Prospect analysis of sensible heat recovery from blast furnace slag
A number of waste heat and energy recovery technologies developed in recent years have been applied in iron and steel enterprises and achieved remarkable results. However, as one of the high-quality waste heat resources, the sensible heat of blast furnace slag has no mature recovery technology, and a large amount of sensible heat of blast furnace slag is wasted. The annual output of blast furnace slag in China is hundreds of millions of tons, and the sensible heat is equivalent to more than 7 million tons of standard coal. Waste heat resources are quite abundant. If it can be effectively recycled, it will be of great significance to the energy saving and sustainable development of China's iron and steel enterprises.
1 sensible heat resources of blast furnace slag
The waste heat resources of iron and steel enterprises are mainly concentrated in the processes of coking, sintering, ironmaking, steelmaking and hot rolling, which are characterized by product waste heat, flue gas waste heat, sensible heat of waste residue and sensible heat of cooling water. According to relevant statistics, various forms of waste heat resources are shown in Table 1.
Table 1 Status of waste heat resources in iron and steel enterprises
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kind
Waste heat quality
Total proportion
Existential form
Application status
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Smoke (coal)
Middle and low
43%
Keywords coke oven flue gas, coal gas, sintering flue gas,
Partial utilization
Sensible heat of qi
Hot blast stove flue gas, converter gas, blast furnace gas
Sensible heat of product
High school and junior high school
Thirty percent
Sinter/pellet, coke, billet, etc.
Partial utilization
Sensible heat of slag
high
10%
Blast furnace slag, steel slag, etc
Make little use of
Sensible heat of cooling water
low
17%
Blast furnace cooling water, etc
Make little use of
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Note: The sensible heat of molten iron is not included in the waste heat statistics.
In all kinds of waste heat resources of iron and steel enterprises, the sensible heat of products and smoke (coal) gas accounts for a large proportion of the total waste heat resources, and the quality of waste heat includes various temperature grades. At present, the technologies that have been developed and applied include dry quenching technology, sintering waste heat recovery technology, converter flue gas waste heat recovery technology, and continuous casting billet hot delivery and hot charging technology. It has achieved obvious effects and benefits. Although the sensible heat of cooling water also accounts for a certain proportion, it belongs to low-temperature waste heat resources, and the economic effect of recovery is poor, and the recovery rate of waste heat is only about 2%. Slag has a high sensible heat level and belongs to high-grade waste heat resources, accounting for about 35% of all high-temperature waste heat resources, of which blast furnace slag accounts for 28% (see figure 1), which has great recycling value. However, due to the difficulty of recovery technology, the recovery rate of sensible heat of blast furnace slag is extremely low at present, and only part of the residual heat of slag flushing water can be utilized, and sensible heat of blast furnace slag is one of the few important waste heat resources that have not been developed and utilized.
The tapping temperature of blast furnace slag is between 1400 ~ 1550℃. Each ton of slag contains (1260 ~1880) ×103 kj of sensible heat, which is equivalent to the calorific value of 60kg standard coal. The discharge rate of blast furnace slag is related to the ore grade. In recent years, China's large and medium-sized iron and steel enterprises have used high-grade iron ore raw materials, and the slag-iron ratio of blast furnace has dropped to about 300 kg/t. Therefore, a blast furnace with an annual output of 900,000 tons of pig iron and a slag-iron ratio of 300 kg/t has an annual slag discharge of 270,000 tons, and the heat released by slag is equivalent to 1.60. According to incomplete statistics, There are about 169 m3 and above large blast furnaces put into operation and under construction in China. It is estimated that the comprehensive production capacity of pig iron is about 320 million tons, so the annual slag output of large blast furnaces in China is close to1000 million tons, and the heat carried is equivalent to 6 million tons of standard coal. However, the actual pig iron output and slag-iron ratio are higher than the calculated values, so the sensible heat energy of blast furnace slag is greater. Although the heat energy of blast furnace slag can not be completely recovered, if it can be partially recovered, its energy-saving benefit is also remarkable and it has great market development potential.
2 Status of sensible heat utilization of blast furnace slag
More than 90% of blast furnace slag in China is extracted by water quenching method and used as cement raw materials. The commonly used water treatment methods include Inba method, Tulafa method and Lhasa method, but without the heat recovery function of slag, the heat of slag is basically lost. The recovery and utilization of blast furnace slag waste heat in China is limited to the waste heat heating of slag washing water. Shougang, Jigang, Xuanhua Steel, Angang, Benxi Steel, Laigang, Angang and other enterprises reported that the waste heat of slag washing water was used to solve part of the heating in the factory or hot water in the bathroom. However, this utilization only accounts for a small part of the total sensible heat of blast furnace slag, and the recovery rate of residual heat is low, only about 10%. Due to the limitation of time and region, this part of energy can only be wasted in summer and southern areas without heating facilities, so its popularization and application are limited. Although the problem of sensible heat recovery from blast furnace slag has been concerned, some iron and steel enterprises, universities and scientific research institutes are also conducting research, but most of them focus on theoretical design and basic experiments. At present, there is no report on the industrial application of physical equipment, and no mature technology has been formed.
Nippon Steel Corporation of Japan began to research and develop the slag waste heat recovery technology in the early 1980s, and it has been tested and applied in large factories, and great progress has been made, but the related technology is still immature. Other countries, such as Britain and the United States, are also developing and testing slag waste heat recovery technology.
3 development status of sensible heat recovery technology of blast furnace slag
At present, the methods of slag heat energy recovery under development can be summarized into two categories: medium heat exchange method and chemical reaction method.
The medium heat exchange method uses blast furnace slag to exchange heat with medium through contact or radiation, and then uses the energy of high temperature medium to generate electricity or use it for other purposes. The technologies developed earlier and achieved certain results mainly include Japanese internal cooling drum method, rotor granulation method, air quenching method and British centrifugal turntable method, as shown in Table 2. The above method can recover 40% ~ 60% sensible heat of blast furnace slag through small-scale test or industrial test, but the technology is immature, some are inefficient, some affect the slag performance and reduce the added value, and some have large equipment investment. , can not be promoted.
Table 2
Summary of sensible heat recovery technology of blast furnace slag
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way
Technical overview
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Drum cooling method
The liquid slag is cooled by the surface of the drum into a slag film, and after the medium in the drum absorbs heat,
(Japan)
It becomes steam, which is cooled by a heat exchanger and recycled to recover heat.
This amount is used for steam turbine power generation. Develop and develop industry in the 1980s.
The test shows that the vitrification rate of finished slag is 95%, and the sensible heat recovery rate of slag is low.
Less than 40%.
Rotating wheel granulation method
Slag falls to the surface of the rotating roller, is thrown out and granulated, and then is sent in.
(Japan)
The fluidized bed of solid medium fully exchanges heat with air and medium and cools it.
The air after heat exchange becomes high-temperature hot air and is sent to the waste heat boiler to recover heat.
Developed in 1970s, the heat recovered from the test is about 60% of the sensible heat of slag.
The finished slag can be used for concrete aggregate.
Air quenching method
The molten slag is granulated and cooled by high-speed blast, and the slag particles undergo secondary flow.
(Japan)
The fluidized bed heat exchanger is cooled to a low temperature and discharged, and introduced into a wind tunnel and a cooling tower.
High temperature air is converted into steam or electric energy for use.
Developed in the 1980s, after semi-industrial test and large-scale production test, hot
The recovery rate is less than 50% and the glass transition rate is more than 95%. But it was Pang who designed the equipment.
Large, fan energy consumption is high.
Centrifugal turntable method
Liquid slag falls on the high-speed rotating cups and plates and is subjected to centrifugal force (or additional force).
(UK)
High-speed airflow), through the water wall and multistage flow.
Recovery of slag heat energy by fluidized bed.
Developed in the 1980 s, the glass phase obtained in the small test is over 95%, and it is predicted.
The recovery rate of industrial sensible heat reaches 60%.
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Chemical reaction method uses sensible heat of blast furnace slag to promote chemical reaction, so as to recover residual heat of blast furnace slag. It has been studied abroad that the high temperature heat of slag can promote the reaction of CH4 and H2O to generate H2 and CO, and the generated gas reacts in the next reactor to generate CH4 and H2O, releasing heat, which can be used for power generation or hot blast stove after treatment. In addition, the experimental study on hydrogen production from biogas by sensible heat is reported: CH4+CO2→2H2+2CO. At present, this method is still in the stage of theoretical research and exploration, which is far from practical application.
Discussion on development of sensible heat recovery technology of blast furnace slag
There are many designs for recovering sensible heat from blast furnace slag, but in order to realize the popularization and application of this technology in industrial production, several preconditions need to be met: (1) At the same time, the performance of blast furnace slag can not be reduced and still meet the mainstream requirements; (2) The recovery system is simple and the investment is low; (3) The sensible heat recovery efficiency of blast furnace slag is high.
The main purpose of sensible heat recovery of blast furnace slag is to reuse secondary energy on the basis of existing production, so the technical development should not affect the availability and added value of slag. As mentioned above, more than 90% of blast furnace slag in China is made into water-quenched slag and sold to cement plants as raw materials. About 70% of the cement produced in China is mixed with different proportions of blast furnace slag. Although blast furnace slag can also be used to make high value-added products such as slag cotton and glass-ceramics, hardened blast furnace slag as cement raw material has a broader market and a large consumption. Therefore, the sensible heat recovery technology of blast furnace slag should be studied mainly for the production process of blast furnace slag as cement raw material, that is, the heat energy of blast furnace slag can be reused, and the properties of treated blast furnace slag can meet the requirements of cement raw material.
At present, the hot spot of sensible heat recovery technology of blast furnace slag is dry recovery, which is more water-saving and environmentally friendly than the existing water-quenched slag method and conforms to the development concept. However, the problems of large equipment investment and low comprehensive efficiency have become the bottleneck of its industrial application, and new technological breakthroughs are needed, such as the air quenching method tested in Japan. In addition, some dry recovery technologies only pay attention to the recovery efficiency of sensible heat, ignoring the influence on the properties of blast furnace slag, which makes the treated slag difficult to meet the requirements of cement raw materials, such as wheel granulation, and is not suitable for industrial promotion. Water quenching process of blast furnace slag is a mature technology, which has been widely used in China. If the existing water-quenched slag process equipment is partially reformed, heat energy recovery equipment is added to realize a certain degree of heat energy recovery, the cost investment can be reduced and it is easier to popularize and implement in iron and steel enterprises. The new INBA method for water slag treatment is to add a condensing equipment system on the basis of the original INBA method, which can recover a large amount of steam generated during slag washing and reduce environmental pollution. Taking this as a lesson, heat recovery can also be further realized.
Although the sensible heat of blast furnace slag is high, it has low thermal conductivity, slow heat transfer and discontinuous slag discharge, so it is difficult to effectively recover its heat energy. In the design, we should first consider ensuring the stability of the heat source, so that the energy can be continuously and stably converted into output; In addition, the appropriate heat exchange medium is selected to improve the heat exchange efficiency. At present, water, air and slag are mostly used for heat exchange, and other inert gases or organic liquids can also be considered as media, or a combination of several media can be used. After that, we should also consider the rational use of recovered heat energy, try our best to match the energy levels and make efficient use. The sensible heat utilization of blast furnace slag should be directly utilized first, such as air and gas preheating, material drying and steam hot water production in the production process. In addition, it is also used for power recovery, such as power generation. For the part of heat energy with low recovery temperature, it can also be used for heating and cooling.
The sensible heat recovery technology of blast furnace slag in China has not yet entered the industrialization stage, so the design and manufacture of large-scale recovery equipment is also one of the main contents of development. The structure and size of sensible heat recovery mechanism of blast furnace slag should not only meet the technological requirements, but also have certain high temperature resistance, wear resistance and corrosion resistance, which is safe and stable in operation and easy to maintain. Therefore, the sensible heat recovery equipment of blast furnace slag should be designed and developed in combination with the actual production to realize standardization and serialization.
The development and popularization of sensible heat recovery technology of blast furnace slag can be implemented step by step. For example, in view of the actual production situation of the existing blast furnace, firstly, a relatively simple recovery system is developed to recover a certain proportion (30% ~ 40%) of heat energy, which can be supplemented when the blast furnace is overhauled or expanded; After successful use and benefit, based on this technology and experience, technical equipment can be upgraded or new recovery methods can be developed, so that the heat energy recovery efficiency can reach a higher level (60% ~ 70%), which can be applied to new blast furnaces and gradually improved and fully popularized in iron and steel enterprises.
5 conclusion
To sum up, China's blast furnace slag has high sensible heat quality and rich resources, and its recycling has a very broad development prospect. The development of sensible heat recovery and utilization technology of blast furnace slag should be based on the premise of satisfying the application of slag, making the performance meet the requirements of cement raw materials, simplifying equipment as much as possible, reducing investment and achieving a certain heat recovery and utilization rate; Combined with the actual situation, based on the existing water slag process, the sensible heat recovery function of blast furnace slag is developed, the recovery process equipment is gradually improved, the efficiency is improved, and the dry recovery technology and other more efficient and environmentally friendly sensible heat recovery technologies are further developed. The recovery and reuse of sensible heat of blast furnace slag involves many aspects such as technology, equipment and production. It is suggested that scientific research institutes, design units and enterprises cooperate in development and realize industrial application as soon as possible.