Coal underground gasification method based on carbon cycle and carbon sequestration

Abstract

The application belongs to the technical field of coal gasification, and discloses a coal underground gasification method and system based on carbon cycle and carbon storage. The method is characterized by the following steps: the underground coal seam is gasified by using a gasification agent rich in carbon dioxide; the coal gas is caustic washed; the caustic washing liquid is regenerated; and the carbon dioxide is converted into insoluble carbonate to fill the underground gasification combustion area. The application can provide continuous carbon dioxide for the coal underground gasification, greatly improve the heat value or the content of effective components of the produced coal gas, fully utilize the heat energy resources carried by the coal gas at the gas drilling hole of the underground gasification furnace, and reduce the carbon dioxide emission during the coal utilization process, thereby having a very good energy-saving and emission-reducing effect.

Description

Technical Field

[0001] This application belongs to the field of coal gasification technology, and in particular relates to an underground coal gasification method based on carbon cycle and carbon sequestration. Background Technology

[0002] Underground coal gasification is a process in which coal buried deep underground is burned in a controlled manner by injecting a gasifying agent into the coal seam. Combustible gas is produced through the thermal and chemical reactions of the coal. It has the advantages of low investment, high efficiency, scientific and environmental protection, and is of great significance for the efficient and comprehensive utilization of coal resources, especially deep coal resources.

[0003] Conventional underground coal gasification typically produces coal gas with high levels of carbon dioxide or nitrogen, and low volume fractions of effective gases (CO, H2, CH4, etc.), resulting in a relatively low calorific value. To increase the volume fraction of effective gases, oxygen enrichment is usually employed to reduce nitrogen content. However, this results in coal gas with a high carbon dioxide content, and the effective component content remains low. Furthermore, using carbon dioxide as a gasifying agent requires the purchase of large quantities of gas, making the energy-saving and emission-reduction effects of underground coal gasification insignificant. Simultaneously, the abandoned combustion chamber after underground coal gasification poses a risk of ground subsidence after long-term (multi-year) vacancy. Summary of the Invention

[0004] To address the problems existing in the prior art, this application provides a method for underground coal gasification based on carbon cycle and carbon sequestration.

[0005] This application is implemented as follows: A method for underground coal gasification based on carbon cycle and carbon sequestration includes:

[0006] Step 1: Inject carbon dioxide-rich gasifying agent into the underground coal seam for gasification. The produced coal gas is collected at the wellhead and subjected to alkaline washing after heat exchange. The alkaline-washed coal gas can be used as product coal gas for subsequent use. The obtained alkaline washing solution is regenerated by contacting calcium oxide to obtain calcium carbonate-rich precipitate and alkaline solution.

[0007] Step 2: The alkaline solution is recycled to heat and decompose the calcium carbonate-rich precipitate to obtain carbon dioxide and calcium oxide-rich material.

[0008] Step 3: The carbon dioxide obtained in the previous step is transported to the underground coal seam as part of the gasifying agent, and the other part reacts with the Ca-rich coal. 2+ Mg 2+ Ba 2+ Cd 2+ Fe 2+ Pb 2+ Zn 2+ Mn 2+ and / or Cu 2+The waste liquid is reacted to generate insoluble carbonates, which are then filled into abandoned underground combustion chambers for storage. Its characteristic is that the heat energy for heating and decomposing the precipitate comes from the heat carried by the gas at the outlet borehole.

[0009] Furthermore, the alkaline washing uses an aqueous sodium hydroxide solution. Although a higher concentration of sodium hydroxide solution enhances its carbon dioxide absorption capacity, a higher concentration also results in stronger corrosiveness and higher equipment requirements. Additionally, industrially commonly used sodium hydroxide solutions have a mass percentage concentration between 20% and 40%. Considering all factors, the mass percentage concentration of sodium hydroxide in the alkaline washing solution is between 20% and 40%, preferably 25% to 35%, and further preferably 30%. The process involves blowing crude coal gas into the sodium hydroxide aqueous solution, controlling the pressure of the alkaline washing environment to be below a pressure difference of 0.2 MPa between the inlet and outlet, and maintaining an inlet temperature below 80°C. The escaping gas is wet coal gas with carbon dioxide removed.

[0010] Furthermore, during the heating and decomposition of the precipitate, the pressure is controlled between 0.01 atm and 40.00 atm, preferably between 0.05 atm and 0.50 atm for depressurized thermal decomposition, or preferably between 10.00 atm and 20.00 atm for pressurized thermal decomposition. When performing depressurized thermal decomposition, the temperature is controlled between 500℃ and 700℃, more preferably 600℃; when performing pressurized thermal decomposition, the temperature is preferably between 700℃ and 900℃. The precipitate undergoing thermal decomposition is mainly composed of calcium carbonate. The decomposition of calcium carbonate is closely related not only to temperature and pressure, but also to particle size, impurities, catalysts, and specific surface area. Under normal pressure, the decomposition initiation temperature of calcium carbonate is greater than 600℃, and the decomposition termination temperature is approximately 900℃, with the decomposition temperature increasing with increasing pressure. When choosing depressurized thermal decomposition precipitates, relatively less thermal energy is required, but the kinetic energy required for subsequent carbon dioxide separation and pressure increase will increase; while when choosing pressurized thermal decomposition precipitates, more thermal energy is required, but less kinetic energy is needed subsequently; the choice can be made flexibly in specific applications.

[0011] Furthermore, during the heating and decomposition of the precipitate, the carbon dioxide gas produced by pyrolysis should be separated in a timely manner to ensure more thorough decomposition of the precipitate. A portion of the separated carbon dioxide needs to be pressurized to a certain pressure before being mixed with oxygen and / or water vapor to form a gasifying agent.

[0012] Furthermore, the separated carbon dioxide can be converted into insoluble carbonates and then backfilled into abandoned underground coal gasification combustion chambers for storage. Methods for converting carbon dioxide into insoluble carbonates include reacting it with concentrated brine from seawater desalination, bittern from salt extraction, and / or waste liquid from phosphate chemical plants, etc., which are rich in Ca. 2+ Mg 2+ Ba 2+ Cd 2+ Fe2+ Pb 2+ Zn 2+ Mn 2+ and / or Cu 2+ When the solution comes into contact with the cations, the above-mentioned cations are converted into corresponding insoluble carbonates. By filling the abandoned underground gasification combustion zone with the insoluble carbonates, carbon dioxide can be sealed and waste liquids from other industries or industrial processes can be treated, while reducing the risk of geological subsidence in the combustion zone. This can achieve multiple technical benefits.

[0013] Another objective of this application is to provide a coal underground gasification system based on carbon cycle and carbon sequestration, comprising:

[0014] The coal underground gasifier includes an air inlet borehole, an air outlet borehole, and a gasification channel; an alkaline washing tank equipped with an air inlet, an air outlet, an alkaline solution inlet, a liquid outlet, and a temperature and pressure monitoring hole; a regeneration tank equipped with a liquid inlet, a feed inlet, a liquid outlet, and a slag outlet; a pyrolysis furnace equipped with a slag outlet, a feed outlet, an exhaust outlet, a heat source inlet for heat exchange in the partition wall, a heat source outlet, and a temperature and pressure monitoring hole; and the necessary pipelines connecting the tank body and the furnace body.

[0015] The drain port and alkali inlet of the alkali washing tank are connected to the inlet and drain port pipelines of the regeneration tank, respectively. The feed port and slag discharge port of the regeneration tank are connected to the discharge port and slag inlet pipelines of the pyrolysis furnace, respectively. The heat source inlet and heat source outlet of the pyrolysis furnace are connected to the gas outlet borehole of the underground coal gasification furnace and the air inlet of the alkali washing tank, respectively. The exhaust port of the pyrolysis furnace is connected to the air inlet borehole of the underground coal gasification furnace.

[0016] The pyrolysis furnace has a multi-section structure. One section uses the heat energy carried by the gas for heat exchange between the sections, while the other section uses an external heat source for enhanced heating.

[0017] A booster pump can be installed between the exhaust port of the pyrolysis furnace and the inlet borehole of the underground coal gasification furnace to increase the pressure of carbon dioxide gas.

[0018] Based on the above technical solutions and the technical problems they solve, the advantages and positive effects of the technical solution to be protected in this application are as follows:

[0019] First, this application proposes a coal underground gasification method based on carbon cycle and carbon sequestration, which can provide a continuous source of carbon dioxide for coal underground gasification. Alkali washing greatly improves the calorific value and effective component content of the produced coal gas. At the same time, heat exchange can make full use of the thermal energy resources carried by the wellhead coal gas.

[0020] Secondly, the reaction between underground coal seams and oxygen-rich carbon dioxide gasifying agents produces coal gas with a relatively high carbon dioxide content. This coal gas undergoes alkaline washing to remove CO2, significantly increasing the effective gas volume fraction, resulting in higher calorific value and greater added value. The alkaline solution that absorbs carbon dioxide is regenerated through a reaction with calcium oxide. The byproduct of this regeneration, calcium carbonate, undergoes thermal decomposition to produce calcium oxide and carbon dioxide. Part of the carbon dioxide produced is used as a gasifying agent in underground coal gasification, while the remainder reacts with calcium-rich... 2+ Mg 2+ Ba 2+ Cd 2+ Fe 2+ Pb 2+ Zn 2+ Mn 2+ and / or Cu 2+ The waste liquid is converted into insoluble carbonates through contact with the gas, which are then filled into the combustion chamber of the underground coal gasification system for storage. This proposed solution significantly improves the quality of underground coal gasification gas, effectively addresses the issue of carbon dioxide emission reduction during fossil fuel utilization, and simultaneously converts carbon dioxide into insoluble salts to fill the combustion chamber of the underground coal gasification system, achieving multiple benefits including carbon dioxide sequestration and mitigating ground subsidence after underground gasification. Attached Figure Description

[0021] Figure 1 This is a flowchart of an underground coal gasification method based on carbon cycle and carbon sequestration provided in an embodiment of this application.

[0022] Figure 2 This is a structural block diagram of an underground coal gasification system based on carbon cycle and carbon sequestration provided in the embodiments of this application.

[0023] Figure 3 This is a thermal decomposition curve of the precipitate under different pressures provided in the embodiments of this application. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0025] like Figure 1 As shown in the embodiment of this application, a method for underground coal gasification based on carbon cycle and carbon sequestration includes the following steps:

[0026] S101 involves injecting a carbon dioxide-rich gasifying agent into an underground coal seam for gasification. The produced coal gas is collected at the wellhead and subjected to alkaline washing after heat exchange. The alkaline-washed coal gas can be used as product coal gas for subsequent applications. The resulting alkaline washing solution is regenerated by contacting calcium oxide to obtain a precipitate rich in calcium carbonate and an alkaline solution.

[0027] S102, alkaline solution is recycled to heat and decompose calcium carbonate-rich precipitates to obtain carbon dioxide and calcium oxide-rich materials.

[0028] S103, the resulting carbon dioxide is transported to the underground coal seam as part of the gasifying agent, and another part reacts with Ca-rich... 2+ Mg 2+ Ba 2+ Cd 2+ Fe 2+ Pb 2+ Zn 2+ Mn 2+ and / or Cu 2+ The waste liquid is reacted to generate insoluble carbonates, which are then filled into abandoned underground combustion chambers for storage. Its characteristic is that the heat energy for heating and decomposing the precipitate comes from the heat carried by the gas at the outlet borehole.

[0029] The alkaline washing method provided in this application uses an aqueous sodium hydroxide solution, wherein the mass percentage concentration of sodium hydroxide is 20%~40%, preferably 25%~35%, and more preferably 30%. Raw coal gas is bubbled into the sodium hydroxide solution, and the pressure of the alkaline washing environment is controlled to be below 0.2 MPa between the inlet and outlet gas pressures, and the inlet gas temperature is below 80°C. The escaping gas is wet coal gas with carbon dioxide removed.

[0030] When heating and decomposing the precipitate, the pressure is controlled between 0.01 atm and 40.00 atm, preferably between 0.05 atm and 0.50 atm for depressurized thermal decomposition, or preferably between 10.00 atm and 20.00 atm for pressurized thermal decomposition; when depressurized thermal decomposition is performed, the temperature is controlled between 500℃ and 700℃, more preferably 600℃; when pressurized thermal decomposition is performed, the temperature is preferably between 700℃ and 900℃.

[0031] Take 10g of each precipitate that has been dried at 110℃ for 24 hours, and conduct thermal decomposition experiments at different pressures and temperatures. Use the same heating rate, maintain the temperature at each node for 1 hour, then cool to room temperature. Weigh the remaining solid, calculate the corresponding mass fraction, and obtain the corresponding thermal decomposition weight loss curves, as shown below. Figure 3 As shown, the pressure range for depressurized thermal decomposition is between 0.05 and 0.5 atm, and the temperature range in which the precipitate loses weight most concentrated is between 500 and 700°C, which is the decomposition temperature range of calcium carbonate; the preferred pressure range for pressurized thermal decomposition is 10 to 20 atm, and the temperature range in which the precipitate loses weight most concentrated is between 700 and 900°C.

[0032] When heating and decomposing precipitates, the carbon dioxide gas produced by pyrolysis should be separated in a timely manner to ensure more thorough decomposition of the precipitates. A portion of the separated carbon dioxide needs to be pressurized to a certain pressure before being mixed with oxygen and / or water vapor to form a gasifying agent.

[0033] The separated carbon dioxide can also be converted into insoluble carbonates and then backfilled into the combustion chamber of abandoned underground coal gasification sites for storage. Methods for converting carbon dioxide into insoluble carbonates include reacting it with concentrated brine from seawater desalination, bittern from salt extraction, and / or waste liquid from phosphate chemical plants, all of which are rich in Ca. 2+ Mg 2+ Ba 2+ Cd 2+ Fe 2+ Pb 2+ Zn 2+ Mn 2+ and / or Cu 2+ When these solutions come into contact with each other, the aforementioned cations are converted into their corresponding insoluble carbonates. Filling abandoned underground gasification combustion zones with these insoluble carbonates not only seals carbon dioxide and treats wastewater from other industries or processes, but also reduces the risk of geological subsidence in the combustion zone, achieving multiple benefits in one fell swoop.

[0034] like Figure 2 As shown in the embodiments of this application, a coal underground gasification system based on carbon cycle and carbon sequestration includes:

[0035] The coal underground gasifier includes an inlet borehole, an outlet borehole, and a gasification channel; an alkali washing tank equipped with an inlet, an outlet, an alkali inlet, a drain outlet, and a temperature and pressure monitoring hole; a regeneration tank equipped with a liquid inlet, a feed inlet, a liquid drain outlet, and a slag discharge outlet; a pyrolysis furnace equipped with a slag discharge outlet, a discharge outlet, an exhaust outlet, a heat source inlet / outlet for heat exchange in the partition wall, and a temperature and pressure monitoring hole; and necessary pipelines connecting the tank and the furnace. The liquid inlet and outlet of the alkali washing tank are connected to the liquid inlet and outlet pipelines of the regeneration tank, respectively. The feed inlet and slag discharge outlet of the regeneration tank are connected to the discharge outlet and slag inlet pipelines of the pyrolysis furnace, respectively. The heat source inlet and outlet of the pyrolysis furnace are connected to the outlet borehole of the underground gasifier and the inlet of the alkali washing tank, respectively. The exhaust outlet of the pyrolysis furnace is connected to the inlet borehole of the underground gasifier.

[0036] The pyrolysis furnace is multi-stage, with one part utilizing the heat energy carried by the gas for heat exchange between the walls, and another part using other methods (such as electric heating, nitrogen-free calcination, etc.) to enhance the heat.

[0037] A booster pump can be installed between the exhaust port of the pyrolysis furnace and the inlet borehole of the underground coal gasification furnace to increase the pressure of carbon dioxide gas.

[0038] Specific implementation of this application:

[0039] Construct an underground coal gasification system based on carbon cycle and carbon sequestration, including an underground coal gasifier, a pyrolysis furnace, an alkaline washing tank, and a regeneration tank. The underground coal gasifier includes an inlet borehole, an outlet borehole, and a gasification channel located within the coal seam and connecting the inlet and outlet boreholes. The pyrolysis furnace is equipped with a slag inlet, a discharge outlet, an exhaust outlet, a heat source inlet / outlet for heat exchange in the partition wall, and temperature and pressure monitoring holes. The alkaline washing tank is equipped with an inlet, an outlet, an alkaline solution inlet, a liquid outlet, and temperature and pressure monitoring holes. The regeneration tank is equipped with a liquid inlet, a feed inlet, a liquid outlet, and a slag outlet. The drain and inlet of the alkali washing tank are connected to the inlet and drain pipelines of the regeneration tank, respectively. The feed inlet and slag outlet of the regeneration tank are connected to the discharge inlet and slag outlet pipelines of the pyrolysis furnace, respectively. The heat source inlet and outlet of the pyrolysis furnace are connected to the gas outlet borehole of the underground gasifier and the gas inlet of the alkali washing tank, respectively. The exhaust outlet of the pyrolysis furnace is connected to the gas inlet borehole of the underground gasifier.

[0040] Coal seam gasification is achieved by injecting an oxygen-enriched carbon dioxide gasifying agent with an oxygen volume fraction of 40% into an underground coal gasifier through an air inlet borehole. The pressure at the gasifier outlet borehole is maintained at 1.5 ± 0.1 MPa, and the pressure difference between the gasifier inlet and outlet boreholes is maintained at 0.3 MPa. Under thermal action, the gasifying agent and coal undergo pyrolysis and gasification reactions to generate crude coal gas rich in carbon dioxide, carbon monoxide, hydrogen, methane, etc.

[0041] The raw coal gas discharged from the gas borehole is collected and enters the heat source inlet of the pyrolysis furnace into the indirect heat exchanger to heat the slag in the pyrolysis furnace. After heat exchange, the coal gas is discharged from the heat source outlet into the alkaline washing tank. Here, a multi-stage pyrolysis furnace is used. The slag entering from the slag inlet is heated and dried in the upper stage of the pyrolysis furnace, and then stored in the middle stage. It is intermittently introduced into the lower stage. In the lower stage, the slag rich in calcium carbonate (i.e., precipitate) is decomposed by enhanced heating to generate solid material rich in calcium oxide and carbon dioxide gas. The temperature in the lower stage of the pyrolysis furnace is controlled at 750°C and the pressure at 10 atm. The carbon dioxide gas is discharged from the exhaust port of the pyrolysis furnace, mixed with an appropriate amount of oxygen, and then fed back into the underground coal gasification furnace. The solid material is discharged from the discharge port at the bottom of the pyrolysis furnace and transported to the regeneration tank.

[0042] The cold coal gas entering the alkaline washing tank comes into countercurrent contact with a 30% sodium hydroxide solution. The temperature inside the tank is controlled at 75±5℃. The solution from the lower part of the tank is raised to the upper part for spraying through a circulation system, where it comes into contact with the gas escaping from the lower solution, further washing away carbon dioxide. The carbon dioxide reacts with sodium hydroxide to produce sodium carbonate and water. The remaining gas is discharged from the outlet of the alkaline washing tank, which is the clean coal gas. By controlling the flow rate of each material stream, the pH of the solution in the lower part of the tank is maintained at 12±0.5. A portion of the sodium carbonate-rich alkaline washing solution is discharged from the drain outlet and transported to the regeneration tank.

[0043] Inside the regeneration tank, materials rich in calcium oxide come into contact with and react with alkaline washing solution rich in sodium carbonate, generating insoluble calcium carbonate solid slag (i.e., precipitate) and sodium hydroxide solution (i.e., alkaline solution). The sodium hydroxide solution is transported from the drain port of the regeneration tank to the alkaline washing tank to achieve the regeneration and circulation of the alkaline solution. The solid slag is discharged from the slag discharge port of the regeneration tank and sent to the pyrolysis furnace.

[0044] As underground coal gasification progresses, the amount of carbon dioxide produced increases. Not all the carbon dioxide emitted from the pyrolysis furnace can be mixed with oxygen to form a gasifying agent for transport to the gasifier. Excess carbon dioxide can be sourced locally and reacted with wastewater rich in calcium and magnesium ions to convert it into insoluble carbonates (i.e., mineralization) for storage. This wastewater can be concentrated brine from seawater desalination, bittern from salt extraction, or desulfurization and denitrification wastewater from a thermal power plant. This proposed method uses desulfurization wastewater from a nearby thermal power plant, neutralized to a slightly alkaline state before contacting it with carbon dioxide gas. The resulting precipitate is then transported via boreholes to an abandoned underground gasification combustion zone for storage.

[0045] The underground coal gasification method and system of this application not only greatly enhance the effective components in the produced coal gas (comparison of coal gas composition and calorific value is shown in Table 1), but also fully utilize the thermal energy carried by the coal gas at the wellhead of the underground gasifier, while reducing carbon dioxide emissions during coal utilization.

[0046] Table 1: Comparison of Gas Components and Calorific Value

[0047]

[0048] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with embodiments.

[0049] Example 1

[0050] In an underground coal seam gasification project, a coal seam with a thickness of 6 meters and a burial depth of 300 meters was selected as the gasification target. By injecting a mixed gasifying agent composed of oxygen, steam, and recycled carbon dioxide, incomplete combustion of the coal underground is achieved to produce coal gas. The coal gas collected at the gas outlet borehole has a temperature of approximately 250℃. After the heat is transferred to the thermal decomposition system in a heat exchanger, the coal gas enters an alkaline washing unit containing sodium hydroxide solution. The sodium carbonate solution generated after the alkaline washing solution absorbs carbon dioxide then contacts calcium oxide to form calcium carbonate precipitate and regenerated alkaline solution.

[0051] The obtained calcium carbonate precipitate is fed into a pyrolysis furnace for decomposition. Part of the released carbon dioxide is reused in coalbed gasification, while the other part reacts with concentrated seawater desalination brine rich in calcium and magnesium ions (or alternatively, bittern wastewater from salt extraction or wastewater from a phosphate plant) to form insoluble carbonates, which are then backfilled into the abandoned combustion chamber, achieving carbon dioxide sequestration. This recycling process effectively reduces carbon emissions while improving gas quality (total effective gas content and calorific value).

[0052] Example 2

[0053] In another engineering example, a 30% sodium hydroxide solution was used as the alkaline washing agent, and crude coal gas at approximately 70°C was bubbled into it. Throughout the process, the inlet and outlet pressure difference was controlled at 0.15 MPa, ensuring stable gas flow while reducing energy consumption. After alkaline washing, carbon dioxide was largely removed, and the resulting wet coal gas mainly consisted of hydrogen, carbon monoxide, and a small amount of methane, which could be directly used as a syngas feedstock.

[0054] After the alkaline washing solution is saturated with carbon dioxide, it reacts with calcium oxide to form calcium carbonate precipitate, which is then regenerated through thermal decomposition, ensuring the long-term utilization of sodium hydroxide and reducing system operating costs.

[0055] Example 3

[0056] In the experimental setup, calcium carbonate precipitate was fed into a thermal decomposition furnace and operated under reduced pressure. The pressure was maintained at 0.1 atm, and the furnace temperature was raised to 500°C. After a continuous reaction of 2 hours, calcium carbonate was decomposed into calcium oxide and carbon dioxide, and the gases were collected through a separation system.

[0057] In another set of comparative experiments, pressurized decomposition conditions were used, with the operating pressure maintained at 10 atm and the temperature controlled at 750℃. Complete decomposition was achieved within 1 hour. The results show that calcium carbonate decomposition can be achieved under different pressures, but the energy consumption and carbon dioxide collection methods differ, and the appropriate method can be selected based on actual engineering requirements.

[0058] Example 4

[0059] During the thermal decomposition process, the carbon dioxide produced is immediately removed through a separation device, promoting the decomposition of insoluble carbonates (i.e., precipitates—calcium carbonate). The separated carbon dioxide is pressurized to 3 MPa by a compressor and then mixed with oxygen and water vapor in a specific ratio to form a new gasifying agent that is continuously injected into the coal seam, ensuring the continuity of the gasification process.

[0060] This not only enables the recycling of carbon dioxide, but also improves the coalbed gasification reaction temperature and gas product composition by adjusting the composition of the gasifying agent, thereby increasing the overall efficiency of the system.

[0061] Example 5

[0062] In a coal underground gasification project in a coastal area, the separated carbon dioxide comes into contact with concentrated brine produced during seawater desalination. The concentrated brine is rich in calcium and magnesium ions. After the carbon dioxide dissolves, it quickly forms calcium carbonate and magnesium carbonate precipitates. These insoluble carbonates are directly backfilled into the abandoned coal seam slag area to form stable mineralized sequestration.

[0063] Meanwhile, this process effectively treats wastewater from seawater desalination, reducing the environmental burden caused by discharge into the ocean. It achieves synergistic optimization between coal mining, energy production, and environmental protection, demonstrating significant application value.

[0064] Example 6

[0065] The system includes an underground coal gasifier and its inlet / outlet boreholes, with the surface portion sequentially connected to an alkaline washing tank, a regeneration tank, and a pyrolysis furnace. Coal gas discharged from the underground gasifier enters the pyrolysis furnace for heat exchange before entering the alkaline washing tank, where carbon dioxide is absorbed. The alkaline washing liquid enters the regeneration tank and contacts calcium oxide from the pyrolysis furnace to form calcium carbonate precipitate. The calcium carbonate precipitate is transported to the pyrolysis furnace, where some of the carbon dioxide produced by its heating and decomposition is returned to the underground gasifier.

[0066] Each unit is interconnected via pipelines. A bidirectional pumping system is installed between the alkaline washing tank and the regeneration tank to ensure liquid circulation; a solid-liquid conveying device is installed between the regeneration tank and the pyrolysis furnace to achieve continuous supply of calcium carbonate. The entire device constitutes a closed carbon cycle system.

[0067] Example 7

[0068] The pyrolysis furnace adopts a three-section structure: the first section is connected to the gas heat exchanger, which uses the waste heat of the gas for indirect heat exchange; the second section is equipped with an external burner for supplemental heating; and the third section is set with a rapid cooling zone for gas separation and to prevent reaction reversal.

[0069] This multi-stage design not only improves thermal energy utilization efficiency but also ensures the stability and completeness of the decomposition reaction through zoned control. Compared with a single-stage furnace, energy consumption is reduced by approximately 15%, and the decomposition rate is increased by approximately 20%.

[0070] Example 8

[0071] A dedicated carbon dioxide booster pump with a working pressure range of 1 MPa to 5 MPa was installed between the exhaust port of the pyrolysis furnace and the air inlet borehole of the underground gasifier. This device can rapidly pressurize carbon dioxide gas at atmospheric pressure, ensuring that the carbon dioxide content and pressure in the gasifying agent meet the coal seam reaction conditions.

[0072] By setting up the pressurization unit, the efficiency of carbon dioxide injection is improved, the reaction temperature of underground coal seams is more stable, gas backflow and leakage problems are avoided, and the overall gasification rate is increased by about 12%.

[0073] Example 9

[0074] In gasification system applications, alkali absorption, regeneration, thermal decomposition, and carbon dioxide recycling are all achieved within continuous pipelines and equipment. An automated monitoring system regulates temperature, pressure, and flow rate in real time to ensure stable gaseous products.

[0075] The system operated for 120 days in a demonstration project, processing approximately 5,000 tons of carbon dioxide, of which 60% was recycled and 40% was stored through mineralization. Operational data shows that the system has high stability and carbon reduction benefits.

[0076] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any modifications, equivalent substitutions, and improvements made by those skilled in the art within the scope of the technology disclosed in this application, and within the spirit and principles of this application, should be included within the scope of protection of this application.

Claims

1. A coal underground gasification system, characterized in that, include: An underground coal gasification furnace including an air inlet borehole, an air outlet borehole, and a gasification channel; An alkaline washing tank equipped with an air inlet, an air outlet, an alkaline solution inlet, a drain outlet, and a temperature and pressure monitoring hole; A regeneration tank equipped with a liquid inlet, a feed inlet, a liquid outlet, and a slag outlet; A pyrolysis furnace equipped with a slag inlet, a discharge outlet, an exhaust outlet, a heat source inlet for heat exchange in the partition wall, a heat source outlet, and a temperature and pressure monitoring hole; The drain port and alkali inlet of the alkali washing tank are connected to the inlet and drain port pipelines of the regeneration tank, respectively. The feed port and slag discharge port of the regeneration tank are connected to the discharge port and slag inlet pipelines of the pyrolysis furnace, respectively. The heat source inlet and heat source outlet of the pyrolysis furnace are connected to the gas outlet borehole of the underground coal gasification furnace and the air inlet of the alkali washing tank, respectively. The exhaust port of the pyrolysis furnace is connected to the air inlet borehole of the underground coal gasification furnace. The above-mentioned underground coal gasification system is used in the following manner: A carbon dioxide-rich gasifying agent is injected into the underground coal seam for gasification. The produced coal gas is collected at the wellhead and then subjected to alkaline washing after heat exchange. The alkaline-washed coal gas is used as product coal gas. The alkaline washing solution is contacted with calcium oxide to generate calcium carbonate precipitate and alkaline solution. Calcium carbonate precipitate is decomposed by heating to obtain carbon dioxide and calcium oxide, wherein the heat energy for heating decomposition comes from the heat carried by the gas at the outlet of the gas borehole. Part of the carbon dioxide produced in the previous step is injected into the underground coal seam as a gasifying agent, and the other part is mixed with calcium-rich... 2+ Mg 2+ Ba 2+ Cd 2+ Fe 2+ Pb 2+ Zn 2+ Mn 2+ and / or Cu 2+ The waste liquid reacts to form insoluble carbonates, which are then filled into abandoned underground gasification combustion zones for storage.

2. The gasification system as described in claim 1, characterized in that, The alkaline washing uses an aqueous solution of sodium hydroxide, wherein the mass percentage concentration of sodium hydroxide is 20%~40%, and is carried out under the conditions that the pressure difference between the inlet and outlet gases does not exceed 0.2 MPa and the inlet gas temperature is below 80°C.

3. The gasification system as described in claim 2, characterized in that, The mass percentage concentration of sodium hydroxide is 25%~35%.

4. The gasification system as described in claim 1, characterized in that, The thermal decomposition of calcium carbonate precipitates is carried out by either reduced pressure or increased pressure. Decomposition conditions were controlled within a pressure range of 0.01 atm to 40.00 atm; When performing decomposition under reduced pressure, the temperature is controlled at 500℃~700℃; When performing pressurized pyrolysis, the temperature is controlled between 700℃ and 900℃.

5. The gasification system as described in claim 4, characterized in that, When performing depressurized pyrolysis, the pressure range is controlled between 0.05 atm and 0.50 atm; when performing pressurized pyrolysis, the pressure range is controlled between 10.00 atm and 20.00 atm.

6. The gasification system as described in claim 4, characterized in that, The carbon dioxide gas produced during the heating and decomposition process needs to be separated in a timely manner.

7. The gasification system as described in claim 6, characterized in that, The separated carbon dioxide gas is partially pressurized and mixed with oxygen and / or water vapor to form a gasifying agent; the remaining carbon dioxide gas is mixed with a substance rich in Ca... 2+ Mg 2+ Ba 2+ Cd 2+ Fe 2+ Pb 2+ Zn 2+ Mn 2+ and / or Cu 2+ The waste liquid reacts to form insoluble carbonates.

8. The gasification system as described in claim 1, characterized in that, The pyrolysis furnace has a multi-section structure, with one section using heat energy carried by gas for heat exchange between the sections, and the other section using an external heat source for enhanced heating.

9. The gasification system as described in claim 1, characterized in that, A booster pump is installed between the exhaust port of the pyrolysis furnace and the air inlet borehole of the underground coal gasification furnace.

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