Talking about energy saving and consumption reduction
rotary kiln of rotary kiln is a thermal equipment for heat treatment of bulk or slurry materials. It is widely used in industrial sectors such as nonferrous metallurgy, ferrous metallurgy, refractory materials, cement, chemical industry and papermaking, especially in some industries. Non-ferrous metal production plays an important role in the treatment of ore, concentrate and intermediate products by sintering and roasting. During the more than 100 years of the rotary kiln, the rotary kiln as a combustion device provides the necessary heat for material calcination. During the production process, high temperature physical and chemical reactions occur in the rotary kiln. The temperature of the material can reach 1400 ~ 1450 °C, the maximum temperature of the flue gas. Up to 1700 °C, due to the low heat exchange efficiency between the hot gas flow in the kiln and the material, a large amount of waste heat is generated. Taking measures to save energy and reduce consumption in rotary kiln is a fundamental and effective way to improve the economic benefits of related industries. It can be considered from the following four aspects: (1) Combustion efficiency of fuel. Mainly to reduce the mechanical incomplete combustion loss of fuel, chemical incomplete combustion loss, etc.; (2) heat dissipation of kiln body heat, that is, to strengthen the heat insulation capacity of the kiln body, reduce the surface radiation heat transfer coefficient; (3) waste heat, waste heat Recycling. The heat loss of the rotary kiln mainly includes clinker cooling heat loss, evaporation moisture loss, kiln ash heat loss, flue gas heat loss, etc.: (4) Auxiliary equipment energy consumption. The following is a discussion of the corresponding energy saving technologies for various aspects.
1. Combustion Loss
of Fuels The types of fuel used in rotary kiln systems can be classified into gaseous fuels, solid fuels, liquid fuels, and mixed fuels. Gas fuels are generally natural gas and coke oven gas. Natural gas is used less in the country because of the high cost. Coke oven gas is a fuel commonly used by steel companies, and is also a fuel suitable for calcining lime in rotary kiln; solid fuel refers to pulverized coal. Because some manufacturers do not have gas supply or gas supply shortage, pulverized coal is generally used as fuel; liquid fuel refers to heavy oil, diesel and other fuels. Because of its high cost, it is hardly used at present; mixed fuel has high coke mixed gas and high Mixing gas, pulverized coal gas mixed fuel, oil and gas mixed fuel, etc., among which the first three mixed fuels are used.
At present, the fuel used in the domestic rotary kiln system can be roughly divided into two categories: pulverized coal and coal gas, and the combustion devices are also different. The energy saving and consumption reduction of the gas and pulverized coal combustion devices are separately discussed below.
1.1 Pulverized coal combustion
According to the thermal measurement statistics, the incomplete combustion heat loss of China's rotary kiln averages 250KJ per kilogram of clinker, accounting for 4% of heat consumption. Among them, the chemical incomplete combustion loss is 34% to 37%, and the incomplete combustion loss of clinker machinery is 54 to 71 KJ / kg. According to the data, the increase of 0.19% CO in the exhaust gas is equivalent to 0.6% loss of fuel. The following problems should be handled:
(1) Select a reasonable coal powder control index. The finer the pulverized coal, the larger the specific surface area, the faster the burning rate and the less incomplete combustion loss. The ash content of coal has a great influence on heat generation and burning speed. China's coal species are diverse and the quality fluctuations are also large. For a high ash coal, a small amount of air should be appropriately reduced and the fineness of pulverized coal to accelerate the combustion and reduce the incomplete combustion can be improved while the strength of the thermal burning zone, to improve
yieldand increase efficiency.
(2) A reasonable excess air ratio. Rotary kiln selection of reasonable exhaust and a certain amount of excess air is the primary condition for complete combustion. Generally, the excess air coefficient is preferably controlled between 1.05 and 1.15.
(3) Reasonable burner structure. Rotary kiln is commonly used in 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 ducts to two air ducts, three air ducts, four air ducts and even five air ducts. Currently, four air duct burners are the most widely used. The function of the single air duct is only to play the role of conveying coal powder, and has no regulating effect on the flame; the second air duct has some adjustment functions, but it is still not ideal; the three air duct burner is mainly composed of pulverized coal air duct and axial wind. The air duct and the radial wind duct are composed, and the shape and length of the flame can be adjusted by changing the ratio of the axial wind and the radial wind; the four-channel burner adds a vortex or a central duct for the mixed combustion of the pulverized coal. The suction of the secondary air, the length of the flame and the adjustment of the shape can all be very effective.
At present, the four-channel burner is the most advanced burner, which can obtain the shape of the flame required in the firing zone, improve the combustion effect, reduce the amount of primary air, increase the secondary air volume of high temperature and participate in combustion, and reduce the heat consumption. . In addition, the four-duct burner has a wide range of adaptation to coal varieties. According to the statistics of the cement industry, the four-channel pulverized coal burner can reduce the heat consumption by 3% to 5% compared with other burners. Therefore, the four-duct structure should be used as much as possible in the selection of the burner. It can be seen that the rational selection of pulverized coal burners is also the key to energy saving.
1.2 Application of low calorific value gas The calorific value of
coke oven gas is 3600 kC al/Nm3, the calorific value of converter gas is 1600kCal/Nm3, the calorific value of blast furnace gas is only 800kCal / Nm3, and the calorific value required for calcining active lime in rotary kiln Generally more than 2 600 kCal / 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. However, converters and blast furnace gas are often forced to be dissipated due to excess. How to use low calorific value gas is an important research topic in rotary kiln production, especially when it occurs. Furnace gas.
Low calorific value gas has been successfully applied in activated lime rotary kiln systems. The main method is to use mixed gas, coal powder and gas co-firing, high-efficiency heat exchanger for preheating. The mixed gas is a mixture of coke oven gas and converter or blast furnace gas in a certain proportion, and the calorific value is reduced after mixing, but can meet the requirements of active lime production; the mixed combustion of coal powder and gas can be based on the calorific value and combustion required for calcination. Temperature regulation, mixing pulverized coal and coal gas; high-efficiency heat exchanger can use waste flue gas to preheat the gas and primary air to 150 ~ 200 °C, this way can improve the physical heat brought by low calorific value gas, can Increase the amount of low calorific value gas used. Therefore, the comprehensive utilization of low calorific value gas can generate huge economic benefits and at the same time reduce environmental pollution caused by mass release. At present, the technology of generating furnace gas by coal gasification and burning in high-temperature flue gas generated by a boiling furnace, thereby generating ultra-high temperature flue gas, has become a major breakthrough in expanding the adaptability of coal types and the production capacity of rotary kiln, as in the above method. The key is to solve the problem that low-calorific value gas combustion is not easy to organize. Therefore, reasonable gas burner structure, 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 passage, and the primary air was passed by the annular passage. The gas pressure was also low, generally only a few thousand Pa, and the gas and air mixture was fixed. The swirling fin and the shrinkage at the front end of the burner are realized. The shape and length of the flame can only be adjusted according to the pressure and flow of the primary air, and the adjustment ability is very poor, and sometimes the operation cannot achieve the desired
combustion effect. In recent years, the use of multi-channel gas burners has become more and more common. Multi-channel burners are generally more than four channels, usually composed of gas passages, central air passages, swirling air passages, radial wind passages, etc.,
swirling wind makes gas and air Mix evenly, the center 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 flame may beaccording to the conditionsthe whole
adjustedoforientation to achieve optimal combustion efficiency, which can result in more stable product quality calcined uniform, but also an appropriate increase in the residence time of the material, Its energy saving effect is also very obvious. At present, China has developed a multi-channel burner suitable for various gas fuels, which can achieve all localization, and the effect is also very good. The price is only 10% of the import. It is recommended to use multi-channel combustion in units using gas fuel in China. Improve combustion and save energy. The use of multi-channel burners generally requires a gas supply pressure of 10 to 20 kPa. The need to build a gas pressure station will increase some of the investment, but it is still very economical in terms of long-term energy conservation and consumption reduction.
2. Heat radiation in the kiln body During the
working process of the rotary kiln, the heat loss from the cylinder to the air accounts for more than 20% of the total heat. The heat of the hot air in the rotary kiln is transferred to the surrounding air through the kiln lining and the cylinder by heat conduction. The smaller the thermal conductivity of the kiln lining, that is, the greater the thermal resistance, the smaller the heat loss, so the heat of the kiln lining is increased. Resistance is an important measure for energy saving and consumption reduction of rotary kiln. Heat dissipation is inevitable, and placing heat exchangers outside the cylinder is another measure to reduce energy consumption.
2.1 The use of different insulated kiln linings to
thicken the kiln lining to reduce heat dissipation, there will be problems that make the kiln lining too thick and affect the safety and efficiency of operation. The kiln lining with different insulation capabilities for different sections is the main way to solve this problem, so as to save energy and improve economic benefits.
In the dry zone, the pre-tropical kiln lining inner surface temperature is less than 1 000 °C, and the kiln gas flow temperature is less than 1200 °C, shale ceramsite insulation brick is used. The thermal insulation performance of the shale ceramsite insulating brick is good. The surface temperature of the kiln is reduced by 70 to 80 °C compared with the clay brick. The outer surface temperature of the clay brick is about 220 °C, and the ceramsite brick is about 150 °C. In addition, ceramsite bricks are replaced by clay bricks in the kiln. Due to the small bulk density, the kiln lining of the same thickness is reduced in weight by 1/2, and its alkali resistance and thermal stability are better than clay bricks.
The decomposition zone adopts alkali-resistant and non-burning composite bricks, which has good thermal shock stability, alkali corrosion resistance, moderate load softening temperature, organic integration of working layer and heat insulation layer, good heat insulation effect, and remarkable energy-saving effect after testing.
The transition zone and the cooling zone are made of high-activity composite brick, which has low thermal conductivity, can effectively reduce the surface temperature of the rotary kiln cylinder, and reduce heat loss. The performance is obviously better than the currently used magnesium-aluminum spinel brick and direct combination. Magnesia chrome brick.
2.2 Use auxiliary insulating materials
to place auxiliary thermal insulation materials with smaller thermal conductivity in the kiln lining of the rotary kiln, and improve the thermal resistance of the kiln lining to reduce the heat loss through the cylinder. The measures taken mainly by changing the shape of the refractory bricks to form shaped refractory bricks, forming a grid-like gap between the kiln lining and the cylinder, as a filling space for the auxiliary heat insulating material. The larger the volume of the cut-off part of the shaped refractory brick, the larger the mesh-like void formed. The more auxiliary insulating material is placed, the better the heat insulation effect, but the shaped refractory brick is cut off. Resulting in a reduction in the radial strength of the refractory brick In addition, the location of the auxiliary refractory material is also important. For cost reasons, it is not necessary to place auxiliary insulation material in the kiln lining of the entire rotary kiln. The surface temperature of the calcined belt cylinder 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 it is best to place auxiliary insulation materials in the high temperature section, ie, the firing zone.
In addition, refractory coatings can be applied to the surface of the kiln to reduce radiation heat dissipation.
2.3 Using preheaters, jackets, etc.
for the cooling of the rotary kiln cylinder, different companies have recycled them according to the actual production.
(1) Preheat the air once. The preheater is installed in the highest temperature zone of the cylinder, so that the air is turbulently moved in the preheater against the surface of the cylinder and the surface of the fin, and the heat exchange effect is enhanced by prolonging the time of the air in the preheater.
(2) Set the water jacket. Two semi-circular water jackets are installed outside the high temperature belt of the rotary kiln to clamp the cylinder. Use the heat of the kiln cylinder to heat the water in the jacket to steam or hot water for heating in the workshop, office or factory bath.
(3) Welded jacket layer for drying. The inclined angle of the rotary kiln and the rotary motion of the cylinder are used, and the guide vanes are welded in the jacket to continuously run the material along the wall of the cylinder and be dried by the heat dissipation of the kiln body.
3. Recycling of waste heat and waste heat
3.1 Reducing the energy consumption of
kiln dust The heat loss of kiln dust is about 1% to 4% of the heat consumption. Although the proportion is not large, the kiln dust itself is a raw material. Neglect. Avoid the opening and closing of the electric precipitator, improve the efficiency of the precipitator, choose the appropriate kiln wind speed, raw material particle gravity and heat exchange equipment type, can reduce the kiln ash escape, thus reducing the kiln dust away The heat.
3.2 Reduce the energy consumption taken by the
water and reduce the energy consumption of the water. The higher the moisture content of the slurry, the greater the heat loss of the slurry water evaporation. To reduce the evaporation heat of the slurry, it is necessary to reduce the moisture of the abrasive slurry and reduce the external water of the pump. The use of slurry diluent to increase the fluidity of the slurry is the key to reducing the heat of evaporation of the slurry water, and is an economical and simple way to achieve energy saving and increase production in the wet process kiln. Theoretically, for every 1% reduction in slurry moisture, the clinker heat consumption can be reduced by 1% tocan be
2%, and the clinkerincreased by 1.5% to 2.5%.
3.3 Clinker cooling heat loss
Material cooling is an important part of the system energy consumption. When the clinker is burned into the cooler, the temperature is about 1000 °C, with a lot of heat. According to statistics, China's rotary kiln production clinker heat loss accounts for 8% of clinker heat consumption, reaching 293 ~ 520 KJ / kg, and abroad is generally 184 ~ 470KJ / kg. In the energy saving and consumption reduction of the rotary kiln, increasing the cooling efficiency of the cooler is critical to ensuring the secondary air temperature entering the kiln. The single-cylinder cooler uses the suction plate to cool the material by natural suction. The cooling effect is poor, the cooling air volume cannot be adjusted, the secondary air temperature is low, and the system energy consumption is high; although the cooling effect of the grate cooler is good, the equipment structure is complicated. There are many moving parts and special materials, and the maintenance is inconvenient; the square multi-wind tower type vertical cooler has a simple structure, and has no moving parts other than the material vibrating feeder. The cooling effect is good, and the material can be quickly cooled from 1000 °C to 100. Around °C, the cooling air is burned as a secondary air into the kiln to participate in the combustion, thereby accelerating the ignition of the pulverized coal into the kiln, causing it to burn completely, thereby reducing the heat loss caused by the incomplete combustion of the mechanical and chemical pulverized coal. The heat is recovered and the energy saving effect is very obvious.
In addition, in the interior of the cooler, it is conceivable to place the heat exchanger, make full use of the heat dissipation of the material, and heat water or air for other uses.
3.4 Flue gas emission heat loss
Flue gas emission heat loss is the largest heat loss, with an average of about 2006 KJ / kg, accounting for about 33% of heat consumption, which depends on the flue gas temperature and the amount of flue gas. Increasing the heat transfer area in the kiln, enhancing the heat exchange effect, and strengthening the seal are the primary factors for reducing the temperature of the flue gas and reducing the emission of flue gas. According to statistics, for every 4 °C reduction in kiln temperature, dry kiln can reduce heat consumption by 3.8 KJ / kg; semi-dry kiln up to 12 KJ / kg; wet kiln up to 5.6 KJ / kg. Less than 0.002kg of standard coal per kilogram of clinker can reduce the amount of smoke of 0.015m 3 and reduce the heat consumption of 4.18 KJ /kg. Therefore, reducing smoke loss is an important way to reduce heat consumption and an important way to improve the efficiency of enterprises.
3.4.1 Waste heat power generation
China's earliest dry-process hollow rotary kiln with waste heat power generation system was adopted in Dalian Cement Plant and Tangshan Qixin Cement Plant. The kiln flue gas discharge temperature is 850 ~ 900 °C, and a waste heat boiler is installed at the kiln tail, and a steam turbine generator set is equipped to drive the steam turbine to generate electricity by using steam generated by the waste heat boiler. In the 1980s, China designed and developed several waste heat power generation kiln, and the waste heat power generation technology has been greatly improved. The operating parameters have been increased to about 215 MPa, and the single installed capacity has reached 3000 kW. The domestic 1500 kW and 3000 kW steam turbine generator sets have basically met. The need for a waste heat power generation system in a cement plant. 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 kiln are increased to 315 ~ 3182MPa, the installed capacity of single unit is 3000kW and 6000kW, and the power generation capacity of tonne clinker is 140 ~ 160kWh. At present, the best waste heat power generation system can produce 180 to 195 kWh of clinker. The electric power is generated at the same time as the rotary kiln production process, which can basically solve the electric load required for production. The self-sufficiency rate of electric power is 80-100%, which plays an important role in reducing electricity expenses and improving the economic benefits of enterprises.
3.4 .2 Preheating materials The. Preheating materials
preheating device outside the kiln is installed at the kiln endwith flue gas are an effective method for rationally utilizing the thermal energy of the exhaust gas. Among the preheating technologies, the most widely used is the suspension preheating technology. Suspension preheating refers to a technique in which low-temperature powder materials are uniformly dispersed in a high-temperature gas stream, heat exchange is carried out in a suspended state, and the materials are rapidly heated and heated, and the raw materials are preheated, gas-solid separation, raw material moisture evaporation, and partial carbon. Physical and chemical processes such as acid decomposition are integrated. The suspension preheating technology fundamentally changes the heat transfer state of the material preheating process, and the preheating and decomposition processes of the material accumulation state in the kiln are separately moved to the suspension preheater and the decomposition furnace in a suspended state. Since the material is suspended in the hot gas stream, the contact area with the gas stream is greatly increased, so the heat transfer rate is extremely fast and the heat transfer efficiency is high. At the same time, the raw meal powder and the fuel are uniformly mixed in the suspended state, and the fuel combustion heat is transferred to the material to be rapidly decomposed. 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 yield of the kiln, low maintenance workload, and low investment per unit product. Suspension preheater can be divided into two types: vertical cylinder preheater and cyclone preheater. With the development and maturity of the cyclone preheater kiln and the kiln decomposition kiln, the development of the vertical preheater kiln has been greatly impacted and has been eliminated.
In addition, a heat exchanger can be placed in the tail flue gas duct to function as an economizer and an air preheater.
4 Auxiliary equipment energy consumption
In the rotary kiln system, the equipment capacity of the fan and pump should be reasonably selected and the rated power of the drag motor. It is necessary to pay full attention to the energy-saving potential of such equipment, to avoid such devices and their motors often in non-rated working conditions, to reduce motor efficiency, reduce power factor, increase power consumption, and take measures to install motor speed control devices when necessary. , improve the efficiency of the equipment.