Developments in EAF Steelmaking - Steel Making - Lecture Notes, Study notes of Metallurgy

Download Developments in EAF Steelmaking - Steel Making - Lecture Notes and more Study notes Metallurgy in PDF only on Docsity! Lecture 16 Developments in EAF steelmaking Contents: Introduction Furnace design Developments in EAF steelmaking a) Furnace design b) Process operating technologies c) Usage of chemical energy d) Carbon injection Requirements for usage of chemical energy Future of EAF steelmaking Key words: Electric steelmaking, bottom stirring, scraps preheating, foamy slag, chemical energy Introduction The growth of electric steel production around the world has been driven by lower investment, higher operational flexibility and easy adoptability to market demand on long or flat products of either plain carbon or alloy steels. Growth has been supported by updating installations and technologies to reduce the electric energy, electrode consumption and tap to tap time. Figure16. 1 shows the developments in Figure 16.1: Trends in EAF steelmaking technologies developments in EAF steelmaking electric steelmaking technologies. Developments in EAF technologies are strongly supported by secondary steelmaking. One can note in the figure that the power consumption has decreased from 630 Docsity.com Kwh/ton of steel to 290kWh/ton. Similarly tap tp tap time has decreased from 180 minutes to 40 minutes and electrode consumption has decreased from arounf 6.2 kg/ton to as low as 1.2 kg/ton within the periods of representation in the figure. This became possible with the several simultaneous developments in the secondary steel-making method. Table shows the various developments Developments in EAF steelmaking Furnace design Process operating technologies Chemical energy Charge materials Split shell design Transformer power DC arc furnace Bottom stirring Foamy slag practice Scrap preheating Oxidation reaction Post combustion oxy fuel burner Carbon injection Directly reduced iron, hot metal Furnace design: i) Construction of hearth and lower side section of the shell of larger diameter than the top opening. This leads to increase in shell volume which results in larger tonnage charge, lower heat losses and improved thermal efficiency. ii) In the split shell design, shell structure is constructed in two sections: lower section which contains hearth and free board allowance for slag, and upper section containing side wall and roof. The two sections are coupled such that the upper section can be repaired easily. This reduces the downtime and increases furnace availability. iii) High powered transformers are the current trends. Most modern furnaces operate at 500 k VA ton⁄ and the trend is towards ultra high power ranging in between 700 k VA ton⁄ to1000 k VA ton⁄ . Developments are in progress to install transformer with 1500 k VA ton⁄ capacity. It is claimed that a 120 tons operating at 180 MVA transformer capacity and by using refining combined burner technology through oxygen gas and carbon injection, it is possible to increase capacity by up to 50%. The largest transformer in AC EAF corresponds to a rated power 0f 240 MVA for 300 ton furnace. iv) Eccentric bottom taping reduces tap times, temperature losses and slag carry over into ladle. The strip producing plants are equipped with eccentric bottom tapping in electric arc furnaces. v) DC (direct current) arc furnaces represent a different concept in arc furnace design. Most DC furnaces are with single electrode where current flows down from the carbon electrode to an anode mounted in the bottom of the furnace. Reduced electrode consumption of the order of 50 to 60 % is the major benefit of a dc furnace compared to a convectional three- phase arc furnace. Noise levels for the dc furnaces are lower. Lower maintenance costs are claimed and refractory costs are less for sidewall but more for the furnace bottom. A dc arc furnace requires an addition of the bottom electrode (anode), a dc reactor, and a thyristor all of which add cost to a dc furnace. The electrode technology limits diameter to a Docsity.com Figure 16.3 Energy balance of an electric furnace steelmaking waste gases and represents about 130 kWh/ton of steel produced. Efficient utilization of thermal energy of exit gas is the key to realize the advantages of preheating of scrap. Batch preheating and continuous preheating are the available technologies. In CONSTEEL, scrap and exit gases move counter current to each other. It is possible to preheat the scrap to ~320℃. Usage of Chemical Energy The high electrical energy costs pushed EAF steelmakers to look for alternative energy sources. One such source is the chemical energy derived from chemical reactions. In recent years the chemical energy supply amounts to 35% to 40% of the total energy in most of the modern EAFs. (See figure 16.3) i) Oxidation reactions The main oxidation reactions are oxidation of iron and carbon besides oxidation of silicon and manganese. The oxidation of iron though generates more energy than oxidation of carbon but iron oxidation results in loss in productivity. Fe + 0.5O2 = FeO; Heat content 6kW/m3O2 C + 0.5O2 = CO; Heat content 3.5kW/m3O2 Hence oxygen injection must be controlled such that iron oxidation is kept minimum. For bath carbon levels above 0.3%, all oxygen reacts with carbon to produce CO. Below 0.3% C, the efficiency of carbon oxidation to form CO drops and more and more FeO is generated in the slag. For scrap carbon levels below 0.1%, FeO levels in the slag can be quite high and represents an unavoidable yield loss. Increased carbon injection is necessary to control slag FeO levels and to prevent excessive refractory wear. Efficiency of heat transfer from oxidation reactions is extremely high due to the fact that these reactions are occurring in the bath. Greater penetrability of oxygen jet ensures the occurrence of oxidation reactions in the bath. Docsity.com ii) Post combustion It is a practice of generating additional energy for melting steel by using the right amount of extra oxygen to combust CO and H2 which evolve within the EAF. Carbon monoxide is generated in an EAF by • Partially combusted hydrocarbons entering the furnace with the scrap • Combustion of charged and injected carbon via C + 0.5O2 = CO • The reduction of FeO by carbon during slag foaming Hydrogen is generated by: • The cracking of hydrocarbons (oil in scrap, methane) • The reduction of water: H2O + CO = H2 + CO2 or H2O + C = H2 + CO In EAF, carbon monoxide and hydrogen may be available at the freeboard, whereas the foaming slag contains carbon- monoxide. It must be noted that oxygen flow should have low velocity to promote mixing with the furnace gases and to avoid scrap oxidation and rebound of oxygen from the scrap to the water cooled panels. Post combustion in the slag typically aims at combustion of 20 to 30% of the CO generated in slag and 70 – 80% at the free board. For post combustion speed of oxygen injection must be low and also uniform distribution of oxygen is required. Iii) Oxy – fuel burner Oxy-fuel burner uses natural gas or oil, together with pure oxygen to produce an extremely high flame temperature. Oxy- fuel burners are used to melt unmelted scrap between the electrodes and to provide heat to cold spot. On most modern UHP furnaces, the primary function of burners is to provide heat to cold spots to ensure even scrap melting and to decrease the melting time necessary to reach a flat bath. Typically industry practice indicates that 0.133 MW of burner rating should be supplied per ton of furnace capacity. Others recommend 32 k Wh/ ton of burner power to eliminate cold spots in a UHP furnace and 50 to 200 kWh/ ton of burner power for low powered furnaces. iv) Carbon injection Injection of carbon brings following benefits: i. For 100 percent scrap practice or when carbon content of the bath is insufficient to produce CO for slag foaming, carbon injection is beneficial. ii. Carbon oxidation produces CO which on post combustion generates thermal energy. It is to be noted that carbon injection requires oxygen injection to onset carbon oxidation. Requirements for chemical energy usage The chemical energy usage requires to develop a device to inject oxygen in different modes: Docsity.com  Hold mode (to prevent plugging)  Burner mode (to heat and melt scrap)  Soft lancing mode (for post combustion)  Supersonic lancing mode (for decarburization and slag foaming)  Carbon injection mode (when slag foaming is required) Injectors are either fixed type or moveable type. Submercible hand lances arc used through the slag door. Large opening in EAF shell is required. Slag and metal splashing restricts the device movement. Excessive repairs and down- time are associated with this technology. An innovative design is CO-jet injectors which are fixed type and can be mounted on the furnace shell. CO-jet injectors are highly flexible in usage. The reader may see the references given in this lecture. The next lecture deals with charge mix in EAF steelmaking. Future of EAF steelmaking The EAF needs a metallurgical reactor that has the largest growth potential both in terms of production capacity and technology evolution. Future EAF will be equipped with all modern technologies- like Ultra high power input (up to1500 kVA/t), latest oxygen and carbon injection technology and design features- like ultra high shell design, heavy mill type components. This combination leads to an Electric Arc Furnace where the tap to tap times can be extremely short and the corresponding productivity reaches the level of larger furnace sizes or converter plants. The two main reasons for this are: • The possibility of a higher electrical power input and • A far higher efficiency of chemical energy, decarburization and scrap preheating compared to the same size (tap weight) standard furnace. It is interesting to compare a conventional 120ton EAF with the ultimate 120 ton EAF. Conventional 120 ton EAF Ultimate 120 ton EAF 2- bucket charge Scrap bucket 130 m3 Furnace volume 145 m3 1-bucket charge Scrap bucket 185 m3 Furnace volume 210 m3 Transformer design upto 1,000kVA/t, 120MVA for 120 ton tapping weight, Secondary voltage up to 1,200V Transformer design upto 1,500kVA/t, 180MVA for 120 ton tapping weight, Secondary voltage up to 1,500V Docsity.com