Removing nitrogen from the air, and then combusting the input fuel in an oxygen-rich environment, results in a highly concentrated flue gas stream (with greater than 80% CO2) which can be further concentrated using physical gas purification techniques such as cryogenic separation.
This concentrated flue gas stream is one of the primary benefits of oxy-fuel combustion. A second advantage stems from the absence of nitrogen which results in the virtual elimination of nitrogen oxide (NOx) emissions. An overall systemic advantage is the reduced size of the entire process, due to the reduced volumes of both the input and exit gases, both of which translate into reduced capital and operating costs.
A variant of oxy-fuel technology, oxy-fuel recycling, can be used to control flame temperatures by recycling a portion of the exit flue gas into the oxygen input gas prior to combustion. By diluting the oxygen in the input gas it is possible to achieve conventional flame and heat transfer characteristics, thus potentially allowing the technology to be retrofit into current power plants. Another emerging variant is hydroxy-fuel combustion, which again provides an opportunity to moderate process temperatures by facilitating the combustion process in an oxygen and steam environment.
The greatest challenge facing oxy-fuel today is to lower the energy penalty (and therefore the cost penalty) involved in producing oxygen, which ranges from 8 to 30% (or perhaps even higher) of the total fuel cost depending on the fuel source and process used (Dillon, 2004). The US Department of Energy is working on improved ion transport membrane (ITM) systems, which are meant for low-cost, large-scale oxygen production. Success would result in a key enabling technology that significantly reduces the energy penalty involved in producing oxygen.
Another important challenge is that current design configurations and materials are unable to operate at the high temperature ranges for oxy-fuel combustion; however, CO2 or steam recycling may mitigate this issue. A final issue is the need to reduce the total energy consumption for CO2 separation and compression. However, this issue is not unique to oxy-fuel, as all four capture systems face this problem.
While oxy-fuel will assist in reducing the size, number and cost of the units required to produce energy, and will make emissions capture easier, its full potential is unlikely to be realized until new high-temperature materials become available for combustors and boilers. Natural Resources Canada’s CanmetENERGY division is working on oxy-fuel combustion systems, and is collaborating with partners who are developing super alloys and other advanced materials. The European Union's Thermie Program on advanced materials is working on materials that will be used in future applications of oxy-fuel combustion, as well as in ultra-supercritical pulverized coal and natural gas systems.
Early-stage commercial demonstrations are needed for oxy-fuel and/or hydroxy-fuel recycle systems, through which researchers could conduct the work needed to better define the science around oxy-fuel combustion, and develop new equipment, design principles and energy system process configurations.