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Oxyfuel Combustion

The Insitute of Energy Systems and Technology at TU Darmstadt performs oxyfuel combustion tests in a 1 MWth CFB plant. During the tests, the ratio of lignite and waste-derived fuels (RDF), as well as the ratio of sand and ilmenite are fixed. Starting with air-firing, the fluidization medium is switched from air to recirculated flue gas mixed with oxygen. During the tests, the oxygen concentration is increased stepwise to a maximum value by reducing the flow of recycled flue gas. The maximum oxygen concentration is either limited by the entrainment of bed meaterial in the riser or by the formation of bed agglomerations due to temperature peaks that are expected at high oxygen concentrations.

A simplified flow diagramm of the oxyfuel combustion process can be seen on the right handside. An air separation unit (ASU) is used to separate atmospheric air into its primary components. In the next step, fuel, recycled flue gas and oxygen form the ASU are added to the boiler. The heat inside the boiler is used to produce high pressure steam, which is converted into electric power. Before leaving the boiler, the gas passes a cyclone where the separation between coarse particles and the flue gas stream takes place. After passing an external particle removal system, a fraction of the flue gas is separated to be recycled to the boiler. The larger fraction of the flue gas goes through a final SO2– and NOx-removal process, whereafter the CO2 is purified and can be compressed.