Process and system for pretreating gaseous effluent for post-combustion CO2 capture

Abstract

The present invention concerns the field of capturing the CO.sub.2 from a gaseous effluent. The incoming gaseous effluent is burned with a fuel, so as to obtain a hot gaseous effluent rich in acidic compounds, and the hot gaseous effluent rich in acidic compounds is cooled to give a cold effluent rich in acidic compounds, which is subsequently used in the step of contacting with an absorbent solution rich in acidic compounds.

Claims

1. Process for separating at least one acidic gas contained in an incoming gaseous effluent, comprising at least the following steps: contacting, in a separation unit, a cold gaseous effluent rich in acidic compounds with an absorbent solution low in acidic compounds so as to obtain a gaseous effluent low in acidic compounds and an absorbent solution rich in acidic compounds, regenerating at least a fraction of the absorbent solution rich in acidic compounds in a regeneration column so as to obtain an absorbent solution low in acidic compounds and an effluent rich in acidic compounds, the absorbent solution low in acidic compounds being used in the step of contacting with the cold effluent rich in acidic compounds, burning, in a combustion device, the incoming gaseous effluent with a fuel, so as to obtain a hot gaseous effluent rich in acidic compounds, cooling the hot gaseous effluent rich in acidic compounds in an exchanger to give the cold effluent rich in acidic compounds used in the step of contacting with an absorbent solution low in acidic compounds, introducing a fluid in high-pressure liquid form into the exchanger, from which a fluid in high-pressure vapour form is withdrawn, and using the fluid in high-pressure vapour form as an energy source for operating the regeneration column by introducing the fluid in high-pressure vapour form into the regeneration column.

2. Process according to claim 1, in which the quality of the hot effluent rich in acidic compounds is controlled with the addition, to the combustion device, of at least one supplementary oxidizer.

3. Process according to claim 2, in which the at least one supplementary oxidizer is dioxygen O.sub.2.

4. Process according to claim 1, in which the step of using the fluid in high-pressure vapour form as an energy source for operating the regeneration column comprises introducing the fluid in high-pressure vapour form into a reboiler of the regeneration column.

5. Process according to claim 4, in which the fluid in high-pressure vapour form has a condensation point 10° C. to 20° C. above an operating temperature of the reboiler.

6. Process according to claim 1, in which the quality of the operation of the regeneration column is controlled with the addition of a complementary fluid in supplementary high-pressure vapour form.

7. Process according to claim 1, which also comprises a step of reheating the incoming gaseous effluent before introducing it, in hot incoming gaseous effluent form, into the combustion device.

8. Process according to claim 7, in which the incoming gaseous effluent is reheated by introducing it into the exchanger, from which it is taken out in hot incoming gaseous effluent form.

9. Process according to claim 1, in which the incoming gaseous effluent is a flue gas from a previous combustion.

10. Process according to claim 1, in which the effluent rich in acidic compounds is stored underground.

11. Process according to claim 1, in which the fuel is a discard from waste or from biomass.

12. Process according to claim 1, in which the at least one acidic gas comprises at least one of the compounds including carbon dioxide CO.sub.2.

Description

LIST OF FIGURES

(1) FIG. 1 illustrates a combustion scheme according to the prior art.

(2) FIG. 2 illustrates a combustion scheme according to a first embodiment of the invention.

(3) FIG. 3 illustrates a combustion scheme according to a second embodiment of the invention.

DESCRIPTION OF THE EMBODIMENTS

(4) FIG. 1 presents a scheme of a process for separating at least one acidic gas contained in a gaseous effluent according to the prior art. Consideration is given to an incoming gaseous effluent from industrial combustion (for example flue gas, industrial gas, etc.) (100) which may contain oxygen in the form of dioxygen (O.sub.2). O.sub.2 may be present in the flue or industrial gas because during industrial combustion it may be in excess in order to ensure both the completion of the industrial combustion and the control of the temperature. The oxygen may also come from leakage of ambient air into the flue gas line, insofar as the combustion devices are generally operated at reduced pressure for safety reasons relative to their environment. This combustion effluent may come from the combustion of fossil, synthetic or natural carbonaceous matter (converted or not). This combustion effluent hence typically contains a substantial amount of acidic compounds (particularly H.sub.2S, mercaptans, CO.sub.2, COS, SO.sub.2, CS.sub.2), for example carbon dioxide (CO.sub.2) from the combustion. The description hereinafter addresses only the carbon dioxide instance, though the process is applicable to all types of acidic compounds (for example H.sub.2S, mercaptans, CO.sub.2, COS, SO.sub.2, CS.sub.2). This CO.sub.2 is absorbed in the separation unit (1001), which is also called an absorber. The effluent low in acidic compounds (200) from the separation unit (1001) is a flue gas whose CO.sub.2 content will have been substantially lowered. The separation unit (1001) provides a CO.sub.2 separation function by adsorption or by absorption.

(5) In the case of CO.sub.2 separation by means of an amine-based solvent, this absorber is a gas/liquid contactor which contacts the incoming gaseous effluent for treatment (100) with a liquid absorbent solution low in acidic compounds (501), also termed an amine solution. This amine solution becomes charged with CO.sub.2 within the separation unit (1001).

(6) The resulting solution is an absorbent solution rich in acidic compounds (500) and rich in CO.sub.2, which is sent to a regeneration column (1002), also termed regenerator. The function of the regenerator is to extract the CO.sub.2 contained in the effluent rich in acidic compounds (500), to form an exit gas (700) comprising the CO.sub.2 extracted. This regeneration may be obtained by thermal effect and lowering of the CO.sub.2 partial pressure in the gas phase at equilibrium with the amine solution in the regenerator. The LP (low-pressure) or MP (medium-pressure) hot utility (400) may be used to carry out the regeneration, for example by means of a reboiler (not shown). The regenerated amine solution (501) is called low-grade amine. It is sent to the absorber (1001) for capturing the CO.sub.2.

(7) The invention lies in a process for separating at least one acidic gas contained in an incoming effluent (100), comprising at least the following steps: contacting, in a separation unit (1001), a cold gaseous effluent rich in acidic compounds (120) with an absorbent solution low in acidic compounds (501) so as to obtain a gaseous effluent low in acidic compounds (200) and an absorbent solution rich in acidic compounds (500), regenerating at least a fraction of the absorbent solution rich in acidic compounds (500) in a regeneration column (1002) so as to obtain an absorbent solution low in acidic compounds (501) and an effluent rich in acidic compounds (700), said absorbent solution low in acidic compounds (501) being used in the step of contacting with said cold effluent rich in acidic compounds (120), burning, in a combustion device (2000), the incoming effluent (100) with a fuel (300), so as to obtain a hot gaseous effluent rich in acidic compounds (110), and cooling the hot gaseous effluent rich in acidic compounds (110) in an exchanger (3000) to give the cold effluent rich in acidic compounds (120) used in the step of contacting with an absorbent solution low in acidic compounds (501).

(8) The steps of contacting in the separation unit (1001) and of regeneration may conform to the prior-art process, particularly as described in relation to FIG. 1. The steps of burning and of cooling of the hot gaseous effluent are steps of deoxygenating the incoming gaseous effluent (100).

(9) FIG. 2 provides a non-limiting description of the process in a first embodiment of the invention, including a number of embodiment options. These embodiment options may be contemplated independently of one another. This figure shows an incoming gaseous effluent (100), also termed combustion flue gas, which in this case is treated in the combustion device (2000). The combustion device (2000) has a dual function. The first function of this combustion device (2000) is to lower the O.sub.2 content of the flue gas (100) so as to reach a value compatible with the operation of the amine selected for operating the separation unit (1001) and the regeneration column (1002). The second function of this combustion device (2000) is to increase the energy content of the combustion flue gas (100) by a substantial increase in its temperature. These two functions are ensured by the burning of a fuel or of a fuel mixture (300). This carbonaceous feed may be the same as the feed whose industrial combustion led to the make-up of the incoming gaseous effluent (100), or a different feed.

(10) In one embodiment of the invention, the fuel (300) may be a discard from wastes or from biomass. Thus, in a context of reducing the environmental footprint, this feed may be, for example and non-limitingly, an industrial waste or a reject from a center for recovery of value from wastes or from biomass such as forestry residues, agricultural by-products or energy crops. In this latter instance of biomass, the downstream capture of CO.sub.2 results in the negative emissions being accounted for, and is particularly favorable in terms of the balance of greenhouse gas emissions. Another oxidizer which may have a positive effect on the emissions is hydrogen. This is because it is a high-energy-value oxidizer and its combustion product, water, can be directly utilized and separated simply from the flue gases. This is, moreover, an advantageous pathway to hybridization of processes between fuel and electricity, as the latter can be easily exploited to give hydrogen by means of devices such as electrolysers. An advantage offered by this hybridization is its flexibility according to the amount of decarbonized electricity available in the networks, in excess or not.

(11) The result of this combustion is a hot effluent rich in acidic compounds (110) with an oxygen content which can be brought below the threshold recommended by the supplier of amine-based solvent, i.e. generally below 10% on a dry basis, and preferably below 6% on a dry basis.

(12) If needed, as for example in the case of a lack of oxidizer that means the energy supply required for regeneration cannot be achieved through combustion, it is possible in one embodiment to employ supplementary air (600). In one embodiment, indeed, the quality of the hot effluent rich in acidic compounds (110) may be controlled with the addition, to the combustion device (2000), of at least one supplementary oxidizer (600). In one embodiment of the invention, said at least one supplementary oxidizer (600) may be dioxygen O.sub.2.

(13) This effluent rich in acidic compounds (110) is cooled in a heat exchange device (3000) so as to produce a high-pressure steam fluid (410), also called hot utility, at sufficient temperature and in sufficient amount to regenerate the amine containing the captured CO.sub.2 from an absorbent solution rich in acidic compounds (500), starting from the fluid in high-pressure liquid form (800). The reason is that, in one embodiment of the invention, the process may also comprise a step of introducing a fluid in high-pressure liquid form (800) into the exchanger (3000), from which a fluid in high-pressure vapor form (410) is withdrawn, which can also be referred to as fluid in pressurized vapor form. In other words, within the exchanger (3000), the fluid in high-pressure liquid form (800) recovers heat energy from the hot effluent rich in acidic compounds (110) to form a hot utility (410) and an effluent rich in acidic compounds (120), which exits it in cooled form.

(14) In one embodiment of the invention, said fluid in high-pressure vapor form (410) may be used as an energy source for operating the regeneration column (1002). The use of the fluid in high-pressure vapor form (410) as an energy source for operating the regeneration column (1002) is particularly useful in the sense that condensing this vapor in the reboiler is an effective way of transmitting heat to the process. As a non-limiting example, said fluid in high-pressure vapor form (410) may be a saturated vapor with a defined condensation point of the order of 10 to 20° C. above the operating temperature of the reboiler intended for regenerating the solution of amines. Thus in the case of a 30 wt % solution of MEA in water, operation may typically take place at 120° C., corresponding to supply of a vapor with a condensation point of 140° C. An air supplement (600) may be used (insofar as is acceptable given the capacity of the unit in terms of flow rate and temperature, especially when an existing plant is adapted to add a CO.sub.2 capture device) to increase the amount of fuel which can be burned to produce the hot effluent rich in acidic compounds (110) so as to increase the energy content of the high-pressure vapor fluid (410).

(15) In one embodiment, this energy provision may also be made up with an external provision of energy in the form of a supplementary high-pressure vapor fluid (401). In one embodiment of the invention, indeed, the quality of the operation of the regeneration column (1002) may be controlled with the addition of a complementary fluid in supplementary high-pressure vapor form (401).

(16) The reason is that the oxygen content of the incoming gaseous effluent (100) is not always enough to bring about the combustion of a sufficient amount of fuel to regenerate the amount of solvent corresponding to the entirety of the CO.sub.2 it is desired to capture. As an example, this is typically the case for an MEA solvent for flue gases containing more than 20% of CO.sub.2 and less than 5% of O.sub.2 on a dry basis. This supplement in the form of the supplementary high-pressure vapor fluid (401) may then be predominant without any loss of the benefit of an energy contribution of the utility in the form of the high-pressure vapor fluid (410). Generally speaking, effort is made, when possible, to harmonize to the greatest degree possible the systems for producing hot utilities in the form of the high-pressure vapor fluid (401) and in the form of the high-pressure vapor fluid (410) in such a way as to optimize the energy balance of the whole system; this may, for example, mean a partial reheating of the fluids used to produce the high-pressure vapor fluid (401) on the pathway of the flue gases between the incoming gaseous effluent (100) and the effluent low in acidic compounds (200) and especially in the heat exchange device (3000) (not shown). In the exchanger (3000), the first function, via exchange of heat with the reheated flow in the form of the hot effluent rich in acidic compounds (110), is to produce the hot utility in the form of the high-pressure vapor fluid (410). The exchanger (3000) may also contain supplementary heat exchangers for producing a flow in the form of the cold effluent rich in acidic compounds (120), which has a temperature compatible with optimal operation of the selected amine, typically below 50° C. for a 30 wt % solution of MEA amine in water.

(17) In one embodiment of the invention, the process may also comprise a step of reheating the incoming gaseous effluent (100) before introducing it, in hot incoming effluent (101) form, into the combustion device (2000).

(18) The incoming gaseous effluent (100) may preferably be reheated by introducing it into an exchanger (3000), from which it is taken out in hot incoming effluent (101) form.

(19) FIG. 3 represents a second embodiment of the invention, which contains an additional step relative to the embodiment of FIG. 2, of reheating the CO.sub.2-rich effluent, the incoming gaseous effluent (100), in a heat exchanger (3000) so as to produce the flow in the form of the hot incoming gaseous effluent (101), the temperature of which is selected to facilitate the ignition of the fuel (300) on contact with the oxygen contained in the hot incoming gaseous effluent (101). Typically, for a known partial pressure of oxygen and a known concentration of a specific fuel, it is possible to define an autoignition point, which can be an element enabling a definition of the minimum temperature the flow of the hot incoming gaseous effluent (101) must reach for attaining complete combustion of the fuel (300). This minimum temperature may also be lowered by using devices such as catalytic burners. The preheating of the incoming gaseous effluent (100) to make hot incoming gaseous effluent (101) may be performed by exchange of heat with a dedicated hot utility (not shown), by thermal integration with the device for producing the supplementary hot utility in the form of supplementary high-pressure vapor fluid (401) where appropriate (not shown) or by integration in effluent charge around the combustion device (2000) such as a dedicated heat exchange zone of the heat exchange device (3000) as shown in FIG. 3. For this latter variant embodiment, within the exchanger (3000), the fluid in high-pressure liquid form (800) and also the incoming gaseous effluent recover heat energy from the hot effluent rich in acidic compounds (110) to form a hot utility (410), a hot incoming gaseous effluent (101) and a cold effluent rich in acidic compounds (120). In other words, the hot gaseous effluent rich in acidic compounds (110) exiting the combustion device may be used for reheating the incoming gaseous effluent (100).

(20) In one embodiment of the invention, the incoming gaseous effluent (100) may be a flue gas from a previous combustion, obtained in particular from an industrial process.

(21) In one embodiment of the invention, the effluent rich in acidic compounds (700) may be stored underground, more particularly in oilfields.

(22) In one embodiment of the invention, said at least one acidic gas may comprise at least one of the compounds including carbon dioxide CO.sub.2.

(23) In one embodiment of the invention, the absorbent solution rich in acidic compounds (500) and the absorbent solution low in acidic compounds (501) are two states of the same solvent, comprising an amine solution with reactive compounds in aqueous solution, said two solutions containing greater or lesser amounts of acidic compounds.

(24) The invention also lies in a system for separating at least one acidic gas contained in an incoming gaseous effluent (100) suitable for implementing the process according to any one of the variants or combinations of variants of the process described above.

(25) To illustrate the proposed solution, we refer to a process for absorbing CO.sub.2 by washing with amines. The performance qualities contemplated are those of the washing process using 40% weight % monoethanolamine (MEA) for capturing CO.sub.2 on a typical combustion flue gas from a coal-fired thermal power plant. These performance qualities were observed experimentally.

(26) For each of the cases presented, the characteristics of the flue gas stream for treatment are as follows:

(27) Conditions of the Incoming Gaseous Effluent (100)

(28) TABLE-US-00001 TABLE 1 Temperature ° C. 150 Pressure atm 1 Volume flow rate Nm.sup.3/h 250,000 on dry basis

(29) Reference case: CO.sub.2 capture in combustion flue gases according to the prior art, cf. FIG. 1.

(30) For simplification, the incoming gaseous effluent (100) is considered to be free of impurities such as SOx or NOx. Its oxygen content is about 5 vol % on a dry basis. The compositions of flue gases at the furnace exit are generally supplied on a dry basis. For the implementation of these examples, we shall consider hydrated flue gases containing 0.043 kg H.sub.2O/kg dry gas (value retrieved from a typical combustion flue gas from a coal-fired thermal power plant). Its composition is as follows:

(31) Flue Gas Composition, 5 vol % of O.sub.2

(32) TABLE-US-00002 TABLE 2 dry basis wet basis vol % Flow rate Flow rate Flow rate Compound dry gas (Nm3/h) (kg/h) (kg/h) wt % CO2 14.12 3.53E+04 7.06E+04 7.06E+04 32.30 N2 80.75 2.02E+05 1.28E+05 1.28E+05 58.78 O2 5.13 1.28E+04 1.05E+04 1.05E+04 4.80 H20 0 0 0 9.01E+03 4.12 Total 100 2.50E+05 2.10E+05 2.19E+05 100

(33) The CO.sub.2 absorber operating by washing with 40 wt % MEA within the separation unit (1001) produces an effluent low in acidic compounds (200), containing 90 wt % less CO.sub.2. Its composition is presented in the table below:

(34) Flue Gas Composition After CO.sub.2 Capture with 40 wt % MEA

(35) TABLE-US-00003 TABLE 3 Composition of treated flue gas Flow rate Compound (kg/h) wt % CO.sub.2 7.06E+03 4.55 N.sub.2 1.28E+05 82.87 O.sub.2 1.05E+04 6.77 H.sub.2O 9.01E+03 5.81 Total 1.55E+05 100

(36) The regeneration column (1002) requires a regeneration energy of 3.02 GJ/t.sub.CO2 captured. In FIG. 1 the vapor stream of the hot utility (400) returns to the regeneration column (1002). This base case as known in the prior art does not specify how the vapor is formed, but sets out the amount of vapor needed to capture 90% of the CO2 in the incoming gaseous effluent. The flow rate of vapor required as hot utility (400) for reboiling the solvent to capture 90 wt % of CO.sub.2 by capture with 40 wt % MEA is indicated in the table below:

(37) Characteristics of the CO.sub.2 Capture with 40 wt % MEA

(38) TABLE-US-00004 TABLE 4 Flow rate of kg/h 2.19E+05 incoming gaseous effluent (100) of which CO.sub.2 flow rate kg/h 7.06E+04 Flow rate of CO.sub.2 captured kg/h 6.36E+04 (90 wt %) Energy needed for regeneration GJ/t.sub.CO2captured 3.02 Energy needed for regeneration kJ/h 1.92E+08 Flow rate of hot utility (400) kg/h 8.86E+04

(39) Case according to one embodiment of the invention: control of the oxygen content of an incoming gaseous effluent (100) containing CO.sub.2 and supply of energy, with preheating of the effluent for treatment, in accordance with the embodiment of FIG. 3.

(40) The performance characteristics of the device are compared over 3 cases with an incoming gaseous effluent (100) whose oxygen content on a dry basis is 5 vol % (same composition as the flue gas in the reference case), 10 vol % and 15 vol %. The flue gas compositions considered are as follows:

(41) Flue Gas Composition—3O.sub.2 Contents Tested

(42) TABLE-US-00005 TABLE 5 Case with 5 Case with 10 Case with 15 vol % of O.sub.2 vol % of O.sub.2 vol % of O.sub.2 Flow rate wt % Flow rate wt % Flow rate wt % Compound (kg/h) (wet) (kg/h) (wet) (kg/h) (wet) CO.sub.2 7.06E+04 32.30 7.06E+04 31.96 7.06E+04 31.64 N.sub.2 1.28E+05 58.78 1.21E+05 54.65 1.13E+05 50.53 O.sub.2 1.05E+04 4.80 2.05E+04 9.26 3.07E+04 13.75 H.sub.2O 9.01E+03 4.12 9.11E+03 4.12 9.11E+03 4.08 Total 2.19E+05 100 2.21E+05 100 2.23E+05 100

(43) The fuel (300) used in the device is methane at 7 bar and 25° C. and has an LHV of 5.003.10.sup.4 kJ/kg. Complete combustion with excess oxygen proceeds until the oxygen content reaches a value of 2 vol % in the flue gas stream exiting the combustion device (2000). The target oxygen content is variable but the aim is to minimize it so as to obtain a flue gas stream whose oxygen content does not cause excessive breakdown of the amine in the separation unit (1001). The lower the oxygen content selected, the greater will be the energy recovered for steam production, but there is also an effect on the amount of fuel consumed. This value can therefore be optimized.

(44) The incoming gaseous effluent (100), which is initially at 150° C., is first heated to 650° C. in a heat exchange device (3000) using the hot effluent rich in acidic compounds (110) at the exit of the combustion device (2000) as hot fluid. The temperature of 650° C. is greater than the ignition temperature of the fuel (300), in this case methane, which is 540° C. This ensures combustion of the fuel with the flue gas for treatment. The temperatures observed for the various cases are presented in the table below. The greater the oxygen content of the incoming flue gas, the more combustion there will be in the device, hence a high exit temperature.

(45) Temperatures Around the Heat Exchange Device (3000)

(46) TABLE-US-00006 TABLE 6 Temperature Case Case Case with 5 with 10 with 15 Reference vol % vol % vol % Stream No. of O.sub.2 of O.sub.2 of O.sub.2 Flue gas Cold entry 100 150 150 150 preheat Cold exit 101 650 650 650 Hot entry 110 906.8 1286 1621 Hot exit Not shown 448 872.9 1243 Heat Hot entry Not shown 448 872.9 1243 recovery, Hot exit 120 150 150 150 vaporization Boiling 800 140 140 140 feed water 3.6 bar steam 410 140 140 140

(47) The flue gas stream exiting the combustion device (200) which has enabled preheating of the flue gas for treatment then makes it possible to produce steam by vaporization of boiling feed water at 140° C. and 3.6 bar (vapour conditions needed for the steam required to operate the regenerator with 40 wt % MEA solvent). This steam stream is then used as hot utility, high-pressure steam fluid (410) by the regeneration column (1002). The amount of steam produced is related to the amount of energy needed for regeneration of the MEA solvent, of 3.02 GJ/t CO.sub.2 captured, which is itself linked to the amount of CO.sub.2 it is possible to capture by absorption in the separation unit (1001). The performance values obtained in the 3 cases are presented in the following table:

(48) Device Performance by the 3 Cases

(49) TABLE-US-00007 TABLE 7 Case with 5 Case with 10 Case with 15 vol % of O.sub.2 vol % of O.sub.2 vol % of O.sub.2 Flow rate of steam kg/h 3.40E+04 9.08E+04 1.49E+05 (410) generated Available energy kJ/h 7.37E+07 1.97E+08 3.22E+08 Energy for GJ/t CO.sub.2 3.02 regeneration captured Flow rate of CO.sub.2 kg/h 2.44E+04 6.51E+04 1.07E+05 captured in the separation unit (1001) Total CO.sub.2 kg/h 7.47E+04 8.15E+04 8.84E+04 flow rate CO.sub.2 flow rate kg/h 2.03E+04 5.43E+04 8.88E+04 avoided Possible CO.sub.2 % 32.69% 79.94% 120.56% capture rate Level of CO.sub.2 % 28.80% 76.85% 125.74% avoided NB: the CO.sub.2 capture rate takes account of the further CO.sub.2 generated by combustion in the combustion device (2000). The amount of CO.sub.2 eventually emitted (after combustion device and CO.sub.2 capture unit) by comparison with the amount of CO.sub.2 which would initially have been emitted without any capture unit, when considered, leads to the concept of the level of CO.sub.2 avoided.

(50) The higher the oxygen content of the flue gas stream for treatment, the greater the amount of steam that will be produced, according to the invention, by heat recovery from the flue gases, and hence the greater the CO.sub.2 capture rate by virtue of the process according to the invention.