Consteel Electric Arc Furnace Process
Consteel Electric Arc Furnace Process
Consteel process is a patented process. It is an innovative energy conservation technology for scrap pre-heating in an electric arc furnace (EAF) developed by ‘Intersteel Technology Inc., Charlotte, North Carolina (since 1994 a part of Techint, Tenova S.p.A.). The first industrial application of the Consteel technology took place in 1989, in Gerdau-Ameristeel Charlotte (USA). Since then there has been a steady industrial acceptance of this technology. Today the Consteel EAF process is a proven and reliable steelmaking technology. It ensures efficient use of energy and raw materials, easy operation and maintenance, as well as environmental friendliness.
In the EAF with the Consteel process, the adopted plant solution is the continuous feeding of EAF by the scrap, pre-heated and conveyed through a conveyor moving in a pre-heating tunnel, in which the exhaust gas from EAF, flowing counter current of metallic charge is combusted by injected air. The continuous feeding and preheating of scrap offer some potential advantages compared to the conventional EAF batch feeding.
The Consteel process boosts productivity and improves energy efficiency with minimum environmental impact. The main highlights in implementing this technology are the conservation measures such as (i) recovery of heat from furnace exhaust gas to pre-heat scrap prior to charging in the furnace, (ii) reduction in specific electrical energy, (iii) reductions in O2 (oxygen), and C (carbon) usage with no burner fuel consumption , (iv) increased production rate due to decreased tap-to-tap time, (v) significant decrease in electrical disturbances on the network, (vi) lower electrode consumption and electrode breakage, (vii) increase in scrap yield, (viii) less dust is evacuated to the bag house, and (ix) cost reductions for logistics, manpower, maintenance and waste product management.
The low power requirement and steady flat-bath operation, under foaming slag in case of the Consteel process, make the Consteel process the ideal EAF technology when the electric network is relatively weak and able to tolerate only very low disturbances. The Consteel process also minimizes noise, dust and polluting emissions, both inside the melt shop (no doghouse is needed) and to the exhaust gas system, which, unlike a conventional EAF, does not need to be oversized to handle burst emissions.
Conventional scrap preheating has been used since a long time primarily in countries with high electricity costs. Conventional scrap preheating involves the use of hot gases to heat scrap in the bucket prior to charging the scrap into the EAF. The source of the hot gases can be either exhaust gases from the EAF or gases produced by burning a fuel gas. Conventional scrap preheating can be accomplished by delivering the hot furnace gases to the scrap charging bucket by piping the exhaust gases from the fourth hole in the EAF to a special hood over the charging bucket. Typically the gases leave the EAF at around 1200 deg C, enter the bucket at around 815 deg C, and leave at around 200 deg C. The amount of preheating depends on the heat transfer to the scrap which is a function of scrap size and time at temperature. Typically the scrap is preheated to a range of 350 deg C to 450 deg C. This amount of preheating typically reduces energy consumption by 40 kWh/ton to 60 kWh/ton, electrode consumption by around 0.35 kg/ton, refractory consumption by around 1.2 kg/ton and tap-to-tap time by around 5 minutes to 6 minutes. Some of the disadvantages with conventional scrap preheating include (i) inconvenient to operate such as scrap sticking to bucket and short bucket life, (ii) poor controllability of preheating due to variation in the exhaust gas temperature and flow rate through various phases in the EAF operation, and (iii) for tap-to-tap times less than 70 minutes the logistics of conventional scrap preheating lead to minimal energy savings which cannot justify the capital expense of a preheating system.
The Consteel process continuously pre-heats and feeds metallic charge (scrap, pig iron etc.) to the EAF while simultaneously controlling the gaseous emissions. The Consteel process achieves the continuous feeding of scrap by means of an inertial conveyor, which moves the scrap from an open section, used for charging (normally by crane), to the furnace, passing through a closed section (tunnel) in which scrap is being heated by process exhaust gas travelling in the opposite direction, towards the exhaust gas extraction system. The energy for pre-heating is provided by hot gas entering the tunnel from the EAF, post combusted by air added in the tunnel. Preheated scrap is continuously fed into the EAF, where it is melted by the immersion in liquid steel. The EAF operates in constant flat bath conditions, a key advantage over conventional batch processes where scrap is melted by the direct action of the electric arc. The EAF gases are sent to the exhaust gas cleaning plant in conditions suitable for the complete combustion of carbon monoxide (CO) and other pollutants without any fuel consumption. The concept of the Consteel process is shown in Fig 1.
Fig 1 Concept of the Consteel process
Process description and plant characteristics
The Consteel process is the process which allows a continuous feeding and preheating of the metallic charge to the EAF. In this process, scrap is charged directly from the scrap yard to the charge conveyor. The scrap is then automatically and continuously conveyed to the EAF while being preheated in the preheater conveyor which is called the tunnel.
The energy for pre-heating is provided by hot gas coming from the EAF, post-combusted by air injected in the tunnel through injectors purposely located on the roof of the tunnel. Together with injected air, uncontrolled air enters EAF and tunnel. In the tunnel, uncontrolled air mainly enters in the zone where EAF and tunnel are connected which is the so called connecting car area. The uncontrolled air contributes to the post-combustion of the exhaust gases from the EAF.
The outlet tunnel gas is then conveyed to an exhaust gas cleaning system. A system of fan and mechanical curtains is placed at the end of the tunnel to avoid the entering of air which is called the dynamic seal.
Being the Consteel process based on the post-combustion of gas coming from the EAF it is strictly connected to the steelmaking process carried out in the EAF, hence to optimize the process the whole system consisting of EAF and tunnel is to be considered.
Management of flow rate in each of the injector is allowed. Air injection can be automatically controlled or manually set. The automatic control is based on the measurement of O2 concentration in the gas at the tunnel outlet. The target value of this parameter is reached and maintained through the plant control system, by proper management of the air flow rates in the six injectors. Fig 2 shows the schematic diagram of the Consteel process.
Fig 2 Schematic diagram of the Consteel process
Special features of the Consteel process
The Consteel process incorporates continuous charging of scrap into the EAF by means of a conveying system which connects the scrap yard with the EAF. No conventional bucket charging takes place. Scrap is loaded onto conveyors by the scrap yard cranes and these conveyors move the scrap in an oscillating motion comprising a slow forward movement and rapid reverse motion which causes the scrap to move together with the conveyor during the forward stroke but to slide on the conveyor surface during the more rapid reverse stroke so producing a net forward travel of the scrap towards the furnace.
Some distance before reaching the furnace the scrap enters a preheating section consisting of a tunnel through which the hot gases exiting the EAF flow in a counter direction to the motion of the scrap. In the preheating section CO in the exhaust gas is burnt by an automatically controlled injection of air, allowing more energy to be recovered to the scrap. During the continuous feeding operations, the steel bath in the EAF is kept liquid and the scrap entering the furnace is melted by immersion in the bath. The electric arc is thus always working on a liquid bath (flat bath conditions), not on solid scrap. In this situation the arc is stable and it is unaffected by the presence of solid scrap as is the case with batch charging.
The EAF with Consteel process can use any type of steel scrap and all the metallic raw material which can be charged in a traditional EAF. In terms of maximum dimensions of scrap, the normal limits specified by the steel scrap specification (1.5 m x 0.5 m x 0.5 m) are compatible with continuous charging through the Consteel process.
The main rule to be followed is that the maximum scrap piece is not to exceed the distance between the tip of the Consteel process conveyor and the electrodes, in order to avoid the chances of an electrical bridge between these parts. This can limit the maximum acceptable piece length on small furnaces whilst large furnaces can be more tolerant. In practice, steel scrap normally available in the market is already compatible with the smallest Consteel EAFs.
In the Consteel process the charge of scrap is continuous, thus scrap density is not affecting the operations as much as it does in a conventional top-charge furnace, where a too light scrap can require more bucket charges, reducing efficiency and productivity, and where too much heavy scrap produces delays in the charge meltdown, if not frequent electrode breakages due to massive scrap cave-ins during the initial melting.
The Consteel process is carried out balancing the power input to the furnace with the mass flow rate of charge materials (similarly to the control logic applied in DRI fed furnaces) and the required mass flow rate is achieved automatically by the control logic of the process, adjusting the feeding rate of the conveyor which is faster for light scrap and slower for heavy scrap. The mass flow rate is controlled by a furnace weighing system and/or by a charge tracking system in the newer installations.
The scrap discharged by the conveyor melts by immersion in a large pool of liquid metal (the hot heel) providing the most favourable conditions for the melting of heavy scrap pieces, like bundles, which are quite troublesome for conventional EAFs. The only requirement is to distribute uniformly the heavy pieces along the charge.
In general, it is much easier to melt heavy scrap pieces in an EAF with the Consteel process than in a conventional EAF of the same size, and the furnace performance is benefitted if the hot heel is correctly sized and the process is combined with bottom stirring with N2 (nitrogen) or Ar (argon). Normally the hot heel is sized as 42 % to 50 % of the tap weight. This mass of liquid metal is maintained across the various heats and acts as a stabilizing ‘thermal flywheel’ for the process. On the very first heat of the furnace campaign, the hot heel is created with the melt down of a bucket charge. When the furnace needs to be drained, the charge is reduced and the hot heel used to tap a full heat. Fig 3 gives the schematic view of the Consteel process line.
Fig 3 Schematic view of the Consteel process line
There are two main characteristics which make the Consteel system different from most other EAF technologies. These characteristics are the combination of preheating and, even more important, continuous charging. Preheating is important to save energy, but the continuous charging has shown to have even greater benefits, namely (i) low production costs, (ii) high productivity, (iii) flexibility, (iv) reduced environmental impact, and (v) greater safety
Continuous charging of scrap distributes the charge throughout the whole power-on period. No bucket charges are used and the conveyor feeds the scrap from the yard directly into the EAF. The EAF roof is always closed and so gas suction constantly takes place through the primary circuit, not through canopies in the secondary circuit. In the furnace, the scrap melts by immersion and the electric arc is working on a flat bath covered by a foamy slag. The EAF control system automatically adjusts the conveying speed to maintain the steel bath at the target temperature and controls the O2 and C injection to maintain the proper foamy slag.
Preheating the charge is effective in reducing energy consumption. The energy savings which can be obtained is a function of the preheating temperature and the melting efficiency. Assuming an average preheating temperature in the range of 400 deg C to 600 deg C, energy savings ranging from 80 kWh/ton to 120 kWh/ton of liquid steel tapped are gained. These values are confirmed by the experience of existing Consteel installations.
The typical heat cycle for an EAF equipped with the Consteel system is shown in Fig 4. These characteristics give Consteel considerable advantages in terms of operational savings and reductions in environmental impact. The operational characteristics of Consteel bring to the working environment lower noise, less dust and the absence of bucket charging with its consequential noise, transport and loss of heat and flue gases when the furnace roof is opened for charging.
Fig 4 Typical heat cycle of the Consteel process
Metallurgically, the liquid steel in the furnace is in better equilibrium and less likely to generate violent reactions. Furnace water-cooled sidewalls, roof, and lances do not suffer leakage problems caused by arcing or scrap impacts, thus minimizing the risk of water in the furnace. All of this contributes to creating a safer and more comfortable working environment compared to the typical standards of the steel industry.
The key characteristic of the latest Consteel process is the introduction of new solutions to increase the amount and efficiency of the chemical energy used in the process. This has been developed through laboratory trials and extensive use of CFD analysis. The new solutions feature wider conveyors to increase the exchange surface, a different tunnel profile to improve the convective heat exchange, and a new tunnel section equipped with burners, to boost chemical energy input. Through the new solutions, the use of chemical energy is controlled, section by section, by continuous measurement of the exhaust gas flow, temperature and composition, with automatic optimization of the relevant operational parameters. The results are a more effective charge pre-heating and lower operating costs.
Comparison with conventional EAF steelmaking
The greatest difference with conventional EAF steelmaking is the better yield, though there are several aspects related with the logistics and maintenance.
The most important operation is the management of the scrap flow from the scrap yard to the furnace. This is by bucket preparation in the conventional EAF and by the continuous charging system in the Consteel process. Raw materials handling in the conventional EAF is normally performed by overhead travelling cranes in sufficient number to have an adequate margin of safety against breakdowns. The number of cranes depends on the number of buckets which are to be prepared in the given time, considering the heat size of the furnace, the scrap density and size of the buckets. The Consteel process adopts a different organization of the scrap yard, normally storing the raw materials at the side of the charging conveyor. The size and number of the charging cranes depend on the maximum scrap feeding rate required by the furnace (Fig 3). The Consteel process simplifies the logistics as it minimizes scrap movements.
The maintenance practice depends upon the scrap yard equipment, furnace bay equipment and EAF furnace, together with slag and dust disposal. The Consteel process has only the overhead travelling cranes for the conveyor charging, which can also perform the weighing operation for each lift. In case of the maintenance of the EAF, this is strongly influenced by the melting process in use. The thermal and chemical stresses which affect the consumable components of the furnace depend mainly on the parameters of the melting process (Fig 5).
Fig 5 Schematic views of a top charge EAF and a Consteel EAF
Electrode consumption has the highest maintenance cost, however, Consteel process has around 15 % lower consumption since it is governed by the lower oxidation rate due to the lower post-combustion ratio inside the furnace. Electrode erosion also depends on the productivity of the melt shop. At the same working condition, Consteel process has a higher productivity, thus the electrode consumption can be considered the same as a conventional EAF with lower productivity. Also, flat bath operation maintains a good stability of the electric arcs and practically eliminates the occurrence of electrode breakages.
The Consteel process also has lower refractory lining wear because operating conditions are smoother than the conventional EAF and it produces less iron oxide in the slag. With Consteel process, provided that slag is foaming correctly, the electric arcs can be completely covered and buried under a protective layer which reduces arc radiation to the furnace refractory for almost the entire power-on period. It also eliminates electric discharge on the furnace roof and shell because panel maintenance is drastically reduced.
With regard to the maintenance of the Consteel scrap conveyor, it is very simple and has been reduced to the level of periodic inspection of the mechanical structure, electric motors and hydraulic equipment, and the planned maintenance of the most critical parts. The refractory lining of the preheating section has no particular stresses and can normally be re-bricked annually.
The connecting-car tip is the most stressed component of the conveying system because it receives both the thermal stress of the melting bath and the mechanical load produced by the conveying of the scrap. It is to be part of a planned maintenance programme and, as per the experience of operating units, has an average lifetime of four months. The suspension rod is easily replaced, usually during the furnace turnaround. Failure analysis shows an average 100 suspension rod breakages a year for a well-charged conveyor.
In the case of dust and slag disposal, the Consteel process achieves lower slag and dust production than a conventional EAF, being strongly dependent on the main characteristics of the process which is the continuous charging and the preheating of the metallic charge. The elimination of the bucket charge reduces dust formation in the canopy hood, and the pre-heating section of the conveyor works like a settling chamber where the dust can deposit on the scrap, promoting dust recycling into the furnace. The overall dust emission of 5 kg/ton to 9 kg/ton of liquid steel is less than that of a conventional EAF.
The flexibility with the charge materials
For the present EAF steelmaking practice, the possibility to easily adapt the metallic charge of the furnace to follow the variations in raw material cost scenarios and market demands is, obviously, very important. The EAF is intrinsically flexible in terms of charge materials. The EAF operating with the Consteel process delivers maximum flexibility in the selection of metallic charge materials, which can be scrap, pig iron, DRI, and hot metal, in all possible combinations and ratios. Continuous charging means that buckets are not used, the conveyor continuously feeds the metal charge directly into the EAF.
The continuous charging process is normally used by EAF processing large amounts of DRI or HBI, since batch processing (top charging by buckets) of these material has proven to be unworkable. This concept has been extended to the processing of scrap with the introduction of the Consteel process. The scrap and power shortage in some countries has led to combine the scrap charge with hot metal. The experience has shown that Consteel process is also the best option for taking advantage of hot metal in EAF steelmaking. It is basically because the Consteel process allows spreading the decarburization of the melt across the entire power-on time, often without major changes in the primary exhaust gas systems and without incurring in decarburization delays which limit the productivity of a traditional top-charge EAF when the hot metal charge rises above around 30 % of the total.
The experiences with hot-metal in EAFs with Consteel process have spanned from 20 % to around 86 %, reaching a point in which the furnace can be operated like an basic oxygen converter, without electric power, with power-on/O2 blowing time around 32 minutes, yet still able to operate on a 100 % scrap charge when needed.
In case of the cost of steel scrap going down and the increased pressure to reduce the emissions of CO2 (carbon di-oxide), the hot metal can be reduced and the use of scrap can be increased due to the flexibility provided by the EAFs with Consteel process. An example of the Consteel flexibility is the 140 t Consteel EAF at Vallourec-Sumitomo Brazil (Brazil), commissioned in 2012, which has been designed to work with a charge mix made of scrap, upto 40 % pig iron and upto 40 % hot metal.
There is an EAF with Consteel process, installed in South Korea, in which the continuous charge of scrap has been combined with the continuous feeding of DRI and HBI. Hence, the Consteel process can seamlessly combine the continuous charging of scrap with any form of virgin iron charge namely hot metal, pig iron, DRI or HBI as shown in Fig 6.
Fig 6 Consteel process with alternative charge materials
As a general rule, DRI and HBI are to be fed to the furnace in the traditional way which is directly through the furnace roof. The hot metal is poured into the furnace through a door or sidewall runner, pig iron, instead, can be mixed with scrap into the charging conveyor. Like pig iron, also iron scrap, in pieces of a few tons each, can be charged through the Consteel process together with scrap.
Benefits of Consteel process
The proven benefits of the Consteel process Includes (i) fast payback, (ii) high flexibility, (iii) environmental friendliness, (iv) safety of the operational personnel, (vi) minimum disturbance to the electric network, (vii) reduction in logistics, maintenance and manpower costs, (viii) improved use of chemical energy, (ix) lower electrode consumption, (x) increase in scrap yield compared to batch-charged furnaces, (xi) continuous control and optimization of operational parameters, and (xii) improved charge control through automated scrap quantity and quality tracking.
In addition, there are environmental benefits. The production of dust in a traditional bucket-charged EAF produces is around 20 kg/ton of liquid steel. The dust production in the equivalent EAF with the Consteel process is around 30 % to 40 % lower. This is because of the natural recycle of the dust deposit on the scrap inside the heating tunnel and of the lower decarburization rate allowed by this process technology. Lower dust production means lower cost for its disposal, which is increasingly expensive due to the more stringent environmental regulations.
The process reduces the PCCD/F and NOx, emissions. Scrap can contain oil, paints, plastic and other substances which can turn into pollutants during the steelmaking process. Some of these substances can become precursors of polychlorinated di-benzo-p-dioxins (PCDD), polychlorinated di-benzo-p-furans (PCDF) and polychlorinated biphenyls (PCD), a class of substances whose emission has been severely restricted by new environmental regulations. The emission of these pollutants is a complex phenomenon which is influenced by several concurrent factors such as the amount of precursors in the scrap charge, the type of process carried out in the furnace and, ultimately, and on the exhaust gas extraction system, which plays a key role, since the emissions are measured at the stack.
It has been demonstrated that given a scrap charge with the same contents of PCDD/F precursors (e.g. polyvinyl chloride plastic), the Consteel process releases a significantly lower amount of PCDD/F in the exhaust gas in comparison with the equivalent discontinuous process, with batch charges.
There is a growing attention also to the emission of NOx and it has been found that the electric arc is the major responsible for these emissions in the EAF process. The amount of NOx emitted per ton of liquid steel produced via a conventional bucket-charged EAF is about 250 g/ton of liquid steel, whilst for a continuous charge this value is just 120 g/ ton of liquid steel. This is due to the operation of the electric arcs under forming slag, which reduces the interaction of theses electric arcs with the atmospheric O2 and N2.
Using the EAF to melt scrap charged by bucket is highly noisier than that melting a continuous charge of DRI. This is because the arcs are burning on flat bath and under a foaming slag. Consteel process extends this type of operation to the processing of scrap, significantly reducing the acoustic emissions of the plant. During the Consteel process operations, the sound intensity level in proximity of the furnace is reported to be around 85 dBA while with the bucket-charged EAF the sound intensity level is generally above 90 dBA.
Some of the operating parameters achieved in operating plants are (i) 30 % to 40 % reduction in dust emissions to be disposed, (ii) 98 % availability, (iii) high productivity at 2.7 ton of liquid steel /hour/MW, (iv) reduced power consumption at 300 kWh/ton of liquid steel, (v) 1 % to 2 % increase in scrap yield, (vi) productivity increase of 33 %, (vii) reduced electrode consumption of 40 %, (vii) reduced dust emissions by around 30 %, (viii) electricity savings estimated to be 60 kWh/t for retrofits, and (ix) annual operating cost savings of USD 1.90 / ton of crude steel (including productivity increase, reduced electrode consumption, and increased yield). Besides this also there are decrease in the electrical disturbance on the network, and lower use of scrap and no burner fuel consumption.