Reduce consumption and improve efficiency of steel rolling heating furnace

Heating furnace is an important thermal equipment in the production process of the steel industry. It plays a very important role in steel rolling production. Its energy consumption accounts for 6% of the total energy consumption of the steel industry. The main task of the heating furnace is to heat the steel billet to the required temperature while ensuring the requirements of the heating process while minimizing the consumption of gas and electricity and reducing the pollution of emissions to the environment.

This article conducts an in-depth and comprehensive study on the intelligent control technology of the heating furnace, adjusts the operating status and process parameters of the heating furnace to achieve the best operating status, improves the stability and reliability of the heating furnace operation, and solves the problem of billet temperature fluctuations. Large, uneven cross-section temperature, high oxidation and burning loss rate, large energy consumption and serious emission pollution.


1. Factors affecting oxidation burning loss and gas consumption


During the heating process of the steel billet in the heating furnace, the surface of the steel billet is oxidized due to the presence of oxidizing gases (O2, H2O, SO2) in the furnace. Generally, the oxidation burning loss rate is as high as 5%-6%, which seriously affects the yield of finished products. A large number of literature studies have shown that furnace temperature, heating time and furnace atmosphere are the main factors affecting the oxidation and burning loss of steel billets.


1.1 Unbalanced furnace temperature control


Due to the large calorific value and pressure fluctuations of the gas, and the poor ability of the system to suppress them, the temperature fluctuations in the furnace are serious. If the heating temperature is too high, it will cause overheating, overburning and severe oxidation of the steel billet, which will affect normal rolling and even produce scrap, resulting in a waste of raw materials and energy. On the contrary, the insufficient heating temperature will affect normal rolling and cause huge wear and tear on the rolling mill and rolling spokes. Therefore, it is necessary to understand the basic knowledge of the billet heating process and formulate a correct heating process system to reduce production defects caused by the heating process.

Heating temperature is the main factor in slab surface oxidation. As the temperature in the heating furnace increases, the slab surface oxidation rate gradually accelerates. The relationship between heating temperature and oxidation amount of plain carbon steel is parabolic. When the temperature is lower than 1000K, the oxidation burning loss of ordinary carbon steel is very small and can generally be ignored; when the temperature is 1000-1100K, the oxidation burning loss is small; when the temperature is 1100-1300K, the oxidation burning loss is slow increases; when the temperature is between 1300-1400K, the diffusion rate of each component gradually accelerates, and the oxidation and burning loss of ordinary carbon steel increases rapidly; when the temperature exceeds 1400K, the oxide scale on the surface of ordinary carbon steel begins to melt, and the oxidation and burning loss increases sharply. increase. At the same time, if the temperature is too high, the gas consumption will increase accordingly, which is a waste of gas.


1.2 Effect of heating time


Under the same heating system, as the heating time prolongs, the amount of oxidation and burning loss on the slab surface gradually increases, and its weight gain follows a parabolic change pattern. The figure shows the relationship between heating time and oxidation amount of 1390-1470K plain carbon steel billet when the oxygen concentration is 3%. It can be seen from the figure that the oxidation amount of the steel billet at each temperature changes with time in roughly the same trend; the oxidation reaction is violent between 0 and 30 minutes, and the oxidation and burning loss of carbon steel increases sharply; after 30 minutes, the oxidation and burning loss of carbon steel gradually slow down. In the early stage of oxidation, the steel matrix is in direct contact with the air, and the oxidation weight gain rate is relatively fast. As the heating proceeds, the dense iron oxide scale generated covers the surface of the steel substrate, making it difficult for external oxidizing media to penetrate, hindering the formation of iron oxide scale, so oxidation The rate slows down, but the overall oxidation amount still increases with time.

steel rolling heating furnace, Reduce consumption


Although the temperature of each sample billet differs by 20K, the increment of oxidation amount is different. In the first 30 minutes of heating, the increase in oxidation amount is small. Still, as heating continues, the increase in oxidation amount gradually expands within 30-60 minutes, and after 60 minutes, the increase in oxidation amount tends to be stable. The increase in oxidation amount increases with the increase in temperature, which shows that the higher the temperature, the greater the oxidation reaction speed, so the residence time of the steel billet in the high-temperature zone needs to be shortened.


1.3 Influence of atmosphere in the furnace


Under normal circumstances, to achieve optimal combustion of gas and air, the air coefficient ranges from 1.02 to 1.10. If there is insufficient air, combustion is insufficient, black smoke is produced, energy is wasted, thermal efficiency is reduced, and the environment is polluted; if there is excess air, the excess air will take away heat, causing waste of gas, lowering the combustion temperature, and increasing oxidation and decarburization of steel. It not only wastes materials but also makes phosphorus removal difficult, seriously affects the surface quality of the product and increases NOx emissions. Therefore, controlling a reasonable air coefficient is crucial. In actual production, while ensuring a reasonable heating temperature in the soaking section, a lower air coefficient should be used to reduce the oxygen concentration near the high-temperature steel billet to reduce its oxidation burn loss rate.


2. Optimization of heating furnace combustion process


2.1 Billet temperature calculation model


The billet temperature calculation model is the key to realizing the optimal furnace temperature setting control system, and the online calculation of the billet temperature is the basis for furnace temperature feedback control. The deviation between the real-time calculated temperature of the billet and the set heating curve of the billet is used as the input of the feedback compensation control, and the heating process of the billet is strictly carried out in accordance with the ideal heating curve.


2.2 Furnace temperature setting adjustment control according to expert rules


Feedback compensation control is used to dynamically compensate the optimal furnace temperature setting value of each section. At the same time, according to the actual measured temperature of the billet coming out of the furnace, expert experience is used to adjust the temperature of each section of the heating furnace.


2.3 Air-fuel ratio self-optimizing algorithm


The optimal ratio of air and gas is an important part of the combustion control of the heating furnace. The reasonable configuration of the air-fuel ratio is conducive to reducing unit fuel consumption. If the air-fuel ratio is too high, the furnace temperature decreases, requiring more fuel to heat the billet, resulting in increased fuel consumption. If the air-fuel ratio is too low, the fuel cannot be burned completely, inevitably leading to excessive fuel consumption. The characteristic of air-fuel ratio self-seeking optimal control is that it automatically searches for the optimal process with extreme nonlinear characteristics of the controlled object without knowing the mathematical system model, ensuring that the calculation function reaches or is close to the optimal value, when the process environment changes, on the premise of ensuring the extreme nonlinear relationship first, the optimal value in the new process environment can be found through the automatic search function. According to the change rate of the furnace temperature deviation, the change in the calorific value of the gas can be reflected. To this end, the furnace temperature deviation and the change rate of the furnace temperature deviation are fuzzy, and fuzzy expert rules are used to optimize the air-fuel ratio.


2.4 Gas and airflow control


In the case of drastic changes in combustion load, the gas and air flow double cross-limiting control method can control the furnace temperature. To limit the set values of the airflow and gas flow of the auxiliary loop controller, the double cross limiting combustion control uses the actual measured values of the airflow and gas flow to control. Through mutual restriction, it effectively prevents rapid changes in the load. Excessive residual air and gas may occur. Ensuring that the combustion system always works in the optimal combustion area can greatly reduce the heat loss caused by over-oxygen combustion and hypoxic combustion, thereby minimizing fuel waste and environmental pollution, achieving energy saving, consumption reduction, and environmental protection. the goal of.


2.5 Furnace pressure control


The furnace pressure control of the heating furnace can adopt a selective control system of exhaust temperature and furnace pressure. When the smoke temperature is normal, the furnace pressure controller can be selected by selecting the direct pressure control method, thereby effectively controlling the furnace pressure to keep it stable. ; When the smoke temperature is extremely high, the smoke temperature controller can be used to control the smoke temperature within the required range allowed by the process.

(1) When the exhaust gas temperature is in the range of 100-200°C, select the controller as the furnace pressure controller to control the opening of the regulating valve, thereby effectively controlling the furnace pressure stability of the heating furnace.

(2) When the exhaust smoke temperature exceeds the range of 200°C, select the controller as the smoke temperature controller to control the opening of the regulating valve so that the smoke temperature can return to the range allowed by the process as soon as possible.

(3) When the controller does not work, track the output of the working controller to achieve fast and undisturbed switching between controllers, thereby meeting the control requirements and achieving effective control of the system.


2.6 Optimization of heating time


According to the different types of steel billets, the corresponding heating curves are reasonably formulated. At the same time, according to the rolling rhythm of the rolling line, the residence time of the steel billet in each section is set, and the time of the billet in the high-temperature section is shortened as much as possible while ensuring the temperature of the steel billet coming out of the furnace. , to reduce oxidation and burning losses and save gas.


3. Waste heat utilization


The exhaust gas from the heating furnace is very hot and takes away a lot of waste heat. To improve the thermal efficiency of the heating furnace and save energy, the waste heat of the exhaust gas should be utilized to the maximum extent. There are two main ways: one is to use the waste heat of the exhaust gas to preheat the air and gas and bring part of the heat back to the furnace to improve the thermal efficiency of the furnace. The equipment used is a heat exchanger or a regenerator; the other is to use the waste heat of the exhaust gas to produce steam to improve the efficiency of the furnace. For heat energy utilization, the equipment used is a waste heat boiler.


4. Implementation results


After an in-depth analysis of the process flow and object characteristics of a heating furnace in a domestic steel plant, an intelligent control system for the heating furnace was designed and implemented based on quantitative feedback theory and fuzzy control theory. The basic information of the heating furnace is shown in Table 1.


Table 1 Basic information of heating furnace

Project Content
Basic size 36.5m*7.8m
Processing power 100w t/a
Fuel Blast furnace gas
Steel type Plain carbon steel, low alloy steel
Pre-heat temperature 900~1050℃
Out of oven temperature 1150~1230℃
Burner form Regenerative type
Furnace section Preheating section, heating section one, heating section two, soaking section


The application of the heating furnace intelligent control system effectively suppresses the influence of factors such as gas calorific value, pressure fluctuations and changes in furnace parameters on the operation of the heating furnace, improves the accuracy of furnace temperature control, and achieves optimal control of the combustion process of the heating furnace. It achieves the purpose of saving gas and reducing oxidation and burning losses, and realizes intelligent combustion of the heating furnace, reducing the work intensity of workers. The comparison of the effects before and after the transformation is shown in Table 2.


Table 2 Comparison of the effects of intelligent combustion modification of heating furnaces

  Control method Oxidation burning loss rate % Gas consumption m³/t
Before transformation Artificial 0.76 351.22
After transformation Automatic steel burning, unattended for 8 hours 0.52 323.56
Effect   -0.24 -27.66


Based on the unit price of steel billet of 2,000 yuan/t and the output of 1 million t/a, calculate the energy loss and energy saving amount:

2000×0.24×100×104+104=4.88 million yuan

According to the gas unit price of 0.09%/m3, the unit consumption is saved by 27.66m3/t, and the output is 1 million t/a.

Calculate the amount of gas energy saving:

0.09×27.66×100×104÷104=2.4894 million yuan

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