A low-rank coal drying system and method coupled with flue gas waste heat and air cooling waste heat

Abstract

This invention relates to the field of coal dry distillation pyrolysis technology, and particularly to a low-rank coal drying system and method that couples flue gas waste heat and air-cooled waste heat. The low-rank coal drying system includes a drying chamber, an air-cooled air box, a waste gas fan, a flue gas heat exchanger, and a drying fan. The drying chamber is located in the drying section, and the air-cooled air box is located in the air-cooling section. The drying fan has a mixing chamber. Utilizing the waste heat from the combustion products of the externally heated pyrolysis furnace—flue gas—and the waste heat from the semi-coke cooling process, two heat sources are used to provide drying air for the coal, achieving efficient pre-drying of the coal entering the furnace. A drying air temperature control system and a coal moisture control system ensure stable drying effect and process safety, achieving precise control of coal moisture while improving the system's energy utilization efficiency.

Description

Technical Field

[0001] This invention relates to the field of coal dry distillation pyrolysis technology, and in particular to a low-rank coal drying system and method that couples flue gas waste heat and air-cooled waste heat for externally heated pyrolysis furnaces. Background Technology

[0002] Externally heated pyrolysis furnaces (dry distillation furnaces) are often used for the dry distillation of low-rank coal to produce semi-coke, coal gas, and coal tar. If the raw coal entering the furnace (especially low-rank coals such as lignite and long-flame coal) has a high moisture content, it will significantly affect the pyrolysis efficiency and product quality. Therefore, measures such as pre-drying the raw coal are often adopted.

[0003] Traditional coal drying processes often employ a single heat source (such as boiler flue gas or hot flue gas) to dry raw coal. For example, the Chinese utility model patent CN202018186U, which discloses a system for drying lignite using waste heat from boiler flue gas, introduces a portion of the high-temperature flue gas from the economizer outlet into a rotary drum dryer to dry the lignite, utilizing the waste heat from boiler exhaust as a heat source, thus achieving energy-efficient drying of lignite. However, such solutions typically require independent drying equipment (such as a rotary drum dryer), additional heat exchangers, or flue bypasses. Not only is the equipment complex and requires a large area, but it also relies solely on flue gas heat, resulting in a single heat source. When the flue gas heat is insufficient, a combustion furnace is needed to provide hot air, leading to still relatively high energy consumption. Furthermore, directly using high-temperature flue gas as the drying medium can introduce problems such as dust and pollutant adhesion, requiring subsequent dust removal treatment.

[0004] With the development of coal drying technology, integrated drying sections have emerged within coal pyrolysis units. For example, CN111518581A discloses a "Vertical Multi-Pipe Segmented Gas-Conducting External Heating Pulverized Coal Drying Method and Apparatus," which sets up a drying section at the top of the pyrolysis furnace, using the furnace's own high-temperature flue gas to indirectly heat the raw coal, achieving dehydration and drying before it enters the furnace, eliminating the need for separate drying equipment. However, this method only utilizes the flue gas from the combustion chamber in the coal pyrolysis unit as a heat source, without fully considering the recovery and utilization of other waste heat. Furthermore, the indirect heating drying method limits drying efficiency due to the lack of direct heat and mass exchange, and the built-in drying section results in a complex structure for the coal pyrolysis unit.

[0005] In addition, traditional coal drying systems still have shortcomings in controlling drying temperature and coal moisture content. For example, when relying solely on flue gas drying, the temperature is not easy to adjust precisely; if the temperature is too high, it can easily cause spontaneous combustion of the coal, and if the temperature is too low, it will lead to insufficient drying.

[0006] Currently, there is a lack of a solution that can couple multiple waste heat sources for coal drying to further improve drying efficiency and energy utilization. Furthermore, how to coordinate multiple heat sources to stably control the drying temperature and achieve precise control of coal moisture content (reducing it from around 20% to around 5%) remains a challenge for the industry. In summary, existing coal pre-drying processes suffer from problems such as a single heat source, insufficient energy recovery, large equipment size, and high control difficulty, and urgently need improvement. Summary of the Invention

[0007] This invention provides a low-rank coal drying system and method that couples flue gas waste heat with air-cooled waste heat. It utilizes two heat sources—the waste heat from the combustion products of the externally heated pyrolysis furnace (flue gas) and the waste heat from the semi-coke cooling process—to provide drying air for coal drying, achieving efficient pre-drying of the coal entering the furnace. The drying air temperature control system and the coal moisture control system ensure stable drying effect and process safety, achieving precise control of coal moisture while improving the system's energy utilization efficiency.

[0008] To achieve the above objectives, the present invention employs the following technical solution: A low-rank coal drying system based on flue gas waste heat and air cooling coupling is connected to an externally heated pyrolysis furnace. The externally heated pyrolysis furnace consists of a drying section, a low-temperature dry distillation section, a medium-temperature dry distillation section, a high-temperature dry distillation section, an air-cooled section, and a water-cooled section arranged sequentially from top to bottom. The low-rank coal drying system includes a drying box, an air-cooled air box, a waste gas fan, a flue gas heat exchanger, and a drying fan. The drying box is located in the drying section, and the drying fan has a mixing chamber and a first hot air inlet, a second hot air inlet, and a mixed hot air outlet connected to the mixing chamber. The drying chamber is located in the air-cooled section. The air-cooled chamber has an air inlet and a hot air outlet. The hot air outlet is connected to the first hot air inlet of the drying fan. The air inlet of the exhaust fan is connected to the hot flue gas outlet of the low-temperature dry distillation section. The air outlet of the exhaust fan is connected to the hot flue gas inlet of the flue gas heat exchanger. The flue gas outlet after heat exchange of the flue gas heat exchanger is connected to the flue gas discharge pipe. The flue gas heat exchanger also has an air inlet and a post-heat exchange air outlet. The post-heat exchange air outlet is connected to the second hot air inlet of the drying fan. The mixed hot air outlet of the drying fan is connected to the hot air inlet of the drying chamber.

[0009] The drying chamber is equipped with multiple layers of material distribution plates and multiple layers of air distribution channels arranged at intervals along the height. The material distribution plates and air distribution channels are staggered in the horizontal direction, and multiple air distribution holes are provided on both sides and bottom of the air distribution channels.

[0010] The top of the drying chamber is equipped with a raw material buffer silo, and the bottom of the raw material buffer silo is equipped with a discharge valve that is connected to the dry distillation section; the drying chamber is equipped with a dehumidification outlet connected to a condensation separation device.

[0011] The air-cooling section is equipped with multiple vertical feeding pipes, and the air-cooling box is located around the vertical feeding pipes. The lower part of the air-cooling box has multiple air intakes along the circumference, and the upper part has a hot air outlet. The height of the air-cooling section is 3 to 5 meters, and the air-cooling box has multiple layers of baffles along the height.

[0012] The flue gas heat exchanger is a shell-and-tube heat exchanger, with an air inlet and a heat-exchange-after air outlet at both ends of the shell side, and a hot flue gas inlet and a heat-exchange-after flue gas outlet at both ends of the tube side.

[0013] A low-rank coal drying system based on flue gas waste heat and air cooling coupling also includes a drying air temperature control system, which consists of a temperature sensor, a flow regulating valve one, a flow regulating valve two, and a controller. The temperature sensor is located on the mixed hot air duct near the mixed hot air outlet of the drying fan, or on the mixed hot air duct near the mixed hot air inlet of the drying box. The flow regulating valve one is located at the first hot air inlet of the drying fan, and the flow regulating valve two is located at the second hot air inlet of the drying fan. The signal output terminal of the temperature sensor is connected to the signal input terminal of the controller, and the signal output terminal of the controller is connected to the control terminals of the flow regulating valve one and the flow regulating valve two, respectively.

[0014] A low-rank coal drying system based on flue gas waste heat and air cooling coupling also includes a coal moisture control system. The coal moisture control system includes a moisture detection device and a controller. The moisture detection device is located at the coal outlet of the drying section. The signal output terminal of the moisture detection device is connected to the signal input terminal of the controller. The signal output terminal of the controller is also connected to the control terminal of the bottom discharge valve of the drying section or the control terminal of the drying fan.

[0015] A method for drying low-rank coal based on the coupling of flue gas waste heat and air cooling includes the following processes: 1) In the drying box at the top of the externally heated pyrolysis furnace, the incoming coal is spread into a thin layer through the distribution plates of each layer, and drying air is supplied through the air distribution holes on the air distribution channel between each layer of distribution plates, so that the coal and the drying air are in full contact; the drying air is a mixed hot air provided by the drying fan. 2) In the air-cooled section at the bottom of the externally heated pyrolysis furnace, the semi-coke flowing out of the high-temperature dry distillation section falls through multiple vertical feed pipes, and the air entering through multiple air inlets flows from bottom to top and exchanges heat with the semi-coke. The hot air with a temperature of 100-130℃ after heat exchange enters the mixing chamber of the drying fan as the first hot air. 3) In the low-temperature dry distillation section at the top of the externally heated pyrolysis furnace, the coal material entering from the drying section flows downward and undergoes low-temperature dry distillation; the hot flue gas discharged from the low-temperature dry distillation section at a temperature of 300-350°C enters the flue gas heat exchanger and exchanges heat with the externally introduced air. After heat exchange, the hot air with a temperature of 150-180°C enters the mixing chamber of the drying fan as the second hot air. 4) In the drying fan, the first hot air and the second hot air are mixed. The mixed hot air with a temperature of 110-120℃ is pressurized by the drying fan and enters the drying box to dry the coal.

[0016] A drying air temperature control system is used to control the temperature of the drying air entering the drying chamber. The controller adjusts the opening of flow regulating valve one and flow regulating valve two according to the detection value of the temperature sensor to adjust the mixing ratio of the first hot air and the second hot air, so that the temperature of the drying air sent into the drying chamber is maintained within the range of 110 to 130°C.

[0017] A moisture detection device is used to detect the residual moisture in the coal flowing out of the drying section; the controller adjusts the opening degree of the feeding valve in the drying section or the air volume of the drying fan according to the moisture detection value. By adjusting the residence time of the coal in the drying section or the drying air volume entering the drying box, the moisture content of the coal is ensured to reach the target value.

[0018] Compared with the prior art, the beneficial effects of the present invention are: (1) Make full use of waste heat and save energy: The waste heat of flue gas from the externally heated pyrolysis furnace and the waste heat during semi-coke cooling are combined and used as the drying medium for coal entering the furnace. This avoids the problem of the traditional coal drying process requiring a separate hot air furnace and consuming fuel, and greatly improves the energy utilization rate. Practical tests have shown that the large volume of drying air generated by the above two types of waste heat can reduce the moisture content of coal entering the furnace from 20% to 5%. Under the premise of ensuring low return gas and high oil yield, the overall energy utilization rate of the system reaches more than 80%.

[0019] (2) Improved carbonization efficiency and product quality: Pre-drying the coal significantly reduces its moisture content, so the coal no longer consumes a large amount of heat to evaporate moisture when entering the carbonization section. This improves the effective heat utilization rate of the externally heated pyrolysis furnace, resulting in a more complete pyrolysis reaction. The volatile matter content is more uniform after drying, which reduces the impact of high-temperature carbonization on product distribution and helps improve key indicators such as semi-coke coking rate and coal gas and tar yield. At the same time, because the drying air is clean hot air, direct contact between the coal and flue gas is avoided, ensuring that the semi-coke and tar products are not contaminated by smoke and dust.

[0020] (3) Improved safety and controllability: The temperature of the drying air can be adjusted by the flow rate of the two hot air streams and precisely controlled at a safe level of around 120℃, thereby reducing the risk of spontaneous combustion of the coal. By setting temperature monitoring points and interlocking to adjust the air volume, closed-loop control of the drying process is realized. When abnormal temperature or dew point is detected, the air volume and the mixing ratio of the two heat sources can be automatically adjusted to avoid the coal being too dry or having too much moisture, thus ensuring safe and reliable operation.

[0021] (4) The system has a compact structure and is easy to integrate: The main body of the low-rank coal drying system can be directly integrated into the top of the externally heated pyrolysis furnace as an independent unit. The flue gas heat exchanger and air-cooled air box can be arranged using the furnace space, without the need for large-scale addition of independent drying equipment. The drying box adopts a multi-layer material distribution and air distribution design, which can achieve efficient drying within a limited height. The overall structure is compact, which helps to reduce the floor space and equipment investment. Attached Figure Description

[0022] Figure 1 This is a front view of a low-rank coal drying system that couples flue gas waste heat with air-cooled waste heat, as described in this invention.

[0023] Figure 2 yes Figure 1 Side view.

[0024] Figure 3 This is a schematic diagram of the internal structure of the drying oven.

[0025] Figure 4 This is a schematic diagram of the drying air temperature control system described in this invention.

[0026] In the diagram: 1-Drying box; 11-Distribution plate; 12-Air distribution channel; 13-Raw material buffer bin; 2-Low-temperature dry distillation section; 3-Medium-temperature dry distillation section; 4-High-temperature dry distillation section; 5-Air-cooled section; 51-Air-cooled air box; 52-Air inlet; 53-Hot air outlet; 6-Water-cooled section; 7-Drying fan; 8-Flue gas heat exchanger; 81-Hot flue gas inlet; 82-Flue gas outlet after heat exchange; 9-Exhaust gas fan; 10-Mixed hot air duct; 101-Temperature sensor; 102-Flow regulating valve one; 103-Flow regulating valve two; 201-Air; 202-Hot air; 203-Air after heat exchange; 204-Hot flue gas; 205-Flue gas after heat exchange; 206-Drying air. Detailed Implementation

[0027] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings: like Figure 1 , Figure 2As shown, the present invention discloses a low-rank coal drying system based on flue gas waste heat and air cooling coupling, which is connected to an externally heated pyrolysis furnace. The externally heated pyrolysis furnace consists of a drying section, a low-temperature dry distillation section 2, a medium-temperature dry distillation section 3, a high-temperature dry distillation section 4, an air-cooled section 5, and a water-cooled section 6 arranged sequentially from top to bottom. The low-rank coal drying system includes a drying box 1, an air-cooled air box 51, a waste gas fan 9, a flue gas heat exchanger 8, and a drying fan 7. The drying box 1 is located in the drying section, and the drying fan 7 is provided with a mixing chamber and a first hot air inlet, a second hot air inlet, and a mixed hot air outlet connected to the mixing chamber. The air-cooled air box 51... 1. A drying box 51 is set in the air-cooled section 5. The air-cooled air box 51 is provided with an air intake 52 and a hot air outlet 53. The hot air outlet 53 is connected to the first hot air inlet of the drying fan 7. The air inlet of the exhaust fan 9 is connected to the hot flue gas outlet of the low-temperature dry distillation section 2. The air outlet of the exhaust fan 9 is connected to the hot flue gas inlet 81 of the flue gas heat exchanger 8. The flue gas outlet 82 after heat exchange of the flue gas heat exchanger 8 is connected to the flue gas discharge pipe. The flue gas heat exchanger 8 is also provided with an air inlet and a heat exchange air outlet. The heat exchange air outlet is connected to the second hot air inlet of the drying fan 7. The mixed hot air outlet of the drying fan 7 is connected to the hot air inlet of the drying box 1.

[0028] The drying chamber 1 is provided with multiple layers of material distribution plates 11 and multiple layers of air distribution channels 12 at intervals along the height. The material distribution plates 11 and the air distribution channels 12 are staggered in the horizontal direction. Multiple air distribution holes are provided on both sides and at the bottom of the air distribution channels 12.

[0029] The top of the drying chamber 1 is provided with a raw material buffer 13, and the bottom of the raw material buffer 13 is provided with a discharge valve that is connected to the dry distillation section; the drying chamber 1 is provided with a dehumidification outlet connected to a condensation separation device.

[0030] The air-cooled section 5 is provided with multiple vertical feeding pipes, and the air-cooled box 51 is located around the vertical feeding pipes. The lower part of the air-cooled box 51 is provided with multiple air intakes 52 along the circumference, and the upper part is provided with a hot air outlet 53. The height of the air-cooled section 5 is 3 to 5 meters, and the air-cooled box 51 is provided with multiple layers of baffles along the height.

[0031] The flue gas heat exchanger 8 is a shell-and-tube heat exchanger, with an air inlet and a heat-exchange air outlet at both ends of the shell side, and a hot flue gas inlet 81 and a heat-exchange flue gas outlet 82 at both ends of the tube side.

[0032] The low-rank coal drying system based on flue gas waste heat and air cooling coupling described in this invention also includes a drying air temperature control system, which consists of a temperature sensor 101, a flow regulating valve one 102, a flow regulating valve two 103, and a controller. The temperature sensor 101 is located on the mixed hot air duct 10 near the mixed hot air outlet of the drying fan 7, or on the mixed hot air duct 10 near the mixed hot air inlet of the drying chamber 1. The flow regulating valve one 102 is located at the first hot air inlet of the drying fan 7, and the flow regulating valve two 103 is located at the second hot air inlet of the drying fan 7. The signal output terminal of the temperature sensor 101 is connected to the signal input terminal of the controller, and the signal output terminal of the controller is connected to the control terminals of the flow regulating valve one 102 and the flow regulating valve two 103, respectively.

[0033] The low-rank coal drying system based on flue gas waste heat and air cooling coupling described in this invention also includes a coal moisture control system. The coal moisture control system includes a moisture detection device and a controller. The moisture detection device is located at the coal outlet of the drying section. The signal output terminal of the moisture detection device is connected to the signal input terminal of the controller. The signal output terminal of the controller is also connected to the control terminal of the bottom discharge valve of the drying section or the control terminal of the drying fan 7.

[0034] like Figure 4 As shown, the low-rank coal drying method based on the coupling of flue gas waste heat and air cooling according to the present invention includes the following process: 1) In the drying box 1 at the top of the externally heated pyrolysis furnace, the incoming coal is spread into a thin layer through the distribution plates 11, and the drying air 206 is supplied through the air distribution holes on the air distribution channel 12 between the distribution plates 11, so that the coal and the drying air 206 are in full contact; the drying air 206 is a mixed hot air supplied by the drying fan 7. 2) In the air-cooled section 5 at the bottom of the externally heated pyrolysis furnace, the semi-coke flowing out of the high-temperature dry distillation section 4 falls down through multiple vertical feed pipes, and the air 201 entering through multiple air intakes 52 flows from bottom to top and exchanges heat with the semi-coke. The hot air 202 with a temperature of 100-130°C after heat exchange enters the mixing chamber of the drying fan 7 as the first hot air. 3) In the low-temperature dry distillation section 2 at the top of the externally heated pyrolysis furnace, the coal material entering from the drying section flows downward and undergoes low-temperature dry distillation; the hot flue gas 204 with a temperature of 300-350℃ discharged from the low-temperature dry distillation section 2 enters the flue gas heat exchanger 8 and exchanges heat with the externally introduced air 201. The air 203 after heat exchange (temperature rises to 150-180℃) enters the mixing chamber of the drying fan 7 as the second hot air; 4) In the drying fan 7, the first hot air and the second hot air are mixed. The mixed hot air (drying air 206) with a temperature of 110-120℃ is pressurized by the drying fan 7 and enters the drying box 1 to dry the coal.

[0035] A drying air temperature control system is used to control the temperature of the drying air entering the drying chamber 1. The controller adjusts the opening of the flow regulating valve 102 and the flow regulating valve 103 according to the detection value of the temperature sensor 101 to adjust the mixing ratio of the first hot air and the second hot air, so that the temperature of the drying air 206 sent into the drying chamber 1 is maintained in the range of 110 to 130°C.

[0036] A moisture detection device is used to detect the residual moisture in the coal flowing out of the drying section; the controller adjusts the opening degree of the feeding valve in the drying section or the air volume of the drying fan 7 according to the moisture detection value. By adjusting the residence time of the coal in the drying section or the drying air volume entering the drying box 1, the moisture content of the coal is ensured to reach the target value.

[0037] The low-rank coal drying system that couples flue gas waste heat and air-cooled waste heat according to the present invention includes the following main components: 1. Drying Box 1: Located in the drying section, which is situated above the externally heated pyrolysis furnace, this box contains coal and allows it to slowly descend. In this invention, the coal descending channel and hot air channel are arranged in an alternating pattern within the drying box 1, ensuring direct and sufficient heat transfer between the drying air 206 and the coal. Preferably, the drying box 1 is equipped with multiple layers of distribution plates 11 and multiple layers of air distribution channels 12. The distribution plates 11 divide the coal into thin layers, and the air distribution channels 12 are located between the distribution plates 11 to evenly distribute the drying air into the coal layers. This structure of the drying box 1 effectively increases the contact area between the coal and the drying air, thereby improving drying efficiency.

[0038] Preferably, the drying chamber 1 is provided with at least 3 layers of material distribution plates 11 and at least 2 layers of air distribution channels 12. The drying chamber 1 is also provided with a dehumidification outlet connected to a condensation separation device to discharge and condense and recover the humid waste gas generated during the drying process.

[0039] 2. Air-cooled air box 51: Used to provide the primary heat source for coal drying, namely air-cooled waste heat; the externally heated pyrolysis furnace has an air-cooled section 5 below the high-temperature dry distillation section 4. The air-cooled section 5 has multiple vertical feed pipes for semi-coke feeding. The air-cooled air box 51 is an air-cooled heat exchange chamber located around the vertical feed pipes. The vertical feed pipes extend downwards from the bottom of the high-temperature dry distillation section 4 through the air-cooled section 5. The semi-coke, after high-temperature dry distillation, is cooled by air as it falls along the vertical feed pipes. Air-cooled air box 51 introduces air 201 through multiple air inlets 52. The air 201 indirectly exchanges heat with the hot semi-coke inside the air-cooled air box 51, and the heated air serves as the primary hot air. After heat exchange in the air-cooled air box 51, the air 201 rises from the ambient temperature (approximately 25°C) to 100–130°C (depending on the semi-coke temperature and air flow rate), becoming a drying gas medium rich in waste heat.

[0040] 3. Exhaust gas blower 9 and flue gas heat exchanger 8: These provide a secondary heat source for coal drying, namely waste heat from the flue gas. The externally heated pyrolysis furnace generates high-temperature flue gas (usually flue gas from burning raw coal gas) during the low-temperature carbonization of coal. The flue gas temperature discharged from the top of the low-temperature carbonization section 2 is approximately 300–350°C. This invention installs a flue gas heat exchanger 8 (gas-to-gas heat exchanger) on the furnace top or side of the externally heated pyrolysis furnace. The hot flue gas 204 discharged from the low-temperature carbonization section 2 is introduced into the flue gas heat exchanger 8 (preferably via the tube side) through the exhaust gas blower 9, where it exchanges heat with the introduced air 201 (at room temperature, preferably via the shell side). The temperature of the air 203 after heat exchange rises from ambient temperature (approximately 25°C) to 150–180°C (depending on the high-temperature flue gas temperature and flow rate), serving as the secondary hot air. The flue gas 205 (below 200°C) after further cooling is then discharged or sent to a subsequent flue gas treatment system.

[0041] 4. Drying Fan 7: The two types of hot air (the first hot air generated by the air-cooled air box 51 and the second hot air generated by the flue gas heat exchanger 8) are mixed in the mixing chamber of the drying fan 7. After adjusting the mixing ratio, a suitable temperature of drying air 206 is formed. This air is pressurized by the drying fan 7 and sent into the drying chamber 1. It flows out through the air distribution holes of each air distribution channel 12 and directly contacts the downward-flowing coal for heat exchange, thereby achieving the drying of the coal entering the furnace. Preferably, the temperature of the drying air 206 is controlled at around 120℃ (preferably 110-120℃) by adjusting the flow ratio of the two types of hot air. This temperature is sufficient to evaporate the moisture in the coal and is below the auto-ignition point of the coal, ensuring safety.

[0042] 5. Drying Air Temperature Control System: This system includes a temperature sensor 101, a flow regulating valve 102, a flow regulating valve 103, and a controller (preferably a PLC controller). The temperature sensor 101 is installed at the mixed hot air outlet of the drying fan 7 or the mixed hot air inlet of the drying chamber 1 to monitor the temperature of the drying air in real time. The controller controls the opening of the flow regulating valves 102 and 103 based on the temperature detection feedback value, adjusting the mixing ratio of the first and second hot air to stabilize the drying air temperature within the set range. Furthermore, a moisture detection device can be installed at the coal outlet of the drying section. The moisture detection feedback value controls the airflow or feeding speed of the drying fan 7 to ensure that the moisture content of the coal entering the furnace reaches the set target value (approximately 5%).

[0043] As an alternative, the flue gas heat exchanger 8 can be a rotary air preheater or a plate heat exchanger to reduce flue gas resistance and improve heat exchange efficiency.

[0044] As an alternative, drying air at different temperatures can be sent to different heights in drying chamber 1 to precisely control the degree of drying of each layer of coal.

[0045] If the initial moisture content of the coal is high (e.g., greater than 30%), the power of the drying fan 7 can be increased or the mixing ratio of the second hot air can be increased. As an alternative, if necessary, a small amount of flue gas (containing inert gas) can be added to the drying air to further prevent coal oxidation. For some low-rank coals that are prone to cracking after heating, a small amount of atomized water can also be sprayed into the drying air to control the heating rate and avoid overheating and cracking of the coal.

[0046] 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.

[0047] Example: In this embodiment, the existing low-rank coal pyrolysis system is modified by adding a low-rank coal drying system to the externally heated pyrolysis furnace.

[0048] In this embodiment, the externally heated pyrolysis furnace comprises, from top to bottom, a raw material buffer silo 13, a drying section, a low-temperature carbonization section 2, a medium-temperature carbonization section 3, a high-temperature carbonization section 4, an air-cooling section 5, and a water-cooling section 6. The coal fed into the furnace is lignite with a particle size of 5–13 mm and a moisture content of approximately 20%. The goal is to dry it to a moisture content below 5% before it enters the low-temperature pyrolysis section 2. The specific implementation process is as follows: 1. Structure of Drying Chamber 1: Drying Chamber 1 is installed in the drying section at the top of the externally heated pyrolysis furnace. Drying Chamber 1 is a rectangular cylindrical high-temperature resistant metal shell, 6 meters high, with its internal cross-sectional dimensions matching the furnace chamber dimensions of the externally heated pyrolysis furnace. The top of Drying Chamber 1 is connected to the raw material buffer silo 13, and the bottom is connected to the inlet of the low-temperature carbonization section 2 via a discharge valve. Four layers of distribution plates 11 are welded and fixed inside Drying Chamber 1, dividing the internal space into five material distribution spaces. Between adjacent two layers of distribution plates, multiple air distribution channels 12 are horizontally staggered with the distribution plates 11. Air distribution holes are opened on both sides and the bottom of the air distribution channels 12 to ensure that the hot air blown from the air distribution holes can cover the coal layer in the corresponding material distribution space. After the coal enters Drying Chamber 1 through the raw material buffer silo 13, it is intercepted and thinned by each layer of distribution plates 11, forming a thin coal layer of approximately 10-20 cm thickness, which is then placed on the lower distribution plate. The space above the coal seam on each layer of the distribution plate 11 is the hot air circulation space, where hot air is blown onto the coal seam to achieve direct gas-solid contact drying. The discharge valve at the bottom of the drying chamber 1 is a star-shaped discharge valve, which is used to control the rate at which the coal enters the low-temperature carbonization section 2, thereby adjusting the residence time of the coal in the drying chamber 1.

[0049] 2. Preparation process of the first hot air: In the original externally heated pyrolysis furnace, the lower part of the high-temperature dry distillation section 4 only has a water-cooled section 6 for semi-coke cooling. In this embodiment, an air-cooled section 5 with a height of 3m is added between the high-temperature dry distillation section 4 and the water-cooled section 6. The air-cooled section 5 has multiple parallel vertical feed pipes to guide the semi-coke from the high-temperature dry distillation section 4 to the water-cooled section 6. A sealed air-cooled box 51 is wrapped around the air-cooled section 5. Five air intakes 52 are opened on the lower part of the side wall of the air-cooled box 51, and one hot air outlet 53 is opened on the upper part (e.g., ...). Figure 2 (As shown). Hot air outlet 53 is connected to the first hot air inlet of drying fan 7 via a pipe. During operation of the low-rank coal drying system, under the action of drying fan 7, air-cooled air box 51 draws in air at an ambient temperature of 25°C from air intake 52. As the air flows along the serpentine channel formed by five layers of baffles within air-cooled air box 51, it indirectly exchanges heat with the semi-coke in the vertical feed pipe. In this embodiment, the semi-coke temperature is 600–700°C, and the airflow of drying fan 7 is 3.64 × 10⁻⁶. 4 At a flow rate of kg / h, the temperature of the hot air in the outlet air cooling box 51 rises to 110℃, serving as the first hot air for drying, characterized by a large volumetric flow rate and a moderate temperature rise.

[0050] 3. Preparation process of the second hot air: The top of the low-temperature dry distillation section 2 of the externally heated pyrolysis furnace is equipped with a hot flue gas outlet. The hot flue gas generated by dry distillation is discharged from the furnace body by the exhaust fan 9. The hot flue gas temperature is about 350℃ and contains a large amount of heat energy. In this embodiment, a flue gas heat exchanger 8 is installed on one side of the furnace top. It is a shell-and-tube gas-to-gas heat exchanger. The tube side is filled with hot flue gas, and the shell side is filled with air 201 at room temperature of 25℃ drawn in by the drying fan 7. The flue gas heat exchanger 8 is made of stainless steel and is designed to handle 10,000 standard cubic meters of flue gas per hour. When the low-rank coal drying system is running, the hot flue gas enters the tube side of the flue gas heat exchanger 8 from the lower side and flows to the flue gas outlet at the upper end of the other end. After transferring heat to the air in the shell side, the temperature drops to 180-200℃. The flue gas discharged from the flue gas heat exchanger 8 is sent to the waste heat boiler for treatment. The air inside the shell is heated to 180°C, and then the heated air is led out through a pipe as a second hot air for drying.

[0051] In this embodiment, taking into account the safety and operational stability of the low-rank coal drying system, the hot air temperature at the outlet of flue gas heat exchanger 8 is designed to be 180°C, and the design temperature of the drying air after mixing with the first hot air is 120°C.

[0052] 4. Preparation and Delivery of Drying Air: First hot air (approximately 110℃, high flow rate) and second hot air (approximately 180℃, low flow rate) enter the mixing chamber of the drying fan 7 through pipes. In the mixing chamber, the two airflows quickly and evenly mix to form dry air at approximately 120℃. The temperature of the mixed dry air can be precisely adjusted by changing the opening of the valves (flow regulating valve 102 and flow regulating valve 103) on the two airflow pipes. To lower the temperature, more first hot air is introduced; to raise the temperature, the mixing ratio of the second hot air is increased.

[0053] In this embodiment, the drying fan 7 is a high-temperature resistant centrifugal fan, and its power is selected according to the required air volume and pressure head. In this embodiment, the design air volume of the drying fan 7 is 40,000 m³ / h. 3 / h, pressure rise to 5kPa. The mixed drying air is pressurized by the drying fan 7 and then sent to the air distribution channels 12 of each layer of the drying chamber 1 through the mixed hot air duct 10. Figure 1 , Figure 3 As shown, air is blown out from the air distribution holes of each air distribution channel 12, directly contacting and exchanging heat with the stationary coal layer. The moisture in the coal is evaporated into water vapor and carried away by the airflow. The dried coal is discharged to the low-temperature dry distillation section 2 through the discharge valve. The top of the drying box 1 is equipped with a dehumidification outlet, which is connected to a small dust collector and condenser to treat the discharged hot and humid air, condensing and recovering a small amount of light tar and water to avoid environmental impact.

[0054] 5. Automatic Control of the Low-Rank Coal Drying System: This embodiment includes a drying air temperature control system and a coal moisture control system to ensure the drying effect of the coal. The main control parameters and control strategies are as follows: Drying air temperature control: A temperature sensor 101, specifically a K-type thermocouple thermometer, is installed on the mixing hot air duct 10 near the drying chamber 1 to monitor the temperature of the drying air entering the drying chamber 1 in real time. The drying air temperature range set in the controller is 110~120℃, and a PID algorithm is used to adjust the flow regulating valve (…). Figure 4 102) Opening and flow regulating valve II ( Figure 4 The opening of flow regulating valve 103 is adjusted accordingly. If the temperature measured by temperature sensor 101 is higher than the set value, the opening of flow regulating valve 103 is appropriately reduced (reducing the second hot air mixing ratio), or the opening of flow regulating valve 102 is increased (increasing the first hot air mixing ratio). Conversely, if the temperature measured by temperature sensor 101 is lower than the set value, the opening of flow regulating valve 103 is appropriately increased (increasing the second hot air mixing ratio), or the opening of flow regulating valve 102 is reduced (reducing the first hot air mixing ratio). This dynamic adjustment method can stably control the drying air temperature within the range of 110–125℃, avoiding the risk of smoldering coal due to excessively high temperatures or insufficient drying due to excessively low temperatures.

[0055] Coal moisture control: A moisture detection device, specifically a near-infrared moisture sensor, is installed at the coal outlet at the bottom of the drying chamber 1. The controller adjusts the opening of the discharge valve at the bottom of the drying chamber 1 based on the measured coal moisture content. If the coal moisture content is higher than the target set value (5%), it indicates insufficient drying. In this case, the controller reduces the opening of the discharge valve to prolong the residence time of the coal in the drying chamber 1, while ensuring stable drying air temperature. If necessary, the drying air temperature can also be temporarily increased (within a safe range) to enhance the drying effect. Conversely, if the detected coal moisture content is significantly lower than 5% and the drying air temperature shows an upward trend, it indicates over-drying or potential spontaneous combustion. In this case, the controller increases the opening of the discharge valve to increase the discharge speed and shorten the residence time of the coal in the drying chamber 1, while simultaneously increasing the proportion of the first hot air in the drying air to cool the drying air.

[0056] Fan Interlocking and Safety: In this embodiment, the drying fan 7 and the exhaust fan 9 are interlocked with the low-rank coal pyrolysis control system via a controller. When the combustion chamber of the externally heated pyrolysis furnace experiences fluctuations in operating conditions (such as abnormal O2 concentration) leading to insufficient waste heat in the flue gas or requiring shut-off, the controller automatically closes the flow regulating valve 103, and all drying air uses the first hot air to avoid interruption of the drying air supply. Correspondingly, when the waste heat of the flue gas can be reused normally, the flow regulating valve 103 is slowly opened, and the mixing ratio of the drying air is readjusted.

[0057] In addition, this embodiment also includes a temperature limit alarm function: if the drying air temperature exceeds 130°C and remains so for a certain period of time, the controller will trigger an emergency alarm function and reduce the supply of drying air or start the backup spray cooling device to ensure the safe operation of the low-rank coal drying system.

[0058] Implementation Results: Through the above control measures, this embodiment successfully reduced the moisture content of the raw coal from approximately 20% to below 5%. The capacity of the drying section matched the processing capacity of the externally heated pyrolysis furnace, achieving continuous and stable operation of the low-rank coal pyrolysis system. After drying, the coal entered the subsequent carbonization section, volatile matter was released smoothly, the semi-coke coking rate reached over 50%, the tar yield was approximately 8%, and the raw coal gas production was approximately 280 m³. 3 / t, consistent with the design target. Due to the full utilization of waste heat from dry distillation, the overall thermal efficiency of the low-rank coal pyrolysis system increased by approximately 15% compared to before the improvement, demonstrating the significant energy-saving effect and practical value of this invention.

[0059] 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 low-rank coal drying system based on flue gas waste heat and air cooling coupling, connected to an externally heated pyrolysis furnace; characterized in that, The externally heated pyrolysis furnace consists of a drying section, a low-temperature carbonization section, a medium-temperature carbonization section, a high-temperature carbonization section, an air-cooled section, and a water-cooled section arranged sequentially from top to bottom. The low-rank coal drying system includes a drying box, an air-cooled box, an exhaust gas fan, a flue gas heat exchanger, and a drying fan. The drying box is located in the drying section. The drying fan has a mixing chamber and a first hot air inlet, a second hot air inlet, and a mixed hot air outlet connected to the mixing chamber. The air-cooled box is located in the air-cooled section and has an air intake and a hot air outlet. The hot air outlet is connected to the first hot air inlet of the drying fan. The exhaust gas fan's inlet is connected to the hot flue gas outlet of the low-temperature carbonization section, and the exhaust gas fan's outlet is connected to the hot flue gas inlet of the flue gas heat exchanger. The flue gas outlet after heat exchange in the flue gas heat exchanger is connected to the flue gas discharge pipe. The flue gas heat exchanger also has an air inlet and a heat-exchange air outlet. The heat-exchange air outlet is connected to the second hot air inlet of the drying fan. The mixed hot air outlet of the drying fan is connected to the hot air inlet of the drying box.

2. The low-rank coal drying system based on flue gas waste heat and air cooling coupling according to claim 1, characterized in that, The drying chamber is equipped with multiple layers of material distribution plates and multiple layers of air distribution channels arranged at intervals along the height. The material distribution plates and air distribution channels are staggered in the horizontal direction, and multiple air distribution holes are provided on both sides and bottom of the air distribution channels.

3. The low-rank coal drying system based on the coupling of flue gas waste heat and air cooling according to claim 2, characterized in that, The top of the drying chamber is equipped with a raw material buffer silo, and the bottom of the raw material buffer silo is equipped with a discharge valve that is connected to the dry distillation section; the drying chamber is equipped with a dehumidification outlet connected to a condensation separation device.

4. The low-rank coal drying system based on the coupling of flue gas waste heat and air cooling according to claim 1, characterized in that, The air-cooling section is equipped with multiple vertical feeding pipes, and the air-cooling box is located around the vertical feeding pipes. The lower part of the air-cooling box has multiple air intakes along the circumference, and the upper part has a hot air outlet. The height of the air-cooling section is 3 to 5 meters, and the air-cooling box has multiple layers of baffles along the height.

5. A low-rank coal drying system based on flue gas waste heat and air cooling coupling according to claim 1, characterized in that, The flue gas heat exchanger is a shell-and-tube heat exchanger, with an air inlet and a heat-exchange-after air outlet at both ends of the shell side, and a hot flue gas inlet and a heat-exchange-after flue gas outlet at both ends of the tube side.

6. A low-rank coal drying system based on flue gas waste heat and air cooling coupling according to claim 1, characterized in that, It also includes a drying air temperature control system, which consists of a temperature sensor, a flow regulating valve one, a flow regulating valve two, and a controller. The temperature sensor is located on the mixed hot air duct near the mixed hot air outlet of the drying fan, or on the mixed hot air duct near the mixed hot air inlet of the drying chamber. The flow regulating valve one is located at the first hot air inlet of the drying fan, and the flow regulating valve two is located at the second hot air inlet of the drying fan. The signal output terminal of the temperature sensor is connected to the signal input terminal of the controller, and the signal output terminal of the controller is connected to the control terminals of the flow regulating valve one and the flow regulating valve two, respectively.

7. A low-rank coal drying system based on flue gas waste heat and air cooling coupling according to claim 6, characterized in that, It also includes a coal moisture control system, which includes a moisture detection device and a controller. The moisture detection device is located at the coal outlet of the drying section, and the signal output terminal of the moisture detection device is connected to the signal input terminal of the controller. The signal output terminal of the controller is also connected to the control terminal of the bottom discharge valve of the drying section or the control terminal of the drying fan.

8. A method for drying low-rank coal based on the coupling of flue gas waste heat and air cooling, implemented based on the low-rank coal drying system based on the coupling of flue gas waste heat and air cooling as described in any one of claims 1 to 7; characterized in that, The process includes the following: 1) In the drying box at the top of the externally heated pyrolysis furnace, the incoming coal is spread into a thin layer through the distribution plates of each layer, and drying air is supplied through the air distribution holes on the air distribution channel between each layer of distribution plates, so that the coal and the drying air are in full contact; the drying air is a mixed hot air provided by the drying fan. 2) In the air-cooled section at the bottom of the externally heated pyrolysis furnace, the semi-coke flowing out of the high-temperature dry distillation section falls through multiple vertical feed pipes, and the air entering through multiple air inlets flows from bottom to top and exchanges heat with the semi-coke. The hot air with a temperature of 100-130℃ after heat exchange enters the mixing chamber of the drying fan as the first hot air. 3) In the low-temperature dry distillation section at the top of the externally heated pyrolysis furnace, the coal material entering from the drying section flows downward and undergoes low-temperature dry distillation; the hot flue gas discharged from the low-temperature dry distillation section at a temperature of 300-350°C enters the flue gas heat exchanger and exchanges heat with the externally introduced air. After heat exchange, the hot air with a temperature of 150-180°C enters the mixing chamber of the drying fan as the second hot air. 4) In the drying fan, the first hot air and the second hot air are mixed. The mixed hot air with a temperature of 110-120℃ is pressurized by the drying fan and enters the drying box to dry the coal.

9. A method for drying low-rank coal based on the coupling of flue gas waste heat and air cooling according to claim 8, characterized in that, A drying air temperature control system is used to control the temperature of the drying air entering the drying chamber. The controller adjusts the opening of flow regulating valve one and flow regulating valve two according to the detection value of the temperature sensor to adjust the mixing ratio of the first hot air and the second hot air, so that the temperature of the drying air sent into the drying chamber is maintained within the range of 110 to 130°C.

10. A method for drying low-rank coal based on the coupling of flue gas waste heat and air cooling according to claim 9, characterized in that, A moisture detection device is used to detect the residual moisture in the coal flowing out of the drying section; the controller adjusts the opening degree of the feeding valve in the drying section or the air volume of the drying fan according to the moisture detection value. By adjusting the residence time of the coal in the drying section or the drying air volume entering the drying box, the moisture content of the coal is ensured to reach the target value.

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