Gas molecules can chemically bond to the surface of some materials. The process is called associative if the molecule bonds in whole to the surface and dissociative if the gas molecule breaks up in order to form a bond. Chemisorbents are often composed of an active surface layer supported by an inert substrate. Proposed systems use small particles as substrates in order to provide large surface area. Regeneration drives the chemical reaction in reverse, often at elevated temperature.

Metal Oxide Air Separation

Air separation allows a pure stream of oxygen to react with the fuel, creating an effluent of only CO2 and water or other useful products such as hydrogen. It is often easier with current technology to separate CO2 from water or hydrogen than from nitrogen. Reactive metal exposed to air will oxidize rapidly. The metal oxidation reaction is highly exothermic. The oxides can then be endothermically regenerated by exposing them to a high temperature reducing environment. The oxygen combines with the fossil fuel to form carbon oxides and varying amounts of hydrogen-containing species depending on the type of fuel. The chemistry and geometry of this separation has allowed recent small-scale studies to obtain a nearly 100% pure stream of oxygen to react with the fuel. Phase separation of water from the resulting effluent could produce a pure stream of CO2. This complete process is commonly called chemical looping separation. Research in metal oxide air separation is focused on cost and the physical and chemical stability of the oxygen carriers over many cycles. The particles usually consist of a reactive oxide and a supporting inert oxide. While various oxygen carrier particles are under consideration, copper, iron, manganese, and nickel are the most promising reactive metals.

No large-scale demonstration has been performed, but models predict that a power system utilizing metal oxide air separation has significant advantages. The lower irreversibilites associated with the regeneration step relative to conventional combustion add to the already low energy requirement of the inherent separation of CO2 from nitrogen. Exergy analyses show the resulting overall energy penalty could be as low as 400 kJ/kg CO2 for a natural gas combined cycle plant, assuming idealized chemical stability of the oxygen carrier.

Dry Chemical Absorbents

Under some conditions, CO2 can undergo a reversible chemical reaction with a dry absorbent material. The chemical reaction can be reversed by changing the conditions, resulting in the release of pure CO2. Sodium carbonate supported on an inert particle has been proposed as such an absorbent. An exothermic reaction of sodium carbonate with CO2 and water held at 60 to 70ºC forms primarily sodium bicarbonate and Wegscheiderite. The products must be heated to 120 to 200ºC to reverse the reaction. Lithium zirconate is also being investigated for its high capacity chemisorbtion separation of CO2 at high temperatures.