Energy saving can be achieved by adjusting the production process structure and energy structure of the steel industry. The improved spray iron coal ratio, increasing the ratio of pellets used for continuous casting process, the thin slab casting and rolling process, rolling billet hot charging process or the like technique may achieve power saving.
The energy consumption of the coking process is 142.21kgce/t, and the energy consumption of the pulverized coal injection process is 20~35kgce/t. The multi-injection of coal powder changes the energy structure of the blast furnace ironmaking, and the use of coke can save 1.5%. This is the structural adjustment center of the blast furnace ironmaking process.
The energy consumption of the pellet process is 42kgce/t, and the energy consumption of the sintering process is 66.38%. The pellets are often used, and the sintering can be energy-saving. At the same time, the iron content of the pellet is higher than that of sintering, and the effect of improving the grade of the furnace can be achieved.
Continuous casting reduces energy consumption by 25% to 50% compared to die casting. Thin slab continuous casting and rolling is 70% more energy efficient than conventional die casting-opening-hot rolling. Continuous casting hot charging and direct rolling technology can save energy. 35%.
2. Energy saving technology in the steel industry
2.1 Reduce fuel consumption
1) Regenerative combustion technology:
Regenerative combustion technology is to heat the combustion air and gas first, reaching 500 ~ 1 000 °C, and then the energy value can reach the standard required by the industrial furnace. After regenerative combustion technology, blast furnace gas can be widely used in baking ladle, ladle, continuous casting tundish and rolling steel heating furnace, and heat treatment furnace. The economic benefits of regenerative combustion technology: a. The blast furnace gas is rationally utilized; b. The furnace that burns heavy oil is converted into blast furnace gas, which brings huge economic benefits to the enterprise; c. The physical heat of the flue gas is fully recovered. [3]
2) Blast furnace pulverized coal injection technology:
The pulverized coal injection in the blast furnace is the central link in the structural optimization of the ironmaking system. It is the general trend of the development of blast furnace ironmaking technology at home and abroad, and is also one of the important technologies for the development of China's steel industry. The advantages of pulverized coal injection in blast furnace: a. instead of coke, reduce the environmental pollution caused by coking process; b. alleviate the shortage of main coking coal in China, optimize the energy structure of ironmaking system; c. blast furnace coal injection can achieve structural energy saving. [3] In 2007, the energy consumption per ton of iron in the coking process of China's key steel enterprises was 123.4 kg of standard coal, and the energy consumption per ton of iron required for coal injection and injection was between 20 and 35 kg of standard coal. Each 1000g of pulverized coal injected into the blast furnace can save 100 kg of standard coal in the iron-making system with energy structure tons.
3) Coal Humidity Control (CMC) Technology
Coal conditioning is to remove some of the moisture from the coking coal before the furnace is installed, keep the moisture of the furnace coal stable at about 6%, and then install the furnace for coking. According to the scale of coke production in 2007, if the coking enterprises in the country promote the implementation of coal conditioning, it can save 3 million tons of standard coal per year, reduce coking wastewater by about 15 million tons per year, and reduce CO emissions by about 16 million tons. [4]
2.2 Strengthening the recovery of secondary energy
1) CDQ technology.
Coke Dry Quenching (CDQ) uses a cold inert gas to exchange red hot coke with hot red coke in a dry quenching furnace. The inert gas that has absorbed heat transfers the heat to the dry quenching boiler to generate steam, and the cooled inert gas is blown into the dry furnace by the circulating fan to cool the red coke. CDQ technology has the functions of energy saving, environmental protection and improving coke quality. In the process of CDQ, 80% of the red sensible heat is recovered, and 15t (4MPa, 450 °C) of medium-pressure steam can be produced per ton of coke; 15~18m3 of quenching water can be saved per ton of coke, [5 The CDQ process does not discharge water vapor containing phenol, cyanide, sulfide and dust to the atmosphere, which improves the environmental quality; the coke strength is improved, which can improve the technical and economic indicators of the blast furnace.
2) Blast furnace gas residual pressure turbine power generation technology.
Top Gas -Pressure Recovery Turbine (TRT) is the use of pressure energy and thermal energy in the blast furnace gas discharged from the top of the blast furnace to convert it into mechanical energy and drive the generator to generate electricity. Most modern blast furnaces use high-pressure furnace tops. In addition to chemical energy, blast furnace gas discharged from the top of the furnace also has certain physical energy. In order to promote the comprehensive utilization of these combustible waste gases, blast furnace gas residual pressure turbine power generation and energy-saving devices are usually adopted. The pressure of the gas can be converted into mechanical energy and drive the generator to generate electricity. The dry TRT device is an important energy-saving and consumption-reducing technology for iron and steel enterprises. It is an energy-saving and environmentally-friendly innovative technology that is supported, encouraged and developed by the state. The amount of electricity generated by this technology is considerable. Baosteel's TRT technology produces 3,617 kWh of ton of iron, which is equivalent to saving 1,418 kgce/t of iron. [5]
3) Blast Furnace Gas Combined Cycle Power Generation (CCPP) Technology:
The thermoelectric conversion efficiency can reach 40% to 45% when heating is not used, which is nearly double the steam conversion efficiency of conventional boilers. With the same amount of gas, CCPP is 70% to 90% more than conventional boiler steam. Moreover, the CO2 emission of this power generation technology is reduced by 45% to 50% compared with conventional thermal power plants, without SO2, fly ash and ash discharge, low NOx emissions, recycling of secondary energy in steel production, and water for conventional coal-fired power plants of the same capacity. About 1/3 of the amount. [3]
4) Converter negative energy steelmaking technology:
In the process of converter smelting, gas recovery can be carried out when the CO content in the gas is more than 30% and the oxygen content is less than 2%. The recovery of the converter gas ton steel is greater than 100 m3, the steam is greater than 60 kg / t, and the recovered materials are fully utilized. It is possible to realize the negative energy steelmaking of the converter. [3]
5) Sintering low temperature waste heat recovery technology
The remaining energy of sintering residual heat accounts for about 10% of the waste heat resource of the whole process, and the residual heat temperature is between 300 and 500oC, which is the focus of low temperature waste heat resource application. Sintering waste heat power generation is an effective way to utilize low temperature waste heat. However, there are some problems at present. During the operation, the operating parameters of the waste heat recovery system will also change due to changes in the working conditions of the sintering machine and the ring cooler. The steam pressure, temperature, and flow rate will also change, which will affect the operating efficiency of the generator set. Moreover, due to the high air leakage rate, the exhaust gas temperature is lowered, and the temperature of the exhaust gas before entering the precipitator is guaranteed to be above the dew point, etc., it is difficult to recycle the residual heat of the sintering. If this technology is developed to fully utilize the residual heat of the sinter, the steel industry can save about 9 million tons of standard coal per year. [4]
6) Converter waste heat steam power generation technology
On the basis of improving the recovery of waste heat from converter flue gas, we will focus on the development of low-pressure (saturated) steam power generation technology. For example, the electricity generation per ton of steel is calculated according to 15kWh. The annual output of steel in the country is 500 million tons, which can generate 7.5 billion kWh per year, equivalent to about 3 million tce, resulting in a profit of more than 4 billion yuan. At the same time, the power generation can be used for residential electricity, thus achieving a social emission reduction of 6.2 million tons of CO2 and a reduction of SO2 of 60,000 tons. The social and environmental benefits are significant. [4]
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