Program Background and Project Benefits
Advanced combustion power generation from fossil fuels involves combustion in a high-oxygen (O2) concentration environment rather than air. This type of system eliminates introduction of most, if not all, of the nitrogen (N2) found in air into the combustion process, generating flue gas composed of CO2, water (H2O), trace contaminants from the fuel, and other gas constituents that infiltrated the combustion system. The high concentration of CO2 (≈60 percent) and absence of nitrogen in the flue gas simplify separation of CO2 from the flue gas for storage or beneficial use. Thus, oxygen-fired combustion is an alternative approach to post-combustion capture for Carbon Capture and Storage (CCS) for coal-fired systems. However, the appeal of oxygen-fired combustion is tempered by a number of challenges, namely capital cost, energy consumption, and operational challenges associated with supplying O2 to the combustion system, air infiltration into the combustion system that dilutes the flue gas with N2, and excess O2 contained in the concentrated CO2 stream. These factors mean oxygen-fired combustion systems are not cost-effective at their current level of development. Advanced combustion system performance can be improved either by lowering the cost of oxygen supplied to the system or by increasing the overall system efficiency. The Advanced Combustion Systems Program targets both of these possible improvements through sponsored cost-shared research into two key technologies: (1) Oxy-combustion, and (2) Chemical Looping Combustion (CLC).
Oxy-combustion power production involves three major components: oxygen production (air separation unit [ASU]), the oxy-combustion boiler (fuel conversion [combustion] unit), and CO2 purification and compression. These components along with different design options are shown below. Based on the different combinations of these components, oxy-combustion can have several process configurations. These different configurations will have different energetic and economic performance.
Today's oxy-combustion system configuration would use a cryogenic process for O2 separation, atmospheric-pressure combustion for fuel conversion in a conventional supercritical pulverized-coal boiler; substantial flue gas recycle; conventional pollution control technologies for SOx, NOx, mercury, and particulates; and mechanical compression for CO2 pressurization. However, costs associated with currently available oxy-combustion technologies are too high. The Advanced Combustion Systems R&D Program is developing advanced technologies to reduce the costs and energy requirements associated with current systems. R&D efforts are focused on development of pressurized oxy-combustion power generation systems, as well as membrane-based oxygen separation technologies.
Washington University in St. Louis is working to improve oxy-combustion technology by developing a staged, high-pressure combustor system. Pressurized oxy-combustion reduces the mass and volume of flue gas, increases heat transfer rates, and makes latent heat recoverable, all of which improves efficiency. Furthermore, pressurization reduces equipment size, potentially reducing capital costs, and prevents air in-leakage, which increases CO2 purity. Fuel-staged combustion is used to manage peak combustion temperatures using excess oxygen as the diluent, allowing the use of conventional boiler materials and eliminating flue gas recycle, which both reduce capital cost. These factors have the ability to drive down the cost of electricity and the cost of CO2 capture relative to conventional coal-fired power plants with post-combustion CCS. Successful development of a laboratory-scale pressurized oxy-combustor, subsequent testing to validate the feasibility of Washington University in St. Louis’ approach, and projected cost and performance analysis of a full-scale system will support design, development, and testing of a small pilot-scale prototype.