How to save energy for frequency converter

Energy saving and emission reduction has become one of the most important scientific and technological innovation tasks for the further development of iron and steel industry. Converter steelmaking is still dominant in contemporary steelmaking production, and it is unlikely to change in the foreseeable future. Although converter steelmaking is the lowest energy consumption process in contemporary iron and steel production and the only process that can achieve "negative" total energy consumption, there are still many problems to be further studied and optimized, such as further reducing the energy consumption and material consumption of the process, realizing more efficient energy conversion and recovery, utilizing secondary energy more effectively, and developing new ways to recover low-temperature waste heat. This paper only talks about the idea of further energy saving and emission reduction in converter steelmaking process, and talks about some principled suggestions.

The optimization of 1 process should be the first focus of further energy saving in steelmaking plant.

Process optimization is mainly reflected in three aspects: compactness, high efficiency and self-control.

1. 1 The analysis, optimization and reorganization of process functions are the most important energy-saving measures to realize the compactness of converter steelmaking production, that is, the minimization of process time and the optimization of connection.

At present, the most important thing is to promote full-scale hot metal desulfurization, pre-desilication and dephosphorization of converter, less slag steelmaking of decarbonization converter and full-scale continuous casting after molten steel refining (if it is near net shape continuous casting, it can directly form an optimized process with rolling). This compact process is bound to have the lowest energy and material consumption. The operation effect of this compact mode may need more perfect statistical analysis, and some processes are still controversial, but the direction of process compactness should not be doubted.

The high efficiency of 1.2 is an important measure to save energy in converter steelmaking.

First of all, compact process and optimized connection matching are the main contents of high efficiency. Secondly, the direction of scale must be adhered to. "Iron and Steel Industry Industrialization Policy" and "Iron and Steel Industry Adjustment and Revitalization Plan" have pointed out the direction, and only large-scale can better save energy, which has also been proved by facts. In addition, we should adhere to the high-speed development direction (hot metal pretreatment, smelting, refining and continuous casting are all applicable). Speed is the most important sign to improve efficiency, and it is also an important factor to reduce material and energy consumption.

1.3 automation is an important guarantee for energy saving in converter steelmaking.

Converter smelting, molten iron pretreatment, molten steel refining and continuous casting all need automatic control to achieve high efficiency. An important idea is to upgrade and improve the equipment according to the requirements of automation, and strictly implement the key technological progress such as system concentration standards and control process (especially the end point), instead of adopting automatic control means when conditions are ripe.

Grasping the above-mentioned technological measures is actually the process of building an optimization platform for clean steel production, which is very important for energy saving and emission reduction or laying a good foundation for product development. In short, process optimization can achieve maximum energy saving.

2. Take further improving the energy conversion efficiency of steelmaking and recycling secondary energy more efficiently as the current important innovation direction, highlight and optimize the existing key energy-saving technologies, and achieve better energy-saving effects.

In the whole modern steel production process, converter steelmaking (including refining and continuous casting) is the process with the highest operating temperature, and it needs to realize efficient energy conversion and recycling under most conditions. As we all know, the energy carriers in this process mainly include furnace gas, molten steel, slag, billet and cooling water. At present, the conversion and recovery of furnace gas waste heat (recovery of gas and steam) and the utilization of billet waste heat (hot charging of continuous casting billet) are very common, but the conversion efficiency and utilization efficiency are quite different. The energy of molten steel is mainly used to meet the needs of the next process (refining and continuous casting), not to mention conversion and recovery. The conversion and recovery of waste heat from slag and cooling water has just started or will be studied.

2. 1 Efficient conversion and recycling of flue gas energy

Efficient conversion and recycling of flue gas energy is the main way to make the energy consumption of converter process "negative". There are two main ways to recover flue gas energy: converting sensible heat and chemical energy of flue gas into converter gas with medium and low calorific value and steam with medium and low pressure for recycling. At present, the main problem is that the amount of recycling is huge, and there are great differences in the value evaluation of utilization methods, which is worth studying and discussing. As for the kinetic energy of flue gas and the waste heat carried by converter gas, there is no corresponding conversion and recovery method.

The recovery of converter gas in China started from 1965, but it was not until 2005 that some large enterprises just started to recover, and the time span was as long as 40 years. Moreover, due to the length of steel recovery time, process stability, equipment level (mainly the automatic control level of smelting end point) and other reasons, the quantity and quality of recovered gas in converter steelmaking plant vary greatly. According to the standard coal gas volume (Nm3) converted from calorific value of 2000kcal/m3, the high level can reach 1 10Nm3/t steel, and the difference level is less than 60 nm3/t steel. The steel of most steel mills < 10 nm3/t is still far behind the steel of Japanese steel mills >110nm3/t. As for the utilization mode of recovered gas, most steel mills use it as fuel for ladle baking and steel rolling heating furnace, and a few steel mills use it for roasting activated lime. The research on using converter gas with CO content as high as 60% or above as chemical raw materials (such as H2 production and synthetic ammonia water) or synthesizing high-quality fuel began as early as 1960s, and experiments have been conducted in recent years, but it has not been applied on an industrial scale so far. Recently, the voice of converter gas and other metallurgical gases used for power generation is very high, but there are few convincing research results in technical and economic comparison.

The waste heat of converter flue gas in China Converter Steel Plant was only recovered by evaporative cooling from water-cooled hood and flue in 1970s, but most of it was recovered by low-pressure saturated steam ≤ 12kg/cm2 and incorporated into the enterprise power pipe network. A few steel mills use medium pressure of 25-45kg/cm2 to recover steam, but it can only be recovered under reduced pressure, which wastes a lot of energy. Because most of them belong to the low-utilization recovery of low-quality steam, there are fewer users, and some enterprises often release the recovered steam again, causing greater waste. Since 1990s, some steel mills have heated the recovered saturated steam into superheated steam, which is used as the steam source of VD and RH steam injection pumps in our factory, and the surplus part has been incorporated into the company's pipe network, which has achieved considerable economic benefits. In recent years, the hot topic is that the recovered converter steam is directly used to generate electricity, and VD and RH are evacuated by mechanical pumps driven by electric energy, which is said to save energy by more than 90%. The world's first mechanical vacuum pump RH device, which is still in practice in Changshou New District of Chongqing Iron and Steel Co., Ltd., will provide the most direct application effect data, which has aroused great concern in the domestic steelmaking industry. In addition, low-quality converter steam used for power generation can improve efficiency, and some problems such as optimization of technology and equipment are still in the research and development stage. The steam recovery rate of converter smelting in China is still low, generally less than 80 kg/t steel, which is far from the level of > 100 kg/t steel in Japan.

The physical heat of flue gas and the chemical heat released in the recovery process of converter gas combustion part are the heat sources to generate steam, so there should be a reasonable combination value between gas and steam recovery, which should be considered by converter steel plants when formulating conversion and recovery targets.

In recent years, wet method, dry method or "semi-dry method" technology is generally used for dust removal, cooling and recovery of converter flue gas, and dry method is generally considered to be more energy-saving. In the early 1990s, China's first converter dry dust removal device was put into operation in Baosteel. It took more than ten years to usher in the upsurge of popularization and application of dry dust removal technology, which is still improving. The discussion on the advantages and disadvantages of the three methods is still going on, which should be a concern of everyone.

2.2 Hot delivery and hot charging of continuous casting billet is an important energy-saving measure to connect steelmaking and steel rolling.

Since the rapid development of continuous casting in China in 1990s, hot delivery and hot charging of continuous casting billets have been gradually popularized in various steel mills, but there is still a big gap. At present, the key point should be to optimize the connection of production plans and strive to improve the direct hot charging ratio. As for the hot charging temperature, it should be controlled according to the variety characteristics and production situation of regenerative heating furnace for steel rolling to ensure the improvement of quality and energy saving efficiency.

This control will become a new research topic whether a large amount of heat energy lost by continuous casting slab cooling on the conveying roller can be recycled.

2.3 Recovery and utilization of slag waste heat

This part of the waste heat energy is very large, but it is basically not recycled at present. According to the idea of on-site treatment and making full use of the residual heat of slag, some steel mills began to design new slag treatment technologies and equipment that can recover the sensible heat of slag, which attracted the attention of converter steel mills.

The proportion of slag treated to achieve stable, reliable and high value-added applications is also increasing. However, there are still many research and development contents in less slag steelmaking technology and steel slag treatment technology to reduce the free CaO in converter slag steadily.

2.4 The recovery and utilization technology of cooling water waste heat is a new subject and difficult problem to further improve the energy conversion and utilization efficiency in converter steelmaking plant.

Up to now, no one has seriously studied the transformation and utilization technology of < 100℃ residual temperature. On the contrary, reducing the cooling water in closed circulation or open circulation from 50-80℃ to about 35℃ for recycling will increase the operation of equipment and the consumption of electric energy. The technical feasibility and economy of waste heat conversion and recovery below 100℃ have long been controversial and lack of reliable certification. But the total amount of heat energy in this part is still considerable!

3 other ideas of energy saving in converter steelmaking plant

3. 1 Heat preservation measures during heating are an important basis for energy saving in converter steelmaking plant.

In this regard, the efforts of converter steel mills are also quite different, and there are few important research results and progress in recent years, which should be paid great attention to again.

Such as ladle, ladle and tundish, is an important prerequisite to reduce temperature loss during operation. Especially under the conditions of longer lining life, thinner working layer and longer operation time, the thermal insulation layer, thermal insulation covering agent and surface capping of molten metal should be designed and optimized with the cooling level of the unit ≤0.5℃/min as the goal. In the case of high efficiency in each process, it is especially necessary to strengthen management to ensure that the number of turns of ladle, ladle and tundish is reduced, so as to reduce the temperature drop loss.

3.2 The process runs stably, realizing the low-temperature system operation of the whole plant, and effectively saving energy and reducing consumption.

Therefore, it is necessary to control the whole process, especially the terminal temperature of each process. Reducing tapping temperature and low superheat continuous casting are important indexes to measure the operation of low temperature system in the whole plant, and each plant should pay close attention to optimization.

3.3 Under the overall framework of energy efficient conversion and utilization, energy flow network and optimization in iron and steel enterprises, the new measures of energy saving and consumption reduction in converter steelmaking plant are studied.

There are mainly the new technologies mentioned above, such as the conversion and utilization of residual temperature ≤ 100℃, the recovery and utilization of furnace temperature waste heat, and the heat dissipation and utilization during the transportation of high-temperature billets. The most reasonable composition of energy flow network (mainly the optimization of transmission and connection temperature system, etc.) is studied. ) between converter steelmaking plant and previous processes (ironmaking, steel rolling); On the premise of ensuring quality, accelerating the research and industrialization of compact technology (especially thin strip continuous casting and rolling) with minimum energy and material consumption is a problem that converter steel mills should pay attention to in the future.

As for the feasibility study of the application of new clean energy in converter steel works, it should also be put on the agenda gradually, and whether the focus should be on wind energy and solar energy.

The above ideas to further promote energy saving in converter steel works are only some principled opinions, and the specific technical measures remain to be discussed. Some may have just begun to study, and even the feasibility is still in doubt and error. Please criticize them. Converter steel mills should and may make new contributions to energy saving and emission reduction of iron and steel enterprises.