Integrated process for CO2 capture and use in thermal power production cycle

Abstract

A process for reducing the amount of CO.sub.2 released into the atmosphere with the exhaust gas stream produced by the combustion of a hydrocarbon fuel in an internal combustion engine (ICE) used to power a vehicle by capturing at least a portion of the CO.sub.2 in a liquid sorbent on board the vehicle, recovering the CO.sub.2 from the sorbent and compressing the CO.sub.2 for temporary storage on board the vehicle, where the process is operated as a semi-closed system in which the liquid sorbent that captures the CO.sub.2 serves as a working fluid and retains the CO.sub.2 during the power generation cycle to produce mechanical energy or work, after which the CO.sub.2 is desorbed for densification and recovery as an essentially pure gas stream and the working fluid is recycled for use in the process.

Claims

1. A process for reducing the amount of CO.sub.2 released into the atmosphere with the exhaust gas stream produced by the combustion of a hydrocarbon fuel in an internal combustion engine (ICE) used to power a vehicle by capturing at least a portion of the CO.sub.2 with a sorbent on board the vehicle, recovering the CO.sub.2 from the sorbent and compressing the CO.sub.2 for temporary storage on board the vehicle, the process characterized by a. passing the hot exhaust gas stream from the ICE through a plurality of heat exchangers in a first heat exchange zone to reduce the temperature of the exhaust gas stream to a value in a predetermined temperature range; b. contacting the cooled exhaust gas stream in an absorption zone with a liquid CO.sub.2 sorbent solution at a temperature within a predetermined temperature range, the solution comprising a liquid solvent in which is dissolved at least one compound that reversibly combines with CO.sub.2 to capture at least a portion of the CO.sub.2 from the exhaust gas stream to provide a CO.sub.2-rich solution; c. separating the CO.sub.2-rich solution from the remaining exhaust gas stream that is of reduced CO.sub.2 content; d. discharging the remaining exhaust gas stream of reduced CO.sub.2 content into the atmosphere; e. pressurizing the CO.sub.2-rich solution and passing it into a boiler for passage in a first heat exchange relation with the exhaust gas stream to raise its temperature to desorb the CO.sub.2 and provide a concentrated CO.sub.2-lean sorbent solution, and to vaporize a portion of the solvent from the sorbent solution to provide a vaporized solvent/CO.sub.2 mixture; f. separating the CO.sub.2-lean sorbent solution from the vaporized solvent/CO.sub.2 mixture in a first separation zone; g. passing the vaporized solvent/CO.sub.2 mixture to a superheating zone where it passes in a second heat exchange relation with the hot exhaust gas stream directly from the ICE to further increase the temperature of the mixture; h. passing the superheated solvent/CO.sub.2 mixture to a turbine and expanding the mixture to a predetermined pressure value; i. passing the hot expanded solvent/CO.sub.2 mixture in heat exchange with the pressurized CO.sub.2-rich solution of step (e); j. passing the solvent/CO.sub.2 mixture to a condensing heat exchanger to lower its temperature to condense substantially all of the solvent vapor to the liquid state; k. separating the condensed solvent from the CO.sub.2 in a second separation zone and mixing all or a portion of the condensed solvent with the sorbent solution upstream of the absorption zone or discharging the solvent from the vehicle; l. recovering the substantially pure CO.sub.2 from the second separation zone and passing it to a compression zone to compress and densify the CO.sub.2 and discharging any remaining water; m. recovering the pressurized pure CO.sub.2 and passing it to an on-board vessel for storage or for further processing to reduce its volume by changing its physical state; n. passing the pressurized CO.sub.2-lean solution from the first separation zone in heat exchange relation to increase the temperature of the pressurized CO.sub.2-rich solution from the absorption zone; o. introducing the pressurized CO.sub.2-lean solution into an expansion device to produce mechanical energy; p. passing the reduced-pressure concentrated CO.sub.2-lean solution from the expansion device to a mixing device through which solvent is added to restore the desired concentration of the sorbent solution; q. cooling the CO.sub.2-lean solution to the predetermined temperature range prior to passing it into the absorption zone; and r. pressurizing the CO.sub.2-lean sorbent solution upstream of the absorption zone.

2. The process of claim 1 in which the solvent is water.

3. The process of claim 2 in which the increase in the temperature of the water/CO.sub.2 mixture in step (g) is in the range of from 200 to 500 C.

4. The process of claim 1 in which the CO.sub.2-rich solution from the absorption zone is passed to the intake of a pump to increase its pressure to a predetermined system pressure.

5. The process of claim 1 in which the first heat exchange zone includes a final heat exchanger passing a cooling fluid at the predetermined temperature of the exhaust gas stream entering the absorption zone.

6. The process of claim 1 in which the mechanical energy output of the turbine and/or the expansion device is used directly to turn one or more pumps and/or one or more CO.sub.2 compressors.

7. The process of claim 1 in which the mechanical energy output of the turbine and/or expansion device is used to generate electricity that is used to power pumps and or compressor motors and/or to charge a storage battery on board the vehicle.

8. The process of claim 1 in which the sorbent solution is prepared from one or more compounds selected from the group consisting of water, amine-functionalized molecules, alkali metal carbonates and bicarbonates, alkaline earth metal carbonates, alkali metal and alkaline earth metal oxides, aqueous ammonia and ammonium carbonate, alcohols, polyethers, amide compounds, molecular sieves, MOFs, COFs.

9. The process of claim 8 in which the polyether is a dimethylether of polyethylene glycol.

10. The process of claim 8 in which the CO.sub.2 absorbing amide compound is N-methyl-2-pyrrolidone.

11. The process of claim 8 in which the solvent is methanol.

12. The process of claim 8 in which the CO.sub.2 absorbing amine is monoethanolamine.

13. The process of claim 8 in which the CO.sub.2 absorbing carbonate is potassium carbonate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be further described below and with reference to the attached drawings in which the same or similar elements are identified by the same number, and in which:

(2) FIG. 1 is a schematic diagram of an embodiment of the process of the invention in a basic cycle in which CO.sub.2 is captured and compressed in a power production cycle;

(3) FIG. 2 schematically illustrates an embodiment of the invention that includes an optional re-heat step;

(4) FIG. 3 schematically illustrates an embodiment of the invention in which the pressure at the turbine exit is reduced to below atmospheric pressure (vacuum) in order to increase expansion power recovery;

(5) FIG. 4 schematically illustrates a fourth embodiment of the invention in which an additional internal heat exchanger extracts heat from the exhaust gas stream; and

(6) FIG. 5 is a screenshot of an Aspen simulation for a process that is similar to the process described in FIG. 2.

DETAILED DESCRIPTION OF INVENTION

(7) As discussed above, the process of the present invention operates as a semi-closed system that captures CO.sub.2 from an exhaust gas stream of an ICE and produces mechanical energy, or work, utilizing a working fluid that contains the CO.sub.2 in the power generation cycle. The process can be used to advantage for CO.sub.2 capture from a mobile source powered by an internal combustion engine (ICE).

(8) Referring to an embodiment of the invention schematically illustrated in FIG. 1, a simplified cycle of the process is depicted in which CO.sub.2 is captured and compressed in a power production cycle.

(9) A lean loaded CO.sub.2 absorbing solution (hereafter referred to as solution) such as aqueous potassium carbonate is transferred via pump (10) as stream (102) to the absorption unit (20) to capture CO.sub.2 from the exhaust gas stream at atmospheric or near atmospheric pressure.

(10) The CO.sub.2 absorption unit (20) can be a direct contact liquid/gas column such as packed column or an indirect contact membrane absorption device such as gas-liquid membrane contactor. For convenience, the description that follows will refer to the practice of the process of the invention in a direct contact absorption unit. However, as will be understood by those of ordinary skill in the art, an indirect absorber can be employed with substantially the same effect.

(11) The hot exhaust gas stream (901) exiting the ICE is first cooled by passage through the superheater (31) and enters the boiler (30) as reduced temperature stream (902). The exhaust gas stream (903) exiting the boiler (30) is further cooled to a predetermined temperature between 30 C. and 100 C. in a heat exchanger (36) and the cooled stream (904) enters the absorption unit (20) where CO.sub.2 is absorbed by the cooled CO.sub.2-lean loaded solution that enters the absorber (20) via stream (102) at a temperature between 30 C. and 100 C.

(12) The remaining exhaust gas (905) leaves the absorber (20) after CO.sub.2 capture and is discharged into the atmosphere.

(13) The CO.sub.2-rich solution leaves the absorber (20) via stream (200) and is pressurized by pump (11) to the high pressure value of the system, e.g., to 4 MPa, and passes as stream (201) to a first internal heat exchanger (34) where it is heated about 100 C. by the CO.sub.2/water stream (403) leaving turbine (51) as will be described in further detail below.

(14) The pressurized CO.sub.2-rich solution (202) exits the internal heat exchanger (34) and passes through a second internal heat exchanger (33) for further heating. The second internal heat exchanger (33) is heated by the high pressure CO.sub.2-lean solution (300). The high pressure CO.sub.2-rich solution (203) then enters boiler (30).

(15) The high pressure CO.sub.2-rich solution (203) is partially evaporated in boiler (30) which is heated by the hot exhaust gas stream (902) downstream of the superheater (31) which is in close proximity to the exhaust manifold of the ICE; the CO.sub.2 and water are vaporized because of their lower normal boiling points.

(16) The high pressure CO.sub.2-rich liquid/gas mixture (205) leaves the boiler (30) at an increased temperature of, e.g., about 210 C., and enters a liquid/vapor separator (40) that separates the gaseous CO.sub.2/water mixture from the remaining high pressure CO.sub.2-lean solution (300).

(17) The high pressure CO.sub.2-lean solution (300) leaves the liquid/vapor separator (40), enters internal heat exchanger (33) and passes as stream (301) to an expansion device (50), e.g., a turbine or throttle valve, where it is expanded to a lower pressure before passing to the liquid header (100) as stream (302). The expansion device (50) recovers power P for the system from the waste heat and provides mechanical energy to pumps (10) and (11).

(18) The CO.sub.2/water vapor mixture (401) exiting the liquid/vapor separator (40) passes through the superheater (31) that is heated by the exhaust gas stream (901) and exits as superheated stream (402) at a temperature of approximately 400 C. and expands in a turbine (51) to produce power, exiting at approximately atmospheric pressure as stream (403).

(19) The power P from the turbine (51) is applied to operate pumps in the system, to compress CO.sub.2 and/or to operate the process utilities, as required.

(20) The low pressure CO.sub.2/water exiting the turbine (51) as stream (403) passes through an internal heat exchanger (34) and exits via stream (406) to another heat exchanger (37) where it is further cooled to approximately 40 C. in order to condense the water. After exiting the heat exchanger (37) via stream (407), the low pressure CO.sub.2/water passes to a separator (41) where the condensed water is separated from the CO.sub.2 gas. The condensed water stream (500) exiting the separator (41) is composed of water with some dissolved CO.sub.2, all or a portion of which can be passed to the liquid header (100) as stream (502); any excess water can be discharged from the system as stream (501).

(21) The liquid solution (100) is further cooled in heat exchanger (35) to the desired CO.sub.2 absorption temperature before it is fed to the suction line (101) of pump (10) that feeds the CO.sub.2 absorber (20).

(22) The vapor stream (600) consisting principally of CO.sub.2 passes from the separator (41) to the compression zone (60) where it is compressed to produce a high-purity CO.sub.2 stream (601). The high-purity CO.sub.2 stream (601) can be passed to on-board storage in mobile applications and to storage and/or a pipeline in the case of stationary or fixed CO.sub.2 sources. Any remaining water is condensed by intercooling and phase separation and discharged from the system as water stream (700).

(23) All or a portion (704) of the condensed water (700) can optionally be returned via a three-way valve (702) to the loop (100) or to the pump suction line (101) in order to control the water content of the lean absorption solution in the process and prevent salt precipitation. Fresh make-up water can also be used for this purpose, alone or in combination with condensed water stream (700). Alternatively, the condensed water (700) can be discharged (706) from the system.

(24) In another embodiment of the invention schematically illustrated in FIG. 2, an optional re-heating step is provided in which the exiting vapor stream is re-heated after a first expansion of the working fluid in order to increase the overall cycle efficiency.

(25) In this embodiment, the hot exhaust gas stream (900) enters the system through heat exchanger (32) where the medium pressure CO.sub.2/water mixture (403) at, e.g., one Mpa, is re-heated to about 400 C. and exits as heated stream (404).

(26) The cooled exhaust gas stream (901) from heat exchanger (32) enters the superheater (31) and follows the same path that was described in FIG. 1.

(27) The superheated CO.sub.2/water stream (402) from superheater (31) is expanded in turbine (51) to a medium pressure of about 1 MPa and exits as stream (403). Stream (403) passes to heat exchanger (32) to be re-heated by the entering exhaust gas stream (900) to a temperature of about 400 C. and then passes as stream (404) to turbine (52). The expanded low pressure stream (405) exits the turbine (52) at approximately atmospheric pressure and passes to internal heat exchanger (34) to exchange heat with the high pressure CO.sub.2 rich solution stream (201), and exits as stream (406).

(28) The process steps of stream (406) are the same as those described above in conjunction with the embodiment of FIG. 1.

(29) The re-heating step is followed by a further expansion step to reduce the irreversibilities in the system and increase the overall system efficiency. Other aspects of the process of FIG. 1, including the use of the condensate stream (700) that may be injected back into the loop via line (100) or (101) as make-up water in order to control the water content in the process and prevent salt precipitation is also applicable to the embodiment of FIG. 2.

(30) In a third embodiment of the invention that is schematically illustrated in FIG. 3, the pressure at the turbine exit is reduced to below atmospheric pressure, e.g., to a vacuum in order to increase expansion power recovery.

(31) This advantage can be realized because the CO.sub.2 water saturation pressure at ambient temperature is less than atmospheric pressure allowing for a higher power recovery from the fluid expansion and an increase in the net power and efficiency of the process of the invention.

(32) The process in FIG. 3 is similar to the first embodiment as described above in connection with FIG. 1, with the difference being that the outlet pressure of stream (403) exiting the turbine (51) is reduced to, i.e., 20 kPa absolute pressure and a pump (12) is added to the process to pressurize the liquid stream (500) to near atmospheric pressure.

(33) The superheated CO.sub.2/water stream (402) leaving the superheater (31) is expanded in turbine (51) to 20 kPa in order to recover the expansion energy. The CO.sub.2/water stream leaves the turbine via stream (403) to enter internal heat exchanger (34) and the CO.sub.2 water stream exits as stream (406).

(34) The CO.sub.2/water stream (406) is further cooled in heat exchanger (37) to achieve the desired separation of the CO.sub.2 by condensing the water. Stream (407) exiting heat exchanger (37) passes to a separator (41) where a CO.sub.2-rich stream (600) is recovered under a vacuum, e.g., 20 kPa, and compressed in the multi-stage compressor (60) to the required outlet pressure and the pressurized stream (601) and passed for storage or further processing.

(35) The condensate stream (500) composed mainly of water is pressurized by pump (12) to the liquid header line (100) pressure, e.g., 100 kPa to complete the cycle. Stream (510) exiting pump (12) is conveyed in whole or in part for addition to stream (100) via stream (502), the excess being discharged from the system as stream (501).

(36) The same vacuum condensation principle can be applied to the re-heat configuration by reducing the outlet pressure of turbine (52), e.g., to 20 kPa, in order to recover additional work energy and increase the efficiency of the process.

(37) In a fourth embodiment of the invention that is schematically illustrated in FIG. 4, the exhaust gas stream (903) is further cooled exchanging heat with the high pressure CO.sub.2-rich solution stream (202) in a step to increase the overall cycle efficiency, capturing more CO.sub.2 or providing more power for a same CO.sub.2 capture rate.

(38) The process in FIG. 4 is similar to the embodiment as described above in connection with FIG. 3, with the difference of the inclusion of an additional internal heat exchanger (39) between heat exchanger (30) and external heat exchanger (36) on the exhaust gas line, and between heat exchanger (34) and heat exchanger (33) on the high pressure CO.sub.2-rich solution.

(39) The exhaust gases leaving heat exchanger (30) in stream (903) heat the high pressure CO.sub.2-rich solution stream (202) exiting heat exchanger (34). The cooler exhaust gas stream (934) leaves heat exchanger (39) to enter heat exchanger (36) and continue the process as described in FIG. 3.

(40) The high pressure CO.sub.2-rich solution stream (202) leaving heat exchanger (34) is heated in heat exchanger (39) by the hot exhaust gases before entering heat exchanger (33) for further heating via stream (222). Afterwards, the high pressure CO.sub.2-rich solution undergoes the same steps described in FIG. 3 of the process.

(41) In yet another embodiment, it is possible to integrate heat exchanger (39) in the re-heat configuration of the system as described in FIG. 2 or in the above atmospheric pressure outlet configuration as described above and represented in FIG. 1 of the invention.

(42) As will be apparent to one of ordinary skill from the above description of the process and system, the fluids circulated to the three heat exchanges (e.g., 33, 34 and 39) can be varied depending upon the operating characteristics and requirements of the process. For example, the thermodynamic characteristics can be adjusted in order to obtain additional power from the turbines (50, 51, 52), as discussed further below.

(43) As described above in connection with the previous embodiments, all or a portion of water stream (700) can be injected back into the loop in line (100) or (101) in order to control the water content of the solution used in the process and prevent salt precipitation. Fresh make-up water can also be used for this purpose, either alone or in combination with water from stream (700).

(44) It is noted that the process shown in FIG. 4 also includes a re-heat step as was previously shown in FIG. 2 to further increase process efficiency.

(45) The process according to the invention can be operated to achieve a predetermined CO.sub.2 capture goal, e.g., 25%, or to produce a predetermined required amount of power.

(46) In a CO.sub.2 capture application, CO.sub.2 compression is the main energy-intensive component of the system and the net power output is the net power produced by the turbines minus the power consumed by the pumps and in the CO.sub.2 compression step or steps.

(47) Since pumps are indispensable to the operation of the system, there can be little or no variation in meeting requirements for the operation of the pump; however, the extent of CO.sub.2 compression can be varied and is dependent on the CO.sub.2 capture rate and/or the on-board storage capacity.

(48) In a power-oriented operational mode with no CO.sub.2 capture rate requirements, the rate can be adjusted according to the desired net power output, e.g., by reducing the CO.sub.2 capture rate to reduce the CO.sub.2 compression power requirement, thereby increasing the net power output of the system.

(49) Alternatively, if the CO.sub.2 capture rate is to be fixed, e.g., within a given range, or not less than a predetermined value, the system should operate at the required CO.sub.2 capture flow rate with no degree of freedom on the net power production.

(50) The choice of the pressure and temperature throughout the system dictates the parameters of the production cycle and the potential CO.sub.2 capture rate. For example, superheating and re-heating can be used to increase the power output and reduce the irreversibilities in the system. As a result, superheating and re-heating do not affect the CO.sub.2 capture rate, but do affect the net power produced.

(51) An important parameter that does affect the CO.sub.2 capture rate is the temperature and pressure of stream (205) exiting heat exchanger (30) and entering separator (40) since the conditions of this stream will determine how much CO.sub.2 and water go into the vapor phase in separator (40).

(52) The temperature and pressure at the outlet of heat exchanger (37), as well as the operating temperature and pressure of separator (41) relate to the actual rate of CO.sub.2 capture because the temperature and pressure of separator (41) control the ratio between the liquid and vapor phase. It is therefore possible to regulate the system's operation to achieve the desired power production and/or level of CO.sub.2 capture and emissions reduction by controlling the temperature and pressure in these devices (37, 41).

(53) The process according to the invention can use, in addition to the heat of the exhaust gas stream, one or more different sources of energy such as engine coolant energy, solar energy, or any other available form of recoverable thermal energy, to support the operation of the heat exchangers (30) and/or (31) and/or (32) and/or (39) to maximize the power production.

(54) Recoverable energy such as kinetic, mechanical and/or electrical energy can be used in the process to increase the output of the turbines and/or operate the CO.sub.2 compressor. Energy recovery systems and devices that are used on all-electric or hybrid motor vehicles can also be employed on vehicles powered by an ICE to provide electrical power directly or through a storage battery or other device.

(55) Any cooling device in the process used to cool a stream with an ambient or external stream, e.g., an air-cooled heat exchanger (36), can be replaced by an energy recovery device, e.g., a thermo-electric device or other device that captures and converts heat to energy while cooling the working fluid stream to the desired temperature, and the recovered energy can be utilized in the process. For example, instead of cooling the exhaust gas stream from 200 C. to 60 C. in a heat exchanger, a thermoelectric device can be utilized to cool stream (903) to the desired temperature while producing electricity from the recovered energy.

(56) The process of the invention can also be modified by changing the position of the pumps or replacing the pumps with ejectors. It is also possible, depending on the type of the absorber (20), i.e., closed type, membrane absorber, or other, to combine pump (10) and pump (11) in a single pump that is either upstream of the absorption unit (20) or, preferably in the location of downstream pump (11) in order to carry out the absorption at a lower solution pressure.

(57) The process of the invention can also employ various processes for CO.sub.2 and water separation such as membranes or other separation means.

(58) The CO.sub.2 absorbing solution used in the process according to the invention can be a water-based solution containing salts and/or amines and/or other molecules that capture CO.sub.2, by either a physical or chemical process. The CO.sub.2 sorbent solution used in the process of the invention can be selected from the following: a. a solvent-based solution containing salts and/or amines and/or other molecules that physically or chemically absorb CO.sub.2; b. a solvent-based or water-based carrier in which solid CO.sub.2 adsorbent particles are dispersed and the CO.sub.2 is adsorbed by the particles at low temperature and desorbed from the particles at high temperatures, the particles being regenerated and recycled, and the liquid carrier also preferably adsorbs or absorbs the CO.sub.2 physically or chemically at low temperatures and desorbs the CO.sub.2 at high temperatures in order to reduce the flow rate and contactor size; c. a colloid fluid or crystalloid fluid reversibly absorbing and/or adsorbing CO.sub.2 and desorbing CO.sub.2 at the appropriate conditions; and d. a mixture of absorbing and adsorbing liquids.

(59) As will be understood from the above descriptions and examples, the process of the invention broadly comprehends the combination of CO.sub.2 capture in an integrated system that reduces irreversibilities and thereby increases the overall efficiency of the processing and operating system.

(60) In addition to increased efficiency and waste heat recovery in mobile applications, the process of the invention includes the advantages of requiring a reduced number of components as compared to separate heat recovery and CO.sub.2 recovery systems. The integrated system saves space and weight on board mobile sources and reduces capital expenditures and operational maintenance costs.

(61) FIG. 5 is a screen shot of an Aspen Plus Simulation flowsheet representing an embodiment of the invention similar to the process that is depicted in FIG. 2.

Example

(62) The process according to the best mode of the embodiment illustrated in FIG. 3 for the practice of the process of the invention for mobile applications will be described in further detail in this example. A lean aqueous potassium carbonate CO.sub.2 absorbing solution is pressurized by pump (10) and introduced as stream (102) into the absorption unit (20) to capture CO.sub.2 from the cooled exhaust gas stream. The CO.sub.2 absorption unit (20) can be a direct contact liquid-gas column or an indirect contact membrane absorption device that operates at atmospheric or near atmospheric pressure.

(63) The hot exhaust gas stream (901) is cooled in passes through superheater (31) and boiler (30). The exhaust gas stream (903) exiting the boiler (30) is further cooled to a predetermined temperature between 30 C. and 100 C., depending on ambient conditions, in a heat exchanger (36) and the cooled exhaust gas stream (904) enters the absorption unit (20) where CO.sub.2 is absorbed by the CO.sub.2-lean solution (102) to complete the absorption.

(64) The remaining portion of the exhaust gas stream (905) of reduced CO.sub.2 content exits the absorber (20) and is discharged into the atmosphere. In an alternative embodiment, prior to its discharge into the atmosphere, the flue gas stream (905) can be reheated, e.g., to expand its volume. The reheating of stream (905) can be accomplished using the heat from stream (903) entering heat exchanger (36). In this embodiment, heat exchanger 36 can be replaced by an internal heat exchanger or the system can incorporate an internal heat exchanger upstream of heat exchanger (36) in which stream (903) provides heat to stream (905).

(65) The CO.sub.2-rich solution (200) exits the absorber (20) and is pressurized by pump (11) to the high pressure value of the system, e.g., to 4 MPa, and passes as pressurized stream (201) to a first internal heat exchanger (34) where it is heated by the CO.sub.2/water stream (403) leaving turbine (51) as will be described in further detail below.

(66) The heated high pressure CO.sub.2-rich solution (202) exits the first internal heat exchanger (34) and passes through a second internal heat exchanger (33) for additional heating. The second internal heat exchanger (33) is heated by the hot high pressure CO.sub.2-lean solution (300) from which CO.sub.2 has previously been recovered. The high pressure CO.sub.2-rich solution (203) then enters the boiler (30).

(67) The pressurized CO.sub.2-rich solution (203) is partially evaporated in boiler (30) which is heated by the hot exhaust gas stream (902); the portion of absorbed CO.sub.2 is desorbed and some water is vaporized because of their lower normal boiling points. As the concentration of the potassium carbonate increases, the boiling point of the solution also rises, so that the solution remains in a flowable liquid state.

(68) The high pressure CO.sub.2-rich solution (205) passes from the boiler at a temperature of about 210 C. and enters a liquid/vapor separator (40) that separates the CO.sub.2/water gaseous mixture from the remaining pressurized CO.sub.2-lean solution.

(69) The pressurized CO.sub.2-lean solution (300) leaves the liquid/vapor separator (40), passes through internal heat exchanger (33) and then as stream (301) enters expansion device (50), e.g., a turbine or throttle valve, where it is expanded to a lower pressure before passing as stream (302) to the lower pressure process liquid header or conduit (100).

(70) The expansion device (50) can be a throttle valve or a turbine that recovers the power P required for the operation of pumps (10), (11) and as in FIG. 4 (12). The expansion device (50) is preferably linked directly to the shaft of the high pressure pump (11). Alternatively, electric power can be recovered to charge a battery that delivers the electricity to drive the pumps. In another embodiment, one or more pumps can be connected to a common drive shaft from the turbine.

(71) The CO.sub.2/water vapor mixture (401) exiting the liquid/vapor separator (40) passes through the superheater (31) that is heated by the hot exhaust gas stream (901) and exits as a superheated CO.sub.2/water mixture (402) at a temperature around 400 C. Stream (402) is expanded in a turbine (51) to the vacuum pressure value of the system, e.g., 20 kPa, and produces power P which is applied as needed to operate pumps in the system, to compress CO.sub.2 and to operate the process utilities

(72) The low pressure CO.sub.2/water mixture leaves the turbine (51) as stream (403) to enter internal heat exchanger (34) and then heat exchanger (37) as stream (406).

(73) The CO.sub.2/water stream (406) is cooled to condense the water to achieve the desired separation of CO.sub.2 and water. Stream (407) exits heat exchanger (37) and passes to separator (41) where a CO.sub.2-rich stream (600) is recovered under vacuum, e.g., 20 kPa.

(74) The vapor stream (600) is composed mainly of CO.sub.2 and passes to the compression zone (60) where it is compressed to provide the compressed high-purity CO.sub.2 stream (601). The high purity CO.sub.2 stream (601) can be passed to on-board storage in mobile applications, and eventually to permanent underground or other storage via pipeline. Any remaining water is condensed by intercooling and phase separation and discharged from the system as waste water stream (700).

(75) The condensate stream (500) from the separator (41) is mainly composed of water with some dissolved CO.sub.2 and is pressurized by pump (12) for introduction into the liquid header line (100) at a pressure of about 100 kPa. Stream (510) exiting pump (12) is passed in whole or in part to sorbent solution stream (100) as stream (502), any excess being discharged from the system as stream (501).

(76) The absorbent solution stream (100) is further cooled in heat exchanger (35) to the predetermined CO.sub.2 absorption temperature and then passed to the suction line (101) of pump (10) for introduction into the CO.sub.2 absorber (20).

(77) No systems of the prior art concerned with reduction of CO.sub.2 emissions contemplate the utilization of CO.sub.2 from exhaust streams as a working fluid in energy recovery systems.

Example

(78) A computer analysis/simulation was prepared using the Aspen Technology program model in lieu of bench testing. The model corresponds generally to the schematic arrangement depicted in FIG. 1. The calculations are based on a 25% CO.sub.2 capture rate with no pressure drop across the equipment.

(79) It will be understood that the results are indicative and although some uncertainties remain, the results provide useful data for the specified condition. The following Table includes the characteristics of the various streams described above for the Aspen Simulation presented in FIG. 5.

(80) TABLE-US-00001 TABLE (Based on Aspen Simulation) Temperature Pressure Vapor Mass Flow Rate Stream ( C.) (kPa) Fraction (kg/sec) 901 600 100 1 1 902 562 100 1 1 903 242 100 1 1 904 35 100 1 1 905 40 100 1 0.91 200 39 200 0 3.5 201 40 4000 0 3.5 202 62 4000 0 3.5 203 222 4000 0.02 3.5 205 241 4000 0.05 3.5 300 241 4000 0 3.34 301 65 4000 0 3.34 302 65 100 0 3.34 401 241 4000 1 0.16 402 400 4000 1 0.16 403 99 100 1 0.16 406 45 100 0.16 0.16 407 40 100 0.14 0.16 500 40 100 0 0.1 600 40 100 1 0.06 601 40 10000 1 0.06

(81) While various exemplary embodiments of the invention have been described above and in the attached drawings, further modifications will be apparent to those of ordinary skill in the art from these examples and the description. The scope of the invention is to be determined with reference to the claims that follow.