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Fuel Cells Fuel cells convert chemical energy to electricity without combustion. This conversion is similar to that of a battery. There are three sections to a fuel cell: • Fuel pre-treatment and processing section – Here, the gas is purged of H2S and other harmful contaminants. The processing section includes a fuel reformer that converts methane to a hydrogen-rich gas used by the fuel cell stack. • Fuel cell stack - The electricity-producing chemical reaction takes place here. • DC to AC power electrical conditioning and controller section - This section converts the DC electricity produced by the fuel cell to grid-standard AC electricity. There are several different types of fuel cells. Each is named for the type of electrolyte used. • Alkaline Fuel Cell – These are the oldest fuel cell types. They were originally used in space shuttles. These fuel cells are easily contaminated by carbon, therefore their application on earth is limited (Anon, 2005e). • Phosphoric Acid Fuel Cell – This type was the first commercially available fuel cell, in 1992. They operate at a temperature between 150 and 220°C (Anon, 2005a). They are reliable sources of energy, with an electrical efficiency of between 36 and 40%. These efficiencies, however, lag behind other fuel cells. Furthermore, the price has not decreased as expected and at least one fuel cell manufacturer is phasing out of this type of fuel cell in favour of the proton exchange membrane (NSTAR, 2005). • Proton Exchange Membrane (PEM) Fuel Cell – These are the leading candidates for use in an automobile as they are capable of starting quickly and match a changing load. This ability to start quickly stems from a relatively low operating temperature of 90°C (Anon, 2005a). Most stationary PEM fuel cells will have a capacity under 10 kW (NSTAR, 2005). A PEM fuel cell has become the first fuel cell to generate electricity from agricultural biogas. The 5 kW unit is being run intermittently by researchers from the University of Minnesota on a dairy farm (Anon, 2005c). • Solid Oxide Fuel Cell (SOFC) and Molten Carbonate Fuel Cell (MCFC) – Both of these fuel cells operate at very high temperatures. MCFC operate between 600 and 700°C, while SOFC function between 650 and1000°C. This high operating temperature translates into high quality exhaust heat, but also a slower start-up time. MCFC range in size from 250 kW to 3 MW units. SOFC are being developed in the 5 kW to multi-megawatt range. The “Fuel to Electricity” efficiencies of both MCFC and SOFC approach 60% (Anon, 2005a). Biogas intended for use in a fuel cell must first be purified. The catalysts of fuel cells cannot tolerate hydrogen sulfide (Minott et al., 2004). Levels higher than 0.5 to 1.0 ppm are unacceptable for a high te mperature fuel cell generator. Water vapour and carbon dioxide are considered fuel diluents (Anon, 2005a). The purified biogas would not need to be further compressed. Methane can be used in the range of 1 to 3.5 kPa (0.14 to 0.5 psi) for a phosphoric acid fuel cell (Anon, 2005e). 15PDF Image | Electricity and Heat Production Using Biogas from the Anaerobic Digestion of Livestock Manure - Literature Review
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