Removal of H2S and CO2 from Biogas by Amine Absorption

Abstract

Due to strategic and environmental reasons, currently, there is an increasing interest in biofuels as alternative energy source. Bio-alcohols and biodiesel are the alternatives been considered for auto-motion while biomass and biogas are the alternatives been considered for electrical power generation. Biogas is a medium-energy content fuel (~22 MJ/kg) derived from the organic material decomposition under anaerobic conditions (Horikawa et al, 2004). It can be obtained from landfills or from bio-digesters that transform manure and biomass into natural fertilizer in farms after 25-45 days of residence time. Due to its gaseous nature and the impossibility of producing it intensively, it is not attractive for large scale power generation. However, recently, a new approach for electric power generation has been emerging. It consists of inter-connecting thousands of small and medium scale electrical plants powered by renewable energy sources to the national or regional electrical grids. It is considered to interconnect the hundreds of the existing small aero generators and solar panels (Pointon & Langan, 2002). Even though, there are still several technical issues to be resolved, this alternative of distributed electrical power generation is being considered as the best alternative to bring electricity to the rural communities located far away from the large urban centers. In this case, the use of the biogas generated in the thousands of existing farms and landfills, as fuel for internal combustion engines connected to an electric generator becomes a very attractive alternative for electric power generation because of its very low cost, high benefitcost ratio and very high positive impact on the environment. Biogas is made up mainly of methane (CH4) and carbon dioxide (CO2). It also contains traces of hydrogen sulfide (H2S). Its composition varies depending on the type of biomass. Table 1 shows its typical composition. The biogas calorific power is proportional to the CH4 concentration. To be used as fuel for internal combustion engines, it has been recommended a CH4 concentration greater than 90% (Harasimowicz et al, 2007). However CO2 has a typical concentration of ~ 40%. This high CO2 concentration reduces the engine power output proportionally to its concentration, limiting the use of biogas in electrical power plants driven by internal combustion engines (Marchaim, 1992). The high content of H2S (~3500 ppm) causes corrosion in the metallic parts at the interior of the engine. The H2S is an inorganic acid that attacks the surface of metals when they are