Oxy Fuel Combustion Method

A potential alternative to absorption technologies would be to combust fossil fuels in pure oxygen instead of air, which contains approximately 78% nitrogen by volume. If nitrogen were removed from the process, flue gas streams would have a much higher concentration of CO2, reducing or eliminating the need for costly CO2 capture. Moreover, NOX emissions (a source of acid rain and an ozone precursor) and the subsequent need for scrubbing would be reduced significantly. Finally, trace pollutants such as NOX and SO2 could potentially be compressed and stored along with CO2, allowing control costs to be “shared” among pollutants and resulting in a zero-emissions power plant.

The main problem with this method is separating oxygen from the air. This is usually completed cryogenically which requires a lot of energy (for a typical 500MW coal-fired power station supplying pure oxygen requires at least 15% of the electricity the plant generates annually). However, a promising new technology called chemical looping combustion is under development. With this technique the oxygen in the air is removed by oxidation of a metallic compound which can be reduced during combustion allowing the oxygen to be released.

With Oxyfuel combustion pulverized coal is burned in 95-99% pure oxygen rather than air. This process requires a recycle stream of flue gas to control temperature in the boiler since pure oxygen combustion would exceed temperature limitations of the boiler. Burning the fuel in this manner produces water and highly pure CO2 exhaust that can be captured at relatively low-cost through cooling and compression that condenses and separates the two byproducts. The high cost of producing oxygen, however, has made this a cost-prohibitive option for commercial use in most power plants. Developing technologies in oxygen and ion transport membranes have the potential to reduce the cost of oxygen production and increase oxy-combustion’s cost-effectiveness.

Oxy-combustion is most often considered for existing coal boilers burning lower sulfur coals without any SO2 and NOx control in the hope that these pollutants can be captured and disposed of with the CO2.

However, disposing of all these pollutants together can be difficult due to the physical properties of the gases along with other potential regulatory and transportation issues. As with post-combustion capture, oxycombustion presents trade-offs. The large power requirements for both oxygen production and operation of the CO2 compressor may lead to a reduction in net capacity and efficiency in the range of 25-30% relative to the same combustion system without CO2 capture.

There are a limited number of commercial scale oxy-combustion technologies that are currently being researched around the world. In North America, the Babcock & Wilcox Power Generation Group, Inc. and Air Liquide successfully operated a 30 MW generator in “full oxy-combustion mode” and are continuing research into the different types of coal mediums and plant designs that optimize carbon capture in different oxy-combustion models. 

In September 2008, Vattenfall began operation of a 30 MW oxy-combustion system in Germany that will be utilized to produce process steam and will eventually have the ability to store captured CO2 in a geological formation. In addition to these projects, the DOE/NETL is currently funding multiple oxy-combustion CO2 emission control projects in the laboratory and in small scale pilot programs along with advanced oxy-combustion system designs and analysis.

Oxy Fuel Projects:

Callide A Project

Vattenfall Schwarze Pumpe

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