A system and method for co-processing fly ash dioxins in cement kilns

Abstract

The present invention discloses a system and method for the coordinated disposal of fly ash dioxins in a cement kiln, the system comprising a hot air unit, a thermal desorption unit, and a decomposition unit connected in sequence. In this system, fly ash in a hot air driven state undergoes dioxin desorption and separation simultaneously in a thermal desorber. Dioxins and dioxin synthesis precursors that are desorbed into the gas phase by hot air heating are separated from the fly ash solids once desorbed, thereby avoiding the secondary synthesis of dioxins and greatly improving the disposal efficiency of fly ash dioxins. At the same time, the excess heat energy of the cement production line is fully utilized for the thermal desorption of fly ash dioxins, further improving the thermal energy utilization efficiency of the cement plant while achieving harmless disposal and resource utilization of fly ash. The thermal desorption units provided by the present invention can be combined into multi-stage thermal desorption units, which can be flexibly adjusted according to actual conditions to be applicable to various cement plants, waste incineration sites, and other scenarios.

Description

Technical Field

[0001] The present invention relates to the field of harmless disposal of fly ash, in particular to a system and method for the coordinated disposal of fly ash dioxins in a cement kiln. Background Art

[0002] Municipal solid waste incineration technology has become one of the most important and effective waste treatment technologies worldwide due to its advantages in harmlessness, resource utilization, and waste reduction. Fly ash generated by incineration of municipal solid waste accounts for approximately 3% to 5% of the total incineration volume. It contains high concentrations of toxic and hazardous substances such as heavy metals and dioxins. The National List of Hazardous Wastes clearly defines it as hazardous waste and requires harmless treatment. Currently, commonly used fly ash treatment technologies include separation and extraction, thermal treatment, and stabilization / solidification. However, with the increasing amount of fly ash generated, conventional stabilized landfill treatment technology is unable to achieve the goals of harmlessness and resource utilization due to limitations such as limited technical maturity, inability to recover industrial salt, large land occupation, and short-term solidification effects.

[0003] Fly ash contains soluble salts and components similar to cement clinker, such as CaO, SiO2, and Al2O3, accounting for approximately 70% of the total fly ash volume. Because fly ash's primary components are similar to cement, it can be used to replace some of the raw materials used in cement production. Consequently, cement kiln fly ash co-processing technology has been widely promoted and applied both domestically and internationally. The conventional co-processing process involves washing the fly ash to recover the industrial salts. The ash is then fed into the cement kiln production line, where it undergoes high-temperature calcination to decompose dioxins and solidify heavy metals, achieving harmless disposal and resource utilization.

[0004] Because fly ash contains dioxins after washing, it is not permitted to be added from the raw material end. Instead, it is typically added to the cement production line's precalciner or at the kiln tail. Improperly controlling the addition rate can easily cause fluctuations and impact cement production conditions. Furthermore, due to the high dioxin content in fly ash, it is difficult to completely decompose the dioxins after high-temperature calcination in the cement kiln, potentially leading to excessive dioxin levels in the kiln tail gas.

[0005] In view of this, a technical solution to the above problem is provided. Summary of the Invention

[0006] In order to overcome the shortcomings of the existing technology, one of the objectives of the present invention is to provide a system for the coordinated disposal of fly ash dioxins in cement kilns with a simple structure and high desorption efficiency. To achieve the above objectives, the present invention provides the following technical solutions:

[0007] The present invention provides a system for co-processing fly ash dioxins in a cement kiln, comprising a hot air unit, a thermal desorption unit and a decomposition unit connected in sequence; wherein,

[0008] A hot air unit, which is an air outlet device of a grate cooler in a cement production line, and provides hot air for conveying fly ash into the thermal desorption unit and heating the fly ash;

[0009] A thermal desorption unit includes a thermal desorber and a dust collector connected to the thermal desorber, wherein dioxins in the fly ash are thermally desorbed in the thermal desorption unit and separated from the fly ash;

[0010] The decomposition unit is a decomposition furnace or cement kiln in a cement production line, connected to the dust collector, and decomposes the separated dioxins.

[0011] Preferably, the thermal desorber is any one of a cyclone dust collector or a gravity settling dust collector, and the dust collector is any one of a high-temperature electrostatic precipitator, a high-temperature ceramic membrane dust collector or a high-temperature metal membrane dust collector.

[0012] Preferably, the hot air temperature in the hot air unit is 550-850°C.

[0013] Preferably, the cement kiln fly ash dioxin co-treatment system further comprises a fly ash recycling unit, and the fly ash recycling unit comprises a fly ash washing unit.

[0014] Preferably, the cement kiln fly ash dioxin co-treatment system comprises at least two thermal desorption units, adjacent thermal desorption units are connected in series, and each of the at least two thermal desorption units is connected to a hot air unit.

[0015] Preferably, the thermal desorption unit further comprises a fly ash conveying device, which is respectively connected to the thermal desorber and dust collector in the thermal desorption unit, for collecting the fly ash separated by the thermal desorber and dust collector, and conveying the fly ash to an adjacent thermal desorption unit.

[0016] Preferably, the fly ash conveying device is any one of a screw conveyor, a scraper conveyor or a tube chain conveyor.

[0017] Another object of the present invention is to provide a convenient, flexible, and resource-efficient method for co-processing fly ash dioxins in cement kilns. To achieve the above object, the present invention provides the following technical solutions:

[0018] The present invention provides a method for co-processing fly ash dioxins in a cement kiln applied to the above system, comprising the following steps:

[0019] The fly ash is transported to the thermal desorber in the thermal desorption unit and heated by hot air to thermally desorb dioxins. The desorbed dioxins enter the dust collector in the thermal desorption unit for separation to obtain flue gas containing dioxins.

[0020] The flue gas containing dioxins treated by the thermal desorption unit is input into the decomposition unit to decompose the dioxins therein.

[0021] Further, the following steps are included:

[0022] The fly ash is sequentially separated through at least two thermal desorption units, and the dioxin-containing flue gas separated by each thermal desorption unit is input into a decomposition unit for decomposition.

[0023] Further, the following steps are included:

[0024] The fly ash after dioxin separation by thermal desorption unit is subjected to multi-stage water washing and evaporation crystallization to recover soluble salts in the fly ash, and the washed fly ash enters the raw meal mill.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] 1) The system provided by the present invention can work in conjunction with a cement kiln, making full use of the hot air generated by the grate cooler of the cement kiln and the high temperature environment of the cement kiln to thermally desorb and completely decompose dioxins, greatly improving the resource utilization efficiency of the cement plant.

[0027] 2) The system and method for co-processing dioxins from fly ash in cement kilns provided by this invention are simple to use. Simply add fly ash to the system's feed port to automatically initiate thermal desorption. Direct heating of the fly ash using hot air ensures uniform heating and high heat transfer efficiency.

[0028] 3) Since the fly ash is always desorbed and separated from dioxins in a hot air circulation environment, the dioxins and dioxin synthesis precursors that are desorbed into the gas phase by hot air heating are separated from the fly ash solids once they are desorbed, avoiding the secondary synthesis of dioxins and greatly improving the disposal efficiency of fly ash dioxins.

[0029] 4) The thermal desorption units provided by this invention can be combined into multi-stage thermal desorption units, allowing for flexible adaptation to various cement plants, waste incineration sites, and other scenarios. Since the fly ash produced after multi-stage thermal desorption contains virtually no dioxins, this not only reduces washing costs but also allows the washed fly ash to be added directly to the cement raw material feed without concerns about fluctuations and impacts on cement production conditions or excessive dioxin production at the kiln tail, thereby improving fly ash resource utilization. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 A schematic diagram of a system for co-processing fly ash dioxins in a cement kiln provided by the present invention;

[0031] Figure 2 A schematic diagram of a two-stage thermal desorption cement kiln system for co-processing fly ash dioxins provided by the present invention;

[0032] Figure 3 A schematic diagram of a three-stage thermal desorption cement kiln system for co-processing fly ash dioxins provided by the present invention. DETAILED DESCRIPTION

[0033] For the convenience of explanation, the scheme of the present invention is described in detail below with reference to the accompanying drawings. Obviously, this is only for the purpose of illustration, and the described embodiments are only part of the embodiments, not all of the embodiments.

[0034] See also Figures 1 to 3 , a cement kiln fly ash dioxin co-treatment system, comprising a hot air unit, a thermal desorption unit and a decomposition unit connected in sequence; wherein,

[0035] A hot air unit, which is an air outlet device of a grate cooler in a cement production line, and provides hot air for conveying fly ash into the thermal desorption unit and heating the fly ash;

[0036] A thermal desorption unit includes a thermal desorber and a dust collector connected to the thermal desorber, wherein dioxins in the fly ash are thermally desorbed in the thermal desorption unit and separated from the fly ash;

[0037] The decomposition unit is a decomposition furnace or cement kiln in a cement production line, connected to the dust collector, and decomposes the separated dioxins.

[0038] The fly ash is transported to the thermal desorber in the thermal desorption unit and heated by hot air to thermally desorb dioxins. The desorbed dioxins enter the dust collector in the thermal desorption unit for separation to obtain flue gas containing dioxins.

[0039] The flue gas containing dioxins treated by the thermal desorption unit is input into the decomposition unit to decompose the dioxins therein.

[0040] This system can work in conjunction with the cement kiln system, using the high-temperature hot air generated by the cement kiln grate cooler to thermally desorb dioxins from fly ash, and using the high-temperature environment of the cement kiln to completely decompose dioxins under conditions of temperatures above 850°C and a residence time of more than 2 seconds. This not only completely treats dioxins, but also further improves the utilization efficiency of the cement plant's thermal energy.

[0041] In one embodiment, the thermal desorber is any one of a cyclone dust collector or a gravity sedimentation dust collector, and the dust collector is any one of a high-temperature electrostatic precipitator, a high-temperature ceramic membrane dust collector or a high-temperature metal film dust collector; wherein the thermal desorber is preferably a cyclone dust collector, and the dust collector is preferably a ceramic membrane dust collector or a high-temperature metal film dust collector.

[0042] In one embodiment, the hot air temperature generated by the hot air unit is 550-850°C, specifically 550°C, 600°C, 650°C, 700°C, 750°C, 800°C, 850°C or any value between 550°C and 850°C.

[0043] In one embodiment, the system for co-processing fly ash dioxins in cement kilns further includes a fly ash recycling unit, and the fly ash recycling unit includes a fly ash washing unit.

[0044] In one embodiment, the thermal desorption unit further includes a fly ash conveying device, which is connected to the thermal desorber and dust collector in the thermal desorption unit, respectively, and is used to collect fly ash separated by the thermal desorber and dust collector and convey the fly ash to an adjacent thermal desorption unit. The fly ash conveying device can be a screw conveyor, a scraper conveyor, or a tube chain conveyor, preferably a screw conveyor.

[0045] It is known that dioxins have a melting point of 303-304°C and a boiling point of 421-446°C. When fly ash is heated to 400-600°C, almost all dioxins in the fly ash are desorbed into the gas phase. The inventors discovered through research that conventional thermal desorption treatments for dioxins often utilize heating and insulation within a certain space. In addition to dioxins, fly ash also contains a large amount of dioxin synthesis precursors, such as aromatic compounds like chlorobenzene and chlorophenol. Desorbed fly ash is highly susceptible to secondary synthesis of dioxins at the thermal desorption temperature, resulting in low thermal desorption efficiency and high treatment costs. Based on this, the inventors provide a fly ash dioxin desorption treatment system that operates under a continuous hot air circulation environment. In this system, fly ash in a hot air driven state undergoes dioxin desorption and separation simultaneously in a thermal desorber. Dioxins and dioxin synthesis precursors that are desorbed into the gas phase by hot air heating are separated from the fly ash solids once desorbed, thus avoiding the secondary synthesis of dioxins and greatly improving the disposal efficiency of dioxins in fly ash.

[0046] In one embodiment, the cement kiln fly ash dioxin co-processing system includes at least two thermal desorption units, with adjacent thermal desorption units connected in series. Each of the at least two thermal desorption units is connected to a hot air unit. Fly ash is sequentially separated by the at least two thermal desorption units, and the dioxin-containing flue gas separated by each thermal desorption unit is fed into a decomposition unit for decomposition.

[0047] It is understood that to ensure that all fly ash is heated to the required temperature, the hot air temperature generated by the hot air unit can be increased, multiple thermal desorption units can be installed, or insulation measures can be added during the fly ash transportation process. The number of multi-stage thermal desorption units can also be freely adjusted based on the actual fly ash processing volume and the required emission requirements. For example, the number of multi-stage thermal desorption units can be 2, 3, 4, 5, or 6.

[0048] In one embodiment, the system includes a first thermal desorption unit and a second thermal desorption unit connected to the first thermal desorption unit, each thermal desorption unit includes a thermal desorber and a dust collector, and the fly ash desorbed and separated by the thermal desorber and dust collector in the first thermal desorption unit enters the thermal desorber of the second thermal desorption unit, and each dust collector is connected to the decomposition unit, and each thermal desorber is connected to the hot air unit.

[0049] In one embodiment, the system also includes a third thermal desorption unit connected to the second thermal desorption unit, and the third thermal desorption unit includes a thermal desorber and a dust collector. The fly ash desorbed and separated by the thermal desorber and dust collector in the second thermal desorption unit enters the thermal desorber of the third thermal desorption unit, and each dust collector is connected to the decomposition unit, and each thermal desorber is connected to the hot air unit.

[0050] In one embodiment, the fly ash after dioxin separation by thermal desorption unit is subjected to multi-stage water washing and evaporation crystallization to recover soluble salts in the fly ash, and the washed fly ash is fed into the raw mill.

[0051] The "Technical Specification for Environmental Protection of Co-processed Solid Waste in Cement Kilns" (HJ662-2013) stipulates that the chlorine content of feed materials should not exceed 0.04%. Because the dioxin treatment method provided by this invention removes the vast majority of dioxins and their chlorine-containing precursors, it reduces water consumption and washing time during washing, lowering washing costs. After washing, the dioxin-free fly ash can be directly added to the cement raw meal, enabling better resource utilization.

[0052] The solution provided by the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0053] See also Figures 2-3 , Figure 2 This is a schematic diagram of a two-stage thermal desorption cement kiln system for co-processing fly ash dioxins provided by the present invention. Figure 3 This is a schematic diagram of a three-stage thermal desorption cement kiln system for co-processing fly ash dioxins provided by the present invention. For the convenience of explanation, the following examples are based on Figures 2-3 The scene shown is carried out.

[0054] Example 1

[0055] An application such as Figure 2The system depicted in the paper describes a method for co-processing dioxins from fly ash in a cement kiln. The hot air unit in the system is the air outlet of a grate cooler in a cement production line, where the hot air generated by the grate cooler is at a temperature of 800°C. The decomposition unit is a decomposition furnace in the cement production line. Cyclone dust collectors are used as thermal desorbers at each stage, and high-temperature electrostatic precipitators are used as dust collectors at each stage. Fly ash is transported between the thermal desorption units via a screw conveyor. The method includes the following steps:

[0056] The fly ash is transported by hot air into the thermal desorber in the first thermal desorption unit for thermal desorption treatment to produce first flue gas and first fly ash. The first flue gas enters the dust collector in the first thermal desorption unit for further treatment to produce second flue gas and second fly ash. The second flue gas is input into the decomposition unit to decompose dioxins therein.

[0057] The first fly ash and the second fly ash enter the thermal desorber in the second thermal desorption unit for thermal desorption treatment to obtain the third flue gas and the third fly ash; the third flue gas enters the dust collector in the second thermal desorption unit for further treatment to obtain the fourth flue gas and the fourth fly ash; the fourth flue gas is input into the decomposition unit to decompose the dioxins therein, and the third fly ash and the fourth fly ash are recycled.

[0058] After secondary thermal desorption, the fly ash is heated to 450°C, where dioxins are desorbed and fed into the decomposition furnace, where they are thoroughly decomposed at temperatures exceeding 850°C and a residence time greater than 2 seconds. The desorbed fly ash then enters a water washing unit, where soluble potassium and sodium salts are recovered through a multi-stage water washing and evaporation crystallization process. After washing, the fly ash can be fed directly into the raw mill, where it can be mixed with other raw materials in cement production to produce cement clinker, or fed into the cement grinding system as a cement admixture, effectively reusing the fly ash as a resource.

[0059] Example 2

[0060] An application such as Figure 3 The system depicted in the present invention describes a method for co-processing fly ash dioxins in a cement kiln. The hot air unit in the system is the air outlet of the grate cooler in the cement production line, where the hot air generated by the grate cooler has a temperature of 700°C. The decomposition unit is the decomposition furnace in the cement production line. Each thermal desorber utilizes a gravity settling dust collector, and each dust collector utilizes a high-temperature ceramic membrane dust collector. Fly ash between each thermal desorption unit is transported via a screw conveyor. Furthermore, an insulation jacket is installed around the screw conveyor transporting the fly ash. The method includes the following steps:

[0061] The fly ash is transported by hot air into the thermal desorber in the first thermal desorption unit for thermal desorption treatment to produce first flue gas and first fly ash. The first flue gas enters the dust collector in the first thermal desorption unit for further treatment to produce second flue gas and second fly ash. The second flue gas is input into the decomposition unit to decompose dioxins therein.

[0062] The first fly ash and the second fly ash enter the thermal desorber in the second thermal desorption unit for thermal desorption treatment to produce a third flue gas and a third fly ash; the third flue gas enters the dust collector in the second thermal desorption unit for further treatment to produce a fourth flue gas and a fourth fly ash; the fourth flue gas is input into the decomposition unit to decompose dioxins therein;

[0063] The third fly ash and the fourth fly ash enter the thermal desorber in the third thermal desorption unit for thermal desorption treatment to obtain the fifth flue gas and the fifth fly ash; the fifth flue gas enters the dust collector in the third thermal desorption unit for further treatment to obtain the sixth flue gas and the sixth fly ash, the sixth flue gas is input into the decomposition unit to decompose the dioxins therein, and the fifth fly ash and the sixth fly ash enter the water washing unit for water washing.

[0064] After three stages of thermal desorption, the fly ash is heated to 550°C, almost completely desorbing dioxins from the fly ash before entering the decomposition furnace for thorough decomposition. The desorbed fly ash then enters a water washing unit for washing. A multi-stage water washing and evaporation crystallization process recovers the soluble potassium and sodium salts. Since the washed fly ash is free of dioxins and free heavy metal ions, it can be directly fed into the raw mill and used as an auxiliary material for cement clinker production after being mixed with other raw materials. Alternatively, it can be fed into the cement grinding system as a cement admixture, effectively realizing the resourceful utilization of fly ash.

[0065] In the above-described embodiment, the thermal desorption unit and cement kiln work synergistically to improve the efficiency of cement kiln resource utilization. The hot air from the grate cooler provides both heat and wind energy, allowing fly ash to undergo two separation and desorption steps within each thermal desorption unit, preventing dioxin re-synthesis. Fly ash is heated and desorbed by multiple thermal desorption units, reaching temperatures exceeding 400°C and containing virtually no dioxins. The desorbed dioxins are then fed into the cement kiln's decomposition furnace for complete decomposition and elimination.

[0066] Compared to Example 1, Example 2 adds a first-stage thermal desorption unit and installs a heat preservation device outside the fly ash device, so that the fly ash ultimately reaches a higher temperature and achieves a better desorption effect. In actual applications, the above embodiments can be combined with the inventive concept of the present invention to achieve the desired effect based on actual needs.

[0067] The above embodiments are only preferred embodiments of the present invention and cannot be used to limit the scope of protection of the present invention. Any non-substantial changes and replacements made by technicians in this field on the basis of the present invention fall within the scope of protection required by the present invention.

Claims

1. A method for co-processing fly ash dioxins in a cement kiln, characterized in that: A cement kiln co-processing system for fly ash dioxins is used for treatment, the system comprising a hot air unit, a thermal desorption unit and a decomposition unit connected in sequence; The hot air unit is the air outlet device of the grate cooler in the cement production line, providing hot air to send the fly ash into the thermal desorption unit and heat the fly ash; The thermal desorption unit includes a thermal desorber and a dust collector connected to the thermal desorber. Dioxins in the fly ash are thermally desorbed in the thermal desorption unit and separated from the fly ash. The decomposition unit is a decomposition furnace or cement kiln in a cement production line, connected to a dust collector to decompose the separated dioxins; The system for co-processing fly ash dioxins in a cement kiln comprises at least two thermal desorption units, adjacent thermal desorption units are connected in series, and each of the at least two thermal desorption units is connected to a hot air unit; The thermal desorber is any one of a cyclone dust collector or a gravity settling dust collector, and the dust collector is any one of a high-temperature electric dust collector, a high-temperature ceramic membrane dust collector or a high-temperature metal membrane dust collector; The following steps are involved: The fly ash is transported to the thermal desorber in the thermal desorption unit and heated by hot air to thermally desorb dioxins. The desorbed dioxins enter the dust collector in the thermal desorption unit for separation to obtain flue gas containing dioxins. The dioxin-containing flue gas treated by the thermal desorption unit is fed into the decomposition unit to decompose the dioxins therein; The fly ash is sequentially separated through at least two thermal desorption units, and the dioxin-containing flue gas separated by each thermal desorption unit is input into a decomposition unit for decomposition.

2. The method for co-processing fly ash dioxins in a cement kiln according to claim 1, characterized in that: The following steps are involved: The fly ash after dioxin separation by thermal desorption unit is subjected to multi-stage water washing and evaporation crystallization to recover soluble salts in the fly ash, and the washed fly ash enters the raw meal mill.

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