Measures for energy saving and consumption reduction in ironmaking system

       Abstract: with the development of society and the reduction of earth resources, higher requirements and more challenges have been put forward for various industries. However, for the blast furnace iron making industry, the gradual reduction of fossil fuels on the earth has led to insufficient resources in the industry, resulting in the slow development of the industry. Therefore, the blast furnace iron making industry urgently needs to optimize energy and resources. We should reduce fuel and power consumption, strengthen secondary energy recovery, and save energy and reduce emissions.

       Key words: energy saving and consumption reduction in blast furnace ironmaking

1、 Foreword

       Since the reform and opening up, China's industry has developed rapidly. By 2017, China had produced 73.2313 million tons of crude steel and 16.6784 million tons of pig iron, accounting for about 50% of the world's total iron and steel. The iron and steel industry is also an industry with high energy consumption. Energy conservation and consumption reduction have become the top priority of the sustainable development of the iron and steel industry.

       Ironmaking system includes ironmaking process, sintering process and coking process. In iron and steel enterprises, the direct energy consumption of this system accounts for about 70% of the total energy consumption, and it is the key link of energy conservation in iron and steel enterprises. At the same time, because the system will also produce a large number of secondary energy such as blast furnace gas and coke oven gas in the production process, the utilization effect of these secondary energy can not only affect the energy (power) medium balance of iron and steel enterprises, but also affect the development of enterprise environmental protection work [1].

       The energy consumption of ironmaking process includes power item, fuel item and recovery item. Power items include electricity, industrial water, purified water, steam, blast, oxygen, nitrogen, compressed air, etc. The fuel item includes blast furnace gas, pulverized coal, coke oven gas and coke, and the recovery item includes blast furnace gas recovery, hot blast furnace flue gas heat recovery, TRT power generation and blast furnace slag waste heat recovery. After deducting the recovery items, the fuel consumption in the iron making system accounts for about 90%, and the power consumption only accounts for about 10% [2]. Therefore, the focus of reducing energy consumption in ironmaking system should be on reducing fuel consumption. In addition, energy recovery also has a great impact on reducing the energy consumption of ironmaking process. It is a very powerful measure to do a good job in energy recovery to reduce the energy consumption of ironmaking process.

2、 Measures for energy saving and consumption reduction in ironmaking system

       1. Energy saving measures for sintering process

       Taking WISCO as an example, WISCO carried out energy-saving transformation of its sintering workshop in 2001, adopting the new process of "thick material layer - fuel separate addition - small ball sintering". The thickness of the material layer was increased from 420mm to 600mm[3], the particle size greater than 3mm in the mixture was increased from 50% to 70%, and the addition proportion of pulverized coal reached about 70%. The consumption of solid fuel was significantly reduced, and the ore consumption was reduced from 48.79kg/t to 46.75kg/t.

       2 energy saving measures for coking process

       Taking WISCO as an example, the coking company of WISCO actively carried out the technical transformation centered on the large-scale coke oven, and built three 55 hole 6m Coke Ovens in 2003, 2004 and 2005 respectively. Many energy-saving and environment-friendly technologies have been adopted for these coke ovens after transformation, such as the chain technology of four large cars and professional furnace structure; The computer adopts the auxiliary equipment of PLC program control for furnace temperature control technology, as well as the instruments of DCS system and the coke quenching system of low moisture coke quenching process of American Steel Union. At the same time, the environmental protection technology of coal loading and dust removal is adopted to realize the zero emission of waste water and smoke control in the whole production system. It has many advantages, such as reducing energy consumption, improving coke strength, good environmental protection conditions and so on, providing a hard guarantee for coking energy conservation and consumption reduction.

3 energy saving measures for blast furnace process

       3.1 implement fine shaking policy
       Take Shougang as an example. Fine shaking is the material basis for stable and smooth energy saving and consumption reduction of blast furnace. In order to improve the quality of raw fuels entering the plant, in 2006, the iron making plant of steel adopted the new mode of supply chain management. The professionals who tested the raw fuels and the supply company went to the raw fuel manufacturer to investigate the quality of raw fuels, so as to "reach out to the periphery" and comprehensively grasp the trend of the quantity and quality of raw fuels entering the plant [4]. For the quality problems of raw fuels, professional personnel react to the blast furnace in time to greatly reduce the impact of the quality fluctuation of raw fuels on the blast furnace.

       3.2 oxygen enriched blast

       The functions of rich blast are as follows: (1) oxygen rich blast can improve smelting intensity and increase output; (2) After oxygen enrichment, the nitrogen content is reduced, which can relatively increase the CO concentration in the furnace belly, which is beneficial to the indirect reaction; (3) Oxygen enriched blast can improve the combustion of injected fuel; (4) Oxygen enriched blast can also improve the calorific value of gas on the furnace top, which is very beneficial to the increase of air temperature [5].

       3.3 continuously improve coal injection ratio

       To inject pulverized coal into the tuyere of the blast furnace, firstly, the abundant non coking coal resources can be used to replace most of the coke in the blast furnace, which can not only alleviate the current shortage of coking coal resources, but also appropriately reduce the environmental pollution caused by coke in the smelting process; Second, because the operation of high coal ratio is to use coal to replace part of coke, it can not only reduce the cost per ton of iron, but also enhance the market competitiveness of blast furnace ironmaking [6]. Of course, the amount of unburned powder must be paid attention to when increasing the coal ratio. The amount of unburned powder is generally about 15%-20% lower than the amount of coal injected. Excess unburned powder will enter the material column with the gas, which will have an adverse impact on the blast furnace. The following figure 1 shows the change trend of coal injection ratio and fuel ratio of Shougang from 2004 to 2008.

       3.4 maintain reasonable gas distribution

       The core of blast furnace ironmaking is to distribute gas reasonably. Because the reasonable distribution of gas can make the furnace condition stable and smooth, can also improve the utilization of gas and reduce the fuel ratio.

4. Increase the utilization of secondary energy

       1) Increase the utilization of waste heat of hot blast furnace flue gas: because the temperature of hot blast furnace flue gas is generally about 250-350oc, the amount of flue gas is large and there is a large amount of waste heat. These waste heat are good secondary energy, which can be recycled to generate other energy, so as to reduce energy consumption in iron making process. Moreover, the recovery of waste heat from hot blast furnace flue gas is an effective measure to improve air temperature and reduce fuel ratio.

       2) Use the pressure of the blast furnace top to increase the power generation capacity of the pressure difference: the pressure energy and heat energy of the blast furnace top gas enable the gas to do work through the turbine expander, convert it into mechanical energy and drive the generator.

Fig. 1 change trend of coal injection ratio and fuel ratio of Shougang from 2004 to 2008

3、 Concluding remarks

       With the development of society and economy, the earth's energy consumption is increasing, and the earth's energy reserves are becoming less and less, which leads to the rise of the prices of various energy sources. As an energy consumption giant, the iron and steel industry must save energy and reduce consumption, especially in the iron making system, and strengthen the utilization of secondary energy and energy recycling to reduce costs before new energy is developed. In addition to the above technical energy conservation and consumption reduction, iron and steel companies must strengthen energy management and reduce unnecessary energy consumption to achieve the purpose of energy conservation.