l6-ae2015_h2_for_ fuelcells.pdf

Processing of Hydrocarbon Fuels 1

Hydrogen Production for Fuel Cells

Anode Exhaust Burner

REFORMER

e-

Air / O2

N2, O2, H2OH2 / H2OCathode ExhaustAnode Exhaust

Anode Cathode

Air

H2O

CO Clean Up

H2, H2O, CO2, N2, CO

CATALYSTS containing nanoscale particles

Catalysts in Fuel Processors and Fuel Cells

FuelH2 / H2O


Fuel processors ■  Main purpose of fuel processing system:

◆  convert a hydrocarbon fuel into a H2-rich gas

Efficiency of fuel reformer system:

η =ΔH (HHV ),H2

ΔH (HHV ), fuel


Fuel Processing

Steam reforming (endothermic, heat transfer limited)CnHm + nH2O → nCO +(n+1/2m) H2

Partial oxidation (exothermic, mass transfer limited)CnHm + 1/2O2 → nCO +1/2m H2

Autothermal reforming (adjustable heat duty)CnHm + xO2+ (2n-2x)H2O → nCO2 +(2n-2x+1/2m) H2

CO RemovalReformer Water Gas Shift Reactor (WGS)

Desulfurizer

< 1 ppm sulfur

H2 (< 10 ppm CO) 10%

CO2,000 ppm

COGasoline

PEM FC

SOFC

Fuel Purification: Desulfurization

CO RemovalReformer Water Gas Shift Reactor (WGS)

Desulfurizer

< 1 ppm sulfur

H2 (< 10 ppm CO) 10%

CO2,000 ppm

COGasoline

PEM FC

SOFC

Natural gas contains mercaptans, added as odorantGasoline and diesel contain significant amounts of sulfur compounds

Sulfur is catalyst poison for fuel reformer and fuel cell catalysts


Sulfur Removal ■  Depending on the type of fuel, different concentrations of

sulfur compounds present. ◆  Methanol is clean ◆  Natural gas, gasoline, and Diesel must undergo desulfurization

process. (typical S content: 30 - 50 ppm) ■  Sulfur must be removed from feed stream to fuel

processor to protect the catalysts. ■  Sulfur must also be prevented form entering the fuel cell.

It could deactivate the electrocatalysts on the electrodes.

Sulfur Tolerance of Fuel Cells

■  PEM <1ppm, ◆  poisoning is cumulative and not reversible

■  PAFC <20 ppm; ◆  poisoned anodes can be reactivated by polarization at high potentials

■  MCFC <10 ppm at anode ◆  Poisoning is reversible

■  SOFC 10 -35 ppm ( higher for all ceramic systems)


S-containing compounds in fuels

Effect of Thiophene on Synthesis Gas Yield


Hydrodesulfurization (HDS)

C4H4S + 3 H2 → C4H8 + H2S

Catalysts: Co-Mo or Ni-Mo supported on silica/alumina

For fuel cell applications, the H2S must be removed!

Model for HDS active site

H.Topsoe, B. S. Clausen, Catal. Rev.-Sci.Eng.26, 395(1984)


Desulfurization of Fuels by Adsorption

Yang et al., U.S. and foreign patents applied.

Sorbent Container

Performance Data: •  Sorbent: three-layers

-  Activated Carbon (12.4 wt%) -  Activated Alumina (23 wt%) -  Ni(II)-Y (64.6 wt%)

•  Gasoline Rate: 50 mL/hr •  Effluent Sulfur Conc.:

~0.3 ppmw •  Operation Cycle: 9-10 hrs

Act

ivat

ed

Car

bon

Act

ivat

ed

Alu

min

a Ni(II)-Zeolite

Absorption by metal oxide sorbents:Oxides of Zn, Cu, Mn, Ti, Fe, Ni and their mixtures

Fixed Bed Adsorption at Room Temperature

Breakthrough of total sulfur from diesel feed at RT for Cu(I)-Y (VPIE) and Selexsorb CDX/Cu(I)-Y (VPIE) adsorbents (Hernández-Maldonado and Yang, J. Am. Chem. Soc. 2004, 126, 992)

Cu(I)-Y (VPIE)Zeolite

Ci

Ct


Steam reforming Objective: extract the maximum quantity of H2 held in water and the hydrocarbon feedstock

Overall reaction:

Actual set of reactions in steam methane reformer:ΔH= +206 kJ/mol

ΔH= - 41 kJ/mol

Steam Reforming Catalysts ■  Nickel ■  Cobalt ■  Ruthenium ■  Rhodium ■  Palladium ■  Platinum

◆  Noble metals are more active than nickel and cobalt, but much more expensive

■  Temperature range: 800 - 900 °C ■  Pressure: near atmospheric ■  Steam/carbon molar ratio = 3.5 ■  Typical conversions reached: 98 - 99.6 % ■  Process primarily used for stationary applications where

large quantities of steam are available


Mechanism of steam reforming 1.  Adsorption of hydrocarbon onto metal sites

Dissociation into carbon and hydrogen species 2.  Adsorption of water on oxide sites

Dissociation of water into oxygen and hydrogen species 3.  Metal sites must facilitate

surface reaction between carbon and oxygen species to produce CO and CO2

the combination of hydrogen surface species into H2

Mechanism of steam reforming Hydrocarbon is adsorbed on dual site Successive α-scission of the C-C bond Resulting C1 species react with adsorbed steam


Activation of C-H bond in methane   S

Nikolla, Schwank, and Linic, J. Catal. 263 (2009), 220-227

Kinetics of methane steam reforming

Reaction is first order in methaneLess agreement on other kinetic parameters

due to diffusion and heat transfer limitationsreported activation energies span a wide rangemisleading pressure effects due to diffusion limitationslarge catalyst pellets result in low effectiveness factors

η = 0.3 at reactor inlet region, and 0.01 at exit

Classic Russian study:N.M. Bodrov, L.O. Apel’baum, M.I. Temkin, Kinet.Catal. 5, 614 (1964)

-d(CH4)/dt = kPCH4/[1+a(PH2O/PH2)+bPCO]@ 800 °C: constant a= 0.5 atm-1

constant b= 2.0 atm-1

@ 900 °C: constant a= 0.2 atm-1

constant b= 0.0 atm-1

Ea = 130 kJ/mol


Ni surface area (m2) per gram of Ni

Ni surface area per gram of catalyst

Effect of nickel content

Side reaction accompanying the steam reforming reaction: Water gas shift

◆  CO + H2O ⇔ CO2 + H2 ΔH = -41 kJ/mol Since both the steam reforming reaction CH4 + H2O ⇔ CO + 3 H2

as well as the water gas shift reaction are reversible, equilibrium is reached very quickly at the high reactor temperatures needed to get good conversion. Reactor effluent contains a mixture of CO, CO2, H2, CH4, and H2O


Steam reforming product distribution depends on: ■ Reactor outlet temperature ■ Operating pressure ■ Composition of feed gas ■ Steam/carbon ratio ■ Nature and packing of catalyst bed

ASPEN® software models allow to predict the thermodynamic equilibrium composition of the reactor effluent as function of temperature and pressure

Equilibrium Concentrations of Steam Reforming Reactant Gases

Larminie and Dicks, Fuel Cell Systems Explained, Wiley, 2003 p.242


Effect of pressure on steam reforming ■  LeChatelier principle:

◆  Equilibrium will shift to the left if P is high (favoring the formation of methane)

◆  The water gas shift reaction, on the other hand, is not affected much by pressure (since there are identical numbers of molecules on both sides of the equation)

CH4 + H2O ⇔ CO + 3 H2

CO + H2O ⇔ CO2 + H2

Carbon formation on reforming catalysts

Thermal cracking CH4→ C + 2 H2

Disproportionation 2CO→ C + CO2

CO reduction CO +H2→ C + H2O


Thermodynamics of methane decomposition: Equilibrium composition of C/4H system

P= 101.13 kPa

Reaction engineering of SMR ■  Since steam reforming is endothermic, reaction must be

run at high T ( > 800 °C) ■  Materials of reactor construction become an issue at

such high T (expensive alloys required) ■  Thick wall tubular reactors used

◆  High fuel consumption to heat the tubes ◆  Critical balance between heat input through tubes and heat of

reaction


Process flow diagram for methane steam reforming

HDS unit

■  High pressure hydrogenation on cobalt-molybdenum catalysts operated at 290 - 370 °C converts thiols to H2S and olefins

■  H2S is stripped using a ZnO absorber bed at 340 - 390 °C


Step-wise steam reforming

Step 1:Decomposition of methane over Ni/ZrO2 catalyst:CH4 → C + 2H2

Step 2:Carbon gasification by steam:C + 2 H2O → CO2 + 2H2

Cyclic process, switching between two reactors

Schematic of Stepwise Reforming System


Partial Oxidation (POX) ■  Alternative to Steam Reforming

◆  Incomplete combustion of fuel with substoichiometric amounts of oxygen or air

◆  Highly exothermic process; T gets very high (1200 - 1500 °C without catalyst)

CH4 + 1/2 O2 → CO + 2 H2 ΔH = - 247 kJ/mol

Note: POX produces less H2 per fuel molecule than steam reforming ( lower efficiency).

Process flow diagram for POX


Partial Oxidation ■  Thermal integration of process streams is a challenge ■  Process does not scale down well to smaller size ■  Control of process is difficult ■  For small-scale configurations, a catalyst (supported Pt,

Ni, or chromium oxide) can be used to increase reaction rate and operate at lower temperature where steel reactors can be used.

Characteristics of POX ■  Exothermic

◆  reactor does not need to be externally heated ◆  more compact and light-weight design possible

■  Maximum allowable O/C ratio = 1 ◆  Generates lots of heat, and gives low H2 selectivity


Autothermal Reforming ■  Combination of steam reforming (SR) with partial

oxidation (POX) ■  Nature of catalyst determines the relative extent of SR

vs. POX reactions ■  SR reaction absorbs part of the heat generated by POX

◆  This limits the T in reactor ◆  Overall, slightly exothermic process

Autothermal reformer

Gas inlet

Reformed gas product

Air and steam inlet

Catalyst

High T catalystInert material

Insulation

Insulation

Additional insulation

Burner

275- 1000 psig


Catalytic Fuel Reformers and SOFC Auxiliary Power Units

H2 and CO production for solid oxide fuel cells in mobile applications

40

On-board Reforming of Fuels

H2&

CO

http //www.ip3.unipg.it/FuelCells/images/fctype_SOFC.jpg

Solid Oxide Fuel Cell

Ni/Ceria-Zirconia/Monoliths

B D Gould, A R Tadd, and J W Schwank, Journal of Power Sources, 164 (2007) 344-350 B D Gould, X Chen and J W Schwank, Journal of Catalysis, 250/2 (2007) 209-221 B D Gould, X Chen, and J W Schwank, Applied Catalysis A: General 334 (2008) 277-290


APU: A device to generate electricity when a vehicle’s power train is off

• Benefits of auxiliary power units:

•  Reduced engine idling •  Reduced fuel consumption •  Reduced emissions •  Reduced engine wear •  Reduced noise pollution

1 liter

0.25 liter

SOFC APUs for Trucks

3

Engine

Transmission

Power steering pump

Oil pump

Air compressor

Heated power seats

Air conditioning

Electronics

SOFCAPU

H2, CO

Fue

l

FuelProc.

SCR S Jain, H-Y Chen, and J Schwank, Journal of Power Sources 160 (2006) 474-484


ATR Schematic Homogeneous

SRPOX

C12

H2O, O2

H2, H2O

CO, CO2, C1-12

Cracking

Combustion

Catalytic

Monolith

Catalyst temperature control strategies during start-up needed to avoid high temperature spikes

T_downstream

T_upstream

T_vaporizer

Tem

pera

ture

ºC

Time (min)

Sintering during startup


ATR Schematic

2 µm 500 nm

Carbon Deactivation

■  Monometallic Ni catalysts ◆  Deactivate severely during steam reforming of hydrocarbons

–  Extended carbon structure formation such as graphitic layers, fullerenes, and nanotubes

50 nm50 nm

Ni

C


Carbon deposition, isooctane ATR

X. Chen, A. R. Tadd and J. W. Schwank, Journal of Catalysis 251 (2007) 374-387

T=650°C, O / C = 0.5, H2O / C=0.8


ATR over Ni/CZO/Monolith: SEM

Upstream

Downstream

Upstream channel surface

Downstream channel surface

Upstream inside wall

Downstream inside wall

Summary: carbon deposition

■  Both homogeneous and oxidative cracking of C8-C12 hydrocarbons into C1-C4 hydrocarbons contribute to overall conversion

■  Partial oxidation and steam reforming of C1-C4 play key roles in forming H2, CO, and CO2

■  C1-C4 hydrocarbons contribute to the major carbon deposition ■  Two types of carbon: coating carbon and filamentous carbon ■  Extent of carbon deposition and morphology show spatial

gradients along flow direction in monolith and strong dependence on type of reaction (ATR, POX, SR)

■  Alloying the nickel surface with tin prevents carbon deposition


Characteristics of ATR ■  Operates at lower O/C ratio than POX ■  Operates at lower T than POX ■  Faster process, quicker starting, better response to

transient operation ■  Good H2 selectivity and yield ■  Once started, surplus heat from other parts of system

can be sent to ATR to increase its efficiency.

Overall Equation for ATR

CnHmOp + x(O2 + 3.76 N2) + (2n - 2x - p) H2O →

nCO2 + (2n-2x-p +m/2) H2 + (3.76x) N2

x = molar ratio of O2/fuel

At elevated T, CO is formed via reverse steam reforming reaction

CO2 + H2 ⇔ CO + H2O


ATR ( cont.)

■  At x=1, feed contains enough O2 to convert all carbon to CO2 without any added H2O ◆  ( exothermic oxidation)

■  With decreasing x, the H2O/fuel ratio increases ◆  Yield of H2 increases

◆  Reaction becomes less exothermic

■  Reaction becomes thermoneutral at x = x0 ◆  X0 = 0.44 for natural gas

■  At x=0, endothermic SR reaction.

CnHmOp + x(O2 + 3.76 N2) + (2n - 2x - p) H2O →

nCO2 + (2n-2x-p +m/2) H2 + (3.76x) N2

Calculated Thermoneutral Oxygen-to-Fuel Molar Ratios (xo) and Maximum Theoretical Efficiencies (at xo) for Common Fuels

Source: Fuel Cell Handbook, DoE

Efficiency = lower heating value of anode gas produced lower heating value of fuel used


Water Gas Shift ■  Products leaving the reformer contain 10 - 15 % CO. ■  This CO concentration is unacceptable for PEMFC or

PAFC. ■  CO concentration in reactor effluent can be decreased

by water gas shift reaction downstream of the reformer: CO + H2O ⇔CO2 + H2

CO RemovalReformer Water Gas Shift

Desulfuriz

er

< 1 ppm sulfur

H2 (< 10 ppm CO) 10%

CO2,000 ppm

COGasoline

PEM FC

SOFC

Characteristics of Water Gas Shift ■  Exothermic, reversible reaction

◆  Lowest CO yields are achieved at low T, ◆  but at low T, conversion is low!

■  Ideally, one would want to operate along the locus of maximum rate. This is, however, not practical in conventional reactors, because of lack of sufficient temperature control and transport limitations.

■  Two stage process used: ◆  High T shift (350 - 550 °C, Fe-Cr catalysts) ◆  Low T shift (150 - 300°C, Cu-Zn/Al2O3 catalysts)


Energy balance for exothermic reaction

H.S. Fogler, Elements of Chemical Reaction Engineering, Prentice Hall, 3rd Ed. , p.471, 1999

Interstage cooling: a strategy to increase conversion

Temperature ( C)

H.S. Fogler, Elements of Chemical Reaction Engineering, Prentice Hall, 3rd Ed. , p.471, 1999


Water Gas Shift

100

200

300

400

500

600

0.01.02.03.04 05.06 0

Tem

pera

ture

(°C

)

Exit CO Content

100

200

300

400

500

600

0.01 02.03.04.05.06.0

Tem

pera

ture

(°C

)

Exit CO Content

HighTemperature

Shift

InterstageCooling

LowTemperature

Shift

Equilibrium Equilibrium

Locus ofMaximum Rate

100

200

300

400

500

600

0.01.02.03.04 05.06 0

Tem

pera

ture

(°C

)

Exit CO Content

100

200

300

400

500

600

0.01 02.03.04.05.06.0

Tem

pera

ture

(°C

)

Exit CO Content

HighTemperature

Shift

InterstageCooling

LowTemperature

Shift

Equilibrium Equilibrium

Locus ofMaximum Rate

Water Gas Shift Catalysts

Early Transition Metal Carbides Catalytic properties Levy and Boudart, 1973 similar to Pt-group Oyama, 1992 Highly active for WGS Thompson et al., 2000 Tolerant to sulfur Manoli et al., 2001

Supported Pt-group metals Au/Ag/Ru on Reducible Oxides

Au/MOx highly active Stephanopoulus et al., 2001 Questions about stability Löffler et al., 2002


Final Removal of CO ■  Preferential Oxidation ■  Purifying H2 by Diffusion through a Pd membrane ■  Selective Methanation

Final Removal of CO

PREFERENTIAL OXIDIZER

AUTOTHERMALREFORMER

10% CO

WATER GAS SHIFT REACTOR

2,000 ppmCO

FuelProcessor

H2(<10 ppm CO)

GasolineDieselJP-8

H2O

O2

Power

<1 ppm sulfur

DESULFURIZER


Preferential Oxidation CO + 1/2 O2 → CO2

■  Catalysts: –  Ru/Al2O3 –  Rh/Al2O3 –  Pt/zeolite ( low selectivity ~62%) –  CoO –  Au/Fe2O3

Small amount of air is added to gas stream ◆  H2 + O2 can form explosive mixtures ◆  Transients are difficult to control

Separation with Pd Membrane

CO

H-H

H

H

H

H+ pure H2


Methanation CO + 3 H2⇔ CH4 + H2O

ΔH = -206 kJ/mol (reverse of steam reforming of methane)

Disadvantage:

consumption of H2 lower efficiency

Pressure Swing Adsorption

II Adsorption

H2, CO

Exhaust: CO

III PurgeI Pressurization IV Desorption

H2

l6-ae2015_h2_for_ fuelcells.pdf

Documents

type of fuel

fuel cell applications

desulfurization of fuels

sulfur removal

fuels effect of thiophene

ppm sulfurh2

clean natural gas

mlhr effluent sulfur