A method for coupling green hydrogen with coal-to-methanol integrated with supercritical CO2 cycle

Abstract

The present invention provides a method for coupling green hydrogen with coal-to-methanol integrated with supercritical CO₂ cycle power generation, including: a renewable energy electrolytic water device, a coal gasification device, a conversion device, a purification device, a methanol synthesis device, a carbon dioxide hydrogenation-to-methanol device, a methanol rectification device, and a supercritical CO₂ cycle device; hydrogen and oxygen required for the whole process are generated by the renewable energy electrolytic water device, a part of the oxygen is input into the coal gasification device to meet the gasification requirement, and the other part enters the supercritical CO₂ cycle device as a combustion-supporting gas, and the hydrogen generated by electrolysis is used to produce methanol with the CO₂ removed by the purification device. Among them, considering the resource utilization of the purge gas in the process of coupling green hydrogen with coal-to-methanol, the purge gas is used as a fuel gas, and through the integration of supercritical CO₂ cycle power generation, the recycling of process materials is promoted, realizing the co-production of methanol and electricity as well as the low-cost separation and capture of CO₂, having the advantages of high efficiency, energy conservation, and emission reduction.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy coupling utilization, and particularly relates to a method for coupling green hydrogen with coal-to-methanol integrated with supercritical CO2 cycle power generation. Background Art

[0002] The coupling of green hydrogen with coal chemical industry, especially the process of coal-to-methanol, has received extensive attention in recent years. By hydrogenating the removed carbon dioxide to methanol, it is considered to be able to well solve problems such as large carbon emissions and low resource utilization rate in this process.

[0003] Although the proportion of the purge gas in the recycle gas of this process is relatively small (5%-15%), in large-scale production, the unutilized purge gas is still an important part of resource waste and environmental pollution. The evacuation of the purge gas will undoubtedly cause a large amount of pollution and resource waste, and burning it in the flare will also cause a large amount of emissions. Recycling hydrogen through pressure swing adsorption to reduce resource waste is one of the current effective ways to utilize the purge gas. However, the cost of pressure swing adsorption is its limiting factor, and the utilization of other resources has not been taken seriously. At the same time, for the oxygen generated in the process of green hydrogen production, a part of it is used as a gasifying agent, but there is still surplus oxygen, which is usually sold as a by-product without effective and reasonable utilization of this part of oxygen in the process. Therefore, how to promote the utilization of the purge gas and surplus oxygen is of great significance for resource recovery, emission reduction, and system efficiency improvement.

[0004] The Allam cycle is a regenerative Brayton cycle that uses supercritical CO2 (sCO2) as the working fluid and burns gaseous fuel and oxygen in the combustion chamber. The main difference from the sCO2 thermodynamic cycle that heats the working fluid through indirect heat exchange is that it uses direct combustion to increase the temperature. Since the combustion is in an atmosphere of CO2 and oxygen, there are no pollutants such as NO x etc., which is conducive to the low-cost capture of CO2. Moreover, the cycle efficiency of sCO2 has obvious advantages in a relatively large temperature range. At present, some studies have combined the sCO2 cycle into the traditional coal-to-methanol process, burning the purge gas as gaseous fuel for power generation, improving the system energy efficiency, promoting resource utilization, and reducing the cost of carbon capture.

[0005] The Allam cycle has very high requirements for the oxygen purity and requires the use of an air separation oxygen production device. The green hydrogen-to-methanol process does not require an air separation device, and the purity of its surplus oxygen can meet this requirement. At present, there is a lack of a design scheme that integrates the supercritical CO2 (sCO2) Allam cycle into the green hydrogen-coupled coal-to-methanol process to promote the consumption and utilization of system resources and further improve the system efficiency. Summary of the Invention

[0006] The object of the present invention is to address the deficiencies of the prior art. Based on the process of coupling green hydrogen with coal-to-methanol, it integrates the supercritical CO2 (sCO2) Allam cycle to promote resource consumption, further improve the system efficiency, and reduce carbon emissions.

[0007] Technical solution of the present invention

[0008] To achieve the above object, the present invention provides a method for coupling green hydrogen with coal-to-methanol integrated with supercritical CO2 cycle power generation, which includes a renewable energy electrolytic water device, a coal gasification device, a conversion device, a purification device, a syngas-to-methanol device, a carbon dioxide hydrogenation-to-methanol device, a methanol rectification device, and a supercritical CO2 cycle device; hydrogen and oxygen required for the whole process are generated by the renewable energy electrolytic water device. The hydrogen enters the carbon dioxide hydrogenation-to-methanol device for methanol synthesis. Part of the oxygen electrolyzed is input into the coal gasification device to meet the gasification requirement, and the other part enters the supercritical CO2 cycle device as a combustion-supporting gas. The raw syngas after gasification enters the conversion device, and through conversion, it meets the reaction requirements of the syngas-to-methanol. The converted gas removes carbon dioxide and sulfides in the purification device. Among them, the removed carbon dioxide reacts with hydrogen in the carbon dioxide hydrogenation-to-methanol device. The purified syngas then enters the syngas-to-methanol device to produce methanol, and then methanol purification is carried out through the methanol rectification device. The unreacted purge gas from the two methanol production processes is transported into the supercritical CO2 cycle device for power generation.

[0009] Due to the instability of renewable energy, the renewable energy electrolytic water device needs to include hydrogen storage and oxygen storage devices to ensure the continuous and stable operation of the process flow.

[0010] The supercritical CO2 cycle device includes a purge gas compressor, a combustion chamber, a gas turbine and a generator, an oxygen compressor, an oxidation gas compressor, a high-temperature recuperator, a low-temperature recuperator, an oxidation gas preheater, a water separator, a CO2 primary compressor, an inter-stage condenser, a CO2 secondary compressor, a condenser, a primary pressurizing pump, and a secondary pressurizing pump

[0011] After the off-gas is compressed to 30.5 MPa by a compressor, it enters the combustion chamber together with the compressed oxidation gas for combustion. The combustion gas and the circulating working medium CO2 are then fed into a gas turbine to perform turbine work. After the work is done, the working medium is split into two parts. One part preheats the mixed gas with the oxidation gas preheater, and the other part successively enters the high-temperature recuperator and the low-temperature recuperator to heat the circulating working medium about to enter the combustion chamber. After leaving the low-temperature recuperator, it converges with another stream of the circulating working medium, and water is separated through a water separator. After a small part of CO2 is captured, it is compressed to 8 MPa by a two-stage compressor with an inter-stage condenser provided between the two stages, and then condensed through a condenser to make it higher than the critical value of CO2. Then it is pressurized to a certain pressure by a primary pressurizing pump. A small stream of CO2 is mixed with oxygen and enters the combustion chamber as an oxidant, and another stream, as the circulating working medium, is pressurized by a secondary pressurizing pump and then preheated through the low-temperature recuperator and the high-temperature recuperator. A small stream is used as the turbine cooling flow to cool the turbine, and the vast majority of the circulating flow enters the combustion chamber to adjust the firing temperature, thus completing the cycle process. The cooling process in the condensation process is all carried out with cooling water.

[0012] On the basis of ensuring the amount of gasifying agent used in the process of coupling green hydrogen with coal-to-methanol, the surplus oxygen is used in this system as the source of pure oxygen without the need for an air separation unit.

[0013] The fuel gas is the off-gas in the methanol synthesis process, mainly composed of CO, H2, and CO2. The combustion products are H2O and CO2, which facilitates the separation of water and the capture of CO2. Among them, when the content of green hydrogen decreases, the off-gas is mainly unreacted CO2, with a lower lower calorific value and a reduced power generation.

[0014] The heat recovery device mainly includes a high-temperature recuperator, a low-temperature recuperator, and an oxidation gas preheater.

[0015] The working pressure in the whole cycle power generation process varies between 3 and 30 MPa, with a large span, and multiple compressors in series are required to achieve this. The highest working temperature, i.e., the turbine inlet temperature, is 1100 °C, and turbine inter-stage cooling is provided.

[0016] The present invention has the following beneficial effects: The present invention comprehensively considers the utilization problem of the off-gas resources generated in the process of coupling green hydrogen with coal-to-methanol, uses the off-gas as the fuel gas, integrates supercritical CO2 cycle power generation, and at the same time provides the pure oxygen required by the process for supercritical CO2 cycle power generation, removes the air separation unit, improves the efficiency of supercritical CO2 cycle power generation, realizes the co-production of methanol and electricity, as well as the low-cost separation and capture of CO2, and has the advantages of high efficiency, energy conservation, and emission reduction.

[0017] The following takes an embodiment to detail the process characteristics of the present invention. Description of the Drawings

[0018] Figure 1Schematic process diagram of the present invention

[0019] Figure 2 Schematic diagram for simulation of the embodiment

[0020] In the figure: 1. Photovoltaic electrolytic water device; 2. Coal gasification device; 3. Shift device; 4. Purification device; 5. Syngas to methanol device; 6. Carbon dioxide hydrogenation to methanol device; 7. Methanol rectification device; 8-1. Flare gas compressor; 8-2. Combustion chamber; 8-3. Gas turbine and generator; 8-4. Oxygen compressor; 8-5. Oxidation gas compressor; 8-6. High-temperature recuperator; 8-7. Low-temperature recuperator; 8-8. Oxidation gas preheater; 8-9. Water separator; 8-10. CO2 primary compressor; 8-11. Inter-stage condenser; 8-12. CO2 secondary compressor; 8-13. Condenser; 8-14. Primary pressurizing pump; 8-15. Secondary pressurizing pump Specific implementation manner

[0021] By using a photovoltaic electrolytic water device to generate the hydrogen and oxygen required for the whole process and connecting the hydrogen pipeline to the carbon dioxide hydrogenation to methanol device, a part of the electrolytically generated oxygen is input into the coal gasification device to meet the gasification requirement, and the other part enters the supercritical CO2 circulation device as the combustion-supporting gas. The coal gasification device is connected to the shift device to meet the reaction requirement of the syngas to methanol through the shift. The purification device is connected to the shift device to remove carbon dioxide and sulfides. The removed carbon dioxide reacts with hydrogen in the carbon dioxide hydrogenation to methanol device. The purified syngas enters the syngas to methanol device to produce methanol, and then methanol purification is carried out through the methanol rectification device. The unutilized flare gas in the two processes is transported into the supercritical CO2 circulation device for power generation.

[0022] After the purge gas is pressurized to 30.5 MPa by a compressor, it enters the combustion chamber as fuel gas. The pure oxygen required for combustion comes from the surplus oxygen after the photovoltaic electrolysis of water to produce hydrogen, except for the oxygen used as a gasifier. To meet the pressure conditions, it needs to be compressed. For safety reasons, part of the circulating working fluid (12 MPa CO2) is mixed with oxygen and then compressed to 30.5 MPa and sent into the combustion chamber. By injecting a large amount of supercritical CO2 recycle stream (accounting for 95% of the oxidizing gas), the combustion temperature is adjusted (at this pressure, sCO2 can significantly enhance the combustion process). When the outlet temperature of the combustion chamber is between 1150 and 1200 °C, the system can operate at the highest efficiency. The combustion gas and the injected circulating working fluid are merged into the gas turbine for turbine work to generate electricity. The inlet turbine pressure of the circulating working fluid and the combustion gas is 30.5 MPa, and it is reduced to 3.4 MPa after cooling and expansion. Among them, an increase in the outlet pressure of the turbine will cause a decrease in power generation, but at the same time, it will also reduce the compression work required for the subsequent compression process. Considering the energy efficiency, there are good effects between 3.4 and 4 MPa; after expansion work, it is divided into two parts. One part preheats the oxygen and CO2 mixed gas with the help of the oxidizing gas preheater and preheats it to 720 °C. The other part enters the high-temperature recuperator and the low-temperature recuperator in sequence to heat the circulating working fluid about to enter the combustion chamber, also heating it to 720 °C. The exhaust gas after leaving the low-temperature recuperator, after converging with another working fluid used to preheat the mixed gas, separates out all the water through a water separator. The remaining CO2 with a purity of more than 95% ensures the stability of the working fluid circulation volume, and the CO2 generated during the combustion process is captured and stored. Then the circulating working fluid is compressed to 8 MPa in two stages, with inter-stage cooling set for the two-stage compression, and then condensed to 31 °C through a condenser, slightly higher than the critical value of CO2. In this state, CO2 can be efficiently compressed in the compressor, but its temperature will not change significantly. The thermal conductivity also reaches the maximum value at the critical density, greatly accelerating the heat exchange process of CO2 and fully leveraging the advantages of the drastic change in the physical properties of the working fluid. Then it is pressurized to 12 MPa by a primary pressure pump. A small stream of CO2 is mixed with oxygen and enters the combustion chamber as an oxidizer. The circulating working fluid is then pumped to 30.5 MPa by a secondary pressure pump. After recovering the heat of the expanded gas through the low-temperature recuperator and the high-temperature recuperator and preheating it to 720 °C, 20% of the circulating working fluid is used as the turbine cooling flow to cool the turbine, and the other part is used as the circulating flow to adjust the combustion temperature, completing this cycle process. The condensation process is cooled with 20 °C cooling water to ensure the compressor operates with low power consumption.

[0023] In the above embodiments, the conversion device is retained, and the green hydrogen produced by photovoltaic electrolysis of water only matches the hydrogen-carbon ratio in the carbon dioxide hydrogenation process. The oxygen generated meets two conditions: the appropriate oxygen-coal ratio in the coal gasification process; and the complete combustion of the fuel gas in the supercritical CO2 cycle process. Therefore, this process should first meet the oxygen required for the gasification process, and adjust the oxygen as an oxidant according to the complete combustion of the combustible gas in the purge gas. The remaining oxygen outside the above process can be sold as an industrial by-product.

[0024] In the above embodiments, the annual output of methanol is 600,000 tons. Through the replacement with renewable energy green hydrogen, the coal consumption is reduced to 0.664 tons of coal per ton of methanol.

[0025] In the present invention, the electricity consumed by the system comes from a photovoltaic power generation unit, which is clean green electricity, reducing carbon emissions and at the same time lowering the electricity cost. The electricity generated through the supercritical CO2 cycle can be partly used to regulate the volatility of photovoltaic power generation.

[0026] The working pressure of the entire cycle varies between 3 and 30 MPa, with a large span, and multiple compressors in series are required to achieve it. The highest working temperature, i.e., the turbine inlet temperature, is 1100 °C. The material problem of the turbine needs to be considered, and a turbine cooling system is set up. The turbine housing can adopt CrMoV, the rotor center part adopts nickel-based material, and the rotor end adopts CrMoV material. The blades are protected by a thermal barrier coating and cooled by convection, with CO2 used as the coolant.

[0027] A method for coupling green hydrogen with coal-to-methanol integrated with a supercritical CO2 cycle provided by the present invention can operate stably through Aspen Plus simulation. The gas temperature (1150 °C) coming out of the combustion chamber is within the above optimal operating range. For simplicity in simulation, a multi-stream heat exchanger is used to replace the high-temperature recuperator, low-temperature recuperator, and oxidation gas preheater in the heat recovery device, and the cyclic simulation parameters are set according to the above description. At the same time, taking the net power generation as the power generation index, the Allam cycle efficiency is obtained from the low calorific value of the purge gas, and the key parameter results of the system are shown in Table 1. After analysis, through the integration of the supercritical CO2 cycle, the energy efficiency of the green hydrogen-coupled coal-to-methanol process is increased by 16.7%, and the total carbon emissions are reduced from 1.59 tCO2 / tMEOH to 1.28 tCO2 / tMEOH. Due to the removal of the air separation unit, the cycle efficiency of the Allam cycle is increased to 66.89%. Based on the above analysis results, the integrated system can effectively promote the consumption and utilization of resources, further reduce carbon emissions, and improve the system efficiency.

[0028] Table 1

[0029]

Claims

1. A method for coupling green hydrogen with coal-to-methanol integrated with a supercritical CO2 cycle, characterized in that It includes a green hydrogen-coupled coal-to-methanol process and a supercritical CO2 power generation cycle process; the green hydrogen-coupled coal-to-methanol process includes a renewable energy electrolyzed water device (1), a coal gasification device (2), a conversion device (3), a purification device (4), a syngas-to-methanol device (5), a carbon dioxide hydrogenation-to-methanol device (6), and a methanol rectification device (7); in addition to using carbon dioxide to improve the methanol production capacity, the green hydrogen-coupled coal-to-methanol process also provides gaseous fuel and oxygen for the supercritical CO2 power generation cycle, and the supercritical CO2 power generation cycle process includes a supercritical CO2 cycle device (8). The renewable energy electrolyzed water device (1) generates the hydrogen and oxygen required for the whole process. The hydrogen enters the carbon dioxide hydrogenation-to-methanol device (6) to synthesize methanol. A part of the oxygen electrolyzed is input into the coal gasification device (2) to meet the gasification requirement, and the other part enters the supercritical CO2 cycle device (8) as a combustion-supporting gas. The crude syngas after gasification enters the conversion device (3), and through conversion, it meets the reaction requirements of the syngas-to-methanol. The converted gas removes carbon dioxide and sulfides in the purification device (4). Among them, the removed carbon dioxide reacts with hydrogen in the carbon dioxide hydrogenation-to-methanol device (6). The purified syngas then enters the syngas-to-methanol device (5) to produce methanol, and then methanol purification is carried out through the methanol rectification device (7). The unreacted purge gas from the two methanol production processes is then transported into the supercritical CO2 cycle device (8) for power generation.

2. The method for coupling green hydrogen with coal to produce methanol by integrating a supercritical CO2 cycle according to claim 1, wherein The renewable energy electrolyzed water device also includes a hydrogen storage and an oxygen storage device.

3. The method for coupling green hydrogen with coal to produce methanol by integrating a supercritical CO2 cycle according to claim 1, wherein The supercritical CO2 cycle device (8) includes a purge gas compressor (8-1), a combustion chamber (8-2), a gas turbine (8-3), an oxygen compressor (8-4), an oxidation gas compressor (8-5), a high-temperature recuperator (8-6), a low-temperature recuperator (8-7), an oxidation gas preheater (8-8), a water separator (8-9), a CO2 primary compressor (8-10), an inter-stage condenser (8-11), a CO2 secondary compressor (8-12), a condenser (8-13), a primary pressurizing pump (8-14), and a secondary pressurizing pump (8-15).

4. The method for coupling green hydrogen with coal to produce methanol by integrating a supercritical CO2 cycle according to claim 3, characterized in that, The off-gas is compressed step by step by the compressor (8-1) to 30.5 MPa, and then enters the combustion chamber (8-2) together with the oxidized gas compressed by the oxygen compressor (8-4) and the oxidized gas compressor (8-5) and the circulating working medium for combustion. The circulating working medium is used to adjust the combustion temperature. The combusted gas is fed into the gas turbine (8-3) to perform turbine work. After the work, it is divided into two parts. One part preheats the mixed gas by means of the oxidized gas preheater (8-8), and the other part successively enters the high-temperature recuperator (8-6) and the low-temperature recuperator (8-7) to heat the circulating working medium about to enter the combustion chamber. After leaving the low-temperature recuperator (8-7), it converges with another stream of circulating working medium, separates out water through the water separator (8-9), captures a small part of CO2, and then is compressed in two stages by the CO2 first-stage compressor (8-10) and the CO2 second-stage compressor (8-12). An inter-stage condenser (8-11) is provided for the two-stage compression, and then it is condensed by the condenser (8-13) to make it higher than the critical value of CO2. After that, it is compressed to a certain pressure by a primary booster pump (8-14). Part of the CO2 is mixed with oxygen and enters the combustion chamber to act as an oxidant, and another stream, as the circulating working medium, is pressurized to the working pressure by a secondary booster pump (8-15), preheated through the low-temperature recuperator (8-7) and the high-temperature recuperator (8-6), and a small stream acts as a turbine cooling flow to cool the turbine. Most of the circulating flow enters the combustion chamber (8-2) to adjust the firing temperature, completing the cycle process. The condensation process is cooled by cooling water throughout.

5. The method for coupling green hydrogen with coal to produce methanol by integrating a supercritical CO2 cycle according to claim 1, wherein, On the basis of ensuring the amount of gasifying agent used in the process of coupling green hydrogen with coal to produce methanol, the surplus oxygen is used for supercritical CO2 cycle power generation. As a source of pure oxygen, no air separation unit is required.

6. The method for coupling green hydrogen with coal to produce methanol by integrating a supercritical CO2 cycle according to claim 1, characterized in that, The fuel gas is the off-gas in the methanol synthesis process and the carbon dioxide hydrogenation process, mainly composed of CO, H2 and CO2, and the combustion products are H2O and CO2, which is convenient for realizing the separation of water and the capture of CO2.

7. The green hydrogen-coupled coal-to-methanol method integrating a supercritical CO2 cycle according to claim 1, wherein, When the content of green hydrogen decreases, the methanol output decreases, and the off-gas is mainly unreacted CO2, resulting in a reduction in power generation.

8. The method for coupling green hydrogen with coal to produce methanol by integrating a supercritical CO2 cycle according to claim 1, wherein, The heat recovery device includes a high-temperature recuperator (8-6), a low-temperature recuperator (8-7) and an oxidized gas preheater (8-8).

9. The method for coupling green hydrogen with coal to produce methanol by integrating a supercritical CO2 cycle according to claim 1, wherein The working pressure of the entire cycle power generation process varies between 3 and 30 MPa, with a large span, and multiple compressors in series are required to achieve it. The highest working temperature, i.e., the turbine inlet temperature, is 1150 - 1200 °C, and turbine inter-stage cooling is provided.

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