In many industrial combustion furnaces used in metallurgy, machinery, building materials and other departments, the temperature of exhaust gas is as high as 600~1100 ℃. In order to make full and effective use of this heat, many researchers have done a lot of research work in this area. It is an effective and profitable measure to use the heat recovery device to recover the waste heat taken away by the flue gas, heat the air and gas for combustion support, and then send them back to the furnace combustion chamber.
In the early days, the heat recovery devices for recovering waste heat for air preheating were mainly inter wall heat exchanger and regenerative heat exchanger. The gas flow direction of the intermittent wall heat exchanger is unchanged, and the working condition is stable, but its preheating temperature is not more than 700 ℃, its service life is short, the heat recovery rate is low, and the exhaust gas still has a high temperature. The preheating temperature of the regenerative heat exchanger can reach 1200 ℃, while the exhaust gas temperature is low, approaching 300 ℃, and the service life is long. The maximum heat recovery rate can reach 70%. However, the regenerator of the early regenerative heat exchanger was made of checkered bricks, with low comprehensive heat transfer coefficient, large volume of the regenerator, long reversing time and large fluctuation of preheating temperature. At the same time, the exhaust temperature of flue gas is still 300~600 ℃. The heat exchange equipment is required to be both heat-resistant and airtight, which makes the structure complex and the operation inflexible. Considering the economy, material performance, thermal efficiency and other factors of the heat exchanger, the heating temperature of the inter wall heat exchanger with good performance can reach about 1000 ℃, and the maximum preheating air temperature can reach 700 ℃. If the preheating temperature is increased again, the problem of high NOx, increased equipment costs due to the expansion of the heat transfer area of the heat exchanger and the life of the heat exchanger itself will occur. The regenerative heat exchanger shows new vitality because of its high efficiency and energy-saving characteristics and the development of the material industry.
2. Origin of High Temperature Air Combustion Technology
In 1982, British Hotwork and British Gas cooperated to develop a compact ceramic ball heat storage system (RCB) for the first time. The system uses ceramic balls as the heat storage body, with a specific surface area of 240m2/m3, so the heat storage capacity is greatly enhanced, the volume of the heat storage body is significantly reduced, the reversing time is reduced to 1~3min, the temperature efficiency is significantly improved (generally more than 80%), and the preheating temperature fluctuation is generally less than 15 ℃. In the following years, a large number of experimental studies have been carried out on the heat storage system and a trial run has been made. The application in stainless steel annealing furnace and walking beam furnace has achieved the expected results and achieved remarkable economic benefits.
Japan began to further develop the RCB after a detailed investigation of its application technology and actual use around 1985. In the early 1990s, NKK and NFK jointly developed a new type of heat accumulator, called High cycle Regenerative Combustion System. The ceramic honeycomb with small pressure loss and larger specific surface area is used to reduce the volume and weight of the heat accumulator. In order to achieve low NOx emissions, the regenerator and burner work together as a whole. Two stage combustion method and flue gas self recirculation method are used to control the intake air, with good results. NKK carried out many tests and analyzed the measured data. The results show that the temperature of the air entering the burner after pre heating is close to the exhaust gas emission temperature. The data shows that when the air preheating temperature reaches 1300 ℃ and the O2 content in the furnace is 11%, the NOx emission is 40kg/m3 [1]. The development of HRS has not only realized the limit recovery of flue gas waste heat and the substantial reduction of NOx emissions, but also triggered a new combustion technology - High Temperature Air Combustion (HTAC).
HTAC technology is different from traditional combustion technology in terms of combustion conditions, reaction mechanism, flame characteristics, etc. The preheating air temperature is above 800~1000 ℃, and the fuel is burned in a high temperature environment with low oxygen content (as low as 2%). Because the flammable range is expanded under high temperature, stable combustion can be guaranteed when the oxygen content is greater than 2%. The combustion process is similar to a diffusion controlled reaction. There is no local high temperature zone, and NOx generation is inhibited in this environment. At the same time, in this low oxygen environment, the combustion flame has different characteristics from the traditional combustion: the flame volume is significantly increased, even to the entire combustion chamber space; Irregular flame shape, no flame interface; The common incandescent flame disappears, and the flame appears as mist; The radiation intensity increases, and the height radiation of the flame decreases. The whole combustion space is like a high temperature strong radiation black body with relatively uniform temperature. In addition, the reaction speed is fast, the heat transfer efficiency of the furnace is significantly improved, and the NOx emission is greatly reduced [2].
3 Working principle and characteristics of HTAC technology
The key technology of HTAC is to adopt efficient regenerative combustion system [3]. The system consists of a combustion chamber, two sets of regenerative burners with the same structure and a four-way valve. The burners can be arranged symmetrically or collectively. Figure 1 shows the schematic diagram of two groups of burners arranged symmetrically. When burner A works, the high-temperature waste gas after heating the workpiece is discharged through burner B, and the heat is quickly transferred to the heat accumulator in the form of radiation and convection. After heat release, the temperature of flue gas drops below 200 ℃ and is discharged through four-way valve. After a certain time interval, the switching valve makes the combustion supporting air flow through the regenerator B, and the regenerator quickly transfers the heat to the air. The air is preheated to more than 800 ℃, and the combustion process is completed through the burner B. At the same time, burner A and heat accumulator A are converted into smoke exhaust and heat storage devices. Through this alternative operation mode, the limit recovery of flue gas waste heat and preheating of combustion supporting air can be realized. The new ceramic honeycomb regenerator can achieve a temperature difference of 50~150 ℃ between the exhaust temperature and the preheated air temperature.
In order to reduce NOx generation, two-stage combustion method and flue gas self recirculation method are adopted. Figure 2 is the schematic diagram of the regenerative burner burner. The center of the burner is an air flow passage. Primary fuel is supplied in the tangential direction around the throat, and secondary fuel is supplied in the direction parallel to the air flow passage at the throat outlet. The combustion of primary fuel (much less than secondary fuel) belongs to oxygen enriched combustion, which will be completed quickly under high temperature conditions. After the combustion flue gas flows through the optimized nozzle, it forms a high-speed gas jet and surrounding entrainment backflow movement, and the furnace is infiltrated
The oxygen content can reach 5%~15%. A large amount of fuel is injected into the furnace in parallel through the secondary gas channel to mix and burn with the flue gas with low oxygen concentration in the furnace. At this time, there is no local hot high temperature zone in the furnace, forming a flame with uniform temperature distribution. Therefore, NOx emission is greatly reduced.
HTAC technology is mainly realized through efficient honeycomb heat storage system, and its characteristics are as follows:
(1) The regenerator has fast heat transfer speed, strong heat storage capacity, short switching time, good dynamic heat transfer and less pressure loss.
(2) The air and gas flow into the furnace is fast, the combustion process such as fuel cracking and spontaneous combustion in the furnace is accelerated, and the chemical reaction rate and combustion efficiency are improved.
(3) The flame starts to burn gradually not in the burner but in the furnace space, with low combustion noise.
(4) Under high temperature conditions, as long as the fuel mixture enters the flammable range, stable combustion in the furnace can be guaranteed.
(5) In the high-temperature and low oxygen environment, combustion produces a large amount of cracking, forming a large amount of C2, which causes a strong thermal radiation effect and enhances the radiation force.
(6) The furnace temperature distribution is uniform, the maximum temperature during combustion is reduced, the average temperature is greatly increased, and the heat transfer efficiency is significantly increased.
(7) The generation of NOx and dioxin is inhibited and the emission is greatly reduced.
(8) Except the regenerative burner and furnace body, other equipment operates at the low temperature end.
4 Application effect of HTAC technology
4.1 Compact structure and low initial investment
The volume of the regenerator and furnace body of the HRS system can be greatly reduced due to the greatly enhanced heat exchange capacity. The waste gas discharged from the heat accumulator (with a temperature of only about 200 ℃) is pumped out by the induced draft fan, and the long flue and chimney requiring refractory lining are removed. The equipment is simplified, and the land area is reduced, so that the initial investment is less. In addition to the construction of new furnaces, HTAC technology is also suitable for the transformation of old furnaces. Regenerative burner is a combination of regenerator and burner, which can be retrofitted by combining the external regenerative burner with the old furnace type. It is only necessary to modify the furnace body slightly based on the original furnace.
4.2 Small temperature difference, good heating quality
After HTAC technology is applied, the temperature distribution in the combustion furnace is uniform, the temperature difference reaches ± 5 ℃, and the lower oxygen content environment in the furnace is very favorable for heating the workpiece. It not only improves the heating speed and quality, but also reduces the oxidation burning loss rate of the workpiece and greatly improves the output of the furnace. In addition, the furnace temperature can be adjusted and controlled conveniently and accurately by adjusting the flow to achieve a balanced furnace temperature to meet different heating requirements.
4.3 Flexible layout and convenient operation
The HRS system is compact in structure, small in size and flexible in layout. It can determine the position and number of burners according to the process requirements and furnace body shape. The burner can be located on the side, top and axial (furnace nose section is required). Pairs of burners can be reversed independently, or multiple pairs of burners can be reversed collectively in sections, so the control is flexible [4]. The four-way valve and control system are both at the low temperature end, so the operation is convenient, safe and reliable.
4.4 Remarkable energy-saving effect
The honeycomb ceramic heat accumulator is used to realize the limit recovery of waste heat of flue gas, and the recovery rate of waste heat of flue gas can reach more than 85%. At the same time, in the high air preheating temperature and low oxygen environment with uniform mixing, once the fuel contacts with O2 molecules, it can burn rapidly. Therefore, the excess air coefficient for complete combustion can be close to 1, greatly reducing the furnace inlet and outlet flow and smoke exhaust loss, and further improving the fuel saving rate. The practical application shows that the fuel saving rate can reach more than 55%.
4.5 Less pollutant discharge
The application of HTAC technology has positive effects on environmental protection: (1) The high efficiency and energy saving of HTAC burner and the sufficiency of combustion process greatly reduce the emission of CO, CO2 and other greenhouse gases in flue gas; (2) The combustion environment of high temperature and low oxygen and the mixing effect of flue gas backflow greatly inhibit the generation of NOx and reduce the emission of NOx to less than 100 mg/m3; (3) The high-temperature environment inhibits the generation of dioxin, and the exhaust gas cools rapidly, effectively preventing the re synthesis of dioxin, so the emission of dioxin is greatly reduced; (4) The flame gradually diffuses and burns in the whole furnace, with low combustion noise.
4.6 Expansion of fuel range of industrial furnace
The development of HTAC technology has greatly expanded the application scope of industrial furnace fuel. It can well burn low calorific value fuel without ignition difficulty and misfire problem, and the fuel variety is not limited to gas or liquid. With the development of high-temperature air related technologies, coal, industrial waste and other solid fuels can also be used. At present, Japan has developed a multi-stage enthalpy extraction technology for high-temperature air gasification, which can handle a variety of calorific value raw materials, including various wastes and biomass combustibles. The use of solid fuel is usually to gasify high-temperature air into gas, purify it, and then use it for high-temperature air combustion.
4.7 Strong applicability and wide application scope
The excellent characteristics of HTAC technology make it a wide range of applications, and it can be used in a variety of industrial furnaces with different process requirements. At present, the furnace types that can use this technology include large and medium-sized pusher type and walking beam type steel rolling heating furnace, soaking pit, hood type heat treatment furnace, radiant tube gas carburizing furnace, ladle baking furnace, glass melting furnace, aluminum melting furnace, forging furnace, etc. The scope covers metallurgy, metal processing, chemical industry, ceramics, textiles and other industries. In addition, HTAC technology is also applicable to enterprises with unstable production and large output fluctuation.
5 Application prospect of HTAC technology in China
China is a major fuel consumer in the world. From the perspective of China's energy status, HTAC technology will have broad application prospects in China.
Industrial furnaces in China consume a large amount of energy. During the "Seventh Five Year Plan" period, the energy consumption of kilns accounted for 1/4 of the total industrial energy consumption in China, accounting for 40% of industrial energy consumption. The average thermal efficiency of industrial furnace is low, only about 20%. The average unit consumption of products is 40% higher than that of developed countries. According to statistics, most of the energy of the kiln is attributed to the loss of smoke exhaust, which is estimated to be equivalent to more than 50 million tons of standard coal every year. In view of this situation, there is a great potential to improve the utilization rate of industrial furnace fuel and the recovery rate of flue gas waste heat so as to achieve energy conservation.
For a long time, the excessive emission of atmospheric harmful substances has been quite serious in China. Among the 10 cities with the most serious atmospheric pollution in the world, China accounts for 7. In order to reduce the emission of air pollutants, the first is to reduce energy consumption, and the second is to control emissions. These two points just conform to the technical characteristics of HTAC, namely, high efficiency, energy saving and low pollution. Therefore, the application of HTAC technology in China is imperative.
From the perspective of China's energy structure, coal and other solid fuels account for a large proportion, while liquid and gas fuels account for a small proportion. However, after the 1980s, the overall development trend was that the proportion of coal-fired and oil fired kilns decreased, while the proportion of gas fired kilns increased significantly. Although HTAC technology is currently only suitable for direct burning of gas and some liquid fuels, with the adjustment of China's energy structure, the implementation of the "West East Gas Transmission Project", the continuous discovery of new natural gas resources in Sichuan, Inner Mongolia and other places, and the further development of HTAC technology, it can be expected that the application of HTAC technology in China will have a rapid development.
6 Conclusion
HTAC technology has the characteristics of high efficiency, energy saving and low pollution. Since it came out, it has been widely concerned by the world industry and enterprises. It has completely broken the traditional combustion mode and entered a new unknown field - high temperature and low oxygen combustion field. It is an energy saving and environment-friendly technology with great vitality, which is worth promoting and developing. For enterprises, it can greatly reduce energy consumption and production costs, and improve the economy and market competitiveness of its operation. HTAC technology is considered as a new strategic technology with creativity, practicality and growth potential.
China's energy situation is not optimistic, and high energy consumption, high pollution and low efficiency are quite serious. With the continuous development of economy, it will face the severe test of energy shortage. Therefore, vigorously promoting the application of HTAC technology in China will bring a historical opportunity for China's rapid development.
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No 2022.08.27Product Recommendation
