design and optimization of biomass power plant

ARTICLE IN PRESSCHERD-1557; No. of Pages 16

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chemical engineering research and design x x x ( 2 0 1 4 ) xxx–xxx

Contents lists available at ScienceDirect

Chemical Engineering Research and Design

j ourna l h omepage: www.elsev ier .com/ locate /cherd

esign and optimization of biomass power plant

esfaldet Gebreegziabhera, Adetoyese Olajire Oyedunb, Ho Ting Lukb,sz Ying Gene Lamb, Yu Zhangb, Chi Wai Huib,∗

Department of Environmental Engineering, The Hong Kong University of Science and Technology, Clear Water Bay,owloon, Hong KongDepartment of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Clearater Bay, Kowloon, Hong Kong

a b s t r a c t

Among the renewable energy sources, biomass offers some benefits due to its low cost and presumed zero-carbon

emission when compared with fossil fuels. However, the moisture content of biomass is often high that lowers

its heating value, reduces the combustion temperature and causes operational problems. Because of these, when

burning biomass for power generation, biomass is often dried prior to the combustion. To lower the drying cost or

to maximize the power output of a biomass power plant, proper heat integration in between the steam power plant

and the drying process has to be considered. In this work, heat integration studies are performed to a biomass power

plant that burns empty fruit bunches (EFB) as fuel. Composite curves of all studied cases are plotted to visualize

the intensity and to identify opportunities of heat integration among the drying and power generation systems. A

multi-stage drying process is proposed that employs steam and waste-heat from the power plant and the drying

process, respectively. Results of this study show that with proper drying and heat integration, the overall efficiency

of a biomass power plant can be significantly improved.

© 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Heat integration; Biomass power plant; Biomass drying

tent and that of fresh microalgae is more than 90% (Kanda and

. Introduction

iomass is one form of renewable energy source used for botheat and power generation through thermochemical and bio-hemical conversion processes like combustion, gasification,yrolysis and anaerobic/aerobic digestion. It includes bothnergy crops and wastes, such as forestry residues (cottontalks, paddy straw, rice husk, sawdust) and a range of othergricultural and industrial by-products, which can all be uti-ized for energy production. Direct combustion is the mostommon thermochemical conversion process used for soliduels due to its low cost and reliability (IEA, 2012). In addi-ion, combustion is the key process in a biomass power plantor generating steam. The steam can be used for industrialrocesses or to produce electricity. The selection and design

Please cite this article in press as: Gebreegziabher, T., et al., Design andhttp://dx.doi.org/10.1016/j.cherd.2014.04.013

f any biomass combustion system mainly depends on the

∗ Corresponding author. Tel.: +852 2358 7137; fax: +852 2358 0054.E-mail addresses: [email protected] (T. Gebreegziabher), [email protected]

T.Y.G. Lam), [email protected] (Y. Zhang), [email protected] (C.W. Hui).ttp://dx.doi.org/10.1016/j.cherd.2014.04.013263-8762/© 2014 The Institution of Chemical Engineers. Published by

characteristics of the fuel to be used, environmental legisla-tion, the costs and performance of the equipment and theenergy and capacity needed (Van Loo and Koppejan, 2008). Thecharacteristics of the fuel affect directly all the selection anddesign parameters. A typical fuel characteristic that stronglyaffects combustion is the moisture content. Most modernbiomass based combustion systems specify a range of accept-able moisture content of the fuel for meeting emissions andefficiency standards. If a solid fuel with specifications outsidethis range is used the system may shut down automatically(BEC, 2011). Biomass materials have a wide range of mois-ture content (on wet basis) ranging from less than 10% forcereal straw up to 50–70% for forest residues (Quaak et al.,1999). Industrial wastes like sludge have high moisture con-

optimization of biomass power plant. Chem. Eng. Res. Des. (2014),

k (A.O. Oyedun), [email protected] (H.T. Luk), [email protected]

Li, 2011). A biomass with high moisture content is not suitable

Elsevier B.V. All rights reserved.

dx.doi.org/10.1016/j.cherd.2014.04.013

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dx.doi.org/10.1016/j.cherd.2014.04.013



2 chemical engineering research and design x x x ( 2 0 1 4 ) xxx–xxx

Nomenclature

BFW boiler feed waterCpda specific heat capacity of dry air (kJ/kg K)Cpv specific heat capacity of dry air (kJ/kg K)Cpv specific heat capacity of water vapor (kJ/kg K)CpW specific heat capacity of liquid water (kJ/kg K)E enthalpy flow (kJ/h)EFB empty fruit bunchei, ej specific enthalpy of a steam/water stream

(kJ/kg)eout,s enthalpy of outlet stream at 100% isentropic

efficiencyHa enthalpy of air (kJ/kg)Ha,F enthalpy of feed air (kJ)Ha,H enthalpy of hot air (kJ)HAD hot air dryerHe enthalpy of EFB (kJ/kg)Hv enthalpy of vapor (kJ/kg)HP high pressuer steamLH latent heat of waterLHVe lower heating value of EFB (MJ/kg)LP pressure of low pressure steam (bar)mda mass flow rate of dry air (kg/h)mde mass flow rate of dry EFB (kg/h)me mass flow rate of wet EFB (kg/h)mevap,ssd evaporation rate at SSD (kg/h)min input flow rate of steam/water (kg/h)mout output flow rate of steam/water (kg/h)MP pressure of high pressure steam (bar)P1, P2, P3 power input for BFW pump 1, 2 and 3Pa pressure of moist air (Pa)PA,F pressure of fresh air feed (Pa)Pout net power output (MW)Pw partial pressure of water vapor in moist air (Pa)Pws saturated partial pressure of water vapor in

moist air (Pa)QCondenser condenser heat duty (kJ/h)QHAD hot air dryer heat duty (kJ/h)Qhtr heat duty (KW)QSSD super heated steam dryer heat duty (kJ/h)RH relative humidity of air (%)si, sj specific entropy of a steam/water stream

(kJ/kg K)Ta air temperature (K)TA,F temperature of feed air (K)Te EFB temperature (K)TEFB,F temperature of EFB feed (K)Tref reference Temperature (K)Tsat saturated temperature of water or vapor stream

(K)TS supply temperature (K) of a process streamTT target temperature (K) of a process streamW mass flow rate (Kg/h)X moisture content (%)XF,d moisture content of solid (kg/kg dB)xg vapor fractionXHAD,in moisture fraction of EFB at the inlet of hot air

dryer (%)XHAD,out moisture fraction of EFB at the outlet of hot air

dryer (%)

XSSD,in moisture fraction of EFB at the inlet of Superheated steam dryer (%)

XSSD,out moisture fraction of EFB at heated steam dryeroutlet of Super (%)

Y moisture content of air (kg/kg dB)�boiler boiler efficiency (%)�overall overall efficiency (%)�pump pump efficiency (%)�turbine efficiency of a turbine (%)

Indexi,j stream number

for direct combustion. It should either be properly dried ormixed with other auxiliary fuels in order to meet the perfor-mance of emission and efficiency specifications of a powerplant. Using dry fuel in combustion systems improves effi-ciency, increases steam production, reduces fuel use, lowersemissions and improves boiler operation (Amos, 1998). Hav-ing multiple advantages however, drying biomass is an energyintensive process and can account for up to 15% of the overallindustrial energy usage with relatively low thermal efficiencyranging between 25 and 50% (Chua et al., 2001). The dryingtemperature is also influenced by the size of the particle tobe dried (Kapseu et al., 2007). The intensive drying energyrequirement, variability of types and size of biomass and widegap of thermal efficiency of drying operation suggests theexistence of a way for improvements especially in views ofrising energy costs and concerns related to greenhouse gasemissions from burning fossil fuels.

Proper utilization of steam or waste-heat from a powerplant for drying can significantly reduce the energy cost. Ina steam power plant, steam extracted from steam turbineat medium (10–40 bar) and low pressure (2–5 bar) are oftenused for boiler feed water (BFW) preheat. Preheating BFWreduces the condenser loss thus increase the overall energyefficiency of the power plant. Similar effect should be appliedfor biomass drying. Drying biomass by steam reduces energyloss at both the condenser and the boiler. In the followingsections, a case study of empty fruit bunch (EFB) drying is pre-sented to demonstrate how the energy efficiency of a biomasspower plant can be improved by proper integrating the steampower plant and biomass drying.

1.1. Heat integration of biomass power plant

After the energy crisis in late 1970s, extensive research effortshave been made in reducing energy consumption or improvingenergy efficiency in process industries. Linnhoff and Flower(Linnhoff and Flower, 1978a,b) introduced a heat integrationapproach, namely “Pinch Analysis”, to target, design and opti-mize heat exchanger networks in chemical processes. Theseworks were then extended to designing commercial powerplants (Linnhoff and Alanis, 1989) and to integrate chemi-cal and utility plants (Hui and Ahmad, 1994). Until recently,researchers are still using these techniques for improving theefficiency of different kind of power plants (Ataei and Yoo,2010; Eskandari and Behzad, 2009; Fu and Gundersen, 2010;Tan et al., 2009). So far, applying pinch analysis to design and


optimize a biomass power plant is rarely discussed in litera-ture.

dx.doi.org/10.1016/j.cherd.2014.04.013


chemical engineering research and design x x x ( 2 0 1 4 ) xxx–xxx 3

stion

phltospio(dflptan

Fig. 1 – Direct combu

Unlike coal, which is often used as a fuel for thermal powerlants, biomass contains much higher levels of moisture. Theigh moisture content increases the difficulty in operation,

owers the combustion efficiency and threatens environmen-al limitations. Most of the latest biomass power plants areften integrated with drying facilities of which biomass ishredded and/or thermally dried before sending it to the boilerlant. In order to sustain the combustion in a boiler, biomass

s often burnt at a moisture level below 55–65 wt.% while theptimum moisture content could be as low as 10–15 wt.%

Ross, 2008). Brammer et al. showed the importance of biomassrying for small-to-medium scale biomass gasification plantsor the production of heat and power, and verified that highevels of moisture content within feedstock not only lowers theerformance of the system but also deteriorates the quality ofhe product gas (Brammer and Bridgwater, 2002). Le Lostec et


l. suggested that, using waste heat of a power plant is an eco-omical solution for drying wood chips (Le Lostec et al., 2008).

Table 1 – Mass and energy balance equations.

Units∑

imin,∑

iEin∑

imo

Condenser m2, E2 m3, E3

Mixer 1 m3 + m18, E3 + E18 m3, E4

Mixer 2 m5 + m17, E5 + E17 m6, E6

Mixer 3 m7 + m15, E7 + E15 m8, E8

BFWP 1 m4, E4 m5, E5



Spliter 1 m9, E9 m10 + m11

Spliter 2 m13, E13 m14 + m15

Spliter 3 m16, E16 m17 + m1

Boiler m11, E11 m12, E12

1st Turbine m12, E12 m13, E13

2nd Turbine m14, E14 m16, E16

3rd Turbine m1, E1 m2, E2

of EFB (Base Case).

We recently proposed a mathematical model to determine theoptimum drying level of biomass (Gebreegziabher et al., 2013).The model incorporates material and energy balances as wellas heat transfer and drying kinetics to determine the optimummoisture content for calorific value enhancement of wood.The performance of a combustion process can be enhancedby the removal of moisture from the feedstock. Three differ-ent drying methods namely steam drying, flue gas drying andvacuum drying were studied by Andersson et al. (Anderssonet al., 2006) in a process where the drying and pelletizing ofbiofuel is integrated to pulp mill. Their study showed the pos-sibility of energy recovery and reduction in CO2 emissionscompared to stand-alone pellets production. Holmberg andAhtila (Holmberg and Ahtila, 2004) in their work on biofuel dry-ing used air as the only drying medium. They utilized steamto preheat air for drying at multi-stage level thereby reducing


the overall drying cost but their work was not extended to theeffects or operation in the steam power plant.

ut,∑

iE �E Remark

QCondenser –– –– –– –P1 �pump = 100%P2 �pump = 100%P3 �pump = 100%

, E10 + E11 – –, E14 + E15 – –

, E17 + E1 – –Qboiler �boiler = 90%W1 �turbine = 80%W2 �turbine = 80%W3 �turbine = 80%

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Fig. 2 – Composite curves Case 1.

Drying is an energy and capital intensive process. Theenergy cost of the drying process can be reduced by utiliz-ing low-grade waste heat (Li et al., 2012) but at the sametime increasing the size of dryer makes it too bulky andexpensive.

Song et al. studied the effect of integrating drying processinto a biomass power plant (Song et al., 2012). Flue gas from theboiler plant was used to dry the biomass feedstock and theirresult shows 3.1% increase in overall efficiency. Their workdemonstrated that the integration of the drying process withthe power plant is beneficial.

In this study, a biomass power plant that burns emptyfruit bunch (EFB) is used to illustrate the integration of amulti-stage drying process with the power plant. Mathe-matical models of the steam power plant and the dryingprocess are developed and applied for designing and optimiz-ing the energy efficiency of the overall plant. Pinch analysisis used to show the effectiveness of the heat integration


of different design options and to identify further improve-ments.

Table 2 – Base Case stream results.

Streams Temp. (K) P (bar) Enthalpy (kJ/kg) Ent

1 481 3 2883

2 319 0.1 2476

3 314 0.1 171

4 314 0.1 171

5 314 3 171

6 330 3 238

7 330 18 240

8 400 18 534

9 401 100 543

10 401 100 543

11 401 100 543

12 900 100 3692

13 666 18 3235

14 666 18 3235

15 666 18 3235

16 481 3 2883

17 481 3 2883

18 303 1 126

2. Problem statement

The Malaysian Government is aiming to reduce 40% of itscarbon dioxide emissions by 2020 by promoting renewableresources like empty fruit bunches (EFB), an abundant wasteproduct of the palm oil milling process, as the primary biomassfuel for power generation (Basiron, 2007; Panapanaan et al.,2009; USDA, 2012). The EFB is often available at 60–70 wt.%moisture content and it is desirable to dry to below 10 wt.%before combustion or gasification (Hasibuan and Wan Daud,2007; Sulaiman et al., 2013).

The objective of this work is to determine the most energyefficient drying scheme by properly integrating the drying andpower generation systems. In this study, moist EFB is driedusing a hot air dryer (HAD) and/or a superheated steam dryer(SSD) before combustion. Steam at different pressure levelsfrom the power plant and the waste-heat from the SSD areconsidered as the heat sources for the drying. The investiga-


tion is to determine the optimal drying level together withthe integration of the power plant and the drying process for

ropy (kJ/kg) Vapor fraction m (kg/h) E (kJ/h)

7.349 1.00 35,287 101,730,6877.812 0.96 35,287 87,382,5780.583 0.00 35,287 6,031,1150.583 0.00 35,387 6,043,6890.583 0.00 35,387 6,054,0350.791 0.00 36,285 8,643,6830.791 0.00 36,285 8,698,3171.600 0.00 40,238 21,487,4011.600 0.00 40,238 21,838,3901.600 0.00 100 54,2731.600 0.00 40,138 21,784,1176.979 1.00 40,138 148,177,2777.157 1.00 40,138 129,862,7557.157 1.00 36,185 117,073,6717.157 1.00 3953 12,789,0847.349 1.00 36,185 104,320,3357.349 1.00 898 2,589,6480.437 0.00 100 12,583

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Table 3 – Problem table of the Base Case.

Stream name TS (K) TT (K) DH (kW)

17 436 407 7.84407 406 266.6406 320 44.63

15 576 480 411480 479 3284479 400 590

2 319 318 28,600318 313 210

5 314 320 319.56320 400 4287.4400 584 12,486.6584 585 19,016585 850 12,084

FG 1200 473 –CW 285 295 –

mbc

3

IpBC(cas

3

Ii1dvba

In the above equations, the specific enthalpy and entropyof a water or vapor stream can be estimated once when the

aximizing the energy efficiency of the overall plant. Aiomass power plant is designed to generate 12.5 MW by theombustion of EFB with initial moisture content of 70 wt.%.

. Case studies

n the Base Case, EFB is combusted directly without drying toroduce steam for power generation. The steam level and theFW water preheating temperatures are selected arbitrarily. Inase 2, the levels of the steam pressure and boiler feed water

BFW) preheating levels are optimized. Cases 3, 4 and 5 areomprised of different drying schemes either with the HADnd/or SSD. These case studies are illustrated in the followingections.

.1. Base Case – direct combustion of EFB

n the Base Case, fresh EFB with a moisture content of 70 wt.%s fed directly to the boiler targeting an output power, Pout, of2.5 MW. Fig. 1 is the process flow diagram of the Base Case,emonstrating all of its relevant components, with assumedalues of temperature and pressure of streams. It includes aoiler, boiler feed water pumps (BFWPs), turbines, mixers and

condenser.


Table 4 – Stream results of Case 2.


1 462 2.4 2846

2 319 0.1 2476

3 314 0.1 171

4 314 0.1 171

5 314 2.4 171

6 394 2.4 506

7 394 19.8 508

8 480 19.8 883

9 481 100.0 892

10 481 100.0 892

11 481 100.0 892

12 900 100.0 3692

13 677 19.8 3256

14 677 19.8 3256

15 677 19.8 3256

16 462 2.4 2846

17 462 2.4 2846

18 303 1.0 126

3.1.1. Modeling the steam power plantMathematical modeling of the steam cycle is based on conser-vation of mass and energy principles. These principles can bedescribed by the following two equations: Eq. (1) is for massbalance and Eq. (2) is for energy balance.

∑imin =

∑imout (1)

∑iEin =

∑iEout + �E (2)

where,∑

imin and∑

imout respectively are mass in and massout of a unit (kg/h);

∑iEin and

∑iEout respectively are energy

flow in and out of a unit (kJ/h); and �E is the change (e.g. poweroutput of a turbine) in energy of a unit (kJ/h)

The energy flow rate, E (kJ/h), is the product of the specificenthalpy, e (kJ/kg), of each stream and the mass flow rate, m(kg/h). The specific enthalpy and other important propertiesof a water or vapor stream such as specific entropy, s (kJ/kg K),saturation temperature, Tsat (K), are calculated by Water97.Water97 is an Add-In for MS Excel which provides a set of func-tions for calculating thermodynamic and transport propertiesof water and steam using the industrial standard IAPWS-IF97(Spang, 2002). It is a useful tool in case of unavailability ofprocess simulators like ASPEN PLUS® (Aspentech, 2014) andProSim Plus (ProSimPlus). The use of Water97 in our study alsoallows us to integrate the steam power plant model with thedryer models on a same platform so that the whole process canbe optimized simultaneously. The following routine equationsof Water97 are thoroughly used in our simulation study.

ei = enthalpyW(Ti; Pi) (3)

si = entropyW(T; P) (4)

ej = xgenthalpySatVapPW(Pj) + (1 − xg)enthalpySatLiqPW(Pj)

(5)

Sj = xgentropySatVapPW(Pj) + (1 − xg)entropySatliqPW(Pj) (6)

Tsat,i = tSatW(Pi) (7)



7.381 1.00 32,102 91,368,1307.810 0.95 32,102 79,472,8840.583 0.00 32,102 5,486,7220.583 0.00 32,202 5,499,2970.583 0.00 32,202 5,506,8801.533 0.00 36,809 18,619,9941.533 0.00 36,809 18,684,2892.394 0.00 42,634 37,652,2762.394 0.00 42,634 38,050,4722.394 0.00 100 89,2492.394 0.00 42,534 37,961,2236.979 1.00 42,534 157,024,0597.146 1.00 42,534 138,495,0687.146 1.00 5825 18,967,9877.146 1.00 36,709 119,527,0817.381 1.00 36,709 104,481,2447.381 1.00 4607 13,113,1140.437 0.00 100 12,583

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Fig. 3 – Composite curves Case 2.

Table 5 – Stream Data of Case 2.

Steam TS (K) TT (K) DH (kW)

17 481 407 39.16407 406 539.81406 330 79.93

15 665 480 480.82480 479 2102.88479 400 382.48

2 318 317 22,392.83317 313 163.84

5 313 330 659.91330 400 2895.06400 583 8504.09583 585 12,951.55585 900 9438.94

FG 1200 473 –CW 285 295 –

temperature, Ti (K), and pressure, Pi (bar), of the stream areknown. Eqs. (3) and (4) are for single-phase state and Eqs.(5) and (6) are for two phase state. xg is the vapor fraction ofthe stream. Eqs. (3)–(7) are valid for temperature and pressureranges of; 273.15 K ≤ T ≤ 1073.15 K and 0 < p ≤ 1000 bar, respec-tively.

The unit models of the steam power plant are derived as inTable 1.

Table 1 Mass and Energy balance equations The boiler feedwater pumps are assumed to operate at 100% isentropic effi-ciency, �pump, defined by Eq. (8).

�pump = (eout,s − ein)(eout − ein)

× 100% (8)

where eout,s is enthalpy of the outlet stream of the pump at100% isentropic efficiency. eout is the actual enthalpy of thestream at �pump.

The turbines are assumed to operate at 80% isentropic effi-ciency. The formula for isentropic efficiency, �turb, is given inEq. (9).

�turb = (ein − eout)ein − eout,s

× 100% (9)

ein and eout are the enthalpy values of the inlet and out-let streams of the turbine at the given isentropic efficiency,respectively and eout,s is the enthalpy value of the outlet streamfor an isentropic expansion.

For the boiler, its heat duty depends on the lower heatingvalue (LHV) of the fuel to be used and the boiler efficiency. Theefficiency of the boiler, �boiler, is assumed to be 90% and theboiler duty, Qboiler (kJ/h), can be calculated as in Eq. (10).

Qboiler = (E12 − E11)�boiler

(10)


The LHV (kJ/kg) of EFB depends on the moisture content, X(%), and is estimated by making use of the data provided by

the Energy Research Center (ECN) of the Netherlands (ERCN,2012) as in Eq. (11).

LHVEFB = 15820 − X ∗ 181.99 (11)

When EFB is totally dried (i.e. 0% moisture), the LHVEFB is15,820 (kJ/kg). The value LHVEFB is declines when the moisturecontent is getting low. This LHVEFB drops below zero when themoisture content is greater than 87% indicating the heat of theEFB is not sufficient for sustaining the combustion process.

Hence, the feed rate of moist EFB, mEFB (kg/h), required togenerate a specific amount of boiler heat duty, Qboiler (kJ/h), canbe calculated by Eq. (12).

mEFB = Qboiler

LHVEFB(12)

Eq. (2) can be used also to calculate the total power out-put, Pout (MW), for the closed cycle of the power plant in Fig. 1for a given amount of EFB feed. Following is the equation forcalculation Pout.


Pout = (W1 + W2 + W3) − (P1 + P2 + P3) (13)

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Fig. 4 – Process flow diagram of HAD integrated power plant.

3To

�

mi

att3ia

.1.2. Objective functionhe overall efficiency, �overall, defined in Eq. (14) was set as thebjective function.

overall =(

Pout ⁄(1−XF,d)mde∗LHVEFB at 0 wt.% moisture

)∗ 100% (14)

In Eq. (14) XF,d is the moisture content of EFB feed (kg/kg),

de is the flow rate of dried EFB (kg/h) and LHVEFB at owt% moisture

s the heating value of EFB at 0 wt.% moisture.The steam entering to the first turbine is assumed to be

t a pressure of 100 bar and a temperature of 627 ◦C (900 K):hese values are fixed for the whole study. The inlet streamso the second and third turbines are initially set at 18 and

bar, respectively in the Base Case but will be optimized dur-


ng the integration study. All the turbines are assumed to haven isentropic efficiency of 80%, and their outlet temperatures

Fig. 5 – Hot air drye

are estimated with their inlet conditions and outlet pressuresusing Eqs. (1), (2), (3), (4) and (9). When a turbine outlet con-tains two phases (e.g. the condensing turbine), Eqs. (5) and(6) are used instead of Eq. (3) and (4). The boiler feed water(BFW) is pumped successively from 0.1 bar at the condenseroutlet to the pressure of LP, MP and 100 bar before sendingto the boiler. The condensing pressure is fixed in the wholestudy. The pressure of LP and MP are the pressure of low pres-sure and medium pressure steam which will be optimizedin different cases. In the Base Case LP and MP are set at3 bar, 17 bar respectively. In order to make sure that the waterentering to BFWP2 and BFWP3 is in its liquid state, the inlettemperatures to these pumps are arbitrary fixed at low tem-peratures in the Base Case at 57 ◦C (330 K) and 127 ◦C (400 K)


respectively. These values will also be optimized in the othercases.

r stream data.

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Table 6 – Design parameters of HAD integrated powerplant.

Parameters Value

Moisture content of the EFB feed, XF 70%Temperature of the EFB feed, TEFB,F (K) 298Pressure of the air feed, PA,F 101,325 PaTemperature of the air feed, TA,F (K) 298Relative humidity of the air feed, RH1 (%) 50Specific heat of air, Cpda (kJ/kg K) 1.006Specific heat of EFB, Cpds, (kJ/kg) 1.2566Specific heat of water vapor, Cpv, (kJ/kg K) 1.89Specific heat of water, Cpw, (kJ/kg K) 4.186Latent heat of water, LHw, (kJ/kg K) 2270

Constraints for the hot air dryer Value

Maximum relative humidity of the exhausted air 95%Minimum LHV of the dried EFB (kJ/kg) 15,000

Table 8 – Stream data of Case 3.

Stream name TS (K) TT (K) DH (kW)

18 422.04 393.86 36.99393.86 392.86 1369.46392.86 388.36 11.87

21 601.14 470.38 578.15470.38 469.38 3651.91469.38 464.88 37.51

19 422.04 393.86 460.69393.86 392.86 17,058.14392.86 388.36 147.91

2 318.96 317.96 10,134.21317.96 313.96 75.48

Hot Air 298 364 17,6925 313.90 388.36 1420.32

388.36 464.88 4274.16464.88 583.65 8654.27583.65 585.00 19,474.79585.00 850.00 12,375.07

The process constraints applied to the Base Case include:

1. �turb = 80%, �boiler = 90% and �pump = 100%2. P12 = 100 bar, P14 = 18 bar, P1 = 3 bar, P2 = 0.1 bar, P16 = 3 bar,

P13 = 18 bar, P5 = 3 bar, P7 = 17 bar, P9 = 100 bar3. Moisture content of EFB, X (%) = 70%4. LHVEFB at 0 wt.% moisture = 15,829 kJ/kg5. m18 = m10 = 100 kg/h6. T6 = 330 K, T8 = 400 K, T12 = 900 K7. Pout = 12.5 MW

The objective is to maximize the overall efficiency, �overall,defined in Eq. (14) by changing the flow rate of steam (m12),first turbine out let temperature (T13), the second turbine out-let temperature (T16), the third turbine outlet temperature (T2)and the outlet temperature of BFWPs. Excel 2010 Standard GRGNon-linear Solver was used to solve the optimization problem.While solving the optimization problem, the stream propertiesof the units are determined automatically by Water97.

3.1.3. Results


The result of energy and material balance of streams is givenin Table 2.

Table 7 – Stream results of HAD integrated power plant.

Streams Temp. (K) P (bar) Enthalpy (kJ/kg) En

1 422 3 2759

2 319 0.1 2436

3 314 0.1 171

4 314 0.1 171

5 314 2 171

6 388 2 483

7 388 14.5 485

8 465 14.5 815

9 466 100 825

10 466 100 825

11 466 100 825

12 850 100 3569

13 601 14.5 3101

14 601 14.5 3101

15 601 14.5 3101

16 422 2 2767

17 422 2 2767

18 422 2 2767

19 422 2 2767

20 388 2 483

21 303 1 126

In the Base Case, to produce 12.5 MW power output, thetotal moist EFB feed rate required is 45,589 kg/h. As shown inTable 2, BFW at 100 bar and 401 K is heated at the boiler to900 K. The enthalpy change of this stream is 3149 kJ/kg result-ing into a high pressure (HP) steam flow rate of 40,138 kg/hfrom the boiler. The combined power output of the three tur-bines is 12.616 MW with an amount of 115.55 kW power forpumping the three BFW pumps. The maximum value of theoverall energy efficiency (�overall) defined in Eq. (14) is calcu-lated to be 20.80% which is much lower than the efficiency ofconventional coal fired power plants.

3.1.4. Pinch analysis to the base caseSome operating conditions in the Base Case were arbitrarilyset and may offer efficiency improvement upon changing. Tofind out how the efficiency can be improved, stream data isextracted and shown in Table 3. The process composite curvesare plotted using a software package for heat exchanger net-work design SPRINT (University of Manchester) and are shownin Fig. 2 assuming flue gas (FG) and cold water (CW) as available


utilities.

tropy (kJ/kg) Vapor fraction m (kg/h) E (kJ/h)

7.07 1.00 16,255 44,845,2367.69 0.94 16,255 39,601,1520.58 0.00 16,255 2,778,3550.58 0.00 16,355 2,790,9300.58 0.00 16,355 2,794,2651.48 0.00 46,488 22,475,8261.48 0.00 46,488 22,541,1672.25 0.00 53,210 43,384,5262.25 0.00 53,210 43,903,4782.25 0.00 100 82,5102.25 0.00 53,110 43,820,9686.84 1.00 53,110 189,533,7437.04 1.00 53,110 164,700,5687.04 1.00 46,388 143,857,2097.04 1.00 6721 20,843,3597.28 1.00 46,388 128,346,8397.28 1.00 18,495 51,171,2867.28 1.00 2239 6,195,8137.28 1.00 27,894 77,175,5531.48 0.00 27,894 13,485,7480.44 0.00 100 12,583

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Fig. 6 – Composite Curves of HAD integrated power plant.

toeiF

Fig. 7 – Layout of super-heated steam drier.

The composite curve in Fig. 2a shows a large gap betweenhe hot and cold composite curves indicating the insufficiencyf BFW preheat by LP and MP. The overall efficiency may benhanced by increasing the amounts of BFW preheat. Fig. 2b


s a balanced composite curve in which utility stream such asG and cooling water is included.

Fig. 8 – Integration of HAD and SS

3.2. Case 2 – Base Case with BFW preheat andoptimum pressure

To improve energy efficiency of a steam power plant, BFWshould be heated to a temperature close to the temperatureof the steam for preheating. For instance, by increasing the LPinputs to Mixer 2, temperature of stream 6 will be heated to atemperature close to the boiling points at P6. To avoid havingvapor at the inlet of BFWP2, the temperature of stream 6 isrestricted to be 5 ◦C below its boiling point. This setting is alsoapplied to Mixer 3.

The steam extraction pressure (MP and LP) of the turbinesis another factor that affects the overall efficiency. Once theextraction pressures are changed, the levels of BFW preheatare varied accordingly. In Case 2, the pressure of MP and LP,and BFW preheating temperature are variables in the opti-mization. In the following case where pressure of LP and MP


are allowed to be changed, their pressures are restricted to be,2 bar ≤ LP ≤ 5 bar and 15 bar ≤ MP ≤ 40 bar.

D to the power plant (Case 4).

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wer
Fig. 9 – Steam po
The optimal solution of Case 2 obtained and its streaminformation is presented in Table 4. The optimal pressure ofMP and LP are at 2.4 bar and 19.8 bar. The overall efficiency ofthis case is improved from 20.80% of that in the Base Case to22.08%.

Stream data for composite curve construction is extractedfrom the optimal results. Table 5 is the stream dataof Case 2 and Fig. 3 is the corresponding composite


curves.

Table 9 – Stream results of HAD and SSD integrated power plan


1 434 3.0 2785

2 319 0.1 2432

3 314 0.1 171

4 314 0.1 171

5 314 2.5 171

6 395 2.5 513

7 395 13.0 514

8 460 13.0 793

9 461 100.0 802

10 461 100.0 802

11 461 100.0 802

12 850 100.0 3569

13 590 13.0 3080

14 590 13.0 3080

15 590 13.0 3080

16 434 2.5 2789

17 434 2.5 2789

18 434 2.5 2789

19 434 2.5 2789

20 395 2.5 513

21 590 13.0 3080

22 590 13.0 3080

23 460 13.0 793

24 303 1.0 126

plant of Case 4.

The preheated (Mixer 1 and Mixer 2) outlet temperaturesof stream 6 and stream 8 were changed to 411 K and 491 K,respectively. As shown in Fig. 3a, utility Pinches created atthese temperatures indicated that BFW preheat is properlydone. The utility pinches are opened up a little bit in Fig. 3bshowing that there might have some potential to furtherimprovement by including FG for BFW preheat. The improve-ment of this option is expected to be small and will not be


included in this study.

t.


7.13 1.00 29,129 81,127,8957.67 0.94 29,129 70,841,5350.58 0.00 29,129 4,978,6130.58 0.00 29,229 4,991,1880.58 0.00 29,229 4,998,3761.55 0.00 33,616 17,234,1781.55 0.00 33,616 17,276,6422.20 0.00 52,324 41,469,6772.20 0.00 52,324 41,985,6792.20 0.00 100 80,2412.20 0.00 52,224 41,905,4376.84 1.00 52,224 186,374,4027.06 1.00 52,224 160,857,0287.06 1.00 37,610 115,844,0867.06 1.00 33,516 103,233,4157.23 1.00 33,516 93,471,4967.23 1.00 33,516 93,471,4967.23 1.00 4387 12,235,8027.23 1.00 – –1.55 0.00 – –7.06 1.00 4094 12,610,6717.06 1.00 14,614 45,012,9422.20 0.00 14,614 11,582,3640.44 0.00 100 12,583

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Fig. 10 – Mass balance of results o

Table 10 – Stream table of Case 4.

TS (K) TT (K) DH (kW)

18 434.36 400.74 87.39400.74 399.74 2659.36399.74 395.24 23.33

21 589.84 465.26 332.31465.26 464.26 2246.57464.26 459.76 22.73

19 434.36 400.74 0.00400.74 399.74 0.00399.74 395.24 0.00

LP-SSD 385 384 8071.222 590 465 1186.15

465 464 8004.15464 460 81.13

2 318.96 317.96 18,126.19317.96 313.96 135.25

Hot Air 298 371 80715 313.93 395.24 2774.02

395.24 459.76 2606.55459.76 583.65 8837.60583.65 585.00 19,150.78585.00 850.00 12,169.17

si(

3

Rtdsaf

3TtL

Since the energy efficiency is improved in Case 2, for theame amount of power output of 12.5 MW, the moist EFB feeds reduced to 42,934.7 kg/h, 5.8% lower than the Base Case45,589 kg/h).

.3. Dryer integration

eduction in moisture content of biomass is necessaryo provide retrofit EFB fuel for the boiler. During thermalrying, EFB is dehydrated through heat transfer with air orteam. Two alternative drying systems, Hot air dryer (HAD)nd Super-heated steam dryer (SSD) are considered in theollowing studies.

.3.1. Hot air dryer (HAD)he hot air dryer section integrated with the power plant for


his study is given in Fig. 4. The air stream is pre-heated by theP steam generated from the power plant. The condensate,

f the drier section of Case 4.

S20, from the hot air dryer is returned and mixed with theBFW at Mixer 2. The heat duty required, Qheater, to change theproperty of air to the state required at the inlet of the dryercan be calculated by making use of energy and mass balancein the highlighted section of Fig. 4.

3.3.2. Air heater model3.3.2.1. Enthalpy value of an air stream. The enthalpy of airstream i, Ha,i (kJ/kg), is calculated by Eq. (15) when the air tem-perature Ta,i, and moisture ratio Yi (kg/kg dry base (dB)) areknown.

Ha,i = CPda(Ta,i − Tref ) + Yi(Cpv × (Ta,i − Tref ) + LH) (15)

Eq. (15) assumes the specific heat of dry air (Cpda) and thespecific heat of water vapor (Cpv) are constant for the giventemperature range and the latent heat of water (LH) is fixed at2270 kJ/kg. The moisture ratio Yi can be calculated by makinguse of the psychometric relations (Gebreegziabher et al., 2013)as in Eqs. (16)–(18).

Pws,i = e((77.3450+0.005Ta,i−7235)/Tai)

T8.2a,i

(16)

Pwi = RHi × Pwsi (17)

Yi = 0.62198Pa

Pa − Pwi(18)

Pws,i (Pa) and Pwi (Pa) are the saturated vapor pressure andthe actual water vapor pressure of the air stream and Pa is theatmospheric pressure. Low pressure (LP) steam is used to heatup the air stream. Assuming the relative humidity (RH) and thetemperature of feed air, TA,F (K), to the air heater are known.The latent heat of the low pressure steam is used for heatingthe air stream and the properties of the hot air are determinedbased on the above equations. The heat requirement, Qheater

(kJ/h), of the heater is calculated using Eq. (19) by making useof airflow rate, Wa (kg/h) and enthalpy of air.


Qheater = Wa(Ha,H − Ha,F) (19)

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ite c
Fig. 11 – Compos
The flow rate (kg/h) of the air, Wa, is computed by Eq. (20).

Wa = (e17 − e20) ∗ m20

(Ha,H − Ha,F)(20)

3.3.3. Hot air dryer modelIn the hot air dryer, the solid contacts hot air directly. Heat istransferred from the hot air to the solid and moisture that iscarried by the solid is vaporized. The evaporation takes awaya significant amount of heat that is transferred to the solid.The HAD is continuously fed and is assumed to be in steadystate. Eqs. (21)–(28) describe material and energy balances ofthe hot air dryer.

Wde,HAD(XHAD,in − XHAD,out) = Wda,HAD(YHAD,out − YHAD,in) (21)

Wde,HAD(Hde,out − Hde,in) + Wde,HAD(XoutHw,HAD,out − XinHw,HAD,out)

= Wda(Hda,HAD,in − Hda,HAD,out)

+ Wda(YoutHv,HAD,out − YinHv,HAD,in) (22)

(Hs,HAD − Hs,H) = Cpdw(Ts,out − Ts,in) (23)

Hw,out = cpw(Ts,out − Tref ) (24)

Hw,in = cpw(Ts,in − Tref ) (25)

(Hda,H − Hda,E) = Cpda(Ta,in − Ta,out) (26)

Hv,out = Cpv(Ta,out − Tref ) + LH (27)

Hv,in = Cpv(Ta,in − Tref ) + LH (28)

Eq. (21) describes the moisture balance. Eqs. (22)–(28)describe the energy balance among the inlet and outletstreams. The specific heat values of the dry air (Cpda), solid(Cpds), water (Cpw) and water vapor (Cpv) are assumed to beconstant within the dryer and the reference temperature is


set at 298 K. In this study, it was assumed that both Ts,out andTa,out have the same value.

urves of Case 4.

The dryer model can be solved by assuming the exhausttemperature of the air stream (TA,E) (Fig. 4); the solid temper-ature (TEFB,HAD) hence then be known. The moisture contentof the solid at the outlet Xout is determined by the moistureremoval requirement of EFB for maximizing its LHV. Once Xout

is determined, Yout can be calculated by Eq. (21) and so on.

3.4. Case 3: integrating a HAD with the steam powerplant

For the steam power plant model, as in Table 1, togetherwith the HAD presented above are solved simultaneously. Theobjective function is the same as in the Base Case, Eq. (14).The design parameters used for the Case 2 are summarizedin Table 6. Beside the constraints used for the Base Case, twomore constraints are added. In all case with dryers, the LHVof the dried EFB is restricted to be 15,000 kJ/kg or above formaintaining a good quality of combustion.

3.4.1. Results of Case 3Stream information at the optimum of Case 3 is presented inTable 7.

The efficiency of Case 3 is increased significantly from22.08% in Case 2 to 27.36%. The LP and MP pressures of thesteam power plant are changed to 2 bar (at the lowest limit)and 14.5 bar respectively. The final moisture content of EFB iscalculated to be 4.5% with a LHV of 15,000 kJ/kg.

The mass balance of the dryer section is given in Fig. 5. Forproducing an output power of 12.5 MW, a total of 34,322 kg/hEFB is fed along with 958,544 kg/h of fresh air for drying.

The stream data of Case 3 is extracted and listed in Table 8.The composite and the balanced composite curves are

shown in Fig. 6. The optimum solution showed Case 3 requiresmore LP for preheating BFW and the air heater. Since the LHVof the EFB is significantly increased, together with the sensibleheat carried by the EFB to the boiler, the overall efficiency ofCase 3 is much higher than the previous cases.

3.5. Super-heated steam dryer


A superheated steam dryer (SSD) uses a higher temperatureheat sources such as a medium pressure steam to operate.

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Table 11 – Stream results of Case 5.

Streams Temp. (K) P (bar) Enthalpy (kJ/kg) Entropy (kJ/kg) Vapor fraction m (kg/h) E (kJ/h)

1 441 3.0 2799 7.167 1.00 25,808 72,246,6752 319 0.1 2432 7.674 0.94 25,808 62,775,7973 314 0.1 171 0.583 0.00 25,808 4,411,0904 314 0.1 171 0.583 0.00 25,908 4,423,6645 314 2.7 171 0.583 0.00 25,908 4,430,5636 398 2.7 525 1.582 0.00 43,632 22,916,8217 398 16.5 527 1.582 0.00 43,632 22,981,6228 471 16.5 842 2.309 0.00 49,664 41,839,1749 472 100.0 852 2.309 0.00 49,664 42,316,192

10 472 100.0 852 2.309 0.00 100 85,20411 472 100.0 852 2.309 0.00 49,564 42,230,98712 850 100.0 3569 6.838 1.00 49,564 176,881,68413 614 16.5 3126 7.026 1.00 49,564 154,938,73414 614 16.5 3126 7.026 1.00 43,532 136,081,18215 614 16.5 3126 7.026 1.00 6032 18,857,55216 441 2.7 2801 7.219 1.00 43,532 121,946,29217 441 2.7 2801 7.219 1.00 29,840 83,591,85218 441 2.7 2801 7.219 1.00 13,692 38,354,44119 398 2.7 525 1.582 0.00 13,692 7,191,25920 –21 –22 441 2.7 2801 7.219 1.00 29,840 83,591,85223 441 2.7 2801 7.219 1.00 4032 11,294,99924 303 1.0 126 0.437 0.00 100 12,583

Ffit

e

W

e1a

Q

c

m

3p

Is8pStdpittt

ig. 7 is a simplified process diagram of SSD. Heat is trans-erred to the dryer indirectly. The moisture with the solid feeds vaporized at a LP condition and can be used for preheatinghe air stream of the hot air dryer.

The mass balance on water removal for SSD can bexpressed as in Eq. (28).

de(Xin,SSD − Xout,SSD) = mevap,ssd (29)

Additional specifications are made with regard to thenergy balance: the outlet temperature of EFB is assumed to be20 ◦C and the energy balance of the dryer can be representeds in Eq. (29).

SSD = mevap,ssd ∗ LH + Wde ∗ (1 − Xin)SSD(Tout,ssd − Tin,ssd) (30)

The steam flow rate required to dry EFB to the required levelan be calculated as in Eq. (30).

MP,SSD = Qssd

(ein = −eout)SSD

(31)

.6. Case 4: HAD and SSD integrated with the steamower plant

n Case 4, both HAD and SSD are used for EFB drying. An overallchematic diagram of the integrated process is given in Figs.

and 9. While Fig. 8 is a detailed flow diagram of the dryingrocess, Fig. 9 shows the integration scheme. Medium Pressureteam (MP) and Low Pressure Steam (LP) are extracted fromhe power plant for preheating BFW and EFB drying. The SSDryer generates a low pressure steam at 1.5 bar and is used forreheating the air of HAD. The boiling point of the steam that

s used for heating up the SSD should be at least 10 K abovehe steam temperature generated at SSD. We also assume both


he generated steam and EFB leaving the SSD are at the sameemperature.

The interconnection of the HAD and SSD acts as multi-stage dryers where EFB losses its moisture upon passing theseunits. Since heat for the SSD is being used twice, energy effi-ciency of the overall plant is expected to be improved.

The conditions and constraints for the power plant and theobjective function are the same as in the previous cases. Forthe drying section the feed air properties and original moisturecontent of the moist EFB feed are the same as in Case 3. Inthis case, the intermediate moisture content of the EFB, i.e. themoisture content at the exit of the HAD is a variable that needsto be optimized. The stream results of the optimal solution arepresented in Table 9 and the mass balance along with heatduties of the drying section is given in Fig. 10.

The overall energy efficiency of power generation integrat-ing SSD is 27.87 which is just slightly higher than that of HADin Case 4. To achieve this efficiency 395,997 kg/h of fresh airfeed and 33,886 kg/h moist EFB should be supplied to HAD.When comparing results with drying using only HAD there isa significant reduction in fresh air and EFB requirement, butthe final moisture content is about 4.5% in both cases. To findout how this case can be improved, stream data are extractedas in Table 10 and composite curves are drawn in Fig. 11.

The composite curves of Case 4 show the ineffectivenessof the heat integration among the two processes. A big tem-perature difference shown between the MP and SSD indicatesthat there might have a chance to replace MP by LP as the heatsource of SSD. This observation is supported by the results ofCase 4 where the pressure of MP is at its lower bound, 13 bar.

3.7. Case 5: HAD and SSD integration using LP steam

With the observation from the composite curves of Case 4, LPis used to replace MP for SSD heating. The integrated processdiagram is modified as in Fig. 12. LP steam is extracted andused for running the SSD. It leaves still as LP steam at lower


temperature and is proposed to be used for preheating the airentering the HAD.

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Fig. 12 – Process flow diagram of Case 5.

Table 12 – Stream data of Case 5.

TS (K) TT (K) DH (kW)

23 441.08 403.68 89.67403.68 402.68 2434.46402.68 398.18 21.47

15 614.49 476.42 554.35476.42 475.42 3238.31475.42 470.92 33.88

18 441.08 403.68 304.48403.68 402.68 8266.70402.68 398.18 72.90

LP-SSD 385 384 7531.32 318.96 317.96 16,062.63

317.96 313.96 119.83Hot Air 298 374 75315 313.92 398.18 2549.23

398.18 470.92 3833.04470.92 583.65 7708.77583.65 585.00 18,177.22585.00 850.00 11,550.54

3.7.1. HAD and SSD integrated plant simulation with LPsteamThe material and energy balances are set in similar fashion asin Case 4 except here LP steam is used in the SSD. The streamresults of the simulation are presented in Table 11 and themass balance along with heat duties of the drying section isgiven in Fig. 13.

The overall energy efficiency of power generation integrat-ing SSD that employs LP steam is calculated to be 29.92% whichis higher by about 2% than that of case 4. This is the maximumefficiency of all the case studies. To achieve this maximum effi-ciency 355,110 kg/h of fresh air feed and 31,576 kg/h moist EFBshould be supplied to HAD with slight reduction from the pre-vious case. The final moisture content is about 5%. The heatcurves of this case study Fig. 14 drawn by extracting streamdata as in Table 12 reflect that there is proper integration asthe hot and cold streams are very close to one another. Thesteam pressure of MP and LP are at 16.5 bar and 2.7 bar respec-tively, both are not on their bounds. Another very importantobservation is that during HAD and SSD integration using LPsteam, the steam generated at SSD can be used alone as sourceof heat for HAD as the heat duty of the heater is zero in Fig. 13.

4. Discussion

Analysis results of the five case studies show that integrationof drying to power generation from EFB increases the overallenergy efficiency. The results obtained through the five casestudies are summarized in Table 13.

After taking drying into consideration, the overall energyefficiency increased from 22.08% to 29.92%. The main reasonleading to the exceptionally low efficiency achieved by theBase Case is due to its high moisture content. Burning EFB with


high moisture content could generate some operation diffi-culties and often be prohibited. Heat integration studies were

performed for all cases to indicate the shortfalls in the integra-tion. For instance, improper BFW water preheat was observedin the Base Case and the replacement of MP with LP in Case 4.The balance composite curve of Case 5 (Fig. 14b) showed thetightest coupling of the hot and cold composite curves amongall cases, indicating the heat is properly integrated among theprocesses.

Throughout the assessments of different integratingoptions, following important observations were identified:

(1) drying is prerequisite to power generation,(2) steam pressures and BFW preheat should be taken into


account when the steam power plant is optimized,

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Fig. 13 – Mass balance of results of drier section of Case 5.

Fig. 14 – Composite curves of Case 5.

Table 13 – Summary of the effect of drying on the overall process efficiency.

System Moist EFB requirement (kg/h) Air for drying (kg/h) Efficiency %

Base Case without dryers 45,589 – 20.08%Base Case with EFP preheat and optimum P 42,934 – 22.08Integration with HAD 34,322 958,544 27.36%Integration with HAD and SSD using MP 33,886 395,997 27.87%

(

(

5

As

Integration with HAD and SSD using LP 31,576

3) the steam power plant should be optimized together withthe drying process,

4) the successive operation of HAD and SSD using LP steam asa heat source resulted in the maximum energy efficiency.

. Conclusions


material and energy balance model of a 12.5 MW biomassteam power plant that utilizes EFB as fuel is presented in

355,110 29.92%

this paper. Water and steam properties in this model werecalculated using Water97 in Excel making the model be ableto simulate and optimize the energy efficiency of the overallprocess. To obtain the optimal solution, the model simulta-neously varied the flow rates, pressures and temperatures ofthe streams in the power plant as well as the drying process.Composite curves of the overall plant are plotted for each case


to indicate the effectiveness of heat integration and to provideinsights for further improvement. Results showed that huge

dx.doi.org/10.1016/j.cherd.2014.04.013



improvement in energy efficiency can be achieved by properdrying of the biofuel. Innovative design of integrating a hot airdryer and a superheated steam dryer is proposed in this study.By proper integrating these two dryers into the power plant,energy efficiency can be improved by nearly 10% comparing tothe case where EFB was not dried.

Acknowledgements

The authors would acknowledge the financial support fromthe Hong Kong RGC-GRF grant (613513), the UGC-Infra-Structure Grant (FSGRF13EG03), the Studentship from EVNGprogram and the International Studentship from the Schoolof Engineering at HKUST.

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