Plant and process for removing carbon dioxide from gas streams
The present invention is based on the realization that the carbon dioxide component of industrial gas streams also containing steam can be processed so to utilize either as latent and/or sensible heat the heat available from the steam component to assist in separating carbon dioxide from the remainder of the gas stream. For example, flue gases produced by power stations burning brown coal, black coal or natural gas inherently contain a useful amount of energy that can be harnessed according to the present invention. According to particular preferred forms of the invention, nitrogen and sulphur constituent such as SO x and NO x , H 2 S and other nitrogen containing compounds may also be removed from the gas stream through direct contact with the absorbing medium and used to produce by-products such as fertilizer material.
1. A process that removes carbon dioxide from a post combustion gas stream such as a flue gas of a power station or processing plant, the gas stream including steam and carbon dioxide, and the process including the steps of:
a) extracting carbon dioxide from the gas stream by contacting the gas stream with an absorbing medium in the form of a solution;
b) volatilizing carbon dioxide from the absorbing medium so as to produce a product stream rich in carbon dioxide;
c) obtaining energy from the gas stream in the form of either one or a combination of i) sensible heat from the gas stream and/or ii) latent heat obtained by condensing steam and using the energy as a heat source to assist in volatilization of carbon dioxide according to step b); and
wherein step a) is carried out at a pressure ranging from 100 to 150 kPa absolute and step b) is carried out at a pressure ranging from 5 to 60 kPa absolute.
2. The process according to claim 1 , wherein step c) involves heat being transferred to the absorbing medium before volatilising carbon dioxide from the absorbing medium according to step b).
3. The process according to claim 1 , wherein step b) is carried out in a stripper and that heat is transferred to the absorbing medium before the absorbing medium is fed into stripper.
4. The process according to claim 1 , wherein step c) involves heat being transferred to the absorbing medium by direct heat transfer in which the gas stream directly contacts the absorbing medium.
5. The process according to claim 4 , wherein the gas stream and the absorbing medium are mixed in an absorber or any other another vessel such as a cyclone separator.
6. The process according to claim 4 , wherein a stream of absorbing medium is discharged from an absorber in which step a) is carried out and is returned or recycled to the absorber via a heat exchanger network that transfers heat to absorbing medium to assist in the volatilisation of carbon dioxide in according to step b).
7. The process according to claim 6 , wherein said stream of absorbing medium discharged from the absorber is split in two sub-streams, whereby one of the sub-streams is recycled or returned to the absorber via the heat exchanger network and that other sub-stream is subject to volatilisation according to step b).
8. The process according to claim 6 , wherein heat is transferred via the heat exchanger network to one or more side streams of the absorbing medium withdrawn from a stripper or supplied to a re-boiler heating absorbing medium located at the base of the stripper.
9. The process according to claim 6 , wherein the exchanger network is a heat pump including a closed loop containing a heat transfer medium, two or more than two heat exchangers that operate as a condenser or condensers and an evaporator or evaporators and a compressor for pressurising the heat transfer medium.
10. The process according to claim 1 , wherein step c) involves heat being transferred to the absorbing medium by indirect heat transfer in which heat is transferred to the absorbing medium via one or more intermediate heat transfer mediums.
11. The process according to claim 10 , wherein heat is transferred by a heat exchanger from the gas stream to a stream of the absorbing medium rich in carbon dioxide that is conveyed from the absorber to the stripper.
12. The process according to claim 10 , wherein heat is transferred via a first heat exchanger from the gas stream to a stream of the absorbing medium lean in carbon dioxide that is conveyed from the stripper to the absorber and heat is transferred via a second heat exchanger from the absorbing medium stream lean in carbon dioxide that has been heated in the first heat exchanger to a stream rich in carbon dioxide that is fed to the absorber.
13. The process according to claim 1 , wherein the absorbing medium is a solution containing any one or a combination of amino acids, alkali carbonates or amines.
14. The process according to claim 1 , wherein step c) involves increasing the temperature of the absorbing medium by a value ranging from 1 to 55° C.
15. The process according to claim 1 , wherein step b) involves reducing the operating pressure to which absorbing medium rich in carbon dioxide is exposed so as to flash carbon dioxide from the absorbing medium.
16. The process according to claim 1 , wherein the process further includes discharging a by-product stream containing the absorbent solution and either one or a combination of sulphur and nitrogen constituents and adding a makeup stream containing fresh alkali mineral to the absorbent.