GEORGIAN ENGINEERING NEWS, №4, 2004 57 FUEL CELL POWER FROM BIOGAS Aladashvili T.Z., Berezhiani M.G., Chkhaidze B.Sh. Dudauri T.V., Orjonikidze M.O., Partskhaladze G. Sh. and Ugrekhelidze V.D. National High Technology Center of Georgia Abstract. Technological achievements and commercial status of different types of fuel cells are reviewed. The use of biogas as a renewable fuel for fuel cells is discussed in general. Two different approaches such as the direct use of biogas in molten carbonate fuel cells and the use of biogas-derived hydrogen in all types of fuel cells have been analyzed on the basis of original thermodynamic computations. The results show that the maximum energy efficiency could be reached when refined biogas was directly used in high-temperature molten carbonate fuel cells. Keywords: biogas, anaerobic digestion, fuel cell, molten carbonates. Introduction Fuel cells have the potential for electricity generation with high efficiency and for reduction of the environmental impact in terms of overall greenhouse gas emissions. Unlike conventional power generators, the chemical energy of the fuel in fuel cells is converted to electrical and thermal energy directly and, therefore, can achieve higher electric efficiency than the conventional combustion process. The electric power generation efficiency in low temperature fuel cells was assessed as about 35-40%, and as 47% for high temperature ones. As an additional benefit, fuel cells contribute to reduction of greenhouse gas emission. However, as long as they are powered by fossil fuels such as natural gas, emissions cannot be entirely avoided. Renewable energy resources have recently received considerable attention as a potential partial substitute for fossil fuels. Among different types of renewable energy resources, biomass derived fuels such as biogas, bio-methanol or bio-ethanol are considered to be very promising alternatives for feeding the fuel cells. Biogas for Fuel Cells - General Overview Biogas is produced through anaerobic digestion of organic wastes. It mainly consists of methane (60-70%) and carbon dioxide (30-40%) with trace impurities of hydrogen sulphide and ammonia. Because of their similar composition, the biogas can be used almost like natural gas, providing a lower calorific value (due to the presence of carbon dioxide) than methane. This is the basis for considering the use of biogas in fuel cells [1]. Fuel cells pose different requirements for the fuel gas composition. These major differences are the consequences of the employed ion-conducting electrolyte, the electrocatalyst used, and the operating temperature. High temperature fuel cells, based on internal reforming of the fuel, are generally much more tolerant towards contaminants than low-temperature ones. High temperature fuel cells can use renewable fuels after some purification, while the low-temperature fuel cells require reforming of biofuels into hydrogen-rich gas and further cleaning-up. The use of biofuels in low-temperature fuel cells requires more technical steps and hence higher investment and operating costs as compared to high temperature fuel cells. Besides, high temperature fuel cells can generate more energy due to higher efficiency by 7-8% [1-2]. Among fuel cell families, high temperature fuel cells such as the Molten Carbonate Fuel Cell (MCFC) and the Solid Oxide Fuel Cell (SOFC) are potentially more efficient for using biofuels as they can use methane gas in an impure form produced by biomass fermentation. Besides, the heat itself can be exploited to convert the methane into hydrogen-rich gas for the fuel stack. The capability to exploit biomass makes such high temperature fuel cells inherently more efficient than lower temperature alternatives depending on some external processing of their fuel [1, 2]. The electrolyte in MCFC-s is a mixture of alkali carbonates, typically Li 2 CO 3 and K 2 CO 3 - sometimes with additions of alkaline earth carbonates, above their melting point at operating temperatures of about 650°C. Therefore, the charge carrier ion is no longer a proton but a carbonate ion, CO 3 -2 , moving from the cathode to the anode. It is a peculiarity of the MCFC that the depletion of carbonate ions from the cathode makes it necessary to recycle CO 2 from the anode to the cathode or, less commonly, to supply