External combustion and external heating type pulverized coal pyrolysis system and pulverized coal pyrolysis method

Abstract

The invention relates to an external combustion and external heating type pulverized coal pyrolysis system and a pulverized coal pyrolysis method. The external combustion and external heating type pulverized coal pyrolysis system comprises a coal dry distillation device, a flue gas heat exchange system and a raw gas system, the flue gas heat exchange system comprises an external coal gas combustion furnace, a high-temperature flue gas distribution system and a flue gas mixing temperature control system; the raw coke oven gas system comprises a raw coke oven gas cooling and collecting system and a raw coke oven gas purifying system; the coal dry distillation device adopts a three-section structure comprising a low-temperature pyrolysis drying section, a medium-high-temperature pyrolysis section and an upgraded coal cooling section, waste gas generated by combustion of coal gas in an external coal gas combustion furnace is introduced into the medium-high-temperature pyrolysis section to indirectly transfer heat with slack coal, and cascade pyrolysis of the raw material slack coal is realized; the inlet flue gas temperature of the medium-high temperature pyrolysis section and the low-temperature pyrolysis drying section is controllable, so that the service life of equipment is prolonged, and the product quality is stabilized; according to the invention, slack coal pyrolysis can be stably and continuously realized, the product quality is good, and the investment yield is high.

Description

Technical Field

[0001] The present invention relates to the technical field of low-rank coal dry distillation, and in particular to an external combustion and external heating type pulverized coal pyrolysis system and a pulverized coal pyrolysis method. Background Art

[0002] Low-rank coal has the characteristics of rich reserves, shallow burial depth and low economic cost of mining. According to statistics, the geological reserves of low-rank coal in the world are as high as 4 trillion tons, accounting for 40% of the world's total coal reserves. In my country, the development and utilization of high-quality coal types such as bituminous coal and anthracite has reached saturation, while the large-scale development and utilization of low-rank coal has just started.

[0003] At present, the industry generally uses internal heating vertical furnaces to upgrade the quality of low-rank coal, which involves injecting a mixture of coal gas and air into the furnace for combustion and distillation. However, this process produces a large amount of coal gas but the quality of the coal gas is not high, and the proportion of invalid components such as nitrogen exceeds 50%. This low-quality coal gas can often only be sent to power plants for combustion and power generation, and cannot be further used for deep processing to produce high-value-added chemicals. In addition, due to the process characteristics of the internal heating vertical furnace, it is impossible to process fine coal with a particle size of less than 13mm. So far, the efficient and graded utilization of fine coal is still a problem that hinders the industrial application of low-rank coal.

[0004] The Chinese patent application with publication number CN105018120A discloses "a continuous dry distillation process and device for low-rank pulverized coal". The low-rank pulverized coal enters the externally heated continuous dry distillation refractory furnace through a double-sealed feeding device and a material buffer bin, and the raw coal gas is collected through a gas collecting device and a gas collecting cavity and discharged from the gas collecting system; the furnace charge after dry distillation is discharged through a discharge device. It realizes the process of continuous dry distillation in an externally heated continuous dry distillation furnace using low-rank pulverized coal as raw material. The raw coal gas produced during the dry distillation process can be quickly discharged, and the light oil yield and the calorific value of coal gas can be increased; during the coke discharge process, the powdered coke can be controlled to be discharged uniformly and continuously, and the quality of the semi-coke can be controlled and adjusted. However, since the furnace body is made of refractory bricks, the heat transfer efficiency of the device is low, resulting in a small unit production capacity and a large overall investment.

[0005] The Chinese patent application with publication number CN112708434A discloses "a modular combined unit-mounted continuous low-rank coal pyrolysis furnace", which can be used for the processing and production of low-rank coal semi-coke, especially for small-size coal and pulverized coal. It has good pertinence and applicability. The overall structure of the pyrolysis furnace is composed of a plurality of furnace body units assembled in a row. Each unit is a furnace body with complete functions, and each unit is constructed by a plurality of detachable functional modules. The modular structure makes the coke oven as a whole disassembleable and replaceable, which is conducive to reducing construction costs and facilitating maintenance. However, the refractory prefabricated modules it uses are difficult to cast as a whole, have complex structures, and have a low yield rate, which makes it difficult to promote actual industrial applications.

[0006] The Chinese patent application with publication number CN111518581A discloses "a vertical multi-tube segmented gas-conducting external heating type pulverized coal dry distillation method and device", which uses a tubular dry distillation process and a tower-type segmented vertical tubular structure to process raw coal. From top to bottom, it is a drying stage, two dry distillation stages, and a cooling stage. The device can improve the quality of semi-coke, has high heat exchange efficiency, and improves resource utilization and production capacity. However, due to the characteristics of the heat-resistant material of the dry distillation stage, the production of the high-temperature stage equipment is unstable. Summary of the invention

[0007] The present invention provides an external combustion and external heating pulverized coal pyrolysis system and a pulverized coal pyrolysis method. The coal dry distillation device adopts a three-section structure of a low-temperature pyrolysis drying section, a medium-high temperature pyrolysis section and a quality-improving coal cooling section. The waste gas generated by the combustion of coal gas in an external coal gas combustion furnace is introduced into the medium-high temperature pyrolysis section to indirectly transfer heat with the fine coal, thereby realizing the step-by-step pyrolysis of the raw material fine coal; the inlet flue gas temperature of the medium-high temperature pyrolysis section and the low-temperature pyrolysis drying section is controllable, which is beneficial to improving the service life of the equipment and stabilizing the product quality; the present invention can stably and continuously realize the pyrolysis of the fine coal, the product quality is good, and the investment return rate is high.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] An external combustion and external heating type pulverized coal pyrolysis system comprises a coal dry distillation device, a flue gas heat exchange system and a raw coal gas system; the coal dry distillation device is provided with a low-temperature pyrolysis drying section, a medium-high temperature pyrolysis section and a quality-improving coal cooling section in order from top to bottom; the flue gas heat exchange system comprises an external coal gas combustion furnace, a high-temperature flue gas distribution system and a flue gas mixing and temperature control system; the raw coal gas system comprises a raw coal gas cooling and gas collection system and a raw flue gas purification system; the maximum heat-resistant temperature of the low-temperature pyrolysis drying section is greater than 900°C; the maximum heat-resistant temperature of the medium-high temperature pyrolysis section is greater than 1400°C; a plurality of coal circulation areas in the low-temperature pyrolysis drying section, the medium-high temperature pyrolysis section and the quality-improving coal cooling section are spaced apart from the flue gas circulation area to form A coal circulation channel and a flue gas circulation channel connected up and down; wherein, the flue gas inlet of the medium and high temperature pyrolysis section is connected to the high temperature flue gas outlet of the external gas combustion furnace through the high temperature flue gas distribution system, the flue gas outlet of the medium and high temperature pyrolysis section is connected to the high temperature flue gas inlet of the flue gas mixing and temperature control system, and the mixed flue gas outlet of the flue gas mixing and temperature control system is connected to the flue gas inlet of the low temperature pyrolysis and drying section; the flue gas outlet of the low temperature pyrolysis and drying section is connected to the flue gas purification system through the outlet flue gas pipe, and the outlet flue gas pipe is also connected to the low temperature return flue gas inlet of the flue gas mixing and temperature control system; the low temperature pyrolysis and drying section and the medium and high temperature pyrolysis section are respectively provided with a plurality of raw coal gas outlets, which are connected to the raw coal gas purification system through the raw coal gas cooling and gas collection system.

[0010] A furnace top silo is arranged at the top feed inlet of the coal dry distillation device, and a coke pusher and a semi-coke conveying device are arranged at the bottom discharge outlet.

[0011] The low-temperature pyrolysis and drying section is made of one or more materials selected from the group consisting of carbon steel, cast steel, heat-resistant stainless steel and silicon carbide ceramics; the medium- and high-temperature pyrolysis section is made of one or more materials selected from the group consisting of refractory materials, cast steel, heat-resistant stainless steel and silicon carbide ceramics; a plurality of baffles are respectively arranged in the flue gas flow channels of the low-temperature pyrolysis and drying section and the medium- and high-temperature pyrolysis section.

[0012] The high-temperature flue gas distribution system consists of a horizontally arranged high-temperature flue gas conveying pipe and a high-temperature flue gas distribution pipe. One end of the high-temperature flue gas conveying pipe is connected to the high-temperature flue gas outlet of an external gas combustion furnace through a high-temperature flue gas pipeline, and the other end of the high-temperature flue gas conveying pipe is connected to the high-temperature flue gas distribution pipe; a plurality of high-temperature flue gas distribution holes are arranged on the top of the high-temperature flue gas distribution pipe corresponding to a plurality of flue gas areas of the medium and high-temperature pyrolysis sections; along the flow direction of the high-temperature flue gas, the high-temperature flue gas distribution pipe is a gradual structure with a gradually decreasing cross-sectional area.

[0013] The high-temperature flue gas conveying pipe is provided with a high-temperature flue gas temperature detection device and a high-temperature flue gas pressure detection device.

[0014] The flue gas mixing and temperature control system consists of a first-stage flue gas duct, a mixing sleeve, a mixing flue gas regulating valve, two-stage flue gas ducts, a first-stage flue gas temperature detection device and a second-stage flue gas temperature detection device; one end of the first-stage flue gas duct is connected to the medium- and high-temperature pyrolysis section flue gas outlet, and the other end of the first-stage flue gas duct is connected to the second-stage flue gas duct through a high-temperature expansion joint; a mixing sleeve is provided on the first-stage flue gas duct, a low-temperature return flue gas inlet is provided on the mixing sleeve, and a mixing gas regulating valve is provided at the low-temperature return flue gas inlet; the other end of the second-stage flue gas duct is connected to the low-temperature pyrolysis drying section flue gas inlet; a first-stage flue gas temperature detection device is provided on the first-stage flue gas duct, and a second-stage temperature detection device is provided on the second-stage flue pipe.

[0015] The raw gas main pipe of the raw gas cooling and gas collection system is equipped with an ammonia water spraying, cooling and washing device; the raw gas purification system is connected to the external gas pipeline through a purified gas pipeline, and a gas blower is provided on the purified gas pipeline; the purified gas pipeline is also connected to the gas inlet pipeline of the external gas combustion furnace, and the gas inlet pipeline is also connected to the lean gas pipeline; a combustion furnace gas regulating valve is provided on the gas inlet pipeline close to the external gas combustion furnace.

[0016] A pulverized coal pyrolysis method comprises the following steps:

[0017] 1) The raw material fine coal enters the low-temperature pyrolysis drying section of the coal distillation device through the coal bunker on the furnace top, goes down along the coal flow channel, and indirectly exchanges heat with the flue gas in the flue gas flow channel in a countercurrent manner to achieve drying and low-temperature distillation; the fine coal after low-temperature distillation enters the medium-high temperature pyrolysis section for pyrolysis and upgrading, and the generated upgraded coal enters the upgraded coal cooling section under the action of gravity and is cooled to room temperature. The cooled upgraded coal is controlled by the coke pusher to control the coke discharge rate and is transported out through the semi-coke conveying device;

[0018] 2) The raw coal fines are distilled at low temperature in the low temperature pyrolysis drying section, and the raw coal gas, tar and water produced by pyrolysis in the medium and high temperature pyrolysis section are collected in sections and uniformly merged into the raw coal gas cooling and gas collection system, and then purified by the raw coal gas purification system to obtain high calorific value coal gas. After being pressurized by the gas blower, part of it is used as external coal gas, and the other part is sent to the external coal gas combustion furnace as the rich coal gas for recycling combustion;

[0019] 3) The fuel gas raw material of the external gas combustion furnace includes recycled combustion rich gas and / or external lean gas. The external gas combustion furnace controls the combustion heat through the combustion furnace gas regulating valve to control the temperature of the generated high-temperature flue gas at 1100-1400°C; the high-temperature flue gas is evenly distributed to each flue gas flow area of ​​the medium and high temperature pyrolysis section through the high-temperature flue gas distribution system, and indirectly exchanges heat with the fine coal in the coal flow area in the medium and high temperature pyrolysis section; the outlet flue gas temperature of the medium and high temperature pyrolysis section is controlled at 800-950°C The flue gas at the outlet of the medium and high temperature pyrolysis section is mixed with the low-temperature return flue gas through the flue gas mixing and temperature control system to control the inlet flue gas temperature of the low-temperature pyrolysis and drying section within the range of 600-900°C; the mixed coal gas entering the low-temperature pyrolysis and drying section is indirectly heat exchanged with the raw material fine coal in countercurrent flow, and the outlet flue gas temperature of the low-temperature pyrolysis and drying section is controlled within the range of 150-250°C; the outlet flue gas of the low-temperature pyrolysis and drying section is pressurized by the flue gas fan, part of which enters the flue gas purification system, and the other part enters the flue gas mixing and temperature control system as low-temperature return flue gas.

[0020] The particle size of the raw material fine coal is less than 13 mm.

[0021] The raw gas cooling and gas collection system is provided with an ammonia water spraying cooling and washing device in the raw gas main pipe to cool the raw gas; the flue gas purification system includes a flue gas desulfurization and denitrification system.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1) The coal dry distillation device adopts a three-stage structure of low-temperature pyrolysis and drying section, medium-high temperature pyrolysis section and quality-improving coal cooling section. The waste gas generated by the combustion of coal gas in an external coal gas combustion furnace is passed into the medium-high temperature pyrolysis section to indirectly transfer heat with the fine coal, thereby realizing the step-by-step pyrolysis of the raw material fine coal;

[0024] 2) The maximum heat-resistant temperature of the low-temperature pyrolysis and drying section is greater than 900°C, and the maximum heat-resistant temperature of the medium-high temperature pyrolysis and drying section is greater than 1400°C; the present invention uses a flue gas mixing and temperature control system to mix low-temperature return flue gas into the high-temperature flue gas output from the medium-high temperature pyrolysis and drying section, and controls the inlet flue gas temperature of the low-temperature pyrolysis and drying section within the range of 600-900°C; the high-temperature flue gas temperature generated by the external gas combustion furnace is controlled within the range of 1100-1400°C by the combustion furnace gas regulating valve, that is, the inlet flue gas temperature of the medium-high temperature pyrolysis and drying section is controllable, which is beneficial to improving the service life of the coal distillation device and stabilizing the product quality;

[0025] 3) The present invention can stably and continuously realize the pyrolysis of fine coal, with good product quality and high investment return rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a structural schematic diagram of an external combustion and external heating type pulverized coal pyrolysis system according to the present invention.

[0027] Figure 2 It is a structural schematic diagram of the high-temperature flue gas distribution system of the present invention.

[0028] Figure 3 It is a structural schematic diagram of the flue gas mixing temperature control system of the present invention.

[0029] In the figure: 1. Furnace top silo 2. Low temperature pyrolysis drying section 3. Medium and high temperature pyrolysis section 4. Upgraded coal cooling section 5. Coke pusher 6. Semi-coke conveying device 7. External gas combustion furnace 8. High temperature flue gas distribution system 801. High temperature flue gas conveying pipe 802. High temperature flue gas distribution pipe 803. High temperature flue gas distribution hole 804. High temperature flue gas temperature detection device 805. High temperature flue gas pressure detection device 806. Hot exhaust gas chamber 807. Coal pyrolysis chamber 9. Flue gas mixing temperature control system System 901. Medium and high temperature pyrolysis section flue gas outlet 902. First stage flue gas pipeline 903. Mixing sleeve 904. Mixing flue gas regulating valve 905. High temperature expansion joint 906. Second stage flue gas pipeline 907. Low temperature pyrolysis drying section flue gas inlet 908. First stage flue gas temperature detection device 908. Second stage flue gas temperature detection device 10. Flue gas fan 11. Raw gas cooling and gas collection system 12. Raw gas purification system 13. Gas fan 14. Combustion furnace gas regulating valve

[0030] A. Raw coalB. Upgraded coalC. Raw gasD. High calorific value gasE. External coal gasF. Rich coal gas for return combustionG. Lean coal gasH. High temperature flue gasJ. Exit flue gas of medium and high temperature pyrolysis sectionK. Inlet flue gas of low temperature pyrolysis drying sectionL. Exit flue gas of low temperature pyrolysis drying sectionM. Flue gas entering flue gas purification systemN. Low temperature return flue gas DETAILED DESCRIPTION

[0031] The specific implementation of the present invention will be further described below in conjunction with the accompanying drawings:

[0032] like Figure 1-Figure 3 As shown, the external combustion and external heating type pulverized coal pyrolysis system described in the present invention includes a coal dry distillation device, a flue gas heat exchange system and a raw gas system.

[0033] The coal dry distillation device adopts a vertical furnace structure, and is equipped with a furnace top silo 1, a low-temperature pyrolysis and drying section 2, a medium-high temperature pyrolysis section 3, a quality-improving coal cooling section 4, a coke pusher 5 and a semi-coke conveying device 6 in sequence from top to bottom.

[0034] The low temperature pyrolysis drying section 2 is composed of multiple coal circulation areas and flue gas circulation areas arranged at intervals, and multiple baffles are arranged in the flue gas circulation area. The maximum heat resistance temperature of the low temperature pyrolysis drying section 2 is greater than 900°C, and the materials used include but are not limited to carbon steel, cast steel, heat-resistant stainless steel and silicon carbide ceramics.

[0035] The medium and high temperature pyrolysis section 3 is composed of multiple coal circulation areas (i.e., coal pyrolysis chamber 807) and flue gas circulation areas (i.e., hot exhaust gas chamber 806) arranged at intervals, and multiple baffles are arranged in the flue gas circulation area. The maximum heat-resistant temperature of the medium and high temperature pyrolysis section 3 is greater than 1400°C, and the materials used include but are not limited to carbon steel, cast steel, heat-resistant stainless steel, and silicon carbide ceramics.

[0036] The flue gas heat exchange system includes a combustion furnace gas regulating valve 14, an external gas combustion furnace 7, a high-temperature flue gas distribution system 8, a flue gas mixing temperature control system 9 and a flue gas fan 10. Among them, the combustion furnace gas regulating valve 14 is arranged on the gas inlet pipe of the external gas combustion furnace 7, the external gas combustion furnace 7 is connected to the high-temperature flue gas distribution system 8 through the high-temperature flue gas pipe, the high-temperature flue gas distribution system 8 is connected to the exhaust gas inlet of multiple hot exhaust gas chambers 806 of the medium and high temperature pyrolysis section 3 through multiple high-temperature flue gas distribution holes 803, the medium and high temperature pyrolysis section flue gas outlet 901 is connected to the flue gas mixing temperature control system 9, the mixed flue gas outlet of the flue gas mixing temperature control system 9 is connected to the low-temperature pyrolysis drying section flue gas inlet 907, and a flue gas fan 10 is arranged on the low-temperature flue gas pipe downstream of the low-temperature pyrolysis drying section flue gas outlet to provide power for the flue gas circulation of the flue gas heat exchange system.

[0037] The high-temperature flue gas distribution system 8 is provided with a high-temperature flue gas delivery pipe 801, a high-temperature flue gas distribution pipe 802 and a plurality of high-temperature flue gas distribution holes 803 in sequence along the high-temperature flue gas flow direction. The high-temperature flue gas delivery pipe is provided with a high-temperature flue gas temperature detection device and a high-temperature flue gas pressure detection device. Each high-temperature flue gas distribution hole 803 corresponds to a hot exhaust gas chamber 806 connected to the medium and high-temperature pyrolysis section, and between two adjacent hot exhaust gas chambers 806 is a coal pyrolysis chamber 807 of the high-temperature pyrolysis section. The high-temperature flue gas distribution pipe 802 is a structure with a gradually changing cross section and a gradually decreasing cross-sectional area along the high-temperature flue gas flow direction.

[0038] In the flue gas mixing temperature control system 9, a first flue gas pipeline 902 is connected to the medium and high temperature pyrolysis section flue gas outlet 901, a mixing sleeve 903 is provided on the first flue gas pipeline 901, a mixing gas regulating valve 904 is provided at the low temperature return flue gas inlet of the mixing sleeve 903, the first flue gas pipeline 902 is connected to the second flue gas pipeline 906 through a high temperature expansion joint 905, and the second flue gas pipeline 906 is connected to the low temperature pyrolysis drying section flue gas inlet 907. A first flue gas temperature detection device 908 and a second flue gas temperature detection device 909 are respectively provided on the first flue gas pipeline 902 and the second flue gas pipeline 906.

[0039] The raw gas system includes a raw gas cooling and collecting system 11, a raw gas purification system 12 and a gas blower 13. The raw gas cooling and collecting system 11 is respectively connected to the multiple raw gas outlets of the low-temperature pyrolysis drying section 2 and the medium-high temperature pyrolysis section 3, and is provided with an ammonia spray cooling and washing device.

[0040] In this embodiment, the pulverized coal pyrolysis process is as follows:

[0041] 1) The raw material fine coal A (particle size <13 mm) enters the coal flow channel of the low-temperature pyrolysis and drying section 2 of the coal distillation device through the furnace top coal bunker 1, and is dried and low-temperature distilled by countercurrent heat exchange with the flue gas, and then enters the medium-high temperature pyrolysis section 3 for further medium-high temperature pyrolysis and upgrading. The generated upgraded coal B flows into the upgraded coal cooling section 4 under the action of gravity and is cooled to room temperature. At the bottom of the upgraded coal cooling section 4, the upgraded coal B is controlled by the coke pusher 5 to discharge the coke, and finally sent out through the semi-coke conveying device 6.

[0042] 2) The raw coal A produces raw gas C, tar and moisture in the low-temperature pyrolysis drying section 2 and the medium-high temperature pyrolysis section 3, which are collected in sections and aggregated to the raw gas cooling and gas collection system 11, and then purified by the raw gas purification system 12 to obtain high calorific value gas D, which is pressurized by the gas blower 13 as external gas E or as rich gas F for return combustion.

[0043] 3) The fuel gas raw materials of the external gas combustion furnace 7 include recycled combustion rich gas F and / or external lean gas G. The external gas combustion furnace 7 controls the combustion heat through the combustion furnace gas regulating valve 14, and the temperature of the generated high-temperature flue gas H is controlled at 1100-1400°C. The high-temperature flue gas H is evenly distributed to the hot exhaust gas chamber 806 of the medium and high temperature pyrolysis section 3 through the high-temperature flue gas distribution system 8, and indirectly exchanges heat with the fine coal in the coal pyrolysis chamber 807 of the medium and high temperature pyrolysis section 3. The temperature of the flue gas J at the outlet of the medium and high temperature pyrolysis section is controlled at 800-950°C. The flue gas J at the outlet of the medium and high temperature pyrolysis section is mixed with the low-temperature recycled flue gas N through the flue gas mixing and temperature control system 9, and the temperature of the inlet flue gas K of the low-temperature pyrolysis drying section is controlled between 600 and 900°C to ensure the service life of the low-temperature pyrolysis drying section 2. The flue gas K at the inlet of the low-temperature pyrolysis and drying section enters the low-temperature pyrolysis and drying section 2 and continues to indirectly exchange heat with the fine coal in countercurrent. The temperature of the flue gas L at the outlet of the low-temperature pyrolysis and drying section after cooling is controlled at 150-250°C. The flue gas L at the outlet of the low-temperature pyrolysis and drying section is pressurized by the flue gas fan 10, part of which is used as the flue gas M entering the flue gas purification system, and the other part is used as the low-temperature return flue gas N entering the flue gas mixing and temperature control system 9.

[0044] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. An external combustion and external heating type pulverized coal pyrolysis system, characterized in that: It includes a coal dry distillation device, a flue gas heat exchange system and a raw coal gas system; the coal dry distillation device is provided with a low-temperature pyrolysis drying section, a medium-high temperature pyrolysis section and a quality-improving coal cooling section in order from top to bottom; the flue gas heat exchange system includes an external coal gas combustion furnace, a high-temperature flue gas distribution system and a flue gas mixing temperature control system; the raw coal gas system includes a raw coal gas cooling and gas collection system and a raw flue gas purification system; the maximum heat-resistant temperature of the low-temperature pyrolysis drying section is greater than 900°C; the maximum heat-resistant temperature of the medium-high temperature pyrolysis section is greater than 1400°C; multiple coal flow areas in the low-temperature pyrolysis drying section, the medium-high temperature pyrolysis section and the quality-improving coal cooling section are spaced apart from the flue gas flow area to form a coal flow area connected from top to bottom. The flue gas flow channel and the flue gas circulation channel; wherein, the flue gas inlet of the medium and high temperature pyrolysis section is connected to the high temperature flue gas outlet of the external gas combustion furnace through the high temperature flue gas distribution system, the flue gas outlet of the medium and high temperature pyrolysis section is connected to the high temperature flue gas inlet of the flue gas mixing temperature control system, and the mixed flue gas outlet of the flue gas mixing temperature control system is connected to the flue gas inlet of the low temperature pyrolysis drying section; the flue gas outlet of the low temperature pyrolysis drying section is connected to the flue gas purification system through the outlet flue gas pipe, and the outlet flue gas pipe is also connected to the low temperature return flue gas inlet of the flue gas mixing temperature control system; the low temperature pyrolysis drying section and the medium and high temperature pyrolysis section are respectively provided with a plurality of raw gas outlets, which are connected to the raw gas purification system through the raw gas cooling and gas collection system.

2. The external combustion and external heating type pulverized coal pyrolysis system according to claim 1, characterized in that: A furnace top silo is arranged at the top feed inlet of the coal dry distillation device, and a coke pusher and a semi-coke conveying device are arranged at the bottom discharge outlet.

3. The external combustion and external heating type pulverized coal pyrolysis system according to claim 1, characterized in that: The low-temperature pyrolysis and drying section is made of one or more materials selected from the group consisting of carbon steel, cast steel, heat-resistant stainless steel and silicon carbide ceramics; the medium- and high-temperature pyrolysis section is made of one or more materials selected from the group consisting of refractory materials, cast steel, heat-resistant stainless steel and silicon carbide ceramics; a plurality of baffles are respectively arranged in the flue gas flow channels of the low-temperature pyrolysis and drying section and the medium- and high-temperature pyrolysis section.

4. The external combustion and external heating type pulverized coal pyrolysis system according to claim 1, characterized in that: The high-temperature flue gas distribution system consists of a horizontally arranged high-temperature flue gas conveying pipe and a high-temperature flue gas distribution pipe. One end of the high-temperature flue gas conveying pipe is connected to the high-temperature flue gas outlet of an external gas combustion furnace through a high-temperature flue gas pipeline, and the other end of the high-temperature flue gas conveying pipe is connected to the high-temperature flue gas distribution pipe; a plurality of high-temperature flue gas distribution holes are arranged on the top of the high-temperature flue gas distribution pipe corresponding to a plurality of flue gas areas of the medium and high-temperature pyrolysis sections; along the flow direction of the high-temperature flue gas, the high-temperature flue gas distribution pipe is a gradual structure with a gradually decreasing cross-sectional area.

5. The external combustion and external heating type pulverized coal pyrolysis system according to claim 4, characterized in that: The high-temperature flue gas conveying pipe is provided with a high-temperature flue gas temperature detection device and a high-temperature flue gas pressure detection device.

6. The external combustion and external heating type pulverized coal pyrolysis system according to claim 1, characterized in that: The flue gas mixing and temperature control system consists of a first-stage flue gas duct, a mixing sleeve, a mixing flue gas regulating valve, two-stage flue gas ducts, a first-stage flue gas temperature detection device and a second-stage flue gas temperature detection device; one end of the first-stage flue gas duct is connected to the medium- and high-temperature pyrolysis section flue gas outlet, and the other end of the first-stage flue gas duct is connected to the second-stage flue gas duct through a high-temperature expansion joint; a mixing sleeve is provided on the first-stage flue gas duct, a low-temperature return flue gas inlet is provided on the mixing sleeve, and a mixing gas regulating valve is provided at the low-temperature return flue gas inlet; the other end of the second-stage flue gas duct is connected to the low-temperature pyrolysis drying section flue gas inlet; a first-stage flue gas temperature detection device is provided on the first-stage flue gas duct, and a second-stage temperature detection device is provided on the second-stage flue pipe.

7. The external combustion and external heating type pulverized coal pyrolysis system according to claim 1, characterized in that: The raw gas main pipe of the raw gas cooling and gas collection system is equipped with an ammonia water spraying, cooling and washing device; the raw gas purification system is connected to the external gas pipeline through a purified gas pipeline, and a gas blower is provided on the purified gas pipeline; the purified gas pipeline is also connected to the gas inlet pipeline of the external gas combustion furnace, and the gas inlet pipeline is also connected to the lean gas pipeline; a combustion furnace gas regulating valve is provided on the gas inlet pipeline close to the external gas combustion furnace.

8. A pulverized coal pyrolysis method based on the external combustion and external heating pulverized coal pyrolysis system according to any one of claims 1 to 7, characterized in that: The process includes the following: 1) The raw material fine coal enters the low-temperature pyrolysis drying section of the coal distillation device through the coal bunker on the furnace top, goes down along the coal flow channel, and indirectly exchanges heat with the flue gas in the flue gas flow channel in a countercurrent manner to achieve drying and low-temperature distillation; the fine coal after low-temperature distillation enters the medium-high temperature pyrolysis section for pyrolysis and upgrading, and the generated upgraded coal enters the upgraded coal cooling section under the action of gravity and is cooled to room temperature. The cooled upgraded coal is controlled by the coke pusher to control the coke discharge rate and is transported out through the semi-coke conveying device; 2) The raw coal fines are distilled at low temperature in the low temperature pyrolysis drying section, and the raw coal gas, tar and water produced by pyrolysis in the medium and high temperature pyrolysis section are collected in sections and uniformly merged into the raw coal gas cooling and gas collection system, and then purified by the raw coal gas purification system to obtain high calorific value coal gas. After being pressurized by the gas blower, part of it is used as external coal gas, and the other part is sent to the external coal gas combustion furnace as the rich coal gas for recycling combustion; 3) The fuel gas raw material of the external gas combustion furnace includes recycled combustion rich gas and / or external lean gas. The external gas combustion furnace controls the combustion heat through the combustion furnace gas regulating valve to control the temperature of the generated high-temperature flue gas at 1100-1400°C; the high-temperature flue gas is evenly distributed to each flue gas flow area of ​​the medium and high temperature pyrolysis section through the high-temperature flue gas distribution system, and indirectly exchanges heat with the fine coal in the coal flow area in the medium and high temperature pyrolysis section; the outlet flue gas temperature of the medium and high temperature pyrolysis section is controlled at 800-950°C The flue gas at the outlet of the medium and high temperature pyrolysis section is mixed with the low-temperature return flue gas through the flue gas mixing and temperature control system to control the inlet flue gas temperature of the low-temperature pyrolysis and drying section within the range of 600-900°C; the mixed coal gas entering the low-temperature pyrolysis and drying section is indirectly heat exchanged with the raw material fine coal in countercurrent flow, and the outlet flue gas temperature of the low-temperature pyrolysis and drying section is controlled within the range of 150-250°C; the outlet flue gas of the low-temperature pyrolysis and drying section is pressurized by the flue gas fan, part of which enters the flue gas purification system, and the other part enters the flue gas mixing and temperature control system as low-temperature return flue gas.

9. A pulverized coal pyrolysis method according to claim 8, characterized in that: The particle size of the raw material fine coal is less than 13 mm.

10. A pulverized coal pyrolysis method according to claim 8, characterized in that: The raw gas cooling and gas collection system is provided with an ammonia water spraying cooling and washing device in the raw gas main pipe to cool the raw gas; the flue gas purification system includes a flue gas desulfurization and denitrification system.

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