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.
Praxair is developing a unique oxy-combustion system that integrates oxygen-separation membrane modules into the boiler, thereby using oxygen as it’s produced, resulting in a significant reduction in the size of the ASU to only what is necessary for coal gasification. In addition, the OTM oxy-combustion system can provide a highly concentrated, sequestration-ready stream of CO2 without the need for a large CO2 post-combustion capture system. These factors have the ability to reduce equipment size and complexity, thereby reducing capital cost, which has the potential to drive down the cost of electricity and the cost of CO2 capture relative to supercritical coal-fired power plants with post-combustion CCS. Successful testing of a development-scale OTM partial-oxidation unit will validate the feasibility of Praxair’s approach, and cost and performance analysis of a full-scale system will support design and development of an OTM oxy-combustion boiler prototype.
This project is a continuation of an ongoing effort that has been performed by Praxair under "Advanced Oxyfuel Boilers and Process Heaters for Cost Effective CO2 Capture and Sequestration" (Contract No.: FC26-01NT41147). The Final Report for the original project provides more detailed discussion of the work.