Abstract

Reducing CO2 emissions of fossil fuel fired power plants by CO2 capture and sequestration (CCS) can mitigate the greenhouse effect. Up to now various technologies have been suggested, but their application results in net-efficiency drops of 10 to 14 %-points. A more promising strategy for reducing CO2-emissions is the oxyfuel-combustion, where fuel is burned with pure oxygen diluted in recirculated flue gas instead of air.

The production of the pure oxygen can be realized by cryogenic means (~10% decrease of the overall efficiency). However, if oxygen separation from air is realized by a ceramic mixed ion electron conducting (MIEC) membrane, it is possible to reduce the efficiency decrease to 5% to 8%. Compared to the cryogenic process, the lower energy demand and the 100% selectivity of the membrane are the main advantages. At temperatures greater than 800°C oxygen is able to permeate through the ceramic membrane. Different integrations of the membrane in the power plant process are possible: 3-end and 4-end:

  1. In the 4-end concept, the membrane operates with hot (850°C) and dust free recirculated flue gas as a sweep stream and with pressurized air (20bar) on the feed side. This operation mode provides a high flux of oxygen due to a low oxygen partial pressure on the permeate side of the membrane. The membrane has to be chemical resistant against flue gas components (e.g. SOx, NOx).

  2. Alternatively the membrane can operate without contact to the flue gas (3-end). In consequence, the membrane must only be stable against air. In this case the air is preheated in the combustion chamber to supply the heat for oxygen permeation. Partial pressure difference is reached by vacuum on the permeate side, pressurized feed air or combination of both.

The presentation will give a short overview about the integration methods of the membrane, the main influencing process parameters and resulting efficiencies for hard and lignite coal fired power plants. An economic analysis of the power plant process and of the membrane unit will show the expected costs. A comparison with other conventional and membrane based CCS techniques will complete the process evaluation.

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