Capture: The first step in the sequestration of carbon dioxide is the separation and capture of CO₂ from power plant flue gas and other stationary sources. CO₂ capture systems are divided into these three categories:

• A post-combustion system separates CO₂ from the flue gases produced by the combustion of the primary fuel in air. These systems normally use a liquid solvent to capture the small fraction of CO₂ present (typically 3-15 percent by volume) in a flue gas stream in which the main constituent is nitrogen (from air). Similar separation processes are already used on a large scale to remove CO₂ from natural gas.
• A pre-combustion system processes the primary fuel in a reactor with steam and air or oxygen to produce a mixture consisting mainly of carbon monoxide and hydrogen (“synthesis gas”). This conversion produces a gas containing mainly hydrogen and CO₂, which can be quite easily separated out. The hydrogen can then be used for energy or heat production. If the CO₂ is stored, the hydrogen is a carbon-free energy carrier that can be combusted to generate power and/or heat. Pre-combustion would be used at power plants that employ integrated gasification combined cycle (IGCC) technology.
• An oxyfuel combustion system uses oxygen instead of air for combustion of the primary fuel to produce a flue gas that is mainly water vapor and CO₂, resulting in a flue gas with high CO₂ concentrations (more than 80 percent by volume). The water vapor is then removed by cooling and compressing the gas stream. However, this process requires that one must first separate oxygen from the air, which is a fairly complex process. As a method of CO₂ storage in boilers, these systems are currently in the demonstration phase.

* IPCC Special Report, 2005