Process and plant for removing acid gases

Abstract

The present invention relates to a process and plant for removing acid gases such as carbon dioxide, sulphur containing compounds and nitrogen containing compounds from gas streams including high and low pressure gas streams. A solvent solution containing alkali carbonates absorbs the acid gases including carbon dioxide and either one or both of sulphur and/or nitrogen containing compounds. The bicarbonate is regenerated into a carbonate form to provide a gas stream rich in carbon dioxide, and sulphur and/or nitrogen containing compounds are recovered.

Claims

1. A process for removing acid gases from a gas stream, the process includes the steps of: a) contacting the gas stream with a solvent solution stream containing alkali carbonate at a concentration ranging from 30 to 60 wt % to absorb acid gases including carbon dioxide and either one or both of sulphur containing compounds or nitrogen containing compounds to form i) a loaded stream including alkali bicarbonate and sulphur and/or nitrogen containing compounds and ii) a second gas stream that is lean in acid gases, and wherein step a) is carried out in at least one contactor and includes precipitating the alkali bicarbonate to form a precipitant in the contactor so that the loaded stream includes the precipitant and a liquid phase including the solvent stream, and wherein step a) is carried out in the at least one contactor at a pressure ranging from 100 to 300 kPa; b) controlling the temperature at which step a) occurs in the contactor by cooling the loaded stream either in situ or by discharging a side stream of the loaded stream from the at least one contactor, cooling the side stream and returning the cooled side stream to the same contactor, such that step a) is carried out in the at least one contactor at an-isothermal temperature in the temperature range of 40 to 95° C, such that the heat of absorption of carbon dioxide and the heat of precipitation of bicarbonate do not affect the amount of acid gas absorption due to temperature effects on mass transfer, and wherein the isothermal temperature is selected based on the concentration of the alkali carbonate in the solvent stream and the required acid gas absorption; c) heating the loaded stream so as to regenerate alkali bicarbonate in the precipitant and the liquid phase and form i) a regenerated stream containing alkali carbonate, ii) a gas stream that is rich in carbon dioxide and iii) an intermediate stream that is lean in alkali bicarbonate and contains sulphur and/or nitrogen compounds in solution; d) recovering from the intermediate stream, that is rich in alkali carbonate, either one or both of sulphur containing compounds or nitrogen containing compounds to form a lean stream, wherein step d) includes recovering either one or a combination of sulphur containing compounds or nitrogen containing compounds by selective sorption using at least one ion exchange resin; and e) recycling a recycle stream that includes the regenerated stream of alkali carbonate and the lean stream for reuse in the solvent solution of step a), in which the recycle stream is lean in bicarbonate and has a carbon dioxide loading in the range of 0.2 to 0.3, wherein the liquid phase of the solvent stream has either one or both of sulphur containing compounds or nitrogen containing compounds and the liquid phase forms at least part of the intermediate stream lean in bicarbonate and from which either one or both the sulphur containing compounds or nitrogen containing compounds are recovered according to step d).

2. The process according to claim 1, wherein the process includes transferring heat from the regenerated stream to the loaded stream so as to heat the loaded stream that is being treated according to step c) and cool the regenerated alkali bicarbonate prior to reuse as the solvent solution according to step e).

3. The process according to claim 1, wherein step c) includes forming at least part or all of the intermediate stream from a split of the regenerated stream containing alkali carbonate.

4. The process according to claim 1, wherein step c) includes forming at least part or all of the intermediate stream from the liquid phase of the regenerated stream.

5. The process according to claim 1, wherein the alkali bicarbonate precipitant is separated from the slurry and fed to a regenerator for regeneration according to step c).

6. The process according to claim 1, wherein step a) and step c) is carried out in two or more contactor stages, and the process includes splitting the solvent solution stream into a plurality of solvent sub-streams and supplying the solvent sub-streams to each one of the respective contactor stages to form loaded sub-streams and conveying the gas stream successively through the contactor stages.

7. The process according to claim 6, wherein the process includes conducting step a) and step c) such that the temperature of the solvent solution stream, or sub-streams, is less than or equal to the temperature at which the partial pressure of carbon dioxide of the sub-stream, is less than the partial pressure of carbon dioxide of the gas stream.

8. The process according to claim 6, wherein step a) and step c) are conducted in up to 5 contactor stages and the solvent solution stream is split into a corresponding number of the solvent sub-streams.

9. The process according to claim 6, wherein the process includes the loaded sub-streams forming slurry streams in one or more contactor stages, in which the slurry includes a solid phase rich in alkali bicarbonate and a liquid phase lean in bicarbonate and containing varying amounts of either one or both of sulphur containing compounds or nitrogen containing compounds.

10. The process according to claim 6, wherein the solvent solution stream or substreams are conveyed in either counter current or co-current to the gas stream in at least one contactor stage.

11. The process according to claim 6, wherein the process includes precipitating alkali bicarbonate from the loaded sub-streams that are discharged from the contactor stages.

12. The process according to claim 11, wherein the step of precipitating alkali bicarbonate includes cooling each sub-stream in a dedicated cooler and/or adding a crystallising agent.

13. The process according to claim 9, wherein the precipitant is separated from the liquid phase of each loaded sub-stream in dedicated separators for each sub-stream.

14. The process according to claim 1, wherein the process is characterised by the use of a promoter, activator or catalyst for enhancing the absorption of acid gases and/or the regeneration of bicarbonate to carbonate.

15. The process according to claim 1, wherein the process is characterised by the use of a promoter, activator or catalyst, and the process includes forming a slurry by precipitating from the loaded stream a precipitant including alkali bicarbonate and a liquid phase having either one or both of sulphur containing compounds or nitrogen containing compounds, and the promoter, activator or catalyst is retained in the liquid phase and free from the precipitant which is treated according to step c).

16. The process according to claim 1, wherein the process is characterised by being without a promoter, activator or catalyst.

17. The process according to claim 1, wherein step c) includes heating the alkali bicarbonate using an auxiliary heating source that is separate from power plant operations, the auxiliary heating source involving the combustion of fossil fuels, and flue gas produced by the auxiliary heating source forms a part of the gas stream contacted with the solvent in step a).

18. The process according to claim 1, wherein the process includes storing bicarbonate precipitant prior to regeneration according to step c) either i) in a slurry form, or ii) when separated from the slurry as a solid form.

19. The process according to claim 18, wherein the process includes storing the bicarbonate in precipitant form during periods of high demand for electrical energy and regenerating stored bicarbonate according to step c) by using the surplus heat from a power plant during period of lower demand for electrical energy or when there is surplus heat energy is available.

20. The process according to claim 1, wherein step d) includes conditioning the intermediate stream by adjusting the oxidation potential of the liquid phase by adding a oxidant so as to oxidise either one or a combination of sulphur containing compounds and or nitrogen containing compounds.

21. The process according to claim 20, wherein up to 20% wt of the intermediate stream is conditioned in step d).

22. The process according to claim 20, wherein up to 5% wt of the intermediate stream is conditioned in step d).

23. The process according to claim 1, wherein step d) includes recovering the sulphur containing compounds by precipitation and recovering the nitrogen containing compounds by sorption.

24. The process according to claim 1, wherein both sulphur containing compounds and nitrogen containing compounds are recovered concurrently in a combined sorption step, or alternatively separately, in which sulphur containing compounds are sorbed in one sorption step and recovered, and nitrogen containing compounds are sorbed in a second sorption step and recovered.

25. The process according to claim 1, wherein the overall loading of carbon dioxide either in loaded stream or in the precipitant is up to 0.75 moles of carbon dioxide per mole of alkali carbonate.

26. The process according to claim 1, wherein the overall loading of carbon dioxide ranges from 0.30 to 0.70 moles of carbon dioxide per mole of alkali carbonate in the loaded stream.

27. The process according to claim 1, wherein the gas stream is a low pressure gas stream and of high temperature such that cooling is required prior to step a) and said cooling is done by a direct contact cooler with re-circulating water.

28. The process according to claim 27, wherein the re-circulating water stream is dosed with a potassium compound as makeup such that sulphur and/or nitrogen compounds are removed in a purge stream as potassium salts.

29. The process according to claim 28, wherein the potassium compounds are removed from the purge stream by any of a number of thermal and/or physical processes to produce concentrated potassium by-products.

30. The process according to claim 1, wherein regeneration of the bicarbonate in step c) is carried out at a pressure ranging from 30 to 1100 kPa absolute.

31. The process according to claim 1, wherein regeneration of the bicarbonate in step c) is carried out at a temperature ranging from 70 to 270° C.

32. The process according to claim 1, wherein the process includes utilising the sulphur containing compounds and/or the nitrogen containing compounds recovered in step d) to produce an agricultural fertilizer.

33. A process for removing acid gases from a gas stream, the process includes the steps of: a) contacting the gas stream with a solvent solution stream containing alkali carbonate at a concentration ranging from 30 to 60 wt % to absorb acid gases including carbon dioxide and either one or both of sulphur containing compounds or nitrogen containing compounds to form a loaded solvent stream including alkali bicarbonate and sulphur and/or nitrogen containing compounds and a second gas stream that is lean in acid gases, wherein step a) is carried out in at least one contactor, and includes precipitating the alkali bicarbonate to form a precipitant in the contactor so that the loaded stream includes the precipitant and a liquid phase including the solvent solution, and wherein step a) is carried out in the at least one contactor at a pressure ranging from 100 to 300 kPa; b) controlling the temperature at which step a) occurs in the contactor in situ or by discharging a side stream of the loaded stream from the at least one contactor, cooling the side stream and returning the cooled side stream to the same contactor, such that step a) is carried out in the at least one contactor at an isothermal temperature in the temperature range of 40 to 95° C, such that the heat of absorption of carbon dioxide and the heat of precipitation of bicarbonate do not affect the amount of acid gas absorption due to temperature effects on mass transfer, and wherein the isothermal temperature is selected based on the concentration of the alkali carbonate in the solvent stream and the required acid gas absorption; c) heating the loaded stream so as to regenerate alkali bicarbonate in the precipitant and the liquid phase and form i) a regenerated stream containing alkali carbonate, ii) a gas stream that is rich in carbon dioxide and iii) an intermediate stream that is lean in bicarbonate and contains sulphur and/or nitrogen compounds in solution; d) recovering from the intermediate stream, that is rich in alkali carbonate, either one or both of sulphur containing compounds or nitrogen containing compounds to form a lean stream, in which the alkali sulphate and nitrate content of the intermediate stream is in the range of 0.6 to 0.8% weight, and wherein step d) includes recovering either one or a combination of sulphur containing compounds or nitrogen containing compounds by selective sorption using at least one ion exchange resin; and e) recycling a recycle stream that includes the regenerated stream of alkali carbonate and the lean stream for reuse in the solvent solution of step a, in which the recycle stream is lean in bicarbonate and has a carbon dioxide loading in the range of 0.2 to 0.3; wherein step c) includes heating the alkali bicarbonate using an auxiliary heating source that is separate from power plant operations, the auxiliary heating source involving the combustion of fossil fuels, and flue gas produced by the auxiliary heating source forms a part of the gas stream contacted with the solvent in step a); and wherein the solvent solution is without a promoter, activator or catalyst, wherein step c) includes transferring heat from the regenerated stream to the loaded stream wherein the process includes storing the bicarbonate in precipitant form during periods of high demand for electrical energy and regenerating stored bicarbonate according to step b) by using the surplus heat from a power plant during period of lower demand for electrical energy, and wherein the solvent solution stream is without a promoter, activator or catalyst to enhance absorption of acid gases from the gas stream.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will now be described with reference to the Figures, of which:

(2) FIG. 1 is a flow diagram of a process and plant for removing acid gases from a gas stream in which alkali bicarbonate is precipitated prior to regeneration according to an embodiment;

(3) FIG. 2 is a flow diagram of a process and plant for removing acid gases from a gas stream in which alkali bicarbonate is essentially retained in solution for regeneration according to an alternative embodiment;

(4) FIG. 3 is flow diagram of a process and plant for removing acid gases from a gas stream in which the gas stream is conveyed successively through three contactor stages and three solvent sub-streams are fed in parallel and discharged from the contactor stages according to a preferred embodiment;

(5) FIG. 4 is a block diagram of the steps for removing impurities such as sulphur and nitrogen containing compounds; and

(6) FIG. 5 comprises sample data including thermodynamic, flowrate and composition data for the process and plant shown in FIG. 3 for recovery of the acid gases from a post combustion gas stream using an alkali carbonate absorbent without a promoter or catalyst, the sample data has been generated by a computer package known as ASPEN which simulates chemical processes.

(7) A description of Tables 1 to 6 is included in the text under the heading DETAILED DESCRIPTION.

DETAILED DESCRIPTION

(8) The present invention is suitable for removing acid gases from gas streams of any scale, but is particularly suited for removing acid gases from large scale gas streams such as and without limitation, gas streams of fossil fuelled fired power stations such as post-combustion gas streams, cement plants, fossil fuel powered processing facilities including pre-combustion gas stream, gas streams of natural gas separating plants and iron smelting plants. In the case of a coal fired power plant, a flue gas can be in the order of 1250 ton per day (TPD).

(9) With reference to FIGS. 1 and 2, a post combustion gas stream 20 containing carbon dioxide, SO.sub.x and NO.sub.x is fed into a contactor or absorber vessel 1 and contacted with a alkali carbonate solvent solution 30 such as potassium carbonate. The precise composition of the gas stream will vary from application to application, and in the case of a coal fired power plant, flue gas stream may have an acid composition in the order of 13% CO.sub.2 227 ppm SO.sub.2, 42 ppm SO.sub.3, and 450 ppm NO.sub.x on a dry basis. Essentially all SO.sub.2 and SO.sub.3 will be absorbed by the solvent, only 10% of NO.sub.x is present as NO.sub.2, of which 30% is absorbed, and the remainder is NO which is unreactive. Based on this type of gas stream composition, approximately 400 TPD of CO.sub.2 and approximately 1200 kg/day of K.sub.2SO.sub.4 captured, and 50 kg/day of KNO.sub.3 will be produced.

(10) The contactor 1 may comprise any form of internal structures including trays and/or packing or open space to facilitate spray contact using sprays or foam matrix contacting methods and devices to maximise contact between the solvent solution and the gas stream.

(11) In the case of the embodiment shown in FIG. 1, the contactor 1 is operated with a solid phase including KHCO.sub.3 forming in the early stages in the K.sub.2CO.sub.3 solvent. This will have the effect of reducing the backpressure of CO.sub.2 from the gas stream vented. One of the key points is that significantly higher loadings, and hence CO.sub.2 holding capacity can be achieved in the solvent, and less energy is required to liberate CO.sub.2 from solid phase including KHCO.sub.3.

(12) In contrast, in the case of the embodiment shown in FIG. 2, the contactor 1 will be operated substantially without a solid phase, and in which case KHCO.sub.3 is substantially retained in solution throughout the process.

(13) When the solvent solution has a K.sub.2CO.sub.3 concentration of 30 wt %, the required loading to begin KHCO.sub.3 precipitation is 0.65 at 50° C. At these conditions the equilibrium partial pressure of CO.sub.2 is roughly 13 kPa. The partial pressure of CO.sub.2 in the flue gas is roughly 13 kPa.

(14) The condition of 30 wt % and 50° C. can be thought of as the limit of feasible operation for the acid gas composition mentioned above. In order to move into steady state operating conditions there are a few adjustable operating parameters:

(15) Increasing the wt % of alkali carbonate in the solvent solution (eg K.sub.2CO.sub.3) has the following consequences: Decreases the loading at which KHCO.sub.3 precipitation occurs. Increases CO.sub.2 holding capacity.

(16) Decreasing the temperature of the alkali carbonate solvent solution has the following consequences: Decreases the loading at which precipitation occurs Decreases the equilibrium partial pressure of CO.sub.2 Decreases absorption reaction kinetics.

(17) Increasing the wt % is the preferred option, as hydrodynamic issues associated with high concentrations (and more with impurities) may be encountered in a slurry based process. The decrease in precipitation loading is significant and advantageous when looking at the driving force between equilibrium pCO.sub.2 and flue gas pCO.sub.2.

(18) In contrast, decreasing absorber temperature below 50° C. may compromise absorption kinetics.

(19) Table 1 below summarizes the back pressure of CO.sub.2 over a given wt % of K.sub.2CO.sub.3 solution at saturation.

(20) TABLE-US-00001 TABLE 1 Overall Liquid Phase K.sub.2CO.sub.3 Solid K.sub.2CO.sub.3 pCO2 Loading Wt % (%) Loading Wt % (kPa) 0.4 40 4.0 0.35 38.4 1.94 0.5 40 8.3 0.41 36.4 2.96 0.6 40 12.3 0.48 34.4 4.75 0.7 40 16.3 0.57 32.3 8.25 0.75 40 18.1 0.62 31.1 11.33 0.8 40 20 0.68 30.0 16.35

(21) In the case when the contactor 1 is operated at 50° C. and the solvent has a 40 wt % at the inlet, CO.sub.2 is absorbed into the solution and surpasses a loading of 0.3, at which point the solution becomes supersaturated with respect to potassium bicarbonate. Assuming solid liquid equilibrium is reached in the contactor 1, Table 2 below provides resulting compositions at overall CO.sub.2 loadings (i.e., bound as either solid or liquid).

(22) TABLE-US-00002 TABLE 2 Temperature K.sub.2CO.sub.3 Loading Eq-pCO.sub.2 Driving (° C.) Wt % limit (kPa) force (kPa) 40 30 0.53 4 9 35 0.35 1 12 40 0.21 0.5 12.5 50 0.09 0.05 12.95 50 30 0.65 13 0 35 0.45 4 9 40 0.31 1.5 11.5 50 0.13 0.2 12.8 60 35 0.56 12.5 0.5 40 0.40 4 9 50 0.19 0.7 12.3

(23) The maximum overall loading for a 40% wt solution is in the region of 0.75 for the given operating conditions. At an overall loading of 0.8 the resulting liquid phase generates a partial pressure of CO.sub.2 which is above pCO.sub.2 of the inlet flue gas.

(24) With reference to FIG. 1, the loaded stream 21 formed at the outlet of the contactor 1 may have any overall loading, but is suitably 0.75. The stream may be in the form of a slurry including bicarbonate solids or a solution without solids. Ideally at least some bicarbonate has begun to precipitate in the loaded stream 21.

(25) The liquid phase of the lean stream 30 would have a pCO.sub.2˜2 kPa. Accordingly, it is envisaged that the lowest pCO.sub.2 achievable in the outlet to the absorber flue gas stream is also ˜2 kPa. This would limit the CO.sub.2 absorption recovery to approximately 85% (in this example).

(26) Precipitation of bicarbonate may occur entirely, partially or not at all in the contactor/absorber 1. If desired, the loaded stream 21 discharged from the contactor 1 may be treated to form a bicarbonate precipitant. As shown in FIG. 1, the treatment may involve cooling the loaded stream in a cooler, such as HE1, or adding crystallisation crystals in a crystallizer 8. Cooling/crystallizing of the slurry prior to solid/liquid separation reduces the loading of the resulting lean solvent stream that is fed back to the absorber 1, and increases the efficiency of CO.sub.2 removal. If the rich solvent loading were decreased by controlling flow rates etc, the loading of the resulting lean solvent also decreases slightly, though not as significantly as reducing the crystallizer temperature.

(27) The slurry is then fed to a first solid/liquid separator 2, in which the solid phase 22 including KHCO.sub.3 is separated from the liquid phase 23. The liquid phase 23 includes sulphur and/or nitrogen containing compounds.

(28) The solid phase 22 is fed to a heat exchanger HE4 in which heat is transferred from a regenerated stream 24 that is discharged from a regenerator 3 to the solid phase 22. The solid phase 22 is discharged from the heat exchanger HE4 at a preheated temperature and fed into the regenerator 3. The solid phase is a further heated in the regenerator, schematically depicted by heating means or reboiler HE3 for volatilising CO.sub.2 from the solid phase and converting alkali bicarbonate back to alkali carbonate and water to form the regenerated stream 24. A stream rich in CO.sub.2 35 is discharged from the regenerator 3 and cooler in recuperative heat exchanger HE5 for storage or utilised as desired.

(29) Set out below in Table 3 are results that show a typical relationship between temperature of the solvent solution and the overall CO.sub.2 separation efficiency. The temperature of the solvent solution can be reduced by means of heat exchanger HE1 in FIG. 1

(30) TABLE-US-00003 TABLE 3 Effect of rich solvent loading and crystallizer temperature for a 40% wt solution. Rich loaded CO.sub.2 loading Percentage stream Crystallizer of lean stream Lean solvent CO.sub.2 at inlet to temperature fed to backpressure removal regenerator 3 (° C.) contactor 1 (CO.sub.2, kPa) efficiency 0.5 50 0.348 1.7 87 0.6 (no cooling) 0.371 1.73 87 0.7 0.399 1.94 85 0.75 0.41 2 85 40 0.343 1.7 87 25 0.252 1 92

(31) At a temperature of 50° C., the aqueous solubility of K.sub.2SO.sub.3 is 53 wt % and KNO.sub.2 is 78 wt %, whereas the aqueous solubility limit of KHCO.sub.3 is 35 wt %, KNO.sub.3 is 46 wt % and K.sub.2CO.sub.3 is 55 wt %. This indicates that the -ite impurity forms are highly soluble and less likely to come out of solution with the bicarbonate.

(32) The liquid phase 23 from the first solid/liquid separator 2 is combined with the regenerated stream 24 of alkali carbonate at mixer 9 and an intermediate stream 25 is split from the combined streams. The intermediate stream 25 is treated in a precipitator to precipitate sulphur and/or nitrogen containing compounds. The remaining portion of the combined stream that is not treated for precipitator i.e., recycle stream 26, is suitably mixed with the lean stream 27 discharge from the precipitator in mixer 7 and recycled back to the absorber 1. Make up water, and/or solvent, 28 may also be added to the process and suitably to the absorber as feed after being suitably temperature controlled in HE6. The ratio at which the combined stream is split into the intermediate stream 25 and the recycled stream 26 will vary, but suitably at least 75% of the combination is split into the recycled stream 26.

(33) In the situation in which intermediate stream 25 of FIG. 1 constitutes approximately 10% of regenerated stream 24 and the process is operated at 40 wt % K.sub.2CO.sub.3 and with a CO.sub.2 loading of 0.25 and a temperature of 50° C. entering the contactor 1, the intermediate stream, may for example have the following composition.

(34) TABLE-US-00004 TABLE 4 for a 40 wt % K.sub.2CO.sub.3 solution Mass % H.sub.2O 56.5 K.sub.2CO.sub.3 28.9 KHCO.sub.3 14.0 K.sub.2SO.sub.4 0.6 KNO.sub.3 <0.1 (trace)

(35) Although not shown in FIG. 1, it is possible that an amount of bicarbonate in solid phase may be mixed with the intermediate stream 25 to provide conditions that further favour the precipitation of sulphur and/or nitrogen containing compounds.

(36) The intermediate stream is fed to a precipitator which includes an oxidizer vessel 4 having a feeding device for bubbling an oxidant such as air or any oxygen containing gas, or even ozone through the stream for oxidising sulphur to sulphate. The feeding device may be any manifold or sparging device. The oxidized intermediate stream 25 is fed to a crystalliser 6 including a cooler HE2 for cooling the stream to form a second slurry including sulphur and, optionally, a nitrogen containing precipitant. The precipitant is separated in solid/liquid separator 5 and the lean stream 27 discharged from the solid/liquid separator 5 is mixed in mixer 7 with the recycled stream 26.

(37) In the case of the embodiment shown in FIG. 2, the bicarbonate component of the loaded stream 21 is retained in liquid phase only and the bicarbonate component is removed by volatilization of carbon dioxide therefrom in regenerator 3. Heat exchanger HE4 transfers heat from regenerated stream 24 as shown, from recycle stream 26, to the loaded stream 21 prior to the regenerator 3. The resulting regenerated stream 24 is lean in bicarbonate and includes dissolved sulphur and/or nitrogen containing compounds. The regenerated stream 24 is split into an intermediate stream 25 and a recycled stream 26 in splitter 36. The intermediate stream 26 is treated in the precipitator as described above in relation FIG. 1 and the lean stream combined with the recycle portion

(38) The precipitator includes an oxidizing vessel 4 having a feeding device for bubbling an oxidant such as air or any oxygen containing gas, or even ozone through the stream for oxidising sulphur to sulphate. The oxidized intermediate stream is then fed to a crystalliser 6 including a cooler HE2 for cooling the stream to form a second slurry including sulphur and/or containing precipitant. The precipitant is separated from a liquid phase in solid/liquid separator 5 to form the lean liquid stream 27. A lean stream 27 discharged from the solid/liquid separator is mixed at mixer 7 with the recycled stream 26 of the combined streams not treated in the precipitator as described above and possibly any makeup. This stream, after being suitably temperature controlled in HE6, is fed to the contactor 1.

(39) In the situation in which intermediate stream 25 of FIG. 2 constitutes approximately 10% of regenerated stream 24 and the process is operated at 30 wt % K.sub.2CO.sub.3 and with a CO.sub.2 loading of 0.20 and a temperature of 25° C. entering the contactor 1, the intermediate stream, may for example have the following composition.

(40) TABLE-US-00005 TABLE 5 for a 30 wt % K.sub.2CO.sub.3 solution Mass % H.sub.2O 67.4 K.sub.2CO.sub.3 23.4 KHCO.sub.3 8.5 K.sub.2SO.sub.4 0.7 KNO.sub.3 <0.1 (trace)

(41) With reference to the embodiments shown in both FIGS. 1 and 2, when the weight of carbonate in solution is in the range of 30 to 75%, the amount of nitrogen containing compounds in solution is low.

(42) Control of nitrogen is possible in a variety of ways, including, but not limited to, purging from the recycled liquid (as in Stream 31), and other separation steps such as precipitation of nitrogen containing compounds in a third precipitator, ion exchange and membrane processes. To counter any loss arising from the purge stream or wetness of the sulphur containing solid phase, additional makeup solvent solution and or water may also be added on an as needed basis at any location in the flow sheet that benefits the operating performance.

(43) FIG. 3 illustrates a preferred embodiment of the present invention and includes bicarbonate precipitation. The preferred embodiment is characterised by three contactor stages in which the gas stream 1 is conveyed successively i.e., in series from contactor stage 1a to 1b, and from 1b to 1c. The lean solvent solution stream 35 is split in three sub-streams 36, 37 and 38 which may have equal or different flowrates. The sub-streams 36, 37 and 38 are fed in parallel to their respective contactor stages i.e., sub-stream 36 is fed to contactor stage 1a and loaded stream 5 is discharged, sub-stream 37 is fed to contactor stage 1b and loaded stream 10 is discharged, and sub stream 38 to contactor stage 1c from which loaded stream 16 is discharged. The number of the contactor stages may be can varied depending on a number variables such as feed gas stream and solvent flowrates, and acid gas composition. As described above under the heading, SUMMARY OF THE INVENTION, dividing the absorption stage from one stage as shown in FIGS. 1 and 2, into multiple stages, as shown in FIG. 3, improves the absorption of acid gases in solution by reducing heat effects and altering the solution to improve mass transfer performance. The contactor stages may contact the gas stream and solvent in counter current flow, co-current flow or a hybrid thereof.

(44) The solid fraction of loaded streams 5, 10 and 16 is 3 wt %, 6 wt % and 11 wt % respectively. The loaded streams are then cooled in coolers HE1a, HE1b and HE1c respectively, to reduce temperature and further crystallize alkali bicarbonate to form slurry streams 7, 12 and 18 respectively. The solid phase and liquid phase of the slurry streams 7, 12, and 18 are each separated in the solid/liquid separators 2a, 2b and 2c respectively. Streams 8, 13 and 19 discharged from the separators are lean in bicarbonate but contain dissolved sulphur and/or nitrogen containing compounds. The solid phases 9, 14 and 20 discharged from the solid/liquid separators 2a, 2b and 2c are fed to a heat exchanger HE4 and are heated therein by regenerated stream 29 discharged from the regenerator 3. Heat supplied by reboiler HE3 transforms alkali bicarbonate to alkali carbonate and produces gas stream 25 rich in carbon dioxide. Moisture in the gas stream may be condensed in condenser HE5 and mixed into the regenerated stream 28 via stream 27. The regenerated stream 29 transfers heat to the leaded stream 21 in heat exchanger HE4, and is the then mixed with lean streams 8, 13 and 19 discharged from the solid/liquid separators 2a, 2b and 2c to produce intermediate product stream 33. Prior to recycling the intermediate stream 33, a portion 40 of the intermediate stream may undergo an impurities recovery step when the gas stream 1 contains sulphur and nitrogen containing compounds.

(45) FIG. 4 illustrates is block diagram of some of the basic steps including i) oxidation to convert the sulphur and nitrogen compounds to an -ite or -ate form, ii) crystallisation of the sulphur compounds which are less soluble that the nitrogen compounds and iii) thereafter ion exchange recovery of the nitrogen compounds. Ion exchange could also be used for sulphur removal rather than precipitation.

(46) It will be appreciated ion exchange may be used to recover both sulphur and nitrogen containing compounds and this is shown in FIG. 4 as a dashed line showing the alternative route. As described above, some of the main benefits of the process and plant shown in FIGS. 3 and 4 include the following. Feeding fresh solvent sub-streams to multiple contactor stages and conveying the gas stream successively through the contactor stages minimises the impact of heat of absorption and heat of crystallisation. By reducing the temperature rise in each contactor, the partial pressure of carbon dioxide of the solvent solution is reduced which maintains the driving force for absorption. Dedicated coolers and solid/liquid separators for each of the contactor stages enables greater acid gas separation efficiency to be achieved by both reducing liquid bi-carbonate levels without feeding to the regenerator and in feeding less water to the regenerator which lowers energy usage, and if used, less thermally sensitive promoter or catalyst may be fed to the regenerator resulting in lower degradation rates. The heating source for the regenerator may be an external heating source, and suitably is a dedicated boiler that does not reduce the power generation capacity of power station that produces the gas stream. Moreover, the combustion products produced by the boiler may be combined with the gas stream feed to the contactor stages.

(47) According to our simulations using ASPEN, the energy usage of the boiler reduces as the degree of bicarbonate precipitation increases. FIG. 2, is an example without precipitation, and the boiler for the regenerator has the highest energy load FIG. 1 includes some precipitation followed by FIG. 3 which has the highest degree of precipitation. Table 6 below provides a summary of the energy used for the respective reboilers in the same contacting area.

(48) TABLE-US-00006 TABLE 6 Energy usage for reboiler (MJ/kg of CO2 captured in Case Description concentrated gas stream) 1 Process of FIG. 2 4.88 without promoter 2 Process of FIG. 1 3.68 without promoter 3 Process of FIG. 3 3.57 without promoter

(49) It will be understood to persons skilled in the art of the invention that many modifications may be made without departing from the spirit and scope of the invention.