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Talking about the Incineration Technology of Hazardous Waste Rotary Kiln

Published 2020/12/31 14:15:35

In recent years, cement kiln co-processing solid waste technology has been surging, and many traditional cement production companies have set foot in solid waste disposal. In order to standardize the prevention and control of cement kiln co-processing solid waste pollution, the state has formulated relevant standards and policies: HJ662-2013 "Cement Kiln Co-processing Solid Waste Environmental Protection Technical Specifications", GB30485-2013 "Cement Kiln Co-processing Solid Waste Pollution Control Standards", "Cement Industry Pollution Prevention Technology Policy" (Ministry of Environmental Protection Announcement 2013 No. 31), "Cement Industry Cleaner Production Evaluation Index System" (National Development and Reform Commission Announcement 2014 No. 3), GB30760—2014 "Cement Kiln Co-processing Solids Waste Technical Specifications, "Cement Kiln Co-processing Solid Waste Pollution Prevention Technical Policy" (promulgated by the Ministry of Environmental Protection in 2017). The new technologies that the policy encourages to research and develop include: pollutant emission reduction technology in the production process of cement kilns that co-process solid waste; high-efficiency utilization technologies in cement kilns that increase the amount of co-processing solid waste, such as large-volume solid waste offline combustion systems; High-efficiency pretreatment technology for co-processing solid waste, such as high-quality refuse derived fuel (RDF) preparation technology, etc. Cement kiln co-processing domestic waste is one of the advanced methods of domestic waste treatment. Different processes have different dioxin generation probability and production amount. The waste of different processes enters the furnace moisture, produces incineration gas (or pyrolysis gas) calorific value, ignition start method, heat carrier entering furnace temperature, slag cooling method, heat Utilization efficiency is also different. Correct analysis of the advantages and disadvantages of various processes is beneficial to improve the technical level of cement kiln co-processing domestic waste and promote the development of this technology.


1 Rotary Kiln Incineration System


1.1 Basic principles


Rotary kiln incinerator, also called rotary kiln, evolved from cement rotary kiln. The main body is a rotatable horizontal cylindrical shell, which is made of steel plate and lined with refractory materials. Protection of pipe axis and water level. With a certain inclination, the solid and semi-solid wastes enter the kiln from the high end (head) through the feeder, and slowly move to the tail as the cylinder rotates. The rotation of the kiln body makes the materials fully contact in the process of combustion and combustion air, and completes the whole process of drying, combustion, combustion, and cylinder block. The characteristics of entering the combustion chamber and fuel slag are discharged by the ash and slag of the aircraft. According to the different flow directions of the furnace gas and solids, or the position of the heat source (burner) in the rotary kiln, the rotary kiln can be divided into two types, upstream and downstream, and preserved in the forward direction of the rotary kiln. The flue gas flow is the same, also called countercurrent. The design of the rotary kiln is suitable for feeding and pretreatment, which can increase the time of flue gas, and is widely used in hazardous waste incineration systems. Countercurrent rotary kiln is more suitable for hazardous wastes (such as sludge) with high moisture content or low calorific value.


1.2 Typical process


The rotary kiln incinerator has a wide range of material applicability, and can simultaneously treat solid, liquid and gaseous hazardous waste. The rotary kiln is a hollow steel cylinder with a slightly inclined lining and refractory bricks, which is usually very long. After pretreatment and compatibility, all kinds of hazardous waste enter the combustion system through different feeding methods. Under the driving force of its own gravity and the continuous rotation of the rotary kiln, the furnace body is turned over and the combustion air is fully contacted to complete the drying (water evaporation), In the process of gasification and combustion, the remaining preheater becomes slag, and the slag is continuously discharged through the water sealer.


2 Analysis and discussion of key equipment


2.1 Vertical incinerator


The vertical incinerator is a furnace type suitable for waste incineration based on the traditional vertical furnace. Vertical incinerators are used in cement kiln co-processing of domestic waste due to their simple structure, convenient operation and moderate processing capacity. It adopts the operation mode of upper feeding, lower discharging, upper air outlet, and lower (or waist) oxygen combustion. It is one of the most basic traditional calciners. The main function of a vertical incinerator is combustion or incineration. The organic ash slag after burning or incineration of domestic garbage enters the cement kiln system directly with incineration gas, and the volume of inorganic slag is less than 5% of the original garbage volume. The solids in the inorganic slag are metal, ceramic chips, bricks and tiles, glass, etc. The metal is sorted and recycled; the remaining inorganic slag is sorted and used as cement raw materials; the organic matter is burned to produce high-temperature flue gas, dust, and incineration gas containing calorific value. The precalciner of the cement kiln supplies heat and degrades dioxins.


2.2 Rotary pyrolysis furnace


The rotary pyrolysis furnace is a garbage pyrolysis furnace developed on the basis of the ATP furnace and the Galet furnace in the oil sands and oil shale refining industry based on the common technology of rotary kilns. Different from the conventional rotary kiln used for calcination, the pyrolysis furnace mixes the solid heat carrier heated to a higher temperature (about 650°C) with the normal temperature waste at the feed end of the pyrolysis furnace to transfer heat to the waste, so that the waste is free of charge. It reaches the pyrolysis temperature (400~450℃) in the state of oxygen or less oxygen, and generates pyrolysis gas and slag with very little dioxin content. The slag will continue to be heated to a higher temperature (650~750℃) at the kiln head (discharge end) and then be screened in the furnace to send part of the high-temperature fine slag to the kiln head feed end to be mixed with garbage for pyrolysis. Part of the pyrolysis gas is used as fuel to supply heat to the pyrolysis furnace; the excess pyrolysis gas is used as fuel to be sent to the cement kiln precalcining furnace and degraded in the precalcining furnace. The pyrolysis gas at the heating end of the pyrolysis furnace is burned to produce high-temperature flue gas (above 800°C) with less oxygen content. The heat is also transferred to the waste during the flow from the kiln head to the kiln end, which also achieves the effect of pyrolysis. The way of waste pyrolysis is based on the above-mentioned solid heat carrier method and high temperature flue gas contact method. During the entire pyrolysis process, the probability of the waste coming into contact with oxygen is extremely small, so the amount of dioxin produced is also small.


2.3 Dryer


The heat source of the garbage dryer comes from part or all of the high temperature exhaust gas (about 250°C) of the grate cooler. Whether it is a grate dryer or a rotary dryer, the average temperature of the garbage in the dryer is about 150°C. The drying gas is used in the grate cooler as the cooling gas and finally enters the kiln, so that the residual heat of the drying gas can be reasonably used.


3 Key process analysis and discussion


3.1 Furnace type and dioxin generation


The difference between pyrolysis furnace and vertical incinerator is pyrolysis and incineration. Although the final inorganic slag is basically the same, the pyrolysis and combustion mechanism are fundamentally different. The former is the chemical decomposition process of heating organic matter in an oxygen-free or hypoxic state to crack polymer hydrocarbon chains into low-molecular hydrocarbon-based fuel gas, medium-molecular fuel oil and carbon black mixture. The latter is hydrocarbon Compounds undergo an exothermic oxidation reaction under oxygen or oxygen-rich conditions to generate CO2 and H2O. Incineration is prone to produce large amounts of dioxins and furans, and dioxins are currently one of the most toxic organic substances in the world. The oxidation reaction of combustion and the chlorine in plastic products are the environmental conditions for the generation of dioxins. Although the high temperature above 800℃ can decompose most of the dioxins, the hot active atoms of heavy metals in the combustion become the catalyst for the reduction of dioxins. Dioxins are regenerated in a temperature environment of 300~500℃. Garbage pyrolysis is the process of thermally cracking garbage by heating it at high temperature in an oxygen-free or anoxic environment. Waste pyrolysis not only eliminates the conditions of oxidation reaction, but also reduces the generation of dioxin precursors, which fundamentally inhibits the generation of dioxin. Since the heavy metals in the garbage do not have the conditions for high-temperature oxidation, it is difficult to generate catalysts that promote the formation of dioxin, so it can be said that the formation of dioxin is eliminated from the source.


3.2 Oxygen consumption


CO2 production The waste of the vertical incinerator co-processing process is not dried and dehydrated before incineration, with a water content of about 20% (higher in summer), and it even needs to be mixed with an appropriate amount of coal to be incinerated. Therefore, the amount of CO2 generated by the garbage that is not pre-dried is high. At the same time, the heating value of the incineration gas is low due to the high water content of the garbage, and the heating value of the incineration gas is lower than 3500kJ/Nm3, which has a certain influence on the stability of the calciner.


Conclusion: In recent years, as the country has increased its supervision of hazardous waste. Large-scale rotary kiln incineration technology will be more widely used in hazardous waste treatment.


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