A supercritical water gasification cascaded cycle power generation system for coal and its operation method

Abstract

The invention discloses a supercritical water gasification cascade cycle power generation system for coal and an operation method thereof. The system includes a supercritical water gasification oxidation unit, a mixed working fluid cycle unit, a steam cycle unit, a cooling carbon capture unit, and a gasification feed water preheating unit. The invention adopts a cascade thermal system of a mixed working fluid Brayton cycle and a steam Rankine cycle to efficiently, cleanly, and low-carbon convert the chemical energy of coal into electric energy required by users, and can realize the recycling of water and the complete capture of carbon dioxide; the core gasification process adopts a partial oxidation self-heating heating method to avoid the generation of external heat sources and additional pollution; based on the principle of cascaded utilization of energy grades, the waste heat of the turbine exhaust is used to drive a steam turbine to generate electricity and preheat the gasification feed water respectively according to the heat transfer temperature difference, effectively reducing the irreversible heat transfer loss and improving the power generation efficiency of the system.

Description

Technical Field

[0001] The present invention relates to the technical field of efficient, clean and low-carbon conversion and utilization of coal, and particularly relates to a coal supercritical water gasification cascade cycle power generation system and an operation method thereof. Background Art

[0002] As a fossil fuel with high cost performance and rich reserves, coal still accounts for a quite large proportion in the primary energy supply of our country. However, the traditional coal utilization method converts the high-quality chemical energy of coal into heat energy, resulting in large energy quality losses and environmental pollution, and is also the main source of carbon emissions. Therefore, in order to achieve efficient, clean and low-carbon utilization of coal, constructing a new low-carbon energy conversion system is of great significance for the transformation of the energy structure of our country.

[0003] Coal supercritical water gasification, as a new technology for clean and efficient conversion and utilization of coal, converts the chemical energy of coal into high-calorific value synthesis gas mainly composed of hydrogen and carbon dioxide in the atmosphere of supercritical water. Elements such as nitrogen and sulfur are converted into inorganic salts and leave the gasification reactor with the slag discharge, without generating air pollution, and it is easy to separate and capture carbon dioxide, having advantages such as high hydrogen production rate, clean process, and strong coal type adaptability.

[0004] The configuration of the coal supercritical water gasification system can be divided into an oxidation utilization type and a separation utilization type. Based on the oxidation utilization of synthesis gas, a new power generation system can be constructed. At present, the energy efficiency of the thermal power generation system based on coal supercritical water gasification technology varies greatly; technical problems such as the regeneration of the mixed working medium of water and carbon dioxide and ultra-high temperature turbines are not considered in the system construction; some studies use external coal-fired, gas-fired boilers or high-quality electric heating and other methods, causing additional environmental pollution and poor energy potential matching of the system. Therefore, in order to avoid the above problems, it is necessary to design a new supercritical water gasification power generation system to fully realize the efficient, clean and low-carbon power generation of coal. Summary of the Invention

[0005] To overcome the problems existing in the above-mentioned prior art, the purpose of the present invention is to provide a coal supercritical water gasification cascade cycle power generation system and an operation method thereof. The system adopts a cascade thermal system of a mixed working medium Brayton cycle and a steam Rankine cycle, efficiently, cleanly and low-carbonly converts the chemical energy of coal into electric energy required by users, and can realize the recycling of water and the complete capture of carbon dioxide; the core gasification process adopts a partial oxidation self-heating heat supply method to avoid the generation of external heat sources and additional pollution; based on the principle of cascade utilization of energy grades, the waste heat of the turbine exhaust drives the steam turbine to generate electricity and preheats the gasification feed water respectively according to the heat transfer temperature difference, effectively reducing the irreversible loss of heat transfer and improving the power generation efficiency of the system.

[0006] To achieve the above purpose, the present invention adopts the following technical solutions:

[0007] A supercritical water gasification cascade cycle power generation system for coal, comprising a supercritical water gasification and oxidation unit, a mixed working fluid cycle unit, a steam cycle unit, a cooling carbon capture unit, and a gasification feed water preheating unit;

[0008] The supercritical water gasification and oxidation unit includes a supercritical water gasification reactor 11 and an oxidation reactor 12; coal is connected to the inlet of the supercritical water gasification reactor 11 through a pipeline; the preheated gasification feed water is connected to the inlet of the supercritical water gasification reactor 11 from the cold-side outlet of the feed water preheater 52; oxygen is connected to the inlets of the supercritical water gasification reactor 11 and the oxidation reactor 12 through pipelines respectively; ash is discharged from the ash outlet of the supercritical water gasification reactor 11 and connected to the external environment through an ash discharge pipeline; the outlet of the supercritical water gasification reactor 11 is connected to the inlet of the oxidation reactor 12;

[0009] The mixed working fluid cycle unit includes a mixed working fluid turbine 21; the outlet of the oxidation reactor 12 is connected to the inlet of the mixed working fluid turbine 21;

[0010] The steam cycle unit includes a waste heat boiler 31, a steam turbine 32, a condenser 33, a circulating water pump 34, and a low-temperature heat exchanger 35; the outlet of the mixed working fluid turbine 21 is connected to the hot-side inlet of the waste heat boiler 31, and the hot-side outlet of the waste heat boiler 31 is connected to the hot-side inlet of the feed water preheater 52; the hot-side outlet of the feed water preheater 52 is connected to the hot-side inlet of the low-temperature heat exchanger 35; the cold-side outlet of the waste heat boiler 31 is connected to the inlet of the high-pressure cylinder of the steam turbine 32; the outlet of the low-pressure cylinder of the steam turbine 32 is connected to the inlet of the condenser 33; the outlet of the condenser 33 is connected to the inlet of the circulating water pump 34; the outlet of the circulating water pump 34 is connected to the cold-side inlet of the low-temperature heat exchanger 35; the cold-side outlet of the low-temperature heat exchanger 35 is connected to the cold-side inlet of the waste heat boiler 31;

[0011] The cooling carbon capture unit includes an exhaust steam cooler 41 and a gas-liquid separator 42; the hot-side outlet of the low-temperature heat exchanger 35 is connected to the inlet of the exhaust steam cooler 41; the outlet of the exhaust steam cooler 41 is connected to the inlet of the gas-liquid separator 42; the liquid-phase outlet of the gas-liquid separator 42 is connected to the inlet of the feed water pump 51; the excess water in the liquid-phase outlet of the gas-liquid separator 42 is output from the system through a drain pipeline; the gas-phase outlet of the gas-liquid separator 42 performs complete carbon dioxide capture;

[0012] The gasification feed water preheating unit includes a feed water pump 51 and a feed water preheater 52; the outlet of the feed water pump 51 is connected to the cold-side inlet of the feed water preheater 52; the cold-side outlet of the feed water preheater 52 is connected to the inlet of the supercritical water gasification reactor 11.

[0013] The operating temperature of the supercritical water gasification reactor 11 is 500–800 °C, and the operating pressure is 23–30 MPa.

[0014] By changing the amount of oxygen entering the supercritical water gasification reactor 11 and the oxidation reactor 12, the oxygen entering the supercritical water gasification reactor 11 undergoes a partial oxidation reaction with coal, and the released heat exactly provides the heat required to heat the gasification feed water to the supercritical state and for the gasification reaction. The entire gasification process reaches self-thermal equilibrium, avoiding the generation of external heat sources and additional pollution.

[0015] The outlet of the oxidation reactor 12 is a mixed working fluid of supercritical water and supercritical carbon dioxide.

[0016] The inlet temperature of the mixed working fluid turbine 21 is 1100–1700 °C, and the exhaust pressure is 0.1 MPa.

[0017] The exhaust steam at the outlet of the mixed working fluid turbine 21 has a relatively high temperature. According to the heat transfer temperature difference, it drives the steam Rankine cycle to generate electricity and preheats the gasification feed water respectively, recovering the exhaust steam waste heat, which can effectively reduce the heat transfer irreversibility and improve the system power generation efficiency.

[0018] The steam cycle unit can adopt a dual-pressure reheat steam cycle to recover the exhaust steam waste heat of the mixed working fluid at a relatively high temperature and reduce the steam exhaust humidity.

[0019] The exhaust steam at the outlet of the mixed working fluid turbine 21 successively passes through the waste heat boiler 31, the feed water preheater 52, and the low-temperature heat exchanger 35 for exhaust steam waste heat recovery. The outlet temperature of the hot side of the low-temperature heat exchanger 35 is 85–100 °C, and the outlet temperature of the exhaust steam cooler 41 is 25–35 °C. The large amount of low-temperature latent heat released by the exhaust steam cooler 41 can be used for heating, further recovering and utilizing the exhaust steam waste heat of the mixed working fluid.

[0020] By adjusting the ratio of the exhaust steam waste heat distributed to the steam cycle and the preheating of the gasification feed water, the temperature of the gasification feed water at the cold side outlet of the feed water preheater 52 can be changed, thereby adjusting the temperature of the mixed working fluid entering the mixed working fluid turbine 21 to meet different turbine inlet temperature limits and improving the operation flexibility of the unit.

[0021] The operation method of the coal supercritical water gasification cascade cycle power generation system is as follows: the gasification feed water is preheated to a certain temperature by the feed water preheater 52 and then enters the supercritical water gasification reactor 11, where it is heated to a supercritical state and undergoes a gasification reaction with the coal to generate a high calorific value gasification synthesis gas; oxygen is connected to the inlet of the supercritical water gasification reactor 11 through a pipeline, undergoes a partial oxidation reaction with the coal, and provides the heat required for heating the gasification feed water to a supercritical state and the gasification reaction, thereby realizing a fuel partial oxidation self-heating heating mode; the gasification synthesis gas enters the oxidation reactor 12 and undergoes an oxidation reaction with pure oxygen to obtain a mixed working medium of supercritical water and supercritical carbon dioxide; the mixed working medium enters the mixed working medium turbine 21 to expand and do work; the exhaust steam of the mixed working medium turbine 21 enters the waste heat boiler 31 to heat the steam circulating water to high-temperature and high-pressure steam; The steam at the cold side outlet of the waste heat boiler 31 enters the steam turbine 32 to expand and do work; the exhaust steam of the steam turbine 32 passes through the condenser 33 and the circulating water pump 34 and enters the low-temperature heat exchanger 35 for further recovering the waste heat of the low-temperature exhaust steam of the mixed working medium; the cold side outlet of the low-temperature heat exchanger 35 enters the cold side inlet of the waste heat boiler 31 for recovering the waste heat of the high-temperature exhaust steam of the mixed working medium and completing the closed steam Rankine cycle; the exhaust steam of the mixed working medium turbine 21 passes through the waste heat boiler 31, the feed water preheater 52 and the low-temperature heat exchanger 35 to be reduced to a certain temperature, and then enters the gas-liquid separator 42 after further cooling through the exhaust steam cooler 41; the gas-liquid separator 42 reaches phase equilibrium, carbon dioxide exists in the gas phase, and water condenses in the liquid phase. The condensed water enters the feed water pump 51 to provide the water required for gasification, and the remaining water is output to the system for recovery, which can achieve complete capture of carbon dioxide and recycling of water.

[0022] The present invention has the following beneficial effects:

[0023] 1) The system uses supercritical water gasification technology to efficiently, cleanly and low-carbonly convert coal chemical energy into the electricity required by users, avoiding the generation of nitrogen and sulfur pollutants, and can achieve water recycling and complete capture of carbon dioxide;

[0024] 2) Based on the principle of cascade utilization of energy grade, the exhaust heat of the mixed working medium turbine drives the steam turbine to generate electricity and preheat the gasification feed water according to the heat exchange temperature difference. The low-temperature latent heat released by the exhaust cooler can continue to supply heat to the outside, making full use of the exhaust heat of the mixed working medium, effectively reducing the irreversible loss of heat transfer, and improving the power generation efficiency of the system;

[0025] 3) The core process of gasification adopts the self-heating heating mode of partial oxidation of fuel, which releases heat just enough to provide the heat required for heating the gasification feed water to the supercritical state and the gasification reaction, avoiding the generation of external heat sources and additional pollution;

[0026] 4) By adjusting the ratio of the exhaust waste heat distributed to the steam cycle and the preheating of the gasification feed water, the feed water preheating temperature can be changed, and then the inlet temperature of the mixed working fluid turbine can be adjusted to meet the inlet temperature limits of different turbine technologies, improving the operating flexibility of the unit. Brief Description of the Drawings

[0027] Figure 1 It is a schematic diagram of the supercritical water gasification cascade cycle power generation system of the present invention for coal.

[0028] Figure 2 It is a curve graph of the system efficiency varying with the concentration of water coal slurry.

[0029] Figure 1 In the figure: 11 is a supercritical water gasification reactor, 12 is an oxidation reactor, 21 is a mixed working fluid turbine, 31 is a waste heat boiler, 32 is a steam turbine, 33 is a condenser, 34 is a circulating water pump, 35 is a low-temperature heat exchanger, 41 is an exhaust gas cooler, 42 is a gas-liquid separator, 51 is a feed water pump, and 52 is a feed water preheater.

[0030] Figure 2 In the figure: Keeping the coal quality and coal quantity consistent, when the inlet temperature of the mixed working fluid turbine is controlled at 1500 °C, it reflects the net power generation efficiency of the system and the variation of the efficiency with the concentration of water coal slurry. Detailed Embodiment

[0031] The present invention will be further described in detail below with reference to the drawings and tables. As Figure 1 shown, a supercritical water gasification cascade cycle power generation system for coal according to the present invention includes a supercritical water gasification and oxidation unit, a mixed working fluid cycle unit, a steam cycle unit, a cooling and carbon capture unit, and a gasification feed water preheating unit;

[0032] The supercritical water gasification and oxidation unit includes a supercritical water gasification reactor 11 and an oxidation reactor 12; coal is connected to the inlet of the supercritical water gasification reactor 11 through a pipeline; the preheated gasification feed water is connected to the inlet of the supercritical water gasification reactor 11 from the cold-side outlet of the feed water preheater 52; oxygen is connected to the inlets of the supercritical water gasification reactor 11 and the oxidation reactor 12 through pipelines respectively; ash comes out from the ash outlet of the supercritical water gasification reactor 11 and is connected to the external environment through an ash discharge pipeline; the outlet of the supercritical water gasification reactor 11 is connected to the inlet of the oxidation reactor 12;

[0033] The mixed working fluid cycle unit includes a mixed working fluid turbine 21; the outlet of the oxidation reactor 12 is connected to the inlet of the mixed working fluid turbine 21;

[0034] The steam cycle unit includes a waste heat boiler 31, a steam turbine 32, a condenser 33, a circulating water pump 34, and a low-temperature heat exchanger 35; the outlet of the mixed working fluid turbine 21 is connected to the hot-side inlet of the waste heat boiler 31, and the hot-side outlet of the waste heat boiler 31 is connected to the hot-side inlet of the feed water preheater 52; the hot-side outlet of the feed water preheater 52 is connected to the hot-side inlet of the low-temperature heat exchanger 35; the cold-side outlet of the waste heat boiler 31 is connected to the high-pressure cylinder inlet of the steam turbine 32; the low-pressure cylinder outlet of the steam turbine 32 is connected to the inlet of the condenser 33; the outlet of the condenser 33 is connected to the inlet of the circulating water pump 34; the outlet of the circulating water pump 34 is connected to the cold-side inlet of the low-temperature heat exchanger 35; the cold-side outlet of the low-temperature heat exchanger 35 is connected to the cold-side inlet of the waste heat boiler 31;

[0035] The cooling carbon capture unit includes an exhaust gas cooler 41 and a gas-liquid separator 42; the hot-side outlet of the low-temperature heat exchanger 35 is connected to the inlet of the exhaust gas cooler 41; the outlet of the exhaust gas cooler 41 is connected to the inlet of the gas-liquid separator 42; the liquid-phase outlet of the gas-liquid separator 42 is connected to the inlet of the feed water pump 51; the excess water in the liquid-phase outlet of the gas-liquid separator 42 is output from the system through a drain pipe; the gas-phase outlet of the gas-liquid separator 42 performs complete carbon dioxide capture;

[0036] The gasification feed water preheating unit includes a feed water pump 51 and a feed water preheater 52; the outlet of the feed water pump 51 is connected to the cold-side inlet of the feed water preheater 52; the cold-side outlet of the feed water preheater 52 is connected to the inlet of the supercritical water gasification reactor 11.

[0037] As a preferred embodiment of the present invention, the operating temperature of the supercritical water gasification reactor 11 is 500–800 °C, and the operating pressure is 23–30 MPa, ensuring that the gasification reaction conditions are above the temperature and pressure critical point of water, and giving full play to the advantages of supercritical water gasification.

[0038] As a preferred embodiment of the present invention, by changing the amount of oxygen entering the supercritical water gasification reactor 11 and the oxidation reactor 12, the oxygen entering the supercritical water gasification reactor 11 undergoes a partial oxidation reaction with coal, and the released heat just provides the heat required for heating the feed water to the supercritical state and the gasification reaction. The entire gasification process reaches self-thermal equilibrium, avoiding the generation of external heat sources and additional pollution.

[0039] As a preferred embodiment of the present invention, the outlet of the oxidation reactor 12 is a mixed working fluid of supercritical water and supercritical carbon dioxide.

[0040] As a preferred embodiment of the present invention, the inlet gas temperature of the mixed working fluid turbine 21 is 1100–1700 °C, and the exhaust pressure is 0.1 MPa. On the basis of meeting the existing turbine technology inlet temperature limit, the inlet gas temperature of the mixed working fluid turbine is increased, the power generation of the mixed working fluid is increased, and the exhaust can be directly subjected to carbon capture subsequently, without the need for additional power consumption such as pressurization.

[0041] As a preferred embodiment of the present invention, the exhaust steam at the outlet of the mixed working fluid turbine 21 has a relatively high temperature, which respectively drives the steam Rankine cycle power generation and preheats the gasified feed water according to the heat transfer temperature difference, so as to recover the exhaust steam waste heat. Based on the principle of cascade utilization of energy grades, the irreversible heat transfer loss can be effectively reduced and the power generation efficiency of the system can be improved.

[0042] As a preferred embodiment of the present invention, the steam cycle unit can adopt a dual-pressure reheat steam cycle to recover the exhaust steam waste heat of the mixed working fluid at a relatively high temperature, improve the energy potential matching between the hot and cold sides of the waste heat boiler, increase the power generation of the steam turbine, and reduce the steam exhaust humidity.

[0043] As a preferred embodiment of the present invention, the exhaust steam at the outlet of the mixed working fluid turbine 21 sequentially passes through the waste heat boiler 31, the feed water preheater 52, and the low-temperature heat exchanger 35 for exhaust steam waste heat recovery. The hot-side outlet temperature of the low-temperature heat exchanger 35 is 85–100 °C, and the outlet temperature of the exhaust steam cooler 41 is 25–35 °C, which is beneficial to improving the capture purity of carbon dioxide. The large amount of low-temperature latent heat released by the exhaust steam cooler 41 can continue to supply heat outward to further recover the exhaust steam waste heat of the mixed working fluid.

[0044] As a preferred embodiment of the present invention, by adjusting the ratio of the exhaust steam waste heat of the mixed working fluid distributed to the steam cycle and the preheating of the gasified feed water, the cold-side outlet temperature of the feed water preheater 52 can be changed, and then the inlet temperature of the mixed working fluid turbine 21 can be adjusted to meet the inlet temperature limits of different turbine technologies and improve the operation flexibility of the unit.

[0045] Such as Figure 1As shown in the figure, the operation method of a supercritical water gasification cascaded cycle power generation system of the present invention is as follows: The gasification feed water is preheated to a certain temperature by the feed water preheater 52 and then enters the supercritical water gasification reactor 11, where it is heated to the supercritical state and undergoes a gasification reaction with coal to generate a high-calorific value gasification synthesis gas; oxygen is connected to the inlet of the supercritical water gasification reactor 11 through a pipeline and undergoes a partial oxidation reaction with coal to provide the heat required to heat the gasification feed water to the supercritical state and the gasification reaction, realizing the fuel partial oxidation self-heating heat supply method; the gasification synthesis gas enters the oxidation reactor 12 and undergoes an oxidation reaction with pure oxygen to obtain a mixed working medium of supercritical water and supercritical carbon dioxide; the mixed working medium enters the mixed working medium turbine 21 to expand and do work; the exhaust steam of the mixed working medium turbine 21 enters the waste heat boiler 31 to heat the steam circulating water to high-temperature and high-pressure steam; the steam at the cold side outlet of the waste heat boiler 31 enters the steam turbine 32 to expand and do work; the exhaust steam of the steam turbine 32 passes through the condenser 33 and the circulating water pump 34 and then enters the low-temperature heat exchanger 35 to further recover the waste heat of the low-temperature exhaust steam of the mixed working medium; the cold side outlet of the low-temperature heat exchanger 35 enters the cold side inlet of the waste heat boiler 31 to recover the waste heat of the high-temperature exhaust steam of the mixed working medium, completing the closed steam Rankine cycle; the exhaust steam of the mixed working medium turbine 21 is reduced to a certain temperature after passing through the waste heat boiler 31, the feed water preheater 52 and the low-temperature heat exchanger 35, and then enters the exhaust steam cooler 41 for further cooling and then enters the gas-liquid separator 42; the gas-liquid separator 42 reaches phase equilibrium, carbon dioxide exists in the gas phase, water condenses and exists in the liquid phase, the condensed water enters the feed water pump 51 to provide the required feed water volume for gasification, and the remaining water is output from the system for recovery, realizing the complete capture of carbon dioxide and the recycling of water.

[0046] As shown in Table 1, when the coal quality and coal quantity are kept consistent and the inlet temperature of the mixed working medium turbine 21 is controlled at 1500 °C, the main logistics and project parameter sizes of the system under different coal water slurry concentrations are shown. By changing the gasification feed water flow rate, the change range of the coal water slurry concentration is 19% - 22%, and different coal water slurry concentrations have an impact on the flow rates of the components of the gasification synthesis gas. Since the inlet temperature of the mixed working medium turbine 21, that is, the outlet temperature of the oxidation reactor 12, is controlled at 1500 °C, the corresponding preheating temperatures of the gasification feed water are different for different coal water slurry concentrations, and the oxygen consumption required for the supercritical water gasification reactor 11 and the oxidation reactor 12 also varies accordingly. The change of the coal water slurry concentration will affect the mixed working medium flow rate and the steam cycle flow rate, and then change the power generation of the mixed working medium turbine and the steam turbine, ultimately affecting the net power generation and efficiency of the system.

[0047] Table 1

[0048]

[0049] As Figure 2As shown, when the concentration of the water-coal slurry decreases from 22% to 19%, the net power generation efficiency of the system increases from 49.8% to 52.0%. The efficiency increases from 48.5% to 50.7%, resulting in a 2.2 percentage point increase in the system efficiency.

Claims

1. A supercritical water gasification cascade cycle power generation system for coal, characterized in that: The system includes a supercritical water gasification and oxidation unit, a mixed working fluid circulation unit, a steam circulation unit, a cooling carbon capture unit, and a gasification feed water preheating unit; The supercritical water gasification and oxidation unit includes a supercritical water gasification reactor (11) and an oxidation reactor (12); coal is connected to the inlet of the supercritical water gasification reactor (11) through a pipeline; the preheated gasification feed water is connected to the inlet of the supercritical water gasification reactor (11) from the cold-side outlet of the feed water preheater (52); oxygen is connected to the inlets of the supercritical water gasification reactor (11) and the oxidation reactor (12) respectively through pipelines; ash comes out from the ash outlet of the supercritical water gasification reactor (11) and is connected to the external environment through an ash discharge pipeline; the outlet of the supercritical water gasification reactor (11) is connected to the inlet of the oxidation reactor (12); The mixed working fluid circulation unit includes a mixed working fluid turbine (21); the outlet of the oxidation reactor (12) is connected to the inlet of the mixed working fluid turbine (21); The steam circulation unit includes a waste heat boiler (31), a steam turbine (32), a condenser (33), a circulating water pump (34), and a low-temperature heat exchanger (35); the outlet of the mixed working fluid turbine (21) is connected to the hot-side inlet of the waste heat boiler (31), and the hot-side outlet of the waste heat boiler (31) is connected to the hot-side inlet of the feed water preheater (52); the hot-side outlet of the feed water preheater (52) is connected to the hot-side inlet of the low-temperature heat exchanger (35); the cold-side outlet of the waste heat boiler (31) is connected to the inlet of the high-pressure cylinder of the steam turbine (32); the outlet of the low-pressure cylinder of the steam turbine (32) is connected to the inlet of the condenser (33); the outlet of the condenser (33) is connected to the inlet of the circulating water pump (34); the outlet of the circulating water pump (34) is connected to the cold-side inlet of the low-temperature heat exchanger (35); the cold-side outlet of the low-temperature heat exchanger (35) is connected to the cold-side inlet of the waste heat boiler (31); The cooling carbon capture unit includes an exhaust steam cooler (41) and a gas-liquid separator (42); the hot-side outlet of the low-temperature heat exchanger (35) is connected to the inlet of the exhaust steam cooler (41); the outlet of the exhaust steam cooler (41) is connected to the inlet of the gas-liquid separator (42); the liquid-phase outlet of the gas-liquid separator (42) is connected to the inlet of the feed water pump (51); the excess water in the liquid-phase outlet of the gas-liquid separator (42) is output from the system through a drain pipeline; carbon dioxide is completely captured at the gas-phase outlet of the gas-liquid separator (42); The gasification feed water preheating unit includes a feed water pump (51) and a feed water preheater (52); the outlet of the feed water pump (51) is connected to the cold-side inlet of the feed water preheater (52); the cold-side outlet of the feed water preheater (52) is connected to the inlet of the supercritical water gasification reactor (11).

2. The supercritical water gasification cascade cycle power generation system for coal according to claim 1, characterized in that: The operating temperature of the supercritical water gasification reactor (11) is 500–800 °C, and the operating pressure is 23–30 MPa.

3. The supercritical water gasification cascade cycle power generation system for coal according to claim 1, characterized in that: By changing the amount of oxygen entering the supercritical water gasification reactor (11) and the oxidation reactor (12), partial oxidation reaction occurs between the oxygen entering the supercritical water gasification reactor (11) and the coal, and the released heat exactly provides the heat required to heat the gasification feed water to the supercritical state and for the gasification reaction. The entire gasification process reaches self-thermal equilibrium, avoiding the generation of external heat sources and additional pollution.

4. The supercritical water gasification cascade cycle power generation system for coal according to claim 1, characterized in that: The outlet of the oxidation reactor (12) is a mixed working fluid of supercritical water and supercritical carbon dioxide.

5. The supercritical water gasification cascade cycle power generation system for coal according to claim 1, characterized in that: The intake temperature of the mixed working fluid turbine (21) is 1100–1700 °C, and the exhaust pressure is 0.1 MPa.

6. The supercritical water gasification cascade cycle power generation system for coal according to claim 1, characterized in that: The exhaust steam temperature at the outlet of the mixed working fluid turbine (21) is high. According to the heat transfer temperature difference, it drives the steam Rankine cycle power generation and preheats the gasification feed water respectively to recover the exhaust steam waste heat, effectively reducing the heat transfer irreversibility and improving the system power generation efficiency.

7. The supercritical water gasification cascade cycle power generation system for coal according to claim 1, characterized in that: The steam cycle unit adopts a dual-pressure reheat steam cycle to recover the high-temperature exhaust steam waste heat of the mixed working fluid and reduce the steam exhaust humidity.

8. The supercritical water gasification cascade cycle power generation system for coal according to claim 1, characterized in that: The exhaust steam at the outlet of the mixed working fluid turbine (21) sequentially passes through the waste heat boiler (31), the feed water preheater (52), and the low-temperature heat exchanger (35) for exhaust steam waste heat recovery. The hot-side outlet temperature of the low-temperature heat exchanger (35) is 85–100 °C, and the outlet temperature of the exhaust steam cooler (41) is 25–35 °C. The large amount of low-temperature latent heat released by the exhaust steam cooler (41) is used for heating to further recover and utilize the exhaust steam waste heat of the mixed working fluid.

9. The supercritical water gasification cascade cycle power generation system for coal according to claim 1, characterized in that: By adjusting the ratio of the exhaust steam waste heat distributed to the steam cycle and the preheating of the gasification feed water, the temperature of the gasification feed water at the cold-side outlet of the feed water preheater (52) is changed, thereby adjusting the temperature of the mixed working fluid entering the mixed working fluid turbine (21) to meet different turbine inlet temperature limits and improving the operating flexibility of the unit.

10. A method for operating the supercritical water gasification cascade cycle power generation system for coal according to any one of claims 1 to 9, characterized in that: The gasified feed water is preheated to a certain temperature by the feed water preheater (52) and then enters the supercritical water gasification reactor (11). It is heated to the supercritical state in the supercritical water gasification reactor (11) and undergoes a gasification reaction with coal to produce high-calorific value gasified synthesis gas. Oxygen is connected to the inlet of the supercritical water gasification reactor (11) through a pipeline and undergoes a partial oxidation reaction with coal to provide the heat required to heat the gasified feed water to the supercritical state and for the gasification reaction, realizing the fuel partial oxidation self-heating heat supply mode. The gasified synthesis gas enters the oxidation reactor (12) and undergoes an oxidation reaction with pure oxygen to obtain a mixed working fluid of supercritical water and supercritical carbon dioxide. The mixed working fluid enters the mixed working fluid turbine (21) to expand and do work. The exhaust steam of the mixed working fluid turbine (21) enters the waste heat boiler (31) to heat the steam circulating water to high-temperature and high-pressure steam. The steam at the cold side outlet of the waste heat boiler (31) enters the steam turbine (32) to expand and do work. The exhaust steam of the steam turbine (32) passes through the condenser (33) and the circulating water pump (34) and then enters the low-temperature heat exchanger (35) to further recover the waste heat of the low-temperature exhaust steam of the mixed working fluid. The outlet of the cold side of the low-temperature heat exchanger (35) enters the inlet of the cold side of the waste heat boiler (31) to recover the waste heat of the high-temperature exhaust steam of the mixed working fluid, completing the closed steam Rankine cycle. The exhaust steam of the mixed working fluid turbine (21) is reduced to a certain temperature after passing through the waste heat boiler (31), the feed water preheater (52) and the low-temperature heat exchanger (35), and is further cooled by the exhaust steam cooler (41) and then enters the gas-liquid separator (42). The gas-liquid separator (42) reaches phase equilibrium, with carbon dioxide existing in the gas phase and water condensing and existing in the liquid phase. The condensed water enters the feed water pump (51) to provide the feed water required for gasification, and the remaining water is output from the system for recovery, realizing the complete capture of carbon dioxide and the recycling of water.

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