MASS AND ENERGY BALANCES OF STAINLESS STEEL EAF

Transcription

1 MASS AND ENERGY BALANCES OF STAINLESS STEEL EAF M. Kirschen 1, H. Pfeifer 1, F.-J. Wahlers 2 1 Institute for Industrial Furnaces and Heat Engineering in Metallurgy, RWTH Aachen, Gerany 2 ThyssenKrupp Nirosta, Bochu, Gerany Key Words: Electric arc furnace, Stainless steel, Off-gas easureents, Mass balance, Energy efficiency, Off-gas extraction ABSTRACT Off-gas easureents were perfored at two EAFs of ThyssenKrupp Nirosta in order to set-up precise ass and energy balances. Results show that, with siilar production paraeters as steel type, scrap and slag forer input, coal addition, cooling syste, tap-to-tap ties, the use of fuel burners and dedusting syste operation drastically affects the easured off-gas coposition and aount of air infiltration into the furnace vessel. A high degree of air infiltration ay result in decreasing the cheical energy load (H 2 and CO content) but increasing the total off-gas enthalpy. On the other hand, without additional oxygen input for post-cobustion, H 2 and CO off-gas concentrations ay achieve considerable values. Mass and energy balances for austenitic heats are presented for two TKN steel ills at Bochu and Krefeld. INTRODUCTION During the last 25 years the EAF steel productivity increased with decreasing nuber of production sites in Gerany. Today, tons per year or 29 % of total crude steel is produced in Gerany by scrap elting using the EAF. The capacity of TKN steel ills located at Bochu and Krefeld of tons per year stainless steel (ferritic and austenitic grades) yields about 1 % of total EAF steel production in Gerany (Fig. 1). Bochu Martensite 3% Krefeld Ferrite 51 % Austenite 44 % Austenite with Mo 28 % Austenite 72 % Austenite with Mo 2 % Fig. 1: EAF steel production ix at ThyssenKrupp Nirosta steel ills [1]

2 TKN operates two EAFs in the steel ills of Bochu and Krefeld. Technical data for both furnaces are suarized in table 1. The Bochu EAF is characterized by an UHP transforer with an apparent electrical power of 135 MVA. Energy recovery has been optiized by use of a shell cooling syste based on stea cooling at a pressure of 23 bars and a teperature of 22 C [2]. The Roof is conventionally water cooled at the Bochu EAF. At the Krefeld EAF, both, shell and roof are stea cooled. The dedusting systes lack a rough separator but yield a lowered furnace pressure via the elbow by buoyancy effects in the vertical hot gas line. This construction grants a restricted off-gas extraction and steel production even in the case of breakdown of the dedusting syste fans. A further optiization of the energy efficiency and the influence of off-gas extraction on the energy balance is subject of the present coparing study. Special focus is set on the off-gas enthalpy because (1) total enthalpy increases with the ass of infiltrated air during elting and (2) shifting cheical to sensible heat load by CO post cobustion iproves the possibility of energy recovery fro the exhaust gas and consequently higher energy efficiency. Table 1: Technical data of the ThyssenKrupp Nirosta EAF s at Bochu and Krefeld Bochu Krefeld Start up date Transforer apparent power [MVA] Priary voltage [kv] Secondary voltage [V] Current [ka] Mean tapping weight [t] Annual productivity [t/y] Shell diaeter [] Electrode diaeter [] Wall cooling syste Stea cooling Stea cooling Roof cooling syste Water cooling Stea cooling Nuber of wall cooling panels Gas burner no yes Priary off-gas extraction [N 3 /h] Secondary off-gas extraction [N 3 /h] Tapping syste Skier Runner Tapping teperature [ C] Additional equipent: Current conducting electrode ars Digital electrode control syste Oxygen lance anipulator Process control coputer MASS AND ENERGY BALANCES In order to assess and optiize yield and energy efficiency of the EAF elting process, ass and energy balances are widely used working tools (Fig. 2). However, the energy balance is based on a reliable ass balance, since, e.g., the calculation of cheical energy depends on the asses of oxidized eleents C, Si, Al, Mn, Ni, Cr and Fe. Soe iportant paraeters for ass balances are coonly not logged at the elt shop. As steel weight and coposition is generally precisely deterined after tapping, the slag weight and coposition is not. Scrap coposition is often estiated, dust coposition

3 and yield, specific refractory and gun ass consuption are statistical averages. The off-gas volue flow at the elbow and the volue flow of infiltrated air is usually unknown. However, these unknowns can be deterined fro eleent ass balances, e.g. the total slag weight fro the CaO balance using the slag cheical analysis and the aounts of CaO input fro refractories and gun aterials. Oxygen 15-5 kg Coal 5-15 kg Fuel 5-1 kg Scrap/Alloys kg Lie/Doloite 25-5 kg Electrodes 1-3 kg Dust 15-2 kg Off-gas (N 2,O 2,CO,CO 2,H 2 ) kg Infiltrated air 1-2 kg Slag 1-2 kg Refractories 3-12 kg Steel 1 kg Fig. 2: Ranges of specific values for EAF ass balances [3] E.g., the slag weight is deterined by the CaO ass balance: x CaO, LieLie xcao,refractoriesrefractories = xcao,slagslag + xcao,dustdust + (1) where x denotes the CaO aount in weight % and the total ass. Dashed sybol denotes statistical average. In the case of high aounts of recycled aterial, e.g. for austenitic heats, CaO input fro skulls has to be taken into account. The usually unknown aount of off-gas can be deterined fro the C balance assuing carbon output solely by steel and off-gas: x x C,Scrap C,Steel Scrap Steel + x + C,Liestone x C,i i= CO, CO2 Liestone off -gas dt + x C,Coal Coal + x C,Electrodes Electrodes = (2) where x CO and x CO2 are easured off-gas ass concentrations. The aount of infiltrated air can be estiated fro the N 2 ass balance assuing that all N 2 of the off-gas is fro infiltrated air. Therefore, easureents of off-gas coposition, teperature and volue flow at the elbow yield valuable inforation in order to set up and iprove EAF ass and energy balances. EAF OFF-GAS MEASUREMENTS OF AUSTENITIC HEATS At the Krefeld EAF, off-gas teperature, coposition and volue flow were easured in the exhaust duct of the dedusting syste. In addition, cheical coposition of the priary EAF off-gas was easured on the roof (Fig. 3). With this set-up, the volue flow of infiltrated air at the gap between furnace elbow and exhaust duct is deterined by C ass balance.

4 Fig. 3: Two off-gas sapling points in the dedusting syste (75t-EAF) At the Bochu EAF, off-gas easureents were conducted using water cooled lances in the gap between the EAF elbow and the exhaust duct of the dedusting syste (Fig. 4). Fig. 4: EAF off-gas and infitrated air volue flows at the elbow

5 Gas Sapling Cooling Water Filter Heater Saple Preparation Detector CO 2,CO, O 2,H 2,CH A Copressed Air Data Acquisition Teperature Pt/Rh6 - Pt/Rh A Cooling Water C Signal Converter Copressed Air Volue Flow A Cooling Water Signal Converter Fig. 5: Instruentation for off-gas easureents The gas saple is filtered, cleaned, cooled and dried (Fig. 5). The CO, CO 2 and CH 4 concentrations are deterined using an infrared analyzer. H 2 and O 2 concentrations are easured by theral conductivity and diaagnetis of the oxygen olecule, respectively. Teperature easureents are done with shielded Pt/Rh6-Pt/Rh3 and Ni/Cr-Ni therocouples, respectively. The volue flow is deterined by easuring the ean dynaical pressure of the gas flow which is proportional to the gas velocity. RESULTS Measured aounts of off-gas coponents for austenitic heats at are plotted in Fig. 6 (145t-EAF) and Fig. 7 (75t-EAF). H 2 O content is calculated to few vol.-% assuing water gas equilibriu (CO + H 2 O = CO 2 + H 2 ) at easured off-gas teperature. 8 Off-gas coposition [Vol.-%] CO CO2 H2 O2 H2O calc 1 st Bucket 2 nd Bucket 3 rd Bucket Oxygen Lancing Tapping Tie [in] Fig. 6: Measured aounts of off-gas coponents vs. tie (145t-EAF, austenitic heat)

6 It is clearly shown, that during elting periods without oxygen blowing the off-gas consists ainly of heated air infiltrating the furnace through various leakages, e.g. slag door, gap between vessel and roof, electrode holes. CO 2 content does rarely exceed 1 % (calcination of liestone). With oxygen blowing the aount of CO and CO 2 increases up to 6 % and 25 %, respectively. The aount of postcobusted CO 2 reains essentially constant at about 2 % due to liited infiltrated air volue flow into the vessel. A rather distinct off-gas pattern is easured at the 75t-EAF (Fig. 7), where a fuel gas burner is active during the first period of elting. H 2 fro fuel gas is detected up to 3%, CO 2 ranges fro 1 % to 15 %. CO in excess during the coplete heat period indicates a rather liit aount of infiltrated air. Off-gas coposition [vol%] st Bucket 2 nd Bucket CO H2 CO2 O Tie [in] Fig. 7: Measured aounts of off-gas coponents vs. tie (75t-EAF, austenitic heat) The teperature of the off-gas increases fro about 1 C to 12 C during elting of the scrap with the electric arc up to 16 C during decarburization by oxygen blowing and partial postcobustion of CO (145t-EAF). The volue flow of the off-gas reains essentially constant during various periods of elting due to constant fan power of the dedusting syste. During swinging out the roof for charging the volue flow at the elbow is reduced by opening an air daper. The noralized off-gas volue flow and ass flow slightly decreases with increasing gas teperature at the elbow (Fig. 8). 5 2 Volue flow per ton [N 3 /h t] Teperature [ C] Tie [in] Fig. 8: Measured off-gas volue flow and teperature (145t-EAF, austenitic heat)

7 With known data of off-gas coposition, teperature and volue flow, the off-gas enthalpy at the elbow is easily deterined (Fig. 9, 145t-EAF). As the sensible heat load of the off-gas is ore or less constant over heating tie, the total power significantly varies fro 1 to 5 MW due to varying aounts of H 2 and CO. On the other hand, the aounts of CO and H 2 deterine the iniu aounts of oxygen for coplete cobustion, either fro infiltrated air or fro oxygen injection. CO cobustion in the furnace further increases energy efficiency by decreasing the off-gas cheical heat [4]. However, with increasing gas and shell teperatures due to coplete CO and H 2 cobustion sensible gas enthalpy and heat flux to the cooling syste ay increase. Enthalpy [kwh] Total Enthalpy Total Enthalpy Flow Sensible Portion Sensible Portion Enthalpy Flow [kw] Tie [in] Fig. 9: Total off-gas enthalpy for an austenitic heat (145t-EAF) After having deterined the off-gas energy of a heat the coplete energy balance is easily set up. Average of nuerous heats erases incertitude fro foration of heel to ass balances. Cheical energy includes exotheric oxidation of Si, Mn, Cr, Ni, Fe and endotheric calcination of liestone (Fig. 1). Masses of oxidized Si, Mn, Cr, Ni and Fe were calculated fro the oxides with cheical analysis of slag, slag forers, refractories and dust. C oxidation coprises energy release fro cobustion of coal, electrode and scrap carbon. For the Krefeld EAF the energy fro fuel gas burners is separately noted (Fig. 11). Off-gas enthalpies were integrated fro easured volue flow, teperature and coposition. Cooling includes the enthalpy of stea which is recovered in part at TKN Bochu and Krefeld. However, precise calculation of the enthalpy sink by stea cooling solely fro volue flow and teperature of the cooling ediu is difficult. Instead, enthalpy of stea is estiated fro the easured energy flux to water cooled roof assuing equal flux to roof and wall in the case of Bochu EAF. For the Krefeld EAF the contribution of the cooling syste is estiated by difference. Fro the resulting value (218 kwh/t), an average heat flux of < 239 kw/ 2 is estiated for the Krefeld EAF, which copares to the easured value of 197 kw/ 2 at the roof of the Bochu EAF. Reference weight for the ass and energy balances in table 2 and Figs. 1 and 11, respectively, is the AOD input weight after deslagging.

8 Fig. 1: Averaged energy balance for 7 heats (austenitic grades, 145t-EAF) Fig. 11: Averaged energy balance for 24 heats (austenitic grades, 75t-EAF) The energy balance of the 75t-EAF Krefeld shows lower total energy input (754 kwh/t) and specific electric energy (494 kwh/t) than Bochu (781 kwh/t and 51 kwh/t, respectively). The enthalpies of austenitic elts differ slightly due to lower tapping teperatures of the heats considered in this study (75t-EAF: 1584 C, 145t-EAF: 1543 C). The energy balances and corresponding ass balances are given in table 2. Specific average slag ass and enthalpy as the cheical energy contribution fro alloy eleents oxidation is lower for the 75t-EAF due to lower oxygen lancing (see table 2).

9 Table 2. Mass- and energy balances for two EAF s of ThyssenKrupp Nirosta 145t-EAF 75t-EAF Scrap and Alloys [kg/t] Lie, Doloite, Liestone [kg/t] Fuel Gas [kg/t] Coal [kg/t] Mean Electrode Consuption [kg/t] Mean Refractory Consuption [kg/t].9.9 Mean Dust Eission [kg/t] Oxygen Consuption (Lance and Oxy-fuel Burner) [kg/t] Infiltrated Air [kg/t] Off-gas [kg/t] Slag Production [kg/t] Electrical Energy [kwh/t] Cheical Energy fro Carbon Oxidation [kwh/t] Cheical Energy fro Fe, Si, Cr, Ni, Mn Oxidation [kwh/t] Oxy - fuel Burner [kwh/t] - 12 Enthalpy Steel [kwh/t] Enthalpy Slag and Dust [kwh/t] 36 3 Enthalpy Off-gas [kwh/t] Furnace Cooling [kwh/t] Radiation and Convection [kwh/t] Total Energy [kwh/t] : 2.9 kg/t is oxygen of the gas fuel burner; reference weight is EAF tapping weight after deslagging High aounts of off-gas (293 kg/t) and infiltrated air (267 kg/t) are obvious for the 145t-EAF resulting in a higher average off-gas enthalpy (121 kwh/t) when copared to the 75t-EAF (123 kg/t, 118 kg/t, 79 kwh/t, respectively). As illustrated in Fig. 6 ainly CO-bearing off gas is produced by oxygen blowing during the second period of heat. However, Fig. 8 deonstrates that continuous off-gas extraction through the elbow of the furnace leads to a ore or less constant off-gas ass flow during the coplete elting period. Therefore, large aounts of air are infiltrated at low teperature and heated during the first period of scrap elting for the 145t-EAF. However, during oxygen blowing the aount of infiltrated air is insufficient for coplete CO cobustion in the furnace shell. The aount of energy recovery fro CO post-cobustion within in the stea cooled hot gas line is not considered for the 145t-EAF, but it is included in the energy balance of the 75t-EAF. A reduced off-gas extraction during the first elting period is suggested for further iniization of the off-gas enthalpy for the 145t-EAF. Due to the buoyancy effect of the vertical hot gas line and ixing with secondary off-gas priary off-gas extraction at the elbow is controlled by the infiltrated air daper and the variable gap between elbow and exhaust duct rather than the fan power. Following this hypothesis tests of different furnace operations are on the way. SUMMARY Measureents of exhaust gas ass flow and coposition at EAF elbow yield valuable inforation about various process paraeters and energy efficiency of stainless steel aking, e.g.: the aount of

10 air infiltration due to furnace and dedusting syste operation. These data perit the refineent of estiates about the off-gas enthalpy in order to set up precise ass and energy balances of the EAF. For two furnaces of stainless steel production sites easured off-gas copositions are significantly different due to use of fuel gas burners. In absence of fuel gas cobustion the aount of infiltrated air increases and affects the energy balance of the furnace. Consequently, off-gas extraction should be adapted to different types of EAF operation and even elting periods in order to optiize (1) infiltration of air into the furnace vessel, (2) energy efficiency of the elting process, and (3) hoogeneous teperature distribution in the furnace shell. REFERENCES [1] F.-J. WAHLERS, M. WALTER, H. ZÖRCHER, Stahl und Eisen, 118, (1998), No. 9, pp [2] H. BROD, F. KEMPKENS, H. STROHSCHEIN, Stahl und Eisen, 19, (1989), No. 5, pp [3] H. PFEIFER, in: K.-H. Heinen (Ed.), "Elektrostahlerzeugung," Verlag Stahleisen, Düsseldorf, (1997), pp [4] E.J. EVENSON, H.D. GOODFELLOW, M.J. KEMPE, Proc. ISS 58 th Electric Furnace Conference, Orlando, (2), pp [5] M. KIRSCHEN, H. PFEIFER, F.-J. WAHLERS, H. MEES, Proc. ISS 59 th Electric Furnace Conference, Phoenix, (21), pp