Experimental study of H2S removal from biogas using impregnated activated carbon for energy recovery in CHP technologies

Abstract

Within the framework of sustainable development, and ever-increasing energy costs, wastewater treatment operators focus on developing on-site energy production: the main approach today is via sewage biogas. In most cases, H2S removal is necessary to meet the requirements of the energy conversion equipment inlet requirements (for cogeneration motors around 300 – 500 ppm, but for fuel cells around 1ppm or below). Therefore to optimise the overall energy and economic balance, it is necessary to dispose of efficient and cost-effective biogas treatments, amongst which biological treatments have promising prospects for H2S removal. A biogas cleaning pilot plant was constructed in a wastewater treatment plant (WWTP) (January 2009) in Spain and has been operated for several projects. The pilot plant treats 10 Nm3/h of biogas and is able to produce both electricity and heat in a high efficient way. The MCFC-CONTEX project (collaborative project under auspices of the European Commission and supported by the Fuel Cell and Hydrogen Joint Undertaking, FCH-JU) is one of the few demonstration projects that studies biogas applications in Fuel Cells. The following project tried to review the state of the art of removal H2S technologies and summarized the experience gathered from the polishing stage of the MCFC-CONTEX project. The biogas treatment was designed to reach the inlet requirements of fuel cells. H2S was identified as the main biogas pollutant and it causes severe and rapid degradation of fuel cell stacks. Examples of industrial biogas treatment units coupled to fuel cells consisting of a main H2S removal stage followed by a final H2S polishing system proved to be successful, thus this was the strategy implemented in the MCFC-CONTEX project. A biotrickling filter was installed, followed by adsorption-based systems, which were chosen for biogas polishing, as they are the only technology that can reduce the concentration of contaminants to the stringent extent of fuel cell’s specifications. Specially, an impregnated activated carbon as the polishing source was tested for H2S removal. The pilot plant was operated for 3 months. The biogas conditions were optimized to 2.5% of O2 content, a relative humidity of 90% and a residence time > 20 seconds. Under these conditions, the impregnated activated carbon showed a removal efficiency > 99.9% and an adsorption capacity of > 80% when the adsorbent started to saturate. Operational expenses were calculated at 0,5 – 1 c€/Nm3. To sum-up, this report shows the state-of-the-art of biogas treatment technologies and focuses on the polishing stage experimentation. Results show that H2S removal technologies are ready to decrease the pollutant concentration from biogas until the inlet requirements for Combined Heat and Power (CHP) Technologies.