Simultaneous removal of CO 2,SO 2,and NO x from flue gas by liquid phase dehumidification at cryogenic temperatures and low pressure

Abstract

Abstract

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 NO _x 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.