Landfill gas processing for improving methane content.

Landfill gas is generated by anaerobic degradation of biodegradable material present in the disposed waste. Generally its composition is adequate for combustion with energy recovery in reciprocating engines, which require the methane content be higher than 39-40% by volume. If this condition is not satisfied the landfill gas is flared. If the methane gas content is lower than the required value, it is possible to pre-process it by removing the inert carbon dioxide present in it and increasing the methane content. An innovative process, based on the carbon dioxide capture by means of accelerated carbonation of alkaline residues – namely bottom ash – was proposed and studied for the above purpose. The process was investigated at a laboratory scale by setting up a process for realizing the direct contact between the bottom ash and the simulated landfill gas in a fixed bed reactor. Different landfill gas compositions were simulated, considering decreasing methane contents (30-25-20 % by volume) and evaluating the possibility of obtaining the required minimum methane content in the exiting gas. Results were obtained about the ability of bottom ash to capture the carbon dioxide and about the time and the processed gas volume after which the bottom ashes are no more able to perform the desired capture; the process is able to increase the concentration of CH4 from 20% in volume to values higher than 40% in volume for about 6 hours, processing about 27 Nl of entering gas per kg of bottom ash. When the bottom ashes exhaust their capture ability they need to be replaced with a new amount. An average of about 36 kg of bottom ash per Nm3 of low methane content landfill gas are required to obtain the minimum methane content at the exit of the process. On the basis of this type of results, a preliminary evaluation of the bottom ash annual requirement for different plant sizes was carried out followed by the economic evaluation of realizing this type of landfill gas processing on industrial scale. For and industrial scale plant, processing 100-1000 Nm3 of poor methane content landfill gas and assuming the same specific bottom ash requirement obtained from the experimental tests, about 30000-300000 t of bottom ash are required for one year operation and the overall cost of the studied enrichment process, is in the range of 0.27-0.08 €/Nm3 of raw landfill gas.