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.
Alstom is developing an oxy-combustion system designed for retrofit to tangentially fired, atmospheric-pressure boilers. This effort advances 2nd Generation Technology by conducting pilot-scale tests on a 5-MWe pilot facility to evaluate cost and performance impacts of the ratio of oxygen to recycled flue gas, injection of pure oxygen, injection direction, and firing system designs. The project represents an important stepping stone between 1st Generation technologies, such as those being employed as part of the FutureGen project, and transformational technologies under development. These transformational technologies have the potential to exceed the DOE/NETL objectives of developing advanced oxy-combustion CO2 capture technologies for coal-fired plants capable of 90 percent carbon capture, near-zero air emissions, zero-liquid discharge, and reduced-water consumption with capture costs of less than $40/tonne of CO2 captured.