Chilled Ammonia CO2 Capture Process Facility at American Electric Power's (AEP) Mountaineer Plant

There are two major types of anthropogenic CO₂ sources:  mobile and stationary.  Mobile sources include things like cars, trucks, trains, boats, and aircrafts that burn fossil fuels and generate CO₂.  Capturing CO₂ from mobile sources is currently impractical.  Stationary sources include power plants and industrial facilities that burn fossil fuels, as well as facilities that produce commodities such as cement and ethanol. CO₂ is already captured at oil and gas processing and chemical production facilities. Capturing CO₂ from power plants is less common; however, it is currently used at several power plants to produce food-grade CO₂.  

In a typical stationary source, CO₂ is generated through combustion or through other processes and becomes part of a mixture of gases leaving the facility.  Using a coal-fired power plant as an example, the gases leaving the plant (called flue gas) are typically made up of approximately 75 percent nitrogen, 10-15 percent CO₂, 8-10 percent water, and small percentages of other constituents, such as oxygen, sulfur, and trace contaminants like mercury.  The challenge comes in separating the CO₂ from the other gases in the mixture.

A question might be, “Why not just store all of the flue gas?”  The answer involves two issues:

1. The total volume of flue gas is large and the cost of compressing and transporting that large volume would be prohibitive.
2. Since most of the volume of flue gas is made up of innocuous nitrogen, storing all of the flue gas would not be practical and overwhelm most of the available storage capacity.

The three general categories of CO₂ capture technologies that can be applied to coal-based power plants are pre-combustion, post-combustion, and oxy-combustion. Pre-combustion capture is applicable to integrated gasification combined cycle (IGCC) power plants, while post- and oxy-combustion capture could be applied to conventional pulverized coal-fired power plants.

Pre-combustion capture is mainly applicable to gasification plants, where fuel (coal, biomass, or coal/biomass mixture) is converted into gaseous components by applying heat under pressure in the presence of steam and sub-stoichiometric oxygen (O₂).

Block Diagram Illustrating Power Plant with Pre-Combustion CO2 Capture.

Post-combustion CO₂ capture is primarily applicable to conventional coal-fired, oil-fired, or gas-fired power plants, but could also be applicable to IGCC and natural gas combined cycle (NGCC) flue gas capture. In a typical coal-fired power plant, fuel is burned with air in a boiler to produce steam that drives a turbine/generator to produce electricity.

Block Diagram Illustrating Power Plant with Post-Combustion CO2 Capture.

Oxy-combustion is applicable to both new and existing coal-fired power plants. Oxy-combustion systems for CO₂ capture rely on combusting coal with relatively pure oxygen diluted with recycled CO₂ or CO₂/steam mixtures. Under these conditions, the primary products of combustion are water and CO₂, with the CO₂ separated by condensing the water and removing any other gas constituents that infiltrated the combustion system. Oxy-combustion produces a highly concentrated CO₂ stream (approximately 60 percent), which is separated from water vapor by condensing the water through cooling and compression.

Block Diagram Illustrating Power Plant with Oxy-Combustion CO2 Capture.

Significant research is underway to reduce the cost of all of these methods of carbon capture.