Syngas as SOFC Fuel

Effect of Tar on Anode Materials

Marcos Millan

Joshua Mermelstein, Esther Lorente, Nigel Brandon

SOFC Symposium, London, 19th April 2013

Overview of this talk

• Introduction to Gasification and Gasification-Fuel

Cell Integration

• Effect of Tar on SOFC Anode Materials

» Experimental Setup

» Experiments with tar model compounds:

Carbon Deposition on SOFC Anode Materials

Effect of Tars and Steam on Anode Performance

Effect of a full syngas mixture on Carbon Deposition

» Influence of Actual Tars and Tar Fractions on Carbon

Formation

• Summary

Introduction: Gasification as a Versatile Process

Feedstocks Gasification Syngas

Applications

Gas

Cleaning

and Shift

Coal

Heavy and

Unconventional

Oils

Biomass

Power

(IGCC,

Fuel Cells)

Hydrogen

Methanol

Fischer-Tropsch

SNG

Gasifier

Tar

Trace

elements

S, N

CO2?

CO

H2

CH4

CO2

Main Types of Gasifier Entrained Flow

(Elcogas Gasifier depicted)

Fluidised Bed

(Gussing ICFB

Gasifier depicted)

Moving Bed

(Lurgi Dry Ash

Gasifier depicted)

Integration of Gasification and SOFC

Why SOFC?

• Higher resistance to gas impurities than other types of fuel cells.

• Good matching in temperatures with downdraft and fluidised bed

gasifiers.

• Potentially good matching in scales.

Sadukhan et al, Chem Eng Sci 65, 6, 1942 (2010)

Theoretical

studies on

performance

analysis

showed

potential CHP

efficiencies of

around 80%.

Effect of Tars on SOFC Operation

Tars are a

complex mixture

of condensable

hydrocarbons

including single

to polyaromatic

compounds.

• How do tars affect SOFC operation and performance?

• What is the maximum amount of tars a SOFC can tolerate?

• What is the influence of operating conditions on carbon

formation?

Some Key Research Questions:

Tar Classification System by Bergman et al. (ECN)

Type Examples

GC undetectable Biomass Fragments,

heavy tar fraction

Heterocyclic compounds Phenol, cresol, quinoline, pyridine

Aromatic (1-ring) Toluene, xylene, ethylbenzene

Light polyaromatic hydrocarbons

(2-3 rings PAHs)

Naphthalene, indene, biphenyl,

anthracene

Heavy Polyaromatic

hydrocarbons (>4 rings PAHs)

Fluoranthene, pyrene, crysene

GC detectable, not identified

compounds

Unknowns

Syring

e P

um

p

Mass Flow

Controllers

H2

CO

CO2

CH4

N2

NC

Humidifier

NC

Exhaust

Autolab

PGSTAT302

RE CE

WE

SE

Operating Conditions

765 °C

Up to 7.5% steam

15 g/m3 tars

Experimental Set-up

Test were carried out

both with a single cell

or a packed bed of

anode material

Carbon Deposition on SOFC Anode Materials

Thermodynamic Predictions Carbon Deposits after 1-hour

exposure to benzene

2.5% steam is equivalent to S:C = 1 Mermelstein et al, Energy&Fuels, 23, 5042, 2009

The addition of steam above S/C = 1 reduced carbon deposition

from tars but did not fully suppress coke formation.

20 40 60 80 100 120 140 160 180 200

-0.4

-0.5

-0.6

-0.7

-0.8

-0.9

-1.0

-1.1

-1.2

Pote

ntial (V

)

Current Density (mA/cm2)

No Tars

Benzene

Toluene

Tar Mix

Effect of Tars and Steam on Anode Performance

Mermelstein et al, Chem Eng Sci, 64, 492, 2009

0 5 10 15 20 25

0

1

2

3

4

5

6

3

0

-1

1

2

Z"

( c

m²)

Z' ( cm²)

Initial

S/C = 1

S/C = 2

S/C = 3

S/C = 2 + 50 mA/cm2

Fuel Cell performance curves after

30-min exposure to tar model

compounds (dry conditions, open

circuit).

Mermelstein et al, J Power Sources, 195, 1657, 2010

However, a combination of high S/C

ratio and current density may

inhibit carbon formation,

improving cell performance.

Effect of a Full SynGas Mixture

Mermelstein et al, J Power Sources, 196, 5027, 2011

Gas

Mixture

Benzene

Conversion

to Carbon

(%)

Benzene

Conversion

(%)

H2 0.51 45

H2 + CO2 0.09 23

H2 + CO2 + CO 0.75 26

H2 + CO2 + CO + CH4 3.9 32

TPO on Ni/CGO exposed

to benzene-containing

syngas for 1 hour.

Gas Composition:

15% H2 / 10% CO2 / 25% CO / 2% CH4 / 5% H2O

Complex interactions

between gas components

were observed.

Influence of Actual Tars on Carbon Deposition

0.00

0.02

0.04

0.06

0.08

0.80

0.90

1.00

Ni/YSZ Ni/CGO

Toluene

Real tar

Ca

rbo

n d

epo

site

d

(mg

C/m

g r

edu

ced

sa

mp

le)

The use of toluene as a model

tar leads to higher carbon

formation than actual coal

gasification tars

0 1 2 30.00

0.02

0.04

0.06

0.08

0.10

Steam content (%)

NiO/YSZ

NiO/CGO

Ca

rbo

n d

epo

site

d

(mg

C/m

g r

edu

ced

sa

mp

le)

Again, there is a clear decrease

(but not complete suppression)

of carbon formation at

increasing steam content

Lorente et al, Int. J. of Hydrogen Energy, 37 (2012), 7271

Influence of Tar Fractions on Carbon Deposition (1)

The tar sample was

fractionated by distillation.

UV-F Spectrum

Size Exclusion

Chromatogram

Gas Chromatograms

Influence of Tar Fractions on Carbon Deposition (2)

Lorente et al, Submitted to Journal of Power Sources.

A larger formation of

deposits resulted from the

lighter tar fractions.

This is consistent with the trend observed for the use of

model compounds and a tar sample.

Deposition between both tar

fractions was similar

despite a higher S/C = 1.21

was used in Fraction 1

experiments (vs. S/C = 0.97

for Fraction 2).

- Although tars can act as a fuel, fouling due to carbon deposition on

the anode reduces its performance.

- The addition of steam above the thermodynamic threshold of carbon

formation reduced carbon deposition but did not eliminate it

completely.

- Applying a load to the cell increases oxygen transport, allowing for

partial oxidation of deposited carbon on the surface of the anode.

- Interaction of tars with syngas components, mainly methane, may

lead to increasing carbon formation.

- Carbon formation is produced from lighter tar fractions to a greater

extent and benzene and toluene represent a “worst-case scenario”.

Summary