Steel is the most important material for the

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1 Improvement of energy efficiency in EAF steelmaking With ever increasing costs for electricity and the environmental penalty from the production of CO 2 during steelmaking, plant operators must continuously seek improvements in energy efficiency. Based on its world-class steelmaking operations and as a supplier of integrated chemical energy packages for EAFs, Badische Stahl-Engineering and Badische Stahlwerke provide useful data on performance improvement and energy and CO 2 reduction. Authors: Andreas Opfermann, Dirk Riedinger, Sebastian Baumgartner and Alexander Grosse Badische Stahl-Engineering GmbH (BSE) Steel is the most important material for the development of a modern society. Its applications are manifold and the convenience of our modern life would not be possible without it. The production of steel, however, emits CO 2 either from the production of electrical energy or the input of chemical energy both of which have a negative impact on the environment so the aim should be minimisation of energy consumption and emissions by optimising the process under local conditions. Figure 1 shows a range of energy sources, ranging between 0.2 and 0.4kg/kWh of emitted CO 2, however, depending on local conditions no free choice may be available. The prices for CO 2 certificates and electrical energy are shown in Figure 2. It is apparent that certificates are linked with the energy price in times of rising energy costs, but they do not follow declining prices, therefore, to minimise CO 2 costs, optimisation of EAF operation is essential. r Fig 1 CO 2 emissions for burning different energy sources ENERGY CONSUMPTION AND EAF OPERATION AT BADISCHE STAHLWERKE (BSW) The energy demand of the EAF may be summarised by the following main parameters: input materials, energy efficiency, operational efficiency and energy recovery systems. Input materials are often dictated by local circumstances. In Europe and North America, for instance, scrap is the main charge material, whereas in China, scrap is supplemented by hot metal, and in India by hot metal and DRI. Due to the availability of natural gas in the Middle East 100% DRI operation is standard there. All these differences have a huge impact on the energy demand but cannot be changed in most cases due to cost optimisation and raw material availability. Also, energy recovery such as by scrap preheating is either r Fig 2 Prices for electrical energy and the CO 2 certificates from Oct 2004 to Jan 2007 a 65

2 Losses during PON Losses during POFF, kwh/min/t Gas wastage during melting and refining During melting 0.4 Electrical energy wastage with poor foaming slag During flat bath 1.7 Performance loss due to non-standard performance Between heats < 30 min 0.5 Between heats 30 min 0.2 r Table 1 Losses during PON and POFF PON (avg 28.5 min) POFF (avg 10.2 min) 90 MVA transformers Set-up time 7.5 min/heat 71.1 MW avg power input with two bucket charge Delays, min/heat Electricity consumption 363kWh/t billet Operational x VLB in side wall with 3.8MW burners Mechanical 0.41 and 1,400-1,800Nm 3 /t O 2 injection 4 x O 2 injectors for post combustion Electrical x C injection Cranes 0.07 O Nm 3 /t billet CH 4 5.1Nm 3 /t billet Use of delay recording and optimisation C injected 8.6kg/t billet Charge carbon 6.8kg/t billet r Table 2 BSW operational data 2008 r Fig 3 Influences on the energy demand of the EAF p Fig 4 Furnace design fixed and/or cannot be used due to input material or environmental issues. Energy efficiency and operational performance can, however, be changed. In Figure 3 the factors affecting efficiency and operational performance are shown. During power-on (PON) the electrical efficiency is influenced by electrode regulation and the arc efficiency in the slag during the flat bath phase. The chemical energy input from oxygen and/or fuel depend on the hardware and the operational excellence of the crew. For instance, burners situated in high positions or injection of oxygen in the off-gas by wrong operation, increase consumption but do not improve the process. Equally, power-off (POFF) delays increase energy consumption, thus optimisation is necessary in the charging, tapping and turnaround in order to minimise delays. The energy losses per minute of delay are shown in Table 1. At BSW there are two 100t AC furnaces using 100% scrap, producing typically 93t of wire rod and rebar grades per heat (2.2Mt in 2008). Our objective is to reach the highest productivity by running high power input with high chemical energy input and optimised delays. The furnace design is shown in Figure 4. Table 2 shows plant performance. In Figure 5 data from steel plants around the world are shown for the production of commodity steel with t tap weight. The PON-times depend on the transformer power and the energy consumption, but note that the POFF-times vary significantly, ranging from 66

3 Fuel Calorific value, MJ/Nm 3 Natural gas Light oil, diesel, kerosine Coke oven gas 17 LPG 128 r Table 3 Calorific value of fuels Element oxidised Reaction energy kwh/kg element Reaction energy kwh/nm 3 O 2 Reactions in steel bath Si Mn Cr Fe-Fe 2 O Fe-FeO C Al Mo S P Reactions in gas phases C CO-CO H r Table 4 Reaction energies r Fig 5 Steel plant comparisons r Fig 6 Oxygen consumption as function of energy consumption a 67

4 r Fig 7 Carbon consumption as function of energy consumption r Fig 9 Use of burners in melting down r Fig 8 Energy consumption for different furnace types 10.6min/heat at BSW up to 34.9min/heat in the worst case. Long POFF-times mean higher energy losses. In Figures 6 and 7 the energy consumption for the same data set are shown as a function of oxygen and carbon input to the furnace. Efficient operation in this case means low combined energy consumption of electricity and chemical inputs. In both cases the gases and carbon are not used efficiently in all cases. The example of BSW shows that high productivity is possible with a moderate oxygen input of 39Nm 3 /t billet and a carbon input less than 15kg/t billet. r Fig 10 Fuel efficiency during melting down IMPROVEMENT OF THE PROCESS BASICS In Figure 8 the energy consumptions for different furnace types from the BSE database with 100% solid material input are shown. The large differences with the AC furnaces relate to the amount of DRI charged and also because, in some cases, special steels are made. The only furnace type which can get down to 300kWh/t is the shaft furnace with up to 80kWh/t of preheating power and use of additional burners in the shaft for heating, if required. With Consteel the preheating power is small (BSE experience approximately 20kWh/t) and burners cannot be used for additional energy input during melting. Irrespective of the furnace type and the material input mix the objective is to optimise the usage of chemical energy and reduce the electrical input. Figure 9 illustrates the reduction in cold spots by use of burners. The calorific value of some potential fuels are shown in Table 3. Figure 10 shows how fuel efficiency from a free burning flame reduces as the scrap height decreases during melting. Figure 11 illustrates how total energy consumption decreases as oxygen input increases. 68

5 r Fig 11 Relationship between oxygen consumption and total energy consumption Injection of oxygen oxidises not only carbon but other elements in the furnace, as indicated in Table 4 and, on average, about 3.5-5kWh of chemical energy can be created/nm 3 O 2. As only about 0.5-1kWh of electrical energy are needed for the production of 1Nm 3 of oxygen, a benefit of 2.5-4kWh/Nm 3 is generated. In Figure 12 the efficiency of the arc and the power of the furnace transformers are shown as a function of the furnace diameter. Foaming of the slag and covering of the arc results in a higher efficiency of electrical energy input in the flat bath phase. It is this covering that makes the production with UHP furnaces possible without destruction of the refractories although high power input is also strongly influenced by the regulation of the electrodes. Without foamy slag arc heat transfer efficiency is only 36%, compared to % with a foamy slag. An effective regulation system improves electrical energy transfer to the charge. An example is shown in Figure 13. With large fluctuations (ie, wide spread as shown on the left-hand side) the current is not efficiently regulated, whereas on the right-hand side these fluctuations are minimised by optimisation resulting in a smaller standard deviation and higher power input. As well as optimisation of the existing system, the use of reliable and efficient tools for chemical energy input help optimise furnace operation. BSE offers a wide range of tools for sidewall and door use as shown in Figure 14. r Fig 12 Energy consumption and furnace design IMPROVEMENT OF FURNACE OPERATION SOME PRACTICAL RESULTS BSE and BSW have wide experience in the optimisation of energy efficiency of steelmaking shops. Examples of projects are given below. a 69

6 r Fig 13 Electrode regulation improvement by decreasing fluctuation Influence of post combustion (PC) on energy consumption Tests were conducted with and without PC on a 90t EAF. Yield increased but the specific energy consumption remained the same due to the higher tap weight. Approximately 2kWh/Nm 3 of PC oxygen was used in addition to the burner operation, but a lower yield resulted. PC can increase the productivity (29.6 min PON without PC, 28.2 min with PC) but this has to be paid for with yield losses. r Fig 14 Chemical energy tools Operation with different tools Tests with the combined use of the Lance Manipulator (LM) and Virtual-Lance-Burner (VLB), as well as with VLB-sidewall injectors only, were conducted on a 50t furnace. From BSE experience the LM is an efficient tool in the door area both for melting and refining. In the first case all metallurgical oxygen was injected via the sidewall and lancing was started early. In the second case lancing was delayed and a special burner and lancing flame using the VLBs was used, with metallurgical oxygen injected by sidewall and door tools. The energy consumption decreased from 411 to 386kWh/t billet with the same amounts of fuel and oxygen used. r Fig 15 Use of hot metal instead of pig iron Operation with the use of hot metal A series of tests were performed replacing solid pig iron with hot metal on a furnace equipped with two O 2 -C LMs and one VLB on the EBT platform. The aim was to reduce energy consumption. The results are shown in Figure 15. Similar to projects in China and India, the performance and the energy consumption decreased tremendously. Influence of carbon charged and optimised oxygen operation In these tests several variables were trialled: ` Charge carbon amount was varied 70

7 r Fig 16 Decarburisation rate with VLBs ` Modified alloying practice ` Optimised VLB operation All the measures resulted in SiMn/FeSi alloy consumption decreases of almost 1kg/t. For casts with <700kg charged carbon per heat, yield increased by 1% when compared to casts with >700kg/t. It was calculated that the efficiency of charge carbon reached 74% in this test period. Other furnace parameters did not alter significantly. Influence of charge carbon positioning In order to minimise the evolution of flames which are produced during scrap charging with carbon on the bucket bottom, tests were performed whereby carbon was packed inside the scrap. However, both the yield and the carbon content in the first sample decreased, ie, the efficiency of the carbon utilisation was lowered. r Fig 17 Installation of VLB and LM in 90t scrap furnace Measurement of oxygen efficiency with VLBs To evaluate the efficiency of the VLBs, decarburisation tests were conducted during commissioning a 124t EAF with scrap plus 20-30% HM in the charge and a 110t EAF with coal-based DRI plus 40-50% HM. Both furnaces have four VLBs with 1,800Nm 3 /h lancing. Figure 16 shows that between 1.60% and 0.2%C in the bath, decarburisation is linear, whereas below 0.2% the decarburisation rate decreases. For the C +O = CO reaction an oxygen efficiency of 74% was calculated. IMPROVEMENT OF FURNACE OPERATION INSTALLATION OF CHEMICAL ENERGY PACKAGES With more than 40 VLB-systems, 187 lance manipulators, and 19 LM.2s (lance manipulator with temperature sampling robot), BSE has a wide experience in a range r Fig 18 Installation of VLB 180t scrap furnace a 71

8 r Fig 19 Installation of VLB, LM and regulation in a 100t scrap charged furnace of chemical energy systems and furnace optimisation. Three installations will now be described as examples of energy efficiency resulting from the chemical packages installed. In a 90t scrap-charged furnace (see Figure 17) gas consumption was slightly increased while the oxygen consumption decreased. There was an improvement of PON-time and power consumption of 10% over the former performance. The same technique was used in the 180t furnace shown in Figure 18. Four existing burners were reused to lower the total investment costs but four additional VLBs were installed. In this case the oxygen consumption was unchanged, but gas consumption was increased slightly in order to achieve an increased burner power. In Figure 19 an optimised operation with new chemical energy tools and new regulation on a 100t furnace is shown. In this case the efficiency of the arc was increased by optimised regulating hardware and improved parameters. This resulted in an energy consumption decrease of 12% while PON-time could be reduced by 19% through increased power input. As well as the optimisation of the chemical energy input by hardware or software, BSE is also optimising furnace operations with existing electrode regulators. In Figure 20 the measurements are shown before and after the optimisation of the regulation of a DC furnace. This optimisation resulted in an increase in power input from 50.6MW to 56.9MW and a decreased PON-time of 36.3 min instead of 39 min. SUMMARY With increasing costs for energy and CO 2 emission certificates, efficient production as well as optimisation of furnace operation are the main objectives of EAF steelmakers. By efficient chemical energy tools electrical energy consumption can be reduced, with optimised electrode regulation the efficiency of the electrical energy transfer can be increased, and with changes in furnace operation further efficiency savings are possible. The tests described from BSE, BSW and its customers have shown improved results following installation of the BSE chemical energy package. Examples of ongoing research by BSE include energy increase and resource efficiency linked to a reduction of CO 2 emissions and scrap preheating and its impact on emissions and energy efficiency. MS r Fig 20 Optimisation of the regulation of a DC furnace Andreas Opfermann, Dirk Riedinger, Sebastian Baumgartner and Alexander Grosse are with Badische Stahl-Engineering GmbH, Kehl, Germany. CONTACT: 72