Carbon dioxide recovery from flue gases of a conventional coal-fired power plant using polymer membranes

Abstract

To reduce the emission of carbon dioxide from a conventional coal-fired power plant, carbon dioxide can be separated from the flue gases by using polymer membranes. In this chapter a theoretical investigation into the technological and economic outlook of this option is described.

At present the best types of membranes available for separating carbon dioxide from nitrogen are non-porous polymer membranes based on polyimide, polydimethylphenyleneoxide, polydimethylsiloxane and cellulose acetate. The carbon dioxide can be recovered by applying a single membrane stage configuration. The disadvantage of such a configuration is that it leads to a carbon dioxide gas that is highly diluted with nitrogen. This is an unwanted situation because the carbon dioxide is a liquid in the conditions (8000 kPa and 10°C) under which it will be transported, whereas nitrogen remains gaseous under these conditions. Moreover, the transport facilities and storage capacity for carbon dioxide will not be utilized optimally.

Three methods for purifying the carbon dioxide are considered. In the first method the carbon dioxide product gas is purified by feeding it to a second membrane unit. This configuration is called the two-stage cascade. The second method makes use of the different phases of nitrogen and carbon dioxide at high pressures. In this method the carbon dioxide product gas is compressed to 8000 kPa and cooled down to 25°C. Subsequently the nitrogen, contaminated with some carbon dioxide vapour, is separated from the condensed carbon dioxide. This nitrogen off-gas is released to the atmosphere. The third method is similar to the second one, except that the nitrogen off-gas is recycled back to the membrane unit.

With a computer program based on a cross-flow permeation model for polymer membranes, the recovery design is optimized to obtain the lowest recovery costs per tonne carbon dioxide avoided.

For the membranes examined, a lowest cost figure of 51 US$ per tonne carbon dioxide avoided is calculated. This figure is found for a polyimide-based membrane in a single membrane stage configuration combined with separation of the carbon dioxide by compression and venting the nitrogen off-gas. In this set-up 75% of the carbon dioxide is recovered and the carbon dioxide is nearly pure. For a 90% recovery either the two-stage cascade or the single membrane stage with recycling of the nitrogen off-gas are attractive routes, leading to recovery costs of about 65 US$ per tonne carbon dioxide avoided. Recovery with polymer membranes is calculated to be 50 to 100% more expensive than recovery using cold distillation or a chemical absorption technique. Although some cost reductions are feasible, membrane separation is not expected to become a competing option for carbon dioxide recovery from flue gases of conventional coal-fired power plants in the near future.