1  Foreword

The energy consumption of blast furnace ironmaking accounts for more than 40% of the energy consumption of iron and steel industry, so the energy saving and consumption reduction of ironmaking process is particularly important. The hot blast stove is one of the important equipment for blast furnace ironmaking. The heat supplied by the hot blast stove accounts for about 30% of the energy consumption of ironmaking production, and the gas consumed accounts for more than 50% of the gas produced by the blast furnace. The most effective and economical energy-saving measures to improve the supply air temperature of hot blast stove.

China is a big country in the world's steel production, with an annual output ranking first in the world. In 2018, China's cumulative output of pig iron was 0.771 billion tons. When smelting a ton of pig iron, about 3,000 m of by-product-blast furnace gas will be produced. Among them, 30%-40% of blast furnace gas is used as heating fuel by hot blast stoves. Modern hot blast stoves require high blast temperatures above 1250 ℃ while reducing CO, NO_XThe discharge of pollutants such as high air temperature, long life and low emissions of hot blast stoves.

Blast furnace gas is characterized by low combustible composition, unstable combustion, low combustion temperature and large amount of flue gas. Hot blast stoves generally adopt stable and enhanced combustion measures to obtain higher vault temperature. In recent years, engineers have found that excessive vault temperature will bring nitrogen oxides (NO_X), resulting in pollution of the atmospheric environment and intergranular stress corrosion on the vault of the hot blast stove. When the vault temperature is ≥ 1420 ℃, NO in the burning product_XThe water vapor in the combustion products condenses into liquid water when the temperature drops below the dew point, NO._XCombined with condensed water to form an acidic corrosive medium, the hot blast furnace shell steel plate intergranular stress corrosion, so the existing conventional hot blast furnace will generally control the vault temperature below 1420 degrees C, designed to reduce NO._XThe amount of generation inhibits the intercrystalline stress corrosion of the furnace shell, but therefore the lack limits the further increase of the air supply temperature of the hot blast furnace. Therefore, the design and development of a change in the conventional hot blast stove combustion process, further improve the supply air temperature, while reducing the low CO, NO_XThe high air temperature, high efficiency and long-life hot blast stove, such as the discharge of pollutants, has become a necessary condition to overcome the above technical problems.

2 Hot blast stove NO_XFormation Mechanism and Control Technology

Nitrogen oxides (NO_x) is one of the main sources of air pollution. The usual NO._xThere are many different forms: n₂O 、 NO 、 NO₂, N₂O₃, N₂O₄and n₂O₅, in which NO and NO2 are important atmospheric pollutants, and a small amount of N₂O. The production and emission of nitrogen oxides are closely related to the combustion mode, especially the combustion conditions such as combustion temperature and excess air coefficient.

During the combustion process, NO is produced._XIt is divided into the following three categories:(1) When burning at high temperature, N in the air₂and o₂NO formed in combustion_X, called thermal type NO_X(2) NO generated by chemical reaction of organic nitrogen in fuel_X, called fuel type NO_X;(3) Rapid NO_xThe formation mechanism of the fuel is that when the excess air coefficient is less than 1, a large amount of NO is rapidly generated in the flame surface._x.

Thermal NO_xThe generation mechanism of Zeldovich was proposed in 1964. Its generation is the result of the following chain reaction caused by the impact of oxygen atoms on nitrogen molecules at high temperatures:

Formula (1)

Among them, the first type plays a leading control role, and the reaction conditions of this type are higher than 1500 degrees C, so NO._xThe generation and temperature are shown in Figure 1. According to this mechanism, the N in the air₂Oxidation at high temperatures is carried out through a set of unbranched chain reactions. The speed of the entire reaction is proportional to the concentration of oxygen atoms. With the increase of temperature, the concentration of oxygen atoms increases and the total reaction speed increases. Since the total reaction is endothermic, heating is beneficial to increase NO_xThe same cooling will make the thermal NO_xThe formation is significantly inhibited.

Figure 1 NO_XRelationship between generation amount and vault temperature

Therefore, the thermal type of NO_xThe generation rate is very related to the combustion temperature, so it is also called temperature type NO._x. Influence of thermal type NO_xThe main factors of the amount of generation are temperature, oxygen concentration and residence time in the high temperature zone, thus controlling the thermal NO_xThe method of generating quantity is summarized as follows: reduce the combustion temperature level to avoid local high temperature; reduce the oxygen concentration; combustion is carried out under the condition of deviating from the theoretical air volume, and the residence time in the high temperature zone is shortened.

3 Low NO_xCombustion technology is to reduce the thermal type NO of hot blast stove._xMain technical measures of emission

NO_xThe formation of the combustion zone is determined by the temperature and excess air coefficient. Therefore, by controlling the temperature and air volume of the combustion zone, it has been achieved to prevent NO_xWe call this technology low-nitrogen combustion technology for the purpose of generating and reducing its emissions. The requirement for low-nitrogen combustion technology is to reduce NO._xAt the same time, the combustion of the hot blast stove is stable, and the CO content of the flue gas cannot exceed the standard. In order to control the combustion process NO_xThe principle of the measures taken to produce the amount of the gas is as follows:(1) reduce the excess air coefficient and oxygen concentration, so that the blast furnace gas burns under anoxic conditions;(2) reduce the combustion temperature to prevent the production of local high temperature zone;(3) shorten the flue gas in the high temperature zone residence time, etc. Low NO_xCombustion technology mainly includes: low excess air coefficient, low NO_xBurner, flue gas recirculation.

4 cone-column composite top-fired hot blast stove with low NO_xIntroduction of combustion technology

4.1 low excess air coefficient operation

Thermal NO_xThe formation of is a slow reaction process, which is formed by the reaction of N in the combustion air with reactants such as O and OH and molecular O. The traditional top-fired hot blast stove in the actual operation of the air excess coefficient is generally> 1.1, cone-column composite top-fired hot blast stove select the air excess coefficient of 1.03, so that the combustion process as close as possible to the theoretical air volume conditions, with the reduction of excess oxygen in the flue gas, can inhibit NO_XThe generation. This is one of the most direct reduction of NO_Xemission methods, generally can reduce the NO_X15% to 20% emissions.

4.2 Development of Low NO for Top-fired Hot Blast Stove_xBurner

4.2.1 Burner three-dimensional space vortex combustion technology

Burner is the key equipment of industrial furnaces, and its performance has a great influence on the output, energy consumption, environmental protection and other economic indicators of industrial furnaces. Low pollution combustion technology, especially low NO_xCombustion technology is one of the important topics for gas technology researchers. Reasonable organization of combustion, design and improvement of the burner structure, improve combustion performance, improve the thermal efficiency of combustion equipment to reduce environmental pollution is the focus and difficulty of the new burner design.

In combustion technology, the rotating jet has the characteristics of rotating steady flow motion, free jet and steady flow, so it is an effective combustion method to strengthen combustion and organize flame form. For NO_xEmission problems, the use of three-dimensional swirl in a reasonable air-fuel ratio technology can significantly reduce NO_xA remarkable feature of the three-dimensional vortex burner is that it can produce a reflux zone, so that the gas reflux, the intensity of the reflux and the size of the reflux zone is an important characteristic index to measure the swirl burner. The larger the area of the reflux zone, the stronger the reflux intensity, the closer the location to the flame root, the better the combustion condition, the more uniform the furnace temperature, the less likely to produce local high temperature, thereby reducing the NO_xThe amount of generation.

Figure 2 is a structural diagram of a cone-column composite top-fired hot blast stove burner. The mixing chamber of the cone-column composite type is provided with a multi-layer annular gas and air ring, and each layer of the ring is provided with a plurality of nozzle structures, which not only reduces the vault height, but also improves the plane swirl mixed flow field of the traditional top-fired furnace burner into a three-dimensional space vortex flow field. The key technology of "air gas space (three-dimensional) swirling flow and plane (tangential circle) fine flow high-speed injection" is adopted, and the gas nozzle is arranged in the cone and frustum of the mixing chamber. The nozzle is divided into multiple layers from different radial directions of the cone surface to a certain rotation angle, and then rotates and mixes with the high-speed trickle ejected from the combustion-supporting air of the multi-ring nozzle at the cylindrical part, which can effectively strengthen the mixing process of high-temperature combustion products and unburned fuel in the reflux zone, and form a stable ignition source and activation center, with good combustion flame stability, energy saving, high efficiency and low pollution, large burner load adjustment ratio, and adjustable flame momentum. The generated reflux zone can transfer heat and active centers to the unburned mixed gas, thereby establishing a stable ignition source, maintaining stable flame combustion, and shortening the residence time of flue gas in the high temperature zone and reducing NO_xThe measured value of the industrial application of the Anke cone-column composite top-fired hot blast stove is 35-76 mg/m.³Far lower than the "iron and steel industry air pollutants ultra-low emission standards" 150 mg/m³emission standards.

Fig.2 Structure diagram of cone-column composite top-fired hot blast stove burner

4.2.2 Low NO_xNumerical Simulation of Burner Combustion

The application of CFD fluid simulation technology can effectively reduce the design cost, it uses numerical calculation method to solve the flow simulation directly, and finds a variety of flow phenomena, including computational hydrodynamics, computational aerodynamics, computational combustion, computational heat transfer, computational chemical reaction flow and other research directions. In recent years, we have used simulation technology to study the flue gas flow and combustion characteristics of hot blast stove vault space, and have carried out simulation research on all kinds of top-burning hot blast stoves. At present, the computer numerical simulation method of computational fluid dynamics has been an important means to carry out the pre-design of the project, and the optimal scheme in this project is selected by simulation method as the basis for cold state experiment and industrial application.

Using the turbulent flow model and combustion model radiation heat transfer model of CFD, the numerical simulation of the cone-column composite top-fired hot blast stove is carried out to obtain the flow field, temperature field and concentration field distribution in the burner.

4.2.3 Low NO_xBurner Temperature Field and Flame State

Fig. 3 is a comparison of the temperature field and flame state in the combustion chamber when the theoretical combustion temperature of the cone-column composite top-fired hot stove is 1380 ℃. Through Y = 0 and Z = 1, it can be seen that the gas is fully burned at a height of 1 meter above the plane of the lattice brick. At the same time, it is clear from the figure that the temperature distribution in the whole combustion chamber is uniform, and there is almost no local high temperature zone. The temperature uniformity on the surface of the lattice brick is more than 99%, and the uniformity of temperature distribution on the surface of the lattice brick is very important for the hot blast stove. The flue gas with uniform temperature distribution can improve the heat transfer efficiency of the lattice brick and prolong the service life of the lattice brick, and the heat storage capacity of the lattice brick can also be fully utilized.

Figure 3 Y = 0Burner longitudinal section temperature distribution

Fig.4 Plane temperature distribution of Z = 1 lattice brick

4.2.4 Low NO_xBurner concentration field and velocity field distribution

Fig. 5 shows the CO concentration distribution of the burner of the cone-column composite top-fired hot blast stove. under the working condition, the gas and air are mixed and burned more fully. the high concentration of CO is mainly distributed above the throat, while the CO concentration on the surface of the lattice brick is close to 0, and the gas is completely burned. Figure 6 Under the action of high-speed three-dimensional vortex mixed airflow, the mixing and entrenment of gas jet and air flow are enhanced, and an obvious recirculation zone is formed in the combustion zone. The central recirculation zone directly entraps the high-temperature flue gas that has begun to burn in the lower part and flows back to the root of the mixed airflow (flared zone), which improves the stability and speed of the combustion flame, reduce the residence time of the mixed gas in the high temperature ignition zone and the stable flame zone, so that the lower temperature of the flue gas and the hot combustion products are mixed as soon as possible to reduce NO._xPollutant emissions, improve combustion efficiency.

Fig.5 Distribution of CO concentration field in longitudinal section of Y = 0 burner

Fig.6 Velocity field distribution in longitudinal section of Y = 0 burner

5 Improve the temperature efficiency of the hot blast stove

There is a big gap between the difference (temperature efficiency) between the highest vault temperature at the end of combustion and the air supply temperature on many hot blast stoves in service: in the traditional hot blast stove with large aperture lattice bricks as heat storage body, the temperature difference reaches 150~180 ℃, and the cone-column composite top-fired hot blast stove ensures that the vault temperature is less than 1400 ℃, by strengthening the heat storage area, the difference between the traditional vault temperature and the air supply temperature is reduced to 80 ℃ ~ 100 ℃, and the air temperature can be increased by 50~80 ℃ under the same vault temperature.

Increase the combustion rate of the hot blast stove, improve the material of the furnace grate, adopt the new structure of the grate, increase the maximum exhaust gas temperature at the end of the hot blast stove combustion to ≥ 400~450 ℃; use the high temperature hot blast stove exhaust gas, through the heat exchanger to preheat the gas and combustion air to ≥ 200~230 ℃.

Therefore, the basic technical route of our hot blast stove design is: to reduce the difference between the vault temperature and the supply air temperature, to achieve 100 percent of the blast furnace gas as the hot blast stove fuel, double preheating gas and combustion-supporting air, to obtain the supply air temperature of 1250 ℃ or more.

6 Conclusions

1) When the vault temperature of the hot blast stove is ≥ 1420 ℃, NO in the burning product_XThe content of the sharp rise, the hot blast furnace shell steel plate intergranular stress corrosion, modern hot blast furnace vault temperature should be controlled below 1400 degrees C, to ensure the service life of the hot blast furnace steel structure.

2) by the NO_xDiscussion of the formation mechanism, the main NO produced during the combustion of blast furnace gas_xFor thermal type NO_xLow NO_xCombustion technology is to reduce the thermal type NO of hot blast stove._xMain technical measures for emission, low NO_xCombustion technology mainly includes: low excess air coefficient, low NO_xBurner, flue gas recirculation.

3) The arrangement of multi-layer burners is adopted, and the burner group is designed as a cone-column composite structure to realize three-dimensional vortex strong mixed combustion. Under the condition of realizing the same air temperature, CO consumption can be reduced and NO can be reduced._xgeneration, making the hot blast stove no_xThe emission standard not only solves the pollution of CO, but also does not produce new polluting gases, making it an environmentally friendly high-temperature air supply equipment.