Our country has a large population and produces a large amount of domestic waste every day. Compared with foreign developed countries, the degree of waste classification in my country is relatively low. The characteristics of garbage (physical properties, calorific value, moisture content, etc.) vary greatly due to regional climate and living habits, and with the improvement of people’s living standards and the acceleration of urbanization, the components of garbage also change year by year. my country’s waste incineration industry started later than foreign countries, and the design of lining refractory materials is mainly based on introduction and digestion, and a material system that meets the requirements for the use of grate furnaces in my country has not yet been established. At present, due to improper selection of lining insulation, wear-resistant, and corrosion-resistant materials, frequent shutdown accidents of waste incinerators in my country have seriously affected the operation rate of waste incinerators, resulting in a large amount of garbage accumulation, causing a series of social and people’s livelihood problems. question.
1. Status quo of harmless treatment of municipal solid waste
The harmlessness and volume reduction of municipal garbage has become a matter of great concern to the government and the public. With the rapid development of my country’s economy and the continuous concentration of population in cities, the urbanization process has accelerated and the quantity and scale have continued to expand, and the total amount of urban domestic waste has continued to increase. In the past ten years, the harmless treatment of urban garbage in my country has grown from scratch, and the rate of harmless treatment has also increased year by year.
2. Zinc oxide volatilization kiln is used for the treatment of zinc leaching slag
The volatilization rotary kiln process is a new process and new equipment in our company. It is characterized by large equipment, large equipment processing capacity, high degree of automation, and relatively stable product quality, but the overall investment is large. Except for some control instruments, the system all adopts domestic equipment. A large number of automatic or semi-automatic controls are used in the volatilization kiln system, which greatly reduces the labor intensity of operators on the premise of saving investment. The whole system is the most advanced volatilization kiln processing zinc leaching slag system in China at present.
With the improvement of people’s living standards, the calorific value of domestic waste is gradually increasing. Traditional landfill and composting treatment methods have poor volume reduction and volume reduction effects, and can no longer meet the needs; while incineration has been developed rapidly due to its advantages of large volume reduction and volume reduction. In recent years, the treatment method of urban garbage in my country has gradually changed from the initial sanitary landfill method to the incineration method, and its proportion is increasing year by year.
As the mainstream technology of waste disposal, there are many waste incineration plants in cities with relatively developed economies, and waste incineration power generation has entered a stage of rapid development in my country. As of February 2019, the number of domestic waste incineration plants in operation in my country (excluding Hong Kong, Macao, and Taiwan) has reached 418, and 167 are under construction, of which mechanical grate furnaces account for more than 80%.
2. Technical characteristics of mechanical grate furnace and requirements for refractory materials
Domestic waste incineration is a very complex physical and chemical reaction process, and the incineration process can usually be divided into three stages: drying, thermal decomposition, and combustion. Incinerator types can be divided into mechanical grate type, fluidized bed type, and rotary type. Among them, mechanical grate type incinerator is the mainstream equipment for large-scale waste incinerators. The biggest advantage of this type of furnace is that the technology is mature, the operation is stable, the amount of ash produced is relatively small, the rate of slag thermal ignition is low, and the adaptability is wide. Most solid waste can be directly burned in the furnace without any pretreatment. At present, the main mechanical grate incinerator technologies used at home and abroad include Germany’s Martin, Japan’s Hitachi Zosen, and Belgium’s Seghers.
Garbage is a heterogeneous mixture of different compositions, and its type, quantity and calorific value are also very different. For this reason, the physical and chemical properties of the lining refractory should be adapted to the requirements of different stages during operation. The working temperature of waste incinerators generally does not exceed 1200 °C, but the gases generated during incineration (such as HCl, SO2, Cl2, CO and alkali metal gases, etc.) have strong erosion on refractory materials. At the same time, during the high-temperature movement of the garbage, the wear and thermal impact on certain parts of the incinerator (such as the bottom of the furnace, the discharge port and the side wall, etc.) are relatively large. Therefore, refractory materials for incinerators are required to have the following characteristics:
1) High strength and good wear resistance to resist the wear of solid materials and the erosion of hot air; 2) Good volume stability and corrosion resistance to resist the corrosion of acidic substances in the furnace; 3) Good heat resistance 4) Good resistance to CO erosion to avoid cracking of furnace lining caused by CO erosion; 5) Good high temperature strength, heat resistance and heat insulation.
3. Application status of mechanical grate furnace refractories
The mechanical grate type garbage incinerator is divided into drying area, burning area and burning area. According to the use position of refractory materials, it can be divided into garbage pusher inlet area, grate side wall, front arch, rear arch, furnace top and slag outlet. The temperature, mechanical strength and atmosphere of the furnace lining in these areas are different, and corresponding refractory materials need to be selected according to the characteristics of their respective working conditions.
The damage forms of the waste incineration furnace wall mainly include the structural damage of the furnace wall caused by thermal stress, and the mechanical wear, adhesion, penetration and erosion of the refractory materials of the furnace wall caused by garbage.
3.1 Furnace wall structure damage
In different incineration stages, the process temperature varies greatly, and the furnace walls of each section are heated unevenly. Among them, the combustion section is the highest temperature area, which mainly has two structures: adiabatic furnace wall and air-cooled furnace wall. The stability of air-cooled furnace wall is obviously better than that of adiabatic furnace wall. But no matter what kind of structure, after a long service time, bulges will appear.
During the operation of the incinerator, the furnace wall is heated, and the temperature inside and outside the furnace wall is different. The temperature difference between the working surface and the non-working surface makes the expansion of the refractory brick to the fire surface larger than that of the back fire surface. At the same time, the temperature distribution of the furnace wall in different combustion areas in the incinerator is uneven, so that the expansion of the furnace wall is larger at the higher temperature part. Because the furnace wall is constrained by the size of the furnace body, the entire furnace wall generates greater internal stress, and the higher the temperature area, the greater the internal stress, which leads to the tendency of the furnace wall in the high temperature area to bulge and bulge into the furnace. The bulge and bulge of the furnace wall are also related to overload operation, process temperature control, frequency of starting and stopping the furnace, operation level and furnace wall design.
When the furnace wall deforms and bulges into the furnace, the tensile bricks will generate tensile stress on the furnace wall and play a role in stabilizing the furnace wall. If the tension member deforms or breaks, the tension force weakens, and the furnace wall gradually deforms under the continuous action of this thermal stress. Different areas have different degrees of protrusion of the furnace wall into the furnace, and serious parts may collapse.
3.2 Damage caused by garbage on refractory materials
Domestic waste has a low calorific value and complex composition. During the incineration process, it is easy to produce problems such as dust accumulation, wear and corrosion, which will adversely affect the safe and stable operation of the incinerator. The composition of municipal solid waste is related to the degree of urbanization. The more economically developed the city, the higher the proportion of combustible and compostable waste in municipal waste. my country’s domestic waste has the characteristics of low calorific value and complex components. There are many factors that affect the operation of equipment during the incineration process. The damage to the refractory lining of the incinerator is firstly the mechanical wear of the slag and hard blocks in the domestic waste to the furnace lining, and the second is the mechanical wear, adhesion and erosion of the refractory lining by the slag. The fly ash of waste incinerators is mainly composed of oxides and chlorides composed of K, Na, Al, Ca, Si, etc. The refractory lining of the incinerator is mainly composed of Al2 O3 and SiO2, and the degree of erosion and damage of the refractory by slag is different in different parts of the incinerator.
(1) Mechanical wear of feed inlet, drying section and other parts
The mechanical wear of the incinerator feed inlet, the unloading platform and the lower part of the incinerator side wall that directly contacts the garbage is serious. The temperature of the parts directly in contact with the garbage such as the feed inlet and the drying section does not exceed 1000 °C, and the wear resistance of ordinary bauxite refractory castables cannot meet the requirements for use.
(2) Fly ash adhesion and coking in the front and rear arch areas
A large amount of fly ash was formed during the combustion process of the garbage, and the molten fly ash came into contact with the refractory material lining the front and rear arches, and reacted and adhered to the front and rear arches. There are two different processes in the attachment of fly ash to the furnace wall: the first deposition process is the formation process of the initial deposition layer. The initial deposit layer is a thin ash layer composed of alkali metal and alkaline earth metal sulfates with high chemical activity, which is composed of ash particles of small size. The initial deposition layer has good thermal insulation properties, and its formation increases the temperature of the furnace wall surface. The second deposition process is the impact of larger ash particles on the initial deposition layer of the furnace wall under the action of inertial force (inertial deposition). When the initial deposition layer is viscous, it can capture the ash particles transported by inertial force and The thickness of the slag layer increases rapidly. The thickness of the ash layer is usually uneven. It is not only related to the structure of the furnace, the position of the combustion center, the aerodynamic characteristics, the temperature characteristics of the furnace and the physical and chemical properties of the garbage, but also has a certain relationship with the composition of refractory materials. If the fly ash is not cleaned in time and the coking of the fly ash is too thick, the refractory material of the furnace wall will be deformed and collapsed under the action of the gravity of the coke layer, which will seriously affect the safe operation of the incinerator.
(3) Ash melting and erosion in the secondary combustion zone
The flue gas outlet of the incinerator is the secondary combustion zone. The main component of the lining refractory in the secondary combustion zone is usually a corundum-mullite castable with an Al2O3 content of about 80% (w), and its main crystal phase is corundum and Mullite. The flue gas temperature of oxygen-enriched combustion in the secondary combustion zone can reach up to 1 400 °C. After the fly ash is melted in this area, it forms slag and adheres to the surface of the refractory material. The slag reacts with the refractory material to form a low melting point substance. These molten low-melting substances are easily washed and worn by high-speed dust-laden fumes, resulting in layer-by-layer reaction, erosion, and thinning of refractory materials, so that the steel plate in the secondary combustion zone will burn red and the furnace will be forced to shut down. The main reasons that lead to the melting of fly ash in the secondary combustion zone and the formation of high-temperature erosion are: 1) The furnace temperature rises due to the increase in the calorific value of the material year by year;
The design size of the flue gas outlet in the secondary combustion zone of the incinerator is related to the design size. For example, the working condition of the secondary combustion zone of the Sanfeng Martin furnace is relatively good, and the design of the secondary combustion zone of the Bargok furnace is small, and the fly ash melts and severely corrodes the refractory material. ; 3) Related to the operation process, in order to reduce the degree of fly ash adhesion and reduce the number of shutdowns to clean the ash, the internal incinerator adjusts the process to move the combustion zone backward, so that the temperature field and flow field of the secondary combustion zone change.
The analysis shows that a small amount of feldspar mineral phase is formed in the reaction layer and the permeable layer. In actual working conditions, the melting point of feldspar minerals is low, and the high temperature is in a molten state. The low melting point material formed by the reaction of slag and refractory material dissolves into the slag, and is gradually washed away by high-speed dusty smoke, eventually causing The lining refractory material is worn through.
(4) Spalling of water wall refractory material
The water wall of the incinerator waste heat power generation boiler is equipped with refractory plastic or castable with a thickness of 70 ~ 110mm. After use, the refractory material other than the end face of the pin head will delaminate and fall off, and there will be small cracks near the fall off area. Under high temperature conditions, the thermal expansion of the metal anchor is greater than that of the refractory material. In the area where the interface stress between the refractory material and the metal pin is the largest, a large number of pins work together to cause delamination of the refractory material, resulting in the refractory material falling off.
4. Refractory materials for waste incineration are facing the situation
At present, there are more than 300 grate-type domestic waste incineration projects in my country, and there are more than 1,000 incinerators of different sizes and types. The problem of individualization of refractory material design schemes for mechanical grate furnaces is prominent. The design of refractory bricks is quite different. There are hundreds of brick types in most cases, and they cannot be used interchangeably, resulting in high production costs and long cycle times.
my country’s waste production is increasing, and its calorific value is increasing year by year. Large-scale waste incinerators are imperative. Therefore, new grate furnace incineration technologies are developing rapidly. The fluctuating calorific value of waste incinerators and the increase in processing capacity lead to a series of new problems in the use of refractory materials. In recent years, the structural design of new grate furnaces has been constantly innovating, and the design of many heavy-duty adiabatic incinerators has gradually changed from adiabatic furnace to air-cooled furnace wall to water-cooled furnace wall. At present, large-scale water-cooled wall structure incinerators with a daily processing capacity of 850 t have been successively started in southern regions where the calorific value of waste is relatively stable. The new large-scale incineration technology has raised new issues for refractory design, product research and construction technology.
According to the “13th Five-Year Plan” National Urban Domestic Waste Harmless Treatment Facilities Construction Plan issued by the National Development and Reform Commission, the proportion of waste incineration in the national urban domestic waste treatment will increase from 31% in 2015 to 54% in 2020. In recent years, driven by the dual forces of national policy subsidies and huge market value profits, more and more market forces have begun to gather in the waste incineration power generation industry, conducting industrial competition of “separation of those who can”. Under such circumstances, many companies often cut corners in engineering operations, pollution emissions, etc. to reduce costs after “staking the land” to get the project, but in the end, the environmental benefits of the project are greatly reduced, which also includes reducing fire resistance. Material design and material selection cost. On March 16, 2019, the Ministry of Ecology and Environment announced the “Regulations on the Use of Automatic Monitoring Data of Domestic Waste Incineration Power Plants for Environmental Management (Trial)”, proposing to take strong supervision of waste incineration enterprises’ emission standards and high-quality operations move. Design material selection defects and environmental protection control pressure make the long-term stable operation of the incinerator more risky.
5 Conclusion
The safe and reliable operation of waste incinerators depends to a large extent on the stable and reliable quality of refractory materials, as well as on the reasonable design, material selection and efficient maintenance of refractory materials. At present, refractory products based on bauxite and clay raw materials cannot meet the needs of large-scale and deteriorating working conditions of incinerators, and the product structure of refractory materials for incinerators has also gradually changed. At the same time, under the situation of increasingly stringent national environmental protection control, the refractory industry is facing unprecedented new opportunities and new challenges. In view of this, it is recommended that the research on refractory materials for waste incinerators be developed in the following directions: 1) Improve the refractory material system for grate-type waste incinerators, rationally design the furnace wall structure, and improve the stability of the furnace wall; 2) The key to developing incinerators Partial refractory products to solve the problems of garbage wear, fly ash adhesion, slag erosion and serious peeling; 3) Study the application of wet spraying, pumping and other construction technologies on incinerators to improve construction efficiency and improve the quality of unshaped products 4) Research on the recycling technology of waste refractory materials after incinerators to realize the recycling of resources.
