Application of regenerative high-temperature air combustion technology Wu Daohong Ou Yangze Chloe Wang Rufang Xiao Zeqiang Keywords: regenerator, high-temperature air, reversing valve, combustion and nitrogen oxides Abstract: This paper briefly introduces the principle, technical advantages and application prospect of regenerative high-temperature air combustion technology in China, focusing on the basic research progress in the field of regenerative high-temperature air combustion technology in China and its popularization, application and development in China's industrial heating industry. 1 Foreword High temperature air combustion technology is called HTAC technology in Japan, the United States and other countries, and HPAC (high preheating air combustion) technology in some western European countries, also known as flameless combustion technology. The basic idea is to make the fuel burn in the atmosphere of high temperature and low oxygen concentration (volume). It includes two basic technical measures: First, a regenerative heat exchanger with temperature efficiency as high as 95% and heat recovery rate as high as 80% is adopted to recover sensible heat in combustion products to the maximum extent and preheat combustion air to obtain high-temperature combustion air at 800 ~ 1000℃ or even higher. The other is to use fuel staged combustion and high-speed airflow to suck combustion products in the furnace, and dilute the oxygen concentration in the reaction zone to obtain a low-oxygen atmosphere with a concentration of 15% ~ 3% (volume ratio). In this high-temperature and low-oxygen atmosphere, the fuel first undergoes a recombination process such as cracking, which leads to completely different thermodynamic conditions from the traditional combustion process, and releases heat energy under the delayed combustion with oxygen-poor gas, so there is no longer a local high-temperature and high-oxygen region in the traditional combustion process. This combustion is a dynamic reaction, and there is no static flame. It has many advantages, such as high efficiency, energy saving, ultra-low NOX emission, etc. It is also called environment-friendly combustion technology [1-2]. High temperature air combustion technology has been attached great importance by developed countries such as Japan, the United States, Sweden, the Netherlands, Britain, Germany, Italy and so on since it came out. Its application in the heating industry has been rapidly promoted, and remarkable energy-saving and environmental protection benefits have been achieved [3]. The development of HTAC technology The development of energy-saving technologies for various industrial furnaces and boilers at home and abroad has gone through two stages: the unused waste heat and the beginning of using the waste heat. In the most primitive era, the waste heat of the furnace was not used, and the heat loss taken away by the flue gas at the furnace head was great, and the thermal efficiency of the furnace was below 30%, as shown in figure 1. Since the 1960s and 1970s, air preheater (or air heat exchanger), a device for recovering flue gas from flue, has been widely used at home and abroad, as shown in Figure 2. This method can reduce the temperature of flue gas, increase the temperature of combustion-supporting air in the furnace, and achieve certain energy-saving effect, but there are still the following problems: (1) The heat recovery is limited, and the thermal efficiency of the furnace is generally below 50%; (2) The air preheater is generally made of metal materials and ceramic materials. The former has a short service life, while the latter has huge equipment and difficult maintenance. (3) From the burner's point of view, after the temperature of combustion-supporting air rises, the volume of the flame zone becomes smaller and smaller, the temperature of the flame center becomes higher and higher, and a local high-temperature zone appears in the furnace, which is easy to overheat the heating products of industrial furnaces, affect the service life of local refractory materials and metal components in the furnace, and even cause accidents such as boiler tube explosion. (4) The increase of combustion air temperature leads to the increase of flame temperature, and the NOX emission is greatly increased (even exceeding 103ppm), which has caused serious pollution to the atmospheric environment.

Fig. 1 schematic diagram of furnace without waste heat fig. 2 schematic diagram of furnace with air preheater. In the early 1980s, British Gas Company and American Hot Work Company developed regenerative burners with great energy-saving potential in industrial furnaces and boilers, resulting in the "first generation regenerative combustion technology" under the condition of high temperature air, which was used in small glass melting furnaces. Since then, this burner has been applied in the steel and molten aluminum industries in the United States and Britain. Although this kind of burner has some problems such as large NOX emission and system reliability, it can make the utilization of flue gas waste heat reach the limit level and has great energy saving benefits, so it has been popularized and applied in the United States, Britain and other countries. After entering the 1990s, domestic and foreign academic circles put the energy-saving and environmental protection of regenerative burners in the position of tackling key problems in science and technology, and conducted in-depth basic research on them, aiming at reducing CO2 and NOX emissions while saving energy. The research team led by Ryuichi Tanaka of Japan Industrial Furnace Co., Ltd. adopted honeycomb ceramic regenerator with low thermal bluntness and achieved good results [1]. Due to the solution of the problems of heat storage materials and high frequency reversing equipment for efficient recovery of flue gas waste heat, the "second generation regenerative combustion technology" under the condition of high temperature and low oxygen, which is now called "high temperature air combustion technology". 3 technical principle and technical advantages of regenerative high temperature air combustion technology principle of regenerative high temperature air combustion is shown in figure 3.

Fig. 3 When the furnace with regenerator is switched to regenerator 1 through reversing valve, it is heated when passing through regenerator (ceramic ball or honeycomb, etc.). ). Normal temperature air is heated to near the furnace temperature in a very short time (generally 50 ~100 C lower than the furnace temperature). After the high-temperature hot air enters the furnace, the gas in the surrounding furnace is pumped away, forming an airflow with oxygen content far lower than 265438+. At the same time, the flue gas after combustion in the furnace is discharged into the atmosphere through another regenerator (see regenerator 2 in the figure). When the high-temperature hot flue gas in the furnace passes through the regenerator, the sensible heat is stored in the regenerator, and then the low-temperature flue gas at 150 ~ 200℃ is discharged through the reversing valve. The reversing valve with low working temperature is switched at a certain frequency, so that the two heat accumulators are in the state of alternating heat storage and heat release, and the common switching period is 30~200 seconds. The birth of regenerative high-temperature air combustion technology has brought new solutions to the problems of uniform temperature distribution, automatic temperature control, enhanced heat transfer, expanded flame combustion range and changed flame combustion mechanism in industrial furnaces. It can be seen from the above that the main advantages of regenerative air combustion technology are: (1) great energy saving potential, with an average energy saving of more than 25%. Therefore, carbon dioxide can be discharged into the atmospheric environment by less than 25%, greatly alleviating the greenhouse effect of the atmosphere. (2) The burning area of the flame is enlarged, and the boundary of the flame almost extends to the boundary of the furnace, so that the temperature in the furnace is uniform, which improves the product quality on the one hand and prolongs the service life of the furnace on the other. (3) For the continuous furnace, the average temperature in the furnace length direction increases, which strengthens the heat transfer in the furnace, resulting in that the furnace size of the industrial furnace with the same output can be reduced by more than 20%, in other words, the output of furnace products with the same length can be increased by more than 20%, which greatly reduces the equipment cost. (4) Because the flame is not generated in the burner, but gradually burns in the furnace space, the combustion noise is low. (5) Using traditional energy-saving combustion technology, the higher the preheating temperature of combustion air, the greater the NOX content in flue gas; However, with regenerative high-temperature air combustion technology, the NOX content is greatly reduced under the condition of high preheating temperature of combustion-supporting air. (6) Oxygen-poor combustion in the furnace leads to the reduction of billet oxidation burning loss. (7) Oxygen-poor combustion in the furnace is beneficial to the generation of reducing flame in the furnace, which can ensure the technological requirements such as ceramic firing and meet the needs of some special industrial furnaces. 4. Basic research in the field of regenerative high-temperature air combustion technology in China 4. 1 Study on the technical mechanism of high-temperature air combustion [1, 4-6]19910/0. In October, under the active advocacy of Professor Xiao Zeqiang, Beijing Shenwu Technology Co., Ltd. was one of the main supporting units. Tsinghua University, Central South University, Northeastern University, Beijing Shenwu Technology Co., Ltd. and other scientific research institutes have conducted a series of studies on the combustion mechanism and low pollution characteristics of high temperature air. The basic idea of high temperature air combustion technology is to make fuel burn in an atmosphere with high temperature and low oxygen volume concentration. It includes two basic technical measures: First, the regenerative heat exchanger with high high high temperature efficiency and high heat recovery rate is adopted to recover the sensible heat in the combustion products to the maximum extent, which is used to preheat the combustion-supporting air to obtain high-temperature combustion-supporting air with a temperature of 800 ~ 1000℃ or even higher. The other is to use fuel staged combustion and high-speed airflow to suck combustion products in the furnace, and dilute the oxygen concentration in the reaction zone to obtain a low-oxygen atmosphere with a concentration of 15% ~ 3% (volume ratio). In this high-temperature and low-oxygen atmosphere, the fuel first undergoes a recombination process such as cracking, which leads to completely different thermodynamic conditions from the traditional combustion process, and releases heat energy under the delayed combustion with oxygen-poor gas, so there is no longer a local high-temperature and high-oxygen region in the traditional combustion process. On the one hand, this combustion mode makes the temperature in the combustion chamber rise and distribute more evenly, which makes the fuel consumption significantly reduced. Reducing fuel consumption also means reducing greenhouse gas emissions such as carbon dioxide. On the other hand, the generation of thermal nitrogen oxides (NOX) is suppressed. Nitrogen oxides (NOX) are one of the important sources of air pollution, and various industrial enterprises are trying to reduce NOX emissions. NOX mainly includes thermal type and fuel type. HTAC burner mainly uses gaseous fuel, in which there are few nitrogen compounds, so fuel-type NOX is rarely produced. According to the formula of thermal NOX formation rate [1], the formation rate of NOX is mainly related to the highest flame temperature and nitrogen and oxygen concentration in the combustion process, in which temperature is the main factor affecting thermal NOX. Under the condition of high temperature air combustion, there is no local high temperature area of traditional combustion because of the increase of average temperature in the furnace; At the same time, the high temperature flue gas in the furnace flows back, which reduces the concentration of nitrogen and oxygen; In addition, the airflow speed is high, the combustion speed is fast, and the residence time of flue gas in the furnace is short. Therefore, the NOX emission concentration is low. 4.2 Study on Thermal Characteristics of Ceramic Ball Regenerator [7] After the new technology of small ceramic ball regenerator came out in the early 1980s, it attracted great attention of China's thermal industry. Since the middle and late 1980s, China has developed a new type of regenerator technology and established a special experimental device for ceramic ball regenerator. The resistance and heat transfer characteristics of ceramic ball regenerator have been systematically studied experimentally, and the basic laws between the resistance and heat transfer characteristics of regenerator and the structural and operational parameters of regenerator have been obtained, which laid a foundation for rational design of regenerator. The experimental ceramic ball regenerator is shown in Figure 4.

Fig. 4 schematic diagram of ceramic ball regenerator 4.2. 1 experimental study on resistance characteristics. The resistance loss of gas flowing through regenerator is an important technical index in regenerator design. Understanding the resistance characteristics of regenerator in cold and hot state is an important prerequisite for rational selection of air supply system and smoke exhaust system equipment for industrial furnaces. 4.2. 1. 1 experimental results of cold resistance characteristics of regenerator The experimental results show that the resistance loss of ceramic regenerator is proportional to the height of regenerator; The resistance loss decreases with the increase of ceramic ball diameter; There is a power function relationship between the resistance loss of gas flowing through the regenerator and the empty tower velocity. According to the experimental results, the cold resistance characteristic equation of ceramic ball regenerator is obtained by regression method: DP-resistance loss; H—— the height of the heat accumulator; E-porosity of regenerator; U-empty tower velocity; D- the diameter of the ceramic ball; M—— dynamic viscosity coefficient of fluid; R-density of fluid; A and b coefficients. 4.2. 1.2 experimental results of thermal resistance characteristics of regenerator The experiment of thermal resistance characteristics of regenerator mainly studies the relationship between resistance loss of air and flue gas per unit length in regenerator and temperature, gas flow rate and ceramic ball diameter. The experimental results show that the influence of temperature on the resistance loss of air and smoke is linear; The resistance loss increases with the increase of empty tower velocity, and its variation law is a power function. The resistance loss decreases with the increase of ceramic ball diameter, and its variation law is approximately inversely proportional. Based on this, the thermal resistance characteristic equation is as follows: where: r0—— is the gas density in the standard state; Coefficient determined by experiment; T-the average temperature of air or flue gas in a period of time; Other symbols have the same meanings as above. 4.2.2 Study on heat transfer characteristics of ceramic ball regenerator The working process of regenerator is that it periodically passes through preheated medium (combustion air or gas) and flue gas, that is, it is in exothermic and endothermic state. In the whole process, the flue gas temperature, air temperature and regenerator temperature are not only functions of time, but also change with different positions. The heat transfer process of ceramic ball regenerator is a complex unsteady heat transfer process including convection, radiation and conduction. Chinese scholars have deeply and systematically studied the main characteristics of periodic unsteady heat transfer process in ceramic ball regenerator. 4.2.2. The temperature distribution characteristics of1ceramic regenerator have mastered the following laws through experiments: a) With the extension of time, the air outlet temperature gradually decreases, and the law is approximately linear; B) In a period, the exhaust temperature increases with time, and its law is approximately linear; C) The surface temperature of the regenerator gradually decreases with the extension of time during cooling, and its law is approximately linear; D) The surface temperature of the regenerator gradually increases with the increase of time during heating, and its law is approximately linear; E) The changes of flue gas temperature and air temperature in the regenerator along the height direction are also approximately linear; F) The change of regenerator surface temperature is basically consistent with the change of air and flue gas. At the same position, the surface temperature of the ball is 40 ~ 60℃ higher than the air temperature and 45 ~ 55℃ lower than the flue gas temperature. When the diameter of the ball is large, the temperature difference between the ball and the gas is large, and when the diameter of the ball is small, the temperature difference between the balls is small. According to the experimental results, the comprehensive heat transfer coefficient of 4.2.2.2 ceramic balls decreases with the increase of commutation time and ball diameter. According to the relevant heat transfer theory and experimental results, Chinese scholars put forward the following expressions of comprehensive heat transfer coefficient: where: k-comprehensive heat transfer coefficient; Ah—— heat transfer coefficient between gas and sphere during heating; AC-heat transfer coefficient between gas and ball during cooling; D- the diameter of the ball; Thermal conductivity of l- sphere; F0— Fourier number: (:thermal conductivity, t: commutation time); Coefficient determined by experiment; . Heat transfer coefficient between 4.2.2.3 ball and gas Through experiments, the relationship between heat transfer coefficient between ball and gas and gas temperature, empty tower velocity and ball diameter is obtained. After mathematical regression of the experimental data, the following relationship is obtained:

Air:

Study on thermal characteristics of honeycomb regenerator with A and B coefficients of 4.3 In the early 1990s, the research group led by Ryuichi Tanaka of Japan Industrial Furnace Co., Ltd. began to adopt honeycomb ceramic regenerator with small thermal bluntness, and achieved good results. Compared with spherical regenerator, honeycomb regenerator has great advantages in specific surface area, weight, pressure loss and commutation time [1]. In China, the industrial application of honeycomb regenerator in regenerative combustion system has been paid more and more attention. Ou Jian et al. [4] studied the thermal characteristics of honeycomb regenerator through numerical simulation, and this paper briefly introduced its research results. 4.3. 1 Stress characteristics of the hole wall of the regenerator In the process of use, the regenerator is easy to be damaged because both sides of the hole wall of the unit are heated or cooled, in addition to the temperature, it is also subjected to various stresses. There are many factors that cause the damage of the regenerator, such as the chemical action of high temperature air and combustion products, the physical action of sudden temperature change and thermal expansion, the airflow scouring and the mechanical action of high temperature load. The above factors often exist at the same time, but there must be the main reason for the specific working environment. After studying the heat accumulator replaced in the production site of a domestic factory, it is found that most honeycomb units have cracks and spalling in different degrees. Obviously, brittle stress fracture is the main reason for this problem. The calculation results show that the honeycomb cell wall is mainly subjected to normal stress during the heating and cooling stages, and the tangential stress and axial stress are less than 1/200 and one ten thousandth of the normal stress, respectively. During heating, the stress points to the wall and squeezes the hole wall of the regenerator, which shows the squeezing stress; During the cooling period, the stress direction of the wall surface points to the fluid, and the fluid produces traction on the wall surface, which shows tensile stress. Obviously, if the stress on the regenerator wall is greater than the maximum stress it can bear, it will lead to stress embrittlement. Frequent changes in the process of heat storage and heat release make the hole wall of regenerator alternately subjected to tensile stress and extrusion stress. The greater the fluid velocity, the greater the stress change; The shorter the commutation time, the greater the influence of alternating tensile stress and extrusion stress on the regenerator. 4.3.2 The research results of heat transfer characteristics of honeycomb regenerator show that the heat transfer between the wall of regenerator and gas is strong, and the long and narrow grid channels have certain influence on the flow and heat transfer. Commutation time has a great influence on the heat transfer characteristics of regenerator. The longer the commutation time, the higher the outlet temperature of flue gas, and the lower the temperature efficiency and heat recovery rate of regenerator. The gas velocity also affects the heat transfer characteristics of the regenerator. The higher the gas velocity, the higher the flue gas outlet temperature and the lower the waste heat recovery rate. 5 Development of Regenerative High Temperature Air Combustion Technology in China In 2002, the national steel output reached10.80 billion tons, and there were more than 1000 heating furnaces in China's metallurgical industry, with an annual capacity of 200 million tons of billet. At present, the average energy consumption of steel rolling heating furnaces in China is 60Kg standard coal/ton steel, and the average fuel consumption of international advanced heating furnaces is 5 1kg standard coal/ton steel. Table 1 lists the technical parameters of 230t/h walking beam heating furnace before and after HTAC technology is adopted in Fukuyama Hot Rolling Plant of NKK Steel Pipe Company of Japan [7]. From the parameters in table 1, it is not difficult to see that the average energy consumption of Fukuyama hot rolling mill of NKK Steel Pipe Company in Japan before transformation is 48.6kg standard coal/ton steel, which is less than that of steel rolling reheating furnace in China 19%. After transformation, the heating furnace of NKK Company saves energy by 25% compared with that before transformation. According to China's annual heating of 65438+ tons of billet, the average energy consumption of steel rolling heating furnaces in China has reached 40kg standard coal/ton of steel, which is equivalent to an average energy saving of 33%. After the transformation, there is only one heating furnace for steel rolling in China, which can consume 2 million tons of standard coal every year. In addition, due to the particularity of the process, the energy utilization rate of heat treatment furnace, ladle, tundish dryer and other equipment is poor at present, and its energy saving potential will be greater. In addition, it will also play an important role in reducing oxidation burning loss, reducing environmental pollution, reducing equipment cost and increasing single furnace output in iron and steel industry. Table 1 230t/h technical parameters of walking beam heating furnace for hot rolling before and after HTAC technology is adopted.

To sum up, the application of regenerative new technology in industrial furnaces can achieve remarkable energy saving and environmental pollution reduction effects. There are many kinds of industrial furnaces in China, and the popularization and application of this new technology in China will bring great economic and social benefits. Since the establishment of Beijing Shenwu Company at the end of 1995, WDH series energy-saving burners have been applied to various industrial furnaces and boilers of nearly 800 enterprises in metallurgy, machinery, petrochemical, ceramics, glass, thermal power generation and other industries, and the equipment status of industrial furnaces and boilers in these industries has been fully understood. Since 1996, our company has actively followed foreign advanced technologies, organized technical experts in combustion, industrial furnaces, thermal automatic control and machinery, and focused on the development and research of the application of regenerative high-temperature air combustion technology in industrial furnaces and boilers. Because the popularization and application of this technology is not only a combustion problem, especially in the field of industrial furnaces, because there are many kinds of industrial furnaces and the process requirements are very different, if it does not match the specific process requirements of industrial furnaces, it is impossible to develop mature products and put them into practical application. Through several years of development and research, great progress has been made in the application research of various industrial furnaces in steel, machinery and non-ferrous metals industries, and the company has been able to provide mature technology for enterprises. Taking the heating furnace for steel rolling as an example, this paper introduces the technology developed by our company. 5. 1 air-gas double preheating Most of the steel rolling heating furnaces in China use mixed gas with low calorific value, converter gas and even blast furnace gas as fuel. In the case of burning low calorific value gas, it is not enough to recover the heat of exhaust gas if only the air is preheated. When burning low calorific value gas and high calorific value gas, and only preheating air and preheating air and gas twice, the recovery and utilization of waste gas heat are shown in Table 2. It can be seen from Table 2 that in the case of burning mixed gas, if only the air is preheated, about 34% of the recoverable heat is still not utilized, which is a pity; At the same time, it can be seen that the effect of double preheating of air and gas when burning low calorific value gas is greater than that when burning high calorific value gas. In addition, when burning low calorific value gas, the flue gas of the furnace can be completely discharged through the air regenerator and the gas regenerator, and the furnace does not need to be equipped with a flue and chimney to discharge excess high temperature flue gas, which simplifies the structure and arrangement of the furnace.