Simultaneous removal of CO2, SO2, and NOx from flue gas by liquid phase dehumidification at cryogenic temperatures and low pressure

The current paper presents a new process for postcombustion carbon capture. An historical survey of the cryogenic techniques used in the chemical industry for the removal of acid vapours is given, with particular emphasis on two Ryan—Holmes-based processes. The paper suggests that these two processes have the potential to achieve CO2 separation from flue gas with high second law efficiency. A further variation on these processes is then proposed enabling the simultaneous capture of CO2, SO2, and NOx from flue gas produced in conventional or combined cycle power stations, burning pulverized coal or heavy bunker fuels. The proposed process absorbs these vapours by dehumidification using a cooled liquid absorbent, at low pressure (close to atmospheric) and cryogenic temperatures. A feature of the process is the use of direct contact heat transfer to perform the major cooling and heating operations thus reducing the plant size and cost while improving efficiency. The paper starts with a discussion of the most suitable absorbent for the process; liquid SO2 is suggested and a simplified analysis of an absorption column is conducted using the technique developed by Souders and Brown. This analysis shows that with only four theoretical stages and an acceptable liquid/gas ratio of two, SO2 is able to absorb CO2 and leave a residual concentration of CO2 in the flue gas equivalent to 90 per cent carbon capture. The final section of the paper describes the use of dilute H2SO4 as a precooling stream to reduce the temperature of the flue gas prior to it entering the CO2 scrubbing section. It is proposed to produce the necessary H2SO4 from the constituents of the flue gas using similar reactions to those of the ‘lead-chamber process’. The H2SO4 stream is also used to reheat the flue gas before it is passed to the chimney. It is shown that an important attribute of the H2SO4 stream is its role as a ‘lean-oil’, reducing SO2 emissions from the power station to well below the levels required by future legislation.