76 BENEFIT OF GAS PURGING IN BOF AND EAF WITH A FOCUS ON MATERIAL EFFICIENCY AND EMISSION REDUCTION. Thomas Kollmann, Marcus Kirschen, Christoph Jandl, Karl-Michael Zettl RHI AG, Vienna Abstract In highly competitive steel markets worldwide, an economical and climate-friendly steelmaking process is required to ensure profitable crude steel production in future. Considering the tightening global emission requirements, which result in rising energy prices and purchasing of emission certificates, raw material and energy efficiency has to be optimized continuously. One option for increasing resource efficiency in the steelmaking process chain is the usage of bottom inert gas purging in the BOF and EAF. Gas purging increases mass and energy transfer in the molten metal, the mixing energy and the bath kinetics, promoting improved reaction capacities for decarburization and dephosphorization. Higher thermal and chemical homogeneity of the liquid metal bath, better process control and increased yield of raw materials by reduction of agent, alloy and oxygen consumptions are essential benefits as well. Introduction Global annual crude steel production is showing a significant increase over the last decades, starting from 00 Mt in 1950s to 1,607 Mt in 013. The trend of the Chinese, world and the rest of the world annual crude steel production over the last 8 years is shown in Figure 1. Focusing on the European steel production market, the share in the total annual crude steel production decreased by 1.8% in comparison to 01. BOF steelmaking route: A wide variety of different process routes are applied for the production of crude steel depending on the availability of raw materials and energy capacities. Crude steel production can be divided into two main distinct steelmaking routes: the integrated blast furnace/bof route based on iron ore and the EAF route using scrap and/or DRI as the main iron carrier. Production of steel at an integrated iron and steel mill is accomplished using several interrelated processes such as coke, lime, sinter, iron and steel production. The global integrated production route is characterized by energy consumption in the range between 15 to 5 GJ with a level in the atmosphere of 1.600 to.500 kg dependent on the calculation method and system boundary [], [3]. To quantify it more precisely, especially for the European steel production sector, average values of 1.800 kg are obtained in total. An overview of the emission cluster for an integrated blast furnace/bof route is listed in Figure. It is seen that the emissions are primarily caused by the sinter and coking plant, blast furnace and the power plant that supplies the required energy to keep up and guarantee steel production [3], [], [5]. Figure : emission per ton of crude steel BF/BOF route [6]. Figure 1: Annual crude steel production [1]. The iron and steelmaking industry is one of the most energy-intensive industries, with an annual energy consumption of about 6x10 9 GJ, 5% of the world s total energy consumption. The steel industry accounts for 3 to % of the total world greenhouse gas emissions. Furthermore, 30% of the total industrial emissions are caused by the iron and steel applications themselves. To realize a significant emission reduction, the iron and steelmaking technologies and raw material concepts currently implemented have to be optimized from the process point of view [1], []. The main share, 80 90% of the total energy consumption, is determined by the production of liquid steel, the remaining 10 0% are caused by the casting and shaping applications. Hot metal is produced by the reduction of iron oxide ores in the blast furnace. Due to the injection of the hot blast using tuyeres, oxygen reacts with coke, petroleum coke or coal, creating a reducing atmosphere (formation of carbon dioxide and carbon monoxide ) to reduce and melt the iron oxides to hot metal based on exothermic reactions. The top gas of the blast furnace shows an average composition of %, %, 5% H and 51% N, which corresponds to a emission range between 380 and 00 kg. emissions are also obtained through the calcination of carbonate fluxes. Limestone (Ca 3 ) and magnesium carbonate (Mg 3 ) will form lime (CaO), magnesium oxide (MgO) and during the burning process in the shaft furnace. The process-related emissions of the calcination process have a level of 785 kg /t lime, while the energy-related emissions show a value of 00 kg /t lime only. The CaO and MgO carriers are needed to balance the acid configuration of the coke and iron ore in 57 th International Colloquium on Refractories 01 Refractories for Metallurgy
77 the blast furnace. For the BOF, the addition of lime is influenced by the [Si] input of the HM and the slag basicity aimed for, focusing on the [P] level after end of blowing. For efficient dephosphorization, a quick slag formation, which shows the highest reaction potential and lime activities, has to be realized. To minimize the wear rate during the initial stage of slag formation, it is necessary to charge or provide an MgO carrier in the form of fluxes and avoid <MgO> dissolution from the lining. Rising (MgO) levels in the slag result in increased viscosity and lower metallurgical reaction potential during refining, but promote a better adherence effect of the slag on the lining using slag splashing or coating. The primary aim of the BOF process is the oxidation of undesired elements ([P], [S], [Mn], [C], [Si]) of the HM to lowest levels and the adjustment of stable and high reproducible temperatures, carbon and oxygen contents after end of blowing, considering lowest levels of reblowing numbers. The BOF is a more efficient decarburization and dephosphorization than desulfurization process. This is caused by the oxidizing atmosphere during refining, where pure oxygen is blown through the top blown-lance onto the liquid metal bath. Regarding exothermic reactions, a heating effect will be generated, melting the fluxes and scrap at least. The refining requires 15 to 0 minutes on average. The BOF off gas is characterized by an average composition of 60%, 15% and 5 % N, leading to emissions of between 70 and 95 kg [6], [7], [8], [9], [10], [11], [1]. EAF steelmaking route: An alternative production route of steel is based on the melting and refining of steel scrap, or combinations of steel scrap with direct reduced iron (DRI), hot briquetted iron (HBI), pig iron (PI) or hot metal (HM) in the electric arc furnace (EAF). The reduction of iron ore to DRI, with reformed gas enriched in carbon monoxide,, and hydrogen, H,is of increasing importance. Reformer gas is produced from natural gas, biomass or gasified coal. 69.6 million t of DRI were produced in 013, with a continuously increasing trend during the last three decades. Due to increased substitution of coal by hydro-carbons as a reducing agent, the DRI/ EAF production route is the most emission-effective steel production route today (Figure 3). The main DRI/HBI production sites and EAF-based production plants are located in emerging markets outside China, e.g. India (1.6 million t DRI), the Middle East (6.7 million t) and North African countries (. million t); 6. million t were produced in Latin America [1]. For the production of high-quality specialty steel grades from scrap with varying quality and chemical composition, compliance with high purity levels is sometimes only achieved with the dilution of unwanted tramp elements such as [Pb], [Cu], [Cr], [Ni], [Mo], and [Sn] with highly pure substituting materials for direct reduced iron and hot metal. For example, high-quality tire cord with [Cu] lower than 0.05% is economically produced by melting mixes of steel scrap and 50 to 100% DRI/HBI. Due to increasing scrap prices world-wide and in regions with restricted availability of high-quality steel scrap, the combination of low-quality scrap grades and highly pure scrap substituting materials is a cost-effective option. DRI/HBI is also used for economical high-quality steel production with low nitrogen and very low phosphorus and hydrogen content. The nitrogen content of tapped steel in the EAF decreases from 0 100 ppm N for 100% steel scrap charges to 15 5 ppm for DRI charges. Low nitrogen limits of the steel products require extensive Ar purging and/or vacuum degassing [13]. While steel scrap is charged with coal to the EAF, DRI/HBI already contributes carbon to the EAF with 1.0 to.5% C from the gas-based reduction process and 0.1 to 0.% C from the coalbased reduction process. Lime and/or dololime are charged as slag formers. Carbon fines are injected to the EAF in order to reduce FeO in the slag and to generate gas for slag foaming. 1.6 to 1.98 GJ of electric energy are required for scrap melting and superheating. Consumption of electrode graphite ranges from 0.8 to 3.5 kg. 0 to 0 m 3 oxygen are injected for decarburization and refining. Consumption of natural gas for gas burners is up to 5 m 3 ; EAFs with scrap pre-heating technologies show higher gas consumption. Figure 3: Specific emission from process data including the contribution from electricity generation as function of DRI to scrap ratio for steel production routes in EAF (charged with scrap and DRI, 565 kg/tdri) and BF/BOF (approx. 15% scrap) [13]. Benefits of bottom purging in the BOF Purging patterns, especially number, arrangement and types of plugs, flow rates and the kind and quality and shifting points of inert gases have a significant influence on the BOF process and the metallurgical results. Those parameters must be strictly coordinated, otherwise the process gets beyond control and targeted metallurgical results cannot be achieved. Bottom purging (Figure ) permits to get closer to or approach the equilibrium at the end of blowing. As a result, the bath kinetics and mixing energy are strengthened, showing lower carbon at lower oxygen levels at the end of blowing without steel bath over-oxidation [1], [15]. Figure : Improved mixing of steel volume due to bottom gas purging in BOF. September th and 5 th, 01 EUROGRESS, Aachen, Germany
78 A typical indicator for an efficient bottom purging performance is the [C]x[O] product, which, compared to a top-blown operated converter, is much lower, more stable and controllable depending on the produced steel grades. An efficient bottom purging system is characterized by [C]x[O] levels between 0 and 5x10 - and also reflected in a less turbulent refining, reduced slopping potential and reblow rate numbers. Here, higher yields and lower slag volumes are obtained and chemically aggressive and too liquid slag will be minimized. Furthermore, the total oxygen consumption is approximately % and the tapping temperature on average 10 C lower in comparison to the original LD process without bottom purging. The charged flux amount is reduced by 5 to 10% by the improved bath kinetics via bottom purging. The bath kinetics can be verified by the calculation of the mixing energy. Figure 5 demonstrates the influence of the calculated mixing energy related to the equation of Nakanishi on the [C]x[O] product distribution for a vessel with and without bottom purging after end of blowing. The non-purging mode shows a very high deviation of the [C]x[O] products in the range between 6 to 5x10 -. An additional bottom purging system installation increases the mixing energy and simultaneously leads to a considerable decrease in the [C]x[O] levels and their fluctuation after end of blowing. Furthermore, the process is more flexible and the adjustment of targeted [C]x[O] products or p levels can be controlled better. Figure 6 shows the behavior of the [C]x[O] products after bottom purging commissioning in comparison to the parallel vessel in operation without bottom purging. As a result, the [C]x[O] levels are decreased by 8 to 10x10 - including a reduction of the [C] levels on average of 0.015% by 00 ppm lower [O] contents in liquid steel bath after end of blowing (Figure 7) [1], [15]. Figure 7: [C]x[O] levels dependent on the process philosophy, i.e. with or without bottom purging [1]. Benefits of bottom purging in the EAF Bottom purging systems based on gas injection through a single tube or multi-hole plugs that are either buried in the EAF hearth ramming mix (i.e., indirect purging) or in contact with the steel melt (i.e., direct purging) have been developed. However, currently, direct purging systems with a multi-hole design represent the majority of bottom purging systems in EAFs in the steel industry worldwide; for example, the RHI direct purging plug (DPP) series (Figure 8). Overall, approximately 9% of EAFs are equipped with bottom gas purging systems today and with a common trend towards more cost-efficient EAF operations in the steel industry, the tendency towards bottom gas purging is increasing. Globally, RHI delivers RADEX DPP plugs to more than 80 customers for EAFs with tap weights between 6 and 50 tons. Figure 5: Influence of the mixing energy on the [C]x[O] levels [1]. Figure 6: Course of [C]x[O] levels with and without bottom purging [1]. Figure 8: Improved mixing of steel volume due to bottom gas purging in EAF. The EAF process benefits realized using direct gas purging systems are related to an overall increased steel bath movement as well as increased mixing between the lower and upper steel melt volumes. The specific reported benefits of RADEX DPP bottom gas purging systems include: 1) Increased thermal and temperature homogeneity in the steel melt: Decreased melting time of scrap and DRI Increased heat transfer during the superheating period Increased efficiency of power transfer Decreased specific electrical energy demand Decreased deviation between the measured steel temperature in the EAF and the ladle furnace Avoidance of skull formation or debris in the EAF hearth after tapping ( clean furnace ) 57 th International Colloquium on Refractories 01 Refractories for Metallurgy
79 ) Increased chemical homogeneity in the steel melt: Increased metal yield Increased use of secondary ferrous raw materials (e.g. DRI, HBI, HM) Decreased deviation between the measured carbon content in the EAF and the ladle furnace Increased yield from alloy addition Increased rate of carbon oxidation, in particular for hot metal charges [C] x [O] levels closer to equilibrium conditions, resulting in lower (FeO) levels in slag, less alloy addition, better alloy prediction, and more stable ladle furnace operations Improved dephosphorization Improved efficiency of oxygen injection 3) Generation of gas bubble columns in the steel melt: Avoidance of instantaneous or retarded boiling in the steel melt In Figure 9 the (FeO) levels are shown for low carbon heats without DPP gas purging and with DPP gas purging. Gas purging improves mixing of steel melt and improves decarburization at the oxygen injectors. Highest (FeO) levels above 5% are avoided by gas stirring at [C] levels below 0.15, indicating higher yield of low carbon steels. Besides improved decarburization of the steel melt, the lower (FeO) level of the process slags provides savings potential for carbon fines injection into the slag. The typical benefits observed in a series of case studies with customers with very specific targets for the RADEX DPP system included a 0.018-0.07 GJ (5 0 kwh ) electrical energy saving, a 0.5 minute decrease in power-on-time, and a 0.5% increase in yield. Bottom gas purging systems are claimed Figure 10: BOF emission classification. ered that for an efficient slag splashing operation, an (MgO) saturation in slag is commonly aimed for, otherwise the splashed slag does not adhere properly to the lining, resulting in higher (MgO) carrier consumption and rising slag viscosity and limited metallurgical slag activity at least. That is why an optimized maintenance strategy has to be adapted to the regional BOF production philosophy to meet the expected vessel lifetime and metallurgical requirements from the economic and environmental point of view [1], [15], [16], [17]. Efficient bottom purging is characterized by an increase in yield by to % and a reduction of flux addition consumption of 5 to 10% in comparison to a BOF operation without bottom purging and excessive slag splashing practice. Furthermore, slag volume is decreased by 8 to 10% and the same target carbon levels can be realized by lower oxygen consumption after end of blowing. Including all significant benefits of bottom purging, a calculation in comparison to a BOF process without bottom purging and an intensive slag splashing practice were carried out. Here, the production situation (tapping weight of 30t per vessel) before and after a successful BOF bottom purging implementation were analyzed, also taking an adapted maintenance strategy and the same produced steel grades into account. The results are listed in Table 1, leading to a total saving of 11. kg of emissions. Table 1: BOF specific emission savings for a 30t BOF due to bottom purging. Emission type With Bottom Purging Without bottom purging Delta Figure 9: Decrease in over-oxidized EAF slag at low carbon levels due to gas purging of steel melt indicating increased oxygen efficiency, higher yield and lower Fe losses. to have the shortest payback time compared to other measures that increase EAF energy efficiency [5]. BOF emission figures and savings due to bottom purging Basically, the BOF emission can be clustered into three main classes (Figure 10), including an average emission range between 70 to 95 kg [11], [1]. Focusing on the three different emission initiators, the largest share is caused by indirect emissions. Here, bottom purging is essential to ensure and guarantee an optimized BOF process decreasing flux addition, oxygen and energy consumption, operating with higher yield and lower slag volumes and reblow rate numbers. Slag maintenance such as slag splashing reduces the vessel availability and also the reaction volume, facing problems in yield, decarburization and dephosphorization. It has to be consid- Direct emissions, kg Emissions from imports of electricity, heat or steam, kg- Indirect emissions, kg Total, kg /t crude steel 5.3 57.6-3.3 7.3 76.8 -.5 1681.8 1786. -10. 1808. 190.6-11. EAF emission figures and savings due to bottom purging emission of the EAF production route comprises direct emissions (Figure ) from the application of charge coal, carbon fines, and natural gas, partly as a reducing agent, (FeO) + [C] = [Fe] + {}, partly for the provision of chemical energy by combustion. Indirect contributions are due to the import of electric ener- September th and 5 th, 01 EUROGRESS, Aachen, Germany
80 gy, use of lime, doloma and basic refractory products, and transport of goods. Specific direct emission values depend significantly on the input materials. For the EAF process, direct emissions range from lower than 0.100 t (use of high quality steel scrap) to 360 0 kg and 0kg (use of high amounts of DRI). The increased material conversion efficiency due to the installation of a DPP gas purging system in a 65 t EAF decreased process-related direct carbon emissions by 0.66 kg C (Table ) as a result of lower coal addition, lower electrode consumption and increased yield. When the decreased burner gas consumption was included, the direct emission reduction was 3.7 kg /t crude steel. Indirect reductions were due to decreased electrical energy demand (i.e., 0.363 kg /kwh el local specific electricity emission value): 3.1 kg. Analogous calculations for optimization trials of an existing DPP bottom gas purging system in a 16 t EAF demonstrated decreasing specific electrical energy demand of 0 kwh,, indicating considerably higher saving potential for electricity related emissions. Table : EAF specific emission savings for a 65 t EAF due to bottom purging. Emission type Direct emissions, kg Emissions from electricity import Indirect emissions, kg Total, kg /t crude steel With Bottom Purging Without bottom purging Delta 63.0 66.7-3.7 169.9 173.0-3.1 50.9 51.8-0.9 83.8 91.5-7.7 Conclusion: Increased concerns over the considerable global climate changes, a minimization of emissions and an increase in energy efficiency are crucial to ensure competitiveness and to lower environmental impacts in future. In order to reduce the emissions from an integrated steel plant, optimization potential from the raw material, process, maintenance philosophy and energy management point of view has to be analyzed and quantified. With a focus on the BOF vessel, an implementation of a bottom inert gas purging system correlates with a significant increase in process efficiency in comparison to an operation using a top-blowing oxygen lance only. Here, the process is more stable and better controllable, resulting in higher yields and improved metallurgical key parameters with lower consumptions of fluxes and oxygen per heat. The saving potential (e.g., direct emissions, emissions from electricity, heat or steam, indirect emissions) of gas purging systems in BOF and EAF is in the order of 3-6% for a typical European steel plant. It is emphasized that Gas purging systems represent an available and proven technological option for steelmaking Gas purging systems are claimed as the most cost-effective available technology for increasing energy efficiency in steelmaking [5] Besides emission reduction potential, further cost savings of gas purging systems are beneficial as increased process safety and control (BOF, EAF), decreased electric energy costs (EAF), and decreased alloy addition (BOF, EAF). Literature: [1] World Steel Association, World Steel in Figures 013. [] Laplace Conseil, Impacts of energy market developments on the steel industry, OECD steel committe, Paris, 013. [3] Gerspacher A., Arens M., Eichhammer W., Zukunftsmarkt Energieeffiziente Stahlherstellung, Karlsruhe, 011. [] Scholz, R., Pluschkell, W., Spitzer K. and Steffen, R. Steigerung der Stoff- und Energieeffizienz sowie Minderung von Emissionen in der Stahlindustrie, Chemie Ingenieur Technik, 00, 76, 1318. [5] Jones, D.L. (01), United States Environmental Protection Agency, Available and emerging technologies for reducing greenhouse gas emissions from the iron and steel industry, North Carolina, 78 p. [6] Margolis, N. and Brindle, R., Energy and Environmental Profile of the U.S. Iron and Steel Industry, Report for U.S. DoE, Energetics Inc, Columbia. [7] Türk, A., Liebmann, L., Steininger K. W., Energy Investment Strategies And Long Term Emission Reduction Needs, Eisern Projekt Bericht, Wien, November 01. [8] Kirschen, M., Badr, K., Cappel, J. and Drescher A., Intelligent refractory system: a cost effective method to reduce energy consumption and emissions in steelmaking, RHI Bulletin No. (010), pp. 3-8. [9] Mathiesen, L. and Maestad O., Achieving global emission reductions by an incomplete climate agreement, Climate Policy and the steel industry, p. 6, 00. [10] Hasanbeigi, A., Price, L. and Aden N., A comparison of iron and steel production energy use and energy intensity in China and the US, Ernest Orlando Lawrence Berkeley National Laboratory, June 011. [11] Larsson, M. and Dahl J., Reduction of the specific energy use in an integrated steel plant the effect of an optimization model, ISIJ International, Vol. 3 (003), No. 10, pp. 166-1673. [1] Neelis, M. and Patel, M. (006), Long-term Production, Energy Consumption and Emission Scenarios for the Worldwide Iron and Steel Industry. Utrecht University, Copernicus Institute, Utrecht, Netherlands. [13] Kirschen, M., Badr, K., Pfeifer, H., Influence of direct reduced iron on the energy balance of the electric arc furnace in steel industry. Energy, Vol. 36 (011), pp. 616-6155. [1] Kollmann, T., Influence of Bottom Purging on the Metallurgical Results, Master s Thesis, University of Leoben, Austria, 010. [15] Nakanishi, K., Nozaki, T., Kato, Y., Suzuki, K., Physical and Metallurgical Characteristics of Combined Blowing Processes, Proceedings of the 65th Steelmaking Conference of AIME, Earrendale, 198, pp. 101-108. [16] Choudhary, S., Ajmani, S., Evaluation of Bottom Stirring System in BOF Steelmaking Vessel Using Cold Model Study and Thermodynamic Analysis, Research and Development Division, Tata Steel, 006, pp. 1171-1176. [17] Messina, C., Slag splashing in the BOF worldwide status, practise and results, Iron and Steel Engineer, Vol. 6 (1996), p.p 17-19. 57 th International Colloquium on Refractories 01 Refractories for Metallurgy