From Biogas to Biomethane: How the Biogas Source Influences the Purification Costs

Abstract

Often left in the background by media, biogas is a renewable energy source with a great potential for development. Its production is based on the anaerobic digestion of organic waste materials, which are usually livestock effluents, municipal and industrial sewage sludge, energy crops, agro-industrial wastes, MSW landfills, etc. To date, biogas is mainly used for the production of electrical energy, thanks to a more favorable regulatory and incentive framework. Installations may adopt a cogeneration arrangement, that allows to recover the heat produced by gas-fired engines for home or industrial reuse. In the near future, however, the biogas properly upgraded to biomethane will be used for transportation or fed into the natural gas grid directly reaching end consumers, both solutions adding value to biogas. The utilization of biogas is in line with the “20- 20-20” targets to be met by the Member States of the European Union by 2020, which provide that 20 % of EU energy consumption will have to come from renewable resources and that the share of energy from renewable sources in the transport sector in 2020 will be at least 10 %. Biomethane is obtained from properly treated biogas. Biogas is a mixture of mainly methane and CO₂ with other contaminants (such as H₂S, nitrogen and oxygen), whose type and amount depend upon the biogas source and determine which cleaning technique (and, to some extent, also upgrading technique) is the most suitable for gas purification. ”Cleaning” is referred to the pretreatment that allows the removal of all pollutants but carbon dioxide, while “upgrading” consists of CO₂ removal. There are no clear guidelines for choosing among the different techniques, mainly in relation with the biomass source which affects the biogas composition. This work performs a techno-economic analysis of the biogas-to-biomethane process using three different biogases produced from sewage sludge, agro-industrial wastes and landfills, using water scrubbing and amine washing for the upgrading process. By means of process simulation with commercial software (such as Aspen Plus^®), a comparison among performances, energy requirements and solvent type is carried out. In the end, an economic assessment of the two upgrading techniques for the three considered biogas sources is performed, taking into account the flow rate of the biogas to be treated.