Oxy-combustion cannot be simply substituted for air combustion in existing fossil-fueled power plants due to differences in combustion characteristics. In order for oxy-combustion to be utilized in existing plants, a thermal diluent is required to replace the nitrogen in air. The oxygen produced from air separation would be mixed with recycled flue gas to approximate the combustion characteristics of air.
For oxy-combustion to be a cost-effective power generation option, a low-cost supply of pure oxygen is required. In the most frequently proposed version of this concept, a cryogenic air separation unit is used to supply high purity oxygen to the boiler. This commercially available technology is both capital and energy-intensive and could raise the cost of electricity from coal-fired plants considerably, in addition to degrading the overall plant efficiency. However, novel technologies currently under development, such as oxygen and ion transport membranes, have the potential to reduce the cost of oxygen production.
Another breakthrough oxy-combustion concept under development is the chemical looping combustion process. Chemical looping splits combustion into separate oxidation and reduction reactions. A metal (e.g., iron, nickel, copper, or manganese) oxide is used as an oxygen carrier which then releases the oxygen in a reducing atmosphere and the oxygen reacts with the fuel. The metal is then recycled back to the oxidation chamber where the metal oxide is regenerated by contact with air. The advantage of using two chambers for the combustion process is that the CO2 is concentrated, once the water is removed, and not diluted with nitrogen gas. The benefit of the process is that no air separation plant or external CO2 separation equipment is required.
DOE/NETL is currently funding multiple oxy-combustion CO2 emission control projects within each of the above mentioned approaches. These R&D efforts are being performed both externally by research organizations and academic institutions, as shown in the table below, and internally through NETL’s Office of Research and Development (ORD), specifically the Separations and Fuels Processing Division and the Office of Computational Dynamics.
The In-House Oxy-Combustion CO2 Control webpage provides detailed information regarding NETL’s internal R&D.