THE DANIELI DANARC PLUS M 2 FURNACE AT ABS MELTSHOP

Transcription

1 THE DANIELI DANARC PLUS M 2 FURNACE AT ABS MELTSHOP Aldo A. Fior φ DANIELI CENTROMET Process Engineer Buttrio, Italy SUMMARY The paper describes the main design features of the DANARC PLUS M 2 furnace in ABS meltshop, focusing on the innovative technologies applied to the project. The overall operating procedures are also illustrated, together with a brief description of the experience accumulated from first heat up to date. 1. INTRODUCTION The installation of the DANARC PLUS M 2 furnace in ABS meltshop is part of an overall remodernisation and expansion of the steelmaking facility to more effectively face the challenging requirements of the future. The ABS actual annual production figures are in the order of 535,tpy in high quality steel, comprising a variety of finished products, from continuously cast blooms and billets or lingots, down to rolled products. As a consequence of an impressive investment plan, ABS will reach an annual production of over 1,,tpy with a substantial decrease in operating costs, and with a broader range of available products. The DANARC PLUS M 2 furnace will face the greater part of future production requirements, while the present furnace will be dedicated to stainless steel production. 2. THE DANARC PLUS M 2 CONCEPT 2.1. Main design characteristics DC Arc Power Supply The electrical power supply for a DC furnace typically comprises two or four separate rectifiers (6-pulse rectifier bridges). The rectifier transformers and the rectifiers are normally arranged to obtain 12-pulse rectifying groups. Each rectifier unit is individually controlled and is also provided with a smoothing reactor in series in order to reduce the instantaneous current peaks at short circuit and to stabilise the arc. The bottom electrode system comprises four billet type electrodes embedded in the refractory lining. The upper part of the electrode is a steel billet in contact with the liquid steel and the lower copper part provides electrical 1 / 6 connections and cooling. The cooled region is located inside the bottom refractory thickness with the aim to raise the liquid-solid boundary, thus limiting the local heat load. The power supply control system regulates simultaneously the upper electrode position, arc voltage and the currents in the four bottom electrodes. Preheating Station The preheating station consists in a water-cooled retention bucket positioned on a car to enable a quick and fully automated batch-charging into the furnace centre of scrap heated to a mean temperature of 6 C. Hot off gases leaving the furnace, conveyed through a mobile water-cooled duct, invest the scrap column from above and transfer heat to the charge, cooling down during their transit and exiting the station at approximately 45 C, thereby avoiding excessive thermal stresses in the retention elements. Hot fumes can by-pass the preheating station according to process or maintenance requirements via a dedicated valve and water-cooled duct, without interrupting furnace operation. Preheating technology, while guaranteeing much lower consumptions in melting, does introduce negative factors related to pollution (higher dioxin emission) and safety (possible formtion of CO pockets, explosions); these aspects must be faced with dedicated treatment units, such as post-combustion chambers, and on-line emission control systems. Furnace design One of the main aspects that distinguish the DANARC PLUS design is its single bucket batch-charging configuration, translating in a high furnace geometry, with a second level of water-cooled panels rising above the traditional shell-single level design (Fig. 1). The consequences that arise from this configuration are multiple: reduced power off time for scrap charging; reduced furnace radiation losses due to scrap charging; higher energy recovery due to second-stage preheating (premelting) in the furnace atmosphere; higher arc efficiency in long arc operation due to extended scrap shielding towards the panels. The alternative energy input is managed by 3 DANARC modules, that guarantee high efficiency while eliminating mobility related problems (blockage or non-standard utilisation, typical of supersonic lances) and air infiltration. DANARC modules are sidewall mounted units comprising two injectors: an upper oxygen injector (Oxyjet) and a lower coal injector (Carbonjet). Both operate also as burners in the initial stages of the meltdown, after which they switch to lance mode, promoting the total combustion of the injected coal.

2 The injectors are inserted in water-cooled copper boxes, eliminating undesired air input. 2 / 6 preheating operations in the design, to face the dictated requirements, and are equipped with burners and even water-spray quenching units if necessary. The introduction of post-combustion chambers in the circuit enables the retention of the off-gases in a high temperature zone for a sufficient time to destroy the toxic organic compounds; high temperature quenching may be added to prevent extensive reformation of the dioxins. Fig. 1: Furnace layout - side view Optimisation of process control, obtainable by appropriate automation parameter identification, is of paramount importance to reduce costs related to off-gas treatment. To overcome the pressure drop across the preheating station, a booster fan is inserted in the fume extraction system integrating its suction power to the main stack fans capacity and varying its operating mode in relation to the process phases Main automation aspects The furnace design takes full advantage of pneumatic insufflation technologies, being equipped with lime injectors; the water-cooled copper tile box configuration was chosen also for these units. Special mention must be given to the innovative panel design, that consists in a double layer of water-cooled panels, inner and outer, where the layer directly exposed to the melting process has a large pitch between tubes. The area between inner and outer layers, as between the inner panel tubes, is initially filled with refractory ramming mix, which during operation is gradually replaced by slag, thereby drastically reducing the heat losses related to direct exposure of the water-cooled elements to furnace heat sources (recovery of up to 3% of the 5 8kWh/t in standard operation due to arc radiation, hot fumes, burner flame, etc.) as well as increasing the panel s operating life. Roof Design The so-called cyclone roof of the DANARC PLUS M 2 furnace consists in a consecutive disposition of spiral sectors of water-cooled tubes that form a hollow toroid at the furnace ceiling. Differential tube pitch in relation to the principal outlet duct s position induce a low-speed uniform vertical gas flow inside the shell, minimising particle entrainment in the fume extraction system and maximising the heat exchange between the hot off-gases and the unmelted scrap inside the furnace (premelting). As for the preheating unit, also the furnace roof is coupled to a car for opening and closing. Postcombustion Chambers and Emission Control Preheating technologies must face emission restrictions, of varying stringency according to different national regulations. Postcombustion chambers are therefore coupled to Great attention has been paid in the project to the implementation of an advanced control system, in order to obtain: better final product quality; higher productivity; lower costs; better working conditions. Apart from the capability of performing as a traditional control system, the challenge that the project automation is facing is to carry out an on-line process control supervised by an autoadaptive expert system that not only controls the process, but generates the process itself, seeking the optimisation of productivity, quality and costs. Process control is monitored by "intelligent" controllers that focus on: postcombustion, controlling module O 2 flow in relation to furnace off-gas temperature and composition (O 2, CO, CO 2 ); preheating, controlling off-gas entry composition, temperature and flowrate, and safe operating conditions, in relation to moveable duct and fume bypass valve position; foaming slag height, controlling adequate coal and lime insufflation in relation to on-line calculation models and measurements correlated to submerged or exposed arc status; secondary postcombustion, controlling postcombustion chamber burners O 2 and CH 4 flowrates in relation to output fume temperature and composition. On-line control avails itself of the dynamic simulation that verifies the correct evolution of the process on the basis of signals from the field (Fig. 2).

3 3 / 6 Expert Supervisor ~ x ^x u' Profile set-points u Dynamic Models Estim. outputs Alarms Real outputs PLC r + e Controllers + + u Real Process y ^x State Identifier Identified states and validated measures Fig. 2: On-line control flow diagram To avail itself of these automation capabilities, the process parameters are continually monitored and registered by a data acquisition system. The system registers signals from the field second by second, and the whole heat is then stored for record The meltdown process To obtain the short tap-to-tap times, the DANARC PLUS meltdown process is oriented towards time optimisation in all operations: furnace charging, boring, preheat positioning and execution, melting, tapping, tap-hole refilling. Furnace charging, as already commented, is carried out in a single batch-charge, thereby diminishing power-off time. When the furnace is ready to charge, the roof car cylinders are actuated, fitting into conical seats and lifting the roof. As the roof translates during opening, the preheating car follows, bringing the shuttle bucket into position above the furnace. Considering the bucket valve opening time being 4sec max., the overall charging operation is very quick as preheating car and roof car are ready to simultaneously move back into their positions for the meltdown phase. As soon as the electrode swings back into position, the boring of the high column of preheated scrap begins, while modules start operating in burner mode (Fig. 3). Fig. 3: Furnace meltdown configuration Meanwhile, during boredown, the empty shuttle bucket is removed from the preheating car and deposited in the scrap charging area. Once these operations are concluded, the second shuttle bucket, charged during the preceding heat and in stand-by, is positioned into the empty car and preheating can start once the fume by-pass valve is closed, directing the furnace off-gas through the scrap charge. In the later stages of the meltdown, the modules shift from burner mode to lance mode. In this phase, post combustion oxygen flow is controlled, as previously described, according to the readings of the continuous off-gas analysis and temperature. Once an adequate liquid bath has been obtained, slag height control is enabled so that arc length can be kept long and superheating time reduced to a minimum. A particularly time-effective solution has been applied to optimise the tapping phase, enabling the furnace to be tapped while the electrode is in the shell. The electrode does not tilt with the furnace because, as a consequence of adopting the high furnace approach, a long electrode is necessary to bore through the whole charge and the electrode column must be fastened directly to the ground, to stiffen its structure. The furnace shell rests on the tiltable platform that is supported on rolls and tilting is commanded by 2 hydraulic cylinders. In this manner, the shell moves backwards, or forwards in deslagging, maintaining the shell's centre of rotation in the vicinity of the electrode aperture in the delta. As tapping comes to its conclusion, the electrode is extracted so that the furnace can be charged for the following heat, once tap-hole filling and refractory inspection have been completed.

4 3. A FOCUS ON THE ABS INSTALLATION 3.1. Particular design characteristics The furnace at ABS presents all the aspects illustrated in the DANARC PLUS M 2 concept. There are however some peculiarities in the project that the authors would like to describe. DC Arc Power Supply The supply system is a 1 x 12 pulse system made up of one transformer and two converters, each feeding two anodes. The secondary circuit dislocation has been designed to reduce deviation effects on the arc by lowering the busbar tunnel. The anode installation is according to Danieli patent, and the experience obtained from furnace operations in Spain and Mexico has been integrated in their design, showing greater thermal resistance and lower anode temperatures. Digital converter control is operative in the system, and a reduction of over 6% flicker disturbance is expected with preheating in operation, along with a drastic reduction in noise emission. In the trials conducted up to date, flicker measurements have shown a reduction of approximately 5%. Preheating Station The bucket s cooling water circuit is fed via quick couplings that are automatically engaged with the positioning of the bucket on the car. The car is equipped with a liftable and rotating scrap bucket cover, that avoids hot scrap fumes escaping the extraction system during movement. The cover is equipped with 4x3.5MW burners with compressed air flame control to eliminate, in certain temperature and %CO ranges, the formation of dangerous CO pockets in the bucket. These burners are integrated in the preheating control illustrated in 2.2. From the bottom exit of the water-cooled scrap bucket, the off gases are directed to the postcombustion chambers via a refractory lined duct to maintain the temperature level and minimise the subsequent re-heating requirements in relation to emission control legislation. Scrap Charging The charging of the water-cooled shuttle buckets is made by standard scrap baskets. The shuttle bucket is positioned on a car that moves into charging position under a truss supported funnel that, apart from containing scrap flow during charging, acts as a dedusting collector. 4 / 6 Automation The automation of the system is entrusted to 6 PLCs, subdivided as follows: furnace automation (movements, hydraulics, cooling, electrode regulation, etc.); alternative energy automation; AC/DC converter automation; water treatment plant automation; additives automation; fume treatment plant automation. The control room is configured for 8 workstations: operator control, process management 1, model and process control, data base and acquisition, technologist, SW maintenance, process management 2, TV monitor control. All operations regarding furnace functioning are monitored by a closed circuit TV system operating on 8 TV cameras displaced in the working zone. Automatic Sampling Equipment The furnace is equipped with an automatic sampling device that apart from taking a sample, measures bath temperature and C content, eliminating working risks and reducing power-off time Main design parameters Electrical energy data Transformer Transformer rated power Primary system rated voltage Short circuit fault level at 21kV Sec. voltage range 78-4V Sec. voltage range at full power 78-64V 2*43.5MVA 21V±1% 1MVA Sec. voltage range at const. sec. current 64-4V Max. sec. curr. for each sec. wind. 4kA Converter Number of bridges 2 Bridge type 6 pulse 2 way Single bridge rated current 48kA Auxiliary energy data Modules Nominal O 2 flow 3*25Nm 3 /h Nominal coal flow 3*3kg/min Burner power 6*3.5MW n 2 burners per module unit (2 injectors) Insufflation data Lime Number of injectors 3 Flowrate per injector 15kg/min

5 Geometrical data Furnace Furnace height 738mm Furnace diameter 58mm Electrode diameter 711mm (28") Electrode stroke 88mm Shuttle bucket Shuttle bucket height Shuttle bucket diameter Fume extraction data Booster Primary booster power Nominal speed Main fans Stack fan power Nominal speed 3.3. Main design performance figures 8mm 55mm 1*63kW 1rpm 3*8kW 1rpm The design performance figures for the installation in ABS are shown below; all specific quantities are relative to tapped liquid steel: Units El.En.cons. [kwh/t] 28 O 2 cons. [Nm 3 /t] 38 CH 4 cons. [Nm 3 /t] 6 Coal inj. [kg/t] 13 Lime inj. [kg/t] 35 Electrode cons. [kg/t].9 Power on time [min] 34 Tap-to-tap time [min] 42 Charge [t] 1 Tapped steel [t] 9 Productivity [t/h] START-UP PHASE Danarc Plus operations at ABS meltshop commenced in mid-december Stage1 of the start-up, aiming at the tuning and consolidation of continuous operations bypassing the preheating station, proceeded into the first half of Stage2 operations, involving the full start-up of the preheating unit, followed, pushing towards the operating procedures that characterise the Danarc Plus M 2 design performance figures. The initial start-up heats proceeded in soft mode at medium-low power levels and initially following an intermittent operation to allow visual inspection of the anode systems. 5 / 6 The furnace s state of readiness at start-up comprised full operativity of all auxiliary equipment, which was duly tested during the initial sessions, giving performances in line with the expectations. Already during the first heats, very satisfying results were obtained from the DC arc power supply, regarding the energetic uniformity obtained with vertical arc operations, noting the total absence of twisting phenomena. This result was considered very promising in view of continuous operations with a high scrap column in the furnace. The solutions illustrated in 3.1. to eliminate electromagnetic induced deviation effects on the arc have proven highly successful, as the furnace s interior is uniformly heated. The bottom anodes were tried up to maximum current for successive heats and no inconveniences arose: anode temperature and demineralised cooling water temperature increase remained steady and low. All heats were carried out starting with a full charge of scrap in the furnace, except from the first heat of each production session, which was molten in two buckets and with limited use of alternative energy in the second bucket. As was clearly expected, scrap charging proved to be a key issue in the optimisation of the furnace s operation, not only in relation to its different level qualities (stratification), but also in terms of time intervals associated to crane operation, shuttle bucket positioning and charging. 5. OPERATIONAL RESULTS At the moment, Danarc Plus operations are regularly scheduled in the ABS production programme, alternating activity with the AC furnace, dedicated to stainless steel production. The furnace is currently undergoing performance tests, and soon the installation will formally be handed over to ABS. Danieli will however continue its presence on the furnace, taking advantage of the extensive data monitoring capabilities and process control innovations as feed-backs to continuously update the design and technological teams for further applications. The following graphs show the melting profile (Fig.4) and preheating profile (Fig.5) of a heat performed by the furnace.

6 O2 & CH4 flows [Nm3/h] Fig. 4: Electrical and chemical energy profile Active power [MW] Electrical and chemical energy profile Time [min] O2_Oxyjet_mod1 [Nm3/h] CH4_Oxyjet_mod1 [Nm3/h] ACTIVE POWER [MW] Electrical energy and preheating profile Time [min] ACTIVE POWER [MW] PH_PDROP [mmh2o] Active power [MW] PH_p_drop [mmh2o] 6 / 6 Regarding melting operations, ABS has requested to use a minimum of 15% charge of heavy home scrap (tundish and ladle skulls, return ingots, etc.). Danieli has complied with this process requirement, and the furnace has shown great flexibility in charge practise, not only regarding the process, that has been influenced partly by this restraint, but also regarding mechanical reliability, thereby surpassing other preheating installations, of a more traditional type. Regarding performances, the best heats with PH, corresponding to a standard charge without turnings, have shown a high energetic recovery, 55-6kWh/t with 2min PH, in accordance with the design calculations. Analysing the average data, Table1 shows the performance figures of the operations carried out so far: Specific Unit Without With Delta consumptions* PH PH Electrical Energy [kwh/t] Oxygen [Nm 3 /t] Natural Gas [Nm 3 /t] Coal [kg/t] Tap Temperature [ C] Average Power [MW] POn [min] * specific quantities on liquid steel Fig. 5: Preheating profile Table 1: Performance figures Regarding the melting profile, the graph shows, together with the active power input, the alternative energy input of the Oxyjet of one of the three modules: the profiles of the remaining two modules are identical. Interesting considerations can be made regarding the preheating profile, in which, together with the active power input, a signal from the preheating circuit is shown, corresponding to the pressure drop across the water-cooled scrap bucket. Preheating operations in ABS have shown that this installation is capable of safely preheating scrap with up to 15% turnings in the charge, thereby reducing the charge cost related to PH operations. This is due to a highly flexible circuit design, that allows partialisation of the gas flow between PH circuit and bypass, whenever this is required. As can be seen, the pressure drop is in the order of 35mmH 2 O, a rather high value, corresponding to the presence of turnings in the lower area of the bucket; a quota of the EAF fumes is flowing through the by-pass. Due to the fact that automatic crane handling must still be implemented, PH operations start after approximately 15min POn. On commenting these figures, comparing with the design performances presented in 3.3, it should be taken into account that the actual charge density is 11kg/m 3 (15% heavy home scrap, mentioned above) while prior to this change in charge practise, average density equalled 88kg/m 3. Operating performances are therefore in line with the design performances, as this variation in charge density implies a greater consumption of approximately 2kWh/t. It should be noted that the furnace operation covers all the range of special steel grades produced by ABS, with the exception of stainless steel (in 1998 equal to 5% of the total). So far, the intense and friendly collaboration between ABS and Danieli in this project have brought splendid results in the new furnace s start-up and performance. The authors believe that continuing in this trend, the remarkable results expected from the DANARC PLUS M 2 technology will become a reality, identifying this innovative furnace as a valid competitor in world-wide steelmaking in the very near future.