Oxy-fuel combustion process

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Oxy-fuel combustion is the process of firing a fossil-fueled power plant with an oxygen-enriched gas mix instead of air. Almost all of the nitrogen is removed from input air, yielding a stream that is approximately 95% oxygen. Firing with pure oxygen would result in too high a flame temperature, so the mixture is diluted by mixing with recycled flue gas. The recycled flue gas can also be used to carry fuel into the boiler and ensure adequate convective heat transfer to all boiler areas. Oxy-fuel combustion produces approximately 75% less flue gas than air fueled combustion and produces exhaust consisting primarily of CO₂ and H₂O (see figure).

Diagram of Oxy-Fuel process and comparison of exhaust gas components.

The justification for using oxy-fuel is to produce a CO₂ rich flue gas ready for sequestration. Oxy-fuel combustion has significant advantages over traditional air-fired plants. Among these are:

• The mass and volume of the flue gas are reduced by approximately 75%.
• Because the flue gas volume is reduced, less heat is lost in the flue gas.
• The size of the flue gas treatment equipment can be reduced by 75%.
• The flue gas is primarily CO₂ suitable for sequestration.
• The concentration of pollutants in the flue gas is higher making separation easier.
• Most of the flue gases are condensable, making compression separation possible.
• Heat of condensation can be captured and reused, rather than lost in the flue gas.
• Because nitrogen from air is not allowed in nitrogen oxide production is greatly reduced.

However, because of the energy and economic costs of producing oxygen, an oxy-fuel power plant is less efficient than a traditional air-fired plant. In the absence of any need to reduce CO₂ emissions, oxy-fuel is not competitive. However, oxy-fuel is a viable alternative to removing CO₂ from the flue gas from a conventional air-fired fossil fuel plant. However, an oxygen concentrator might be able to help, as it simply removes nitrogen.