Raw gas purification system and process for multi-section gas export and tar separation coupling

Abstract

The invention relates to a crude gas purification system and process with multi-section gas export and tar separation coupled, the crude gas purification system is composed of a multi-section gas guide collection unit, a spray cooling unit, a tar ammonia water separation unit and a deep purification unit, and the crude gas purification system is designed for an external heating type pyrolyzing furnace and can treat multiple paths of crude gas at the same time; the raw coke oven gas guided out of the multi-section dry distillation furnace is instantly sprayed and cooled, temperature reduction of the raw coke oven gas and precipitation and separation of most tar and ammonia water are achieved, then the tar and the ammonia water are further separated and recycled through centrifugation or sedimentation, and finally micro-mist tar and fine dust are removed through an electrical tar precipitator. One part of the clean coal gas is used as remelting fuel, and the other part of the clean coal gas is used for other purposes; the raw gas purification system is compact in structure, short in technological process, high in automation degree and capable of adapting to intermittent or fluctuation operation of the external heating type pyrolyzing furnace, and stable purification is achieved.

Description

Technical Field

[0001] This invention relates to the field of raw coal gas purification technology, and in particular to a raw coal gas purification system and process that couples multi-stage coal gas extraction with tar separation. Background Technology

[0002] During the dry distillation (pyrolysis) of coal, raw coal gas (also known as unpurified coal gas) is produced, which contains a large amount of coal tar droplets, naphthalene, ammonia, water vapor, and dust. Direct emission or combustion of unpurified raw coal gas will cause serious pollution and equipment scaling. Therefore, raw coal gas purification is a critical step in the coal pyrolysis industry, and typically includes steps such as dust removal, tar and water separation, and desulfurization and deammoniation to obtain clean coal gas and recoverable tar byproducts.

[0003] Currently, the purification technology for coke oven gas (similar to raw coal gas) is relatively mature. The traditional process flow includes steps such as gas-liquid cooling, tar separation, ammonia-water separation, electrostatic oil removal, desulfurization, and benzene washing. The commonly used primary cooling and tar separation processes include the following steps: (1) Indirect heat exchange cooling: A primary cooler (such as a horizontal tube primary cooler) is used to cool the raw coal gas with circulating water in the heat exchange tube, causing most of the tar and water vapor in it to condense and precipitate out, forming a mixture of crude tar and ammonia water. After this process, the temperature of the raw coal gas usually drops from 80-85℃ to about 20-30℃.

[0004] (2) Gravity / centrifugal separation of tar and ammonia: The tar-ammonia mixture obtained by condensation is sent to a separation tank for standing or separated by a three-phase centrifuge to extract the two phases of tar and ammonia (there are also records in the literature of using a disc centrifuge to quickly separate tar, ammonia and tar residue). After standing or centrifuging to remove impurities, the tar is sent to a storage tank. Part of the ammonia is recycled and the remainder is treated as wastewater for further treatment.

[0005] (3) Electrostatic Tar Precipitation: To remove the fine tar droplets remaining in the cooled raw coal gas, an electrostatic tar precipitator is often used. The raw coal gas passes through a high-voltage electrostatic field, causing the tar droplets to become charged and deposited on the collecting electrode, achieving further purification. After electrostatic precipitation, the tar content in the raw coal gas can be reduced to tens of mg / m³. 3 level.

[0006] The above is a classic coke oven gas purification process. However, considering the characteristics of externally heated pyrolysis furnaces (or externally heated dry distillation furnaces) (small or decentralized layout), there are still some new challenges and room for improvement when using the above process, as follows: (1) The need for multiple outlets for raw coal gas: Externally heated pyrolysis furnaces often adopt a multi-section vertical tube bundle structure with raw coal gas outlets at different heights to extract the raw coal gas generated in each section in a timely manner. For example, the Chinese patent application CN111518581A, which discloses "A vertical multi-pipe segmented gas-conducting externally heated pulverized coal dry distillation method and apparatus", sets up gas collection boxes between the drying section and each dry distillation section to export raw coal gas. The advantage of doing so is that it can reduce the gas flow rate in the furnace, reduce dust entrainment, and allow tar to be exported in each section, thereby improving the recovery quality. That is, the raw coal gas purification system of the externally heated pyrolysis furnace needs to adapt to the characteristics of multi-section coal gas convergence and effectively collect and process the raw coal gas from multiple gas inlets.

[0007] (2) Coupling of tar separation and waste heat utilization: Traditional coke oven gas cooling mostly uses indirect cooling with circulating water. However, for smaller-scale externally heated pyrolysis furnaces, direct contact cooling (such as spray cooling) is preferred. On the one hand, it is more efficient and the equipment is more compact. On the other hand, it can be combined with ammonia spray to remove tar and ammonia at the same time. Practice has proven that using ammonia spray to directly cool raw gas can significantly remove impurities such as tar, naphthalene, and ammonia from the raw gas while cooling. For example, the "Direct Cooling of Raw Gas After Coke Oven Gas Collection Pipe and Tar-Ammonia Separation Process" published in Chinese patent application CN102277203A directly introduces raw gas into a spray tower after the gas collection pipe. It is cooled to below 80°C by spraying with low-temperature ammonia water, and the tar and ammonia water are initially separated. The mixed liquid after spraying enters the separation device, and the tar, ammonia water, and tar residue are rapidly separated by centrifugation and other means. Compared to traditional indirect cooling methods, direct spray cooling offers advantages such as high cooling efficiency, low gas pressure drop, and low equipment cost. It also provides purification, significantly reducing the content of tar, ammonia, and hydrogen sulfide in the gas. However, spray cooling also introduces an additional ammonia water treatment load, requiring effective tar-ammonia water separation and recycling measures.

[0008] (3) Compact and integrated system: Compared with the raw gas purification system of large coke ovens, small externally heated pyrolysis furnaces require the raw gas purification system to be as compact as possible, i.e., highly integrated, and fully coupled with the furnace body. For example, cooling, tar separation, and electrostatic precipitator can be combined into one module or set up adjacent to each other to reduce pipeline and heat loss. In addition, some of the purified raw gas often needs to be returned to the combustion system of the externally heated pyrolysis furnace to provide a heat source (i.e., as recycled gas). Therefore, the raw gas purification system also needs to consider coupling with the combustion heating system to ensure that the recycled gas is clean, combustible, and has a stable supply.

[0009] In summary, while existing raw coal gas purification technologies are relatively mature, the raw coal gas purification system for externally heated pyrolysis furnaces still needs improvement. Summary of the Invention

[0010] This invention provides a raw coal gas purification system and process that couples multi-stage coal gas extraction with tar separation. Designed for externally heated pyrolysis furnaces, it can simultaneously process multiple streams of raw coal gas. The raw coal gas extracted from the multi-stage dry distillation furnace is immediately sprayed and cooled to achieve a drop in raw coal gas temperature and the separation of most of the tar and ammonia. Then, centrifugation or sedimentation is used to further separate and recover the tar and ammonia. Finally, an electrostatic precipitator removes micro-mist tar and fine dust to obtain clean coal gas (part of which is used as remelting fuel, and the other part is used for other purposes). The raw coal gas purification system has a compact structure, short process flow, and high degree of automation. It can adapt to the intermittent or fluctuating operation of externally heated pyrolysis furnaces and achieve stable purification.

[0011] To achieve the above objectives, the present invention employs the following technical solution: A multi-stage raw coal gas purification system coupled with tar separation is disclosed for use in an externally heated pyrolysis furnace. The externally heated pyrolysis furnace is provided with a drying section, a low-temperature dry distillation section, a medium-temperature dry distillation section, and a high-temperature dry distillation section, arranged sequentially from top to bottom. The raw coal gas purification system comprises a multi-stage gas guiding and collecting unit, a spray cooling unit, a tar-ammonia-water separation unit, and a deep purification unit. The multi-stage gas guiding and collecting unit includes raw coal gas collection pipes connected to the raw coal gas outlets of each stage of the externally heated pyrolysis furnace. The spray cooling unit includes a spray cooling tower, and the raw coal gas collection pipes are connected to the spray cooling tower via raw coal gas pipelines. The gas inlet is connected to the raw coal gas pipeline, and a gas blower is installed on it. The tar-ammonia-water separation unit includes an oil-water separation device and a centrifugal separation device. The tar-ammonia-water mixture outlet of the spray cooling tower is connected to the oil-water separation device and the centrifugal separation device in sequence through pipelines. The deep purification unit includes an electrostatic precipitator. The raw coal gas inlet of the electrostatic precipitator is connected to the raw coal gas outlet of the spray cooling tower. The clean coal gas outlet of the electrostatic precipitator is connected to the return coal gas pipeline and the external coal gas supply pipeline. The return coal gas pipeline is connected to the combustible gas inlet of the burner on the externally heated pyrolysis furnace. A gas blower is installed on the return coal gas pipeline.

[0012] The multi-stage gas collection unit consists of a raw coal gas collection pipe and a gas collection box / cyclone dust collector. The drying section, low-temperature dry distillation section, medium-temperature dry distillation section and high-temperature dry distillation section of the externally heated pyrolysis furnace are respectively equipped with raw coal gas outlets. Each raw coal gas outlet is connected to the raw coal gas collection pipe through a corresponding raw coal gas outlet pipe. Each raw coal gas outlet pipe is equipped with a gas collection box / cyclone dust collector.

[0013] The top of the spray cooling tower is equipped with an ammonia water spray device, the inside of the tower is equipped with gas-liquid contact packing, and the bottom is equipped with a conical liquid collection plate. The ammonia water spray device is connected to the ammonia water tank and ammonia water circulation pump outside the tower through a circulating ammonia water pipeline to form an ammonia water circulating spray system. An ammonia water cooler and a filter device are installed on the circulating ammonia water pipeline.

[0014] The oil-water separation device is a primary sedimentation tank, and the centrifugal separation device is a three-phase centrifuge. The three-phase centrifuge is equipped with a tar outlet, an ammonia water outlet, and a tar residue outlet. The tar outlet is connected to the tar tank through a tar pipeline, and the ammonia water outlet is connected to the ammonia water tank through an ammonia water pipeline.

[0015] The electrostatic tar precipitator consists of a shell, high-voltage electrodes, precipitation plates, and a high-voltage power supply. Multiple high-voltage electrodes and precipitation plates are installed inside the shell. The high-voltage electrodes are connected to the high-voltage power supply outside the shell, and the precipitation plates are grounded to form an electrostatic tar removal system. The lower side of the electrostatic tar precipitator has a raw coal gas inlet, the top has a clean coal gas outlet, and the bottom has a tar residue outlet.

[0016] A raw coal gas purification system with multi-stage coal gas extraction and tar separation coupling also includes an automatic control system; the automatic control system includes a controller and an online gas analyzer and a flow control valve installed on the return coal gas pipeline, the online gas analyzer being interlocked with the flow control valve through the controller.

[0017] A raw coal gas purification process coupling multi-stage gas extraction and tar separation includes the following steps: 1) During the process of coal entering the external heating pyrolysis furnace for dry distillation, the raw coal gas in the drying section and at least two of the dry distillation sections (low temperature, medium temperature, and high temperature) is extracted from the furnace and collected in the raw coal gas collection pipe after dust removal and deceleration. It is then pressurized by the coal gas blower and transported to the spray cooling tower. 2) Inside the spray cooling tower, the high-temperature raw coal gas at 300-350℃ comes into direct contact with the low-temperature circulating ammonia water sprayed above. The temperature of the raw coal gas drops to 75-85℃, and most of the tar vapor condenses into droplets and is captured by the ammonia water. The resulting tar-ammonia water mixture is collected at the bottom of the tower, and the ammonia and some sulfides in the raw coal gas are absorbed by the circulating ammonia water. 3) The tar-ammonia mixture at the bottom of the spray cooling tower flows into the primary settling tank by gravity. After initial stratification by gravity settling, it enters the three-phase centrifuge to separate tar, ammonia, and tar residue. The tar is collected in the tar tank, and the ammonia is collected in the ammonia tank. After cooling, it is used as circulating ammonia for spray cooling in the spray cooling tower. The tar residue is discharged centrally. 4) The raw coal gas after spray cooling is sent to the electrostatic precipitator, where the trace amounts of tar droplets and fine dust entrained in the raw coal gas are further removed under the action of a high-voltage electric field. The purified coal gas is discharged from the purified coal gas outlet. The tar residue collected at the bottom of the electrostatic precipitator is sent to the bottom of the spray cooling tower and combined with the tar residue collected after spray cooling for recycling. 5) A portion of the clean coal gas discharged from the electrostatic precipitator is returned to the heating and combustion system of the externally heated pyrolysis furnace as recycled coal gas. The controller monitors the composition and calorific value of the clean coal gas in real time through an online gas analyzer and interlocks the flow regulating valve to provide a stable heat source for coal dry distillation. The remaining clean coal gas is supplied externally or vented for combustion as needed. 6) When the output or composition of raw coal gas fluctuates, adjust the raw coal gas outlet temperature of the spray cooling tower within the set range.

[0018] The tar obtained after separation by a three-phase centrifuge has a water content of less than 5%, and the ammonia concentration in the ammonia water is above 3%; the tar content in the purified coal gas is less than 20 mg / Nm³. 3 .

[0019] When the calorific value of the clean coal gas is insufficient or the supply is unbalanced, the automatic control system is used to switch the recycle fuel system or supplement external gas to maintain the stable dry distillation temperature in the furnace. When the raw coal gas purification system fails, the extraction of raw coal gas is reduced or the raw coal gas is directly burned and released until the raw coal gas purification system returns to normal operation.

[0020] Compared with the prior art, the beneficial effects of the present invention are: (1) Thorough tar separation and high gas cleanliness: Through multi-stage separation of "spray cooling + centrifugation + electrostatic precipitation", the tar content in raw coal gas is significantly reduced (below 10 mg / Nm³). 3 This process is far superior to traditional single-step processes that only involve cooling or electrostatic precipitation. Impurities such as naphthalene and ammonia in the raw coal gas are also largely removed, increasing its subsequent utilization value and reducing the likelihood of crystallization and blockage. The final output of clean coal gas is close to city gas standards and can be used as clean fuel.

[0021] (2) Improved tar yield and quality: Raw coal gas is promptly extracted from each dry distillation section, avoiding prolonged stagnation and cracking in high-temperature zones, thus increasing tar yield. Simultaneously, the rapid cooling function of the spray tower allows tar to condense and be absorbed by ammonia water as early as possible, reducing tar escape and secondary reaction losses. The centrifuged tar has low water content, few impurities (after centrifugal slag removal), and good quality, facilitating storage, transportation, and sales. Compared with traditional processes, the overall tar recovery rate is significantly improved, avoiding tar deposition and waste in pipelines, and converting more tar into recyclable products.

[0022] (3) Achieving energy conservation, emission reduction, and heat recovery: The spray cooling of this invention utilizes the ammonia water from the raw coal gas itself as a medium, eliminating the need for a large cooling device. After transferring some heat to the ammonia water, the hot ammonia water can be further utilized for waste heat recovery (e.g., preheating other media in a heat exchanger) or discharged from the cooling tower, simplifying the process. Compared with indirect water cooling, direct spray cooling reduces one heat exchange barrier, resulting in lower cooling water consumption. In addition, most of the acidic gases such as hydrogen sulfide and hydrogen cyanide in the raw coal gas are neutralized and absorbed by the ammonia water, reducing the subsequent desulfurization load and achieving pollutant reduction.

[0023] (4) The system is compact, highly coupled, and highly automated: The raw coal gas purification system of the present invention has tightly connected units. The spray tower, separator, and electrostatic precipitator can be arranged in the same frame or module, which takes up little space and is conducive to the transformation of existing equipment. Equipped with a DCS automatic control system, the spray pump and fan frequency can be automatically adjusted according to the temperature and pressure of the raw coal gas, and tar discharge and ammonia replenishment operations can be performed automatically to achieve unattended continuous operation. The process flow is short, and the main purification process of raw coal gas can be completed almost instantaneously, which reduces the probability of equipment and pipelines being contaminated by tar, requires little maintenance, and is reliable in operation. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the raw coal gas purification system described in this invention.

[0025] Figure 2 This is a cross-sectional view of the spray cooling tower described in this invention.

[0026] Figure 3 This is a schematic diagram of the tar-ammonia water separation unit described in this invention.

[0027] Figure 4 This is a schematic diagram of the structure of the electrostatic tar precipitator described in this invention.

[0028] In the diagram: 1-Externally heated pyrolysis furnace; 11-Drying section; 12-Low-temperature carbonization section; 13-Medium-temperature carbonization section; 14-High-temperature carbonization section; 15-Cyclone dust collector; 16-Raw coal gas collection pipe; 2-Spray cooling tower; 21-Raw coal gas inlet (of the spray cooling tower); 22-Ammonia water spray device; 23-Conical liquid collection tray; 24-Raw coal gas outlet (of the spray cooling tower); 3-Tar ammonia water separation device; 31-Primary sedimentation tank; 32-Three-phase centrifuge; 4-Electrostatic tar precipitator; 41-Shell; 42-High-voltage electrode; 43-Electrostatic precipitator plate; 44-High-voltage power supply; 5-Gas blower one; 6-Tar storage tank; 7-Ammonia water circulation tank; 8-Online gas analyzer; 9-Gas blower two; V-Flow control valve; P1-Tar residue pump; P2-Ammonia water circulation pump. Detailed Implementation

[0029] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings: like Figure 1As shown, the present invention discloses a multi-stage coal gas extraction and tar separation coupled raw coal gas purification system for an externally heated pyrolysis furnace 1. The externally heated pyrolysis furnace 1 is provided with a drying section 11, a low-temperature dry distillation section 12, a medium-temperature dry distillation section 13, and a high-temperature dry distillation section 14, arranged sequentially from top to bottom. The raw coal gas purification system consists of a multi-stage gas guiding and collecting unit, a spray cooling unit, a tar-ammonia-water separation unit, and a deep purification unit. The multi-stage gas guiding and collecting unit includes raw coal gas collection pipes 16 connected to the raw coal gas outlets of each stage of the externally heated pyrolysis furnace 1. The spray cooling unit includes a spray cooling tower 2, and the raw coal gas collection pipes 16 are connected to the spray cooling tower 2 via raw coal gas pipelines. The raw coal gas inlet 21 of the spray cooling tower 2 is connected to the raw coal gas pipeline by a gas blower 5; the tar ammonia water separation unit includes an oil-water separation device and a centrifugal separation device, and the tar ammonia water mixture outlet of the spray cooling tower 2 is connected to the oil-water separation device and the centrifugal separation device in sequence through a pipeline; the deep purification unit includes an electrostatic precipitator 4, the raw coal gas inlet of the electrostatic precipitator 4 is connected to the raw coal gas outlet 26 of the spray cooling tower 2, the clean coal gas outlet of the electrostatic precipitator 4 is connected to the return coal gas pipeline and the external coal gas pipeline, the return coal gas pipeline is connected to the combustible gas inlet of the burner on the externally heated pyrolysis furnace, and a gas blower 9 is installed on the return coal gas pipeline.

[0030] The multi-stage gas guiding and collecting unit is used to separately guide and collect the raw coal gas from different height sections of the externally heated pyrolysis furnace 1 into a total raw coal gas flow; such as Figure 2 As shown, the spray cooling tower 2 in the spray cooling unit is equipped with a raw coal gas inlet 21 and a raw coal gas outlet 26. A circulating ammonia water spraying device is installed inside the tower to allow the raw coal gas to directly contact and cool with low-temperature ammonia water, reducing the temperature of the raw coal gas from its initial high temperature at the outlet to approximately 80°C and causing tar droplets to precipitate. The tar-ammonia water separation unit is equipped with an oil-water separation device (such as a primary settling tank 31) and a centrifugal separation device (such as a three-phase centrifuge 32) to separate the tar-ammonia water mixture collected by the spray cooling tower 2 into tar and ammonia water phases, recovering pure tar and circulating ammonia water. The deep purification unit is equipped with an electrostatic precipitator 4 or an equivalent demisting device to further remove residual fine tar mist and impurities from the cooled raw coal gas, reducing the tar content in the purified gas to 10 mg / m³. 3 After purification, a portion of the clean coal gas is returned to the externally heated pyrolysis furnace 1 for use as combustion gas, while the other portion is supplied externally as coal gas products.

[0031] The multi-stage gas collection unit consists of a raw coal gas collection pipe 16 and a gas collection box / cyclone dust collector 15. The drying section 11, low-temperature carbonization section 12, medium-temperature carbonization section 13, and high-temperature carbonization section 14 of the externally heated pyrolysis furnace 1 are each equipped with a raw coal gas outlet. Each raw coal gas outlet is connected to the raw coal gas collection pipe 16 via a corresponding raw coal gas outlet pipe. Each raw coal gas outlet pipe is equipped with a gas collection box / cyclone dust collector 15 to reduce the raw coal gas flow rate and remove entrained large particles of dust (solid coal powder or black dust). Powered by the gas blower 5, the raw coal gas from multiple outlets is collected and transported into the spray cooling unit 2. Using multi-point gas collection allows for timely collection of gas generated in each stage, reducing gas retention in the furnace and mitigating the adverse effects of high-temperature pyrolysis on tar. Compared to the traditional method of exporting raw coal gas through a single outlet, this significantly reduces the dust content of the raw coal gas and improves the tar collection rate.

[0032] The spray cooling tower 2 is the core equipment of the raw coal gas purification system. It is used to directly cool the raw coal gas and simultaneously remove most of the tar and moisture. The tower body is preferably a vertical cylindrical structure, and the interior is divided into a gas inlet zone, a spray cooling zone, and a gas outlet zone from top to bottom. The raw coal gas enters the tower tangentially from the upper side to avoid direct impact on the internal structure. The top of the spray cooling tower 2 is equipped with an ammonia water spray device 22, and the inside of the tower is equipped with gas-liquid contact packing. The bottom is equipped with a conical liquid collection plate 23. The spray cooling tower 2 also has a dust removal function, where part of the dust in the raw coal gas is wetted and settled. To improve the spraying efficiency, multiple layers of packing or baffles can be installed inside the tower to prolong the gas-liquid contact time, and multiple nozzles can be configured to ensure coverage of the entire airflow cross section.

[0033] The ammonia water spraying device 22 is connected to the ammonia water tank 7 and the ammonia water circulation pump P2 outside the tower through a circulating ammonia water pipeline, forming an ammonia water circulating spraying system; an ammonia water cooler and a filter device are installed on the circulating ammonia water pipeline. The circulating ammonia water is a low-temperature ammonia water solution separated by the tar ammonia water separation unit, and its temperature is preferably 20-40℃; when the raw coal gas enters the spray cooling tower 2, the circulating ammonia water is atomized by the nozzle and sprayed out to contact the raw coal gas in a countercurrent manner. Through direct cooling, the temperature of the raw coal gas is rapidly reduced to below 85℃, while most of the tar (more than 70%) and water vapor in the raw coal gas are condensed and absorbed, removing most of the ammonia, moisture, phenol, etc., achieving the effect of "one-step coarse purification". The resulting ammonia water tar mixture accumulates at the bottom of the tower.

[0034] Oil-water separation devices are used to separate a mixture of tar and ammonia into oil (tar), water (ammonia), and solids, thus achieving three-phase separation; for example... Figure 3As shown, this invention employs a combination of gravity sedimentation and centrifugal separation. The preferred oil-water separation device is a primary sedimentation tank 31, and the preferred centrifugal separation device is a three-phase centrifuge 32. The three-phase centrifuge 32 has a tar outlet, an ammonia water outlet, and a tar residue outlet. The tar outlet is connected to a tar tank 6 via a tar pipeline, and the ammonia water outlet is connected to an ammonia water tank 7 via an ammonia water pipeline. The circulating ammonia water contains ammonia from the coal gas, with an NH3 concentration of 1%–5%. During the spray cooling process, it can neutralize the acidic components in the raw coal gas and promote the dissolution and capture of tar. The bottom of the spray cooling tower 2 has a tar-ammonia water mixture outlet connected to the tar-ammonia water separation unit, from which the collected tar-ammonia water mixture is output for subsequent separation. The primary settling tank 31 is used for gravity settling and stratification of the tar-ammonia water mixture discharged from the bottom of the spray cooling tower, separating it into an upper layer of coarse tar and a lower layer of ammonia-rich water. The upper layer of coarse tar overflows into a three-phase centrifuge 32, which separates the tar-ammonia water mixture into three phases—pure tar, circulating ammonia water, and tar residue—through high-speed rotation. The pure tar is output to the tar tank 6, which is equipped with a slag discharge and heating / insulation device to periodically remove a small amount of slag, outputting clean tar product. The circulating ammonia water is output to the ammonia water tank 7 for standby, and the separated tar residue is output to a slag tank for periodic discharge and treatment.

[0035] At least a portion of the ammonia-rich water separated by the three-phase centrifuge 32 is reused in the spray cooling tower 2 as circulating ammonia water, while the remainder is discharged as "residual ammonia water" to the wastewater treatment or ammonia recovery process. The circulating ammonia water output from the three-phase centrifuge 32 is cooled to the required spray temperature by an ammonia water cooler and filter, and fine tar residue or solid impurities are filtered out to ensure the quality of the circulating ammonia water. After cooling and purification, the circulating ammonia water is returned to the top of the spray cooling tower 2 for spraying, thus achieving the recycling of ammonia water. To ensure the quality of the circulating ammonia water, an ammonia water conditioning tank can also be installed. For example, before the circulating ammonia water is reused, fine tar residue can be removed using air flotation or a filter tank to prevent nozzle clogging. During the ammonia water circulation, if the NH3 content is insufficient, it can be supplemented by adding an ammonia water conditioning tank; generally, no additional addition is needed, as it is provided by the ammonia dissolved in the furnace gas itself.

[0036] Of course, tar-ammonia-water separation can also be achieved without centrifugation, using the traditional "tar-ammonia-water separation tank + neutralization tank + clarification tank" method. However, this method is not only space-consuming but also slow, making it less efficient than centrifugation. Using centrifugation can accelerate oil-water separation, shorten residence time, and eliminate the need for a large-volume clarification tank, resulting in a more compact overall system layout.

[0037] Although spray cooling has removed most of the tar from the raw coal gas, in order to ensure the cleanliness of the coal gas (tar mist content <50 mg / m³), 3 Dust content <5 mg / m 3 Even for processes like these, electrostatic tar removal is still required. A honeycomb high-voltage electrostatic device (containing grounded honeycomb tubes and a central discharge wire) or a tubular high-voltage electrostatic device is preferred. For example... Figure 4 As shown, the electrostatic tar precipitator 4 consists of a shell 41, high-voltage electrodes 42, precipitation plates 43, and a high-voltage power supply 44. Multiple high-voltage electrodes 42 and precipitation plates 43 are installed inside the shell 41. The high-voltage electrodes 42 are connected to the high-voltage power supply 44 outside the shell 41, and the precipitation plates 43 are grounded, forming an electrostatic tar removal system. Raw coal gas, after being cooled by spraying and separated from oil, enters from the bottom of the electrostatic tar precipitator 4 and is evenly distributed to each electrostatic precipitator element by an airflow distributor. The high-voltage electric field applied inside (approximately 50-60kV DC high voltage) charges the residual tar droplets and fine particles, causing them to deposit on the precipitation plates 43 and collect into droplets that flow down. The electrostatic tar precipitator 4 has a raw coal gas inlet on one side at the bottom, a clean coal gas outlet at the top, and a tar residue outlet at the bottom. The collected tar residue is sent to the tar tank 6 or the conical collection tray 23 at the bottom of the spray cooling tower via pipelines and a tar residue pump P1. After electrostatic tar removal, the tar content in the clean coal gas output is reduced to below 10 mg / Nm³, and it contains virtually no visible droplets.

[0038] Multiple electrostatic precipitators 4 can be connected in parallel to meet the required gas volume. In this invention, because the raw coal gas has already undergone a significant reduction in temperature and dust and oil content through a pre-treatment spray cooling process, the operating conditions of the electrostatic precipitator 4 are optimized: the raw coal gas temperature is below 25℃, the humidity is high but the tar mist concentration is low, making it easier to achieve high demisting efficiency and avoiding the risk of discharge ignition. Tests have shown that the tar content of the clean coal gas after electrostatic precipitation can be reduced to approximately 5–10 mg / m³. 3 Compared to traditional processes, this significantly reduces costs. The purified gas, known as clean coal gas, has a relatively good calorific value (approximately 17–20 MJ / Nm³). 3 (Low calorific value).

[0039] The present invention discloses a multi-stage gas extraction and tar separation coupled raw coal gas purification system, which further includes an automatic control system. The automatic control system includes a controller, an online gas analyzer 8, and a flow control valve V installed on the recycled gas pipeline. The online gas analyzer 8 is interlocked with the flow control valve V via the controller. The automatic control system also distributes the purified coal gas proportionally into recycled coal gas and externally supplied coal gas. The recycled coal gas is sent to the combustion heating system of the externally heated pyrolysis furnace 1 as heating fuel, while the externally supplied coal gas is delivered to the gas utilization equipment or flare. The online gas analyzer 8 monitors the calorific value and composition of the purified coal gas and, through linkage with the controller, adjusts the opening of the flow control valve V to ensure a stable supply of recycled coal gas and safe system operation.

[0040] The raw coal gas purification process of the present invention, which couples multi-stage coal gas extraction and tar separation, includes the following steps: 1) During the process of coal entering the externally heated pyrolysis furnace 1 for dry distillation, the raw coal gas in at least two of the dry section 11, low temperature dry distillation section 12, medium temperature dry distillation section 13, and high temperature dry distillation section 14 is extracted out of the furnace, collected in the raw coal gas collection pipe 16 after dust removal and deceleration, and then pressurized by the coal gas blower 5 and transported to the spray cooling tower 2. 2) Inside the spray cooling tower 2, the high-temperature raw coal gas at 300-350℃ comes into direct contact with the low-temperature circulating ammonia water sprayed above, and the temperature of the raw coal gas drops to 75-85℃. Most of the tar vapor condenses into droplets and is captured by the ammonia water. The resulting tar-ammonia water mixture is collected at the bottom of the tower, and the ammonia and some sulfides in the raw coal gas are absorbed by the circulating ammonia water. 3) The tar-ammonia mixture at the bottom of the spray cooling tower flows into the primary settling tank 31 by gravity. After initial stratification by gravity settling, it enters the three-phase centrifuge 32 to separate tar, ammonia and tar residue. The tar is collected in the tar tank 6 and the ammonia is collected in the ammonia tank 7. After cooling, it is used as circulating ammonia for spray cooling in the spray cooling tower 2. The tar residue is discharged in a centralized manner. 4) The raw coal gas after spray cooling is sent to the electrostatic precipitator 4, where the trace amounts of tar droplets and fine dust entrained in the raw coal gas are further removed under the action of a high-voltage electric field. The purified coal gas is discharged from the purified coal gas outlet. The tar residue collected at the bottom of the electrostatic precipitator 4 is sent to the bottom of the spray cooling tower 2 and combined with the tar residue collected after spray cooling for recycling. 5) A portion of the clean coal gas discharged from the electrostatic precipitator 4 is returned to the heating and combustion system of the externally heated pyrolysis furnace 1 as recycled coal gas. The controller monitors the composition and calorific value of the clean coal gas in real time through the online gas analyzer 8 and interlocks the flow regulating valve V to provide a stable heat source for coal dry distillation. The remaining clean coal gas is supplied externally or vented for combustion as needed. 6) When the output or composition of raw coal gas fluctuates, adjust the raw coal gas outlet temperature of spray cooling tower 2 within the set range, stabilize the tar separation effect, ensure the constant quality of output coal gas, and balance the calorific value supply required for combustion in the furnace.

[0041] Furthermore, by installing temperature, pressure, and liquid level detection devices in each unit, the ammonia spray volume, circulating ammonia temperature, blower air volume, and the distribution ratio of clean coal gas (e.g., maintaining the proportion of recycled coal gas at 50%–70% of the total clean coal gas) can be automatically adjusted, enabling the various units of the system to work collaboratively. For example, in the spray cooling unit, by controlling the circulating ammonia spray volume and temperature, the raw coal gas can be reduced from its temperature upon entering the spray cooling tower (300–350°C) to its outlet temperature (80±5°C), and the tar content in the tar-ammonia mixture condensed from the spray cooling can reach more than 90% of the coal tar yield.

[0042] The tar obtained after separation by a three-phase centrifuge has a water content of less than 5%; the ammonia concentration in the ammonia water is above 3% to enhance the absorption of hydrogen sulfide and hydrogen cyanide in the raw coal gas; and the tar content in the purified coal gas is less than 20 mg / Nm³. 3 The gas is free of visible dust, thus meeting the requirements for reprocessing and external gas supply. After purification by the raw coal gas purification system described in this invention, the reprocessed coal gas has extremely low tar content, preventing carbon buildup or blockage in burners and heat exchangers. Corrosive components such as sulfur and ammonia are also significantly reduced (partial ammonia is recovered with ammonia water, while hydrogen sulfide dissolves in water; further removal can be achieved by subsequent washing with lime water if necessary). Furthermore, the separated tar product has low water and solid content, allowing it to be sold directly as a chemical raw material or fuel oil, increasing the company's economic benefits.

[0043] When the calorific value of the clean coal gas is insufficient or the supply is unbalanced, the automatic control system is used to switch the recycle fuel system or supplement external gas to maintain the stable dry distillation temperature in the furnace. When the raw coal gas purification system malfunctions or fails (such as the spray cooling tower temperature exceeding the limit or the electrostatic precipitator malfunctioning), the extraction of raw coal gas is reduced or the raw coal gas is directly burned and released to ensure the safe operation of the system until the raw coal gas purification system returns to normal working condition.

[0044] The key technological features of the multi-stage coal gas extraction and tar separation coupled raw coal gas purification system described in this invention are as follows: 1. Multi-stage gas extraction: Open the raw coal gas outlet valves of the drying section 11 and each dry distillation section of the externally heated pyrolysis furnace 1, and start the coal gas blower 5 to extract the raw coal gas from the furnace at a certain negative pressure. Ensure that raw coal gas is extracted from each section and collected in the raw coal gas purification system. Adjust the valves on each raw coal gas outlet pipe to balance the gas volume and prevent over-extraction or unstable airflow in any section. This step can be carried out continuously during the dry distillation process to achieve continuous discharge of raw coal gas.

[0045] 2. Direct Spray Cooling: The circulating ammonia water is cooled to below 40℃ (using a heat exchanger or cooling tower), and then pressurized by the circulating ammonia water pump and sprayed into the spray cooling tower 2 through nozzles. The raw coal gas introduced simultaneously comes into direct contact with the ammonia water droplets, and the temperature drops from high temperature to below 85℃. Adjusting the ammonia water spray rate can control the raw coal gas outlet temperature and saturation humidity. Condensate is collected at the bottom of the tower.

[0046] 3. Oil-water separation and recovery: The tar-ammonia-water mixture collected at the bottom of the tower is sent to a centrifugal separator. After separation, three phases are obtained: tar, ammonia-water, and tar residue. The tar is stored in tar tank 6, and the ammonia-water is returned to the spray cooling tower for recycling. The tar residue is periodically discharged for treatment. During this process, the ammonia-water density and pH value are monitored. If the ammonia concentration decreases, ammonia is added to adjust the concentration or a small amount of stripped ammonia-water is introduced to maintain the ammonia content and ensure the absorption capacity for hydrogen sulfide, etc.

[0047] 4. Fine purification: After cooling and oil removal, the raw coal gas is sent to the electrostatic precipitator 4 for further purification. Maintain stable operating parameters of the electrostatic precipitator 4 (such as voltage and current). Periodically discharge the tar collected by electrostatic precipitation and monitor the tar content in the purified coal gas. If an increase in tar content is detected, check the operating status of the spray cooling tower 2 and the electrostatic precipitator 4.

[0048] 5. Utilization of Clean Coal Gas: The composition and calorific value of clean coal gas are analyzed in real time. The clean coal gas required for heating the externally heated pyrolysis furnace 1 is drawn to the furnace burner by the gas blower 29, and the remainder is discharged to subsequent processes or supplied externally through valves. By configuring an online calorific value meter and flow meter, a stable supply of recycled coal gas is ensured. When the calorific value of the clean coal gas is high and the production is surplus, it can also be supplied externally as fuel gas.

[0049] The following are optional alternatives to the present invention: 1) For large-scale externally heated pyrolysis units, the spray cooling unit can be equipped with two-stage spray cooling towers connected in series. The first stage uses circulating ammonia water to cool the raw coal gas to about 60°C, and the second stage uses cold ammonia water separated from oil and water to further cool the raw coal gas to ambient temperature before it enters the deep purification unit. This allows for more thorough cooling of the raw coal gas, but the disadvantage is that the structure is more complex. For general externally heated pyrolysis units, a single spray cooling tower is sufficient.

[0050] 2) When the tar yield is extremely high (>10%), a steam heating step can be added before centrifugation to heat the tar-ammonia-water mixture to 80-90°C to reduce the viscosity, which helps with centrifugation.

[0051] 3) Electrostatic precipitators can also be replaced by fiber mist filters as deep purification devices. The choice can be made based on actual needs, but electrostatic precipitators have lower maintenance costs and are more suitable for continuous operation.

[0052] 4) If acidic gases need to be removed, an alkaline scrubbing tower can be added after the electrostatic precipitator to absorb H2S, HCN, etc. The interface of the raw coal gas purification system of this invention is open, which can be easily connected to the desulfurization unit to ensure that the coal gas meets the emission or utilization standards.

[0053] The raw coal gas purification system of the present invention, which is a multi-stage coal gas extraction and tar separation coupling system, has a compact structure and flexible and adjustable process. It can efficiently purify raw coal gas, recover tar and ammonia water, and realize the recycling of coal gas. It has significant application value and environmental benefits in the fields of low-rank coal pyrolysis and coal chemical industry.

[0054] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the examples. The following examples are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention, including simple variations or equivalent substitutions, are all within the scope of protection of the present invention.

[0055]

Example

[0056] The process flow for purifying raw coal gas is as follows: 1. Multi-stage gas collection: The externally heated pyrolysis furnace 1 has a total of 5 raw coal gas outlets (1 at the top of the drying section and 1 at the top of each stage of the dry distillation section). The raw coal gas from each stage is led out through its corresponding raw coal gas outlet pipe (in this embodiment, stainless steel heat-resistant pipes are used) and then converges into a DN300 raw coal gas collection pipe 16 on the furnace side. To perform preliminary dust removal on the raw coal gas, a small cyclone dust collector 15 is installed on each raw coal gas outlet pipe to remove coal dust carried in the raw coal gas. A gas blower 5 is used to draw the raw coal gas from the raw coal gas collection pipe 16, maintaining a slight negative pressure inside the furnace (approximately -100 Pa), while simultaneously sending the raw coal gas to the downstream purification process.

[0057] In this embodiment, the gas blower 5 is equipped with a frequency converter to control the air volume to match the gas production rate of the externally heated pyrolysis furnace 1. The raw coal gas parameters in the raw coal gas collection pipe 16 are: temperature of approximately 300℃, pressure of approximately 100-200Pa, and tar mist concentration on the order of tens of g / Nm³.

[0058] Through multi-point gas guiding, the raw coal gas from the furnace top and various functional sections can be extracted in a timely manner. At this time, the tar is still in a high-temperature gaseous state and will not condense and deposit in the furnace or raw coal gas pipeline. Cyclone dust removal can remove most of the solid particles. The raw coal gas collection pipe 16 adopts a vertical pipe form. The raw coal gas reaching the raw coal gas collection pipe 16 has a reduced gas velocity, and the dust in it further settles (returning to the continuous cooling section of the externally heated pyrolysis furnace 1). The combination of these two methods achieves a dust removal rate of over 90%. Therefore, the dust content of the raw coal gas entering the subsequent purification process is <1g / Nm³. 3 .

[0059] 2. Spray Cooling and Primary Tar Separation: After being pressurized by the gas blower 5, the raw coal gas cools to approximately 280°C and enters the upper part of the spray cooling tower 2 tangentially. Simultaneously, circulating ammonia water is drawn from the ammonia water tank 7, pre-cooled to approximately 30°C by a heat exchanger, and then pumped to the top of the spray cooling tower 2 at a flow rate of 50 L / min by the circulating ammonia water pump P1. It is then atomized through eight clogging-resistant nozzles and sprayed downwards. The initial source of the circulating ammonia water is surplus ammonia water from upstream coking, with an initial ammonia content of approximately 3% (mass content), which will be subsequently enriched through circulation. Inside the spray cooling tower 2, the raw coal gas comes into counter-current contact with the downward-sprayed ammonia water droplets from bottom to top. A large amount of tar vapor condenses into microdroplets upon contact with the condensation and is captured by the ammonia water. The temperature of the raw coal gas rapidly drops to approximately 82°C (measured at the top outlet of the tower), approaching the saturation temperature of water (80–85°C). Some of the water vapor in the raw coal gas condenses, while ammonia and hydrogen sulfide dissolve in the ammonia water.

[0060] In this embodiment, the spray cooling tower 2 is equipped with two layers of packing (ceramic Pall rings, each layer 1 m high) to enhance gas-liquid contact and ensure more thorough cooling. The conical collecting tray 23 at the bottom of the tower is used to collect the tar-ammonia-water mixture. By adjusting the spray ammonia water volume and temperature, the outlet temperature of the raw coal gas at the top of the tower is maintained at 80±5℃, and the outlet temperature of the tar-ammonia-water mixture at the bottom of the tower is approximately 65℃. Under these conditions, most of the tar in the raw coal gas is condensed. Based on the condensate production, approximately 180 L of the tar-ammonia-water mixture is collected per hour, of which tar accounts for 20% of the volume (approximately 36 L / h), equivalent to approximately 30 kg / h by mass, which matches the coal yield.

[0061] The spray cooling process removes most of the dust and naphthalene from the raw coal gas, and also absorbs NH3 (approximately 1% by volume) and H2S (approximately 0.5% by volume) into the ammonia water. According to publicly available literature, direct cooling can reduce the tar content in the coal gas to 100 mg / m³. 3 In this embodiment, the amount of tar carried out by the cooled raw coal gas is very low, and no obvious oil mist condensation is observed in the pipeline downstream of the tower. Testing revealed that the composition of the raw coal gas downstream of the tower (volume dry basis) is approximately H2: 30%, CH4: 20%, CO: 10%, CO2: 5%, N2: 2%, with the remainder being hydrocarbons, etc., exhibiting a high flammability; the tar mist content is below 50 mg / m3.

[0062] 3. Tar-ammonia water separation and recovery: The tar-ammonia water mixture flowing out from the bottom of spray cooling tower 2 is pumped into primary settling tank 31 (capacity 1m³ in this embodiment). 3 After standing for 1 hour, coarse tar floats to the top, ammonia water forms the bottom layer, and a small amount of solids and heavy tar residue remains at the bottom. The upper oil phase overflows into a three-phase centrifuge 32 for continuous oil, water, and residue separation. The three-phase centrifuge 32 is set to a speed of 3000 rpm and a throughput of 0.5 m³ / h.3 After centrifugation, approximately 35 L / h of clean tar (water content <3%) was separated, approximately 140 L / h of ammonia water (containing approximately 4% NH3 by volume, as the ammonia water absorbed some of the ammonia from the furnace gas) was separated, and a small amount of solid tar residue (mainly dust and heavy components) was separated at approximately 0.5 kg / h (intermittently discharged from the slag outlet of the three-phase centrifuge 32).

[0063] Clean tar flows into a sealed tar tank 6 and is periodically transported off-site for storage. Circulating ammonia water flows into an ammonia water tank 7; a portion of it is cooled by a heat exchanger and then returned to the top of the spray cooling tower 2 by the ammonia water circulation pump P2 for reuse; excess ammonia water is sent to the ammonia recovery section as a byproduct. The total amount of ammonia water used in the spray cooling tower 2 is approximately 0.3 m³. 3 To maintain the liquid balance of the ammonia spraying system, ammonia water discharged from the furnace can be replenished periodically. Both the tar tank and the ammonia water tank are equipped with level gauges, which are interlocked with the pumps to prevent overflow or shortage.

[0064] 4. Deep Purification of Tar by Electrostatic Precipitation: In this embodiment, the raw coal gas (approximately 80°C, saturated with moisture, containing a small amount of mist droplets) after treatment by the spray cooling tower 2 is introduced into two parallel tubular electrostatic tar precipitators 4 (one operational and one standby). Each precipitator is equipped with 24 sedimentation tubes of ø100mm×2m length, supplied with 50 kV DC voltage by a high-voltage power supply. The raw coal gas flow rate is controlled at approximately 0.3m / s per tube, and the residence time is approximately 6-7 seconds, during which the micron-sized tar mist remaining in the raw coal gas is captured by electrodes. The temperature of the discharged clean coal gas drops below 30°C (the outer shell of the electrostatic tar precipitator has a water-cooling device), essentially reaching ambient temperature. Every 4 hours, the two electrostatic tar precipitators 4 are switched. The offline precipitator is de-energized and vibrated for 5 minutes to shake off the accumulated tar, which flows along the tube wall into the bottom tank and is discharged into the tar tank 6 through a drain valve. The tar content in the purified coal gas is <10mg / Nm³. 3 Dust content <5mg / Nm 3 The clean gas is virtually free of oil mist or particles, achieving a high level of cleanliness. At this point, the clean gas has a low dew point and no obvious tendency to condense, making it perfectly suitable for direct combustion as fuel.

[0065] 5. Clean gas reuse and emission: The flow rate of clean gas, measured by a flow meter, is approximately 480 Nm³. 3 / h (a small portion is lost or dissolved during spray cooling and electrostatic precipitation), its calorific value, as determined by online chromatography, is approximately 18 MJ / Nm³. 3 This meets the fuel gas requirements. The automatic control system, based on the heating needs of the externally heated pyrolysis furnace, directs 60% (approximately 288 Nm³) of the produced clean coal gas. 3The gas is fed to the burner of the externally heated pyrolysis furnace 1 via gas blower 29. The combustion system is equipped with safety valves and anti-backfire devices, ensuring stable combustion. The remaining 40% of the purified gas is piped to the plant's gas pipeline network to supply nearby boilers for combustion. During system commissioning, excess purified gas is introduced into the flare for combustion and venting. By adjusting the flow control valve V and the frequency of gas blower 29, the proportion of recycled gas is maintained within the range of 50% to 80%, and can be flexibly adjusted. In this embodiment, the purified gas can be directly and efficiently reused, reducing the external fuel requirement of the externally heated pyrolysis furnace 1. During the entire operation of the raw gas purification system, no tar blockage of the pipeline was found, and there were no obvious odorous exhaust emissions, making it environmentally friendly.

[0066] 6. Control and Safety: This embodiment is equipped with an automatic control system for closed-loop control of key parameters. For example, when the raw coal gas outlet temperature of spray cooling tower 2 is >90℃, the spray volume is automatically increased or the raw coal gas flow rate is reduced (linked to the speed control of the gas blower 5); an alarm is triggered when the current of the electrostatic precipitator 4 abnormally increases; and so on. In terms of safety, all equipment is grounded and explosion-proof, and flame arresters are installed on the gas pipelines. The ammonia water circulation system is a slightly positive pressure system, well-sealed, and with no ammonia leakage. Actual measurements show that the H2S in the raw coal gas is 3000 mg / m³ when entering spray cooling tower 2. 3 The concentration dropped to 1000 mg / m³ at the time of exiting the tower. 3 To further desulfurize the gas, an alkaline scrubbing tower is added to the clean gas pipeline (due to ammonia absorption).

[0067] In this embodiment, the tar yield is 8.2% (higher than the 6% to 7% of conventional processes). The clean coal gas is used as fuel for the externally heated pyrolysis furnace 1, which greatly saves fuel costs. The system operates stably, and the coal gas has a high degree of purification, meeting the requirements of clean production.

[0068] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A multi-stage coal gas extraction and tar separation coupled raw coal gas purification system, used in an externally heated pyrolysis furnace, wherein the externally heated pyrolysis furnace is provided with a drying section, a low-temperature dry distillation section, a medium-temperature dry distillation section and a high-temperature dry distillation section from top to bottom; characterized in that, The raw coal gas purification system consists of a multi-stage gas collection unit, a spray cooling unit, a tar-ammonia-water separation unit, and a deep purification unit. The multi-stage gas collection unit includes raw coal gas collection pipes connected to the raw coal gas outlets of each section of the externally heated pyrolysis furnace. The spray cooling unit includes a spray cooling tower; the raw coal gas collection pipes are connected to the raw coal gas inlet of the spray cooling tower via a raw coal gas pipeline, and a gas blower is installed on the raw coal gas pipeline. The tar-ammonia-water separation unit includes an oil-water separation device and a centrifugal separation device; the tar-ammonia-water mixture outlet of the spray cooling tower is sequentially connected to the oil-water separation device and the centrifugal separation device via pipelines. The deep purification unit includes an electrostatic precipitator; the raw coal gas inlet of the electrostatic precipitator is connected to the raw coal gas outlet of the spray cooling tower, and the clean coal gas outlet of the electrostatic precipitator is connected to a return coal gas pipeline and an external coal gas supply pipeline. The return coal gas pipeline is connected to the combustible gas inlet of the burner on the externally heated pyrolysis furnace, and a gas blower is installed on the return coal gas pipeline.

2. The raw coal gas purification system with multi-stage coal gas extraction and tar separation coupling according to claim 1, characterized in that, The multi-stage gas collection unit consists of a raw coal gas collection pipe and a gas collection box / cyclone dust collector. The drying section, low-temperature dry distillation section, medium-temperature dry distillation section and high-temperature dry distillation section of the externally heated pyrolysis furnace are respectively equipped with raw coal gas outlets. Each raw coal gas outlet is connected to the raw coal gas collection pipe through a corresponding raw coal gas outlet pipe. Each raw coal gas outlet pipe is equipped with a gas collection box / cyclone dust collector.

3. The raw coal gas purification system with multi-stage coal gas extraction and tar separation coupling according to claim 1, characterized in that, The top of the spray cooling tower is equipped with an ammonia water spray device, the inside of the tower is equipped with gas-liquid contact packing, and the bottom is equipped with a conical liquid collection plate. The ammonia water spray device is connected to the ammonia water tank and ammonia water circulation pump outside the tower through a circulating ammonia water pipeline to form an ammonia water circulating spray system. An ammonia water cooler and a filter device are installed on the circulating ammonia water pipeline.

4. The raw coal gas purification system with multi-stage coal gas extraction and tar separation coupling according to claim 1, characterized in that, The oil-water separation device is a primary sedimentation tank, and the centrifugal separation device is a three-phase centrifuge. The three-phase centrifuge is equipped with a tar outlet, an ammonia water outlet, and a tar residue outlet. The tar outlet is connected to the tar tank through a tar pipeline, and the ammonia water outlet is connected to the ammonia water tank through an ammonia water pipeline.

5. The raw coal gas purification system with multi-stage coal gas extraction and tar separation coupling according to claim 1, characterized in that, The electrostatic tar precipitator consists of a shell, high-voltage electrodes, precipitation plates, and a high-voltage power supply. Multiple high-voltage electrodes and precipitation plates are installed inside the shell. The high-voltage electrodes are connected to the high-voltage power supply outside the shell, and the precipitation plates are grounded to form an electrostatic tar removal system. The lower side of the electrostatic tar precipitator has a raw coal gas inlet, the top has a clean coal gas outlet, and the bottom has a tar residue outlet.

6. The raw coal gas purification system with multi-stage coal gas extraction and tar separation coupling according to claim 1, characterized in that, It also includes an automatic control system; the automatic control system includes a controller and an online gas analyzer and a flow control valve installed on the reflux gas pipeline, and the online gas analyzer is interlocked with the flow control valve through the controller.

7. A raw coal gas purification process coupling multi-stage gas extraction and tar separation, implemented based on the raw coal gas purification system coupling multi-stage gas extraction and tar separation as described in any one of claims 1 to 6; characterized in that, Includes the following steps: 1) During the process of coal entering the external heating pyrolysis furnace for dry distillation, the raw coal gas in the drying section and at least two of the dry distillation sections (low temperature, medium temperature, and high temperature) is extracted from the furnace and collected in the raw coal gas collection pipe after dust removal and deceleration. It is then pressurized by the coal gas blower and transported to the spray cooling tower. 2) Inside the spray cooling tower, the high-temperature raw coal gas at 300-350℃ comes into direct contact with the low-temperature circulating ammonia water sprayed above. The temperature of the raw coal gas drops to 75-85℃, and most of the tar vapor condenses into droplets and is captured by the ammonia water. The resulting tar-ammonia water mixture is collected at the bottom of the tower, and the ammonia and some sulfides in the raw coal gas are absorbed by the circulating ammonia water. 3) The tar-ammonia mixture at the bottom of the spray cooling tower flows into the primary settling tank by gravity. After initial stratification by gravity settling, it enters the three-phase centrifuge to separate tar, ammonia, and tar residue. The tar is collected in the tar tank, and the ammonia is collected in the ammonia tank. After cooling, it is used as circulating ammonia for spray cooling in the spray cooling tower. The tar residue is discharged centrally. 4) The raw coal gas after spray cooling is sent to the electrostatic precipitator, where the trace amounts of tar droplets and fine dust entrained in the raw coal gas are further removed under the action of a high-voltage electric field. The purified coal gas is discharged from the purified coal gas outlet. The tar residue collected at the bottom of the electrostatic precipitator is sent to the bottom of the spray cooling tower and combined with the tar residue collected after spray cooling for recycling. 5) A portion of the clean coal gas discharged from the electrostatic precipitator is returned to the heating and combustion system of the externally heated pyrolysis furnace as recycled coal gas. The controller monitors the composition and calorific value of the clean coal gas in real time through an online gas analyzer and interlocks the flow regulating valve to provide a stable heat source for coal dry distillation. The remaining clean coal gas is supplied externally or vented for combustion as needed. 6) When the output or composition of raw coal gas fluctuates, adjust the raw coal gas outlet temperature of the spray cooling tower within the set range.

8. The raw coal gas purification process coupled with multi-stage coal gas extraction and tar separation according to claim 7, characterized in that, The tar obtained after separation by a three-phase centrifuge has a water content of less than 5%, and the ammonia concentration in the ammonia water is above 3%; the tar content in the purified coal gas is less than 20 mg / Nm³. 3 .

9. The raw coal gas purification process coupled with multi-stage coal gas extraction and tar separation according to claim 7, characterized in that, When the calorific value of the clean coal gas is insufficient or the supply is unbalanced, the automatic control system is used to switch the recycle fuel system or supplement external gas to maintain the stable dry distillation temperature in the furnace. When the raw coal gas purification system fails, the extraction of raw coal gas is reduced or the raw coal gas is directly burned and released until the raw coal gas purification system returns to normal operation.

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