GREENHOUSE GAS EMISSION MITIGATION POTENTIAL OF DIFFERENT TECHNOLOGIES USED FOR CONVERTING BIOGAS INTO EFFECTIVE ENERGY IN GERMANY

Abstract

The upgrading of biogas to achieve natural gas quality for direct feeding into the supply grid has been frequently discussed as art alternative to its utilization for local energy production in block heat power plants (BHPP) using cogeneration. In this study, we compared the local use of upgraded biogas for producing electricity and heat with its energetic conversion at a distance away from the biogas plant under the conditions present in Germany. A total of seven systems were compared theoretically with the direct feeding of upgraded biogas into the grid. These systems involved either a BHPP or one of three types of fuel cell (molten carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC, proton electrolyte membrane fuel cell (PEMFC)] or a hybrid system consisting of a SOFC and a gas turbine. The degree of efficiency in direct gas feeding was essentially better (71.3% secondary energy out of primary energy) than the decentralized forms of energetic conversion [45.7 (BHPP) -57.8% (hybrid system)]. The reason for this is that in Germany, natural gas, which is substituted through direct gas feeding, is primarily used for heating purposes. In comparison, however, the greenhouse gas emissions (GHG) reduction potential of local cogeneration (254-360 g CO(2)/kWhb(iogas)) was much higher than that of direct gas feeding (187- to 228-g CO(2)/kWh(biogas)). This was found to be trite even when no useful heat could be used on the site of the biogas plant. The best results were achieved with the hybrid system. By considering the effects of a reduction in the CO(2) production of the German power supply mix oil the potential emissions reduction of all eight systems investigated, it could also be shown that the hybrid system again had the greatest GHG emissions reducing potential. These results indicate that the direct feeding of upgraded biogas is not the best option for the utilization of this commodity with regards to climate protection in the German situation. Indeed, the results indicate that the application of innovative and efficient technologies, such. as the hybrid system consisting of SOFC and a gas turbine, call generate essential improvements in emissions reductions in biogas plants ill comparison to the presently used BHPPs. For this reason, the use of fuel cells in German biogas plants should be promoted.