Experimental validation of hydrogen sulphide removal from biogas using biotrickling process at pilot plant level

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-powered SOFC pilot plant was constructed in a Waste Water Treatment Plant (WWTP) (January 2009) in Spain and was operated until the end of the project (June 2012). The pilot plant treats 10 Nm3/h biogas and produces both electricity and heat in a high efficient way. The BIOCELL project (funded by the LIFE+ program of the European Commission, LIFE07 ENV/E/000847) is one of the few demonstration projects that studies biogas applications in SOFC. The following project tried to review the state of the art of removal H2S biological technologies and summarized the experience gathered from the biotrickling filter of the BIOCELL project. The biogas treatment was designed to reach the inlet requirements of the fuel cell. 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 BIOCELL project. In particular, a biotrickling filter was installed as it is one of the most interesting technologies for the main removal because of its low operating costs compared to other physical – chemical treatments. On the other hand, adsorption-based systems (iron oxides + biogas drying + activated carbon) were chosen for biogas polishing as they are the only technology which can reduce the concentration of contaminants to the stringent extent of fuel cell’s specifications. The biotrickling filter was operated for more than 12 months. The operating conditions were optimized to temperature 30ºC, pH 1.5 and residence time > 100 seconds. Over the long term and under these conditions, it showed a removal efficiency of 70% and an availability of 80% because of progressive filter clogging due to elemental sulphur accumulation. Operational expenses were calculated at 0,5 – 1 c€/Nm3. This report includes operational guidelines and recommendations for re-design. To sum-up, this report shows the conclusions of the biogas treatment technologies from BIOCELL project, which shows that are ready to meet fuel cell inlet requirements for long periods of time at a reduced cleaning cost.