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. 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. Current status of harmless disposal of hazardous waste in my country
Hazardous waste disposal in my country started in 1990, and the relevant management system was initially formed in 1996. It was not until 2008 that the “National Hazardous Waste List” was formed. In 2013, the Supreme People’s Court and the Supreme People’s Procuratorate made a judicial interpretation that “illegal discharge, dumping, and disposal of more than 3 tons of hazardous waste will be criminalized”, which is regarded as the key point for the launch of my country’s hazardous waste treatment industry. In the first half of 2016, the “Action Plan for Soil Pollution Prevention and Control” and “National List of Hazardous Wastes (2016 Edition)” and other special plans related to hazardous waste treatment were promulgated and implemented one after another. In 2017, the “13th Five-Year Plan for National Hazardous Waste Standardized Management Supervision and Assessment Work Plan” was promulgated. In 2018, the Ministry of Ecology and Environment launched the “Waste Cleaning Action 2018” plan, and the Ministry of Industry and Information Technology launched the Yangtze River Economic Belt Industrial Green Development Action. On November 12, 2019, the General Office of the Ministry of Ecology and Environment issued the “Hazardous Waste Incineration Pollution Control Standard (Second Draft for Comments)”, and on March 10, 2020, the national standard “Technical Requirements for Solid Waste Vitrification Treatment Products (Draft for Comments)” was publicly solicited for comments, which accelerated the development of the hazardous waste treatment industry and further catalyzed the development opportunities of the hazardous waste treatment industry.
With the development of industry, the amount of hazardous waste discharged in the production process is increasing. In 2016, 214 large and medium-sized cities generated 33.446 million tons of industrial hazardous waste, of which comprehensive utilization accounted for 45.3% of the total utilization and disposal, and disposal and storage accounted for 43.8% and 10.9% respectively. Effective utilization and disposal are the main ways to deal with industrial hazardous waste. In 2018, 200 large and medium-sized cities generated 46.430 million tons of industrial hazardous waste. It is estimated that in 2020, due to the epidemic, hazardous waste will increase to nearly 100 million tons.
2. my country’s hazardous waste incineration technology
Practice has proved that rotary kiln or cement kiln incineration technology can treat industrial hazardous waste in solid, liquid and gaseous states, and is the most widely used disposal technology for hazardous waste today. As of July 2019, my country has obtained 75 cement kiln collaborative disposal projects with hazardous waste business licenses (excluding Hong Kong, Macao and Taiwan), with an approved scale of 4.6252 million tons. 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 process, 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. According to statistics, there are about 800 processing production lines in my country’s rotary kiln hazardous waste incineration system, with processing capacity ranging from 20t/d to 120t/d. In 2018, my country’s hazardous waste incineration disposal capacity reached 6.16 million tons/year, of which professional rotary kiln incineration accounted for 53%, mainly distributed in Zhejiang, Jiangsu, Shandong, Hebei, Fujian and other places.
3. 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 lining structure design. First of all, it is necessary to ensure the stability of the overall lining of the refractory lining under dynamic high-temperature operation to prevent refractory bricks from 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 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 adopted. Because the thermal conductivity of the 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 effect, 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 designed to 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 alumina 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 joints, and the scrap rate is high.
In view of this, the groove type composite brick design can be adopted, the working layer material remains unchanged, and the tail is slotted and filled with thermal insulation materials such as nano-board. This groove brick type composite brick is not only easy to produce and construct, but also can achieve the effect of reducing the outer wall temperature of the cylinder by about 30~50℃.
In addition, some rotary kilns use lightweight castables or fiberboard as partition materials, and the working layer uses heavy castables (metal nail anchoring) double-layer casting structure, 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. 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 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. And 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 perform secondary combustion treatment on the flue gas. The combustible components, fly ash particles, dioxins, etc. are burned 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 thermal insulation materials. Among them, refractory materials refer to the working layer heavy castables, plastics or refractory bricks, etc., insulation materials refer to insulation castables and insulation bricks, etc., and thermal insulation materials refer to calcium silicate boards, ceramic fiber boards, nano boards, etc. Current secondary combustion chamber lining
Design mainly includes three methods:
(1) Option 1: Including refractory materials, thermal insulation materials and thermal insulation materials; this three-layer structure design takes the total thickness of 450mm as an example, and the outer wall temperature is 80~90°C.
(2) Option 2: Including refractory materials and 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) Option 3: Includes refractory materials and thermal insulation materials. This solution is between the first two solutions. 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 of corundum-mullite can meet the usage requirements. It is recommended that the working layer of the burner area be made of corundum or mullite. Chrome corundum material. In comparison, the three-layer structure design of Plan 1 has good stability and is not prone to shutdown for maintenance due to air leakage or cross-fire, resulting in high-temperature corrosion or over-temperature deformation of the cylinder.
3) Other parts
The waste heat boiler mainly requires the use of some wear-resistant castables in the ash hopper, header, top seal and outlet flue. Among them, the ash hopper is mostly made of double layers of insulation materials and wear-resistant castables, and the total thickness is generally 200~250mm.
The quench tower uses acid-resistant castables or acid-resistant clay. In the area about 2 meters below the top where the temperature is higher, 25mm insulation material can be used as the insulation layer, and the total thickness of the lining is 100mm.
4. Typical rotary kiln hazardous waste process and refractory material design optimization
It is recommended that according to the on-site investigation, the damage to the refractory materials is mainly concentrated in the rotary kiln body area. Hazardous waste incineration mainly includes the following typical treatment processes. For this, the following design optimization suggestions are proposed.
1) Processing materials with high moisture content
The moisture content of the materials entering the kiln is high, and the refractory material at the feed end is severely spalled by heat. Refractory materials such as mullite, corundum-mullite, and corundum silicon carbide with good thermal shock stability are suitable for this area.
2) The kiln head waste liquid treatment volume is large and the atomization is not good.
Generally, the kiln head uses an atomizing gun to process the 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 be peeled off and damaged. Mullite, corundum, mullite, Corundum silicon carbide materials are also suitable for this. During operation, it is generally necessary to increase the frequency of checking the atomization equipment, and at the same time, it is appropriate that the waste liquid treatment volume does not exceed 30%.
3) The processing capacity of hard materials such as iron sheets (barrels) is large
Materials such as iron barrels have high hardness and the rotation speed of the rotary kiln is slow. The repeated rotation of materials in the kiln causes serious mechanical wear in the feeding area. Choosing corundum silicon carbide refractory materials with high strength and better wear resistance will produce good results.
4) Materials containing high amounts of low melting point salts
When the material contains more than 5% K and Na salts, the melting point is low and it is easy to form high-temperature slag, which will penetrate and corrode the refractory materials seriously. When high-temperature slag occurs, it is more appropriate to choose chromium-zirconium corundum as the refractory material in the high-temperature area. Generally, the Cr203 content ranges from 3 to 10%.
5) Materials with high fluorine content
The fluorine content in the material is relatively high, reaching 1 to 4%, because F and Si in the refractory material can easily react to form SiF4 gasification, causing structural damage to the lining matrix and rapid erosion; the impurity content in the refractory material, especially the fluorine content, must be strictly controlled. For the silicon oxide content, the Cr203 content is 9~15% chromium corundum material, which can meet the normal operation needs of the rotary kiln.
5. Conclusion
Hazardous waste incineration is an emerging field. There are many types of hazardous wastes with complex physical properties and calorific values. Rotary kilns and secondary combustion chamber linings can be designed in various forms according to needs. As the country tightens 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 material researchers should pay more attention to the characteristics of hazardous waste materials and working conditions (incineration temperature, flue gas composition), etc.
