Fly ash low-temperature pyrolysis equipment and process using low-pressure steam preheating

Abstract

The present application relates to fly ash treatment technical field, specifically to a kind of fly ash low-temperature pyrolysis equipment and process using low-pressure steam preheating, the pyrolysis equipment includes feeding unit, pyrolysis unit and discharge cooling unit;Feeding unit includes fly ash storage and feed pipe;Pyrolysis unit includes jacketed preheating screw conveyor and electromagnetic heating screw conveyor;Jacketed preheating screw conveyor includes preheating shell, screw conveying main body, preheating jacket;Preheating jacket input end is also connected with outside heat unit by gas inlet pipe;Electromagnetic heating screw conveyor includes spiral shell and electromagnetic induction coil and insulation layer;In spiral shell inside wall, along material conveying direction, there is also partitioned multiple thermocouples;Pyrolysis unit is connected with inert gas protection unit;Discharge cooling unit includes two-stage series water-cooled screw conveyor;The present application greatly reduces power consumption, avoids equipment corrosion and ash deposition problem caused by flue gas heating, and realizes in-situ collaborative resource disposal in plant.

Description

Technical Field

[0001] This invention relates to the field of fly ash pyrolysis treatment technology, specifically to a low-temperature pyrolysis equipment and process for fly ash that utilizes low-pressure steam preheating. Background Technology

[0002] Fly ash from municipal solid waste incineration is listed in the National Hazardous Waste List (HW18) due to its enrichment of heavy metals and dioxins. Dioxins are highly toxic and environmentally persistent, and how to achieve efficient and harmless disposal of fly ash is a pain point in the industry.

[0003] 1. Solidification landfill technology: It carries risks of occupying land and having questionable long-term stability. Dioxins and heavy metals are not eliminated, and it is only a risk transfer.

[0004] 2. Co-processing in cement kilns: High chlorine content (usually >10%) in fly ash can exacerbate equipment corrosion and affect the cement setting quality. Moreover, due to the limitations of cement plant layout, transportation costs are high and environmental risks are significant.

[0005] 3. High-temperature melting technology: Although it can completely vitrify heavy metals and decompose dioxins, the operating temperature is as high as 1300-1500℃, and the energy consumption cost is extremely high, making it difficult to promote on a large scale.

[0006] 4. Defects of existing low-temperature pyrolysis technology: Pure electric heating: Although the temperature control is precise, heating fly ash at room temperature to the pyrolysis range of 350-450℃ consumes a huge amount of electricity and has high processing costs.

[0007] Direct / indirect heating of flue gas: Using high-temperature flue gas (above 600℃) from incinerators can easily lead to ash accumulation, tar condensation and corrosion on the inner wall of the heating jacket. Not only does the heat exchange efficiency decline rapidly, but due to the lack of high-temperature and high-efficiency dust removal equipment, the equipment maintenance cycle is short (usually requiring shutdown for ash cleaning every 3-6 months), and the annual effective operating time of the system is less than 80%. Summary of the Invention

[0008] In order to address the shortcomings and deficiencies of existing fly ash pyrolysis treatment technologies, this invention provides a low-temperature fly ash pyrolysis device that utilizes low-pressure steam preheating to reduce power consumption, avoid equipment corrosion and ash accumulation caused by flue gas heating, and achieve in-situ co-processing within the plant.

[0009] The present invention achieves the above objectives by adopting the following technical solution: A low-temperature pyrolysis device for fly ash preheated by low-pressure steam is disclosed. The pyrolysis device includes a feeding unit, a pyrolysis unit, and a discharge cooling unit. The feeding unit includes a fly ash storage bin and a conveying pipe located at the output end of the fly ash storage bin. A variable frequency rotary valve and a first-stage airlock valve are sequentially installed on the conveying pipe. The pyrolysis unit includes a jacketed preheating screw conveyor and an electromagnetically heated screw conveyor. The jacketed preheating screw conveyor includes a preheating shell, a screw conveyor body, and a preheating jacket. The preheating shell is distributed on both sides, the screw conveyor body is located inside the preheating shell, and the preheating jacket is a fully enclosed steam jacket located outside the preheating shell. The input end of the preheating jacket is also connected to an external heat unit via an air inlet pipe. The external heat unit can be either the steam extraction pipeline of a waste incineration power plant turbine or low-pressure steam from a boiler drum. The fly ash is kept in the screw conveyor body for 45-90 minutes, and the latent heat of steam is used to indirectly heat the fly ash from room temperature to 120-180℃. The electromagnetically heated screw conveyor includes a screw shell and an electromagnetic induction coil wound around the outside of the screw shell, with an insulation layer covering the outside of the electromagnetic induction coil. Multiple thermocouples are also arranged in sections along the material conveying direction on the inner wall of the screw shell. The thermocouples employ PID zoned power control to stabilize the fly ash temperature at 350-450℃ and maintain this temperature for 45-90 minutes during pyrolysis. The preheating shell and the screw shell are connected via a discharge pipe. The pyrolysis unit is also connected to an inert gas protection unit. The discharge cooling unit includes a two-stage water-cooled screw conveyor connected in series: a primary water-cooled screw conveyor and a secondary water-cooled screw conveyor. The primary water-cooled screw conveyor is connected to the electromagnetically heated screw conveyor via a pipeline. The primary water-cooled screw conveyor is connected to a cooling water unit, allowing cooling water to flow sequentially through the jackets of both water-cooled screw conveyors, indirectly exchanging heat with the high-temperature pyrolysis fly ash before being sent to the power plant cooling tower via a return water pipeline for cooling.

[0010] As a preferred technical solution, the pressure of the low-pressure steam is maintained at 0.5-1.68 MPa and the temperature at 270-306℃.

[0011] A further preferred technical solution: the output end of the conveying pipe leads to the inner cavity of the preheating shell; the air inlet pipe and the preheating jacket are connected in a sealed manner by a second-stage airlock valve.

[0012] A further preferred technical solution: The cooling water unit can be any of the cooling water supply pipelines for the induced draft fan and primary air fan of the waste incineration power plant; the pipeline of the cooling water unit already has a stable flow rate and suitable pressure, and can be used as the cooling water source for the fly ash cooling spiral, without the need for an additional independent circulation pump station.

[0013] A further optimized technical solution: The total cooling area of ​​the two-stage water-cooled screw conveyor is large enough to rapidly cool the fly ash temperature to below 80°C within 10 minutes, quickly crossing the dioxin resynthesis temperature window of 250-450°C.

[0014] A further preferred technical solution: The inert gas protection unit includes a nitrogen source, a protective gas pipe connected to the nitrogen source, and a flow regulating valve group installed on the protective gas pipe; wherein, the protective gas pipe interfaces are respectively arranged in the middle of the preheating shell and the discharge end of the spiral shell to maintain a slightly positive pressure oxygen-free environment in the equipment; wherein, an oxygen content sensor is also installed in the preheating shell, so that when the oxygen content detection value in the preheating shell is <1%, the fly ash enters the preheating shell through the unloading valve.

[0015] A further preferred technical solution: The jacketed preheating screw conveyor and the electromagnetic heating screw conveyor are respectively connected to a tail gas co-treatment unit through exhaust pipes on one side; wherein, the tail gas co-treatment unit adopts a pyrolysis gas dust collector, which removes dust from the tail gas and introduces it into either the power plant incinerator air supply system or the combustion chamber for high-temperature incineration and deodorization.

[0016] A process for a low-temperature pyrolysis device for fly ash preheated with low-pressure steam is described, wherein the fly ash is pyrolyzed using the aforementioned pyrolysis device, and the pyrolysis process steps include: Step S1: Oxygen-free feeding: Activate the nitrogen protection of the inert gas protection unit. After the oxygen content sensor of the jacketed preheated screw conveyor detects a value of <1%, the fly ash enters the screw conveyor body of the jacketed preheated screw conveyor through the unloading valve. Step S2: Steam preheating: saturated steam with a pressure of 0.5-1.68MPa and a temperature of 270-306℃ from an external heat unit is introduced into the preheating jacket of the preheating screw conveyor; the screw speed is adjusted by the variable frequency motor of the screw conveyor body to control the residence time of fly ash in the preheating section to be 45-90 minutes, and the latent heat of steam is used to indirectly heat the fly ash from room temperature to 120-180℃; Step S3: Precision electric heating pyrolysis: The preheated fly ash enters the electromagnetic heating screw conveyor through the discharge pipe; the electromagnetic induction coil of the electromagnetic heating screw conveyor works to directly heat the screw shell and the fly ash material inside the screw shell; at the same time, through the temperature control of thermocouples and multi-zone PID technology, the temperature of fly ash inside the screw shell is stabilized at 350-450℃ and maintained at this temperature for 45-90 minutes. Step S4: Rapid cooling and dioxin suppression: The high-temperature fly ash after pyrolysis by the electromagnetically heated screw conveyor enters a two-stage water-cooled screw conveyor, which rapidly reduces the temperature to below 80°C within 10 minutes, quickly crossing the dioxin resynthesis temperature window of 250-450°C to prevent the secondary generation of dioxins during the cooling process. Step S5: Discharge: After being cooled by a two-stage water-cooled screw conveyor, the fly ash is discharged and then transported to the fly ash silo by a bucket elevator for later use. Step S6: In-situ destruction of exhaust gas: Dust-laden and VOCs-laden exhaust gas generated by the pyrolysis of the jacketed preheated screw conveyor and the electromagnetic heating screw conveyor is first transported to the pyrolysis gas dust collector through pipelines. After dust removal, it is sent back to either the power plant incinerator's air supply system or the combustion chamber for incineration, so as to utilize the power plant's existing flue gas purification system for treatment and complete the discharge operation without secondary pollution.

[0017] The advantages of this invention compared to existing technologies are as follows: This low-temperature pyrolysis equipment replaces the electrically heated low-temperature section (20℃→180℃) with low-pressure steam preheating from the turbine extraction pipe or boiler drum of a waste incineration power plant. The energy consumption cost of the preheating section is only 20%-30% of that of electric heating. Calculations show that the comprehensive power consumption per ton of fly ash can be reduced by 35%-45%, significantly improving the project's economic efficiency. The reliability and continuous operation cycle of this low-temperature pyrolysis equipment are extended: using clean steam as the preheating medium completely eliminates the corrosion and ash accumulation problems of HCl, SOx, and tar in high-temperature flue gas on the heat exchange wall. The annual effective operating time of the system can be increased from less than 80% in traditional flue gas heating schemes to over 95%. The low-temperature pyrolysis equipment features precise temperature control and coking prevention: the steam saturation temperature is constant (determined by pressure), and there is no risk of local overheating in the preheating section, fundamentally avoiding the spiral coking and blockage problem caused by the premature melting of low-melting-point salts (such as KCl and NaCl) in the fly ash within the 200-300℃ range; at the same time, the electric heating section adopts electromagnetic induction, with a temperature control accuracy of ±5℃; the low-temperature pyrolysis equipment has high environmental and safety standards: multi-stage airlocks combined with nitrogen micro-positive pressure ensure that the oxygen content in the pyrolysis core area is ≤0.1%, inhibiting the formation of dioxin precursors from a mechanistic perspective; the dioxin residual toxicity equivalent of the treated fly ash is ≤50 ng-TEQ / kg (superior to EU and Chinese landfill entry standards), and the fly ash does not leave the plant area, avoiding the leakage risk of hazardous waste transfer. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the overall structure of the present invention; Figure 2 This is a process flow diagram of the low-temperature pyrolysis of fly ash according to the present invention.

[0020] In the diagram: 1. Feeding unit; 11. Fly ash storage bin; 12. Conveying pipe; 13. Discharge valve; 14. First-stage airlock valve; 15. Second-stage airlock valve; 2. Pyrolysis unit; 21. Jacketed preheating screw conveyor; 211. Preheating shell; 212. Screw conveyor body; 213. Preheating jacket; 214. Air inlet pipe; 215. Oxygen content sensor; 22. Electromagnetically heated screw conveyor; 221. Screw shell; 222. 223 Electromagnetic induction coil; 224 Insulation layer; 225 Thermocouple; 23 Discharge pipe; 24 External heating unit; 3 Discharge cooling unit; 31 Primary water-cooled screw conveyor; 32 Secondary water-cooled screw conveyor; 4 Inert gas protection unit; 41 Nitrogen source; 42 Protective gas pipe; 43 Flow regulating valve group; 5 Cooling water unit; 6 Tail gas co-treatment unit; 61 Pyrolysis gas dust collector; 62 Exhaust pipe. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] It should be noted that, in specific embodiments of the present invention, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any actual relationship or order between these entities or operations. Furthermore, terms such as "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, the use of phrases such as "comprising one" to define an element does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0023] In the description of this invention, unless otherwise explicitly specified and limited, the terms "installed," "connected," "linked," and "equipped" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; or they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0024] Example: Figures 1 to 2 As shown: A low-temperature pyrolysis device for fly ash preheated by low-pressure steam is disclosed. The pyrolysis device includes a feeding unit 1, a pyrolysis unit 2, and a discharge cooling unit 3. The feeding unit 1 includes a fly ash storage silo 11 and a conveying pipe 12 located at the output end of the fly ash storage silo 11. A variable frequency rotary valve 13 and a first-stage airlock valve 14 are sequentially installed on the conveying pipe 12. Preferably, a weighing screw conveyor can also be installed on the conveying pipe. This arrangement is intended to stably meter and convey the fly ash into the system while isolating it from outside air.

[0025] In this embodiment, the pyrolysis unit 2 includes a jacketed preheating screw conveyor 21 and an electromagnetic heating screw conveyor 22. The jacketed preheating screw conveyor 21 includes a preheating shell 211, a screw conveying body 212, and a preheating jacket 213. The preheating shell 211 is distributed on both sides, and the screw conveying body 212 is disposed inside the preheating shell 211. The screw conveying body 212 includes a conveying screw and a motor; the motor can be a variable frequency motor, which is existing technology and will not be described in detail. The preheating jacket 213 is a fully enclosed steam jacket disposed outside the preheating shell 211. The input end of the preheating jacket 213 is also connected to an external heat unit 24 via an air inlet pipe. The external heat unit 24 can be either the steam extraction pipe of a waste incineration power plant turbine or low-pressure steam from a boiler drum. The low-pressure steam is maintained at a pressure of 0.5-1.68 MPa and a temperature of 270-306°C. The fly ash is kept in the screw conveyor body 212 for 45-90 minutes, and the latent heat of steam is used to indirectly heat the fly ash from room temperature to 120-180℃. The output end of the conveying pipe 12 leads to the inner cavity of the preheating shell 211; the air inlet pipe 214 and the preheating jacket 213 are connected in a sealed manner through a second-stage airlock valve 15. In this embodiment, the condensate generated by the preheating screw conveyor during the preheating stage is returned to the air preheater for steam return water.

[0026] This low-temperature pyrolysis equipment replaces the electrically heated low-temperature section (20℃→180℃) with preheating from the low-pressure steam external heat unit in the steam extraction pipeline of the waste incineration power plant turbine or boiler drum. The energy consumption cost of the preheating section is only 20%-30% of that of electric heating. Calculations show that the comprehensive power consumption per ton of fly ash can be reduced by 35%-45%, significantly improving the project's economic efficiency. Simultaneously, the reliability and continuous operation cycle of this low-temperature pyrolysis equipment are extended: using clean steam as the preheating medium completely eliminates the corrosion and ash accumulation blockage problems of HCl, SOx, and tar in high-temperature flue gas on the heat exchange wall; the annual effective operating time of the system can be increased from less than 80% of traditional flue gas heating schemes to over 95%. Furthermore, the temperature control of this low-temperature pyrolysis equipment is precise and prevents coking: the steam saturation temperature is constant (determined by pressure), and there is no risk of localized overheating in the preheating section, fundamentally avoiding the spiral coking blockage problem caused by the premature melting of low-melting-point salts (such as KCl and NaCl) in the fly ash within the 200-300℃ range.

[0027] In this embodiment, the electromagnetically heated screw conveyor 22 includes a screw shell 221 and an electromagnetic induction coil 222 wound around the outside of the screw shell 221, with an insulation layer 223 covering the outside of the electromagnetic induction coil 222. Multiple thermocouples 224 are also arranged in sections along the material conveying direction on the inner wall of the screw shell 221. The thermocouples 224 employ PID zoned power control to stabilize the fly ash temperature at 350-450℃ and maintain this temperature for 45-90 minutes during pyrolysis. The PID temperature regulation control technology is existing technology and will not be elaborated upon; its purpose is to achieve precise temperature control in the heating section. The preheating shell 211 and the screw shell 221 are connected by a discharge pipe 23. This arrangement facilitates the material conveying of preheated fly ash. An electromagnetic switch valve can be installed on the discharge pipe for control.

[0028] In this embodiment, the pyrolysis unit 2 is also connected to an inert gas protection unit 4. Specifically, the inert gas protection unit 4 includes a nitrogen source 41, a protective gas pipe 42 connected to the nitrogen source 41, and a flow regulating valve group 43 installed on the protective gas pipe 42. The interfaces of the protective gas pipe 42 are respectively located in the middle of the preheating shell 211 and at the discharge end of the spiral shell 221 to maintain a slightly positive pressure oxygen-free environment in the equipment. An oxygen content sensor 215 is also installed inside the preheating shell 211. Once the detected oxygen content in the preheating shell 211 is <1%, fly ash is allowed to enter the preheating shell 211 through the discharge valve 13.

[0029] In this embodiment, one side of the jacketed preheating screw conveyor 21 and the electromagnetic heating screw conveyor 22 are respectively connected to the exhaust gas co-treatment unit 6 via exhaust pipes 62. The exhaust gas co-treatment unit 6 employs a pyrolysis gas dust collector 61, which removes dust from the exhaust gas and introduces it into either the power plant incinerator's air supply system or the combustion chamber for high-temperature incineration and deodorization. This low-temperature pyrolysis equipment boasts high environmental and safety standards: multi-stage airlocks combined with nitrogen micro-positive pressure ensure that the preheating oxygen content is <1%, and the oxygen content in the pyrolysis core area is ≤0.1%, thus inhibiting the formation of dioxin precursors mechanistically; the dioxin residual toxicity equivalent of the treated fly ash is ≤50 ng-TEQ / kg (superior to EU and Chinese landfill entry standards), and the fly ash does not leave the plant area, avoiding the leakage risk during hazardous waste transfer.

[0030] In this embodiment, the discharge cooling unit 3 includes a two-stage water-cooled screw conveyor 31 and a two-stage water-cooled screw conveyor 32 connected in series. The first-stage water-cooled screw conveyor 31 is connected to the electromagnetically heated screw conveyor 22 via a pipeline. The first-stage water-cooled screw conveyor 31 is connected to a cooling water unit 5, allowing cooling water to flow sequentially through the jackets of the two water-cooled screw conveyors. After indirect heat exchange with the high-temperature pyrolysis fly ash, the water is sent to the power plant cooling tower via a return water pipeline for cooling. The cooling water unit 5 can be any type of cooling water supply pipeline for the induced draft fan or primary air fan of the waste incineration power plant. These fans require continuous cooling of the bearings or hydraulic systems during operation. The pipeline of the cooling water unit 5 already has a stable flow rate and suitable pressure, making it suitable as a cooling water source for the fly ash cooling screw, without the need for a new independent circulating pump station. The two-stage water-cooled screw conveyor has a large enough total cooling area to rapidly cool the fly ash temperature to below 80°C within 10 minutes and complete the operation of quickly crossing the dioxin resynthesis temperature window of 250-450°C.

[0031] A process for a low-temperature pyrolysis device for fly ash preheated with low-pressure steam is described, wherein the fly ash is pyrolyzed using the aforementioned pyrolysis device, and the pyrolysis process steps include: Step S1: Oxygen-free feeding: Start the nitrogen protection of the inert gas protection unit 4. After the oxygen content sensor of the jacketed preheated screw conveyor 21 detects a value of <1%, the fly ash enters the screw conveyor body 212 of the jacketed preheated screw conveyor 21 through the unloading valve 13. Step S2: Steam preheating: saturated steam with a pressure of 0.5-1.68MPa and a temperature of 270-306℃ from an external heat unit is introduced into the screw preheating jacket 213 of the preheating screw conveyor; the screw speed is adjusted by the variable frequency motor of the screw conveyor body 212 to control the residence time of fly ash in the preheating section to be 45-90 minutes, and the latent heat of steam is used to indirectly heat the fly ash from room temperature to 120-180℃; Step S3: Precision electric heating pyrolysis: The preheated fly ash enters the electromagnetic heating screw conveyor 22 through the discharge pipe 23; the electromagnetic induction coil 222 of the electromagnetic heating screw conveyor 22 works to directly heat the screw shell 221 and the fly ash material inside the screw shell 221; at the same time, through the temperature control of thermocouple 224 and multi-zone PID technology, the temperature of fly ash in the screw shell 221 is stabilized at 350-450℃ and maintained at this temperature for 45-90 minutes; Step S4: Rapid cooling and dioxin suppression: The high-temperature fly ash after pyrolysis by the electromagnetically heated screw conveyor 22 enters the two-stage water-cooled screw conveyor, which rapidly reduces the temperature to below 80°C within 10 minutes, quickly crossing the dioxin resynthesis temperature window of 250-450°C to prevent the secondary generation of dioxins during the cooling process. Step S5: Discharge: After being cooled by a two-stage water-cooled screw conveyor, the fly ash is discharged and then transported to the fly ash silo by a bucket elevator for later use. Step S6: In-situ destruction of exhaust gas: Dust-laden and VOCs-laden exhaust gas generated by pyrolysis of jacketed preheated screw conveyor 21 and electromagnetic heating screw conveyor 22 is first transported to pyrolysis gas dust collector 61 through pipelines. After dust removal, it is sent back to either the power plant incinerator air supply system or the combustion chamber for incineration, so as to utilize the power plant's existing flue gas purification system for treatment and complete the discharge operation without secondary pollution.

[0032] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A low-temperature pyrolysis device for fly ash preheated by low-pressure steam, characterized in that: The pyrolysis equipment includes a feeding unit (1), a pyrolysis unit (2), and a discharge cooling unit (3); the feeding unit (1) includes a fly ash storage bin (11) and a conveying pipe (12) installed at the output end of the fly ash storage bin (11), and a variable frequency star-shaped discharge valve (13) and a first-stage airlock valve (14) are installed sequentially on the conveying pipe (12); the pyrolysis unit (2) includes a jacketed preheating screw conveyor (21) and an electromagnetic heating screw conveyor (22); the jacketed preheating screw conveyor (21) includes a preheating shell (211), a screw conveyor body (212), and a preheating jacket (213); the preheating The shell (211) is distributed on the left and right, the screw conveyor body (212) is set inside the preheating shell (211), and the preheating jacket (213) adopts a fully enclosed steam jacket and is set outside the preheating shell (211); the input end of the preheating jacket (213) is also connected to an external heat unit (24) through an air inlet pipe (214). The external heat unit (24) can be either the steam extraction pipe of the waste incineration power plant turbine or the low-pressure steam of the boiler drum; and the fly ash is kept in the screw conveyor body (212) for 45-90 minutes, and the latent heat of steam is used to indirectly heat the fly ash from room temperature to 120-180℃; The electromagnetic heating screw conveyor (22) includes a screw shell (221) and an electromagnetic induction coil (222) wound around the outside of the screw shell (221), and an insulation layer (223) is also covered on the outside of the electromagnetic induction coil (222); multiple thermocouples (224) are also arranged in sections along the material conveying direction on the inner side wall of the screw shell (221); the thermocouples (224) adopt PID zone power regulation control to complete the pyrolysis operation of stabilizing the fly ash temperature at 350-450℃ and maintaining it at this temperature for 45-90 minutes; the preheating shell (211) and the screw shell (221) The pyrolysis unit (2) is connected to the discharge pipe (23); the pyrolysis unit (2) is also connected to the inert gas protection unit (4); the discharge cooling unit (3) includes a first-stage water-cooled screw conveyor (31) and a second-stage water-cooled screw conveyor (32) connected in series. The first-stage water-cooled screw conveyor (31) is connected to the electromagnetic heating screw conveyor (22) through a pipeline; and the first-stage water-cooled screw conveyor (31) is connected to a cooling water unit (5) so that the cooling water flows through the jacket of the two-stage water-cooled screw conveyor in sequence, and after indirect heat exchange with the high-temperature pyrolysis fly ash, it is sent to the power plant cooling tower through the return water pipeline for cooling treatment.

2. The fly ash low-temperature pyrolysis equipment using low-pressure steam preheating as described in claim 1, characterized in that: in, The pressure of the low-pressure steam is maintained at 0.5-1.68 MPa, and the temperature is maintained at 270-306℃.

3. The fly ash low-temperature pyrolysis equipment using low-pressure steam preheating as described in claim 2, characterized in that: The output end of the feed pipe (12) leads to the inner cavity of the preheating shell (211); the air inlet pipe (214) and the preheating jacket (213) are connected in a sealed manner by a second-stage airlock valve (15).

4. The low-temperature pyrolysis equipment for fly ash preheating using low-pressure steam as described in claim 3, characterized in that: in, The cooling water unit (5) is any one of the cooling water supply pipelines for the induced draft fan and primary air fan of the waste incineration power plant; wherein the pipeline of the cooling water unit (5) has a stable flow rate and appropriate pressure.

5. The fly ash low-temperature pyrolysis equipment using low-pressure steam preheating as described in claim 4, characterized in that: in, The total cooling area of ​​the two-stage water-cooled screw conveyor is large enough to rapidly cool the fly ash temperature to below 80°C within 10 minutes and complete the operation of quickly crossing the dioxin resynthesis temperature window of 250-450°C.

6. The fly ash low-temperature pyrolysis equipment using low-pressure steam preheating as described in claim 5, characterized in that: The inert gas protection unit (4) includes a nitrogen source (41), a protective gas pipe (42) connected to the nitrogen source (41), and a flow regulating valve group (43) set on the protective gas pipe (42); wherein, the interfaces of the protective gas pipe (42) are respectively arranged in the middle of the preheating shell (211) and the discharge end of the spiral shell (221) to maintain a slightly positive pressure oxygen-free environment in the equipment; wherein, an oxygen content sensor (215) is also provided in the preheating shell (211) so that when the oxygen content detection value in the preheating shell (211) is <1%, the fly ash enters the preheating shell (211) through the unloading valve (13).

7. The fly ash low-temperature pyrolysis equipment using low-pressure steam preheating as described in claim 6, characterized in that: One side of the jacketed preheating screw conveyor (21) and the electromagnetic heating screw conveyor (22) are respectively connected to the tail gas co-treatment unit (6) through the exhaust pipe (62); wherein, the tail gas co-treatment unit (6) adopts a pyrolysis gas dust collector (61), which introduces the tail gas into either the power plant incinerator air supply system or the combustion chamber after dust removal and performs high-temperature incineration deodorization operation.

8. A process for a low-temperature pyrolysis device for fly ash preheated with low-pressure steam, characterized in that: The fly ash is pyrolyzed using the above-mentioned pyrolysis equipment. The pyrolysis process includes the following steps: Step S1: Oxygen-free feeding: Start the nitrogen protection of the inert gas protection unit (4). After the oxygen content sensor of the jacketed preheated screw conveyor (21) detects <1%, the fly ash enters the screw conveyor body (212) of the jacketed preheated screw conveyor (21) through the unloading valve (13). Step S2: Steam preheating: saturated steam with a pressure of 0.5-1.68MPa and a temperature of 270-306℃ from an external heat unit is introduced into the preheating jacket (213) of the preheating screw conveyor; the screw speed is adjusted by the variable frequency motor of the screw conveyor body (212) to control the residence time of fly ash in the preheating section to be 45-90 minutes, and the latent heat of steam is used to indirectly heat the fly ash from room temperature to 120-180℃; Step S3: Precision electric heating pyrolysis: The preheated fly ash enters the electromagnetic heating screw conveyor (22) through the discharge pipe (23); the electromagnetic induction coil (222) of the electromagnetic heating screw conveyor (22) works to directly heat the screw shell (221) and the fly ash material inside the screw shell (221); at the same time, through the temperature control of the thermocouple (224) and multi-zone PID technology, the temperature of the fly ash inside the screw shell (221) is stabilized at 350-450℃ and maintained at this temperature for 45-90 minutes; Step S4: Rapid cooling and dioxin suppression: The high-temperature fly ash after pyrolysis by the electromagnetic heating screw conveyor (22) enters the two-stage water-cooled screw conveyor, which rapidly reduces the temperature to below 80°C within 10 minutes, quickly crossing the dioxin resynthesis temperature window of 250-450°C to prevent the secondary generation of dioxins during the cooling process; Step S5: Discharge: After being cooled by a two-stage water-cooled screw conveyor, the fly ash is discharged and then transported to the fly ash silo by a bucket elevator for later use. Step S6: In-situ destruction of exhaust gas: Dust-containing and VOCs-containing exhaust gas generated by pyrolysis of jacketed preheated screw conveyor (21) and electromagnetic heating screw conveyor (22) is first transported to pyrolysis gas dust collector (61) through pipeline. After dust removal, it is sent back to either the power plant incinerator air supply system or the combustion chamber for incineration, so as to utilize the existing flue gas purification system of the power plant to complete the discharge operation without secondary pollution.

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