Offgas Treatment Downstream the Gas Processing Unit of a Pulverised Coal-fired Oxyfuel Power Plant with Polymeric Membranes and Pressure Swing Adsorption

Abstract

Due to climate change it is necessary to reduce anthropogenic climate gas emissions. The application of carbon capture and storage (CCS) technologies could be a suitable approach to lower the specific CO2 emissions from coal-fired power plants. One of these CCS technologies is the Oxyfuel process. In the Oxyfuel process the coal is burned in a mixed atmosphere of O2 and recycled flue gas. The flue gas thus generated has a high CO2 concentration, because of the missing air nitrogen. Still the dried flue gas consists of approximately 15 mol-% impurities (O2, N2, Ar, NOx and SOx). To increase the CO2-purity the flue gas is treated in a gas processing unit (GPU). Two promising technologies to perform the gas processing are partial condensation and distillation. Both are well known and available at industrial scale. Using these technologies about 90% of the CO2 can be separated. The remaining part of the CO2 leaves the GPU with the offgas.To increase the overall capture rate of the CO2 in the Oxyfuel process the offgas from the GPU can be treated in either a pressure swing adsorption (PSA) cycle or a polymeric membrane (PM) cycle. These cycles generate a CO2-enriched GPU-recycle stream and an exhaust gas stream which consists of the residual impurities. The CO2-enriched GPU-recycle can be fed back to the GPU or mixed with the CO2 product stream of the GPU. The exhaust gas stream with the impurities has a high content of O2 and could be refed to the air separation unit (ASU) to increase the efficiency of the overall process. The additional gas treatment in the PSA- or the PM-cycle has influences on the specific energy demand of the GPU, the CO2 capture rate, the composition of the CO2 product stream and the overall process efficiency.In the work presented here the feed gas of the GPU is the flue gas of a large scale bituminous coal-fired Oxyfuel power plant.The plant model is based on the actual state-of-the-art power plant technology. For the GPU two different reference process cases are modelled. One case with a distillation of the CO2 and one case with a partial condensation of the CO2 are considered. For both cases the GPU process is externally cooled. These reference cases are compared then with a distillation and a partial condensation which have an additional offgas treatment by PSA or PM. For the offgas treatment with membranes, polymeric membranes are considered due to their high CO2/O2-selectivity and high permeability. For the offgas treatment with PSA a multiple bed cycle is modelled to assure continuous operation of the plant. The overall CO2 capture rate, the specific energy demand and the composition of the CO2 product stream are calculated for the reference cases, the distillation with PSA or PM and the partial condensation with PSA or PM. With these results the potential of these technologies for the GPU shall be compared with the reference cases. Furthermore a recycle of the O2-containing gas stream to the ASU is modelled in the overall process model. This recycled gas stream can be used to reduce the energy demand of the ASU. The influence of the offgas treatment is evaluated by calculating the net efficiency of the overall process.