Produzione e distribuzione di elettricità e calore
Autors: Marco Cavana, Pierluigi Leone, Elena De Luca
Production and Distribution of Electricity and Heat
Biomethane is the fuel gas produced from biomass whose properties are comparable to natural gas ones. 90% of current biomethane production is through the upgrading of biogas produced by anaerobic digestion. The remaining 10% comes from the methanation of syngas produced by thermal gasification of solid biomass. This report will focus on the biogas upgrading to biomethane, which mainly consists in the removal of CO2 form biogas. In fact, the biogas from anaerobic digestion contains between 30 ÷ 60 % of CO2 together with smaller amounts (traces) of contaminants such as hydrogen sulfide and ammonia that have to be cleaned away. The composition of the biogas mainly depends on the initial composition and type of the organic matter (feedstock). In order to be allowed in the natural gas value chain (either for grid injection or for transportation or both), the biomethane must have quality parameters similar to the ones of fossil natural gas usually transported by the network. The biomethane final product is a mixture mainly composed by CO2 (1-3%) and CH4 (97-99%). Most of the new installations of biomethane are based on biomass feedstocks coming from wastes: either agricultural residues (also manure) or even more often organic fraction of municipal solid wastes. Wastes form food and beverage industries are also a valuable stream of feedstock. There are 6 main upgrading technology that will be described in details in this report: Water Scrubbing, physical scrubbing, chemical scrubbing, pressure swing adsorption, membrane separation and cryogenic separation. The main energy input requirement for all these alternatives is electrical energy, to power mainly pumps and compressors. Water scrubbing and physical scrubbing are very similar. In case of physical scrubbing, dedicated solvents are used in order to enhance the dissolution (and separation of CO2). Chemical scrubbing is usually also known as ammines scrubbing from the most common chemical it is employed. Differently from the other scrubbings, in this case there is the need for thermal energy in order for the regeneration of the ammines to take place. The two separation techniques are based on the different permeability of polymers by CO2 and CH4 (membranes) or the different thermophysical properties (cryogenic). As for the European framework, the most common upgrading technology is membrane separation (covering the 39% of the installed plant). It is followed by water scrubbing and chemical scrubbing with a share of 22% and 18% respectively. The rest of the market is covered by pressure swing adsorption (12%), physical scrubbing (1%) and cryogenic separation (1%). As for the sizes of the biomethane plants, at European level the range spans between less than 50 Nm3/h to more than 1,000 Nm3/h, with more than 50% of the plants to be concentrated in the range of 50-250 Nm3/h and 250-500 Nm3/h.