OPERATING EXPERIENCE AT A 170 t EAF WITH EPC SCRAP PREHEATING SYSTEM BY KNUT RUMMLER* DR. HEINZ MÜLLER** INTRODUCTION In the recent years, the growth of EAF steelmaking and the corresponding push for improved productivity and energy saving has seen a blurring of the boundary between oxygen and electric steelmaking and a departure from the pure batch processing operating philosophy associated with early EAF steelmaking. 1 These developments include hot heel operation, continuous scrap feeding, and oxygen injection, scrap preheating and post combustion (Figure 1). Due to the relatively high electrical energy prices all over the world, reduction of electrical energy consumption is of great importance today. Motivated by this need, KR Tec GmbH developed a new generation of preheating technology which prototype was installed in Toscelik/Turkey in April 2012. In March 2013 this technology was acquired by INTECO special melting technologies GmbH. The paper describes the main design features of the EAF and EPC System focusing on the innovative technology applied to the project. The overall operating procedures are illustrated together with a brief summary of the experience gained from one year of operation. The EPC design allows the recovery and transfer of the heat from the off-gas to the scrap while focusing mainly on an increase of the productivity using the energy more efficient. Keywords: Environment, Scrap Preheating, Continuous Scrap Charging, Energy Saving, Productivity Increase * President of KR Consulting GmbH, 77694 Kehl, Melanchthonweg 4, Germany, Phone: +49 7851 88 67 133, Fax: +49 7851 88 67 134, E-mail: k.rummler@kr-tec.net ** Executive Vice President of INTECO special melting technologies GmbH, A-8600 Bruck an der Mur, Wiener Straße 25, Austria, Phone: +43 3862 53 110 108 / Fax: +43 3862 53 844 / Email: heinz.mueller@inteco.at P a g e 1
Figure 1: EAF improvement since 1965 SCRAP PREHEATING AT EAF WITH EPC SYSTEM In the steelmaking process, by scrap melting through the electric arc furnace route, substantial reduction in electric energy consumption and associated increase in furnace productivity can be realized with Scrap Preheating Technology which pre-heats the scrap to about 700 C by making use of the sensible heat carried in the furnace off gas (Figure 2). 2 In this respect, KR Tec GmbH, Germany developed an environmentally friendly and high efficiency scrap preheating system to be superior over the existing systems developed so far. This challenge led to new and superior Environmental Preheating and Continuous Charging (EPC ) System. The EPC System combines the advantages of 100% scrap preheating and continuous scrap feeding through its chambers, without the need of EAF roof opening. EPC prevents totally, any dust emission and heat loss during furnace charging stage, as it is the case normally for other operations. The EPC - EAF is a new generation, economical and environmentally friendly electric arc furnace. Considerable reduction in electric energy consumption, increased productivity, meeting strict environmental regulations, less dust load within the melt shop, flicker reduction & harmonic disturbance reduction are some of the important features of the new and superior EPC System. P a g e 2
Figure 2: Scrap preheating effect DESCRIPTION OF EPC SYSTEM The EPC System combines the advantages of the preheating efficiency of the preheating chamber and the continuous scrap feeding (Figure 3). The system is a new generation of ecological arc furnace. In the EAF field, in a sense power saving and scrap preheating is synonymous. Various technologies have been developed to effectively preheat the scrap by the exhaust gas. One of the issues of the EPC System is to charge the scrap independently of the electric arc furnace taking into consideration the environmental aspects. The EPC System is installed beside the EAF and the preheated scrap can be charged continuously by the telescopic feeder system in the melting chamber during power on. Even during charging of the scrap basket into the EPC System, the system is always closed and only a small amount of dust can escape. The scrap basket will be charged into a charging hopper in the upper part of the EPC System which is in waiting position. While being in this position the front wall of the charging hopper is closing the preheating chamber and the melting process in the EAF and preheating doesn t have to be interrupted. After filling of the charging hopper by the scrap basket a slide gate on top of the EPC System will be closed. Then the charging hopper will be pushed by two hydraulic cylinders to the preheating chamber and the scrap is falling inside the preheating chamber where it will be preheated. If the charging hopper is in front position the back wall of it is closing the preheating chamber. A special design of the off gas duct together with a water cooled regulation flap allows to control the preheating effect in the system. The furnace is operated with flat bath conditions during the entire power-on time and its roof is never opened during normal operation. Nevertheless, if necessary, P a g e 3
the furnace can also work in the conventional operating mode without the EPC System. In this case, a by-pass connects the roof to the hot gas line and the sidewall opening in the furnace shell is closed by a water-cooled panel. Figure 3: EAF with EPC System BENEFITS OF EPC SYSTEM VERSUS THE CONVENTIONAL EAF MINIMUM DUST EMMISION During charging procedure the system/preheating chamber is always closed which reflects a minimum of pollution in the melt shop. ENERGY SAVING The EPC reduces the electric energy consumption by approx. 100 kwh/t compared to the conventional EAF (Figure 2). INDEPENDENT SCRAP CHARGING Charging of the scrap basket is done with power-on and independently from the furnace operation. This improves the operation and reduces the power - off time LOW DOWNTIMES & MAINTENANCE No critical mechanical parts such as fingers, conveyors and water cooled parts which may cause unforeseen stoppages and maintenance. HIGHER PRODUCTIVITY Due to shorter power-on and power-off times the productivity of the furnace can be increased by 20 % compared to the conventional EAF. P a g e 4
HIGHER RETURN ON INVESTMENT The EPC System features lower conversion cost due to the preheating effect. Furthermore higher productivity because of less power-on and power-off are assured. LESS FLICKER Related to the flat bath operation, preheated scrap and the constant energy input, a reduced flicker and less noise generation will be achieved. PROCESS DESCRIPTION OF EPC SYSTEM Considering these limitations the EPC System for continuous scrap flow and heat recovery from the off-gas counter flow in the preheating chamber was developed. The main advantage is to take advantage of charging the scrap independently from any melt down situation and from the energy content of the hot waste gases always into a hopper which is separated from the off - gas stream. The scrap is charged through the preheating chamber and flows down into the vessel counter wise to the off - gas flow. In addition to the energetic advantages the scrap column serves as a coarse filter for the off - gas loaded with dust. A major part of the dust is already captured in the scrap inside the preheating chamber. The aims for the process are: reduced dust emission of the process increased productivity of furnace reduced electrical energy input reduced energy losses through the off-gas reduced energy losses during charging reduced dust load for the filter system reduced electrode consumption CHARGING PROCEDURE FOR EPC SYSTEM USING 100% SCRAP The charging procedure is based on a "HOT HEEL OPERATION" to ensure a high preheating effect of the scrap. Charging will be effected by 3 baskets independently of the melting process (Figure 4). The complete scrap volume will be charged through the charging chamber linked behind the preheating chamber, using only one type of scrap basket (rectangular shaft basket). While the previous heat starts the refining phase, approx. 70% of the total scrap weight of the next heat will be charged through the charging hopper into the preheating chamber. P a g e 5
The hot waste gases in combination with post-combustion ensure a maximum preheating effect of the scrap. After the heat has been finished and tapped, the telescopic feeder system starts its operation and pushes continuously the well preheated scrap through the opening in the upper shell into the furnace vessel. As the furnace vessel is not filled completely, a larger amount of hot heel is necessary to start melting in order to protect the refractory lining in the bottom and slag line. At the same time the charging crane is already waiting with the third basket above the charging chamber. Immediately after the charging hopper can move backward into charging position, the next basket will be charged. By this method 100 % of the scrap will be preheated in the EPC System. Figure 4: Charging procedure of EPC System Melt down phase: The melt down time can be reduced to a minimum by offering in parallel primary energy (carbon, oxygen) and electrical energy. In order to melt scrap and heat up the liquid phase to 1630 C, an enthalpy of about 380 kwh/t is required. 2 However, owing to losses through slag, off-gas, cooling water and radiation, an additional energy of over 200-250 kwh/t is required, depending on furnace design and productivity. Taking into consideration the energy balance (Figure 5), an essential part of the energy input can be achieved by the exothermic oxidation of Si, C, Mn, Al and Fe. By addition of carbon this energy portion can be increased. Energy is not only to be transported to the scrap but also has to be P a g e 6
transferred into the scrap. The governing factor is the thermal conductivity of scrap and not the amount of energy. The benefits of the EPC System operation are the intensive and intelligent use of the exothermic reactions. The oxidation of C contained or charged with the scrap, pig iron or HBI gives C + O = CO - 2,85 kwh/kg C and takes place in any furnace. The key point of the EPC System process is the use of the post-combustion reaction 2 CO + 1/2O2 = 2 CO2-6,55 kwh/kg C for heating the scrap column inside the preheating chamber. 3 The secondary oxidation of the CO gases (post-combustion), which is the more energy containing one, is subject to certain restrictions. - Temperature: The temperature must be below approx. 800 C. In presence of Fe the CO2-molecule is not stable above this temperature and mainly Fe would be oxidized. 4 - Time: High off gas speed at conventional furnaces blasts the reactive gas out of the furnace, so that the reaction energy is lost. In the EPC System the reaction takes place at low gas speed inside the scrap column of the preheating chamber and the energy is immediately transferred to the scrap. The EPC System delivers both, low gas speed and high degree of post combustion. Also a high productivity is achieved by the EPC System, compared to any conventional furnace. Figure 5: Energy balance EAF P a g e 7
Melting and Refining phase: During the whole melting and refining phase the steel bath is covered with foaming slag. Best transport of energy is assured by submerging the electrodes into the slag. The melting time will be reduced. Since time is also needed for metallurgical operation refining time cannot be reduced significantly. The modern EPC System allows using this energy for preheating of 100% of the scrap. For this a charging hopper installed beside the upper part of the preheating chamber, which can be moved inside so that the hot off-gas from the melt has to pass through the scrap column and the CO-content is post - combusted inside the scrap. Advantages of the process: - Each melt is preheated at maximum, according to off-gas heat. - During the phase where the off-gas is still cold, oxygen and carbon injection will bring additional energy into the furnace. The off-gas heat quantity can be controlled by the temperature at the preheating chamber exit. - The preheating chamber serves as a coarse filter and holds a major part of the dust back (approx. 30%). THE EPC SYSTEM AS A POST COMBUSTION CHAMBER In all ultra high power furnaces there is a danger of uneven energy distribution. The combined use of electrical energy and chemical energy in form of fossil combustibles has been proven as to be the best solution in order to reach an equal melting by creating a large number of heat sources and in order to avoid an energy consuming local overheating. Chemical energy is introduced into the EPC System by the use of carbon and oxygen injected through water cooled lances installed in the upper shell of the EAF. The use of carbon carriers (e.g. as hot metal, pig iron, HBI or coal) can only be feasible if the carbon is completely burnt to carbon dioxide (CO2), and if the formed heat is transferred to the scrap. The post combustion of the carbon monoxide (CO) that is normally created during refining of steel to form carbon dioxide (CO2) cannot be complete inside the hot furnace vessel. Furthermore the heat transfer to the scrap by the hot gases is extremely low in conventional furnaces. In the relatively cold preheating chamber, however, the post combustion can be completed. These correlations make clear why the EPC System must be in direct structural connection with the furnace vessel. P a g e 8
THE EPC SYSTEM AS A DUST FILTER AND GAS COOLER Besides the indirect environmental benefits of the EPC System due to its low energy requirement, the preheating and charging chamber design has also important direct positive effects on the environment. During charging of the scrap basket into the charging hopper of the EPC System, the system is always closed / airtight and only a small amount of cold dust, comparable to the charging procedure at the scrap yard can escape. The scrap column inside the preheating chamber acts as a pre - filter for the coarse dust of the off gases. Since in conventional high power furnaces 18-20 kg/t of dust is generated, this amount drops to 12-14 kg/t in the EPC System. Furthermore the amount of zinc in the dust and therefore its recycling possibility is increased. The dust sediment on the scrap is directly brought back to the melt which explains partially the higher yield of approx. 1,0 %. EPC OFF GAS DUCT The off-gases leave the EPC System through two openings arranged on top of the preheating chamber connected to the off gas bypass in front of the system. In case the preheating effect is too high a water cooled flap in the bypass system will be opened and the off gas can be guided directly to the drop out box in order to interrupt the preheating of the scrap. The gases are guided to a vertical drop-out box installed beside the EPC System. The rear end of the drop-out box is designed as dust collector. DESIGN ASPECTS OF THE EPC SYSTEM The EPC System is divided into 2 parts, i.e.: - Independent and airtight charging chamber - Preheating and airtight scrap feeding chamber beside the electric arc furnace (Figure 6). P a g e 9
Figure 6: EPC LAYOUT The system consists of a charging chamber in the upper part where the charging hopper is actuated by two hydraulic cylinders. The telescopic scrap feeding system is installed in the lower part and actuated by three hydraulic cylinders. All hydraulic cylinders are installed laterally in a protected manner and are not exposed to any major thermal load. The charging hopper and the telescopic feeder system are installed so that charging impacts of the scrap can be absorbed without any problem. It is proposed to design the system so that also special conditions can be fulfilled: Charging of special scrap: In case special scrap is to be charged which cannot be charged through the preheating chamber, for example very big pieces, sculls, etc., the EAF roof can also be opened like in a conventional furnace and such special scrap might be charged directly into the vessel. Changing of vessel: For changing the vessel for bottom repair or other reasons, the EPC System can be moved away to give room for such a change. EPC System travelling system The system is designed to offer sufficient charging volume according to scrap density of 0.5 t/m³. The system is incorporated in a travelling frame, which allows movement for charging and maintenance. The travelling frame is made of heavy duty steel structure and supported by 4 wheels on rails. The movement is actuated by a hydraulic cylinder. Special interlocks and a movable stopper are provided to position the system in the requested positions. The rails are resting on a heavy duty support structure/foundation arranged beside the furnace. For media and P a g e 10
power supply to the system a cable track or festoon system will be provided. A weighing system is installed between the travelling frame and the wheels for a precise scrap charging control. At the top of the EPC System above the charging chamber, a transferable water cooled hood is installed which serves as a closing device as well as a charging funnel for scrap into the charging hopper. The closure gate is driven by two hydraulic cylinders and guided by wheels which are moving in a frame. The structure has its own water distribution system which is part of the system structure. ENVIRONMENTAL ASPECTS As already mentioned above, the EPC System has compared with a conventional furnace and other preheating systems many advantages in view of environmental aspects: - No charging into the open vessel, thus no major dust formation and no pollution through organic substances which do not burn completely. - Minimum dust content inside the meltshop, which means reduced capacity at the secondary line of the filter plant - The dust contains rather a lot of zinc and can normally be recycled in an economic manner. - The overall dust quantity is considerably decreased (approx. 30%). - Less energy consumption means less CO 2 content at the stack and a further environmental protection (approx. 25%). P a g e 11
A FOCUS ON THE PROTOTYPE OF THE 1 st EPC SYSTEM APPLICATION The furnace at Toscelik presents all the aspects illustrated in the EPC System concept. The existing 155 t EAF was upgraded by KR Tec GmbH in March 2012 with a 170 t EPC System (Figure 7). The furnace was converted within four weeks shut down and started up on 5 th of April 2012. Already during the first heats, very satisfying results were obtained. The existing 155 MVA Transformer was reused with an average power input of approx. 102 MW at the previous conv. 155 t EAF before the revamp and the upgraded 170 t EPC EAF. Figure 7: 170 t EAF with EPC System Preheating operations in Toscelik has shown that this installation is capable of safely preheating scrap with up to 25% HBI in the charge, thereby reducing the charge cost related to preheating operation. This is due to a highly flexible design, that allows splitting of the off gas flow between preheating chamber and EPC bypass, wherever this is required. Analysing the average data, the below table shows the performance figures after 4 month of operation using 90% scrap and 10% HBI (Figure 8). P a g e 12
Figure 8: EPC Performance Regarding performances, the best heats with preheating, corresponding to a conventional operation has shown a high energy recovery up to 100 kwh/t liquid using 100% scrap. Analyzing the average data of one month (Figure 9, 27 days in August 2012) shows the excellent production figures reached so far. On commenting these figures, comparing to the previous conventional EAF (Figure 10) and the upgraded EPC EAF (Figure 11), it was a higher productivity of approx. 25% with the EPC System achieved, by a reduction of the electrical energy consumption of < 75 kwh/t liquid using 90% scrap/10% HBI. Figure 9: 170 t EPC EAF Productivity August 2012 P a g e 13
Figure 10: Productivity 15.02.2012 with conventional 155t EAF Figure 11: Productivity 13.08 2012 with EPC System CONCLUSION The mechanical EAF development regarding the melting time was more or less finished at the beginning of the 90ies. From that time the main steps were mainly the optimization of the electrical systems and the chemical processes. In almost all cases the further goal is to minimize the electrical energy input and to maximize the energy efficiency in the process. Thus, several technologies have attempted to maximize the use of chemical energy into the process. These P a g e 14
processes are highly dependent on achieving pseudo equilibrium where oxygen has completely reacted with fuel components (carbon, CO, natural gas, etc.) to give the maximum achievable energy input to the process. Other processes have attempted to maximize the use of the energy that is put into the furnace by recovering energy in the off - gases (Shaft furnace, Consteel, EOF). These processes are highly dependent on good heat transfer from the off - gas to the scrap. This requires that the scrap and the off - gas contact each other in an optimal way. All of these processes have been able to demonstrate some benefits. The key is to develop a process that will show process and environmental benefits without having a high degree of complexity and without affecting productivity. There is no perfect solution that will meet the needs of all steelmaking operations. Rather, steelmakers must prioritize their objectives and then match these to the attributes of various furnace designs. The authors are convinced that the proven results of the EPC System technology will be recognized by the market as a new generation of scrap preheating technology for electric steelmaking. REFERENCES 1. Dr. Geoffrey Brooks, Developments in Electric Arc Furnace Steelmaking, Department of Materials Science Engineering, McMaster University; Hamilton, Canada, 2001, page 81 2. Andrea Fontana, EAF Scrap Preheating Technologies, High Temperature Processing Symposium 2012, Swinburne University of Technology, Australia. 3. Dr. Geoffrey Brooks, Developments in Electric Arc Furnace Steelmaking, Department of Materials Science Engineering, McMaster University; Hamilton, Canada, 2001, page 88 4. Hiroaki Nakano, New Scrap Preheating System for Electric Arc Furnace Nippon Steel Technical Report No. 79, 1999, page72. P a g e 15