RU2552807C1 - Metal scrap heating method - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to ferrous metallurgy, namely to steel making in electric arc furnaces. The metal scrap heating method for steel making in arc furnaces includes the use of a scrap heating chamber with a gas return circuit, gases are withdrawn from the top part of the chamber by means of recirculation smoke exhauster, and a part of return gases by means of the smoke exhauster of flue gases are supplied to a chimney and discharged into atmosphere. The method provides preparation of heat carrier in the heat generator by burning of natural gas with oxidiser which is obtained by mixing of part of return gases with oxygen, adding of the heat carrier together with another part of return gases into the bottom part of the scrap heating chamber, and also withdrawal from the output of the heat generator of a part of gases of the heat carrier where their temperature is sufficient for decomposition of dioxine, with their subsequent holding at this temperature, cooling by injected water down to the temperature excluding repeated synthesis of dioxine, and supply of the cooled heat carrier gases into the flue gas path with ensuring heat exchange with oxygen which is supplied for obtaining of the named oxidiser.

EFFECT: method allows to avoid emission of dioxine with flue gases, reduce consumption of natural gas for heat carrier preparation due to recovery of heat of flue gases to oxidiser, reduce water consumption cooling of flue gases due to reduction of yield of these gases because of recirculation, avoid steam explosions in the arc furnace.

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Description

The invention relates to the field of ferrous metallurgy, specifically to the preparation of metal scrap for the production of steel in an electric arc furnace.

From the level of technological development, a method is known (Kalmykov VA., Karasev VP. Steel electrometallurgy: Textbook. St. Petersburg: Publishing House of St. Petersburg State Technical University, 1999. 292 p.) Low-temperature heating of metal scrap with exhaust gases of an electric arc furnace. Hot exhaust gases from the electric arc furnace are sucked through a basket with metal scrap. Cooled gases from the basket are mixed with gases from the furnace, a mixture of these gases enters the exhaust fan and is fed into the gas cleaning system.

This method has the following disadvantages:

- large energy losses of exhaust gases between the furnace and the basket for metal scrap;

- filling gas ducts with dust;

- lack of pre-heating time on high-performance furnaces;

- a significant time of power outage from the furnace when loading metal scrap;

- loading metal scrap should be carried out exactly in accordance with the heating cycle;

- communication with equipment in front of the furnace reduces the flexibility of the process;

- finally, if there is even a small amount of plastic in the metal scrap, then at temperatures above 200 ° C, at which the metal scrap is heated in this method, it begins to burn with the release of phenols and dioxins. And emissions of phenols and dioxins are not removed by conventional gas treatment systems in steelmaking furnaces.

From the level of technological development there is a known method of using the heat of the exhaust furnace gases for heating and drying metal scrap, which involves the use of a shaft heater (Mini-metallurgical plants: Monograph / M 54 Smirnov AN, Safonov VM, Dorokhova LV , Tsuprun A.Yu. - Donetsk: Nord-Press, 2005. - 469 pp. (Tables 53, Figures 73). Http://do.rulitru.ru/v20520/?download=1, US patent 5117438, US patent US 5645791, RF patent 2205234). Shaft arc steelmaking furnaces (for example, designs of the Fuchs Systemtechnik) were developed on the basis of a conventional arc furnace, the volume of which was increased with the help of a so-called shaft mounted above the roof of the furnace. The mine, through which up to 60% of the charge is loaded into the furnace, can be diverted to the side for conventional filling with a bucket. The shaft cross section is limited, so large-sized metal scrap, not intended for heating, is loaded with the first bucket into the furnace bath. Subsequent tubs of medium-sized metal scrap and the products of the shredder installation are loaded through the shaft. Metal scrap is heated in the shaft of the furnace due to the heat of the exhaust gases and with the help of gas-oxygen burners located in the lower part of the shaft. After loading the first basket into the mine, the process of melting the charge and heating the metal scrap in the mine with exhaust gases and burners begins.

This method has the following disadvantages:

- the impossibility of heating and drying large-sized metal scrap;

- the upper layers of metal scrap in the mine in the winter do not heat above 0 ° C. At the same time, the mechanics of the movement of metal scrap in the mine is such that when unloading the mine, the upper layers are the first to enter the bath. This occasionally leads to steam explosions in the winter;

- in the volume of the mine heater of the mine chipboard there are zones in which optimal conditions for the formation of dioxins are formed. Since the gases move from bottom to top, after the formation of dioxins, they do not decompose and go to the gas purification system and then to the atmosphere (Rale V.T. Improving the thermal work and design of the shaft heater of an arc steel furnace. Specialty 05.16.02 - “Metallurgy of ferrous, non-ferrous and rare metals ". Abstract of dissertation for the degree of candidate of technical sciences. Chelyabinsk, 2010, http://susu.ac.ru/upload/298/fc/common/28/341/Raila avtoreferat.pdf).

A known method of heating metal scrap Consteel (Consteel system is gaining recognition. "Metals of Eurasia", No. 5, 2011, http://metall-news.blogspot.ru/2012/08/consteel.html), which is a process when which continuously feeds and preheats a metal charge (heavy and light metal scrap, cast iron, hot-briquetted iron, etc.) in an arc steel-smelting furnace (DSP) with simultaneous monitoring of gaseous emissions. Metal scrap is loaded by cranes into horizontal batch conveyors. Before entering the steelmaking furnace, the metal scrap enters the preheating section - the tunnel furnace - and is heated in this zone by the hot gases of the particleboard, which move in the opposite direction. During the operation of continuous supply of metal scrap, the molten steel bath in the chipboard is supported by foamed slag and with a predetermined constant temperature or specific energy consumption. The metal scrap gets into the ″ swamp ″, the dimensions of which will always be sufficient so that it sinks into it. Metal scrap in the furnace is not subjected to radiation from an electric arc, but is surrounded by liquid steel, so the bulk of the heat to melt the metal scrap comes from convection and thermal conductivity (and not through arc radiation), which is a much less efficient way to transfer energy to the metal scrap during the melting process . An electric arc is located under the foamed slag. Energy is transferred more efficiently to the bath of molten metal and slag, reducing the degree of radiation exerted on the panel and the lining. Since the arc is covered with foamed slag during the entire time that the electric power is turned on, it is more stable than when it is exposed to atmospheric influence and works with a higher power factor, increasing the active power relative to the reactive one.

The disadvantage of this method is that during the operation of the conveyor there is a low efficiency of heating the metal scrap with exhaust gases from the furnace: high-temperature gases go along the top of the conveyor, but at the bottom where the charge is supplied, the gases go at a lower temperature. In winter, this temperature is not enough to evaporate the moisture in the metal charge, which from time to time leads to steam explosions.

A known method of heating metal scrap (Yu.N. Tuluevsky, I.Yu. Zinurov, VG Shver. New features of Consteel furnaces. Electrometallurgy, No. 6, 2011, p.22-27.), Which is a modification of the conveyor countercurrent systems for heating metal scrap with furnace gases (Consteel method) by inverting the flow of furnace gases in the conveyor volume and equipping the conveyor with additional gas-air or oxygen-gas burners. This method is primarily aimed at increasing productivity, because In the Consteel process, productivity is limited by heat transfer to the solid charge from the liquid bath side. Burners create dynamic torches, due to their high-speed pressure, penetrating the thickness of the metal scrap on the conveyor and effectively warming it. The temperature of the metal scrap increases, the required amount of heat, which must be transferred to the metal scrap to melt in the furnace, decreases, and due to this, the bottleneck in the heat transfer process is expanded. To remove the furnace gases, a fourth opening is organized in the roof and the gases are removed as in a conventional chipboard, through a rolling clutch and a cooled duct. Thermophysically, such a “direct-flow” unit (the terminology of the source, denoting the difference from the “counter-current” heat exchange between metal scrap and furnace gases on the Consteel conveyor) is equivalent to a conventional chipboard equipped with gas-oxygen burners. The advantage of the direct-flow process is the continuity provided by conveyor filling. According to the source, this modification allows you to bring the performance and efficiency of Consteel furnaces to record parameters for chipboards or even exceed them.

The disadvantage of this method is the insufficient purification of exhaust gases from dioxins, since metal scrap, entering the furnace, is somehow heated to high temperatures, passing all intermediate temperatures. If metal scrap contains plastic, rubber and oil, then dioxins are still formed and removed from the furnace along with the exhaust gases.

The closest in technical essence and the achieved result is a method of preheating metal scrap (FERROUS SCRAP PREHEATING SYSTEM. PHASE III = FINAL REPORT. Work Performed Under Contract No. DE-AC02-89CE40874. Prepared For: US Department of Energy Washington, DC Prepared By : Surface Combustion, Inc. Maumee, Ohio 43537, Surface Contract Number RX-6127.http: //www.osti.gov/bridge/purl.cover.jsp? Purl = / 373823-EEAqsm / webviewable / 373823.pdf). The method involves recirculating the gases leaving the scrap heating chamber, which enter the flue reactor, where they are burned with a certain amount of natural gas, and the combustion products under the influence of the recirculation smoke exhauster return to the heating chamber from the same side of the chamber from which they were taken, i.e. e. from above. The excess of gases generated in the smoke reactor under the action of a fan enters the chimney and is released into the atmosphere.

The disadvantage of this method is that the afterburning of organic compounds released from scrap requires a very large amount of energy, which makes the method uneconomical. Moreover, this does not lead to the exclusion of dioxins from the composition of the exhaust gases, since even if they decompose at high temperatures in a smoke reactor, then with subsequent cooling of the gases on the way to the atmosphere, dioxins are synthesized. In addition, the supply and intake of recirculation gases from one side of the heating chamber, from above, does not exclude the occurrence of steam explosions when loading icy scrap, which is unacceptable in metallurgical production.

The main task solved by the invention is the creation of a safe, economical, reliable method of heating metal scrap with a reduced consumption of natural gas, which allows for the complete neutralization of organic compounds, including dioxins formed when heating metal scrap with organic inclusions, eliminating the harmful effects of these compounds to the environment.

This goal is achieved by the fact that in the method of heating metal scrap, involving the use of a recirculation circuit for gases taken from the upper part of the chamber for heating the scrap using a recirculation smoke exhauster, some of the recirculation gases using an exhaust gas exhaust exhauster are led to the chimney and then to the atmosphere, it is provided for: preparation of a heat carrier, introduced together with the recirculation gases into the lower part of the scrap heating chamber, and obtained by burning natural gas with an oxidizing agent, the oxidizing agent being oluchaetsya mixing portion with oxygen gas recirculation; the selection of gases intended for emission from the outlet of the heat generator, where they have a high temperature sufficient to decompose dioxins, and their subsequent exposure at this temperature for 2 s; "Hardening" of the aged exhaust gases by injected water, due to which the resynthesis of dioxins is prevented; final cooling of the “quenched” exhaust gases by transferring heat to oxygen, which is mixed with a portion of the recirculation gases in the preparation of the oxidizing agent.

Figure 1 presents an example installation that implements this method of heating metal scrap. Arrows illustrate the direction of gas movement. Figure 1 also shows the values of excess pressure at the nodal points.

The installation that implements the method comprises a scrap heating chamber 1, a first smoke exhauster 2, first and second pipes 3 and 4, respectively, a second exhaust fan 5, a third and fourth pipes 6 and 7, respectively, a heat generator 8, a fifth pipe 9, a sixth pipe 10, a seventh pipe 11, holding chamber 12, eighth conduit 13, quenching water injection unit 14, ninth conduit 15, heat exchanger 16, tenth conduit 17. The first conduit 3 connects the cover of the scrap heating chamber 1 to the inlet of the first smoke exhauster 2, the second conduit 4 connects to the output of the first smoke exhauster 2 with a fifth pipe 9 connecting the outlet of the heat generator 8 with the place of the coolant inlet to the scrap heating chamber 1. The second pipe 4 is connected to the second input of the heat generator 8 using the sixth pipe 10, the oxygen output is connected to the same input via the tenth pipe 17 the path of the heat exchanger 16, the input of which is connected to the oxygen supply system using the ninth pipeline 15, while natural gas is supplied to the first input of the heat generator 8. The seventh pipeline 11 connects the zone of the fifth pipeline 9, located between the outlet of the heat generator 8 and the connection point of the second pipeline 4, with the input of the holding chamber 12, the output of which is connected by the eighth pipe 13 to the input of the exhaust gas path of the heat exchanger 16, the output of which is connected to the third pipeline 6 with the inlet of the second smoke exhauster 5, the output of which is connected to the chimney using the fourth pipeline 7 (not shown in FIG. 1). A water injection unit 14 is connected to the eighth pipeline 13.

Installation works as follows. Under the action of the first smoke exhauster 2, flue gases through the first and second pipelines 3 and 4, respectively, mixed with the coolant from the heat source coming in through the fifth pipe 9, are fed to the place of the coolant inlet to the heating chamber of scrap 1 and are sucked up from the bottom through a metal scrap loaded into the chamber heating scrap 1, forming a flue gas recirculation loop. Part of the recirculation gases from the second pipeline 4 through the sixth pipeline 10 is supplied to the second input of the heat generator 10, where also the tenth pipeline 17 is supplied with heated oxygen from the oxygen path of the heat exchanger 16, and oxygen is supplied to the input of the oxygen path of the heat exchanger 16 through the ninth pipeline 15. Natural gas, supplied to the first input of the heat generator 8, burns in it with oxygen ballasted with recirculation gases, forming a coolant with a temperature of 1200 ° C, coming from the output of the heat generator to the heels conduit 9. Before place in the coolant inlet of the scrap preheating chamber 1 with the coolant temperature 1200 ° C is mixed with recycle gases from the fifth conduit 9 to a second conduit 4, whereby the coolant temperature is lowered to 400 ° C. From the zone of the fifth pipeline 9, located between the outlet of the heat generator 8 and the connection point of the second pipeline 4, the coolant with a temperature of 1200 ° C through the seventh pipeline 11 enters the holding chamber 12, where it is for 2 s, after which it enters the holding chamber 12 eighth pipeline 13. As a result of keeping the exhaust gases from the holding chamber 12 at a temperature of 1200 ° C for 2 s, the dioxins contained in the exhaust gases completely decompose. In the same eighth pipeline 13, water is supplied through the injection unit 14, which leads to rapid cooling of the exhaust gases to a temperature not exceeding 300 ° C, i.e. to their "hardening", which completely eliminates the resynthesis of dioxins. Cooled to a temperature of 300 ° C, the exhaust gases enter the inlet of the exhaust gas path of the heat exchanger 16, where they give part of the heat to oxygen moving along the oxygen path, forming a recovery circuit. Cooled to a temperature of 150 ° C exhaust gases from the outlet of the exhaust gas path of the heat exchanger 16 through the third and fourth pipelines 6 and 7 under the influence of the second exhaust fan 5 are fed to the chimney and are discharged into the atmosphere.

The method is distinguished by novelty, which follows from a comparison with the prototype, inventive step, since it does not follow explicitly from the existing level of technology, it is practically easy to implement, since it uses an indissoluble combination of methods and their technical implementations that are known and often used in the steel industry.

The essence of the method is illustrated by the following.

Drying and heating of metal scrap with an additional coolant before loading it into the furnace gives significant energy savings in the furnace and reduces the melting time, and also eliminates the possibility of steam explosions.

Drying and heating of metal scrap is carried out in modified filling baskets due to heat transfer from the combustion products of natural gas.

At a heating temperature of not more than 200 ° С, the spent heat carrier - the products of fuel combustion, saturated with moisture from the drying of metal scrap, do not contain harmful impurities and are discharged into the atmosphere or into the flue gas duct. In this case, the energy invested in metal scrap is ~ 26 kW × hour per tonne of charge, or ~ 23 kW × hour per tonne of molten steel (suitable coefficient ~ 0.87). It turns out that at an energy cost of ~ 300 kW × hour per ton and an arc heating efficiency of ~ 80%, energy savings due to such heating of metal scrap are up to ~ 10%. The reduction in the duration of the melting period is approximately the same. If in conventional modern chipboards with an energy ratio of ~ 0.8 MVA / t, the tap-to-tap time is about 40 minutes, and the melting period is ~ 15 minutes, then the reduction in the melting cycle can be up to ~ 10% × 15 = 1.5 min, or 1.5 / 40 = 3.7% - the furnace productivity increases by this value. The heat transfer from the coolant to the metal charge during gas heating of metal scrap in baskets is most effective, because hot gases are pumped through the metal scrap from the bottom up. Let the efficiency of this heating be ~ 50%. Then, to heat metal scrap to 200 ° C, ~ 5.4 nm ^{3 of} natural gas per tonne of charge is needed. At current prices for natural gas in Russia, this is equivalent to a cost of ~ 4 kW × hour of electricity, while the energy savings reach 26 kW × hour. Thus, when heating metal scrap to 200 ° C and a suitable coefficient of 0.87, the cost of electricity per ton of molten steel is reduced by ~ 26 / 0.87≈30 kW × hour (10%), the total cost of energy carriers (taking into account additional costs for natural gas) is reduced at equiv. 30-4 = 26 kW × hour per ton of molten steel, the melting time is reduced by ~ 1.5 min.

The heating temperature can be increased to 400 ° C. Then all the technical and economic parameters of the steelmaking process are doubled. The heating temperature of the metal scrap is limited only by the structural stability of the modified filling basket. According to the operating experience of gas installations for heating metal scrap in baskets, for baskets made of 09G2S steel, the temperature of the coolant 400-450 ° C does not cause any problems. However, if there is plastic, rubber, oil, and other organic impurities in a cheap commodity metal scrap, heating in an oxidizing atmosphere can be accompanied by the formation of dioxins, furans, phenols, benzapyrenes, and other harmful organic substances. In this case, direct discharge of the spent coolant into the workshop space, into the atmosphere, or into the furnace gas duct is unacceptable.

There is an opinion that at high temperatures in particleboard all these harmful substances completely burn out. However, the maximum emissions occur at relatively low charge temperatures - 300 ... 600 ° C. Then the furnace gases have a low temperature, and efficient afterburning of gases in the furnace gas duct is impossible.

It is known that for the decomposition of dioxins it is necessary to create a temperature of at least 1200 ° C, a residence time of at least 2 s and an oxygen content of at least 6% (Directive No. 2000/76 / EC of the European Parliament and of the Council of 4 December 2000 on waste incineration) . After decomposition, the gases must be “quenched" - quickly cooled to a temperature of no higher than ~ 300 ° C, when it becomes impossible to re-synthesize dioxins.

There are no such conditions during the heating up of metal scrap neither in the furnace volume, nor in the afterburner, or in the furnace gas duct. This means that in real arc furnaces, after filling a fresh portion of the metal charge, dioxins, furans and benzopyrenes enter the gas treatment system, pass through bag filters and are released into the atmosphere.

Even with basement in the furnace, there are zones with low temperatures, insufficient for the decomposition of dioxins.

Direct solutions related to the controlled heating of the furnace gases to 1200 ° C by burning an additional amount of natural gas directly in the furnace gas duct followed by quenching by water injection do not lead to the desired results. Such decisions are quite expensive, both in terms of operating costs and the cost of reconstruction. Suppose, for example, on a 100-ton chipboard, the volume of gases after the afterburner is 100 thousand nm ³ / h, and their temperature is 800 ° C. Then, for heating up to 1200 ° C, ~ 1500 nm ³ / h of natural gas is required. In addition, for "hardening" it is necessary to bring and evaporate ~ 50 t / h of water. The cost of such decisions is due to the fact that the heat supplied for the decomposition of dioxins is not used in the future, on the contrary, it is necessary to deal with it by supplying and evaporating a large amount of water for “quenching”. Another reason for the high costs is that in order to neutralize dioxins, you have to process the entire volume of chipboard exhaust gases.

The proposed method allows ecologically safe calcination of the metal charge, completely removing organic matter, directly during preheating in modified filling baskets. Then the processing of metal scrap in an arc furnace becomes environmentally friendly, and no reconstruction of the existing gas treatment systems of arc furnaces is required.

According to the operating experience of metal scrap heating installations, heating it in a basket with a capacity of 60 tons requires gas heating with a peak power of 5 MW and a time of ~ 20 minutes. When oxygen is used as an oxidizing agent, the maximum consumption of exhaust gases for neutralization is 1500 nm ³ / hour (and not 100 thousand nm ³ / hour, as when processing the entire volume of furnace gases). These gases already in the process of preparation of the coolant are heated to 1200 ° C, and additional fuel for their heating is not required. For "quenching" it is necessary to evaporate a maximum of 0.7 t / h of water.

To service a 100-ton furnace operating with two fillings, 2 continuously operating drying and heating systems for the metal charge in baskets are required (http://st.rf/catalog/detail.php? SECTION_ID = 60 & ELEMENT_ID = 172). Then the peak gas consumption is 2 × 500 = 1000 nm ³ / h, oxygen - 2000 nm ³ / h, water - 2 × 0.7 = 1.4 t / h. The corresponding average parameters are about half that.

The technical result achieved is the complete elimination of dioxins in the exhaust gases, a decrease in the consumption of natural gas due to the recovery of heat from the exhaust gases to the oxidizing agent, a decrease in the water consumption for “quenching” of the exhaust gases due to a decrease in the consumption of these gases due to their recirculation, and the elimination of steam explosions in connection with the movement of the coolant from the bottom up in the heating chamber of metal scrap and providing predominant heating of the lower parts of the volume of scrap, primarily interacting with high bath-temperature of the steel melt.

Claims (1)

A method of heating metal scrap for steel production in an electric arc furnace, comprising using a scrap heating chamber with a recirculation circuit for gases taken from the upper part of the chamber using a recirculation smoke exhauster, some of the recirculation gases using an exhaust gas exhaust exhauster being led to the chimney and then to an atmosphere that differs the fact that they carry out the preparation of the coolant in the heat generator by burning natural gas with an oxidizing agent, which is obtained by mixing part of the recirculation gases with oxygen , introducing the coolant together with another part of the recirculation gases into the lower part of the scrap heating chamber, taking part of the coolant gases from the outlet of the heat generator, where they have a temperature sufficient to decompose the dioxins, followed by holding them at this temperature, cooling by injection of water to a temperature that excludes repeated the synthesis of dioxins, and the supply of chilled coolant gases to the exhaust gas path, providing heat exchange with oxygen, which is supplied to produce the said oxidizing agent.