Syngas Fuel Cell
Development of analytical techniques and scrubbing options for contaminants in gasifier streams intended for use in fuel cells
From the article:
One goal of the Chariton Valley Biomass Power Project is the development of an integrated switchgrass gasifier/fuel cell (IGFC) power plant. Phase I of this activity involved testing the gasification of switchgrass in a 4.5 tonne/day (5 ton/day) fluidized bed reactor on the campus of Iowa State University (ISU). Simulation of the IGFC was performed by FuelCell Energy (formerly Energy Research Corporation) based on the results of the gasification tests at ISU. Further development of an IGFC power plant and eventual construction of such a plant will require additional equipment to couple the two technologies.
Gas generated during gasification of switchgrass contains contaminants detrimental to the operation and/or useful life of the fuel cell. Successful integration of gasifiers with fuel cells mandates the conditioning of the producer gas to levels stipulated by fuel cell manufacturers. This report brings together the different aspects associated with IGFC power plant.
Although no testing of an IGFC has taken place, manufacturers and fuel cell experts have estimated the impacts of certain contaminants on the performance and expected life cycle of a fuel cell. Carbon monoxide is the primary contaminant for use in a proton exchange fuel cell. Phosphoric acid fuel cells are sensitive to sulfur, chloride, ammonia, and carbon monoxide. Molten carbonate and solid oxide fuel cells suffer adverse affects from sulfur and chloride.
Detection and quantification of the contaminants in the producer gas requires carefully designed sampling and analytical techniques. Every gas species of interest has unique characteristics for sampling, transport, and detection. Although methods have been identified, much more testing needs to be done to confirm the accuracy of the various methods. Presently it appears direct gas stream analysis with Drager tubes is the method of choice for determination of hydrogen sulfide. Analyses of ammonia by fluorometric or ion electrode methods appear to be the most promising. Ion mobility spectroscopy and Drager tubes offer two potential methods of analyses for hydrogen chloride. Hydrogen cyanide is a difficult species to measure and at this time no methodology has been chosen.
Other contaminants include particulate and tar (condensable hydrocarbons). Hot gas cleanup technology removes contaminants while preserving the sensible energy in the producer gas. However, hot gas cleanup technology is not commercially available for biomass applications. Ceramic filters are susceptible to breakage due to thermal and mechanical shock and exhibit intolerance to certain minerals. Metal filters are an alternate hot gas cleanup technology but have not yet been fully developed. Tarry substances in the gas stream may plug both ceramic and metal filters if not operated at sufficient temperatures. Alternatively, soot may form if the filters are operated at too high of temperatures. Moving bed filters are able to capture particles on granular material at elevated temperatures. Its capability for removing trace contaminants, however, is essentially unexplored.
The use of a catalytic reactor downstream of the gasification reactor has proved an effective approach to elimination of tar. A variety of catalysts have shown significant ability to destroy tar in gasifier streams. These catalysts include dolomite, nickel and alumina based catalysts, and various proprietary catalysts.
Testing of a moving bed granular filter to remove dust from hot producer gas has demonstrated promising results. A filter of 0.914 m (36 in.) diameter was able to remove 97 - 98% of the 60 g/m3 dust loading from a 345 nm3 /h (200 ft3 /min) gas flow. Pressure drop never exceeded 80 mm water (3.0 in) water in these tests. Most of the dust penetrating the filter was 2 – 3 µm in size. Virtually none of the exiting dust was from the silica pebbles used as granular media in the filter, as confirmed by elemental analysis of dust entering and exiting the filter. However, we do not appear to have operated under steady state conditions of the filter; additional testing is required.
A tar cracking system consisting of a guard bed and catalytic reactor was designed for the purpose of improving the quality of producer gas from an air-blown, fluidized bed biomass gasifier. All three metal catalysts (ICI 46-1, Z409, and RZ409) proved effective in eliminating heavy tars (>99% destruction efficiency) and increasing hydrogen concentration by 6-11 vol-% (dry basis). Space velocity had little effect on gas composition while increasing temperature boosted hydrogen yield and reduced light hydrocarbons (CH4 and C2H4), thus suggesting tar destruction is controlled by chemical kinetics.
More research and development is needed before integration of a gasifier to a fuel cell can be attempted. Although the moving bed granular filter and catalytic reactors show great promise for gasifier stream conditioning, the gas quality is still not sufficient for direct use in a fuel cell. Testing and development of the moving bed granular filter and the catalytic reactors will continue. Gas analytical techniques will be tested and implemented for further characterization of the producer gas.
