Blast furnace blower process and frequency conversion transformation

1. Brief introduction of modern blast furnace

The process of iron-making is essentially the process of reducing iron from the iron-containing compounds such as ore in the natural state, which is usually divided into two processes: sintering and ironmaking. Sintering is a method to block iron ore powder and provide concentrate for blast furnace. It is a process of using iron ore powder, flux, fuel and ore to make block smelting raw materials in a certain proportion. The main purpose of the ironmaking process is to economically and efficiently obtain liquid pig iron that meets the required temperature and composition.

The blast furnace has the advantages of good technical and economic indicators, simple process, large production capacity, high labor efficiency and low energy consumption, so the pig iron produced by this method accounts for the vast majority of the total pig iron production in the world. Blast furnace is the most important equipment in the ironmaking link, its transverse section is round ironmaking! The fireplace is filled with iron ore, coke, slag making flux (limestone) from the top of the furnace, and blows the preheated air from the tuyere located in the lower part of the furnace. At high temperature, the carbon in the coke is burned with the oxygen in the air to produce carbon monoxide and hydrogen. In the rising process of the furnace, the iron is reduced. The molten iron is released from the outlet, the slag generated is discharged from the slag outlet, and the gas is discharged from the top of the furnace, and can be used as fuel for hot blast furnace, heating furnace, coke oven and boiler.

2. Introduction to the production process of blast furnace blower

Blast furnace blower is the most important power equipment in the process of blast furnace ironmaking. It not only directly provides the oxygen required for blast furnace smelting, but also provides the gas power needed to overcome the high resistance of furnace charge column. It first through the air filter from the atmosphere to collect clean air, by the blast furnace blower pressure into the hot air inlet, by the hot air heating the air after the lower outlet, to ensure the blast furnace combustion coke and spray fuel required oxygen, also have a certain wind pressure to overcome the whole system and material column resistance, and keep the blast furnace-fixed top pressure, in order to transport gas. Most of the blowers used in modern large and medium-sized commercial furnaces are centrifugal or axial flow blowers driven by a three-phase asynchronous motor. In recent years, there is also a tendency of gradually using a large-capacity synchronous motor to drive the blower. Blast furnace smelting requires that the blower can supply a certain amount of stable air to ensure that the blast furnace burns a certain amount of carbon. The size of the required air volume is not only proportional to the furnace capacity, but also related to the strengthening degree of the blast furnace, which is generally equipped according to the air volume of the unit furnace capacity 2.1—2.5m3/min. But in fact, many blast furnaces consider the development of production, the capacity of the fan is greater than this proportion, so the inlet 

air valve and outlet vent valve are generally used to control the air volume and air pressure, in order to meet the needs of the ironmaking process and prevent the surge of the fan.

The schematic diagram of the process flow is shown in Figure Figure 2-1.

Fig. 2-1 Schematic diagram of blast furnace smelting process

1一 skip；skip car ;2一Pending inclined bridge;3一blast furnace:4一Iron, slag mouth; 5一Air inlet of the blast furnace blower;6:—  Cowper blast air heater；Cowper stove；hot-blast furnace；hot-blast stove :

7一Gravity force dust removal;8一 venturi :9—column washer ；10—chimney;11-Hot air furnace inlet

Due to the throttling loss of the blast furnace blower, the efficiency of the fan is low and the power loss is large, so the energy saving space of the high voltage frequency converter is huge. However, because of the blast furnace blower as the most critical equipment of blast furnace production, not only the motor power capacity is large, but also need to ensure continuous operation. If the blower fails in the process of blast furnace production, especially in the molten iron, it will cause slag irrigation accident, resulting to the blast furnace blocking, production and coke ratio rise, resulting in huge economic losses. Therefore, at present, there are very few cases of high voltage frequency conversion transformation of blast furnace blower, the main reason is that users have doubts about the reliability of high voltage frequency converter.

3. Frequency conversion energy saving transformation of blast furnace blower

3.1 Working condition introduction

Shanxi Yongheng Industry and Trade Co., Ltd. was founded in 1998, is rebuilt from the original "Jishan County local state-owned ironworks", belongs to the private joint-stock enterprise. Now there is a 450m3 ironmaking blast furnace, two 220m3 ironmaking blast furnaces, two sintering machines, a JN4340-type coke oven, and a cement production line. Annual production of 900,000 tons of pig iron. In recent years, the company actively responded to the national industrial policy, accelerated technological progress, at the same time, actively carried out energy conservation and emission reduction, realize the waste gas (material) recycling, formed a coal-coke-chemical and coal-coke-iron-electricity (cement) two circulation production lines.
The 450m3 iron-making blast furnace fan adopts 6500kW / 10kV three-phase asynchronous motor, and the stator string water resistance mode is started, and the impact current is large and lasts for a long time. And limited by the grid capacity, in each start all need to plant all other electrical equipment, in the blast furnace fan to start after other equipment, the operation is troublesome, so sometimes when even equipment maintenance is not iron also keep blast furnace fan, and the air volume adjustment, thus caused the waste of electricity. Because the frequency converter has the function of soft start and soft stop, the starting current is small, which can solve the big problem when the blast furnace fan starts, and then the frequency converter can save electricity, so the factory leaders of Shanxi Eternal Industry and Trade Co., Ltd. decided to implement the frequency conversion transformation.

3.2 Transformation scheme of blast furnace blower

The blast furnace blower is the power center of the blast furnace production system. Once the fan cannot operate normally, it will not only affect the production, but also cause heavy losses. In addition, the working environment of the speed regulating system is relatively bad; therefore, the high voltage speed regulating system supporting the blast furnace fan should be careful and meticulous through the cooperation of the technical personnel of both parties.2.14 parameters of 50 m 3 blast furnace fan and supporting motor.

Table 1. Basic parameters of blast furnace fan

2.2Technical scheme of frequency conversion transformation of blast furnace

Figure 2-2 In the bypass cabinet, there are three high voltage isolating switches. In order to ensure that the power is not sent back to the output end of the frequency converter, K2 and K3 adopt electromagnetic interlock operating mechanism to realize electromagnetic interlock. When K1 and K3 are closed and K2 is disconnected, the motor operates frequency operation; when K1 and K3 are disconnected and K2 is closed, the frequency converter is isolated from high voltage for repair, maintenance and debugging.

                                                                                       Figure 2-2 Primary loop diagram of the manual bypass cabinet

The bypass cabinet must be linked with the superior high voltage circuit breaker DL. When DL closes, the bypass isolation switch and frequency conversion output isolating switch are absolutely not allowed to prevent arc pulling phenomenon and ensure the safety of operators and equipment. Fault switch: connect the "high voltage split" signal of the frequency converter with the "frequency conversion input" signal of the bypass cabinet, and connect in parallel with the high voltage switch switch circuit. In the inverter input state, when the inverter fails, break the high voltage input of the inverter is invalid. Protection: keep the original protection of the motor and its setting value unchanged.

2.3 Installation of frequency converter blast furnace blower as the most critical equipment in the steel plant, its light shutdown will lead to the shutdown of the iron plant, heavy will lead to accidents, so it is necessary to ensure the stable operation of the blast furnace blower. The field heat dissipation treatment of frequency converter is an important link. High-power frequency converter as a relatively large heat source, its running heat dissipation is particularly important.

The blast furnace blower is located near the blast furnace, and the surrounding dust amount is relatively large, especially the conductive dust, so there are only two kinds of cooling methods to choose the inverter: (1) air cooling and air conditioning are used together; (2) liquid cooling (generally water cooled). Since the cooling components of high voltage inverter are all conductors, indirect liquid cooling must be used when designing the liquid cooling system, which makes the design of the cooling system extremely complicated and the cost is high; one problem in the liquid cooling system may cause the paralysis of the whole cooling system, so the stability is also low.

On the contrary, air cooling has: (1) simple design; (2) redundant design can be adopted; (3) combined with air conditioning cooling, which can make the operating environment of frequency converter relatively closed and ensure the hygiene of frequency converter. After comprehensive consideration, we choose air cooling on the cooling mode of 6500kW / 10kV frequency converter, and use it with air conditioning cooling. A complete high voltage frequency converter is divided into four parts: bypass cabinet, transformer cabinet, power unit cabinet and control cabinet.

Transformer cabinet and power unit cabinet almost account for the all the heat value of the whole machine. The operating environment of the transformer cabinet required by the power unit cabinet is much lower, and the heat value of such a large frequency converter transformer is not small. In order to save the number of air conditioning used, the transformer cabinet and power unit cabinet are placed separately in the structural design of the frequency converter.

The specific method is: the transformer cabinet and the power unit cabinet are separated by 700mm, and after the installation of the inverter, the middle uses heat insulation materials, so that a frequency converter is placed in two houses.

Transformer cabinet and bypass cabinet together and adopt forced air cooling, and then the air duct is installed on the top of the transformer cabinet to collect the transformer heat production directly outside the installation environment; the power unit cabinet and the control cabinet are installed in a relatively independent and closed room; forced air cooling is also installed in the room to reduce the ambient temperature and ensure the operating environment of the power unit cabinet and control cabinet. In order to ensure the safe operation of the frequency converter, the air conditioning installation requires sufficient cooling capacity and allowance. Because there is no heat of the transformer, the air conditioning is designed according to the heat value of 3% of the rated power of the frequency converter:
3 The use effect of the frequency converter

The frequency conversion transformation of the blast furnace blower has the following advantages:
（1）Electricity saving effect is obvious
Before the frequency converter is not installed, the high voltage motor is started by stator string water resistance, and the wind pressure is adjusted through the air valve. In this way, the motor is always running in the rated state. After the inverter is installed, the air door is fully open, and the wind pressure is realized by adjusting the frequency of the inverter. To achieve the purpose of power saving, the following quantitative analysis of the power saving. Quantitative calculation: the price is calculated as 0.45 yuan, and the annual power consumption of motor is 330 days:
WP=1.732×10×366×0.9×24×330 =45185247.36（Kw.h）
Power consumption of frequency converter:
WF=1.732×10×270×0.95×24×330 =35185233.6（Kw.h）
Maximum power consumption of air conditioning:
WC=1.732×380×17.5×0.85÷1000×7×24×330 =542764.807（Kw.h）
Annual savings:
F= (WP－WF－WC) ×0.45
=(45185247.36－35185233.6－542764.807) ×0.45
=9457248.953×0.45
=4255762（RMB）
To sum up, after the frequency conversion transformation, the energy-saving effect is very significant.

（2） The soft start function of frequency converter solves the problem of blower start
Eternal industry and trade has three factories: iron factory, sintering workshop and steel mills. One transformer is common in the three plants with a capacity of 25000 kVA. The 6500kW / 10kV blast furnace blower starts with stator string water resistance, and the impact current is large, the duration is long, which makes the blast furnace blower start to stop the production, and stop all the large electrical equipment, and then start. Such a troublesome start-up method leads to try not to stop the blast furnace blower during the equipment maintenance, which not only makes the maintenance of the machine is not convenient but also virtually caused a lot of waste of electricity. After using the frequency converter, the inverter drives the motor to start from 0.5Hz, and the starting current does not exceed the rated current, which not only makes the blast furnace blower start and stop, but also saves the power, and reduces the impact on the motor, fan and the whole blast furnace system.

（3） Reduce the motor vibration and noise
Due to the high power of the 6,500 kW motor, the noise pollution is particularly serious during operation. However, as an important link, the blast furnace blower must be monitored 24 hours a day, and this environment is bound to cause serious harm to the physical and mental health of workers. Minimizing the vibration of the motor and fan can not only improve the working environment of the monitoring personnel, but also extend the life of the equipment, reduce the equipment failure rate and maintenance, maintenance costs.

4.  Epilogue；peroration

After the frequency conversion transformation of the blast furnace blower, it not only achieves a good energy saving effect, but also improves the stability of the whole blast furnace system by a step. This is one of the few domestic super high power high voltage frequency converter in the blast furnace fan application, the transformation is very successful, but also shows that the domestic high voltage frequency converter has the production of super high power high voltage frequency converter technology. With the country more and more attention to energy conservation and emission reduction work, enterprises through various measures to reduce production costs, among which frequency conversion technology has played a key role, achieved obvious economic and social benefits, adapted to the national construction of resource-saving society trend.