Oxy-fuel is the burning of fuel with pure oxygen instead of the mix of oxygen, nitrogen, and carbon dioxide in air. When fuel is burned using pure oxygen, any carbon dioxide content can be stored without the need to remove nitrogen, which is a critical step in purifying waste products in power plants. Gases released by power plants are 75% nitrogen, which needs to be removed from any carbon dioxide steam. The most promising application is in coal-burning power plants, but gas turbine, oxygen production, and welding applications may also benefit.
Coal power plants can be reconfigured to burn oxy-fuel without changing the boiler design, although the burn temperature is high for most boilers. The oxy-fuel combustion process results in coal with a much higher combustion temperature, but this can be controlled by mixing oxygen with steam or flue gas from other plant processes. Nitrogen is present in the boiler only at low levels, and little nitrogen or nitric oxide is produced to contaminate the air.
Flue gas resulting from the burning of fossil fuels with oxy-fuel is free of nitrogen. Carbon dioxide and water are the main components of the gas, which can be concentrated into streams of nearly pure carbon dioxide. The benefit is that the remaining gas can be compressed, dried, and purified much faster and more inexpensively than traditional methods before being moved to storage.
The gas turbine cycle is improved through the use of oxy-fuel, but because gas turbine blades do not tolerate high temperatures well, the turbines require a redesign to operate with oxygenated fuel. High-temperature flue gas leaving a modified turbine actually makes the steam cycle more efficient. Oxy-fuel welding and cutting also provides welders with greater control over how much heat is generated, so the temperature of a welding zone can be maintained at safe levels. Weld bead size as well as shape are more easily controlled, and the changes needed to use oxy-fuel in welding are rather simple.
Full applications of oxy-fuel are still being developed. The technology has been tested in the United States, Canada, Europe, and Japan, and the lesser complexity and risks of the technology are appealing. Demands for clean burning coal plants and power production facilities that are safer for the environment could spark a large increase in oxygen-based fuel systems as the technology progresses. The production of oxygen and separation of it from air at a lower cost than cryogenic methods might also be possible.
By Andrew Kirmayer