HOW TO USE THERMAL INSULATION REFRACTORY MATERIALS WITH LOW THERMAL CONDUCTIVITY TO REDUCE THE HEAT LOSS OF THERMAL EQUIPMENT?

The lining of thermal equipment is constructed of refractory materials, which play a key role in heat loss, and refractory materials play a key role in it. At high temperatures, refractory materials must not only maintain stability, but also keep heat as little as possible. Therefore, refractory materials must have thermal insulation properties. OK, less heat storage.
1. Heat loss of thermal equipment
Thermal equipment is generally a large energy consumer. The higher the temperature, the more energy consumption. The thermal efficiency is very low in most cases, and the thermal energy utilization rate is less than 30%. The heat loss of thermal equipment generally has the following items:
(1) The heat dissipated by each part of the surface of the thermal equipment body can reach 10% to 40% of the unit energy consumption of the product;
(2) The heat storage loss of the thermal equipment body is less important to the thermal equipment that operates continuously, and the heat storage loss of the thermal equipment that operates intermittently reaches 5% to 25%;
(3) The heat loss of water cooling, such as the water cooling pipe of the swill rail of the continuous steel rolling heating furnace, is not wrapped with refractory materials, and the heat loss is more than 25%;
(4) The heat loss due to poor sealing of joints, holes and furnace doors, for example, the heat loss of electric arc furnace doors is more than 35%;
(5) Heat loss from smoke exhaust.
The above heat losses are all related to refractory materials, especially (1)~(4) have a great relationship with the thermal insulation performance of refractory materials. The basic way to reduce the heat loss on the surface of the furnace body is to select appropriate thermal insulation materials to reduce the surface temperature of the furnace body. When the furnace temperature is constant, the outer surface temperature mainly depends on the thickness of the furnace wall and the thermal conductivity of the furnace wall material. Increasing the thickness of the furnace wall will lead to an increase in the heat storage of the furnace body, which can increase the heat storage loss. Therefore, the rational use of thermal insulation materials has become the best choice.
In recent years, my country’s thermal insulation materials have developed rapidly. There are not only shaped products of various materials, different bulk densities, and different thermal conductivity, but also corresponding amorphous refractory materials, refractory fibers and products of various materials, silicon calcium board, nano-insulation board, etc. These thermal insulation products have different appearance specifications, physical and chemical indicators, different thermal insulation effects, and different market prices. Therefore, the lining design should be carried out according to the use conditions of thermal equipment. Collect original data, including temperature parameters (hot surface temperature of thermal equipment, cold surface temperature), physical constants (thermal conductivity of thermal insulation materials, bulk density, maximum operating temperature), economic parameters (refractory material prices, fuel prices) , calorific value, utilization coefficient, etc.), then calculate the energy-saving effect, analyze and compare, select suitable thermal insulation materials and formulate a reasonable plan.
2. Examples of reducing heat dissipation loss of thermal equipment
(1) Thermal insulation of the ladle
At present, the average energy consumption of my country’s steel industry is 50% higher than that of Japan, and 30% higher for large enterprises. Ladle is an important thermal equipment in the steel industry. In order to keep the ladle warm, through the calculation of the heat dissipation of the ladle and the research on the thermal insulation materials, it is found that the inner lining of the ladle should be built with four-layer materials, that is, the inner surface of the steel shell should be coated with energy-saving paint, and the inner surface should be 10mm nano-insulation board, and then Inward is 75mm high-strength nano-micron insulating castable, and then inward is the working layer. The working layer of slag line adopts magnesia-carbon brick with low thermal conductivity, and the working layer of molten pool adopts corundum-spinel quality unburned brick. This method is applied to the 120t refining ladle, so that the temperature of the ladle shell at the slag line is about 225 °C, the temperature of the ladle shell at the molten pool is about 200 °C, and the cladding shell is about 170 °C. This energy-saving structure has achieved good results: ①High-strength nano-micron castables and low thermal conductivity working layer can effectively protect the nano-board, keep it within a safe working temperature for a long time, and significantly improve the service life of the thermal insulation layer and the permanent layer; ②Completely It can reduce the temperature of the cladding by more than 100 °C, improve the service life of the cladding, reduce the gas used for baking the bun, significantly slow down the temperature of the molten steel, reduce the tapping temperature, improve the metal yield, improve labor productivity, and achieve energy saving, environmental protection and the purpose of reducing costs.
(2) Low thermal conductivity composite brick for cement rotary kiln passing wave zone
Cement rotary kiln is a high-energy-consuming thermal equipment, especially in the front and rear transition zones. The refractory lining is not protected by kiln skin and directly contacts the cement material. The temperature of the kiln body is high, which increases heat loss and fuel consumption and reduces the kiln body. and the service life of the supporting roller, while making the refractory material easy to damage. In order to reduce heat dissipation and safety hazards, a three-layer structure of working layer, thermal insulation layer and thermal insulation layer is adopted. If three kinds of refractory bricks with different thermal conductivity are used for masonry, the accident of falling bricks from the inner lining is often prone to occur when the rotary kiln is running. Therefore, the low thermal conductivity multi-layer composite brick is studied, that is, the brick adopts a three-layer structure: working layer (silicon mullite brick thickness 0.140m), thermal insulation layer (light mullite brick thickness 0.035m), the bonding interface of these two layers adopts The combination method of sinusoidal surface, and the third layer is the thermal insulation layer (ceramic fiber board containing ZrO2, thickness 0.025m). The stress concentration of the multi-layer composite brick is less, and the comprehensive thermal conductivity of the multi-layer composite brick is reduced from 2.74 to 1.50W/(m·K) of the original silica molybdenum brick, which reduces the temperature of the kiln shell by 50~70℃.
(3) The 260t steelmaking converter of Anshan Iron and Steel adopts 20mm-thick nano-insulation board instead of 40mm-thick polycrystalline fiber insulation board to optimize the furnace lining structure,
The furnace capacity ratio is increased, and the steel output is increased to lower the temperature of the furnace shell by more than 11 °C. There is no pulverization phenomenon during the entire converter operation process, and there is no falling of lining bricks. At the same time, it also reduces the smelting time and reduces the consumption of molten iron. .
(4) High thermal conductivity silicon carbide ramming material for water-cooled pulverized coal gasifier
The water wall of the pulverized coal gasifier is lined with silicon carbide ramming material with high thermal conductivity. At high temperature, the slag hangs on the lining of the silicon carbide ramming material. Due to the high thermal conductivity of silicon carbide, the slag touches the inner The lining rapidly condenses, and as the temperature decreases, the thermal conductivity decreases (see Table 1). Inside and outside the furnace are hot slag, solid slag, silicon carbide refractory, water wall, inert gas protective layer, high alumina amorphous refractory material, and outer protective layer. This reduces heat loss in the furnace.
3. Matters needing attention when choosing thermal insulation materials
In the high temperature industry, there are many examples of using thermal insulation materials to save energy and protect the environment. The thermal insulation material has high porosity (over 40%~85%), low bulk density (less than 1.5g/cm3), and low thermal conductivity (less than 1.0W/(m·K)). However, when choosing these thermal insulation materials, pay attention to the following issues:
(1) Thermal conductivity of thermal insulation material (λ)
Thermal conductivity is also called thermal conductivity, and its reciprocal 1/λ is thermal resistance. The smaller the thermal conductivity, the better the thermal insulation effect. It is well known that air has the lowest thermal conductivity.
The thermal conductivity of solid materials is much larger than that of gases, so the pores of solid materials can significantly reduce the thermal conductivity of materials, so the insulation material must be high porosity. The higher the porosity, the smaller the λ value.
In addition, the pore size also has a certain influence on the λ value. At low temperature, the thermal conductivity of the thermal insulation material decreases with the increase of the pore size, and the thermal conductivity above 800℃, especially above 1000℃, increases rapidly with the increase of the pore size. Therefore, the high temperature takes the thermal insulation material with a small pore size, and the low temperature takes the thermal insulation material with a large pore size. When the porosity is the same, the thermal conductivity of the microstructure in the gas-phase continuous phase is smaller than that of the solid-phase continuous phase, and the pores in the fiber material are continuous like the solid phase, so the thermal conductivity of the refractory fibers and products is small. In the solid phase of thermal insulation materials, the thermal resistivity of the material varies greatly due to the difference in chemical mineral composition. Generally, the more complex the crystal structure, the lower the thermal conductivity, and the thermal conductivity of the glass in the solid phase is lower than that of the crystal phase. As the temperature increases, the thermal conductivity of the glass phase increases; the temperature of the crystalline phase increases, while the thermal conductivity decreases. The United Kingdom has developed an ultra-fine SiO2 composite thermal insulation material with a bulk density of about 0.24g/cm3, and its thermal conductivity is lower than that of all thermal insulation materials, even lower than that of still air.
(2) Heat resistance of heat insulating material
Some thermal insulation materials are used at a lower temperature. For example, nano-insulation boards are used in 100t steel ladle of Angang Steel. Exceeding the use temperature will be deformed under pressure, resulting in the deformation of the lining, which not only deteriorates the thermal insulation performance, but also brings safety hazards. Therefore, it has been suggested that the thermal insulation material mainly depends on the shrinkage deformation at a certain temperature, not the refractory degree. Internationally, the temperature at which the reburning shrinkage is not more than 2% is generally used as the temperature range for the use of thermal insulation materials, and it is also one of the differences between thermal insulation materials and pure refractory materials.
(3) Strength of thermal insulation material
Due to the high porosity and low relative strength, such as the above-mentioned nano-insulation board, the thermal insulation effect is good, the porosity is high, and the strength is low. In order to ensure the transportation and construction needs, the insulation material must have a certain strength. Especially for some thermal insulation products that are in direct contact with the flame, it is very important to improve the strength. As the bulk density increases, the strength increases. When the bulk density is the same, the solid-phase connection is stronger than the gas-phase connection, which is related to the pore size. Reducing the pore size is an effective technical measure to improve the strength of thermal insulation materials.
(4) Atmosphere and thermal insulation material
Many thermal equipment are lined with thermal insulation materials, and various protective atmospheres are also commonly used, such as CO, CO2, H2, N2, etc. Al2O3-SiO2 series refractories in hydrogen, SiO2 is returned to metal silicon and water vapor, Al2O3 is very stable, so in hydrogen, alumina insulation materials should be selected. Aluminum silicate fibers contain 3% to 4% of Cr2O3, which is easily reduced in a hydrogen reducing atmosphere, so aluminum silicate fibers containing chromium oxide should not be used in a reducing atmosphere.
(5) Insulation method
On the thermal equipment in intermittent operation, the thermal insulation layer (refractory fiber veneer) can be directly laid on the hot surface of the furnace lining, which can achieve the best energy saving effect. Better than the thermal insulation effect of the inner wall (hot surface).