Hazardous waste (hereinafter referred to as hazardous waste) is of various types, complex composition, and serious regional differences. Improper treatment will bring a series of livelihood and social problems. Practice has proved that high-temperature incineration is an effective way to safely dispose of hazardous waste and reduce its volume. Among them, the rotary kiln incineration process is one of the most effective disposal processes for hazardous waste treatment, which has the advantages of treating many types of hazardous waste and large processing volume.
In recent years, there have been many kiln shutdown accidents caused by refractory damage, which has seriously affected the stable operation rate of the hazardous waste incineration production line, and put forward urgent needs for the optimization of the refractory lining design and efficient operation of the rotary kiln hazardous waste incinerator.
1. Rotary kiln body
The most critical equipment of the rotary kiln incinerator is the rotary kiln body. Because it is a dynamic operation equipment, it has high requirements for the lining structure design. First of all, it is necessary to ensure the stability of the overall lining of the refractory lining under the dynamic high-temperature state operation to prevent the refractory bricks from falling bricks and drawing. For the hazardous waste rotary kiln system, due to the relatively slow speed, generally below 1 rpm, different structures such as single-layer bricks, double-layer bricks, composite bricks and integral casting can be selected according to the plant design and energy-saving requirements. The total thickness of the rotary kiln refractory material is generally 250~300mm. The material of the working layer refractory bricks is generally different according to the processing temperature and the type of hazardous waste. High-alumina, corundum mullite and chromium-zirconium corundum refractory materials can be selected. When a single-layer structure with a thickness of 230~300mm is used, since the thermal conductivity of the refractory material is mostly above 1.7~2.0, the temperature of the outer wall of the kiln in the later high-temperature zone reaches above 350℃, but the overall structure is stable, which is suitable for rotary kiln projects outdoors.
The double-layer structure of insulation bricks and refractory bricks is used. Because the thermal conductivity of the insulation bricks is low, it can reduce the temperature of the kiln skin. The temperature of the outer wall of the kiln can be controlled at around 220℃ or even below, with good energy-saving effects, but the construction requirements are high, which is suitable for indoor projects. It should be noted that the strength of the insulation brick is generally required to be greater than 20MPa, and the thickness of the insulation brick cannot be less than 50mm.
In order to achieve the stability of a single-layer brick and reduce the temperature of the outer wall, a composite brick can be designed, that is, the working surface is a corundum brick or a chrome corundum brick, and the back is a hollow alumina ball, high aluminum or clay, and the thickness of the insulation layer is about 50~70mm. This product has low production efficiency, and because the thermal expansion coefficient and sintering shrinkage rate of the working layer and the insulation layer are different, micro cracks are easily generated at the connection, and the scrap rate is high.
In view of this, a groove-type composite brick design can be adopted, the working layer material remains unchanged, and the tail is slotted and filled with insulation materials such as nanoboard. This groove brick-type composite brick is not only easy to produce and construct, but also can achieve the effect of reducing the temperature of the outer wall of the cylinder by about 30~50℃.
In addition, some rotary kilns use lightweight castables or fiberboards as insulation materials, and the working layer uses a heavy castable (metal nail anchoring) double-layer casting structure, which can reasonably control the temperature of the outer wall of the rotary kiln. This structure uses metal parts to connect the refractory material and the cylinder as one, and there is no problem of refractory bricks falling off, but the welding quality of metal parts and the drying and discharge of moisture in the castable are the key.
Comparatively speaking, the refractory brick solution is easy to repair and replace, and is less affected by environmental and human factors. Now the rotary kilns of new projects are basically outdoor designs. With the increase in production and environmental protection requirements, the single-layer structure has better stability. The groove composite brick method can be considered in the high-temperature zone to reduce the outer wall temperature. Moreover, from the perspective of corrosion protection, the outer wall temperature of the cylinder is controlled in the range of 160~320℃, which is more reasonable.
2. Secondary combustion chamber
The main function of the secondary combustion chamber is to carry out secondary combustion treatment of flue gas, and the combustible components, fly ash particles and dioxins are incinerated and decomposed. Physical and chemical reactions exist at the same time, and the reaction is violent. The secondary combustion chamber generally includes refractory materials, thermal insulation materials and insulation materials. Among them, refractory materials refer to heavy castables, plastics or refractory bricks in the working layer, thermal insulation materials refer to thermal insulation castables and thermal insulation bricks, and thermal insulation materials refer to calcium silicate boards, ceramic fiber boards and nano boards. At present, the design of the inner lining of the secondary combustion chamber mainly includes three methods: (1) Scheme 1: including refractory materials, thermal insulation materials; this three-layer structure design, taking the total thickness of 450mm as an example, the outer wall temperature is 80~90℃. (2) Scheme 2: including refractory materials and thermal insulation materials. The outer wall temperature of this structure is relatively high. Taking the total thickness of 305mm as an example, the temperature reaches 150~180℃. (3) Scheme 3: including refractory materials and thermal insulation materials. This scheme is between the first two schemes. Taking the total thickness of 270mm as an example, the outer wall temperature is about 110~140℃. Generally speaking, the temperature of the secondary combustion chamber is about 1100~1200℃, and the local temperature of the burner even reaches above 1300℃, so the working layer can meet the use requirements by choosing corundum mullite. It is recommended to use corundum or chrome corundum materials for the working layer of the burner area. In comparison, the three-layer structure design of Scheme 1 has good stability and is not easy to stop the furnace for maintenance due to leakage or cross-fire, resulting in high-temperature corrosion or over-temperature deformation of the cylinder.
3. Other parts
The waste heat boiler part mainly needs to use some wear-resistant castables, including the ash hopper, header, top seal and outlet flue. Among them, the ash hopper uses more double-layer results of insulation material and wear-resistant castable, and the total thickness is generally 200~250mm.
The quenching tower uses acid-resistant castables or acid-resistant cement. The temperature in the area about 2 meters below the top is higher, and 25mm insulation material can be used as the insulation layer, and the total thickness of the lining is 100mm.