What is Oxy-fuel Combustion?
Oxy-fuel combustion is short for oxygen fired pulverized coal combustion. With any type of combustion or burning, two things are required: oxygen and fuel such as coal, wood, or oil. In a traditional power plant, the pulverized coal used for fuel is fired, or burned, in a boiler and the oxygen comes from injected air. This air is approximately 21% oxygen (O₂) and 79% nitrogen (N₂). Oxy-fuel combustion uses the same process except the air portion is an oxygen-enriched gas mix instead of air. In the gas mix, almost all nitrogen is removed from the air injected into the boiler, yielding a stream that is about 95% oxygen.

Removing nitrogen from the gas mix has both benefits and challenges. The benefit is that you get a higher concentration of oxygen that burns better. On the other hand, the challenge is that burning with pure oxygen results in a very high flame temperature in the boiler. This is important because conventional boilers are not designed to handle extremely high temperatures.

	Click on diagram below to see larger view.
	Oxygen Combustion with Recycled Flue Gas (RFG). The red lines show changes to a con- ventional PF Plant. Diagram courtesy of CCSD.

One way to address this problem is to mix the enriched oxygen stream with recycled flue gas (RFG). The RFG dilutes the oxygen stream and decreases the flame temperature to a level similar to that of a conventional air-fired boiler. But the justification for using oxy-fuel is that it produces a CO₂ flue gas ready for sequestration. And the RFG can be used to carry fuel into the boiler providing another benefit. Normally, for this type of system, gas injected into the boiler with pulverized coal must be preheated to meet operating conditions in the boiler. As the RFG is already a hot gas stream, preheating is not required. After the combustion process, sulfur, water, particulate, and other gases can be removed from the flue gas. The remaining gas consists mostly of carbon dioxide (CO₂), which can be processed for sequestration. The diagram to the right shows the general process.

	Click on chart below to see larger view.
	Oxy-fuel and conventional processes comparisons.

Why use Oxy-fuel Combustion?
Oxy-fuel combustion has significant advantages over traditional air-fired combustion, partly because of increased emphasis being placed on carbon sequestration. Carbon sequestration is the process of capturing carbon emissions, primarily in the form of carbon dioxide (CO₂), from power plants and storing it in a permanent location so it is not released into the atmosphere. Oxy-fuel combustion produces about 75% less flue gas, by volume, than conventional air-fueled combustion and consists primarily of CO₂ and water (H₂O). The chart to the right compares the Oxy-fuel and conventional processes.

	Click on chart below to see larger view.
	Graph taken from Rivers, Keith B., PE, Babcock and Wilcox, Reducing Utility Coal Fired Boiler’s Emsissions to Nearly Nothing.

Additional benefits of Oxy-fuel Combustion
Along with the carbon sequestration benefits, another benefit is that the mass and volume of the flu gas is reduced in Oxy-fuel combustion; when you reduce the volume, you reduce the amount of heat lost in the flue gas increasing efficiency and requiring less treatment equipment to process the gas.

Finally, as the process removes all nitrogen from the air before it is injected into the boiler, the production of nitrogen oxide is reduced, substantially. The chart to the right shows just how much that reduction can be. However, even with all the positives of oxy-fuel combustion, there are still challenges for the technology.

Challenges to using Oxy-Fuel Combustion
The greatest challenge facing oxy-fuel combustion today is the cost to produce and supply pure oxygen to the burner in the combustion process. Because of the energy and economic costs to produce the oxygen, the oxy-fueled plant is less efficient than the air-fired plant.

Another issue is the cost to retrofit conventional coal boilers so they use oxy-fuel combustion techniques. Some estimates say the cost increase can be as high as 30%. Invariably, consumers must balance the higher utility bill against a cleaner environment. However, oxy-fuel is a viable method to remove CO₂ from the flue gas and reduce nitrogen oxide emissions. We need to continue to advance the technology

Advances in Oxy-Fuel Combustion
Currently, the Department of Energy sponsors two projects that demonstrate oxy-fuel combustion in existing coal-fired power plants. The projects, valued at nearly $10 million, are expected to help speed up the timeline for commercialization of oxy-fuel combustion technology through slipstream or pilot plant testing.

There are no commercial oxygen combustion power plants operating today due mainly to the high cost of producing oxygen. Two projects do show promise for reducing those costs when compared to existing CO₂ capture systems: one project by Babcock and Wilcox and one by the BOC Group Inc. Both began in 2005. Reducing the cost and technical risk of oxy-fuel combustion technology moves the nation closer to cleaner, more efficient power generation from coal.

Babcock and Wilcox (B&W)—On a $3.5 million, 2-year project, B&W and its team conducted pilot-scale tests across a range of coal types. The team plans to optimize the oxy-fuel combustion process and demonstrate that boilers retrofitted with oxy-fuel combustion technology are a cost-effective approach for CO₂ capture, coupled with much lower nitrogen oxide emissions. Their research will include multiple types of coal on different types of combustors. The initial study (report) was for one type of coal.

In the report B&W, the company stated that oxy-combustion with flue gas recirculation had been successfully demonstrated. The NOx emissions were 65% lower than an air-fired case and flue gas volume had been reduced by 70%.

BOC Group, Inc.—The BOC Group captured CO₂ by combining oxy-fuel combustion technology with recycled flue gas. To reduce the cost of oxygen production, BOC applied its CAR (Ceramic Autothermal Recovery) oxygen production process that uses the mineral Perovskite to absorb oxygen and subsequently release it in a circulating fluidized bed and pulverized coal pilot-scale firing configuration. The total value of this three-year project is $6.1 million.

Full-scale application of oxy-fuel combustion technology is still under development. However, laboratory and theoretical work has provided an initial understanding of the design parameters, operational considerations required, and the potential of the technology. What we can conclude from the findings is that using Oxy-fuel combustion reduces NOx emission by 25-50%; there are no technical barriers to using RFG; and preliminary cost evaluations are becoming comparable with other technologies. And now there are a number of pilot-scale tests and demonstrations in the U.S. as well as Europe, Japan, and Canada.