hydrogen (National Academy
of Science, 2004). The oxygen gas is separated and stored for other purposes. In some
cases, depending on the required purity, a heater can be used to evaporate the water after
the gas-liquid separations unit. The PEM electrolyser has low ionic resistances and
consequently a high current density of >1.6 A/cm2 can be reached. It should be noted that
the efficiency of a PEM electrolyser is 55-75% (Sorensen, 2005, Turner et al., 2008). The
chemical reactions at the cathode and cathode are:
Anode: 2H2O O2 + 4H+ + 4e
- (3.18)
Cathode: 4H+ + 4e
- 2H2 (3.19)
The complete chemical reaction is the same as for alkaline electrolyzers:
H2O H2 + ½ O2 (3.20)
3.3.3.4. Solid Oxide Electrolysis Cells
Solid oxide electrolysis cells (SOECs) are based on solid oxide fuel cell (SOFC)
fundamentals. In order to split water molecules, energy is required. This energy can be
provided by an electrical or thermal energy source. However, electrical energy in the
form of direct current is essential due to polarity of molecules of water. In solid oxide
electrolysis cells, part of the electrical energy required is replaced by thermal energy.
According to the anode and cathode overpotentials, higher temperature operation results
in higher electrolyser efficiency (Hino, 2004, Utgikar and Thiesen, 2006). For instance,
an increase in temperature from 350 to 1100 K reduces the overall required electrical
energy, by approximately 30% (Utgikar and Thiesen, 2006). A solid oxide electrolysis
cell is comparable to the alkaline system in which oxygen ions move through the
38
electrolyte, parting the hydrogen in the incoming steam stream (National Academy of
Science, 2004).
One of the most important advantages of using this system is the physical phase
of its electrolyte, which is solid. The solid electrolyte is not corrosive and it does not have
problems in flow distribution (Turner et al., 2008, Hino, 2004). However, due to a higher
temperature of operation, the required materials are more costly (Turner et al., 2008).
SOEC materials are very similar to those which are being developed and used for solid
oxide fuel cells. These materials are zirconium-oxide based ceramic (YSZ) electrolytes.
For the anode, the material is nickel, which contains YSZ. The cathode is made of metal
which is enhanced with lanthanum oxides (Turner et al., 2008, Hino, 2004). The
efficiency of the high temperature electrolyser is very dependent on the conditions of
operation such as: temperature and the thermal source.
The efficiency of a solid oxide electrolyser cell, as a function of electrical input, is
reported as 85–90% (National Academy of Science, 2004). Nevertheless, the efficiency
of the solid oxide electrolyser cell can decrease significantly if the thermal source is
considered. For instance, a solid oxide electrolyser cell, which can be linked with nuclear
reactors for thermal input energy, is able to reach an efficiency of 65%. Instead of
investigating traditional sources of energy such as combustion or nuclear, renewable
sources of energy such as solar and wind are promising to result in higher efficiencies
(Yalcin, 1989).
A solid oxide electrolyser can be combined with a solid oxide fuel cell for co-
generation of energy and electricity (Joensen, 2002). In this hybrid system, a solid oxide
fuel cell and solid oxide electrolyser cell are integrated into the same stack and fed from
39
the same fuel, such as natural gas. The feasibility of their system was proven as their
results show an efficiency of up to 69% (Joensen, 2002).
3.4. Photovoltaic Panels
Photovoltaics allow customers the opportunity to produce electricity in a clean, quiet and
reliable way. Photovoltaic systems are included in photovoltaic cells as devices that
transform light energy directly into electricity. Since the source of light is typically the
sun, they are usually named solar cells. The word photovoltaic originates from ―photo,‖
denoting light, and ―voltaic,‖ which involves generating electricity. Thus, the
photovoltaic procedure is ―producing electricity directly from sunlight.‖ Photovoltaics are
often mentioned as PV.
For some systems where insignificant amounts of electricity are required, like
emergency call boxes, PV systems are cost acceptable even when grid electricity is not
far away. When applications require greater amounts of electricity and are located away
from current power lines, photovoltaic systems can be the least expensive, most feasible
option. PV cells transform sunlight into electricity without any air or water pollution. PV
cells are prepared by a minimum of two layers of semiconductor material. One layer has
a positive charge, the other negative. When light goes into the cell, some of the photons
from the light are captured by the semiconductor atoms, releasing electrons from the
cell’s negative side to pass over an external circuit and back to the positive side. This
stream of electrons produces electric current.
40
Load
External
Circuit
Encapsulate seal
Top electrical contact
P-Type material
(boran-doped silicon)
P/N junction
N-Type material
(phoshorous-doped
silicon)
Base contact
Figure 3.5: Basic solar cell construction (modified from ASES, 2012).
To increase their usefulness, numerous individual PV cells can be brought
together in a sealed, weatherproof package called a module. When two modules are
connected together in series, their power is doubled while the current remains constant.
When two modules are connected in parallel, their current is doubled while the voltage
remains constant. To obtain the preferred voltage and current, modules are connected in
series and parallel into a PV array. The adaptability of the modular PV system permits
developers to construct solar power systems that can encounter a wide variability of
electrical requirements, regardless of large or small (Joshi et al., 2009).
Some consumers are using PV as a green and reliable energy source although it is
frequently more expensive than power accessible from conventional electric utilities.
These consumers can complement their energy requirements with electricity from their
residential utility when the PV system is not providing enough energy (at nights or on
41
cloudy days) and can transfer extra electricity back to their residential utility when the PV
system is producing more energy than required.
Cell Module Array
Figure 3.6: Photovoltaic cells, modules and arrays (modified from ASES, 2012).
While PV systems might need a considerable investment, they can be more
economical than paying the costs related to the electric utility grid. A network-connected
PV system will need a function interactive DC to AC inverter. This unit will transform
the direct current (DC) electricity which is created by the PV array into alternating
current (AC), which is the electricity needed for loads such as radios, televisions and
refrigerators. Utility interactive inverters are integrated with safety structures which are
required by electric utilities (Joshi et al., 2009).
42
DC Load
AC Load
Inverter
BatteryChange controller
Photovoltaicarray
Figure 3.7: Mechanisms of typical off-grid PV system (modified from ASES, 2012).
For a self-sufficient PV system, users should determine whether they want to use
the direct current (DC) from the PV or convert the power into alternating current (AC).
Utilization and lights for AC are usually cheaper, but the transformation of DC power
into AC can cost up to 20 percent of the power generated by the PV system (ASES,
2012). To store electricity from PV, batteries will be required. The batteries in PV
systems are dissimilar to car batteries. The most appropriate batteries for use with PV
systems are secondary or deep cycle batteries. There are two types of deep cycle
batteries: lead acid, which need the addition of water, and encapsulated electrolyte (or
gel-cell) batteries. Furthermore, PV systems require suitable wiring, adjustments and
fuses for safety, controllers to protect the batteries from becoming overcharged or
43
completely discharged, diodes to permit current to stream in the right track, and
foundation instruments to protect against lightning attacks.
There are some technical challenges related to photovoltaic systems. The most
important is its relatively low efficiency. The efficiencies of solar photovoltaic panels are
9-13%; depending on semiconductor materials. In this thesis, solar panels are used to
provide electrical energy for PEM electrolysers. The produced hydrogen is used in the
methanol synthesis reactor. Therefore, low efficiency of solar panels is one of the
challenges related to methanol synthesis production. The other challenge is design of PV
systems with curved PV arrays; which are not available. An optimal design will help
designers to produce curved panels with respect to the solar radiation angle. The
methanol synthesis plant uses solar panels. This plant can be used in different
environmental conditions. However, the solar radiation angle is different in different
places. Therefore, a suitable design tool is required for different region.
44
Chapter 4: Systems Description
A schematic of solar based methanol production is provided in Figure 4.1. Carbon
dioxide is extracted from an industrial plant which can be a cement or power plant. In
order to produce methanol, a suitable ratio of hydrogen must be added. In this system,
hydrogen is supplied from a PEM water electrolyser. Solar panels are investigated to
provide the required electricity for the electrolyser. A PEM electrolyser is chosen because
of its higher efficiency compared to an alkaline electrolyser. Solar panels are utilized, in
order to produce synthetic fuel from a sustainable and renewable source of energy. Part
of the required water is supplied by feedback from the methanol rector. The supplied
water is passed through the chemical reactor, in order to have a role as the reactor cooling
system and also to become warm enough (T=353 K) to increase the efficiency of the
electrolyser. The feed gas needs energy to reach its reaction temperature. This required
energy can be obtained from heat recovery from the exhaust gas from the boiler of the
plant.
4.1. PV Panels
Photovoltaics are known for producing electricity power by using solar cells to transform
energy from the sun into a stream of electrons. The photovoltaic influence refers to
photons of light stimulating electrons into a higher state of energy, permitting them to act
as charge exporters for an electric current. The term photovoltaic states the balanced
45
functioning of a photodiode in which current through the device is due to the transduced
light energy. Practically all photovoltaic devices are some type of photodiode.
Solar cells generate DC electricity from sunlight, which can be utilized to power
equipment or to recharge a battery. A useful application of photovoltaics is to power
orbiting satellites and other spacecraft. However, today most photovoltaic modules are
utilized for network connected power generation. In this case, an inverter is needed to
adapt the direct current to alternating current. PV panels can also be used for off-grid
power for remote dwellings, boats, recreational vehicles, electric cars, roadside
emergency telephones, remote sensing, and cathodic protection of pipelines. To increase
their usefulness, individual PV cells are unified together in a sealed, weatherproof
package called a module. When two modules are connected together in series, their
power is doubled while the current remains constant. When two modules are connected in
parallel, their current is doubled while the voltage remains constant. To obtain the
preferred voltage and current, modules are connected in series and parallel into a PV
array. The adaptability of the modular PV system permits developers to construct solar
power systems that encounter a wide variability of electrical requirements.
4.2. PEM Electrolyser
PEM electrolysers are based on the latest advances in PEM fuel cell technology. The
materials of electrodes of PEM electrolysers are typically platinum, iridium, ruthenium,
and rhodium. The material of the membrane is usually Nafion, which not only splits the
two-half cells, but performs as a gas separator. In PEM electrolysers, water is split at the
anode to protons and oxygen. The protons are transferred via the membrane to the
cathode. In the cathode, protons are converted into hydrogen. The oxygen gas is
46
separated and stored for other purposes. In some cases, depending on the required purity,
a heater can be used to evaporate water after the gas-liquid separations unit. The PEM
electrolyser has low ionic resistance and consequently high current density.
Solar Energy
Solar Electricity
Hydrogen Production Plant (PEM)
P = 0.1 MPa298<T (K)<353
Industrial Unit
CO2 from flue gas
Methanol Production
Plant
Fresh Water Feedback Water
Methanol
H2
Oxygen Production
Water
CH3OH
Qout(1)
QoutQin(1)
Supply Water
Heat Recovery
Hydrogen Storage
Mixing Chamber
Mixing Chamber
O2
Figure 4.1: Schematic of solar based methanol and hydrogen production.
Through the electrolysis process, electricity and heat are brought to the PEM
electrolyser to operate the electrochemical reactions. Water is heated in order to reach the
temperature of the PEM electrolyser by passing water through the chemical reactor.
Hydrogen is produced in the cathode, while oxygen gas is produced at the anode. In the
PEM electrolysis reaction, water plus electricity and heat converts to hydrogen and
oxygen. A hydrogen storage system is considered in order to provide the required amount
of hydrogen to the methanol production plant during the night and cloudy days. It is
assumed that the rate of hydrogen for both the methanol synthesizer and storage are the
same.
47
4.3. Methanol Synthesis Reactor
The schematic of the methanol synthesizer is shown in Figure 4.2. This system is
modified from Rihko-Struckmann et al. (2010). The methanol synthesis process was
examined for the following conditions.
Four adiabatic reactor units, in cascade configuration, are considered by
assuming equilibrium conversion with the defined stoichiometric reactions.
Methanol Reactor Cascade
CO2, H2, CH3OH, H2O and CO2
Qin(1)
Heat recovery
from plant
Comp-feed
Comp-2
CO2, H2, CO2
Separation
Water
Methanol
Comp-1
Qout(1)
Electrolyser water supply
Qout(2)
Cooling water
Qout(3)
Cooling water
Qout(3)
Cooling water
H2 and CO2
Mixing Chamber
Figure 4.2: Schematic of the methanol synthesizer
The input temperature is 493 K in each adiabatic reactor. Intermediate cooling
between the reactors was simulated by locating heat exchangers between the
reactor units.
48
The direct reaction of carbon monoxide with hydrogen also occurs in order to
produce methanol. This reaction is the most important reaction in the presence of
Cu/ZnO2/Al2O3 catalysts in the large scale production of methanol. The overall reactor
pressure is 5 MPa, and a pressure drop of 25 kPa is assumed in each reactor and heat
exchanger.
The designated pressure and temperature ranges relate to usual industrial
conditions for low pressure methanol synthesis. After the reactor section, the pressure is
decreased to 1.2 MPa. The remaining feed compounds (carbon monoxide, carbon dioxide
and hydrogen) are separated in the main flash unit; reprocessed and moved back in a
second-step to the reactor under a pressure of 5 MPa. The new hydrogen and carbon
dioxide feed is brought into the system at ambient pressure (0.1 MPa) and compressed to
3 MPa. The compressors are assumed to work polytropicly with a compression efficiency
of 0.8 and work efficiency of 0.9. This kind of reactor with its recycling loop reaches a
high conversion of CO2 which is about 40%. The remaining carbon dioxide and hydrogen
in the production stream of methanol and water are removed in a flash unit at 323 K
before the final methanol distillation, in order to avoid low temperatures in the distillation
column condenser. Since methanol formation reactions are exothermic, heat is generated
in the reactors. Consequently, the temperature of the product flow increases in each
reactor unit. The heat is brought after each reactor unit through the heat exchangers after
the reactors. Moreover, heat exchangers are used to decrease the temperature of the
compressed feed gas (Rihko-Struckmann et al., 2010). Mechanical work is applied for the
compression of the feed and the recycle gas to the reaction pressure at 5.0 MPa. The
cooling system is required to cool down the reactor and increase the efficiency of the
49
system. Warming up the feed water before the electrolyser will increase the efficiency of
the electrolyser. Therefore, feed water can absorb heat from the reactor. On the other
hand, feed gases need energy to reach the reaction temperature. This supply energy can is
recovered from the exhaust gas from the plant boiler (Rihko-Struckmann et al., 2010).
The production of methanol has gone through numerous developments and
alterations. Among the alternations, the prevalent adjustment was a switch from high-
pressure to low-pressure synthesis. Both high-pressure and low- pressure procedures
required heterogeneous catalytic conversion to methanol from synthesis gas normally
initiated from natural gas or, otherwise, from coal. The quality and structure of synthesis
gas varies widely, depending on the procedure of conversion as well as the quality of the
feedstock. Thus, a diversity of commercial designs have replicated and included these
differences. Consequently, it is crucial that the chemistry of synthesis gas conversion is
well understood in the synthesis of methanol. It should be noted that syngas can be
produced by a mixture of carbon dioxide and hydrogen, steam reforming of natural gas
and also from gasification of coal. The syngas from natural gas is more pure, compared to
coal gasification (Lee et al, 1994).
50
Chapter 5: Energy and Exergy Analyses
In this chapter, thermodynamic analysis will be performed for three systems, namely:
PV system;
PEM electrolyser; and
methanol synthesis process.
Each system is analysed individually and the results are investigated to evaluate the
integrated structure. Thermodynamic analyses are performed for each system. Energy and
exergy analyses are performed for the water electrolyser plant. Based on the results from
the electrolyser and the amount of carbon dioxide from flue gases, the methanol plant is
designed. To observe the other parameters of the methanol synthesis process, a
parametric study is investigated. To evaluate the integrated system, an exergy analysis is
performed based on the total exergy input and output.
5.1. General Equations
For a control mass system exchanging heat and work with its environment, during a
change of state from 1 to 2, there is a change in energy as follows:
∫
(5.1)
The energy E has a significant physical meaning by representing all forms of
energy of the system at a given state. The forms of energy include kinetic or potential
51
energy, energy of the moving molecules, energy associated with the structure of the atom,
chemical energy, electrical energy, or other forms of energy. It is convenient to represent
all of the forms of energy of the system, except kinetic and potential energies, in the form
of the system internal energy.
or
(5.2)
Substituting Equation (5.2) into Equation (5.1), writing kinetic and potential
energies in their common forms, and integration from state 1 to state 2, a general form of
the energy equation for a control mass system is obtained:
(
)
(5.3)
Equation (5.3) is used to derive a time-dependent equation in the development of
the energy equation for the control volume:
(5.4)
Although Equation (5.4) is derived for a control mass system, it can be used
exactly in the current form for a control volume system. However, since there is a
possibility of mass transfer to/from the boundaries of the control volume, proper
consideration regarding the conservation of mass must be given.
With several possible mass flows, the conservation of mass is
∑ ∑ (5.5)
52
where and are inlet and outlet mass flow rates. The final general form of the
energy equation is described by Equation (5.6).
∑ (
) ∑ (
) (5.6)
The general, transient energy equation is applicable in modeling of a system. Depending
on the type of process, the terms in Equation (5.6) may be reduced or subdivided further.
Through similar steps, the second law of thermodynamics for a control volume
system can be given in the form of the thermodynamic property, entropy.
∑ ∑ ∑
(5.7)
The exergy equation is defined based on the principle laws of thermodynamics.
For a control volume system or process, the exergy equation is in the form of
∑(
)
∑ ∑ (5.8)
where ∑(
) is the exergy rate transferred by heat,
is the
exergy rate transferred by shaft or boundary work, ∑ ∑ is the exergy rate
transferred by flow and is the exergy rate of destruction.
The term exergy destruction is also called irreversibility:
(5.9)
Energy analysis is performed based on the thermodynamic state of each point/flow of the
system, and with Equation (5.6). Performing an exergy analysis requires the
understanding of the exergy of flows and streams.
53
5.2. Exergy of Flows and Streams
The exergy content of a flow is a summation of its chemical and physical exergy values.
Physical exergy is an extent of the temperature and pressure difference from the
environment/reference state. Chemical exergy is measured according to a reference
environment; such as nature, the oceans, or the earth (T0, P0 in Table 6.1) . Its value is
defined based on the equilibrium of the chemical content of the substance, the flow or the
system with the components available in the environment. Any difference between the
chemical components of the flow, or the system from the reference environment, initiates
the appearance of chemical exergy.
5.2.1. Reference Environment
The intensive property of a reference environment, in which exergy is evaluated, helps
determine the exergy of a stream. The reference environment acts an infinite system and
a sink or source of energy and material flow. Exergy of the reference environment is
zero, as with the exergy of any stream and system with the same temperature, pressure
and chemical composition as the reference environment. The ability of a stream or a
system to perform work is measured based on its deviation from the reference
environment. The natural environment is not necessarily a reference environment, since
its properties change by the processes. Thus several reference environments have been
proposed (Dincer and Rosen, 2007). Determination of the reference environment depends
on the process and the tools for analyzing the processes.
Although it is required to calculate the exergy content of each point or
thermodynamic state of the process in exergy analyses, another important concern is the
54
change of exergy content while the system undergoes a process or a change of state.
Therefore, for system undergoing a change in state from 1 to 2, the exergy difference is
calculated as:
(5.10)
If there is no change in the chemical composition of the flow or the system, the
change in chemical exergy of the process is zero. Thus, in the exergy analysis of such
systems, only the change in physical exergy is taken into consideration.
5.2.2. Exergy of Material Flow
The exergy content of a material flow is a combination of the flow and material
characteristics. Chemical exergy of the material is considered in exergy calculations, and
the physical exergy is measured as a function of the thermodynamic state of the flow
(Dincer and Rosen, 2007). The exergy of kinetic and potential energy of the flow is the
same as the calculated values for these energies. Therefore,
(5.11)
where is potential energy rate and is kinetic energy rate. Also the following
equations will be used:
∑ (5.12)
(5.13)
where is the change in Gibbs energy of the chemical compound, is the specific
reference chemical exergy of each component of the chemical compound of the flow, and
is the molar amount of each component.
55
5.2.3. Exergy of Work
The exergy of shaft or electricity work equals the amount of work. However, the exergy
of work due to the change of the control volume from V1 to V2, Wnet, is
(5.14)
in which is work done by the system due to the change in volume.
5.2.4. Exergy of Thermal Energy
Consider a system having heat interaction with its environment. If the temperature of the
control mass is assumed to be constant, the minimum work required by the control mass
and its environment to bring the control mass to the next state is the thermal exergy as
follows:
(
) (5.15)
which is the rate of heat transferred to the system.
For a sustainability analysis, a useful tool is exergy analysis, specifying the
sources of irreversibilities, and the potential for improvement. Re-writing the exergy
balance equation, Equation (5.8), one may obtain:
Exergy input − Exergy output − Exergy consumption = Exergy accumulation (5.16)
If the process is steady state, the exergy accumulation is zero. Exergy destruction
is a measure of entropy generation in the system, which is shown in Equation (5.8) as
exergy destruction. The exergy output accounts for all exergies leaving the system, either
in the form of useful exergy or waste. This includes the exergy loss due to heat loss to the
environment through the system boundaries.
56
5.3. Overall Efficiencies
Efficiency is a measure of the effectiveness of a system or process regarding the ability of
the system or process to produce useful work with minimum input energy and waste. In
thermodynamic systems, efficiency is defined as the ratio of useful produced
energy/work over the total energy input to the system. The quantity and quality of energy
streams are different, according to the second law of thermodynamics. Therefore,
defining the efficiencies of systems based on the exergy contents of the outputs and
inputs can provide a useful insight in defining the performance of systems.
Energy and exergy efficiencies of steady state processes can be written as (Dincer
and Rosen, 2007)
(5.18)
(5.19)
Exergy efficiencies differentiate the losses due to irreversibilities from effluent
losses, thus they show the potential for improvement in the system by decreasing the
effluent losses.
5.4. Procedures for Energy and Exergy Analyses
A useful procedure for performing energy and exergy analyses involves the following
steps (Dincer and Rosen, 2007).
Subdivide the processes into many desired sections so that the forms of processes
are considered for each section.
57
Determine all quantities such as work and heat transfer rates by performing mass
and energy balances on the sections.
Define the reference environment (dead state condition) based on the all materials
of the processes and the ranges of temperature and pressure in the system.
Evaluate energy and exergy values, relative to the selected reference-environment
model.
Perform exergy balance equation on each section, and determine the rates of
irreversibilities.
Define the efficiency of the processes considering the magnitude of the streams
and purpose of each process, and evaluate the values of efficiencies.
Interpret the results, and make appropriate conclusions and recommendations,
relating to such issues as design changes, and retrofit plant modifications.
5.5. Electrochemistry Analysis of PEM Electrolyser
The overall reaction of water splitting is given by
In the anode, oxidation occurs according to
⁄
In the cathode the following reaction takes place
58
The outlet flow rate of H2 can be determined by
(5.20)
where is the current density, F is the Faraday constant and is the rate of
consumed in the reaction. The flow rate of exiting oxygen can be expressed as:
(5.21)
The rate of electric energy input to the electrolyser is given by
(5.22)
(5.23)
where is the rate of electric energy input, is the rate of electric exergy
input, is the reversible potential, which is related to the difference in free energy
between reactants and products, and can be obtained by the Nernst equation. Also,
, and are the activation overpotential of the anode, activation
overpotential of the cathode, and the ohmic overpotential of the electrolyte, respectively.
5.6. Ohmic Overpotential
The ohmic overpotential of the proton exchange membrane is related to the resistance of
the membrane to hydrogen ions crossing over it. The ionic resistance is a function of
humidification, thickness and temperature of the proton exchange membrane. The local
ionic PEM conductivity of the membrane can be expressed as (Guran et al., 2000):
[ ] [ ] * (
)+ (5.24)
59
where is the distance in the membrane measured from the cathode membrane interface
and is the water content at a location x in the membrane. The value of can be
calculated in terms of the water content at the photon exchange membrane edges (Guran
et al., 2000):
(5.25)
where D is the membrane thickness, and are the water contents at the anode-
membrane and the cathode-membrane interface, respectively. In the water mixing
chamber, is exists and stimulates to be transferred from the anode. Therefore,
represents the water content at the cathode-membrane interface. The overall ohmic
resistance can be calculated as
∫
[ ]
(5.26)
According to Ohm’s law, the following equation can be written for the ohmic
overpotential:
(5.27)
5.7. Activation Overpotential
The activation overpotential, , caused by a deviation of net current from its
equilibrium, and also an electron transfer reaction, must be differentiated from the
concentration of the oxidized and reduced species. The conditions of electrolysis have an
important role in the difference between reversible and irreversible electrode processes.
60
In this study, the following equations will be used for the activation overpotentials
(Butler-Volmer equation) (Hamann et al., 2007):
*[ (
) (
)+ (5.28)
where is the exchange current density and subscripts a and c refer to the anode and
cathode, respectively. Also, is the charge transfer coefficient (symmetry coefficient) for
the anodic or cathodic reaction, usually close to 0.5. The Butler-Volmer equation treats
the coefficient differently depending on whether the reaction is cathodic or anodic. A
value of of 0 is ideal for anodic reactions and is ideal for cathodic reactions. In
this thesis, it is assumed that . Also, is the number of electrons involved per
reaction. For water electrolysis, z is 2 (Esmaili et al., 2012). Equation (5.28) can be
written as
(
) (5.29)
The exchange current density is a significant parameter in calculating the
activation overpotential. It characterizes the electrode’s capabilities in the
electrochemical reaction. A high exchange current density implies a high reactivity of the
electrode, which results in a lower overpotential. The exchange current density for
electrolysis can be expressed as (Thampan et al., 2007):
(
) (5.30)
where
is the pre-exponential factor and is the activation energy for the anode
and cathode.
The total exergy can be expressed as (Dincer and Cengel, 2001)
61
(5.31)
where and are the physical and chemical exergies (Rosen and Dincer,
1997).
5.8. Exergy and Energy Analyses of Electrolyser
Exergy analysis is a method that uses conservation of mass and energy principles
together with the second law of thermodynamics for the analysis, design and
improvement of energy and other systems. The exergy method is a useful method for
improving energy-resource use, as it enables the locations, types and magnitudes of
losses to be identified and efficiencies to be determined. The energy and exergy
efficiencies are defined as (Ni et al., 2008)
(5.32)
(5.33)
where and represent the energy and exergy efficiencies. Also, is the lower
heating value of hydrogen, is the outlet flow rate of hydrogen, and
are the rates of electric and thermal energy input for the electrolyser
respectively, and and are the rates of electric exergy input and the
rate of thermal exergy input, respectively. Lastly, is the exergy content of hydrogen.
5.9. Methanol Production Analysis
Methanol synthesis occurs in 2 steps:
62
1. CO2 + H2 CO + H2O (RWGS Reaction) (5.34)
2. CO + 2H2 CH3OH (5.35)
The overall reaction occurs according to
3. CO2 + 3H2 CH3OH + H2O (5.36)
In order to solve the problem, the equilibrium conversion degree is defined as
(5.37)
(5.38)
where is an equilibrium conversion degree for each chemical reaction, and is the
number of moles for each component before and after the reaction . It
should be noted that the conversion degree refers to an initial total of mols in the
mixture, but not selected components (these components can be water, carbon dioxide,
carbon monoxide, hydrogen and methanol). Reaction (1) changes its direction for
combinations containing initially even a small quantity of CO. Under these
circumstances, carbon dioxide becomes a reactant in reaction (3) and a product in
reaction (1). Therefore, the definition of the conversion degree with respect to carbon
dioxide would be inconvenient.
The following steps are used to solve the problem to find the methanol yield:
(5.39)
(5.40)
63
(5.41)
(5.42)
(5.43)
The equilibrium conversion degrees,
and
, can be found by solving a non-
linear system and equilibrium constant of two reactions as follows:
(
)
(
)(
)
(5.44)
(
)
(
)(
)
(5.45)
where and are equilibrium constants for each reaction. The equilibrium
constant is a useful definition to express the concentrations of reactants and products in
units other than partial pressures. , , ,
and are molar fractions of
carbon monoxide, water, carbon dioxide, hydrogen and methanol in equilibrium. The
equilibrium constant can be expressed in terms of partial pressures, and denoted as .
The relation between and is defined as
(
)
(5.46)
where is the difference between product and reactant coefficients in a balanced
chemical reaction.
64
According to the kinetics of methanol synthesis by Lim et al. (2009), for
reactions 5.34 - 5.36 are written as
(5.47)
(5.48)
(5.49)
(5.50)
(5.51)
(5.52)
It should be noted that the equilibrium constant is a function of temperature. Also,
, , and are the gas constant, reaction pressure, atmospheric pressure and
temperature, respectively.
5.10. Solar Energy Analyses
Photovoltaics provide the opportunity to produce electricity in a clean, quiet and reliable
way. Photovoltaic systems are included in photovoltaic cells, as devices that transform
light energy directly into electricity. Since the source of light is typically the sun, they are
usually named solar cells. The word photovoltaic originates from ―photo,‖ denotation
light, and ―voltaic,‖ which means to generating electricity. Thus, the photovoltaic
procedure is ―producing electricity directly from sunlight.‖ Photovoltaics are often
65
mentioned as PV. Solar panels are used to supply the required energy of the electrolyser.
The following equation is given to calculate the power produced by a PV module:
(5.53)
where is the produced energy by PV panels. Also, , , , and are the
solar cell efficiency, incident solar intensity, packing factor of a solar cell, and transitivity
of glass and area of PV module, respectively. Table 5.1 shows the solar PV parameters.
Table 5.1: Solar PV parameters
Parameter Values
400
0.83
0.12
0.95
5.11. Energy and Exergy Efficiencies of the Integrated System
According to the system configuration, the required electricity and thermal energy for
hydrogen and methanol production come from solar panels and heat recovery of the
plant, respectively. In this section, energy and exergy analyses will be investigated in
order to evaluate the system. Exergy analysis is a method that uses conservation of mass
and energy principles together with the second law of thermodynamics for the analysis,
design and improvement of energy and other systems. The exergy method is useful for
improving energy-resource use, as it enables the locations, types and magnitudes of
losses to be identified and efficiencies to be determined. To enhance the understanding of
66
the performance assessment of the system, the energy and exergy efficiencies of the
system are defined and calculated as follows:
(5.54)
(5.55)
where is the exergy rate of the produced methanol and is the exergy
rate of solar energy. Equations (5.53) and (5.54) establish the energy and exergy rates of
the produced methanol.
(5.56)
(5.57)
where HHVmethanol is the higher heating value of methanol which is equal to 726 kJ/mol
and is the chemical exergy of the liquid methanol which is equal to 718
(Arons et al., 2004).
Also, the energy and exergy rate of solar energy can be calculated as follows:
(5.58)
* (
)+ (5.59)
where , and are the ambient temperature, incident solar intensity and area of PV
module, respectively. Also, is sun surface temperature, which is assumed as 5778
K.
67
The analyses related to the system in chapter 4, has been performed in this
chapter. In Chapter 6, the results of these calculations will be discussed and figures will
be shown to illustrate the results.
68
Chapter 6: Results and Discussion
In this chapter, results will show the effects of temperature and current density on
hydrogen production. The analyses related to methanol synthesis at different
temperatures and pressures will be conducted in the second stage. Next, based on the
optimum point of hydrogen production as well as the available extracted carbon dioxide
from the plant, the final designed methanol plant will be modeled and efficiencies related
to this designed plant will be studied.
6. 1. PEM Electrolyser Analyses
An electrochemical model will be used to simulate the performance of the electrolyser.
The electrolyser has two platinum electrodes.
Table 6.1: Design parameters for the PEM electrolyser.
Parameters Values Reference
(kPa) 101
(kPa) 101
T (K) 353
Eact,a (kJ/mol) 76 (Thampan et al., 2001)
Eact,c (kJ/mol) 85.5 (Thampan et al., 2001)
14 (Ni et al., 2008)
10 (Ni et al., 2008)
D ( 100
0.5 (Chan and Xia, 2002)
2 (Chan and Xia, 2002)
T0 (K) 300
P0 (kPa) 101
69
The electrolyte is made of Nafion, a perfluorosulfonic (PFSA) polymer, which is
widely used as an electrolyte in fuel cells and electrolysers. At 353 K, the values of
exchange current density for the anode and cathode are 1.0 105 and 10 ,
respectively. Table 6.1 gives other parameters.
Figure 6.1 shows the overpotentials related to platinum electrodes and ohmic
losses when the current density is changing. Figure 6.1 is obtained based on equations
(5.28)-(5.34), and each overpotential is then shown separately. It can be observed that
membrane losses are less than the activation overpotentials. Membrane losses are based
on Ohm’s law. It can be concluded from Figure 6.2 that for a high current density, the
activation overpotentials remain nearly constant, while the ohmic overpotential is rising.
Figure 6.1: The overpotentials with platinum electrodes and ohmic losses.
0
0.2
0.4
0.6
0.8
0 2 4 6 8 10
Ove
rpo
ten
tia
ls (
V)
Current Density (kA/m2)
cathode activation overpotential
Anode activation overpotential
Ohmic overpotential
70
Figure 6.2 shows the total cell potential versus current density at different
temperatures. The total cell potential is predicted from Equation (5.27) and the Nernst
equation. V0 is the reversible potential, which is related to the difference of free energy
between reactants and products. In Figure 6.3, the role of temperature in the total cell
potential is shown. These results are obtained from equations (5.27)-(5.33). The reactant,
as well as the electrodes, will be activated easier at higher temperatures, resulting in
lower activation energies. According to equations (5.27)-(5.33), the exchange current
density will increase when temperature is increasing and the activation overpotential will
decrease. It should be noted that the electrolyser is working at standard pressure, and
temperatures above 353 K are not recommended.
Figure 6.2: Total cell potential versus current density at different temperatures.
Figure 6.3 compares the thermal energy input and heat production caused by
irreversible losses. According to the irreversibilities, if the thermal energy demand
0
0.5
1
1.5
2
2.5
0 2 4 6 8 10
Cell
Po
ten
tia
l (V
)
Current Density (kA/m2)
T=300 K
T=323 K
T=353 K
71
(required heat for water splitting reaction) yields (Sgen)≥T∆S then Qheat,PEM=0. Therefore,
there is no need for external sources of heat in this electrolyser. For other temperatures,
the same result will occur. Figure 6.4 shows the same issue at 353 K.
Figure 6.3: Comparison between thermal energy demand and heat production caused by
irreversible overpotential losses (T = 300 K).
Figure 6.4: Comparison between thermal energy demand and heat production caused by
irreversible overpotential losses (T = 300 K).
0
50
100
150
200
250
0 2 4 6 8 10
Th
erm
al E
ne
rgy (
kJ/m
olH
2)
Current Density (kA/m2)
2F(ohmic overpotential +electrode activationoverpotentials)TΔS
0
25
50
75
100
125
150
175
200
0 2 4 6 8 10
Th
erm
al E
ne
rgy (
kJ/m
olH
2)
Current Density (kA/m2)
2F(ohmic overpotential +electrode activationoverpotentials)TΔS
72
Figure 6.5 shows how the cell potential varies when temperature is changing for a
constant current density. The practical rages of electrolyser temperature and cell
potentials are 300-360 K and 1-2.5 V, respectively (Ni et al., 2008). The cell potential is
decreasing when temperature is increasing in a linear fashion, i.e., Equations (5.31)-
(5.33). It can be noted that less current density will result in less overpotential as well as
cell potential.
As mentioned above, an increasing temperature will reduce the overpotentials and
cell potential. The energy efficiency is defined as the amount of energy output (hydrogen)
per amount of energy input. Energy input is related to the cell potential and
overpotentials. By increasing the temperature, these overpotentials will decrease, which
will increase the efficiency (Figure 6.6).
The energy efficiency of the electrolyser is found to decrease with an increase in
membrane thickness. This is due to a thicker electrolyte membrane, has higher ohmic
overpotential and higher operating potential (Figure 6.7).
Figure 6.5: Change of cell potential when temperature is changing for a constant amount
of current density.
1.95
2
2.05
2.1
2.15
2.2
290 310 330 350 370
Cell
po
ten
tia
l (V
)
Operating Temeprature (K)
J=8 kA/m
J=5 kA/m
J=2 kA/m
2
2
2
73
Figure 6.6: Change of energy efficiency at different temperatures.
Figure 6.7: Effect of membrane thickness on PEM electrolyser performance.
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10
En
erg
y E
ffic
ien
cy (
%)
Current Density (kA/m2)
T= 353 K
T= 323 K
T= 300 K
1
1.25
1.5
1.75
2
2.25
0 2 4 6 8 10
Cell
Po
ten
tia
l (V
)
Current Density (kA/m2)
L= 200 μm
L=100 μm
L= 50 μm
74
It can be observed that the anode activation overpotential controls the overall
voltage loss due to low reactivity of the anode material. This is expressed by the low
anode exchange current density (J0,a). Figure 6.8 shows the effect of anode exchange
current density on electrolyser plant efficiency. The energy efficiency of the electrolyser
increases considerably with increasing J0,a as the electrolyser cell potential decreases
significantly with J0,a (Figure 6.9). Thus, it is important to improve the anode catalyst to
increase J0,a.
Figure 6.8: Effect of electrode exchange current density on electrolyser efficiencies.
0
20
40
60
80
100
0 2 4 6 8 10
En
erg
y E
ffic
ien
cy (
%)
Current density J (kA/m2)
Anode exchange current density= 1.0 × 10 A/m
Anode exchange current density= 1.0 × 10 A/m
Anode exchange current density= 10 A/m
2
2
2
-2
-5
75
Figure 6.9: Effect of electrode exchange current density on cell potential.
Figure 6.10 shows the effect of temperature on the electrical energy inputs. A
high operating temperature is suggested for hydrogen production by the electrolyser
plant. However, the operating temperature normally does not go beyond 373 K since
liquid water is required for high ionic conductivity of the PEM electrolyte.
Figures 6.11a and 6.11b show the consequence of current density on the energy
and exergy efficiencies of the electrolyser plant at a temperature of 353 K. The energy
efficiency and exergy efficiency curves are close to each other (Figure 6.5b). Both
efficiencies decrease at an increasing rate with J. The electrical energy input, and the
energy output of hydrogen production, are illustrated and presented in Figure 6.5a. The
energy inputs and energy outputs increase at different rates with increasing J. The energy
0
0.5
1
1.5
2
2.5
0 2 4 6 8 10
Cell
Po
ten
tia
l (V
)
Current density J (kA/m2)
Anode exchange current density= 1.0 × 10 A/m
Anode exchange current density= 1.0 × 10 A/m
Anode exchange current density= 10 A/m
-5
-2
2
2
76
efficiency decreases with J because the total energy input increases with J at a rate higher
than the energy output (hydrogen produced).
Figure 6.10: Effect of temperature on electrical energy inputs.
(a)
0
5
10
15
20
25
0 2 4 6 8 10
Ele
ctr
ica
l E
ne
rgy I
np
ut (k
W)
Current Density (kA/m2)
T= 300 K
T= 323 K
T= 353 K
0
2.5
5
7.5
10
12.5
15
17.5
20
0 2 4 6 8 10
En
erg
y (
kW
)
Current Density (kA/m2)
Electrical energy input
Energy output (hydrogen)
77
(b)
Figure 6.11: Energy and exergy (a) rates and (b) efficiencies versus current densities.
6.2. Methanol Synthesis
Methanol synthesis reactions, both carbon monoxide and carbon dioxide hydrogenation,
are thermodynamically not driven at low pressures and high temperatures, as shown by a
plot of Kp versus temperature in Figure 6.12. Decreasing the temperature of the reaction
is kinetically unfavourable, since it makes a significant reduction in the reaction rate.
Therefore, the synthesis reaction must be conducted at comparatively high temperatures
and high pressures. On the other hand, higher pressure conditions may result in higher
capital costs, larger energy requirements and strict operational conditions. Moreover, a
higher temperature increases the thermal deactivation of the catalyst. Furthermore,
temperature increases because of the exothermic heat of reaction.
50
55
60
65
70
75
80
85
90
95
0 2 4 6 8 10
Eff
icie
ncy (
%)
Current Density (kA/m2)
Energy efficiency
Exergy efficiency
78
Figure 6.12: Temperature dependence of equilibrium constants for principle reactions.
In order to increase the efficiency of methanol synthesis, a recycle loop should be
designed in order to recover the unconverted gases. This loop is essential to increase the
methanol conversion as it was shown that a recycle loop provides economic benefits
(Rihko-Struckmann et al., 2010). According to Figure 6.6, based on Equations (5.51) and
(5.52) (for chemical reactions (5.38) and (5.39)), by increasing the temperature, products
of Equation (5.38) will increase; however, Equation (5.39) is more likely to occur in the
in the reverse direction. Therefore, the usual operating conditions for the methanol
synthesis reaction occur in temperature and pressure ranges of 460 K to 520 K and 5 to 10
MPa.
-10
-5
0
5
10
15
100 300 500 700 900
Lo
g K
Temperature (K)
Carbon dioxide + Hydrogen =Monoixde + Water
Monoxide + 2 Hydrogen =Methanol
79
Figure 6.13: Methanol yield at different temperatures and pressures
.
Figure 6.13 represents the methanol yield at different temperatures and pressures.
Figure 6.13 is based on Equations (5.44)-(5.57). From the diagrams, it is shown that both
temperature and pressure have a considerable effect on the equilibrium conversion
degrees of direct methanol synthesis and the reverse water-gas shift reaction, and also the
equilibrium mole fractions of the components.
The detailed results of the computational model are used to plot the curves which
are illustrated as follows. Figures (6.13)-(6.19) show isobars in the co-ordinate system
ξeq
1, ξeq
2 and xeq
i versus temperature for the initial gaseous mixture without CO and H2O.
80
Figure 6.14: Dependence of equilibrium conversion degree of RWGS reaction on
temperature at different pressures
.
Figure 6.15 Dependence of equilibrium mole fraction of CO on temperature at different
pressures
.
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
400 450 500 550 600 650 700
RW
GS
Con
ve
rsio
n
Temperature (K)
P= 30 MPa
P= 10 MPa
P= 5 MPa
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
400 450 500 550 600 650 700
Mo
le F
ractio
n o
f C
O
Temperature (K)
P= 30 MPa
P= 10 MPa
P= 5 MPa
81
The equilibrium conversion degree ξeq
1 of the main reaction increases distinctly
with increasing pressure and decreases strongly as the temperature is increased. The
decrease in ξeq
1 is almost linear with a steep slope to the curve at pressures of 5-15 MPa,
within a temperature range of 453 K to 543 K, which is a typical methanol synthesis
condition (Skrzypek et al., 1990). In contrast, the equilibrium conversion degree ξeq
2 of
the side reaction increases with increased temperature and decreases as the pressure
increases.
Figure 6.16: Dependence of mole fraction of CO2 on temperature at different pressures
.
The curves of the equilibrium mole fractions of methanol and carbon monoxide
are analogous to those of the respective equilibrium conversions. The isobars for the
equilibrium mole fractions for carbon dioxide and water were found. The equilibrium
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
400 500 600 700
Mo
le F
ractio
n o
f C
O2
Temperature (K)
P= 30 MPa
P= 10 MPa
P= 5 MPa
82
mole fraction of carbon dioxide increases initially with temperature, reaches a maximum
value, followed by a decrease. When it reaches this temperature, reaction (5-35) takes
place in a reverse direction, producing more carbon monoxide and it makes the mole
fraction of carbon dioxide decrease. In contrast, the equilibrium mole fraction of water
decreases initially, reaches to a minimum value, followed by an increase with an
increased temperature. By comparison, Figures (6.20)-(6.26) present isobars for the
analogous co-ordinate system, but for an initial gaseous mixture with a large amount of
carbon monoxide.
Figure 6.17: Dependence of equilibrium mole fraction of H2 on temperature at different
pressures
.
It is observed that if carbon dioxide is present in the synthesis gas, the equilibrium
conversion degree ξeq
2 becomes negative, which means the water-gas shift reaction
proceeds.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
400 450 500 550 600 650 700
Mo
le F
ractio
n o
f H
2
Temperature (K)
P= 30 MPa
P= 10 MPa
P= 5 MPa
83
Figure 6.18: Dependence of equilibrium mole fraction of methanol on temperature at
different pressures
.
Figure 6.19: Dependence of equilibrium mole fraction of water on temperature at
different pressures
.
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
400 450 500 550 600 650 700
Mo
le F
ractio
n o
f M
eO
H (
%)
Temperature (K)
P= 30 MPa
P= 10 MPa
P= 5 MPa
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
400 450 500 550 600 650 700
Mo
le F
ractio
n o
f W
ate
r
Temperature (K)
P= 30 MPa
P= 10 MPa
P= 5 MPa
84
When the reactions contain no carbon monoxide and consist only of carbon
dioxide and hydrogen, ξeq
2 is always positive. Based on the detailed results of the
computations, the effects of the initial concentrations were examined both in the absence
of carbon monoxide, and with carbon monoxide and hydrogen. The equilibrium
conversions ξeq
1 and ξeq
2 were then analysed.
Figure 6.20: Dependence of equilibrium conversion degree of methanol reaction on
temperature at different pressures
.
0
0.05
0.1
0.15
0.2
0.25
0.3
400 450 500 550 600
Me
tha
no
l C
on
ve
rsio
n
Temperature (K)
P= 30 MPa
P= 10 MPa
P= 5 MPa
85
Figure 6.21: Dependence of equilibrium conversion degree of WGS reaction on
temperature at different pressures (
).
Figure 6.22: Dependence of equilibrium conversion mole fraction of CO on temperature
at different pressures
.
0
0.05
0.1
0.15
0.2
0.25
400 450 500 550 600
WG
S R
ea
ctio
n C
on
ve
rsio
n
Temperature K
P= 30 MPa
P= 10 MPa
P= 5 MPa
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
400 450 500 550 600
Me
tha
no
l C
on
ve
rsio
n
Temperature (K)
P= 30 MPa
P= 10 MPa
P= 5 MPa
86
Figure 6.23: Dependence of equilibrium mole fraction of CO2 on temperature at different
pressures
.
Figure 6.24: Dependence of equilibrium mole fraction of H2 on temperature at different
pressures
.
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
400 450 500 550 600
Mo
le F
ractio
n o
f C
O2
Temperature (K)
P=30 MPa
P= 10 MPa
P= 5 MPa
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
400 450 500 550 600
Mo
le F
ractio
n o
f H
2
Temperature (K)
P= 30 MPa
P= 10 MPa
P= 5 MPa
87
Figure 6.25: Dependence of equilibrium mole fraction of methanol on temperature at
different pressures
.
The effect of the initial carbon dioxide concentrations was examined both in their
absence as well with its existence in the synthesis gas. The respective isotherms and
isobars in the co-ordinate system of ξeq
1 or ξeq
2 versus initial amount of carbon dioxide
are presented in Figures (6.27)-(6.30). From these figures, it can be seen that the effects
of the initial carbon dioxide concentration are significant when the synthesis gas contains
no carbon monoxide. A significant increase in ξeq
1 and ξeq
2 is observed when increasing
the initial mole fraction of carbon dioxide. When the synthesis gas contains carbon
monoxide, the corresponding curves are more flattened. It indicates a relatively low effect
of the initial carbon dioxide concentration on the synthesis gas containing carbon
monoxide.
0
0.1
0.2
0.3
0.4
0.5
0.6
400 450 500 550 600
Mo
le F
ractio
n o
f M
eth
an
ol
Temperature (K)
P= 30 MPa
P= 10 MPa
P= 5 MPa
88
Figure 6.26: Dependence of equilibrium mole fraction of water on temperature at
different pressures
.
The effect of initial carbon monoxide concentration on the equilibrium conversion
degree is shown in Figures 6.31 and 6.32. The presence of carbon monoxide in the
synthesis gas is advantageous in direct methanol synthesis from carbon dioxide. A large
increase in the equilibrium conversion ξeq
1 to methanol is seen in Figures 6.31 and 6.32,
while the increased initial concentration of carbon monoxide decreases ξeq
2. The RWGS
reaction proceeds in the reverse direction. Owing to the change in direction of the RWGS
reaction, the advantage increases the equilibrium conversion degree ξeq
1 for the main
reaction. It is interesting that the functions of ξeq
1 and ξeq
2 versus the initial mole fraction
of carbon monoxide are represented as straight lines.
0
0.02
0.04
0.06
0.08
0.1
0.12
400 450 500 550 600
Mo
le F
ractio
n o
f W
ate
r
Temperature (K)
P= 30 MPa
P= 10 MPa
P= 5 MPa
89
Figure 6.27: Effect of initial mole fraction of CO2 on equilibrium conversion degree of
methanol
.
Figure 6.28: Effect of initial mole fraction of CO2 on equilibrium conversion degree of
the RWGS reaction
.
0
0.02
0.04
0.06
0.08
0.1
0.12
0 0.05 0.1 0.15 0.2 0.25
Me
tha
no
l C
on
ve
rsio
n
Mole Fraction of CO2
P= 15 MPa
P= 10 MPa
P= 5 MPa
0
0.005
0.01
0.015
0.02
0 0.05 0.1 0.15 0.2 0.25
RW
GS
Con
ve
rsio
n
Mole Fraction of CO2
P= 15 MPa
P= 10 MPa
P= 5 MPa
90
Figure 6.29: Effect of initial mole fraction of CO2 on equilibrium conversion degree of
methanol
.
Figure 6.30: Effect of initial mole fraction of CO2 on equilibrium conversion degree of
the WGS reaction
.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0 0.05 0.1 0.15 0.2 0.25
Me
tha
no
l C
on
ve
rsio
n
Initial Mole Fraction of CO2
P= 15 MPa
P= 10 MPa
P= 5 MPa
0
0.02
0.04
0.06
0.08
0.1
0.12
0 0.05 0.1 0.15 0.2 0.25
WG
S C
on
ve
rsio
n
Initial Mole Fraction of CO2
P= 15 MPa
P= 10 MPa
P= 5 MPa
91
Figure 6.31: Effect of initial mole fraction of CO on equilibrium conversion degree of
methanol
.
Figure 6.32: Effect of initial mole fraction of CO on equilibrium conversion degree of the
WGS reaction
.
0
0.05
0.1
0.15
0.2
0.25
0 0.05 0.1 0.15 0.2 0.25
Me
tha
no
l C
on
ve
rsio
n
Initial Mole Fraction of CO
P= 15 MPa
P= 10 MPa
P= 5 MPa
-0.05
0
0.05
0.1
0.15
0.2
0.25
0 0.05 0.1 0.15 0.2 0.25
WG
S R
ea
ctio
n C
on
ve
rsio
n
Initial Mole Fraction of CO
P= 15 MPa
P= 10 MPa
P= 5 MPa
92
Figure 6.33: Effect of initial mole fraction of H2 on equilibrium conversion degree of
methanol
.
Figure 6.34: Effect of initial mole fraction of H2 on equilibrium conversion degree of the
WGS reaction
.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0 0.2 0.4 0.6 0.8 1
Me
tha
no
l C
on
ve
rsio
n
Initial Mole Fraction of H2
P= 15 MPa
P= 10 MPa
P= 5 MPa
0
0.02
0.04
0.06
0.08
0.1
0.12
0 0.2 0.4 0.6 0.8 1
WG
S R
ea
ctio
n C
on
ve
rsio
n
Initial Mole Fraction of H2
P= 15 MPa
P= 10 MPa
P= 5 MPa
93
Figure 6.35: Dependence of the amount of methanol in liquid products at equilibrium on
initial composition of synthesis gas and temperature (P = 5 MPa):
.
The effect of the initial hydrogen concentration is presented in Figures 6.33 and
6.34. This effect is significant. For a higher initial hydrogen concentration, there is a
larger increase in ξeq
1 and decrease in ξeq
2.
The dependence of the amount of methanol in the liquid products (methanol and
water) on the initial composition of the synthesis gas and temperature is shown in Fig
6.35. If there is no carbon dioxide in the synthesis gas, the mole fraction of methanol in
the liquid products does not exceed 0.5. A significant increase in the methanol
concentration is observed as the initial carbon monoxide concentration is increased. The
curves in Fig 6.35 show a characteristic maximum with respect to temperature.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
400 450 500 550 600
Mo
le F
ractio
n o
f M
eth
an
ol in
Liq
uid
Temperature (K)
Condition A
Condition C
Condition D
Condition B
94
6. 3. Integrated System of Solar Based Methanol and Hydrogen
Production
The methanol plant discussed in this thesis is the same as the plant studied by Rihko-
Struckmann et al. (2010). In this analysis, it is assumed that the required energy can be
provided by the heat recovered from a plant boiler exhaust gas. Supply water for an
electrolyser is passed through the chemical reactor to serve as the reactor’s cooling
system and increase the efficiency of the electrolyser. The required energy for the
compressors is supplied by the solar panels. A hydrogen storage plant is also considered
to store hydrogen during daylight and provide the required amount of hydrogen during
the night. Based on the available carbon dioxide from the plant, design parameters for
the PEM electrolyser (Table 5.2) and solar panels (Table 5.1), Figure 6.36 and Figure
6.37 are obtained. It should be noted that solar based hydrogen and methanol production
is modeled according to optimum temperatures for hydrogen production and methanol
reactor conditions, which have been specified by Rihko-Struckmann et al. (2010).
It is assumed that 140 Kg of carbon dioxide is extracted per hour from the
industrial plant. It is observed that 32.5 kW is required for starting the chemical reactor. It
should be noted that 32.5 kW can be supplied from solar energy; however, the source of
energy should have temperatures above 493 K.
95
PV Panels(17728 m2)
Solar Electricity
Hydrogen Production Plant (PEM)
P = 0.1 MPa
T = 353 K
Industrial Unit
CO2 from flue gas (rate: 0.86 (mol/s))
Treactor= 493 KMethanol
Production Plant
Fresh Water Feedback Water
Methanol
H2 (rate: 1.04 (mol/s))
Oxygen Production
Water
CH3OH (rate: 0.34 (mol/s))
Qout(1) = 4.4 kW
Qout =32.5 kWQin(1)
Supply Water
Heat Recovery
Hydrogen Storage
Mixing Chamber
Mixing Chamber
O2
H2 (rate: 1.04 (mol/s))
H2O (rate: 0.34 (mol/s))
(rate: 0.7 (mol/s))
(rate: 1.04
(mol/s))
Figure 6.36: Schematic of solar based methanol and hydrogen production (designed for
0.86 CO2 mol/s).
Methanol
Reactor
Cascade
CO2, H2, CH3OH, H2O and CO2
Qin(1) = 32.5 kW
Heat Recovery
from Plant
Comp-
feed
Comp-2
CO2, H2,
CO2
Separation
Water
Methanol
Comp-1
Qout(1) = 4.4 kW
Electrolyser
Water
Supply
Qout(2) = 10 kW
Cooling
Water
Qout(3) =4 kW
Cooling
Water
Qout(4)= 15.5 kW
Cooling Water
Flash
CO2, H2,
P=0.1MPa
3 MPa5 MPa, T=300 K
1.2
MPa
Wcomp-feed = 52 kW
Wcomp-2 = 43 kW
Wcomp-1 = 34.1
Treactor =
493 K
Mixing Chamber
Figure 6.37: Schematic of the methanol plant (designed for 0.86 CO2 mol/s).
96
9. 4. Energy and Exergy Efficiencies
Figure 6.38 shows the exergy destruction in each component. It is observed that 92% of
the total exergy destruction occurs in solar panels. This amount of exergy destruction is
due to the low efficiency of solar panels. Figure 6.38 is drawn based on 0.86 mol/s of
extracted CO2.
Exergy losses are considered in this thesis. The methanol production plant
consists of five heat exchangers and three compressors. Exergy destruction during
methanol production is significant because of the thermal and mechanical processes. The
exergy destruction in a methanol production plant is 6%.
Exergy destruction in the hydrogen plant is relatively insignificant. This plant has
PEM electrolysers. These electrolysers are working at 353 K and 0.1 MPa. The exergy
efficiency of each component is between 70%-60%.
Figure 6.38: Exergy destruction in hydrogen, methanol and solar energy plants (T0= 300
K).
2%
6%
92%
Exergy Destruction ofHydrogen Production Plant(65.6 kW)
Exergy Destruction ofMethanol Production Plant(184 kW)
Exergy Destruction of SolarEnergy Plant (2990 kW)
97
Figure 6.39 shows the total exergy destruction in the system, and the exergy
efficiency of the integrated system versus ambient temperature. It is observed that the
total exergy destruction of the system decreases with an increase in ambient temperature,
significantly. Although the exergy efficiency of the system is low, the system is using a
sustainable and renewable source of energy. Solar panels are investigated to provide the
total energy demand of the system. It should be noted that the exergy efficiency of the
system increases significantly as ambient temperature increases.
Figures 6-40 and 6-41 show the energy and exergy efficiencies of the system
versus solar intensity at different electrolyser temperatures. It is observed that the
electrolyser temperature is playing a key role in the efficiencies of the system.
Figure 6.39: Exergy destruction in the system and related efficiency versus ambient
temperature.
3.30
3.31
3.32
3.34
3.35
3.36
3.37
3.38
3.40
3.23
3.23
3.24
3.24
3.24
270 290 310 330 350 370
Exe
rgy E
ffic
ien
cy (
%)
Exe
rgy D
estr
uctio
n (
MW
)
Ambient Temperature (K)
Exergy Destruction (MW)
Exergy Efficiency (%)
98
The efficiencies of the overall system can be reduced by 30% if the electrolyser
operates at 300 K. This is due to low energy and exergy efficiencies of the electrolyser at
low temperatures. Thus, more energy is needed to produce appropriate amounts of
hydrogen in order to satisfy the methanol synthesis plant requirements. Consequently, the
exergy efficiency of the overall system decreases at low temperatures. It is also observed
that the efficiencies of the system are highly dependent on the intensity. The system
efficiencies can be increased if the solar intensity is 600 W/m2 instead of 250 W/m
2.
Figure 6.40: Energy efficiency of the system versus solar intensity at different
electrolyser temperatures.
0
0.5
1
1.5
2
2.5
3
3.5
200 300 400 500 600 700
En
erg
y E
ffic
ien
cy (
%)
Solar Intensity (W/m2)
Electrolyser Temperature= 300 K
Electrolyser Temperature= 353 K
99
Figure 6.41: Exergy efficiency of the system versus solar intensity at different
electrolyser temperatures.
Figures 6.40 and 6.41 show the variations of energy and exergy efficiencies of the
system. Due to low efficiencies of solar panels, the efficiency of the total system is low.
Thus, another definition can be used to provide a better perspective of the system.
According to this definition, input energy of the system is the energy that leaves the solar
panels. Figures 6.42 and 6.43 are shown based on this definition.
0
0.5
1
1.5
2
2.5
3
3.5
4
200 300 400 500 600 700
Exe
rgy E
ffic
ien
cy (
%)
Solar Intensity (W/m2)
Electrolyser Temperature= 300 K
Electrolyser Temperature= 353 K
100
Figure 6.42: Dependence of the energy efficiency of the system on the electrolyser
temperature for different system capacities .
Figure 6.43: Dependence of the exergy efficiency of the system on the electrolyser
temperature for different system capacities.
10
15
20
25
30
35
40
45
290 300 310 320 330 340 350 360 370
En
erg
y E
ffic
ien
cy (
%)
Electrolyser Temperature (K)
Capacity of System= 0.34 mol/sMethanol Production
Capacity of System= 0.6 mol/sMethanol Production
10
15
20
25
30
35
40
45
290 300 310 320 330 340 350 360 370
Exe
rgy E
ffic
ien
cy (
%)
Electrolyser Temperature (K)
Capacity of System= 0.34 mol/sMethanol Production
Capacity of System= 0.6 mol/smethanol production
101
It can be observed from Figures 6.41 and 6.42 that by increasing the capacity of
the system, with respect to the related methanol production, energy and exergy
efficiencies of the system increase significantly. Galindo and Badr reported an energy
efficiency of 17-22% for a similar system. They did not consider the efficiency of the
electrolyser and its energy supply in the calculations. Since the energy supply is
renewable and sustainable, the input energy is only the required energy for extracting
carbon dioxide from an industrial plant (Galindo and Badr, 2007). Saito and colleagues
(Satio et al., 1997) from the National Institute for Resources and Environment from Japan
also reported 30-35% energy efficiency for methanol production and hydrogen
production from solar energy. According to their definition, the input energy is the
energy that leaves the solar panels. It should be noted that they considered an alkaline
electrolyser in their calculations, which has a lower energy efficiency to compare with
PEM electrolyser (Satio et al., 1997).
102
Chapter 7: Conclusions and Recommendations for Future
Studies
7.1: Conclusions
This thesis presented a new design of a methanol production plant, which is linked with
solar based hydrogen production, and analyzed thermodynamically in order to offer a
promising way to reduce carbon dioxide emissions. A solar based integrated system for
hydrogen and methanol production was investigated. Energy and exergy analyses of the
hydrogen production plant, thermodynamic assessment of a methanol synthesis plant and
exergy analysis of the integrated solar based system for hydrogen and methanol
production were performed.
The analysis of hydrogen production was performed to investigate further design
parameters for methanol synthesis. This analysis shows the effects of temperature and
current density on hydrogen production. Thermodynamic parameters of the methanol
synthesis plant, such as temperature and pressure, are having an important role in
methanol production. Based on fundamentals of physical chemistry, the optimum
temperature of methanol synthesis was used for the final design of the methanol plant.
In this thesis, energy and exergy analyses were performed in order to evaluate the
system. Energy and exergy analyses determine the conditions of the system for a larger
scale of methanol production. For the electrolyser, the energy inputs and outputs increase
at different rates with increasing current density. The energy efficiency of the electrolyser
103
decreases with current density because the total energy input increases with current
density at a rate higher than the energy output (hydrogen produced). An increasing
temperature will reduce the overpotentials and cell potential which will increase the
efficiency. It is also concluded that the increasing pressure increases the methanol
production; however, methanol conversion occurs at 493 K precisely. The efficiencies of
the system can be reduced by 30% if the electrolyser operates at 300 K. this is due to
having low energy and exergy efficiencies of the electrolyser at low temperatures. Thus,
more energy is needed to produce appropriate amounts of hydrogen in order to satisfy the
methanol synthesis plant requirements. It should be noted that the efficiencies of the
system are highly dependent on the solar intensity. The system efficiencies can be
increased if the solar intensity is 600 W/m2 instead of 250 W/m
2. It is also concluded that
by increasing the capacity of the system, with respect to methanol production, the energy
and exergy efficiencies of the system can increase significantly.
7.2. Recommendations for Future Studies
This thesis may help decision makers to increase their investment in synthetic fuel
production, using sustainable hydrogen production and carbon capturing systems. Every
synthetic fuel production system has its own pros and cons, which should be compared to
each other with respect to thermodynamic aspects.
Additional detailed studies on the effect of different catalysts in the methanol
synthesis process should be investigated. Better catalysts can be used to maximize the
process efficiency. Also, larger scale plants should be studied in more detail.
104
Variable outcomes might be provided from different sources of renewable energy. This
concern should be studied comprehensively with different carbon capturing procedures.
The results should be evaluated and compared in order to find the most efficient system
for specific operating conditions.
A more detailed feasibility study of this system is recommended for future
studies. Feasibility studies intend to accurately determine the strong points and
weaknesses of the existing system. This study could be enhanced with respect to
opportunities and concerns with the environment, resources, and eventually the prospects
for success. This research should consider economical parameters as well as
thermodynamic conditions that could be enhanced with an exergy-based economic
analysis in order to broaden the horizons of this technology.
105
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