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 treatment processes for hazardous waste treatment, with the advantages of treating many types of hazardous waste and large treatment volume. Since the 1990s, my country has gradually begun to use rotary kiln incineration systems to treat hazardous waste. In recent years, there have been many kiln shutdown accidents caused by refractory damage, which has seriously affected the stable operation rate of hazardous waste incineration production lines, and put forward urgent needs for the optimization of the refractory lining design and efficient operation of rotary kiln hazardous waste incinerators.
1. China’s hazardous waste incineration technology
Practice has proved that rotary kiln or cement kiln incineration treatment technology can treat industrial hazardous waste in solid, liquid and gaseous states, and is the most widely used treatment technology for hazardous waste today.
At present, the hazardous wastes co-processed by cement kilns mainly include solid hazardous wastes, waste liquids and RDF alternative fuels, etc., which are mainly processed from the preheater, decomposition furnace, rear kiln mouth and kiln head cover. Compared with the traditional landfill and incineration treatment methods, cement kilns have obvious advantages in the co-treatment of wastes, but due to the late start, the system construction in many aspects such as technology, operation and supervision is not yet sound. Due to the increasingly strict control of cement process and product quality, many hazardous wastes are prohibited from being co-processed in cement kilns, and the co-treatment of hazardous wastes causes great changes and fluctuations in operating conditions, flue gas composition and material components, aggravating the corrosion of cement kiln cylinders and ringing or crusting in the system, shortening the service life of refractory materials, and seriously affecting the long-term stable operation of cement kilns.
The rotary kiln incinerator is a professional hazardous waste incineration device. The main equipment consists of a rotary kiln, a secondary combustion chamber, a waste heat boiler, a quenching tower, a deacidification and dust removal system. After pretreatment, various types of hazardous waste enter the combustion system through different feeding methods. Under the action of their own gravity and the continuous rotation of the rotary kiln, the materials and the combustion-supporting air are fully contacted to complete the drying, gasification and combustion processes, and the remaining part is finally quenched into ash.
According to the different flow directions of furnace gas and solid, or the position of the burner in the rotary kiln, the rotary kiln incinerator can be divided into two forms: countercurrent and downstream. Among them, the burner heat source is at the feed end of the rotary kiln, and the solid and flue gas have the same direction, which is called downstream; conversely, the burner heat source is at the discharge end, and the solid and flue gas have opposite directions, which is called countercurrent. At present, the hazardous waste incineration system is mainly based on the design of downstream rotary kilns. Countercurrent rotary kilns are more suitable for hazardous wastes with high moisture content or low calorific value. The rotary kiln incinerator has the characteristics of wide applicability and can simultaneously process solid, liquid and gas hazardous wastes. Under the increasingly stringent environmental protection requirements, professional rotary kiln hazardous waste incinerators have obvious advantages.
2. Design of refractory lining of 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 design of the lining structure. First of all, the stability of the overall lining of the refractory lining under dynamic high-temperature operation should be ensured to prevent problems such as refractory bricks falling off 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 chrome-zirconium corundum refractory materials can be selected. When using a single-layer structure with a thickness of 230~300mm, since the thermal conductivity of refractory materials 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 projects where the rotary kiln is outdoors.
Using a double-layer structure of insulation bricks and refractory bricks, because the thermal conductivity of insulation bricks is low, it can play a role in reducing 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 here that the strength of insulation bricks is generally required to be greater than 20MPa, and the thickness of insulation bricks cannot be less than 50mm.
In order to achieve the stability of single-layer bricks and reduce the temperature of the outer wall, composite bricks can be designed, that is, the working surface is corundum bricks, chrome corundum bricks, and the back is alumina hollow balls, high aluminum or clay, and the thickness of the insulation layer is about 50~70mm. This product has low production efficiency, and because the working layer and the insulation layer have different thermal expansion coefficients and sintering shrinkage rates, micro cracks are easily generated at the joints, resulting in a high scrap rate.
In view of this, a groove-type composite brick design can be used, with the working layer material unchanged, and the tail slotted and filled with insulation materials such as nano-board. This groove brick-type composite brick is not only easy to produce and construct, but also can reduce the outer wall temperature 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 double-layer casting structure with heavy castables (metal gripper anchoring), which can reasonably control the outer wall temperature of the rotary kiln. This structure uses metal parts to connect the refractory material and the cylinder body, 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.
In comparison, the refractory brick solution is easy to repair and replace, and is less affected by environmental and human factors. At present, the rotary kilns of new projects are basically designed outdoors. 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, plastic 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 inner lining design of the secondary combustion chamber mainly includes three methods:
Scheme 1: including refractory materials, thermal insulation materials and insulation materials; this three-layer structure design, taking the total thickness of 450mm as an example, the outer wall temperature is 80~90℃.
Option 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℃.
Option 3: including refractory materials and thermal insulation materials. This option is between the first two options. Taking the total thickness of 270mm as an example, the outer wall temperature is about 110~140℃.
Generally speaking, the temperature of the second combustion chamber is about 1100~1200℃, and the local temperature of the burner even reaches more than 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 Option 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 includes the ash hopper, header, top seal and outlet flue, which need to use some wear-resistant castables. Among them, the ash hopper uses more double layers of insulation materials and wear-resistant castables, 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, so 25mm insulation material can be used as the insulation layer, and the total thickness of the lining is 100mm.
3. Typical rotary kiln hazardous waste process and refractory material design optimization
According to the on-site investigation, the damage of refractory materials is mainly concentrated in the rotary kiln body area. Hazardous waste incineration mainly includes the following typical treatment processes. In view of this, the following design optimization suggestions are proposed.
1) Treatment of materials with high moisture content
The moisture content of the materials entering the kiln is high, and the refractory materials at the feed end are severely thermally spalled. Refractory materials with good thermal shock stability such as mullite, corundum mullite, corundum silicon carbide, etc. are suitable for this area.
2) Large amount of waste liquid treatment at the kiln head and poor atomization
Generally, the kiln head uses an atomization gun to treat waste liquid. The incineration area is mainly in the drying section. If the atomization effect is not good due to the influence of the equipment, the refractory materials in this area will also peel off and be damaged. Mullite, corundum mullite, and corundum silicon carbide materials are also suitable for this. During operation, the frequency of checking the atomization equipment should generally be increased, and the waste liquid treatment volume should not exceed 30%.
3) Large amount of hard materials such as iron sheets (barrels)
The hardness of materials such as iron barrels is large, the speed of the rotary kiln is slow, and the repeated rotation of materials in the kiln causes serious mechanical wear in the feeding area. The use of corundum silicon carbide refractory materials with high strength and better wear resistance is good.
4) Materials with high content of low melting point salts
When the material contains more than 5% of K and Na salts, the melting point is low, and it is easy to form high-temperature slag, which penetrates and corrodes the refractory materials seriously. When high-temperature slag appears, it is more appropriate to select chromium-zirconium corundum as the refractory material in the high-temperature zone. Generally, the Cr203 content ranges from 3% to 10%.
5) Materials with high fluorine content
When the fluorine content in the material is high, reaching 1~4%, F and Si in the refractory material can easily react to form SiF4 gasification, causing the lining matrix structure to be destroyed and eroded rapidly; the impurity content in the refractory material, especially the silica content, must be strictly controlled. The use of chromium corundum materials with a Cr203 content of 9~15% can meet the normal operation needs of the rotary kiln.
Hazardous waste incineration is an emerging field, in which the types of hazardous waste are complex and numerous, and the physical properties and calorific values vary greatly. The linings of rotary kilns and secondary combustion chambers are designed in various forms according to needs. As the country increases its supervision of hazardous waste, large-scale rotary kiln incineration technology will be more widely used in hazardous waste treatment, posing challenges to the selection and design of refractory materials for rotary kiln incinerators. Compared with traditional high-temperature kilns, refractory researchers should pay more attention to the characteristics of hazardous waste materials, operating conditions (incineration temperature, flue gas composition), etc.