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Oxyfuel combustion involves burning fossil fuel in nearly pure oxygen rather than in air. This produces a nitrogen free flue gas with water vapour and a high concentration of carbon dioxide as its main components. This makes it easy to further concentrate the flue gas to an almost pure stream of CO2.
The Oxyfuel combustion process eliminates nitrogen from the flue gas by combusting the fuel in a mixture of oxygen and recycled flue gases. The recycled flue gases mainly consist of carbon dioxide and water vapour and are used to create combustion conditions similar to those of air.
After combustion, the flue gas is cleaned. A sequence of steps removes ash and other particles, water and sulphur. Inert gases, such as oxygen and argon, can also be removed.
The cleaned flue gas primarily consists of CO2 and water vapour. By cooling the flue gas, the water vapour condenses thereby creating an almost pure CO2 stream. The CO2 can be compressed, dried and further purified before being transported to a storage site.
A challenge of Oxyfuel combustion is improving the necessary technology for air separation needed to produce oxygen. Air separation is highly energy demanding, thus reducing overall plant efficiency. Technical improvements are currently being explored to reduce costs. Oxyfuel combustion is the technology that seems to have the best potential for commercial use in the kind of large lignite-fired power plants that Vattenfall operates.
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