Absorption of NOx and SO2 in the Flue Gas Treatment of an Oxy-fuel Power Plant, Report No. Nr.T2007-304
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The oxy-fuel process is regarded as one of the promising CO2 capture technologies. In the process fuel is burned in an atmosphere of nearly pure oxygen and recycled flue gases so that a CO2-enriched flue gas is produced. The CO2-conditioning step that follows produces a stream of liquid carbon dioxide whose purity may be of importance for the storage application considered. The compressions necessary for liquefaction of CO2 as well as for the cryogenic separation of oxygen from air are energy-intensive processes. In order to avoid further reduction in the efficiency of an oxy-fuel power plant, conventional energy-intensive desulphurization and denitrification should be avoided if possible. At the same time however, the consequences of storage of impurities (e.g. SO2, NOx) together with the CO2 have not been explored in detail. No regulatory framework for impurity limits for liquid CO2 is available either. As it happens, the conditions that arise in the flue gas treatment path of an oxy-fuel process open up the possibility for an absorption process for SO2 in the CO2-conditioning step. The chemical mechanism for this absorption process is known as the ‘Lead Chamber Process’ for production of sulfuric acid. In the process NOx in the flue gas is used as a catalyst to oxidize SO2 to H2SO4(aq). In this work, an extensive literature survey was performed in order to obtain reasonable assumptions for the SO2-absorption process. Based on these assumptions, simulation of the SO2-absorption process and a additional absorption process for NOx is performed, in order to determine the necessary boundary conditions and the achievable cleaning efficiency for SO2 and NOx. The chemical kinetics used are applied in steady-state models of two separate packed absorption columns with a countercurrent stream of recirculated condensation liquid as absorbent. Almost all SO2 can be absorbed in the first column, and no limitation on the concentration has been observed. A cleaning target of 180 ppm has been chosen for the NOx-absorption column. For both columns, optimal temperature and pressure levels have been identified for which the residence times are minimal so as to achieve the desired reductions. Based on these calculations, a minimum in the total residence time of both absorption columns was identified. The duration depends on the NOx concentration in the flue gas and is lowest for 750-1000 ppm NOx. This is a high level for oxy-fuel combustion. Thus, if the suggested treatment process is applied, it would not only eliminate the need for efforts in primary NOx-reduction but would require an increase in NOx formation in the combustion. No additives are needed and the products are 70 mass% sulfuric acid and up to 60 mass% nitric acid.
Energi , Termisk energiteknik , Kemiteknik , Energy , Thermal energy engineering , Chemical Engineering