Economics of CO₂ and Mixed Gas Geosequestration of Flue Gas Using Gas Separation Membranes

Greenhouse gas emission sources generally produce mixed gases. Previous studies of CO 2 capture and storage have typically examined only sequestration of pure CO 2 . This paper analyzes the cost of separating a gas mixture from a power station flue gas stream and injecting it into an offshore subsurface reservoir. The costs of separating and storing various gas mixtures were analyzed at two extremes. One extreme in which the entire flue gas stream containing both CO 2 and N 2 is stored. The other extreme in which as much CO 2 is separated as is technically possible using gas membrane capture coupled with chemical absorption. The results indicate that for the gases investigated, using a gas membrane capture system, the lowest sequestration cost per tonne of CO 2 avoided occurs when a mixed gas with a CO 2 content of about 60% is sequestered. Lower costs and higher tonnages of CO 2 avoided can be achieved using an amine based absorption capture system. At the lowest cost point, and for most of the range of cases studied, the cost of capture is significantly greater than the cost of storage. However, this depends on the source of the CO 2, the distance between the source and the injection site, and the reservoir into which CO 2 is injected.