Tunnel kiln waste heat recovery system and method for high-purity magnesium oxide continuous calcination process

Abstract

The invention discloses a tunnel kiln waste heat recovery system and method for a high-purity magnesium oxide continuous calcination process, and mainly relates to the technical field of energy conservation. The system comprises two tunnel kilns which are oppositely arranged, wherein each kiln body sequentially comprises a preheating section, a sintering section and a cooling section. And high-temperature flue gas discharged by the sintering section is collected through the high-temperature flue gas recovery system and is fed into the preheating section of the tunnel kiln on the opposite side, so that cross-kiln heat energy utilization is realized. The cooling section collects released medium-temperature hot air, the medium-temperature exhaust fan is used for guiding the medium-temperature hot air into the air-water heat exchanger, and the hot air is converted into hot water at the temperature of 70-90 DEG C and used for plant area heating and production. Compared with the prior art, by oppositely arranging the tunnel kilns, the flue gas conveying path is reduced, the heat efficiency is improved, the energy consumption is reduced through graded utilization of heat energy, and the tunnel kiln has remarkable energy-saving and emission-reducing effects and wide engineering application value.

Description

Technical Field

[0001] The present invention relates to the technical field of energy conservation, and in particular to a tunnel kiln waste heat recovery system for a high-purity magnesium oxide continuous calcination process. Background Art

[0002] In traditional high-purity magnesium oxide calcination processes, the calcination process inside the kiln generates large amounts of high-temperature flue gases, which are typically discharged directly, resulting in significant heat loss. While existing waste heat recovery systems exist, these often suffer from low heat recovery efficiency, complex systems, and an inability to effectively utilize heat energy from different temperature ranges. Therefore, a more efficient and comprehensive heat recovery and utilization system is urgently needed. Summary of the Invention

[0003] To achieve the above objectives, the present invention provides the following technical solutions: A tunnel kiln waste heat recovery system for the continuous calcination of high-purity magnesium oxide (MgO) consists of two parallel, opposing tunnel kilns. The two tunnel kilns share the same structure and process flow, but with opposite directions of flow. Each kiln contains a preheating section, a firing section, and a cooling section, arranged sequentially. Material is pushed into the kiln from the front end of the preheating section, where it is heated in stages and calcined. It is cooled in the cooling section and ultimately discharged from the rear end. The firing section of each tunnel kiln is equipped with a high-temperature flue gas cross-kiln utilization system that collects high-temperature flue gas from the firing section and feeds it to the preheating section of the opposing tunnel kiln. Furthermore, a closed hot air collection hood is installed at the top of the cooling section of each tunnel kiln to capture the medium-temperature hot air (150–300°C) released during the cooling process. This air is directed by a separate medium-temperature exhaust fan into an air-to-water heat exchanger for indirect heat exchange with the plant's heating circulating water system. The sensible heat of the hot air is converted into hot water at 70–90°C, which is used for domestic and industrial heat needs.

[0004] The entire system is coordinated and operated by the PLC automatic control module. By real-time collection of key parameters such as the preheating section outlet temperature, the air-water heat exchange outlet temperature and the tunnel kiln top pressure, it dynamically adjusts the operating frequency of the high-temperature and medium-temperature exhaust fans and the pipeline air damper opening to ensure that the system achieves the optimal balance between heat recovery, energy efficiency utilization and kiln operation stability.

[0005] Furthermore, the high-temperature flue gas cross-kiln utilization system includes a ventilation structure formed by multiple exhaust holes evenly distributed axially along the firing section's dome, and vertical exhaust troughs arranged perpendicularly thereto. High-temperature gases collected at each extraction point are channeled through a high-temperature refractory flue, subsequently passed through a particle trap, and then pumped at a constant pressure by a high-temperature, heat-resistant centrifugal fan. The treated high-temperature flue gas is then transported above the preheating section of the opposite tunnel kiln, achieving cross-kiln thermal energy coupling and treating the temperature rise of the material before entering the kiln, thereby optimizing the distribution of the firing heat load.

[0006] Furthermore, dust collection can use one or both of cyclone dust removal and ceramic bag dust removal to capture dust particles.

[0007] On the other hand, the present invention also provides an application method of the above system, comprising the following steps: S1, the high-temperature flue gas emitted from the firing section is collected by the high-temperature flue gas collection system at the top of the firing section, and is transported to the preheating section of the tunnel kiln on the opposite side, where the high-temperature flue gas is used to preheat the material in the tunnel kiln on the opposite side; S2, which collects the 150–300°C medium-temperature hot air released during the cooling process at the top of the cooling section, uses a medium-temperature exhaust fan to direct the hot air into an air-water heat exchanger, and indirectly exchanges heat with the plant's heating circulating water system, converting the sensible heat of the hot air into 70–90°C hot water for domestic and production heat energy needs; S3 collects key parameters in real time through the PLC control system, adjusts the operating frequency of high-temperature exhaust fans and medium-temperature exhaust fans, and ensures the optimization of heat recovery and energy efficiency.

[0008] Furthermore, the high-temperature flue gas collection step in the above method includes arranging a plurality of exhaust holes evenly distributed axially on the dome of the firing section, and collecting the flue gas through a longitudinal smoke exhaust trough arranged perpendicular to the exhaust holes, and then leading the flue gas out through a high-temperature refractory flue, and after purification through a particle collection device, it is sent to the preheating section of the tunnel kiln on the opposite side.

[0009] Furthermore, the medium-temperature hot air collection step in the above method includes introducing the collected medium-temperature hot air into an air-water heat exchanger through a medium-temperature exhaust fan, exchanging heat with the factory heating system, and converting the sensible heat of the hot air into hot water at 70-90°C.

[0010] Compared with the existing technology, the beneficial effect of the present invention is that, by arranging tunnel kilns in opposite directions, the high-temperature flue gas transmission path is effectively shortened, and the convection and radiation losses in the heat transfer process are significantly reduced; at the same time, the orderly recovery of hot air in the cooling section and the connection with the heating system enable high-grade and medium-grade thermal energy to be utilized in a graded manner, thereby improving the thermal efficiency and energy utilization rate of the tunnel kiln system as a whole, and having significant energy-saving, emission reduction and engineering promotion value. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 2 is a system diagram of an embodiment of the present invention. DETAILED DESCRIPTION

[0012] The following will clearly and completely describe the technology in the embodiments of the present invention in conjunction with the accompanying drawings in the embodiments of the present invention; it is obvious that the embodiments described are only part of the embodiments of the present invention, not all of the embodiments; based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making any creative efforts should fall within the scope of protection of the present invention. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other unless there is a conflict.

[0013] Reference Figure 1 The present invention provides a tunnel kiln waste heat recovery system for a high-purity magnesium oxide continuous calcination process. The system includes a bidirectional tunnel kiln (tunnel kiln A and tunnel kiln B), and its structure is as follows: Figure 1 As shown in the figure, each tunnel kiln consists of a preheating section, a firing section, and a cooling section, arranged in opposing directions. Material is fed by a feeding trolley 2 along track 11 from the preheating section of the tunnel kiln, then passes through the firing section and is then transported along the track from the cooling section outlet to the side of the tunnel kiln for unloading. The following is a detailed structure of the system.

[0014] Tunnel kiln A includes a preheating section A1. A feeding trolley 2 enters this section, where it is preheated by recovering high-temperature flue gas from the firing section of tunnel kiln B. This section collects flue gas through exhaust vents 3 (located in the dome of firing section A) and funnels it through longitudinal exhaust ducts 4 to the main refractory flue 5, ensuring efficient transfer of the high-temperature flue gas to the preheating section of the opposite kiln. In firing section A6, the preheated material from feed trolley 2 enters firing section A6 for high-temperature calcination. High-temperature flue gas is collected at the top of firing section A6 through exhaust holes 3 and directed through exhaust ducts to the preheating section of kiln B on the opposite side, utilizing cross-kiln heat energy. In cooling section A7, the calcined magnesium oxide cools down. A closed hot air collection hood is installed at the top of the cooling section to collect the 150–300°C hot air released during the cooling process. This hot air is then directed into an air-to-water heat exchanger and converted into hot water at 70–90°C.

[0015] Tunnel kiln B includes a preheating section B8. The high-temperature flue gas from the firing section of tunnel kiln A is guided to the preheating section B8 of tunnel kiln B through the longitudinal flue gas duct 4. It is used to heat the material on the feeding trolley 2 to ensure that the material reaches the appropriate temperature before entering the kiln. Firing section B9: The preheated material enters firing section B9 for high-temperature calcination. Similar to tunnel kiln A, the top of firing section B collects the high-temperature flue gas from firing section B9 through exhaust holes 3 and directs it into the preheating section of the opposite kiln A, optimizing the transfer of heat energy across the kiln. Cooling section B10, similar to cooling section A, also features a hot air collection hood at the top to collect the medium-temperature hot air released during the cooling process. This collected hot air is directed by a separate medium-temperature exhaust fan into an air-to-water heat exchanger, where it is converted into hot water.

[0016] Specifically, the high-temperature flue gas recovery system includes multiple equally spaced exhaust holes 3 above the firing section (firing section A and firing section B) of each tunnel kiln. Each exhaust hole 3 collects the high-temperature flue gas emitted from the firing section. The exhaust holes are connected to a longitudinal exhaust duct 4, which runs along the length of the firing section and collects the collected flue gas into a main refractory flue 5. The flue gas is then discharged through the main refractory flue 5 and purified by a flue gas treatment device 12 (such as a cyclone dust collector or ceramic bag filter) before being pumped by a high-temperature, heat-resistant centrifugal fan 13. The purified high-temperature flue gas is then guided through a transverse duct to the preheating section (preheating section B or A) of the opposite tunnel kiln, where it heats the material. Since the two tunnel kilns are arranged opposite each other, the high-temperature flue gas transport path is effectively shortened, reducing heat transmission losses.

[0017] Specifically, enclosed hot air collection hoods are installed at the top of cooling sections A7 and B10 to collect the medium-temperature hot air (150–300°C) released during the cooling process. This collected hot air is channeled through a duct system to a separate medium-temperature exhaust fan. The fan feeds the hot air into an air-to-water heat exchanger, where it indirectly exchanges heat with the plant's heating water system, converting the sensible heat of the hot air into hot water at 70–90°C, providing heat energy for daily life and production within the plant.

[0018] The entire system is automatically regulated through a PLC control module, which monitors key parameters such as the preheating section outlet temperature, the air-water heat exchanger outlet temperature, and the kiln top pressure in real time. Based on this feedback, the PLC system automatically adjusts the speed of the high-temperature and medium-temperature exhaust fans, as well as the opening of the pipeline dampers, to ensure the system achieves an optimal balance between heat recovery, energy efficiency, and kiln operation stability.

[0019] The specific application of the system provided in this embodiment includes the following steps: 1. Material input and preheating process Material input: The feeding trolley 2 is pushed into the kiln from the front end of the preheating section and enters the preheating section A1 of the tunnel kiln.

[0020] Preheating section A1 collects high-temperature flue gas from the firing section of tunnel kiln B, gradually heating the material until it reaches the appropriate temperature before entering the firing section. During this process, exhaust holes 3 are evenly distributed axially along the dome of firing section A6. The flue gas is collected through longitudinal exhaust ducts 4 and collected in the main refractory flue 5 for recovery.

[0021] 2. Firing section and flue gas recovery In the firing section A6, the feed carriage 2 undergoes high-temperature treatment, gradually increasing its temperature to 1000–1100°C. High-temperature flue gas is discharged at the top of the firing section and collected through exhaust holes 3. After being collected through longitudinal exhaust ducts 4, the flue gas is discharged through high-temperature refractory flues 5 and enters the flue gas treatment device 6.

[0022] Flue gas passes through a cyclone dust collector and ceramic bag filters to remove dust particles and ensure that flue gas emissions meet environmental standards. The purified, high-temperature flue gas is pumped at a constant pressure by a high-temperature, heat-resistant centrifugal fan 7 into the preheating section B8 of tunnel kiln B, where it is used to heat the feed trolley 2.

[0023] Through the exhaust holes 3 provided in the longitudinal exhaust pipe 4, the high-temperature flue gas from the firing section A6 is introduced into the preheating section of the tunnel kiln B8 on the opposite side, thereby increasing the preheating temperature of the material and realizing cross-kiln utilization of heat energy.

[0024] 3. Cooling section and hot air recovery The calcined magnesium oxide product is cooled in the cooling section A7. A hot air collection hood is installed at the top of the cooling section to collect the medium-temperature hot air (150-300°C) released during the cooling process.

[0025] The hot air released from cooling section A7 is introduced into an air-to-water heat exchanger through a medium-temperature exhaust fan, where it undergoes indirect heat exchange with the plant’s heating circulating water system, converting the sensible heat of the hot air into hot water at 70–90°C, which is then used for the plant’s living and production heat energy needs.

[0026] Specifically, the system automatically adjusts itself through a PLC control module, collecting real-time data on key parameters such as the preheating section outlet temperature, the air-water heat exchanger outlet temperature, and the kiln top pressure. Based on this data, the system automatically adjusts the speed of the high-temperature and medium-temperature exhaust fans, as well as the opening of the pipe dampers, to ensure an optimal balance between heat recovery, energy efficiency, and kiln operation stability.

[0027] When the temperature of the preheating section reaches the set value, the PLC system will automatically adjust the operating frequency of the fan to ensure that the heat load of each section is reasonably distributed, thereby improving the heat recovery efficiency.

[0028] Specifically, the flue gas recovery system incorporates a cyclone dust collector and a ceramic bag filter (or a combination of both) to effectively remove dust particles from the flue gas. The treated flue gas meets environmental emission standards and is then pumped to a chimney or other treatment facility via an induced draft fan.

[0029] This embodiment significantly improves heat recovery efficiency through high-temperature flue gas recovery and medium-temperature hot air recovery. Specifically, the high-temperature flue gas recovery efficiency reaches over 85%. The 8,000 Nm³ / h of high-temperature flue gas generated in firing section A is fed into the preheating section of the tunnel kiln on the opposite side through the high-temperature flue gas recovery system, successfully recovering approximately 400kW of heat energy. The 6,000 Nm³ / h of medium-temperature hot air released in the cooling section is converted into 70-90°C hot water through an air-water heat exchanger and used in the plant's heating system, achieving 200kW of heat recovery. Through the effective utilization of heat energy, this system reduces overall fuel consumption by approximately 30%, equivalent to saving 120,000 Nm³ of natural gas (approximately US$10,000 in fuel costs) annually and reducing carbon dioxide emissions by approximately 250,000kg, providing a sustainable solution for the plant to achieve its energy conservation and emission reduction goals.

Claims

1. A tunnel kiln waste heat recovery system for a high-purity magnesium oxide continuous calcination process, characterized in that: The system includes two parallel tunnel kilns arranged opposite each other, each tunnel kiln including a preheating section, a firing section and a cooling section arranged in sequence; the firing section of each tunnel kiln is equipped with a high-temperature flue gas collection system for collecting the high-temperature flue gas generated by the firing section and sending it to the preheating section of the tunnel kiln on the opposite side, thereby realizing cross-kiln utilization of heat energy; a closed hot air collection hood is provided on the top of the cooling section of each tunnel kiln for collecting the medium-temperature hot air released during the cooling process, and the hot air is introduced into an air-water heat exchanger through a medium-temperature exhaust fan for indirect heat exchange with the factory heating circulating water system, thereby converting the sensible heat of the hot air into hot water at 70-90°C for use in domestic and production heat energy needs.

2. The system according to claim 1, wherein: The temperature of the high-temperature flue gas is 850-1100°C.

3. The system according to claim 1, wherein: The cooling section hot air collecting hood is used to collect medium temperature hot air of 150-300°C.

4. The system according to claim 1, wherein: The high-temperature flue gas collection system includes a plurality of exhaust holes uniformly distributed along the axial direction of the arch of the firing section, and an exhaust structure formed by a longitudinal exhaust trough arranged perpendicular to the exhaust holes. The collected high-temperature gas is centrally discharged through a high-temperature refractory flue, and after the dust is removed by a particle capture device, it is pumped by a high-temperature heat-resistant centrifugal fan at a stable pressure to transport the treated high-temperature flue gas to the top of the preheating section of the tunnel kiln on the opposite side, realizing the cross-kiln coupling of thermal energy and the temperature rise treatment of the raw materials before entering the kiln, thereby optimizing the firing heat load distribution.

5. The system according to claim 4, characterized in that The particle collection device is a cyclone dust collector or a ceramic bag dust collector, or a combination of a cyclone dust collector and a ceramic bag dust collector.

6. The system according to claim 1, wherein: The system is coordinated and operated by a PLC automatic control module. The control module dynamically adjusts the operating frequency and pipeline damper opening of the high-temperature exhaust fan and the medium-temperature exhaust fan by real-time collecting key parameters such as the outlet temperature of the preheating section, the outlet temperature of the air-water heat exchanger and the pressure at the rear of the kiln, ensuring that the system achieves the best balance between heat recovery, energy efficiency utilization and kiln operation stability.

7. A method for recovering waste heat from a tunnel kiln in a continuous calcination process of high-purity magnesium oxide, using the system according to any one of claims 1 to 6, characterized in that: The following steps are involved: S1, the high-temperature flue gas discharged from the firing section is collected by the high-temperature flue gas collection system at the end of the firing section, and is transported to the preheating section of the tunnel kiln on the opposite side, where the high-temperature flue gas is used to preheat the raw materials in the tunnel kiln on the opposite side; S2, which collects the 150–300°C medium-temperature hot air released during the cooling process at the top of the cooling section, uses a medium-temperature exhaust fan to direct the hot air into an air-water heat exchanger, and indirectly exchanges heat with the plant's heating circulating water system, converting the sensible heat of the hot air into 70–90°C hot water for domestic and production heat energy needs; S3 collects key parameters in real time through the PLC control system, adjusts the operating frequency of high-temperature exhaust fans and medium-temperature exhaust fans, and ensures the optimization of heat recovery and energy efficiency.

8. The method according to claim 7, characterized in that The high-temperature flue gas collection step includes arranging a plurality of exhaust holes evenly distributed axially on the dome of the firing section, collecting the flue gas through a longitudinal exhaust trough arranged perpendicular to the exhaust holes, and then leading the flue gas out through a high-temperature refractory flue, and after purification through a particle capture device, sending the flue gas to the preheating section of the tunnel kiln on the opposite side.

9. The method according to claim 7, characterized in that The medium-temperature hot air collection step includes introducing the collected medium-temperature hot air into an air-water heat exchanger through a medium-temperature exhaust fan, exchanging heat with the factory heating system, and converting the sensible heat of the hot air into hot water at 70-90°C.

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