Energy and economic analysis of a water scrubbing based biogas upgrading process for biomethane injection into the gas grid or use as transportation fuel

Abstract

This works describes the biogas upgrading process to substituted natural gas (SNG), specifically called biomethane, for grid and vehicle application. The physical processes of carbon dioxide absorption and desorption respectively in a pressurized water scrubber and a stripper are described by means of the Henry’s law. Both the water scrubber and CO2 stripper columns are designed by means of the Transfer Units Method in order to handle the biogas stream that is available from an anaerobic digestion of manure.In particular, a case study of a small scale plant located in the centre of Italy is analysed with the aim to assess the energy conversion efficiency and economics of the upgrading process for grid injection and vehicle application. It is found that the whole process efficiency is respectively 89.8% in the case of grid injection and 87.3% in the case of vehicle application.Concerning with the economic analysis of the plant, the capital and operating costs of the plant are determined and the Italian legislative framework is included to account for subsidies. Results show that the unit production cost of biomethane for grid injection is 0.54 €/Sm3 while it is 0.73 €/Sm3 in the case of vehicle application.