Fuel Cells in Energy Technology

(9)

Werner Schindler

Department of Physics – Nonequilibrium Chemical Physics

TU München

summer term 2013

- Source

- Distribution

- CO poisoning

- Emissions

(true zero, CO2)

Focus on

- Hydrogen

- Methanol

(liquid, high energy

density)

2

CO Poisoning of Pt Catalyst

Remove CO from the fuel !

3

CO Poisoning of Pt/Ru Catalyst

Remove CO from the fuel,

or find CO tolerant

catalysts !

4

Hydrogen Production

Industrial production of hydrogen

- from natural gas by steam reforming

- from methane decomposition

- from oxidation of hydrocarbons (oil)

- from coal

- from methanol by steam reforming

- as a side product of chemical syntheses

- from biomass

- from water by electrolysis

Water electrolysis is one of the techniques with highest

energy demand.

(Partial) Oxidation of hydrocarbons is exothermic.

5

Sources for hydrogen

6

Hydrogen Production Cost

cheap expensive

(No distribution / transportation cost included!)

7

Hydrogen Production Cost (different sources)

8

Catalytic steam reforming

Source: R. O'Hayre et al.: Fuel cell fundamentals. 2nd ed., Wiley, 2009 9

Source and distribution

available for natural gas, but reforming required to achieve a H2 rich fuel

10

Some background reading on fuel reforming

J. Larmine, A. Dicks:

Fuel cell systems explained, ch. 8, pp. 229

John Wiley & Sons, 2nd Ed. 2003

ISBN-13: 978-0387344447

Photographs of the book covers from www.amazon.de

R. O'Hayre, W. Colella, Suk-Won Cha, F.B. Prinz:

Fuel Cell Fundamentals, ch. 11, pp. 371

John Wiley & Sons, 2nd Ed., 2009

ISBN-13: 978-0470258439

M. Kaltschmidt, H. Hartmann, H. Hofbauer:

Energie aus Biomasse

Springer, 2nd Ed., 2009

ISBN-13: 978-3540850946

11

Despite

- increasing energy demand (China, India, etc.)

- definitely decreasing resources

- and as a result definitely increasing prices

12

Hydrogen production from hydrocarbons

- Remove impurities from the (natural) supply

- Utilize efficient processes (choose appropriate fuel supply)

- Maximize hydrogen output

- Remove CO and CO2 from the output

13

Principal chemical reactions

Reactions with water / hydrogen:

CnHm + n H2O → n CO + (n+m/2) H2 DH > 0 (steam reforming)

CO + H2O → CO2 + H2 DH = -38 kJ / mol (water-gas shift)

CO + 3 H2 → CH4 + H2O DH = -206 kJ / mol (methanization)

Oxidations:

CnHm + (n+m/4) O2 → n CO2 + m/2 H2O DH << 0 (complete combustion)

CnHm + n/2 O2 → n CO + m/2 H2 DH < 0 (partial combustion)

CO + ½ O2 → CO2 DH = -286 kJ / mol (CO oxidation)

Carbon formation:

2 CO → C + CO2

CnHm → n C + m/2 H2

CnHm → Olefines → Polymers → Coke (thermal cracking, pyrolysis)

Depending on reaction educts and operation conditions the following processes for

hydrogen production are distinguished:

Steam-Reforming: CnHm + n H2O → n CO + (n+m/2) H2 DH > 0 (endotherm)

Partial Oxidation: CnHm + n/2 O2 → n CO + m/2 H2 DH < 0 (without steam, exotherm)

CnHm + n/2 O2 → n CO + m/2 H2 DH < 0 (with steam, autotherm)

CnHm + n H2O → n CO + (n+m/2) H2 DH > 0 14

Hydrogen production via fuel reforming -

overview

• Steam reforming (SR)

• Partial oxidation (POX) reforming

• Autothermal reforming (AR)

• Gasification

• Anaerobic digestion (AD)

15

1. Catalytic Steam Reforming

using light hydrocarbons which vaporize completely without formation of carbon

Required for protection of

catalysts in the reformer

(basically metal oxide sorbents like ZnO

which remove H2S to a few ppm

due to sulphurization)

Ni based catalyst

CnHm + n H2O → n CO + (n + m/2) H2

CO + H2O → CO2 + H2

CO + 3 H2 → CH4 + H2O

n=1, m=4: methane

strongly endothermic –

energy supplied from combustion

process of gas or oil 16

Steam reforming of methane

Source: J. Larminie, A. Dicks: Fuel cell systems explained, 2nd ed., Wiley, 2003

Equilibrium concentrations of steam

reformation reactant gases as a

function of temperature at 1 bar and a

fixed steam to carbon ratio

CH4 + H2O → CO + 3H2 DH = 206 kJ/mol CO + H2O → CO2 + H2 DH = -41 kJ/mol Catalysts:

• Ni or noble metals (reforming)

• Water gas shift at T > 400 °C

(HTS): Fe3O4/Cr2O3 catalyst

• Water gas shift at T < 270 °C

(LTS): Cu/ZnO catalyst

17

Methanol is a promising candidate to substitute fossil hydrocarbons

Reverse methanol synthesis reaction:

CH3OH → CO + 2H2 ∆H = 91.7 kJ / mol

(endothermic > 700 °C without catalyst, at 300 - 450 °C with CuNi or ZnCr alloy catalysts)

Watergas shift reaction

CO + H2O → CO2 + H2 ∆H = -41.0 kJ / mol

Steam reforming

CH3OH + H2O → CO2 + 3H2 ∆H = 50.7 kJ / mol

• Catalysts CuO-ZnO or CuO-Cr2O3

• Molar ratio of water to methanol between 0.67 and 1.5

• Excess steam lowers carbon formation

• 1.5 m3 H2 per kg methanol

• Up to 2000 m3/h

Methanol steam reforming

18

Methanol oxidation complicated pathway

-> reform and use hydrogen

A commercial fuel cell running on methanol

Source: http://www.ultracellpower.com/sp.php?rmfc (May 17, 2009)

Reformed methanol fuel cell (RMFC)

19

A conceptual micro-scale methanol fuel

processor

Source: J. Larminie, A. Dicks: Fuel

cell systems explained, 2nd ed.,

Wiley, 2003

20

Catalytic steam reforming in the Haldor

Topsoe heat exchange reformer

Source: J. Larminie, A. Dicks: Fuel cell

systems explained, 2nd ed., Wiley, 2003

Haldor Topsoe is a Danish

catalyst company.

http://www.topsoe.com/

ca. 675°C

ca. 830°C

Designed for PAFC systems.

Heat for the reforming reaction is

provided by the combustion of lean

anode exhaust gas.

21

Two examples of external reformers

Source: R. O'Hayre et al.: Fuel cell fundamentals. 2nd ed., Wiley, 2009

Honda Home Energy Station

ca. 2 m3 H2 per hour from methane

Pacific Northwest National Laboratory

microfuel processor

22

2. Partial oxidation hydrocarbons

The complete combustion of propane does not yield hydrogen:

C3H8 + 5 O2 3 CO2 + 4 H2O

In a partial oxidation a hydrocarbon is oxidized with less than the stoichiometric amount

of oxygen (incomplete combustion):

Propane C3H8 + 3/2 O2 3 CO + 4 H2

General hydrocarbon CxHy + 3/2 O2 x CO +y/2 H2

Source: R. O'Hayre et al.: Fuel cell fundamentals. 2nd ed., Wiley, 2009 23

3. Autothermal reforming is a combination

of steam reforming and partial oxidation

Endothermic steam reforming including water gas shift reaction:

CH4 + 2 H2O(l) CO2 + 4 H2 DH = +253.4 kJ/mol

Exothermic partial oxidation:

CH4 + 1/2 O2 CO + H2 DH = -35.7 kJ/mol

The stoichiometry of the sum reaction can be adjusted to give a reaction

with zero reaction enthalpy:

CH4 + 1.115 H2O(l) + 0.44 O2 CO2 + 3.11 H2 DH = 0 kJ/mol

(Assumption: reactants and products enter reactor at 298 K and 1 bar)

For a detailed discussion see:

R. O'Hayre et al.: Fuel cell fundamentals. 2nd ed., Wiley, 2009, pp.379 24

Reforming

Hotspot Fuel Processor

• based on methanol

• produces 6000 l H2 per hour

• can supply1 kW fuel cell

• achieves 20 s after start-up 75% production

General Motors Truck

with gasoline reformer (arrow)

25

Summary on steam reforming, partial

oxidation and autothermal reforming

Source: R. O'Hayre et al.: Fuel cell fundamentals. 2nd ed., Wiley, 2009 26

27

A comparison of steam reforming, partial

oxidation and autothermal reforming

Source: R. O'Hayre et al.:

Fuel cell fundamentals. 2nd

ed., Wiley, 2009

27

C + H2O → CO + H2 H = 131.4 kJ/mol

Heat for this endothermic reaction is supplied by the direct combustion of a portion of the

coal.

Ash content, composition, agglomeration, sulphur content make it a complicated process

4. Coal gasification

Source: K. Kordesch, G. Simader: Fuel cells and their applications, VCH, Weinheim, 1996 28

Gasification of biomass

Source: M. Kaltschmidt: Energie aus Biomasse. 2nd ed., Springer, 2009 29

5. Production of biogas from manure

Source: R. A. Zahoransky: Energietechnik, Vieweg/Teubner, 2009 30

Yield of biogas from digestion of biomass

Data from: R. A. Zahoransky: Energietechnik, Vieweg/Teubner, 2009

Digestion temperature 30 °C

FM: fresh mass

31

Typical composition of biogas

Data from: R. A. Zahoransky: Energietechnik, Vieweg/Teubner, 2009 32

Most gasifiers produce hydrocarbons

(primarily CH4)

Initial reaction step reforms these

into CO and H2:

CH4 + H2O → CO + 3 H2 ∆H = 206.3 kJ/mol

Adjustment of CO to H2 ratio via the water-gas

shift reaction:

CO + H2O → CO2 + H2 ∆H = -41.2 kJ/mol

After this point different paths for methanol and

hydrogen production

Hydrogen and methanol from solid

biomass

Source: K. Kordesch, G. Simader: Fuel cells and their applications, VCH, Weinheim, 1996 33

Fuel cell operation temperature

may be less important when a

reforming process is considered

in a complete system! 34

35

SOFC – advantages of internal reforming of natural gas:

- Cost efficiency, minimization of components in SOFC systems;

- Increase of efficiency;

- Heat consumption by endothermic steam reforming process

lowers the necessity for air cooling of the stack;

- Faster load response.

SOFC – disadvantages of internal reforming of natural gas:

- Carbon formation in the anode chamber;

- Changes in the temperature distribution in the stack, and

large temperature gradients due to the gas flow.