Biogas

Use of agricultural biogasfor electrical power generation

with fuel cells

Dr. Doris Schmack

FAL workshop Biomass fermentation as basis for high quality fuel for fuel cell applications

fundamentals and special aspects25.-27.02.2004

The Project

Aims:

Screening of different gas cleaning technologies

Cleaning of agricultural biogas up to fuel cell quality

Operation of a molten carbonate fuel cell with cleaned biogas

Project timeframe: 2002-02-01 to 2003-11-30

Funded by:

Project partners:

Under the scientificsupervision of:

Introduction

Conventional CHP Fuel Cells

Waste Heat 80 C 400 C

Noise Emissions strong little

NOxPollutants SO2 little

Development Status state-of-the-art under development

Electrical Efficiency 35 % 50 %

Overview

Fuel cells

Gas cleaning target Biogas composition Specifications of fuel cells

Gas cleaning Technologies Experiments Results Gas cleaning concepts

Operation of the laboratory fuel cell

Summary and perspective

Fuel cell types

Alkaline FC (AFC)

Polymer Electrolyte FC(PEFC)Phosphoric acid FC (PAFC)

Molten Carbonate FC(MCFC)

Solid Oxide FC (SOFC)

H2

H2O

CO2

O2

H2

H2

H2

H2O

H2O

H2O

H2O

CO2

O2

O2

O2

OH-

H+

CO32-

O2-

room temp.

30-80 C

180-220 C

650 C

1000 C

l

o

w

t

e

m

p

e

r

a

t

u

r

e

f

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e

l

c

e

l

l

s

h

i

g

h

t

e

m

p

e

r

a

t

u

r

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f

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l

c

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l

l

s

Anode Cathode

CH4 Reforming of biogas necessary:

CH4 + 2 H2O CO2 + 4 H2 reforming reactionCH4 + H2O CO + 3 H2CO + H2O CO2 + H2 Shift reactionReforming at high temperatures.Low temperature FC: external reforming.

S compounds harmful to Ni catalysts in reformer must always be removed!

CO2 dilution effects worse kinetics lower cell voltage

Fuel cells:General aspects

AFC pH sensitive CO2 harmful (both in fuel and oxidizer) Operation on pure H2/O2

PAFC CO2 < 3 Vol% Separation of CO2 necessary

PEFC competing adsorption on anode catalyst CO from reforming harmful high standards for shift reaction

MCFC operating temperature approx. 650C high quality heat availableCO2 is reactant higher cell voltage higher overall efficiency

Fuel cells:Individual aspects

O2

CH4, H2O

Reactions in MCFC

Catalyst

CH4 + 2 H2O CO2 + 4 H2

H2 + CO3-- H2O + CO2 + 2 e-

CO2 + O2 + 2 e- CO3--

Anode / Catalyst

Matrix w/ electrolyte

Cathode / Catalyst

CO3--

off gas

Reformingreaction

Anode reaction

Cathode reaction

2

e

-

H2OCO2

CO2

CO2

MTU CFC laboratory stack

Stack with 10 single cells

Operating temperature 650 C

Maximal power 300 Watt

Biogas consumption < 3 l/min

Overview

Fuel cells





Biogas compositionin Haimhausen

Major components Minor components

CH4 50-60 % H2S < 1000 ppm

CO2 35-40 % COS < 1,5 ppm

H2O TD < 25 C NH3 < 50 ppm

O2 < 2 % Siloxanes n. d.

N2 < 5 % HalogenatedHydrocarbons

n. d.

Gas cleaning targets

raw biogasHaimhausen

specificationMTU Note

H2O rH up to 100 % < 60 % condensation,corrosion, protection ofactivated carbon

H2S ppmV norm.

Sulfur compounds

... contaminate the MCFC reforming catalyst

Sulfur compounds in the biogas in Haimhausen:

H2S approx. 50-100 ppmV, peaks 300-1500 ppmV strong periodic changes easy to remove with many technologies

COS approx. 0.3-1.5 ppmV (in Haimhausen) little variation with time most established technologies fail

Higher sulfur compounds not analyzed

COS

New in biogas context Detection by own GC analyses Concentration (up to 1.5 ppmV) high enough to poison

catalyst

Problem: Conventional technologies fail most activated carbons iron oxides

But:Low concentration use of expensive adsorbents possible

Overview

Fuel cells





Gas cleaning technologies

H2S COS CO2 H2O other

Biolog. Desulfurization

Activated carbon

Iron oxide/hydroxide

Iron complexes

Tenside scrubbing

Amine process

Glycol dehydration

Selexol

PSA

Water scrubbing

Membrane separation

Gas cooling

Gas cleaning:Analytical equipment

Biogas analyzer SSM 6000 CH4, CO2, O2 and H2S

Gas chromatograph GC 3800 trace analysis of H2S and COS

Humidity sensor GMH 3350 H2O

Laboratory methods NH3

Biological desulfurization

Oxidation of H2S to elemental sulfur bysulfur bacteria after addition of air/oxygen

very effective at high H2S concentrations cleaning down to approx. 50 ppmV simple technology, minimal investment und operating

costs State-of-the-art at

most current plants

Other H2S cleaning stepscan start at 50-100 ppmV

Desulfurization with activated carbon

Effective elimination of H2S down to

Activated carbon:Elimination of H2S

0,01

0,1

1

10

100

16.06. 17.06. 18.06. 19.06. 20.06. 21.06. 22.06. 23.06. 24.06.

p

p

m

H2S RohgasH2S Reingas

00,2

0,4

0,6

0,8

1

12:57 13:26 13:55 14:24 14:52 15:21 15:50

C

O

S

[

p

p

m

]

vor Reinigung

nach Reinigung

Activated carbon:Elimination of COS

0

0,2

0,4

0,6

0,8

1

12:00 12:14 12:28 12:43 12:57 13:12 13:26

C

O

S

[

p

p

m

]

vor Reinigungnach Reinigung

0

0,2

0,4

0,6

0,8

1

13:12 14:24 15:36 16:48

C

O

S

[

p

p

m

]

vor Reinigung

nach Reinigung

0

0,2

0,4

0,6

0,8

1

06.08. 09.08. 12.08. 15.08. 18.08. 21.08.

C

O

S

[

p

p

m

]

vor Reinigungnach ReinigungAK 4AK 3

AK 2AK 1

Desulfurization withiron oxides

Good elimination of H2S down to

Desulfurization withiron oxides

0,1

1

10

100

1000

30.01. 31.01. 01.02. 02.02. 03.02. 04.02.

H

2

S

[

p

p

m

]

H2S Reingas

H2S Rohgas

Iron complexes

Contact with iron complex solutions in column formation of elemental sulfur at presence of oxygen

cleaning down to approx. 5 ppmV H2S low priced iron complexes available regenerable, minimal operating costs moderate investment costs

Drawbacks: separation of sulfur, plugging chemical stability

Commercial processes available (e. g. LO-CAT)

Iron complexes:experiments

Efficiency about 95 % Suited for high H2S loads / bigger plants Secondary cleaning step necessary for FC operation

1

10

100

1000

12.12. 13.12. 14.12. 15.12. 16.12. 17.12. 18.12. 19.12.

H

2

S

[

p

p

m

]

H2S Rohgas H2S Reingas

Amine process

Selective removal of CO2 and/or H2S Dependant on pressure and amine Best amine for pure desulfurization: MDEA

Conventional design: 6-10 bars pressure good cleaning performance high costs at small plants

Aim: unpressurized process, cost reduction

Question: Still enough cleaning performance?

Unpressurized amine process: experiments

efficiencies about 80 % still relatively high investment costs rich gas must be treated separately

1

10

100

1000

03.12. 04.12. 05.12. 06.12.

H

2

S

[

p

p

m

]

H2S - Rohgas H2S - Reingas

Membrane processes

Use of the selectivity of suited membranes for separation of CO2, H2S and H2O

No regeneration of separation medium necessary High operating, medium investment costs

Drawbacks: high pressure (energy consumption) selectivity vs. permeability methane loss with permeate (up to 30 % of costs)

very expensive

Gas drying

gas coolingelectrical energy for cooling, icing up

Selexol PSA/TSA (pressure/temperature swing adsorption)

high pressure high energy demand Glycol dehydration with TEG

unpressurized, use of CHP/FC spill heat

Aims of gas drying: avoid condensation (pipe plugging) protection against corrosion protection of other units like activated carbon

Possible processes:

Gas drying:Glycol dehydration

Gas drying by contact with triethylene glycol (TEG)

9 Closed, low-maintenance system9 Use of CHP/FC spill heat low operating costs

9 Dewpoint reduction under 10 C(single stage design)

9 reliable inhibition ofcondensation

9 sufficient drying foroperation of activated carbon

Resultsglycol dehydration

0

5

10

15

20

25

30

09.05.03 10.05.03 11.05.03 12.05.03 13.05.03

T

a

u

p

u

n

k

t

[

C

]

vor Trocknung

nach Trocknung

3 heat exchangers higher investment costs higher electrical energy demand higher total costs than gas cooling

before drying

after drying

d

e

w

p

o

i

n

t

[

C

]

Gas cleaning concepts

1000

100

10

1

0,1

50 150 500

H

S

[

p

p

m

]

2

plantsize

[m/h]

Overview

Fuel cells





Fuel cell operation

Overview

Fuel cells





Summary

Screening of gas cleaning technologies

Overview over possibilities and costs

Compliance with specifications of gas quality

Cost-effective gas cleaning possible

Multi level gas cleaning concept

Very promising fuel cell performance with biogas

Interruption of fuel cell operation very harmful for MCFC

Perspective

Further examination of certaingas cleaning technologies

Production of gas cleaning units

HotModule operation on biogas

Cost reduction - funding

Thank you!

Documents

Biogas