A membrane based process for the upgrading of biogas to substituted natural gas (SNG) and recovery of carbondioxide for industrial use

A low pressure carbon molecular sieve (CMS) membrane based process to upgrade biogas from anaerobic digestion of agricultural waste to a substitute natural gas (SNG) has been tested on a pilot scale. The data extracted from the pilot plant was used to estimate membrane permeance and ideal selectivity of the CO2/CH4 gas pair. Four semi-commercial modules were tested. The results show that by using a membrane cascade and by carefully choosing the CMS modules, it is possible to produce a stream containing at least 90 vol-% CH4 and a by-product containing at least 60 vol-% CO2. The substituted natural gas can be mixed with NG in the national grid and the latter by-product is intended for the production of liquified CO2, suitable for use in greenhouses. At a pressure level of 8-16 barg, this process could offer simplicity and less investment and maintenance than other technologies.