Systems and methods for reducing the energy requirements of a carbon dioxide capture plant

Abstract

Systems and methods for reducing the energy requirements for carbon dioxide capture are described. Heat from system processes, such as steam condensation and hot flue gas, is utilized to heat reflux liquid utilized in release of carbon dioxide from absorbent solvent.

Claims

1. A method for reducing an energy requirement of a CO.sub.2 capture process, comprising: condensing at least a portion of an upper product stream of a stripper to produce a condensed stream; feeding at least a first portion of the condensed stream to the stripper as a reflux stream; heating a second portion of the condensed stream in a heat exchanger via heat exchange contact with a heat transfer medium to produce steam; feeding the steam to the stripper; withdrawing a solvent stream from the stripper; heating the solvent stream from the stripper in a reboiler to produce a reboiler stream, returning the reboiler stream to the stripper; and cooling and condensing at least a portion of a second heat transfer medium via heat exchange contact with the solvent stream in the reboiler to produce a reboiler condensate, wherein the heat transfer medium comprises the reboiler condensate.

2. The method of claim 1, wherein the upper product stream comprises CO.sub.2.

3. The method of claim 2, wherein the upper product stream further comprises steam.

4. The method of claim 1, wherein the stripper s configured to receive a CO.sub.2-rich solvent stream.

5. The method of claim 1, wherein condensing at least the portion of the upper product stream comprises introducing the upper product stream into a condenser.

6. The method of claim 1, further comprising: separating at least a portion of the condensed stream to produce a CO.sub.2 product stream and a condensate stream; and wherein the at least the portion of the condensed stream comprises at least a portion of the condensate stream.

7. The method of claim 6, wherein separating at least a portion of the condensed stream comprises introducing the portion of the condensed stream into an accumulator.

8. A method for reducing an energy requirement of a CO.sub.2 capture process, comprising: condensing at least a portion of an upper product stream of a stripper to produce a condensed stream; feeding at least a first portion of the condensed stream to the stripper as a reflux stream; heating a second portion of the condensed stream in a heat exchanger via heat exchange contact with a heat transfer medium to produce steam; and feeding the steam to the stripper; withdrawing a solvent stream from the stripper; heating the solvent stream from the stripper in a reboiler to produce a reboiler stream; returning the reboiler stream to the stripper, and cooling and condensing at least a portion of a second heat transfer medium via heat exchange contact with the solvent stream in the reboiler to produce a reboiler condensate, wherein the second heat transfer medium comprises low pressure steam, and wherein the heat transfer medium comprises the reboiler condensate.

9. The method of claim 8, wherein the upper product stream comprises CO.sub.2.

10. The method of claim 9, wherein the upper product stream further comprises steam.

11. The method of claim 8, wherein the stripper is configured to receive a CO.sub.2-rich solvent stream.

12. The method of claim 8, wherein condensing at least the portion of the upper product stream comprises introducing the upper product stream into a condenser.

13. The method of claim 8, further comprising: separating at least a portion of the condensed stream to produce a CO.sub.2 product stream and a condensate stream, wherein separating at least a portion of the condensed stream comprises introducing the portion of the condensed stream into an accumulator; and wherein the at least the portion of the condensed stream comprises at least a portion of the condensate stream.

14. The method of claim 8, wherein the heat transfer medium comprises an upstream flue gas stream.

15. The method of claim 1, wherein the heat transfer medium comprises an upstream flue gas stream.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a schematic of one embodiment of the inventive concept, in which heat from flue gas is utilized to heat reflux that is returned to a stripper.

(2) FIG. 2 is a schematic of another embodiment of the inventive concept, in which heat from condensed low pressure steam is utilized to heat reflux that is returned to a stripper.

(3) FIG. 3 illustrates a method of the inventive concept, in which a product from a stripper is treated for return to the stripper.

(4) FIG. 4 illustrates a method of the inventive concept in which a product from a stripper is treated with heat from a second product of the stripper for return to the stripper.

DETAILED DESCRIPTION

(5) It should be noted that while the following description is drawn to systems and methods for improving the energy efficiency of recovering CO.sub.2 from flue gases, various alternative configurations are also deemed suitable and may be employed to treat any suitable source of CO.sub.2 containing gas streams, such as streams from combustion processes in the oil and gas industry, cement plants, lime kiln exhausts, engine exhausts, fermentation processes, hydrogen production plants, ammonia production plants, processing of phosphates, and so forth. One should appreciate that compounds other than CO.sub.2 may be recovered, including (but not limited to) CO, ammonia, nitrogen oxides, sulfur oxides, volatile organic carbon compounds, and chlorofluorocarbons, from gas streams containing such compounds.

(6) One should also appreciate that the disclosed techniques provide many advantageous technical effects including reduction in the application of high temperature heat to solvents utilized in CO.sub.2 recovery (thereby minimizing their degradation), reduction of water consumption in the CO.sub.2 recovery process by generating stripping steam utilized for solvent recovery from process water, and reduction of the temperature of flue gases prior to CO.sub.2 capture, thereby reducing the need for active cooling of flue gas prior to entering a CO.sub.2 capture unit.

(7) The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.

(8) The inventive subject matter provides apparatus, systems and methods in which one can remove CO.sub.2 from flue gases using a solvent system and regenerate the CO.sub.2 capture solvent in an energy efficient manner. In one embodiment of the inventive concept, a boiler heated by fossil fuel combustion generates steam that may be used for power generation, for example by providing high pressure steam that is directed through a series of turbines used to generate electric power. It is contemplated, however, that systems and methods of the inventive concept may also be applied to combustion processes utilized in the coal, gas, and/or petroleum industries (such as, for example, from a reformer furnace, a gas turbine, a water heater, a steam generator, a reboiler, and a liquefied natural gas heater).

(9) Such processes generate hot flue gas that contains CO.sub.2, release of which into the atmosphere is being increasingly regulated. Therefore, there is considerable interest in the removal of CO.sub.2 from such flue gas prior to being exhausted to the atmosphere; the CO.sub.2 may then be utilized in other processes or sequestered to prevent environmental release. CO.sub.2 may be captured from flue gas in a CO.sub.2 capture unit. Typically, such devices provide contact between the flue gas and an absorptive media that solvates or otherwise capture CO.sub.2. Typical liquid absorptive solvents include amines or similar compounds, often admixed with water. However, any commercially suitable solvent could be used. Uptake of CO.sub.2 by such a solvent generates a rich solvent (i.e. a solvent rich in CO.sub.2). For peak efficiency, such processes should be optimized for temperature and pressure that provides optimal CO.sub.2 uptake by the solvent, which often requires cooling the hot flue gas from combustion processes (for example using refrigerated contact coolers) to bring the flue gas to optimal temperature.

(10) Following absorption of CO.sub.2 from flue gases, CO.sub.2 is removed from the rich solvent using a stripper. Temperature and pressure conditions within the stripper decrease the solubility of CO.sub.2 in the solvent; typically, the temperature of the absorptive solvent is increased in order to release the absorbed gas. In addition, strippers generally include internal packing or similar structure to increase surface area and expose more solvent to the internal atmosphere. Heat may be provided in the form of steam, which may be introduced through a lower portion of the stripper so that steam and solvent interact in a counterflow fashion. The resulting lean solvent may be collected from the lower part of the stripper (from where it may be returned for use in a CO.sub.2 capture unit) while the released CO.sub.2 can be collected from the upper portion. In practical terms it is often necessary to pass the solvent through the stripper repeatedly in order to remove all (or essentially all) of the absorbed CO.sub.2, so solvent is typically refluxed through the stripper.

(11) The process of removing CO.sub.2 from the rich solvent therefore requires considerable heat energy, which is supplied to the solvent, at least in part, by a reboiler that is in fluid communication with the stripper. As noted above, it is a common practice to utilize a portion of the steam that is produced by a fossil fuel driven boiler to provide this heat, however this practice directly impacts the efficiency of the plant. Systems and methods of the inventive concept utilize heat energy produced by system processes to alternatively provide heat for the reboiler and thereby improve plant efficiency.

(12) One embodiment of the inventive concept is shown in FIG. 1. Rich solvent (100) from a CO.sub.2 capture unit (not shown) is directed to a stripper (105). A semi-lean solvent (110) is collected from a point below the packing and directed to a reboiler (120), where it is heated. Heat energy is preferably supplied to the reboiler (120) in the form of low pressure steam (130), although any commercially suitable heat exchange media could be used. Such steam may, for example, be collected from the exhaust of a turbine. Examples of suitable turbines include, for example, a low pressure turbine from a series of high, mid, and low pressure turbines utilized for power generation and a turbine used to power a compressor. Utilization of such sources of low pressure steam also advantageously minimizes the impact on power generation. In addition, the use of steam raised from process water advantageously maintains the water balance of the plant by eliminating the need to inject water from an outside source into the system.

(13) At least a portion of the low pressure steam condenses in this process, generating a reboiler condensate (135), and transferring the resulting heat of condensation from the condensing steam to the solvent. Utilization of this phase change for energy transfer advantageously reduces exposure of the solvent to high temperatures, such as those that are experienced when solvent is heated using high pressure steam, which minimizes degradation of the solvent. This results in vaporization of a portion of the semi-lean solvent, and the resulting two phase solvent (125) is returned to the stripper (105). This releases stripping steam within the stripper (105), where the substantially CO.sub.2 free lean solvent (140) is removed from a lower portion of the stripper (105) and may be directed to a CO.sub.2 capture unit. In some embodiments of the inventive concept the lean solvent (140) is removed from the bottom of the stripper (105). Stripping steam rises through the stripper (105), carrying released CO.sub.2 through the upper part of the stripper (105) as a CO.sub.2 saturated stream (145). It is contemplated that CO.sub.2 may be released from other components of the CO.sub.2 saturated stream (145) by cooling in a condenser (150), and subsequently separated from liquid components in an accumulator (165). In typical overhead accumulator (165) configurations, product CO.sub.2 (155) is collected from an upper portion and product condensate (160) is collected from a lower portion. This condensate (160) may be primarily water, but may include amines or other compounds utilized in the capture of CO.sub.2.

(14) Condensate (160) may be distributed to one or more destination processes, optionally with the aid of a pump (167). Embodiments of the inventive concept may include one or more valves and/or pump(s) and an associated controller that permits control of the distribution of the condensate (160) in order to optimize plant operations. It is contemplated that condensate (160) may be directed to two or more destination processes and/locations simultaneously. In some embodiments of the inventive concept, at least a portion of the condensate (160) can be directed to the stripper (105) as reflux (170). In other embodiments of the inventive concept, at least a portion of the condensate (175) can be directed to other parts of the plant. In still other embodiments of the inventive process, at least a portion of the condensate (180) can be directed to a reflux heater (185).

(15) A reflux heater (185) may be used to heat the condensate (160) to generate steam (190) utilizing heat from a suitable source. Suitable sources include, for example, hot flue gas (195) and a reboiler condensate and may be transferred to the condensate (160) via a heat exchanger. Where the source is a flue gas, the flue gas may, for example, originate in a boiler utilized to generate steam for power generation, but may alternatively be obtained from other combustion sources such as a burner, a reformer furnace, a gas turbine, a water heater, a steam generator, a reboiler, and/or a liquefied natural gas heater. It is contemplated that one or more flue gas sources may be combined to provide a pooled flue gas source for the reflux heater (185). This process reduces the temperature of the flue gas to produce a cooled flue gas (197), which may be routed to other parts of the CO.sub.2 capture process; this cooling of the flue gas advantageously reduces the size and/or duty cycle of cooling units utilized in the processing of hot flue gas prior to introduction to a CO.sub.2 capture unit. Steam (190) produced in the reflux heater (185) may directed to the stripper (105), where it may be used to aid in the CO.sub.2 removal process.

(16) Another embodiment of a system of the inventive concept is shown in FIG. 2. Rich solvent (200) from a CO.sub.2 capture unit (not shown) is directed to a stripper (205). A semi-lean solvent (210) is collected from a point below the packing and directed to a reboiler (220), where it is heated. Heat energy may be supplied to the reboiler (220) in the form of low pressure steam (230). Such steam may, for example, be collected from the exhaust of a turbine. Examples of suitable turbines include, but are not limited to, a low pressure turbine from a series of high, mid, and low pressure turbines utilized for power generation and a turbine used to power a compressor. Utilization of such sources of low pressure steam advantageously minimizes the impact on power generation.

(17) At least a portion of the low pressure steam condenses in this process, generating a reboiler condensate (235), and transferring the resulting heat of condensation from the condensing steam to the solvent. Utilization of this phase change for energy transfer advantageously reduces exposure of the solvent to high temperatures, such as those that are experienced when solvent is heated using high pressure steam, which minimizes degradation of the solvent. This results in vaporization of a portion of the semi-lean solvent, and the resulting two phase solvent (225) is returned to the stripper (205). This releases stripping steam within the stripper (205), where the substantially CO.sub.2 free lean solvent (240) is removed from a lower portion of the stripper (205) and may be directed to a CO.sub.2 capture unit.

(18) In some embodiments of the inventive concept, the lean solvent (240) is removed from the bottom of the stripper (205). Stripping steam rises through the stripper (205), carrying released CO.sub.2 through the upper part of the stripper (205) as a CO.sub.2 saturated stream (245). CO.sub.2 may be released from other components of the CO.sub.2 saturated stream (245) by cooling in a condenser (250), and subsequently separated from liquid components in an accumulator (265). In typical overhead accumulator (265) configurations, product CO.sub.2 (255) is collected from an upper portion and product condensate (260), condensate is collected from a lower portion. This condensate (260) may be primarily water, but may include amines or other compounds utilized in the capture of CO.sub.2.

(19) Condensate (260) may be distributed to one or more destination processes, optionally with the aid of a pump (267). Embodiments of the inventive concept may include one or more valves and/or pump(s) and an associated controller that permits control of the distribution or the condensate (260) in order to optimize plant operations. In some embodiments of the inventive concept, at least a portion of the condensate (260) can be directed to the stripper (205) as reflux (270). In other embodiments of the inventive concept, at least a portion of the condensate (275) can be directed to other parts of the CO.sub.2 capture process (not shown). In still other embodiments of the inventive process, at least a portion of the condensate (280) can be directed to reflux heater (285). The reflux heater (285) may use the condensate (260) to generate steam (290) utilizing heat from a suitable source. In such an embodiment heat may be supplied to the reflux heater (285) by condensate (235) produced by prior heat transfer in a reboiler (220). Heat transfer from the condensate (235) generates a sub-cooled condensate (237). Steam (290) produced in the reflux heater (285) may be directed to the stripper (205), where it may be used to aid in the CO.sub.2 removal process.

(20) An embodiment of a method of the inventive concept is shown in FIG. 3. At least a portion of an upper product (310) obtained from a stripper (300) may be condensed in condenser (320) to produce a condensed stream. Part of this condensed stream from condenser (320) may be returned to the stripper (300) as a reflux stream (330). Another part of this condensed stream may be provided to a heat exchanger (335) that transfers heat from a heat transfer medium (340) to produce steam (350) that is returned to the stripper (300). The heat transfer medium may be flue gas, produced by an upstream boiler or other device and/or devices as described above. Alternatively, the heat transfer medium may be a condensate from a reboiler. In some embodiments of the inventive concept the condensed stream may be separated into a CO.sub.2 product (325) and a condensate stream (327), where the condensate stream forms at least part of the material that is provided to the heat exchanger (335) for production of steam (350).

(21) Another embodiment of a method of the inventive concept is illustrated in FIG. 4. An upper product stream (410) and a solvent stream (420) are obtained from a stripper (400). At least a portion of the upper product stream (410) may be condensed in condenser (430) to produce a condensed stream. A portion of this condensed stream from condenser (430) may be returned to the stripper (400) as a reflux stream (440). A second portion of this condensed stream may be provided to a heat exchanger (450) to produce steam (470) that is returned to the stripper (400). The solvent stream (420) may be heated in a second heat exchanger (480) using a second heat transfer medium (460); the resulting cooled second heat transfer medium (465) may in turn be utilized by the heat exchanger (450) to produce at least a portion of the steam (470) utilized by the stripper (400). The heat transfer medium (460) may be may be, for example, a low pressure steam. In some embodiments of the inventive concept the condensed stream may be separated into a CO.sub.2 product (435) and a condensate stream (437), where the condensate stream forms at least part of the material that is provided to the heat exchanger (450) for production of steam (470).

(22) In embodiments of the inventive concept where reboiler condensate (235) is used as a heating medium in a reflux heater (285), the condensate can return to the power plant colder in the current art, however, net steam demand is reduced. Typical reductions in steam demand for the present invention may be around 6%. In the case of a power plant, embodiments of the inventive concept reduce the need for low pressure steam to heat the solvent, which can advantageously increase the power output of the generator. This advantageously mitigates the impact of any retrofitting that would be required for a low pressure steam turbine during the installation of a carbon capture plant utilizing an embodiment of the inventive concept.

(23) As used herein, and unless the context dictates otherwise, the term coupled to is intended to include both direct coupling (in which two elements that are coupled to each other contact each other) and indirect coupling (in which at least one additional element is located between the two elements). Therefore, the terms coupled to and coupled with are used synonymously.

(24) It should be apparent to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the scope of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms comprises and comprising should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Where the specification claims refers to at least one of something selected from the group consisting of A, B, C . . . and N, the text should be interpreted as requiring only one element from the group, not A plus N, or B plus N, etc.