to produce electric power and use of the CO2 separated from the hydrogen to increase oil production.

The committee elaborates below on CO2 separation, oxygen production, co-capture of CO2 and other pollutants, and CO2 compression.

CO2 Separation

The first step in CCS is CO2 capture from a gas mixture. CO2 is currently separated at commercial scale where H2 is generated from natural gas for petroleum-refining processes or for the manufacture of ammonia. However, the capture of CO2 from large power plants has not yet been demonstrated. Commercial precombustion separation is typically based on physical adsorption in solvents (Selexol, Rectisol), while post-combustion separation involves chemical absorption (amines). Adsorption on solids such as activated carbon has also been demonstrated, but typically at small scale. Cryogenic and membrane separations are development frontiers. Selectivity and throughput rate are critical, though CO2 purity can be much lower than in relatively demanding applications such as food production.

Typical separations have significant energy requirements, which are reflected in their costs. In chemisorption in an amine, for example, the gas mixture contacts the liquid-amine solution and CO2 transfers to the liquid and reacts with the amine molecules, which releases significant amounts of heat. Nitrogen and other components remain in the gas phase, which is separated physically from the liquid. The amine solution containing CO2 is then heated to release the CO2. The combination of heat-transfer requirements and energy required to remove the CO2 from the amine is a significant component of the cost of the separation.

In some settings (ammonia manufacturing, hydrogen production, and natural gas processing, for example), the CO2 separation must be performed in order to make the product; thus the incremental cost of separation is zero. In others (electric power generation or cement, iron, or steel production), incremental separation costs are significant, and the requirements for energy to accomplish the separation lead to significant reductions in the overall thermal efficiency of the power plant.

Oxygen Production

Two of the three strategies described above (gasification and oxyfuels) involve an oxygen input, and the cost of oxygen is a significant component of the total cost. The oxygen demand with oxyfuels is about three times higher than with gasifica-



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement