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Discussion on Incineration Technology of Hazardous Waste in Rotary Kiln

Published:2021-01-01

In recent years, the technology of cement kiln collaborative disposal of solid waste is surging, many traditional cement production enterprises have set foot in solid waste disposal. In order to standardize the prevention and control of solid waste pollution in cement kiln collaborative disposal, the state has formulated relevant standards and policies: hj662-2013 technical specification for environmental protection of solid waste in cement kiln collaborative disposal, gb30485-2013 standard for pollution control of solid waste in cement kiln collaborative disposal, technical policy for pollution prevention and control of water sludge industry (Announcement No. 31, 2013 of Ministry of environmental protection), cement industry (No Technical index for collaborative disposal of solid waste (gb3073, 2014) and technical index for collaborative disposal of solid waste (gb3073, 2014) issued by the Ministry of cement industry. The new technologies encouraged by the policy include: pollutant emission reduction technology in the production process of cement kiln for collaborative disposal of solid waste; efficient utilization technology of cement kiln to improve collaborative disposal of solid waste, such as offline combustion system of large dosage of solid waste; efficient pretreatment technology for collaborative disposal of solid waste, such as high-quality refuse derived fuel (RDF) preparation technology, etc. Collaborative disposal of domestic waste in cement kiln is one of the advanced methods of domestic waste treatment. Different processes have different generation probability and quantity of dioxin. Different processes have different moisture content of garbage, calorific value of incineration gas (or pyrolysis gas), ignition starting mode, temperature of heat carrier, slag cooling mode and heat utilization efficiency. The correct analysis of the advantages and disadvantages of various processes is conducive to improving the technical level of domestic waste collaborative disposal in cement kiln and promoting the development of this technology.

1 rotary kiln incineration system

1.1 basic principle

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

1.2 typical process

Rotary kiln incinerator has a wide range of material applicability, which can simultaneously treat solid, liquid and gas hazardous waste. A rotary kiln is a hollow steel cylinder with a slightly sloping lining and firebrick, usually very long. After pretreatment and compatibility, all kinds of hazardous wastes enter the combustion system through different feeding methods. Under the driving force of self gravity and continuous rotation of rotary kiln, the furnace body turns over and fully contacts with combustion air to complete the drying (water evaporation), gasification and combustion process. Finally, the residual preheater forms slag, and the slag is discharged continuously through the water seal machine.

2 Analysis and Discussion on key equipment

2.1 vertical incinerator

Vertical incinerator is a new type of incinerator based on traditional vertical incinerator. Vertical incinerator has been applied in the process of domestic waste collaborative disposal in cement kiln because of its simple structure, convenient operation and moderate treatment capacity. It adopts the operation mode of upper feeding, lower discharging, upper discharging and lower (or waist) oxygen supply combustion, which is one of the most basic traditional calciners. The main function of vertical incinerator is combustion or incineration. The volume of inorganic slag is less than 5% of the original garbage volume. The solid matters in the inorganic slag are metal, ceramic, tile, glass, etc., and the metal matters are sorted and recycled; the remaining inorganic slag is sorted and used as cement raw materials; the high temperature flue gas, dust and incineration gas with calorific value generated by the combustion of organic matters are sent to the precalciner of cement kiln for heating and dioxin degradation.

2.2 rotary pyrolysis furnace

Rotary pyrolysis furnace is a kind of garbage pyrolysis furnace developed on the basis of ATP furnace and galot furnace of oil sand and oil shale refining industry based on the common technology of rotary kiln. Different from the conventional rotary kiln for calcination, the pyrolysis furnace mixes the solid heat carrier heated to a higher temperature (about 650 ℃) with the normal temperature garbage at the feed end of the pyrolysis furnace to transfer the heat to the garbage, so that the garbage can reach the pyrolysis temperature (400 ~ 450 ℃) in the state of no oxygen or less oxygen, and generate the pyrolysis gas and slag with little dioxin content. After the slag is continuously heated to a higher temperature (650 ~ 750 ℃) at the kiln head (discharge end), part of the high temperature fine slag is screened in the furnace and sent to the kiln head feed end again for mixed pyrolysis with garbage. Part of the pyrolysis gas is used as fuel to supply heat for the pyrolysis furnace, and the surplus pyrolysis gas is sent to the precalciner of cement kiln as fuel and degraded in the precalciner. The pyrolysis gas at the heating end of the pyrolysis furnace produces high-temperature flue gas (above 800 ℃) with less oxygen content after combustion. The heat is also transferred to the garbage during the flow process from the kiln head to the kiln tail, which also achieves the effect of pyrolysis. The way of waste pyrolysis is based on the solid heat carrier method and high temperature flue gas contact method. During the whole pyrolysis process, the probability of contact between garbage and oxygen is very small, so the amount of dioxin is also small.

2.3 dryer

The heat source of the garbage dryer comes from part or all of the high temperature exhaust gas of the grate cooler (about 250 ℃). No matter grate dryer or rotary dryer, the average temperature of garbage in dryer is about 150 ℃. The drying gas is used in grate cooler as cooling gas and finally enters the kiln, so that the waste heat of drying gas can be reasonably utilized.

3. Analysis and Discussion on key process

3.1 furnace type and dioxin formation

The function difference between pyrolysis furnace and vertical incinerator is pyrolysis and incineration. Although the final inorganic slag is basically the same, the mechanism of pyrolysis and combustion is essentially different. The former is a chemical decomposition process in which the organic matter is heated in the absence of oxygen or oxygen to split the high molecular hydrocarbon chain into low molecular hydrocarbon based gas, medium molecular fuel and carbon black mixture, while the latter is an exothermic oxidation reaction of hydrocarbons in the presence or enrichment of oxygen to generate CO2 and H2O. A lot of dioxins and furans are easily produced by incineration, and dioxins are one of the most toxic organic compounds in the world. The oxidation reaction of combustion and chlorine in plastic products are the environmental conditions for the formation 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, which can regenerate dioxins at the temperature of 300 ~ 500 ℃. Garbage pyrolysis is a process of heating garbage at high temperature in anaerobic or anoxic environment. The pyrolysis of MSW not only eliminates the formation conditions of oxidation reaction, but also reduces the formation of dioxin precursors, which fundamentally inhibits the generation of dioxin. Because the heavy metals in garbage have no high-temperature oxidation conditions, it is not easy to form catalysts to promote the formation of dioxins, so it can be said that dioxins can be eliminated from the source.

3.2 oxygen consumption

The amount of CO2 generated by the vertical incinerator collaborative disposal process is about 20% of the water content (higher in summer) without drying and dehydration before incineration, and it even needs to mix an appropriate amount of coal in the garbage to be incinerated. Therefore, the amount of CO2 generated is high without pre drying. At the same time, due to the high moisture content of the waste, the calorific value of the incineration gas is lower than 3500kj / Nm3, which has a certain impact on the stability of the calciner.

Conclusion: in recent years, with the increase of national supervision on hazardous waste. Large scale rotary kiln incineration technology will be widely used in hazardous waste treatment.

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