NEW APPROACH TO BLAST FURNANCE SLABS HEATING OPTIMIZATION

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1 NEW APPROACH TO BLAST FURNANCE SLABS HEATING OPTIMIZATION * Malindžák Dušan, * STRAKA Martin & * * KOŠTIAL Imrich Ústav logistiky priemyslu a dopravy, FBERG TU Košice Ústav riadenia informatizácie, FBERG TU Košice Abstract The present article describes a new approach to the optimization of heating of slabs in a push furnaces (PF). The new approach consists in the three-level hierarchical control system, namely: a) the control of charging the PF in order to ensure the optimum slabs sequence in the critical zones of the PF, which contributes to the creation of more favourable conditions for their heating; b) based on the structure and temperature of slabs in the PF the optimum slab heating curve in the PF is formed dynamically; c) the corrective control ensuring the optimum heating curve. This article is related to the article [1], which describes methods at all three levels of control, and it aims to point out and describe particular gains in the area of saving costs and increasing quality. Keywords: standard and non-standard batch, hot mill schedule, charging schedule, optimum heating curves. 1. INTRODUCTION At present push furnaces, which are used for heating of slabs to a rolling temperature before their rolling in hot rolling mills, are charged cyclically with one slab at a time in the sequence in which they are scheduled to be rolled in the hot mill. Mainly in the case of a great number of orders for only small numbers of slabs this charging system results in the fact that in zones of the intensive heating there are slabs of various qualities and temperatures, which eventually increases heating costs and worsens heating conditions and quality. This issue may be solved by creating a hierarchical system of control, a combination of the logistic approach and optimal process control on the basis of the calculation of slabs temperature in the PF, dynamically created heating curves and standard correction feedback control. The possibility of solving the above mentioned issues is to apply a new strategy of charging push furnaces (PFs) in batches. Gains are obtained due to the fact that before the charging, the slabs are grouped into the batches groups, whose total width is approximately equal to the length of a zone with the maximum heating intensity (maximum energy input), thus ensuring homogeneity of the charge slabs in zones of the PF from viewpoint of heating, and subsequent minimization of thermal losses, which leads to savings in heating costs, the reduction of metal burn-off, and the heating quality improvement.

2 Fig. 1 Relations between control levels Relation 1 from level III to level II 2. PROBLEMS RESULTING FROM THE CURRENT METHOD OF CHARGING The PFs are used to heat slabs to the rolling temperature before their rolling in the hot rolling mills. The current method of slabs charging is cyclical, i.e. slabs are gradually pushed in (and simultaneously pushed out) to push furnaces one by one, in the order in which they are rolled in the hot rolling mill. However, sequential charging system has a lot of disadvantages: a) Alternation of warm and cold slabs (heating of a heterogeneous charge) in the furnace zones, while the energy input for each zone must ensure the heating of the slab which is the worst from the heating viewpoint (e.g. the coldest one) to the required temperature. This results the overheating of warmer slabs, and increasing the heat losses. The warm slabs reach the scaling temperature earlier and are exposed to a higher temperature for a longer period of time. Thus the output temperature of warm slabs is above the required rolling temperature. b) Alternation of qualities if there are slabs of various qualities in the particular zones, they have various heating requirements, and this contributes to the similar losses as described in point a). c) The hot charge share the increase of the hot charge share cannot be taken to advantage because warm slabs are divided among several push furnaces and they are not cumulated in one PF, where it would be possible to decrease the fuel input and thus save the heating energy.

3 d) Late or early supply of the hot charge. When the hot charge is late, it can be charged into the PF only sometimes in the future to fit the rolling schedule which defines the rules for the rolled slabs sequence. In the meantime the charge slabs gets cold, and energy losses occur as it is necessary to heat the slabs again in the future. If the hot charge is supplied earlier than planned in the operative charging (or rolling) schedule, it must wait until its planned rolling time, and thus loses its heat. 2.1 Heterogeneous charge heating losses Losses occurring during the heating of the heterogeneous charge can be divided into: a1) Losses from temperature jumps, a2) Losses from failure to keep the optimum heating curve, a3) Losses from the overheating of warm slabs to the output temperature higher by about 20 C, a4) Heat losses of incoming warm slabs, a5) Losses caused by the increased burn-off of warm slabs. Losses are determined for the reference rolling mill with 3 push furnaces. a1) Losses from temperature jumps (S TS ) The current method of the cyclic charging with one slab at a time causes the occurrence of slabs of various qualities and temperature in the second and third zone of the PF, where slabs obtain the major part of the thermal energy. If warm slabs (min. temperature of 150 C) alternate with cold ones (eg. 10 C), the higher the amount of orders for a small number of slabs and the higher number of PFs, the more intensive the alteration. Slabs belonging to one group (order) are distributed among a higher number of push furnaces. Losses resulting from the alternation of warm and cold slabs depend on the number of temperature jumps and are shown in Fig. 2. Fig. 2 The effect of temperature jumps on the gas consumption Total heat losses in the case of 2 temperature jumps are about 1% of the supplied energy. [1] S TS = ,- Є/year a2) Losses from failure to keep the optimum heating curves of warm slabs If only cold slabs are heated in one zone, i.e. the charge is homogenous from the temperature viewpoint, the heating curve corresponds to Fig. 2. The strategy is that the slabs achieve the scale formation temperature (800 C) as late as possible, or as close to the end of the heating process as possible, so that the time of the scale formation is as short as possible.

4 Fig. 3 Optimum heating curve of cold slabs In the case of the warm slabs heating, i.e. if only warm slabs, for example those with the input temperature of 300, are heated in the given zone of the PF, the heating curve corresponds to Fig. 4. Fig. 4 Heating curve of warm slabs However, if warm and cold slabs are heated together, then cold slabs must be heated to the rolling temperature of 1250 C, and the heating curve corresponds to Fig. 5. Fig. 5 Heating curves of common heating of warm and cold slabs in one zone In this case we cannot keep the optimum heating curves of warm slabs in accordance with Fig. 4, but the heating shall proceed in accordance with the curve (dash line) in Fig. 5. It causes three problems: a) the final temperature of warm slabs shall be higher by approximately 20 C, i.e. about 1270 C, b) slabs shall reach the scale formation temperature about 30 minutes earlier (compared with the optimum curve), which increases the percentage of scale approximately by 15% compared with the amount of scale in case of the optimum heating, c) heat losses in the case of combined heating correspond to the hatched area in Fig. 6.

5 Fig. 6 Heat losses in case of the warm slabs heating if there are warm and cold slabs in one zone (S MIX ) For example, if the PF charge contains minimally 30 % of warm slabs, approximately half of 30 %, i.e. 15 % of them, is overheated. The hatched area represents min. 5 10%. So, for example, at ton of the annual production of slabs, 30 % represents ton, 15 % of unnecessarily heated slabs equals to t. If only 5 % of energy is supplied unnecessarily, and if 1 the heating of 1 ton from 0 C to 1250 C cost approximately 8 Є, in case of warm slabs whose core has the temperature of approximately 150 C, it is about 6 Є/t (1 GJ of natural gas is approximately 5Є), i.e = = = Є S MIX = Є a3) Losses from the overheating of warm slabs to higher output temperature (S P ) If we consider that the heating to the temperature of 1270 C requires approximately 1.6 % more energy than the heating to 1250 C, which in case of 1 t represents 8 Є/ = = 0.13 Є for /1t. With the volume of t / = = Є/year. S P = Є/year. a4) Heat losses of incoming warm slabs (S TVB ) If warm and cold slabs are mixed, the heat with which a slab enters the PF, is lost. In fact of about 1/2 of this energy is lost. If the average temperature of incoming slabs is for example 300 C (the core of slabs may be of higher temperature), about 1/8 of energy needed to heat slabs to the temperature of 1250 C is lost. If we consider the saving of 1/8 of costs needed to heat t of warm slabs (if the share of the hot charge is up to 30 %). S TVB = *1/8. 8 = Є per year a5) Losses caused by the increased warm slab burn-off (S TZP ) The increase of losses caused by the metal burn-off result from the following: - warm slabs in furnace reach the oxidation temperature earlier (approximately 30 minutes), - the oxidation temperature is higher than if only warm or cold slabs are heated in one zone, - the output temperature of warm slabs from the PF is higher, which results in higher burn-off. In case of 30 % share of warm slabs and current burn-off rate of 1.5 %, the burn-off is reduced by 10 % minimally, 1.5% = 0.015, 10 % reduction is

6 Burn-off of cold slabs 1.5% With the production volume of 4 mil. ton 1.5% out of which 10 % is 0.15 %, which represents = = t 3. INCREASE OF THE HOT CHARGE SHARE A sequence of slabs charged to the PF is joined into a group of slabs with the identical temperature or quality. The created batch is charged into one furnace in the slab cadence intervals. The sum of the slabs widths in the batch is dynamically formed according to the length of the zone with the maximum input. A batch created in this way is so called standard batch. For example, if the length of the zone with the maximum input (mostly the second zone) is 5.5 m (let the length of the entire PF be 33 m), the length of the standard batch shall be formed dynamically within the interval between 5 and 6 m. The batches shall be charged to PFs cyclically. The operation time of PFs is divided into the time of charging, when in the interval of the rolling cadence the slabs from the same batch are charged to one PF (e.g. 5 slabs at the cadence of 90 seconds are charged in 450 seconds. Other PFs, e.g. No. 2, No. 3 and No. 4 are in the heating mode during that time. Subsequently the next batch is charged to PF No. 2 and next to PF No. 3). Changing the order of charging individual push furnaces enables to solve situations of early and late supply of warm slabs to PFs, which results in the increased number of warm slabs charged into PFs. When warm slabs are supplied earlier than defined in the PF charging schedule, they are charged earlier and do not get cold; or when they are supplied later, they are charged in the time of their supply, but they may exit the PF according to the schedule [1]. This increases the amount of warm slabs in the charge by minimum of 20 %. Gain With the volume of mil. ton/year 20 % slabs represents T, and this amount shall be heated from the temperature of 300 C, not from the temperature of e.g. 20 C. It represents about 1/4 of the heating. If the heating of 1 t of slabs from 0 to 1250 C is approximately 8 Є, then 1/4 is 2 Є (t). 2 (Є) = 1.6 mil Є / per year 4. GAINS IN THE AREA OF OPTIMUM HEATING CURVES OFFERED BY THE MODEL 1. A heating curve is formed dynamically. The heating is performed under optimal conditions from the energy consumption and material loss viewpoint. The optimal trajectory is defined for each slab. The main advantage is that this trajectory does not have a fix configuration, but it considers an actual situation in front of the furnaces, in the furnaces and in the rolling mill. All this ensures optimum heating parameters for the considered conditions. In accordance with the furnace output (the minimum time of staying at high temperature, at the heating of 1.5 mil. ton, applies for both warm and cold slabs. 2. Heating curves are adjusted dynamically depending on the charge composition. 3. The heating model has two-level hierarchy. - 1 st level prediction model (strategy) - 2 nd level correction model. 4. In case of batch charging, dynamic creation of heating curves is depending on the duration of slab staying in the push furnace.

7 5. In case of combining the heating model with the charging model the optimum heating strategy is created. The total savings resulting from these five measures represent at least 5 % of heating costs, which is about Є. SUMMARY OF GAINS AT THE ANNUAL PRODUCTION VOLUME OF TM 1. Heat losses (gains of the solution) when alternating warm and cold slabs and gains resulting from the new solution 1.a From solving the issue of temperature jumps b Failure to keep the optimum heating curves c Losses from overheating of warm slabs d Heat losses of incoming warm slabs e Losses caused by the increased burn-off - time longer by approximately 30 min at the oxidation temperature higher oxidation temperature of warm slabs Gains from the increase of the share of warm slabs in the charge Gains from the optimum heating curves Total... Gains Є/year CONCLUSION The present article describes the new approach of solving the slab heating optimization in push furnaces. Qualitatively new is the control system structure hierarchical three level system of the optimal control : - charging control - represent a qualitatively new solution, - creation of dynamic - optimum heating curves on the basis of the push furnaces charge composition also represents a new solution, - corrective control based on the heuristic approach. The gains from such a system at the annual production of t are estimated to Є/year. LITERATURE [1] D. Malindžák, I. Koštial, M. Straka, New approach to push furnaces heating optimization, Košice, 2008 [2] D. Malindžák, I. Koštial, M. Straka, Optimalizačný model vsádzania brám do narážacích pecí divíznaho závodu Teplá valcovňa, Košice, 2005 [3] D. Malindžák, I. Koštial, M. Straka, Models for push furnaces thermal optimization of Mittal Galati, Košice, 2006 [4] D. Malindžák, J. Spišák, The charging logistic of push furnaces VSŽ a.s. Košice, Acta Montanistica Slovaca. roč. 1, č. 2, 1996, s ISSN [5] D. Malindžák, Heuristic-logistic model for production scheduling of TŠP 1700 at Oceľ s.r.o. VSŽ a.s. Košice, Publications of the University of Miskolc, Series C Mechanical Engineering, Miskolc, 1995, University of Miskolc, vol. 45 (1995), p [6] D. Malindžák, J. Spišák, M. Straka, Research report of Simul steel project, proposal No 2004-TGS9-138, system analysis for simulation model creation, Košice-Vaasa, january 2008

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