Flue gas treatment for CO2 capture

This report is in two sections: the first describes the flue gas and its treatment in post-combustion capture and the second covers the flue gas in oxyfuel combustion capture. The main components of the flue gas in pulverised coal combustion for postcombustion capture are CO2, N2, O2 and H2O, and air pollutants such as SOx, NOx, particulates, HCl, HF, mercury as well as other contaminants. The most widespread method of post-combustion capture of CO2 is by solvent absorption. Coal-fired power plants already have units to remove air pollutants prior to CO2 capture, such as FGD, SCR, and ESP. However, some pollutants will be absorbed in the solvent with the CO2 and can lead to the formation of heat stable salts. The FGD process may need to be improved to reduce SOx emissions to an acceptable level for post-combustion capture. The flue gas stream from oxyfuel combustion is composed mainly of CO2 and H2O. It will also contain O2, SOx, NOx, HCl, Hg, any diluents from the O2 stream, and various inerts such as N2 and Ar. Condensing, compression and cooling of the flue gas are required to obtain CO2 of an acceptable purity for transport and storage. Various processes are being developed to raise the purity of CO2 produced from oxyfuel combustion. Air Products have developed a membrane to separate CO2 from a feed gas; Air Liquide have developed a compression and purification unit; Linde are developing a fast catalytic deNOx process and an alkali based wash unit for removal of NOx and SOx; and Praxair are developing oxygen enhanced coal combustion to reduce NOx emissions without resorting to complete oxyfuel combustion. Finally, the ThermoEnergy Integrated Power System enables the condensing heat exchanger to collect particulates, acid gases and mercury into a condensed phase. Despite this ongoing research, at present the issue of CO2 optimum product purity is a question that does not have a satisfactory answer. ASU air separation unit B&W Babcock and Wilcox CAPEX capital expenditure CCR carbonation calcination reaction CCS carbon capture and storage CPU compression and purification unit daf dry ash free DEA diethanolamine DECC Department of Energy and Climate Change (UK) DOE Department of Energy (US) EDTA ethylenedinitrilo-tetra-acetic-acid EOR enhanced oil recovery ESP electrostatic precipitator FGD flue gas desulphurisation HHV higher heating value IEA GHG IEA Greenhouse Gas R&D Programme LCPD Large Combustion Plant Directive LHV lower heating value MEA monoethanolamine mg/m3 milligramme per cubic metre MHI Mitsubishi Heavy Industries MMBtu million British thermal units NETL National Energy Technology Laboratory (US) ng/J nanogramme/Joule NOx nitrogen oxides (NO and NO2) O&M operation and maintenance OPEX operating expenditure PC pulverised coal PM particulate matter ppmv parts per million by volume RFG recycled flue gas SCR selective catalytic reduction SDA spray dryer absorber TIPS ThermoEnergy Integrated Power System 2 IEA CLEAN COAL CENTRE Acronyms and abbreviations