New generation in pre heating technology
- 1. NEW GENERATION IN PRE‐HEATING TECHNOLOGY FOR ELECTRIC STEEL MAKING HIGHER PRODUCTIVITY WITH REDUCED POWER Knut RUMMLER ‐ KR Tec GmbH – Managing Director Akif TUNABOYLU ‐ CVS TECHNOLOGY ‐ Technical Coordinator Dogan ERTAS ‐ CVS TECHNOLOGY –Chief, Project and Contracting Department In the steelmaking process, by Early scrap preheaters used scrap melting through Electric Arc 1. INTRODUCTION independent heat sources. The Furnace route , substantial Today's steelmakers are seeking for scrap was usually heated in the reduction in electric power clever solutions to achive more scrap bucket. Energy savings consumption and associated economical, ecological and flexible reported from this type of increase in furnace productivity operations. These should be realized preheating were, as high as, 30 can be realized with Scrap Pre‐ with less maintenance intensive kWh/ton with associated heating Technique which pre‐ equipments. These are the main reductions in electrode and heats the scrap to about 700 °C objectives for maintaining commercial refractory consumption due to by making use of the sensible success and competitiveness within this reduced tap-to-tap times. heat carried in the furnace off sector. Continuing increase in electric gas. In this respect, CVS energy costs, strict rules impossed on As EAF fourth hole offgas systems Technology has a co operation atmospheric CO2 emissions and ever were developed, attempts were with KR Tec GmbH, which tighter environmental regulations for made to use the EAF offgas for developed an ‘’environmentaly land and waters lead the steelmakers to scrap preheating. A side benefit friendly’’ and ‘’high efficiency’’ decrease their energy consumptions and reported was that the amount of scrap preheating system to be to recycle the waste materials and baghouse dust decreased because ‘’superior’’ over the existed medias. Scrap preheating has been used the dust was sticking to the scrap systems developed so far. This for over 30 years to offset electrical steel during preheating. Scrap challenge led to the raise of a melting requirements. It normaly preheating with furnace offgas is new and superior ‘’ Environmetal involves the use of EAF hot off gas to difficult to control due to the Pre‐heating and Continuous heat scrap in the bucket, prior to its variation in offgas temperature Charging (EPC) System’’. The EPC charging into furnace. The source of the throughout the heat cycle. In System combines the advantages hot gas can be either solely the off gas addition, a temperature gradient of 100% scrap preheating and from EAF and/or gas from suplementary forms within the scrap being continuous scrap feeding through burner(s). The primary energy preheated. Temperatures must be its chambers, without the need of requirement for the EAF is for heating controlled to prevent damage to EAF roof opening. EPC prevents the charged scrap to its melting point. the scrap bucket and in order to totaly, any dust emission and Thus, energy can be saved, if scrap is prevent burning or sticking of fine heat loss during furnace charging charged to the furnace hot. Preheating scrap within the bucket. stage, as it is the case normally of scrap also eliminates the possibility of Scrap temperatures can reach for other operations. charging wet scrap into EAF and this 315–450°C, (600–850°F), though; eliminates the possibility of an explosion this will only occur at the hot end in the furnace, in case wet scrap deeps where the offgas first enters the in liquid steel . Hence, preheating scrap preheater. Savings are typically also improves plant safety and accidental only in the neighborhood of 18– equipment damage. Scrap preheating 23 kWh/ton. In addition, as reduces EAF electrical energy operations become more efficient consumption and increases melt shop and tap-to-tap times are productivity. decreased, scrap preheating
- 2. operations become more and more excessive water cooling heat difficult to maintain. Eventualy, scrap losses from the furnace. handling operations actually started - HIGHER PRODUCTIVITY: Due producing reduced productivity and to shorter power-on and increased maintenance costs. power-off times. The Some of the benefits attributed to scrap productivity of the furnace preheating are increased productivity by can be increased by 20 % 10–20%, reduced electrical compared to the conventional consumption, removal of moisture from EAF. the scrap, and reduced electrode and - LONGER EAF ROOF & ROOF refractory consumption per unit DELTA LIVES: Due to, there is production. Some drawbacks to scrap no need for opening/closing preheating are that hazardous volatiles the furnace roof for charging are evolved from the scrap, creating and electric arc is always odors and necessitating a post- away from the roof, less arc combustion chamber downstream. damage results at roof WCP and the minimized thermal In addition spray quenching following shock additionally helps in post-combustion is required to prevent extending roof delta life. recombination of dioxins and furans. - HIGHER RETURN ON Depending on the preheat temperature, INVESTMENT: The EPC buckets may have to be refractory lined. System features lower conversion cost due to the 2. DESCRIPTION OF EPC SYSTEM preheating effect. Environmental Pre-heating and Furthermore higher Continuous Charging has many productivity because of less adventageous : power-on and power-off times are assured. Depending on - MINIMUM DUST EMMISION: During the scrap quality, some yield charging procedure the system is gain can also be expected. always in a closed and airtight - LESS FLICKER: Related to situation which results in minimum the flat bath operation, pollution level in the meltshop preheated scrap and the - ENERGY SAVING: The EPC reduces constant energy input, a the electric energy consumption by reduced flicker and harmonics approx. 100 kWh/t compared to the level is reached. This also conventional EAF. leads to less arc noise - INDEPENDENT SCRAP CHARGING: generation. Charging of the scrap basket is done - SAVINGS FOR SCRAP with power-on and independently TREATMENT: The above from the furnace operation. This saving does not take into improves the operation and reduces consideration the additional the power off time. Eliminating the saving of approx. 10.00 EUR / need for EAF roof opening t liquid, due to the fact the substantially reduces the heat loss EPC - System does not from furnace. require any special scrap ‐ LOW DOWNTIMES / MAINTENANCE treatment. & LESS HEAT LOSS FROM WCC: No critical water cooled mechanical parts such as fingers, no need for conveyors and no extra water cooled parts requirements which may cause unforeseen stoppages, need intensive maintenance, and lead to
- 3. The new and superior EPC design The Charging with scrap bucket is only The scrap basket will be considers the most flexible done, for one time, and at the beginning charged into the drawer of operational activities. the first heat of an operations cycle or in EPC while it is positioned case of an emergency situation. inside waiting deck. The main features are: During charging, and when the Emission from steel making process is one of the biggest problems. Emission charging drawer is positioned - Flat bath operation. inside waiting deck, front wall of - Controlled scrap charging control regulations are worldwide getting the drawer closes and isolates the rate through telescopic feeder tighter. In the EAF field, in a sense, preheating chamber and hence system. power saving and scrap preheating are synonym. Various technologies have the melting process in EAF and - Continuous charging during the preheating don’t have to be been developed to effectively preheat power on. interrupted. the scrap by the furnace exhaust gas. - Scrap charging rate is tuned One of the issues of the EPC System is After filling the drawer by the according to melting to charge the scrap independent of the raised scrap basket, the slide power/preheating electric arc furnace melting by taking gate on top of the EPC system is temperature. into consideration the environmental closed. - Uniform and well controlled aspects. The preheating chamber of EPC bath temperature. EPC, currently developed, is a is installed beside the EAF upper shell shaft type- preheat furnace, - Well controlled preheating and the preheated scrap resides in this temperature. based on AC technology. The charged continuously, by the telescopic furnace will maintain a large hot - Minimized off gas volume feeder system, into EAF for melting. This heel ,(nearly 40 %), so that related to airtight system. is while the furnace power is on. Even uniform operating conditions can - Environmental benefits : during charging of the scrap basket into be maintained. Steel is tapped out - Charging with closed system, the drawer positioned in the waiting periodically via a bottom taphole into a separate compartment deck , the preheating chamber is closed in the furnace. (EAF roof is closed, dedusting with the drawer’s front wall and hence The scrap charging system primary line is on) the furnace and preheating chambers - Minimum fume emission are totaly isolated. This ensures little or consists of two main components, the preheat chamber and the during scrap charging no dust escape during furnace charging. charging deck inside which a - Cleaner and safer working The scrap basket will be charged into the drawer operates. area drawer of EPC by opening top slide gate - 30% less off gas and while the charging drawer is The scrap is fed into the upper - 30 % less dust at the filter positioned in the waiting deck. After part of the chamber from a - Reduced arc noise level, charging, top horizontal slide gate is receiving hopper. The exhaust (melting preheated scrap, closed and the charged scrap inside gas from the furnace flows up melting flat bath conditions drawer is in waiting position. Due to through the chamber, preheating under foamy slag) melting and preheating chambers are the scrap. Scrap preheat - Direct preheating, charged isolated during charging EPC, melting temperatures as high as 800°C can be achieved. Gas exit scrap is exposed to very high and preheating don’t have to be interrupted. Then, the drawer is temperatures from the chamber is temperatures around 200°C. At the base of the - EPC respects to most forwarded by two hydraulic cylinders towards over the preheating chamber, preheat chamber, there are two environmental standards. horizontally, and the scrap falls screw type pushers. These smoothly into the preheating chamber operate in two stages, allowing where it gets preheated. When the scrap feed into the furnace at a drawer is over preheating zone, its rear constant rate. Offgas leaves the wall is closing the preheating chamber. A top of the preheat chamber and special design of the off gas duct flows to a bag filter. Some gas together with a water cooled regulation can be recycled to the furnace to flap allows to control the preheating regulate the inlet gas temperature effect in the preheating chamber. to the preheater.
- 4. OPERATIONAL MODES OF EPC Scrap is fed continuously to the furnace All of these processes have been 1. Refining Phase of EAF until the desired bath weight is achieved. able to demonstrate some This is followed by a short refining and benefits. The key is to develop a Preheating of 1st bucket of next super heating period followed by tapping process that will show process heat in pre‐heating chamber of the heat. Power input is expected to and environmental benefits Charging of next bucket into be almost uniform throughout the heat. without having a high degree of charging drawer Most furnace operations are fully complexity and without affecting automated. Charging rate of scrap into productivity. There is no perfect the preheater chamber is fully solution that will meet the needs automated based on the scrap height in of all steelmaking operations. the chamber as well as temperature of Rather, steelmakers must the gas. Furnace feeding rate is prioritize their objectives and then interrelated to this, and to the actual match these to the attributes of power input. Carbon and oxygen various furnace designs. It is injections are controlled based on the important to maintain focus on 2. Preheating inside EPC depth of foamy slag. the following criteria: Start feeding of preheated scrap 1. CONCLUSION 1. To provide process flexibility. after tapping Without doubt, current trends in EAF 2. To increase productivity while Pre‐heating chamber half empty design indicate that high levels of both improving energy efficiency. drawer can move to electrical and chemical energy are likely 3. To improve the quality of the waiting/charging position to be employed in future furnace finished product. 3. Charging Preheating Chamber designs. The degree to which one form 4. To meet environmental Moving of charging drawer into of energy is used over another will be dependent on the cost and availability of requirements at a minimum cost. pre‐heating chamber the various energy forms in a particular With these factors in mind, the Preheating of 2nd bucket of next location. There are many new processes following conclusions are drawn: heat in pre‐heating chamber for steelmaking which are now being 1. The correct furnace selection Off gas flap closed commercialized. In almost all cases the will be one that meets the specific goal is to minimize the electrical energy requirements of the individual input and to maximize the energy facility. Factors entering into the efficiency in the process. Thus, several decision will likely include technologies have attempted to availability of raw materials, maximize the use of chemical energy availability and cost of energy into the process. These processes are sources, desired product mix, highly dependent on achieving level of post furnace 4. Preheating inside EPC pseudoequilibrium where oxygen has treatment/refining available, completely reacted with fuel components capital cost and availability of a Scrap bucket in waiting position (carbon, CO, natural gas, etc.) to give trained workforce. Continuous feeding of scrap the maximum achievable energy input to 2. Various forms of energy input during power on the process. Other processes have should be balanced in order to attempted to maximize the use of the give the operation the maximum energy that is input to the furnace by amount of flexibility. This will help recovering energy in the offgases (Fuchs to minimize energy costs in the shaft furnace, Consteel, EOF, IHI Shaft). long run, i.e. the capability of These processes are highly dependent running with high electrical input on good heat transfer from the offgas to and low oxygen or the converse. the scrap. This requires that the scrap and the offgas contact each other in an 3. Energy input into the furnace 5. Charging to Upper Hopper EPC optimal way. needs to be well distributed in Start feeding of preheated scrap order to minimize total energy requirements. Good mixing of the Opening of sliding gate on top bath will help to achieve this goal. Charging of next bucket into 4. Oxygen injection should be charging hopper distributed evenly throughout the tap-to-tap cycle in order to
- 5. minimize fluctuations in offgas 9. Operating practices will temperature and composition. Thus, continue to evolve and will not postcombustion operations can be only seek to optimize energy optimized and the size of the offgas efficiency in the EAF but will seek system can be minimized. In addition, to discover the overall optimum fume generation will be minimized and for the whole steelmaking facility. slag/bath approach to equilibrium will be Universally, the most important greater. factor is to optimize operating 5. Injection of solids into the bath and costs for the entire facility and into the slag layer should be distributed not necessarily one operation in across the bath surface in order to the overall process chain. Along maximize the efficiency of slag foaming with added process flexibility operations. This will also enable the slag comes greater process and bath to move closer to equilibrium. complexity. This in turn will This in turn will help to minimize flux require greater process requirements and will improve the understanding so that the process quality of the steel. may be better controlled. Much more thought consequently must 6. The melting vessel should be closed enter into the selection of electric up as much as possible in order to furnace designs and it can be minimize the amount of air infiltration. expected that many new designs This will minimize the volume of offgas will result in the years ahead. As exiting the furnace leading to smaller long as there is electric furnace fume system requirements. steelmaking, the optimal design 7. Scrap preheating provides the most will always be strived for. likely option for heat recovery from the By the consideration of above offgas. For processes using a high items, CVS Technology and KR degree of chemical energy in the Tec companies has developed furnace, this becomes even more new patented solution in Pre- important, as more energy is contained heating technology. in the offgas for these operations. In order to maximize recovery of chemical energy contained in the offgas, it will be necessary to perform post-combustion. Achieving high post-combustion efficiencies throughout the heat will be difficult. Staged post-combustion in scrap preheat operations could optimize heat recovery further. 8. Operations which desire maximum flexibility at minimum cost will result in more hybrid furnace designs. These designs will take into account flexibility in feed materials and will continue to aim for high energy efficiency coupled with high productivity. For example operations with high solids injection, iron carbide or DRI fines, may choose designs which would increase the flat bath period in order to spread out the solids injection cycle. Alternatively, a deeper bath may be used so that higher injection rates can be used without risk of blow through.