The combustion of fossil fuels in nearly pure oxygen, rather than air, presents an opportunity to simplify carbon dioxide ( CO2) capture in power plant applications. Oxy-combustion power production provides oxygen to the combustion process by separating oxygen from air. However, the capital cost, energy consumption, and operational challenges of oxygen separation are a primary challenge of cost-competitive oxy-combustion systems. Oxy-combustion system performance can be improved by two means:
1) by lowering the cost of oxygen supplied to the system and
2) by increasing the overall system efficiency. The research and development (R&D) within the National Energy
Technology Laboratory’s (NETL) Transformative Power Generation Program is aimed at strategies to improve oxy-combustion system efficiency and reduce capital cost, offsetting the challenges of oxygen production.
In an oxy-combustion process, a pure or enriched oxygen stream is used instead of air for combustion. In this process, almost all the nitrogen is removed from the air, yielding a stream that is approximately 95 percent oxygen. Hence, the volume of flue gas, which is approximately 70 percent CO2 by volume, from oxy-combustion is approximately 75 percent less than from air-fired combustion. The lower gas volume also allows easier removal of the pollutants (sulfur oxide [SOx], nitrogen oxide [NOx], mercury, particulates) from the flue gas. Another benefit is that because nitrogen is removed from the air, NOX production is greatly reduced.
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. Oxy-combustion systems can be configured differently with these components, resulting in different energetic and economic performances.
Advanced oxy-combustion systems can be configured in either low- or high-temperature boiler designs. In low-temperature designs, flame temperatures are like that of air-fired combustion (~3,000°F), while flame temperatures exceed 4,500°F in the advanced high-temperature design. Low-temperature designs for new or retrofit applications recycle combustion products to lower the flame temperature to approximate the heat transfer characteristics of air-fired boilers. In high-temperature oxy-fuel combustion processes, fuel and oxygen are mixed at the burner undiluted with recycled flue gas, except to motivate coal for coal-fired systems. This process can result in a high flame temperature (>4,500°F), which enhances heat transfer in the radiant zone of the boiler. This process also results in more viable heat in the radiant zone of new or existing boilers and can result in a reduction in fuel demand at constant steam generation rates. High-temperature designs in new construction applications use increased radiant heat transfer to reduce the size and capital cost of the boiler. Furthermore, advanced emissions control systems for the removal of acid gases can enable recovery of latent heat in the flue gas.
Today’s state-of-the-art oxy-combustion systems would use a cryogenic process to supply oxygen; 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 CO2 compression. The Transformative Power Generation Program is developing advanced technologies to reduce the cost and improve the performance associated with current systems. R&D efforts are focused on developing pressurized oxy-combustion power generation systems. Currently, NETL supports several oxy-combustion projects in collaboration with industry and academia, ranging from lab- and bench-scale testing to verification testing at pilot scale. These projects are focused on understanding oxy-fuel combustion at high temperatures and pressures, verifying system design and operation concepts, and improving performance of ancillary system components.