Predictions of the impurities in the CO2 stream of an oxy-coal combustion plant

Whilst all three main carbon capture technologies (post-combustion, pre-combustion and oxy-fuel combustion) can produce a CO2 dominant stream, other impurities are expected to be present in the CO2 stream. The impurities in the CO2 stream can adversely affect other processes of the carbon capture and storage (CCS) chain including the purification, compression, transportation and storage of the CO2 stream. Both the nature and the concentrations of potential impurities expected to be present in the CO2 stream of a CCS-integrated power plant depend on not only the type of the power plant but also the carbon capture method used. The present paper focuses on the predictions of impurities expected to be present in the CO2 stream of an oxy-coal combustion plant. The main gaseous impurities of the CO2 stream of oxy-coal combustion are N2/Ar, O2 and H2O. Even the air ingress to the boiler and its auxiliaries is small enough to be neglected, the N2/Ar concentration of the CO2 stream can vary between ca. 1% and 6%, mainly depending on the O2 purity of the air separation unit, and the O2 concentration can vary between ca. 3% and 5%, mainly depending on the combustion stoichiometry of the boiler. The H2O concentration of the CO2 stream can vary from ca. 10% to over 40%, mainly depending on the fuel moisture and the partitioning of recycling flue gas (RFG) between wet-RFG and dry-RFG. NOx and SO2 are the two main polluting impurities of the CO2 stream of an oxy-coal combustion plant and their concentrations are expected to be well above those found in the flue gas of an air-coal combustion plant. The concentration of NOx in the flue gas of an oxy-coal combustion plant can be up to ca. two times to that of an equivalent air-coal combustion plant. The amount of NOx emitted by the oxy-coal combustion plant, however, is expected to be much smaller than that of the air-coal combustion plant. The reductions of the recirculated NOx within the combustion furnace by the reburning mechanism and the char-NO reactions are the main reason for a smaller amount of NOx emitted by the oxy-coal combustion plant. The concentration of SO2 in the flue gas of an oxy-coal combustion plant can be up to six times to that of an equivalent air-coal combustion plant if the recycling flue gas is not desulphurized. The flue gas volume flow rate of an oxy-coal combustion plant is much smaller (<20%) than that of an equivalent air-coal combustion plant, which is a significant advantage for the purification of the flue gas.