Oxy-fuel

Oxy-fuel combustion is the process of burning a fuel using pure oxygen instead of air as the primary oxidant. Since the nitrogen component of air is not heated, fuel consumption is reduced, and higher flame temperatures are possible. Historically, the primary use of oxy-fuel combustion has been in welding and cutting of metals, especially steel, since oxy-fuel allows for higher flame temperatures than can be achieved with an air-fuel flame.

There is currently research being done in firing fossil-fueled power plants 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, or staged combustion. 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).

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:

Economically speaking this method costs more than a traditional air-fired plant. The main problem has been separating oxygen from the air. This process needs lots of energy, nearly 15% of production by a coal-fired power station can be consumed for this process. However, a new technology which is not yet practical called chemical looping combustion can be used to reduce this cost. In chemical looping combustion, the oxygen required to burn the coal is produced internally by oxidation and reduction reactions, as opposed to using more expensive methods of generating oxygen by separating it from air.

At present 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.

In industries other than power generation, oxy-fuel combustion can be competitive due to higher sensible heat availability.

Oxy-fuel combustion is common in various aspects of metal production.

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