Rotary kilns are thermal equipment for heating bulk or slurry materials. They are widely used in nonferrous metallurgy, ferrous metallurgy, refractory materials, cement, chemical industry and papermaking, etc., especially in the production of some nonferrous metals. They are used for sintering, roasting and other heating treatments of ores, concentrates and intermediate products. In the more than 100 years since the advent of rotary kilns, rotary kilns have been used as combustion devices to provide the necessary heat for material calcination. During the production process, materials undergo high-temperature physical and chemical reactions in the rotary kiln. The material temperature can reach 1400-1450℃, and the maximum flue gas temperature can reach about 1700℃. Due to the low heat exchange efficiency between the hot air flow and the material in the kiln, a large amount of waste heat is generated. Taking measures to save energy and reduce consumption in rotary kilns is a fundamental and effective way to improve the economic benefits of related industries. It can be considered from the following four aspects:
(1) Fuel combustion efficiency. The main purpose is to reduce the mechanical incomplete combustion loss and chemical incomplete combustion loss of fuel;
(2) Heat radiation heat dissipation of the kiln body, that is, to strengthen the thermal insulation capacity of the kiln body and reduce the surface radiation heat transfer coefficient;
(3) Recycling of waste heat and waste heat. The heat loss of the rotary kiln mainly includes the heat loss of clinker cooling, the heat loss of evaporating water, the heat loss of kiln ash, the heat loss of flue gas, etc.:
(4) Energy consumption of auxiliary equipment. The following discusses the corresponding energy-saving and consumption-reducing technologies for various aspects.
1. Fuel combustion loss The types of fuel used in the rotary kiln system can be divided into gas fuel, solid fuel, liquid fuel and mixed fuel. Gaseous fuels are generally natural gas and coke oven gas. Natural gas is less used in China due to its high cost, while coke oven gas is a fuel commonly used by steel enterprises and is also suitable for rotary kiln calcining lime. Solid fuel refers to pulverized coal. Since some production enterprises do not have gas sources or have tight gas supply, pulverized coal is generally used as fuel. Liquid fuel refers to heavy oil, diesel and other fuels, which are almost not used at present due to their very high cost. Mixed fuels include high-coke mixed gas, high-speed mixed gas, pulverized coal gas mixed fuel, oil and gas mixed fuel, etc. Among them, the first three mixed fuels are used more. At present, the fuels used in domestic rotary kiln systems can be roughly divided into two categories: pulverized coal and gas, and their combustion devices are also different. The energy saving and consumption reduction of gas and pulverized coal combustion devices are discussed separately below.
1.1 Pulverized coal combustion According to thermal measurement statistics, the average heat loss of incomplete combustion of rotary kilns in my country is 250KJ per kilogram of clinker, accounting for 4% of the heat consumption. Among them, the chemical incomplete combustion loss is 34% to 37%, and the mechanical incomplete combustion loss of clinker is 54 to 71 kJ/kg on average. According to the data, an increase of 0.19% of CO in the exhaust gas is equivalent to a loss of 0.6% of fuel. The following issues should be handled well:
(1) Select reasonable coal powder control indicators. The finer the coal powder, the larger the specific surface area, the faster the combustion speed, and the smaller the incomplete combustion loss. The ash content of coal has a great influence on the calorific value and combustion speed. my country has a variety of coal types and the quality fluctuations are also large. For coal with high ash content, a smaller primary air volume and appropriately reduced coal powder fineness should be used to accelerate its combustion and reduce incomplete combustion. At the same time, it can increase the thermal intensity of the burning zone, increase production, and increase benefits.
(2) Reasonable excess air coefficient. The selection of reasonable exhaust and a certain amount of excess air for rotary kilns is the primary condition for complete combustion. Generally, it is better to control the excess air coefficient between 1.05 and 1.15.
(3) Reasonable burner structure. The rotary kiln generally adopts pulverized coal burners, and its technology is also very mature. With the development of rotary kiln coal-fired technology, pulverized coal burners have developed from single air duct to two air ducts, three air ducts, four air ducts and even five air ducts. At present, the four-air duct burner is the most widely used. The function of the single air duct is only to transport pulverized coal, and it has basically no regulating effect on the flame; the two air duct has some more regulating functions, but it is still not ideal; the three-air duct burner is mainly composed of pulverized coal duct, axial air duct and radial air duct, and the shape and length of the flame can be adjusted by changing the ratio of axial wind and radial wind; the four-air duct burner adds vortex wind or central air duct, which can play a very good effect on the mixed combustion of pulverized coal, the suction of secondary air, and the adjustment of flame length and shape. At present, the four-air duct burner is the most advanced burner. It can obtain the flame shape required in the burning zone, improve the combustion effect, reduce the amount of primary air, increase the amount of high-temperature secondary air to participate in combustion, and play a role in reducing heat consumption. In addition, the four-air duct burner has a wide range of adaptability to coal varieties. According to statistics from the cement industry, the use of four-channel pulverized coal burners can reduce heat consumption by 3% to 5% compared with other burners. Therefore, the four-channel structure should be used as much as possible in the selection of burners. It can be seen that the reasonable selection of pulverized coal burners is also the key to energy saving and consumption reduction.
1.2 Application of low calorific value gas The calorific value of coke oven gas is 3600kCal/Nm3, the calorific value of converter gas is 1600kCal/Nm3, and the calorific value of blast furnace gas is only 800kCal/Nm3. The calorific value required for the rotary kiln to calcine active lime using gas is generally greater than 2600kCal/Nm3. Coke oven gas is the most suitable fuel. However, the cost of coke oven gas is relatively high, and the supply often cannot meet the needs of active lime production, while converter and blast furnace gas are often forced to release in large quantities due to excess. How to use low calorific value gas is an important research topic in rotary kiln production, especially the use of generator gas. Low calorific value gas has been successfully applied in the active lime rotary kiln system. The main methods are mixed coal gas, mixed combustion of coal powder and coal gas, and preheating with high-efficiency heat exchangers. Mixed coal gas is a mixture of coke oven gas and converter or blast furnace gas in a certain proportion. The calorific value is reduced after mixing, but it can meet the requirements of active lime production; mixed combustion of coal powder and coal gas can be adjusted according to the calorific value and combustion temperature required for calcination, and mixed combustion of coal powder and coal gas; high-efficiency heat exchangers can use waste flue gas to preheat coal gas and primary air to 150-200℃. This method can increase the physical heat brought in by low calorific value coal gas and increase the amount of low calorific value coal gas used. Therefore, the comprehensive utilization of low calorific value coal gas can produce huge economic benefits and reduce environmental pollution caused by large-scale release. At present, the technology of generating generator gas through coal gasification and burning it in the high-temperature flue gas generated by the boiling furnace to generate ultra-high temperature flue gas has become the main breakthrough for expanding the adaptability of coal types and the production capacity of rotary kilns. Like the above methods, the key is to solve the problem that the combustion of low calorific value gas is not easy to organize. Therefore, the reasonable gas burner structure and the aerodynamic field and combustion chamber performance have become the focus of research. In the past, the gas burners used in China were mostly simple two-channel structures. The gas passed through the central channel and the primary air for combustion passed through the annular channel. The gas pressure was also low, generally only several thousand Pa. The mixing of gas and air was achieved by the swirl sheet and the shrinkage fixed at the front end of the burner. The shape and length of the flame can only be adjusted according to the pressure and flow of the primary air. The adjustment ability is very poor, and sometimes the operation cannot achieve the ideal combustion effect. In recent years, the use of multi-channel gas burners has become more and more common. Multi-channel burners generally have more than four channels and are usually composed of gas channels, central wind channels, swirl wind channels, radial wind channels, etc. The swirl wind makes the gas and air mix evenly. The central wind can adjust the length of the flame, and the radial wind is used to adjust the shape of the flame. Therefore, in the actual production process, the multi-channel burner can adjust the flame in all directions according to the working conditions to achieve the best combustion effect. As a result, the quality of the calcined finished product can be more stable and uniform, and the residence time of the material can be appropriately increased. Its energy-saving effect is also very obvious. At present, multi-channel burners suitable for various gas fuels have been developed in China, which can be fully localized, and the use effect is also very good. The price is only 10% of the imported ones. It is recommended that units using gas fuels in China use multi-channel combustion as much as possible to improve the combustion effect and save energy and reduce consumption. The use of multi-channel burners generally requires a gas supply pressure of 10-20kPa, and a gas pressure station needs to be built, which will increase some investment, but it is still very economical in terms of long-term energy saving and consumption reduction.
2. Heat radiation heat dissipation of the kiln body During the operation of the rotary kiln, the heat loss dissipated into the air through the cylinder accounts for more than 20% of the total heat. The heat of the hot air flow in the rotary kiln is dissipated to the surrounding air through the kiln lining and the cylinder in the form of heat conduction. The smaller the thermal conductivity of the kiln lining, that is, the greater the thermal resistance, the smaller the heat dissipation loss. Therefore, improving the thermal resistance of the kiln lining is an important measure for energy saving and consumption reduction of the rotary kiln. Heat dissipation is inevitable. Installing a heat exchanger outside the cylinder is another measure to reduce energy consumption.
2.1 Use different insulation kiln linings. Using thickened kiln linings to reduce heat dissipation will cause the kiln lining to be too thick, affecting the safety and efficiency of operation. Using kiln linings with different insulation capacities for different sections is the main way to solve this problem, which can save energy and improve economic benefits. Shale ceramsite insulation bricks are used in the drying zone and preheating zone where the inner surface temperature of the kiln lining is less than 1000℃ and the air flow temperature in the kiln is less than 1200℃. The thermal insulation performance of shale ceramsite insulation bricks is good. The outer surface temperature of the kiln body is 70-80℃ lower than that of clay bricks. The outer surface temperature of the clay brick cylinder is about 220℃, and that of ceramsite bricks is about 150℃. In addition, ceramsite bricks are built into the kiln instead of clay bricks. Due to their small bulk density, the weight of the kiln lining of the same thickness is reduced by 1/2. At the same time, their alkali resistance and thermal stability are better than clay bricks. The decomposition zone adopts alkali-resistant unburned composite bricks, which have good thermal shock stability, alkali erosion resistance, moderate load softening temperature, organic integration of the working layer and the thermal insulation layer, good thermal insulation effect, and significant energy saving effect after testing. The transition zone and cooling zone adopt high-activity combined composite bricks, which have low thermal conductivity, can effectively reduce the surface temperature of the rotary kiln cylinder and reduce heat loss. The performance is significantly better than the currently used magnesia-alumina spinel bricks and direct-bonded magnesia-chrome bricks.
2.2 Use auxiliary insulation materials. Place auxiliary insulation materials with lower thermal conductivity in the kiln lining of the rotary kiln to increase the thermal resistance of the kiln lining, thereby reducing the heat loss through the cylinder. The measures taken are mainly to change the shape of the refractory bricks to form special-shaped refractory bricks, and form a grid-like gap between the kiln lining and the cylinder as the filling space for the auxiliary insulation materials. The larger the volume of the cut-off part of the special-shaped refractory bricks, the larger the grid-like gaps formed, the more auxiliary insulation materials are placed, and the better the insulation effect. However, if the cut-off part of the special-shaped refractory bricks is too large, the radial strength of the refractory bricks will be reduced. In addition, the placement of the auxiliary refractory materials is also very important. For cost considerations, it is not necessary to place auxiliary insulation materials in the entire kiln lining of the rotary kiln. The surface temperature of the cylinder in the firing zone is the highest, and the heat loss through the cylinder in this section is the largest, which can reach more than 35% of the total heat loss of the cylinder. Simulation studies have shown that placing auxiliary insulation materials in the high-temperature section, namely the firing zone, is the best. In addition, refractory coatings can be applied to the surface of the kiln to reduce radiation heat dissipation.
2.3 Using preheaters, jackets, etc. to dissipate heat from the rotary kiln cylinder, different companies have recycled and reused them according to actual production conditions.
(1) Preheating the air. Install the preheater in the highest temperature zone of the cylinder, so that the air in the preheater moves turbulently against the surface of the cylinder and the surface of the heat sink. By extending the time the air stays in the preheater, the heat exchange effect is enhanced.
(2) Install a water jacket. Install two semicircular water jackets outside the high-temperature zone of the rotary kiln to clamp the cylinder. Use the heat dissipation of the kiln cylinder to heat the water in the jacket into steam or hot water, which is used for heating workshops, offices or factory baths.
(3) Weld the jacket layer for drying. Use the inclination angle of the rotary kiln and the rotation of the cylinder, and weld guide blades in the jacket to make the material run continuously along the cylinder wall and be dried by the heat dissipation of the kiln.
3. Recycling of waste heat
3.1 Reduce the energy consumption of kiln dust The heat loss of kiln dust accounts for about 1% to 4% of the heat consumption. Although the proportion is not large, kiln dust itself is a raw material and cannot be ignored. Avoid turning the electrostatic precipitator on and off, improve the efficiency of the dust collector, and choose the appropriate wind speed in the kiln, raw material particle weight and type of heat exchange equipment, which can reduce the escape of kiln dust and thus reduce the heat carried away by kiln dust.
3.2 Reduce the energy consumption of water reduction Reduce the energy consumption of water slurry The higher the water content of the slurry, the greater the heat consumption of evaporation of the slurry water. To reduce the heat of evaporation of slurry water, it is necessary to reduce the water content of the grinding slurry and reduce the external water of the pump. Using slurry diluent to improve the fluidity of slurry is the key to reducing the heat of evaporation of slurry water, and it is an economical and simple way to achieve energy saving and production increase in wet kilns. Theoretically, every 1% reduction in slurry water content can reduce the heat consumption of clinker by 1% to 2% and increase the production of clinker by 1.5% to 2.5%.
3.3 Heat loss of clinker cooling Material cooling is an important link that affects the energy consumption of the system. The temperature of the burned clinker entering the cooler is about 1000℃, carrying a large amount of heat energy. According to statistics, the heat loss of clinker produced by rotary kilns in my country accounts for 8% of the heat consumption of clinker, reaching 293-520KJ/kg, while it is generally 184-470KJ/kg abroad. In the energy saving and consumption reduction of rotary kilns, improving the cooling efficiency of the cooler is very critical to ensure the secondary air temperature entering the kiln. The single-drum cooler uses a lifting plate to cool the material by natural suction, which has poor cooling effect, the cooling air volume cannot be adjusted, the secondary air temperature is low, and the system energy consumption is high; although the grate cooler has good cooling effect, the equipment structure is complex, there are many moving parts and special materials, and it is inconvenient to maintain; the square multi-wind tower vertical cooler has a simple structure, and there are no other moving parts except the discharging vibrating feeder. It has a good cooling effect and can quickly cool the material from 1000℃ to about 100℃. After heat exchange, the cooling air enters the kiln as secondary air to participate in combustion, thereby accelerating the ignition of the coal powder entering the kiln and making it burn completely, thereby reducing the heat loss caused by the mechanical and chemical incomplete combustion of the coal powder. At the same time, a large amount of heat energy is recovered, and the energy-saving effect is very obvious. In addition, inside the cooler, you can consider placing a heat exchanger to make full use of the heat dissipation of the material, heating water or air, etc. for other purposes.
3.4 Heat loss from flue gas emissions Heat loss from flue gas emissions is the largest item in heat consumption, averaging about 2006KJ/kg, accounting for about 33% of heat consumption, which depends on flue gas temperature and flue gas volume. Increasing the heat transfer area in the kiln, enhancing the heat exchange effect, and strengthening the seal are the primary factors for reducing flue gas temperature and reducing flue gas emissions. According to statistics, for every 4°C reduction in kiln tail temperature, the dry kiln can reduce heat consumption by 3.8KJ/kg; the semi-dry kiln can reach 12KJ/kg; and the wet kiln can reach 5.6KJ/kg. Using 0.002kg less standard coal per kilogram of clinker can reduce the flue gas volume by 0.015m3 and the heat consumption by 4.18KJ/kg. Therefore, reducing flue gas losses is an important way to reduce heat consumption and improve enterprise benefits.
3.4.1 Waste heat power generation my country first adopted dry hollow rotary kilns with waste heat power generation systems in Dalian Cement Plant and Tangshan Qixin Cement Plant. The kiln has a flue gas temperature of 850-900℃. A waste heat boiler is installed at the end of the kiln and equipped with a steam turbine generator set. The steam generated by the waste heat boiler drives the steam turbine to generate electricity. In the 1980s, my country independently designed and developed several waste heat power generation kilns. The waste heat power generation technology has been greatly improved, and the operating parameters have been improved to about 215MPa. The installed capacity of a single unit has reached 3000kW. The domestically produced 1500kW and 3000kW steam turbine generator sets basically meet the needs of cement plants.
The need for waste heat power generation system. With the continuous advancement of technology, the operating parameters of the waste heat power generation system of 700t/d and 500t/d waste heat power generation kilns have been improved to 315-3182MPa, the installed capacity of a single unit has reached 3000kW and 6000kW, and the power generation per ton of clinker is 140-160kWh. At present, the best waste heat power generation system can generate 180-195kWh of power per ton of clinker. Generating electricity during the production process of the rotary kiln can basically solve the power load required for production, and the power self-sufficiency rate is 80-100%, which plays an important role in reducing electricity expenditure and improving the economic benefits of the enterprise.
3.4.2 Preheating materials Setting up an external preheating device at the end of the kiln and preheating materials with flue gas is an effective way to reasonably utilize the heat energy of the exhaust gas. Among the preheating technologies, the most widely used is the suspension preheating technology. Suspension preheating refers to the technology that low-temperature powder materials are evenly dispersed in high-temperature airflow, heat exchange is carried out in a suspended state, and the materials are quickly heated and heated. It integrates physical and chemical processes such as raw material preheating, gas-solid separation, raw material moisture evaporation, and partial carbonate decomposition. The suspension preheating technology fundamentally changes the heat transfer state of the material preheating process, and moves the preheating and decomposition process of the accumulated material in the kiln to the suspension preheater and the decomposition furnace respectively. Since the material is suspended in the hot air flow, the contact area with the air flow is greatly increased, so the heat transfer speed is extremely fast and the heat transfer efficiency is very high. At the same time, the raw material powder and the fuel are evenly mixed in a suspended state, and the heat of fuel combustion is transferred to the material, causing it to decompose rapidly. Therefore, the preheating technology greatly improves the production efficiency and thermal efficiency, and has the advantages of low heat consumption per unit product, high unit volume output of the kiln, less maintenance workload, and less investment per unit product. Suspension preheaters can be divided into two categories: vertical drum preheaters and cyclone preheaters. With the development and maturity of cyclone preheater kilns and external decomposition kilns, the development of vertical drum preheater kilns has been greatly impacted and has been eliminated. In addition, a heat exchanger can be placed in the tail flue gas duct to play the role of economizer and air preheater.
4. Auxiliary equipment energy consumption In the rotary kiln system, the equipment capacity of fans and pumps and the rated power of their drag motors should be reasonably selected. We should pay full attention to the energy-saving potential of such equipment, avoid such devices and their motors often being in non-rated working conditions, reduce motor efficiency, reduce power factor, and increase power consumption. If necessary, measures such as installing motor speed control devices can be taken to improve the efficiency of the equipment.
Rotary kilns are thermal equipment for heating bulk or slurry materials. They are widely used in nonferrous metallurgy, ferrous metallurgy, refractory materials, cement, chemical industry and papermaking, etc., especially in the production of some nonferrous metals. They are used for sintering, roasting and other heating treatments of ores, concentrates and intermediate products. In the more than 100 years since the advent of rotary kilns, rotary kilns have been used as combustion devices to provide the necessary heat for material calcination. During the production process, materials undergo high-temperature physical and chemical reactions in the rotary kiln. The material temperature can reach 1400-1450℃, and the maximum flue gas temperature can reach about 1700℃. Due to the low heat exchange efficiency between the hot air flow and the material in the kiln, a large amount of waste heat is generated. Taking measures to save energy and reduce consumption in rotary kilns is a fundamental and effective way to improve the economic benefits of related industries. It can be considered from the following four aspects:
(1) Fuel combustion efficiency. The main purpose is to reduce the mechanical incomplete combustion loss and chemical incomplete combustion loss of fuel;
(2) Heat radiation heat dissipation of the kiln body, that is, to strengthen the thermal insulation capacity of the kiln body and reduce the surface radiation heat transfer coefficient;
(3) Recycling of waste heat and waste heat. The heat loss of the rotary kiln mainly includes the heat loss of clinker cooling, the heat loss of evaporating water, the heat loss of kiln ash, the heat loss of flue gas, etc.:
(4) Energy consumption of auxiliary equipment. The following discusses the corresponding energy-saving and consumption-reducing technologies for various aspects.
1. Fuel combustion loss The types of fuel used in the rotary kiln system can be divided into gas fuel, solid fuel, liquid fuel and mixed fuel. Gaseous fuels are generally natural gas and coke oven gas. Natural gas is less used in China due to its high cost, while coke oven gas is a fuel commonly used by steel enterprises and is also suitable for rotary kiln calcining lime. Solid fuel refers to pulverized coal. Since some production enterprises do not have gas sources or have tight gas supply, pulverized coal is generally used as fuel. Liquid fuel refers to heavy oil, diesel and other fuels, which are almost not used at present due to their very high cost. Mixed fuels include high-coke mixed gas, high-speed mixed gas, pulverized coal gas mixed fuel, oil and gas mixed fuel, etc. Among them, the first three mixed fuels are used more. At present, the fuels used in domestic rotary kiln systems can be roughly divided into two categories: pulverized coal and gas, and their combustion devices are also different. The energy saving and consumption reduction of gas and pulverized coal combustion devices are discussed separately below.
1.1 Pulverized coal combustion According to thermal measurement statistics, the average heat loss of incomplete combustion of rotary kilns in my country is 250KJ per kilogram of clinker, accounting for 4% of the heat consumption. Among them, the chemical incomplete combustion loss is 34% to 37%, and the mechanical incomplete combustion loss of clinker is 54 to 71 kJ/kg on average. According to the data, an increase of 0.19% of CO in the exhaust gas is equivalent to a loss of 0.6% of fuel. The following issues should be handled well:
(1) Select reasonable coal powder control indicators. The finer the coal powder, the larger the specific surface area, the faster the combustion speed, and the smaller the incomplete combustion loss. The ash content of coal has a great influence on the calorific value and combustion speed. my country has a variety of coal types and the quality fluctuations are also large. For coal with high ash content, a smaller primary air volume and appropriately reduced coal powder fineness should be used to accelerate its combustion and reduce incomplete combustion. At the same time, it can increase the thermal intensity of the burning zone, increase production, and increase benefits.
(2) Reasonable excess air coefficient. The selection of reasonable exhaust and a certain amount of excess air for rotary kilns is the primary condition for complete combustion. Generally, it is better to control the excess air coefficient between 1.05 and 1.15.
(3) Reasonable burner structure. The rotary kiln generally adopts pulverized coal burners, and its technology is also very mature. With the development of rotary kiln coal-fired technology, pulverized coal burners have developed from single air duct to two air ducts, three air ducts, four air ducts and even five air ducts. At present, the four-air duct burner is the most widely used. The function of the single air duct is only to transport pulverized coal, and it has basically no regulating effect on the flame; the two air duct has some more regulating functions, but it is still not ideal; the three-air duct burner is mainly composed of pulverized coal duct, axial air duct and radial air duct, and the shape and length of the flame can be adjusted by changing the ratio of axial wind and radial wind; the four-air duct burner adds vortex wind or central air duct, which can play a very good effect on the mixed combustion of pulverized coal, the suction of secondary air, and the adjustment of flame length and shape. At present, the four-air duct burner is the most advanced burner. It can obtain the flame shape required in the burning zone, improve the combustion effect, reduce the amount of primary air, increase the amount of high-temperature secondary air to participate in combustion, and play a role in reducing heat consumption. In addition, the four-air duct burner has a wide range of adaptability to coal varieties. According to statistics from the cement industry, the use of four-channel pulverized coal burners can reduce heat consumption by 3% to 5% compared with other burners. Therefore, the four-channel structure should be used as much as possible in the selection of burners. It can be seen that the reasonable selection of pulverized coal burners is also the key to energy saving and consumption reduction.
1.2 Application of low calorific value gas The calorific value of coke oven gas is 3600kCal/Nm3, the calorific value of converter gas is 1600kCal/Nm3, and the calorific value of blast furnace gas is only 800kCal/Nm3. The calorific value required for the rotary kiln to calcine active lime using gas is generally greater than 2600kCal/Nm3. Coke oven gas is the most suitable fuel. However, the cost of coke oven gas is relatively high, and the supply often cannot meet the needs of active lime production, while converter and blast furnace gas are often forced to release in large quantities due to excess. How to use low calorific value gas is an important research topic in rotary kiln production, especially the use of generator gas. Low calorific value gas has been successfully applied in the active lime rotary kiln system. The main methods are mixed coal gas, mixed combustion of coal powder and coal gas, and preheating with high-efficiency heat exchangers. Mixed coal gas is a mixture of coke oven gas and converter or blast furnace gas in a certain proportion. The calorific value is reduced after mixing, but it can meet the requirements of active lime production; mixed combustion of coal powder and coal gas can be adjusted according to the calorific value and combustion temperature required for calcination, and mixed combustion of coal powder and coal gas; high-efficiency heat exchangers can use waste flue gas to preheat coal gas and primary air to 150-200℃. This method can increase the physical heat brought in by low calorific value coal gas and increase the amount of low calorific value coal gas used. Therefore, the comprehensive utilization of low calorific value coal gas can produce huge economic benefits and reduce environmental pollution caused by large-scale release. At present, the technology of generating generator gas through coal gasification and burning it in the high-temperature flue gas generated by the boiling furnace to generate ultra-high temperature flue gas has become the main breakthrough for expanding the adaptability of coal types and the production capacity of rotary kilns. Like the above methods, the key is to solve the problem that the combustion of low calorific value gas is not easy to organize. Therefore, the reasonable gas burner structure and the aerodynamic field and combustion chamber performance have become the focus of research. In the past, the gas burners used in China were mostly simple two-channel structures. The gas passed through the central channel and the primary air for combustion passed through the annular channel. The gas pressure was also low, generally only several thousand Pa. The mixing of gas and air was achieved by the swirl sheet and the shrinkage fixed at the front end of the burner. The shape and length of the flame can only be adjusted according to the pressure and flow of the primary air. The adjustment ability is very poor, and sometimes the operation cannot achieve the ideal combustion effect. In recent years, the use of multi-channel gas burners has become more and more common. Multi-channel burners generally have more than four channels and are usually composed of gas channels, central wind channels, swirl wind channels, radial wind channels, etc. The swirl wind makes the gas and air mix evenly. The central wind can adjust the length of the flame, and the radial wind is used to adjust the shape of the flame. Therefore, in the actual production process, the multi-channel burner can adjust the flame in all directions according to the working conditions to achieve the best combustion effect. As a result, the quality of the calcined finished product can be more stable and uniform, and the residence time of the material can be appropriately increased. Its energy-saving effect is also very obvious. At present, multi-channel burners suitable for various gas fuels have been developed in China, which can be fully localized, and the use effect is also very good. The price is only 10% of the imported ones. It is recommended that units using gas fuels in China use multi-channel combustion as much as possible to improve the combustion effect and save energy and reduce consumption. The use of multi-channel burners generally requires a gas supply pressure of 10-20kPa, and a gas pressure station needs to be built, which will increase some investment, but it is still very economical in terms of long-term energy saving and consumption reduction.
2. Heat radiation heat dissipation of the kiln body During the operation of the rotary kiln, the heat loss dissipated into the air through the cylinder accounts for more than 20% of the total heat. The heat of the hot air flow in the rotary kiln is dissipated to the surrounding air through the kiln lining and the cylinder in the form of heat conduction. The smaller the thermal conductivity of the kiln lining, that is, the greater the thermal resistance, the smaller the heat dissipation loss. Therefore, improving the thermal resistance of the kiln lining is an important measure for energy saving and consumption reduction of the rotary kiln. Heat dissipation is inevitable. Installing a heat exchanger outside the cylinder is another measure to reduce energy consumption.
2.1 Use different insulation kiln linings. Using thickened kiln linings to reduce heat dissipation will cause the kiln lining to be too thick, affecting the safety and efficiency of operation. Using kiln linings with different insulation capacities for different sections is the main way to solve this problem, which can save energy and improve economic benefits. Shale ceramsite insulation bricks are used in the drying zone and preheating zone where the inner surface temperature of the kiln lining is less than 1000℃ and the air flow temperature in the kiln is less than 1200℃. The thermal insulation performance of shale ceramsite insulation bricks is good. The outer surface temperature of the kiln body is 70-80℃ lower than that of clay bricks. The outer surface temperature of the clay brick cylinder is about 220℃, and that of ceramsite bricks is about 150℃. In addition, ceramsite bricks are built into the kiln instead of clay bricks. Due to their small bulk density, the weight of the kiln lining of the same thickness is reduced by 1/2. At the same time, their alkali resistance and thermal stability are better than clay bricks. The decomposition zone adopts alkali-resistant unburned composite bricks, which have good thermal shock stability, alkali erosion resistance, moderate load softening temperature, organic integration of the working layer and the thermal insulation layer, good thermal insulation effect, and significant energy saving effect after testing. The transition zone and cooling zone adopt high-activity combined composite bricks, which have low thermal conductivity, can effectively reduce the surface temperature of the rotary kiln cylinder and reduce heat loss. The performance is significantly better than the currently used magnesia-alumina spinel bricks and direct-bonded magnesia-chrome bricks.
2.2 Use auxiliary insulation materials. Place auxiliary insulation materials with lower thermal conductivity in the kiln lining of the rotary kiln to increase the thermal resistance of the kiln lining, thereby reducing the heat loss through the cylinder. The measures taken are mainly to change the shape of the refractory bricks to form special-shaped refractory bricks, and form a grid-like gap between the kiln lining and the cylinder as the filling space for the auxiliary insulation materials. The larger the volume of the cut-off part of the special-shaped refractory bricks, the larger the grid-like gaps formed, the more auxiliary insulation materials are placed, and the better the insulation effect. However, if the cut-off part of the special-shaped refractory bricks is too large, the radial strength of the refractory bricks will be reduced. In addition, the placement of the auxiliary refractory materials is also very important. For cost considerations, it is not necessary to place auxiliary insulation materials in the entire kiln lining of the rotary kiln. The surface temperature of the cylinder in the firing zone is the highest, and the heat loss through the cylinder in this section is the largest, which can reach more than 35% of the total heat loss of the cylinder. Simulation studies have shown that placing auxiliary insulation materials in the high-temperature section, namely the firing zone, is the best. In addition, refractory coatings can be applied to the surface of the kiln to reduce radiation heat dissipation.
2.3 Using preheaters, jackets, etc. to dissipate heat from the rotary kiln cylinder, different companies have recycled and reused them according to actual production conditions.
(1) Preheating the air. Install the preheater in the highest temperature zone of the cylinder, so that the air in the preheater moves turbulently against the surface of the cylinder and the surface of the heat sink. By extending the time the air stays in the preheater, the heat exchange effect is enhanced.
(2) Install a water jacket. Install two semicircular water jackets outside the high-temperature zone of the rotary kiln to clamp the cylinder. Use the heat dissipation of the kiln cylinder to heat the water in the jacket into steam or hot water, which is used for heating workshops, offices or factory baths.
(3) Weld the jacket layer for drying. Use the inclination angle of the rotary kiln and the rotation of the cylinder, and weld guide blades in the jacket to make the material run continuously along the cylinder wall and be dried by the heat dissipation of the kiln.
3. Recycling of waste heat
3.1 Reduce the energy consumption of kiln dust The heat loss of kiln dust accounts for about 1% to 4% of the heat consumption. Although the proportion is not large, kiln dust itself is a raw material and cannot be ignored. Avoid turning the electrostatic precipitator on and off, improve the efficiency of the dust collector, and choose the appropriate wind speed in the kiln, raw material particle weight and type of heat exchange equipment, which can reduce the escape of kiln dust and thus reduce the heat carried away by kiln dust.
3.2 Reduce the energy consumption of water reduction Reduce the energy consumption of water slurry The higher the water content of the slurry, the greater the heat consumption of evaporation of the slurry water. To reduce the heat of evaporation of slurry water, it is necessary to reduce the water content of the grinding slurry and reduce the external water of the pump. Using slurry diluent to improve the fluidity of slurry is the key to reducing the heat of evaporation of slurry water, and it is an economical and simple way to achieve energy saving and production increase in wet kilns. Theoretically, every 1% reduction in slurry water content can reduce the heat consumption of clinker by 1% to 2% and increase the production of clinker by 1.5% to 2.5%.
3.3 Heat loss of clinker cooling Material cooling is an important link that affects the energy consumption of the system. The temperature of the burned clinker entering the cooler is about 1000℃, carrying a large amount of heat energy. According to statistics, the heat loss of clinker produced by rotary kilns in my country accounts for 8% of the heat consumption of clinker, reaching 293-520KJ/kg, while it is generally 184-470KJ/kg abroad. In the energy saving and consumption reduction of rotary kilns, improving the cooling efficiency of the cooler is very critical to ensure the secondary air temperature entering the kiln. The single-drum cooler uses a lifting plate to cool the material by natural suction, which has poor cooling effect, the cooling air volume cannot be adjusted, the secondary air temperature is low, and the system energy consumption is high; although the grate cooler has good cooling effect, the equipment structure is complex, there are many moving parts and special materials, and it is inconvenient to maintain; the square multi-wind tower vertical cooler has a simple structure, and there are no other moving parts except the discharging vibrating feeder. It has a good cooling effect and can quickly cool the material from 1000℃ to about 100℃. After heat exchange, the cooling air enters the kiln as secondary air to participate in combustion, thereby accelerating the ignition of the coal powder entering the kiln and making it burn completely, thereby reducing the heat loss caused by the mechanical and chemical incomplete combustion of the coal powder. At the same time, a large amount of heat energy is recovered, and the energy-saving effect is very obvious. In addition, inside the cooler, you can consider placing a heat exchanger to make full use of the heat dissipation of the material, heating water or air, etc. for other purposes.
3.4 Heat loss from flue gas emissions Heat loss from flue gas emissions is the largest item in heat consumption, averaging about 2006KJ/kg, accounting for about 33% of heat consumption, which depends on flue gas temperature and flue gas volume. Increasing the heat transfer area in the kiln, enhancing the heat exchange effect, and strengthening the seal are the primary factors for reducing flue gas temperature and reducing flue gas emissions. According to statistics, for every 4°C reduction in kiln tail temperature, the dry kiln can reduce heat consumption by 3.8KJ/kg; the semi-dry kiln can reach 12KJ/kg; and the wet kiln can reach 5.6KJ/kg. Using 0.002kg less standard coal per kilogram of clinker can reduce the flue gas volume by 0.015m3 and the heat consumption by 4.18KJ/kg. Therefore, reducing flue gas losses is an important way to reduce heat consumption and improve enterprise benefits.
3.4.1 Waste heat power generation my country first adopted dry hollow rotary kilns with waste heat power generation systems in Dalian Cement Plant and Tangshan Qixin Cement Plant. The kiln has a flue gas temperature of 850-900℃. A waste heat boiler is installed at the end of the kiln and equipped with a steam turbine generator set. The steam generated by the waste heat boiler drives the steam turbine to generate electricity. In the 1980s, my country independently designed and developed several waste heat power generation kilns. The waste heat power generation technology has been greatly improved, and the operating parameters have been improved to about 215MPa. The installed capacity of a single unit has reached 3000kW. The domestically produced 1500kW and 3000kW steam turbine generator sets basically meet the needs of cement plants.
The need for waste heat power generation system. With the continuous advancement of technology, the operating parameters of the waste heat power generation system of 700t/d and 500t/d waste heat power generation kilns have been improved to 315-3182MPa, the installed capacity of a single unit has reached 3000kW and 6000kW, and the power generation per ton of clinker is 140-160kWh. At present, the best waste heat power generation system can generate 180-195kWh of power per ton of clinker. Generating electricity during the production process of the rotary kiln can basically solve the power load required for production, and the power self-sufficiency rate is 80-100%, which plays an important role in reducing electricity expenditure and improving the economic benefits of the enterprise.
3.4.2 Preheating materials Setting up an external preheating device at the end of the kiln and preheating materials with flue gas is an effective way to reasonably utilize the heat energy of the exhaust gas. Among the preheating technologies, the most widely used is the suspension preheating technology. Suspension preheating refers to the technology that low-temperature powder materials are evenly dispersed in high-temperature airflow, heat exchange is carried out in a suspended state, and the materials are quickly heated and heated. It integrates physical and chemical processes such as raw material preheating, gas-solid separation, raw material moisture evaporation, and partial carbonate decomposition. The suspension preheating technology fundamentally changes the heat transfer state of the material preheating process, and moves the preheating and decomposition process of the accumulated material in the kiln to the suspension preheater and the decomposition furnace respectively. Since the material is suspended in the hot air flow, the contact area with the air flow is greatly increased, so the heat transfer speed is extremely fast and the heat transfer efficiency is very high. At the same time, the raw material powder and the fuel are evenly mixed in a suspended state, and the heat of fuel combustion is transferred to the material, causing it to decompose rapidly. Therefore, the preheating technology greatly improves the production efficiency and thermal efficiency, and has the advantages of low heat consumption per unit product, high unit volume output of the kiln, less maintenance workload, and less investment per unit product. Suspension preheaters can be divided into two categories: vertical drum preheaters and cyclone preheaters. With the development and maturity of cyclone preheater kilns and external decomposition kilns, the development of vertical drum preheater kilns has been greatly impacted and has been eliminated. In addition, a heat exchanger can be placed in the tail flue gas duct to play the role of economizer and air preheater.
4. Auxiliary equipment energy consumption In the rotary kiln system, the equipment capacity of fans and pumps and the rated power of their drag motors should be reasonably selected. We should pay full attention to the energy-saving potential of such equipment, avoid such devices and their motors often being in non-rated working conditions, reduce motor efficiency, reduce power factor, and increase power consumption. If necessary, measures such as installing motor speed control devices can be taken to improve the efficiency of the equipment.