FIELD OF THE DISCLOSURE

The presently disclosed subject matter relates to systems and methods for generation of power, such as electricity, that operate at desirable efficiencies that are achieved through additive heating of at least part of a recycle CO2stream in a heat exchanger using a further heat source. Particularly, heat from the further source can be derived at least in part from compression of at least a portion of the recycle CO2stream.

BACKGROUND

Conventional means of power production from combustion of a fuel typically lack the ability to simultaneously achieve high efficiency power generation and carbon capture (e.g., for sequestration or other use). One publication in the field of high efficiency power generation with carbon capture, U.S. Pat. No. 8,596,075 to Allam et al., provides for desirable efficiencies in closed cycle combustion systems using CO2as the working fluid. Such systems in particular benefit from the recognized usefulness of heating a recycle CO2stream in a recuperative heat exchanger using heat from the hot turbine exhaust as well as adding further heat from a source other than the turbine exhaust. Despite such advances, there is yet a growing need in the art for improved systems and methods for power generation that provide increased efficiency with capture of CO2and other fuel and combustion derived impurities.

SUMMARY OF THE DISCLOSURE

The present disclosure provides systems and methods of power production with improved efficiency. The systems and methods can utilize CO2as a working fluid and can be configured for capture of CO2and other fuel and combustion derived impurities. The present improvements have been identified in relation to the introduction of low temperature level heating to a recycle CO2stream in an elevated pressure, oxy-fuel combustion system and method that also utilizes recuperative heating of the recycle CO2stream with heat from the hot turbine exhaust. The low temperature level heating can be described herein in terms of being “additive heating.” As such, it is understood that the additive heating is low temperature level heat from a source other than the hot turbine exhaust. In other words, the additive heating is not heat that is recuperated from the hot turbine exhaust. The present disclosure in particular identifies means for obtaining and transferring the additive heating in a closed cycle or partially closed cycle oxy-fuel combustion system and method that provides a power production output that exceeds the heating capacity of the heat from the fuel alone and thus provides an advantageous increase in efficiency.

In some embodiments, the present disclosure relates to systems and methods for generation of power, such as electricity, that operate at desirable efficiencies that are achieved through the combustion of a fuel (e.g., a carbonaceous or carbonaceous fuel) in oxygen at elevated pressure in the presence of a recycle CO2stream followed by expansion of the product gas through a power producing turbine and cooling of the turbine exhaust in a recuperative heat exchanger, which heats the previously compressed recycle CO2stream. Improved efficiency of power generation can be obtained by additive heating of at least part of the recycle CO2stream in a heat exchanger using additive heating, which can be, for example, heat derived at least in part from compression of at least a portion of the recycle CO2stream.

In various embodiments, power production can be achieved utilizing a closed cycle or partially closed cycle system in which CO2is utilized as the working fluid. In such systems, a fossil fuel (e.g., natural gas) or a fuel derived from a fossil fuel (e.g., syngas derived from coal or other solid carbonaceous fuel) is completely combusted in a combustor using substantially pure oxygen as the oxidant to give an oxidized stream of predominantly CO2, H2O, excess O2, and a quantity of impurities derived from oxidized components in the fuel or oxidant, such as SO2, NOx, Hg, and HCl. Solid fossil fuels, such as coal, lignite, or petroleum coke, that contain non-combustible ash may be converted to a gaseous fuel by partial oxidation in a single stage or multi-stage system. Such system, for example, may comprise a partial oxidation reactor. Alternatively, for example, such system may comprise a partial oxidation reactor and an ash and volatile inorganic component removal system. Such systems further comprise combustion of the fuel gas with oxygen in the combustor of the power production system. A preheated recycle CO2stream is mixed in the combustor with the combustion products derived from combustion of the fuel gas. Any combustor adapted for operation under conditions otherwise described herein may be used, and the recycle CO2stream may be introduced to the combustor by any means to be further heated by the combustion and, if desired, to quench and thereby control the temperature of the exit stream. In some embodiments, one or both of a POX reactor and the combustor may utilize, for purposes of example only, a transpiration cooled wall surrounding the reaction or combustion space, and the preheated recycle CO2stream may pass through the wall to both cool the wall and to quench and thereby control the temperature of the exit stream. The transpiration flow promotes good mixing between the recycle CO2and the hot combusted fuel gas streams. Other types of combustors, however, may also be used, and the present disclosure is not limited to the use of transpiration cooled combustors. Although certain fuel types are exemplified above, it is understood that other fuels (e.g., hydrogen) may be utilized in the combustor. Likewise, the advantages flowing from the use of additive heat may be applied to systems utilizing non-combustion heating in part or in total. For example, use of solar systems such as described in U.S. Pat. Pub. No. 2013/0118145, the disclosure of which is incorporated herein by reference, is also encompassed by the present disclosure.

The combined combustion products and preheated recycle CO2leaving the combustor are at the temperature required for the inlet to a power-producing turbine. The CO2power cycle can use a pressure ratio across the turbine from 5 to 12 in some embodiments, although greater pressure ratios (e.g. at least 20) may be used in other embodiments, such as when utilizing a plurality of expansion turbines. A turbine inlet pressure of about 100 bar (10 MPa) to about 500 bar (50 MPa) can be used in some embodiments. The oxygen supplied to the combustor can be either substantially pure O2or O2diluted with CO2. In some embodiments, mixing of the O2and CO2can be useful to control the adiabatic flame temperature of the combustion reaction. As a non-limiting example, the molar concentration of O2in the combined O2/CO2stream can be about 10% to about 50%, about 15% to about 40%, or about 20% to about 30%. The hot turbine exhaust can be cooled in an economizing heat exchanger, which in turn preheats the high pressure CO2recycle stream.

The efficient operation of the system is critically dependent on the optimization of the heat exchange. To achieve a high efficiency, a large quantity of additive heat can be added into the high pressure recycle stream at the cold end of the heat exchanger, such as at a temperature level from about 100° C. to about 400° C. This low temperature level heat may be derived in some embodiments from the air compressors of a cryogenic oxygen plant, which compressors may be operated wholly or in part with their pressure ranges in a high-pressure ratio adiabatic mode so that the compressed air is raised in temperature at the stage discharge to a point in the range of about 100° C. to about 400° C. and so that heat transfer from the compressed air stream to the pressurized recycle CO2process stream can be easily accomplished. For example, a side stream flow taken from the high pressure CO2recycle flow in the economizer heat exchanger can be heated against cooling compressed air to a required temperature of about 100° C. to about 400° C. Systems and methods for such oxyfuel combustion, low level heat production, and low level heat transfer are described in U.S. Pat. Nos. 8,596,075, 8,776,532, 8,986,002, 9,068,743, U.S. Pat. Pub. No. 2010/0300063, U.S. Pat. Pub. No. 2012/0067054, U.S. Pat. Pub. No. 2012/0237881, and U.S. Pat. Pub. No. 2013/0104525, the disclosures of which are incorporated herein by reference in their entireties.

The present disclosure provides further means for introducing heat into a recycle CO2high pressure stream at a temperature of about 100° C. to about 400° C. and thus increase the effectiveness of an economizer heat exchanger and increase the overall efficiency of the power production system and method incorporating the present heat transfer means. In particular, the present disclosure provides for the use of a portion of the heat of compression from a recycle CO2compressor as the additive heating necessary to increase the overall efficiency of a power production system and method.

Previous proposals have been made to optimize the performance of a power production cycle using high pressure CO2as the working fluid. For example, Bryant et al. (“An Analysis and Comparison of the Simple and Recompression Supercritical CO2Cycles” May 2011 presentation at the supercritical CO2power cycle workshop in Boulder, Colo.), the disclosure of which is incorporated herein by reference, describes Brayton cycles for power generation using recuperator heat exchanger with CO2as the working fluid. The paper defines the efficiencies of two cycles in terms of operating parameters and shows the conditions under which the second cycle gives a higher efficiency than the first, simple cycle.

The first, simple cycle from Bryant et al. is shown inFIG. 1. Therein, hot CO2in line7that has been compressed in a near adiabatic, non-intercooled compressor1is further heated in a recuperator heat exchanger4. The hot CO2then passes through line8to the heater3where it is heated either directly by combusting a fuel14with oxygen13, or by some means of external heating. The further heated CO2then passes through line9into a power producing turbine2where it is expanded to a lower pressure producing shaft work (illustrated by arrow15). The turbine exhaust stream10passes to the recuperator heat exchanger4where it cools releasing heat to the high pressure recycle stream. The turbine exhaust is then cooled in a pre-cooler5where heat is rejected to a cooling stream11that exits via line12before finally re-entering the compressor1in line6.

The second cycle from Bryant is shown inFIG. 2, which is identical to the cycle shown inFIG. 1apart from the addition of a second compression stage16in which part of the low pressure turbine exhaust stream17leaving the low pressure return circuit at the exit from the recuperator heat exchanger4abefore the pre-cooler5is compressed in its hot condition in compressor16leaving through line18. This stream enters the recuperator heat exchanger4bafter mixing with the main high pressure recycle stream leaving4aat its corresponding temperature and is heated in heat exchanger section4bagainst hot turbine exhaust stream10. The effect of the additional compression is to inject a large amount of heat into the recuperator heat exchanger from the second compressor which takes as its inlet flow a CO2stream at an elevated temperature which is greater than the inlet temperature of the main CO2compressor.

The teaching of Bryant et al. reveals a disadvantage in that the heat input by-passes the first stage of the recuperator heat exchanger4a. The much larger specific heat of the high pressure CO2stream that is being heated in heat exchanger4acompared to the low pressure turbine exhaust stream cooling in heat exchanger4ameans that the heat transfer in heat exchanger4ainto the high pressure stream must be maximized to achieve a close temperature approach. This is not achieved since the heated compressed CO2stream bypasses heat exchanger4a. What is required for the maximum efficiency is to arrange a hot gas compression system in which the compressed CO2cools down in the heat exchanger and increases the available heat transfer to the high pressure CO2stream. The present disclosure relates in part to means for overcoming this disadvantage.

Although the cycles from Bryant et al. illustrated inFIG. 1andFIG. 2represent the known prior art for hot CO2compression, they are only suitable for use in simple Brayton cycle arrangements that use a main CO2compressor without intercoolers giving a high discharge temperature. This in turn causes the cooling turbine discharge flow leaving the recuperator heat exchanger4ato also be at a high temperature so the heat rejected in the pre-cooler heat exchanger is also high. It is therefore apparent that optimum efficiencies will only be achieved at low pressure ratios in this hot compression cycle which are shown to be in the range of 2 to 4 with optimum main compressor inlet pressures near the critical pressure of CO2. Higher pressure ratios lead to excessive heat losses in the system. The Bryant et al. cycles shown inFIG. 1andFIG. 2also fail to account for system details such as the presence of liquid water separation in the compressor inlet lines6following cooling in the heat exchanger5against ambient cooling means.

The Bryant et al. cycle inFIG. 2has several further limitations. For example, the Bryant et al. cycle efficiency significantly decreases as the pressure ratio is increased since the main and recompression compressors are essentially adiabatic in operation with no intercoolers between stages. The studies reported by Bryant et al. show that the optimum pressure ratio for a turbine inlet temperature of 750° C. is 2.2 at a turbine inlet pressure of 100 bar and 3.3 at a turbine inlet pressure of 250 bar. Low pressure ratios require very high CO2flow-rates in the system for a given power output leading to high capital costs. On the contrary, the present disclosure provides cycles with high pressure ratios and high turbine inlet pressures resulting in high efficiency and low capital cost.

Systems and methods useful according to the present disclosure can utilize pressure ratios of about 5 or greater, such as about 5 to about 30. In some embodiments, pressure ratios preferably can be in the range of about 5 to about 12. The present systems and methods also can utilize intercooled main CO2recycle compression systems. The high pressure ratio favors a turbine inlet pressure above the CO2critical pressure of 7.38 MPa and a turbine discharge pressure below this pressure. These higher pressure ratios give high efficiencies of 50% to 60% for natural gas fueled systems with significantly lower circulation rates of CO2per kW of net power output. Systems and methods useful according to the present disclosure also preferably utilize a very considerable input of additive heat at a temperature level of, for example, greater than 100° C., and particularly in the range of about 100° C. to about 400° C. or about 100° C. to about 300° C. The presently disclosed systems and methods are particularly beneficial in providing for utilization of a portion of the heat of compression from the main recycle CO2compressor as this additive heating.

In some embodiments, the present disclosure provides a method of generating power. For example, the method can comprise various combinations of the following steps: passing a compressed, heated recycle CO2stream into a combustor; combusting a fuel with oxygen in a combustor in the presence of the recycle CO2stream to produce a CO2containing stream; passing the CO2containing stream through a turbine to expand the CO2containing stream, generate power, and form a turbine exhaust stream comprising CO2; withdrawing heat from the turbine exhaust stream comprising CO2; dividing the cooled turbine exhaust stream to form a first turbine exhaust portion and second turbine exhaust portion; separating water from the first turbine exhaust portion to form a main recycle CO2stream; compressing the main recycle CO2stream; compressing the second turbine exhaust portion adiabatically with no intercooling between compressor stages to form a heated, compressed second turbine exhaust portion; withdrawing heat from the heated, compressed second turbine exhaust portion; separating water from the cooled, compressed second turbine exhaust portion to form a secondary recycle CO2stream; combining the main recycle CO2stream and the secondary recycle CO2stream to form a total recycle CO2stream; cooling the total recycle CO2stream to form a high density CO2stream; compressing the total recycle CO2stream in a second compression stage using a fluid pump; heating the total recycle CO2stream with heat withdrawn from the turbine exhaust stream; and further heating the total recycle CO2stream with heat withdrawn from the heated, compressed second turbine exhaust portion to form the compressed, heated recycle CO2stream. In some embodiments, the two compressed CO2streams following heat withdrawal from the secondary recycle CO2stream can be combined, then the combined stream can be cooled followed by liquid water separation. In some embodiments, the second turbine exhaust stream may be compressed in multiple stages with heat transfer between one or more of the stages. For example, the second turbine exhaust stream may undergo multi-stage compression including uncooled compression stages (x) and intercooled compression stages (y), wherein x and y independent may be an integer of 1 or more, 2 or more, or 3 or more (e.g., 1 to 5 or 2 to 4). In some embodiments, the second turbine exhaust stream may be pressurized (and heated) in x uncooled compression stage(s) to an intermediate value, the pressurized gas can be utilized to provide the heat of compression to a recuperative heat exchanger so as to be cooled, the cooled gas can be dewatered, and the gas can be passed back to undergo the remaining y intercooled compression stage(s) prior to joining with the first turbine exhaust stream.

In further embodiments, the present disclosure provides a power generating system. For example, the system can comprise: a combustor; a power production turbine; one or more heat exchangers; a first cooling flow path through the one or more heat exchangers; a heating flow path through the one or more heat exchangers; a flow separator in communication with the first cooling flow path through the one or more heat exchangers; a first compressor in communication with the flow separator; a second cooling flow path through the one or more heat exchangers, the second cooling flow path being in communication with the compressor; one or more water separators; a second compressor; and a pump. In particular, the heating flow path through the one or more heat exchangers is downstream from the pump and upstream from the combustor; and the heating flow path through the one or more heat exchangers is in a heating arrangement with the first cooling flow path and the second cooling flow path through the one or more heat exchangers. In some embodiments, the first cooling flow path and the second cooling flow path can be defined by separate and independent water separation components and/or pump components. When two or more heat exchangers are used, the heat exchangers may be in series.

In some embodiments, the present disclosure can relate to a method for heating a recirculating gas stream. As an example, such method can comprise the following steps: passing a gas stream G at a pressure P1and a temperature T1through a recuperative heat exchanger such that the gas stream is cooled to a temperature T2that is less than T1; separating the gas stream G into a first fraction G1and a second fraction G2; compressing the gas stream fraction G1to a pressure P2that is greater than P1; compressing the gas stream fraction G2to a pressure P3that is greater than P1so as to heat the gas stream fraction G2to a temperature T3that is greater than T2; withdrawing the heat from the compressed gas stream fraction G2; combining the gas stream fraction G1and the gas stream fraction G2to form a combined recirculating gas stream GC; pumping the recirculating gas stream GCto a pressure P4that is greater than P2and greater than P3; and passing the recirculating gas stream GCto the recuperative heat exchanger such that the gas stream GCis heated by the cooling gas stream G; wherein the heat withdrawn from the compressed gas stream fraction G2is added to the recirculating gas stream GCafter pumping to pressure P4. In further embodiments, the method for heating a recirculating gas stream may comprise any one or more of the following statements in any combination thereof.

The temperature T3can be about 100° C. to about 400° C.

The pressure P2of gas stream fraction G1and the pressure P3of gas stream fraction G2can each separately be about 40 bar (4 MPa) to about 100 bar (10 MPa).

The pressure P4of the recirculating gas stream GCcan be about 100 bar (10 MPa) to about 500 bar (50 MPa).

The mass ratio of gas fraction G1to gas fraction G2based on the total mass of gas stream G can be about 50:50 to about 99:1, or can be about 50:50 to about 90:10, or can be about 50:50 to about 70:30 or can be about 70:30 to about 90:10.

The recirculating gas stream GCafter passing through the recuperative heat exchanger and receiving the heat from the compressed gas fraction G2can have a temperature T4that is within 50° C. of T1.

The gas stream fraction G2can be compressed with multi-stage compression with no intercooling.

After withdrawing heat from gas stream fraction G2, the gas stream fraction G2can be further compressed before combining with gas stream fraction G1.

The recuperative heat exchanger can comprise three heat exchangers or three heat exchange sections in series. In such embodiments, heat can be transferred in a first heat exchanger or heat exchange section operating in temperature range R1, a second heat exchanger or heat exchange section operating in temperature range R2, and a third heat exchanger or heat exchange section operating in temperature range R3with the temperature relationship of R1>R2>R3.

The gas stream G can be separated between the first heat exchanger or heat exchange section and the second heat exchanger or heat exchange section.

The gas stream G can separated between the second heat exchanger or heat exchange section and the third heat exchanger or heat exchange section.

The heat withdrawn from the compressed gas stream fraction G2can be added to the recirculating gas stream GCin one or both of the third heat exchanger or heat exchange section and the second heat exchanger or heat exchange section.

The method further can comprise adding heat to the recirculating gas stream GCafter pumping to pressure P4. In such embodiments, the added heat can be derived from one or both of an air separation unit and a gas turbine.

The method further can comprise passing the heated recirculating gas stream GCfrom the recuperative heat exchanger to a combustor that combusts a fuel with oxygen to form a combustion product stream.

The gas stream G can be a turbine exhaust stream.

In some embodiments, the present disclosure further can relate to a method of generating power. As an example, the method can comprise the following steps: combusting a fuel with oxygen in the combustor in the presence of a recycle CO2stream to produce a CO2containing combustion stream; passing the CO2containing combustion stream through a turbine to expand the CO2containing combustion stream, generate power, and form a turbine exhaust stream; withdrawing heat from the turbine exhaust stream; dividing the turbine exhaust stream to form a first turbine exhaust portion and second turbine exhaust portion; separating water from the first turbine exhaust portion to form a main recycle CO2stream; compressing the main recycle CO2stream; compressing the second turbine exhaust portion to form a heated, compressed second turbine exhaust portion; withdrawing heat from the heated, compressed second turbine exhaust portion; separating water from the cooled, compressed second turbine exhaust portion to form a secondary recycle CO2stream; combining the main recycle CO2stream and the secondary recycle CO2stream to form a combined recycle CO2stream; compressing the combined recycle CO2stream; heating the combined recycle CO2stream with heat withdrawn from the turbine exhaust stream; and further heating the combined recycle CO2stream with heat withdrawn from the heated, compressed second turbine exhaust portion. In further embodiments, the method can comprise one or more of the following statements in any combination.

The CO2containing combustion stream can have a temperature of about 500° C. to about 1,700° C. and a pressure of about 100 bar (10 MPa) to about 500 bar (50 MPa).

The pressure ratio across the turbine can be about 5 to about 12.

The heat can be withdrawn from the turbine exhaust stream in a recuperative heat exchanger comprising three or more sections or comprising three or more individual heat exchangers.

Heating the combined recycle CO2stream with heat withdrawn from the turbine exhaust stream and further heating the combined recycle CO2stream with heat withdrawn from the heated, compressed second turbine exhaust portion can be carried out in the recuperative heat exchanger.

The mass ratio of the first turbine exhaust portion to the second turbine exhaust portion based on the total mass of the turbine exhaust stream can be about 50:50 to about 99:1.

The heat withdrawn from the heated, compressed second turbine exhaust portion can be in a temperature range of about 100° C. to about 400° C.

The main recycle CO2stream and the second turbine exhaust portion can be independently compressed to a pressure of about 40 bar (4 MPa) to about 100 bar (10 MPa).

The combined recycle CO2stream after heating with heat withdrawn from the turbine exhaust stream and further heating with heat withdrawn from the heated, compressed second turbine exhaust portion can have a temperature that is within 50° C. of the temperature of the turbine exhaust stream.

The second turbine exhaust portion can be compressed adiabatically with no intercooling between compressor stages.

In some embodiments, a power generating system according to the present disclosure can comprise the following: a combustor configured to exhaust a combustion stream; a power production turbine configured to receive and expand the combustion stream and form a turbine exhaust stream; a recuperative heat exchanger configured to receive the turbine exhaust stream; a flow separator configured to separate the cooled turbine exhaust stream into a first gas stream and a second gas stream; a first compressor configured to receive and compress the first gas stream; a second compressor configured to receive and compress the second gas stream; a pump configured to pressurize the first gas stream and the second gas stream in combination, the pump positioned downstream from the first compressor and the second compressor; a first flow path through the recuperative heat exchanger configured for passage of the turbine exhaust stream; a second flow path through the recuperative heat exchanger configured for passage of the pressurized first gas stream and second gas stream in combination; a third flow path through the recuperative heat exchanger configured for passage of the compressed second gas stream; wherein the first flow path and the third flow path are configured for heating the second flow path. In further embodiments, the system can include any one or more of the following statements in any combination.

The recuperative heat exchanger can comprise a series of three or more heat exchangers or a series of three or more heating sections.

The system further can comprise one or more separators configured for separating at least water from one or both of the first gas stream and the second gas stream.

The first compressor can comprise a multi-stage, intercooled compressor.

The second compressor can comprise an adiabatic, multi-stage compressor with no intercooling between compressor stages.

