Challenges of Biogas Upgrading to Fuel Cell Quality

摘要

The utilisation of biogases, such as fermentation, sewage or landfill gas for power and heat generation requires mandatory tailor-made purification processes for the removal of all harmful pollutants. Biogenous gases, especially landfill gas (LFG), contain a wide range of minor compounds - e.g. sulphur-containing compounds, silicones, aromatics or chlorocarbons - in concentrations high enough to considerably damage or even rapidly destroy any downstream power generation device. Biogas utilisation has been mainly limited to power generation leaving most of the heat produced practically unused. Therefore, the application of more efficient systems like fuel cells has gained in recent years lots of impulse in both scientific and technical fields bringing these devices closer to commercialisation. Among all fuel cell types, high- temperature fuel cells (e.g. MCFC or SOFC) seem to be best suited for biogas utilisation due to their high power efficiency, feeding gas versatility, high operating temperatures and their lower purity demands. This contribution gives an overview of state-of-the-art biogas upgrading technologies and will figure out the challenges of biogas purification and the needs of further improvements, respectively. Finally, the author tries to draw future prospects and briefly introduce novel approaches regarding fuel cell purity demands and cost efficiency. Adsorption devices are the most applied gas cleaning technology due to their simple process design. Due to competing adsorption phenomena, this technology poorly fulfils gas quality demands and cost efficiency requests. Gas cooling or gas scrubbing technologies in conjunction with adsorption devices do not meet the cost efficiency nor the operational safety targets. These purification systems normally target single compound classes e.g. sulphur-containing compounds (H2S, mercaptans), halogenated and no- halogenated volatile organic compounds (VOCs) and silicon-organic compounds (siloxanes). Fraunhofer UMSICHT researches on cost-effective catalytic gas cleaning devices able to eliminate all biogas impurities by transforming them into a simple compound class "acid gases" without compromising the methane contained in biogases. These acid gases (HCl, HF, SO2) can be easily removed by adsorption on alkalised materials, resulting in a high- purity gas. The search for materials for the selective removal of siloxanes from biogases revealed an activated alumina for hot gas cleaning devices and a modified silica for retrofit applications (adsorption devices) as suitable materials.