Hot Gas Conditioning: Recent Progress with Larger-Scale Biomass Gasification Systems; Update and Summary of Recent Progress Page: 74 of 103
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Table 4.9. General fuel cell characteristics (EG&G, et al, 2000; Amos, 1998; Brown, et
al, 1998)
Fuel Cell Type Typical Allowable Concentrations of Gas-Phase Constituents:
Operation
("C) CO CO2 Sulfur Hydrocarbons
PEM 80 <5 ppm < 50 ppm <1 ppm Restrictivea
Alkalineb 120 < 50 ppm < 50 ppm <1 ppm < 300 ppm
Phosphoric Acid 200 <1% Diluentd <1 ppm Diluentd
Molten 650 Unrestrictede Unrestrictede <0.5 ppm <10%f
Carbonate
Solid Oxide 1000 Unrestrictede Unrestrictede <1 ppm <10%f
aVariable depending on construction, usually <100 ppm
bDesigned primarily for use with pure hydrogen and oxygen
cTotal CO + C02 concentrations <50 ppm required. Cell particularly sensitive to CO2
dServes primarily as diluent; low concentrations preferred
eUnrestricted within the range of values typical for gasification
fHigher concentrations in the fuel stack impact economics due to dilution
In all cases where biomass gasifiers are coupled with fuel cells, effective removal of
particulates and tars from the gas stream will be required. For fuel cells operating at low-
temperature, tars would present condensation problems similar to those in other types of
systems. In the higher temperature systems, tar decomposition could lead to carbon
deposition on catalyst surfaces. Data are presently not available to determine whether
thermal tar cracking at about 1000 C in solid oxide types would be sufficient to allow
continuous operation. Unreacted tars also represent a loss of fuel content and result in
lower overall electric conversion efficiencies.
The high sensitivities of the alkaline and PEM types of fuel cells to CO and CO2 make them
unlikely candidates for use with biomass thermal gasification systems. Phosphoric acid fuel
cells are commercially available and have somewhat higher tolerances for CO. However,
their use with typical biomass products would still require extensive shift reactions and CO
removal. Others, particularly the molten carbonate and solid oxide types have little
sensitivity to these components and are potentially suitable for use with biomass gasifiers.
Molten carbonate cells, for instance, have been operated in short duration tests of coal-
derived gases (EG&G, et al, 2000). Both of these types also permit the presence of air and
nitrogen in the fuel gas, which may provide the opportunity for air-blown gasification
technologies.
The tolerance of fuel cells to low molecular weight hydrocarbons also varies. In most cases,
reforming will reduce the concentrations of hydrocarbons. In the low-temperature systems,
unconverted light hydrocarbons primarily dilute the product gas. In high temperature cells,
unconverted hydrocarbons may compete with hydrogen for active sites on catalyst surfaces
and reduce reaction rates. However, the catalytic and thermal reactions occurring in the
high temperature systems also result in reforming of part of the product. As a result, the
higher temperature systems are likely to tolerate unreacted hydrocarbons that reach the fuel
cell stack, unless those hydrocarbon concentrations become excessive. The ability of the
high temperature systems to crack tars and similar materials is not well documented.
While more detailed research is needed, the basic gas conditioning requirements for
biomass gases delivered to the fuel cell stack will be similar or only slightly less complex
than those for synthesis gases. The final gas compositions will clearly be different for the
two applications and the individual gas conditioning step processes may also be somewhat61
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Stevens, D. J. Hot Gas Conditioning: Recent Progress with Larger-Scale Biomass Gasification Systems; Update and Summary of Recent Progress, report, September 1, 2001; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc715374/m1/74/: accessed April 27, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.