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Practical experience in a twin shell EAF with supersonic oxygen/carbon injection and scrap preheating via oxy-fuel burnersThe original twin shell furnace system at ArcelorMittal Differdange had higher electrical energy consumption than desired. Modifications to equipment and the process led to significant improvements in furnace reliability, efficiency and productivity, and the plant was able to reach operating and cost-efficiency levels comparable to the most modern of furnaces.

The twin shell DC furnace at ArcelorMittal Differdange (AMD) is equipped with a 160MVA transformer, spray

cooled shell with a total inside volume of 150m3, an inside diameter of 7.6m and a liquid steel capacity of 170t. Tapping weight is 155t.

OpEraTing iSSuES priOr TO rEvampSeveral problems reduced both the furnace productivity and efficiency. One was poor performance of the supersonic burners which were affected by burner box water leaks and clogging. This led to long downtime and consequent energy losses through burner maintenance. Frequent water leaks also represented a safety risk. Each furnace had four supersonic refining burners on boxes, two regular burners and two post-combustion oxygen injectors on the wall panels (see Figure 1).

JETBOX inSTaLLaTiOnBecause of these problems AMD decided to test an alternative system from PTI. Both systems work to bring the burner and oxygen jets close to liquid steel in order to enhance efficiency. This has become the industry standard over recent years, however there are substantial differences in the systems in terms of reliability and operation.

The first PTI JetBOx™ was installed on one of the shells in 2007. Initial results indicated significantly improved reliability, with reduced maintenance delays and increased furnace efficiency. Based on these test results AMD decided to replace all the original combined burners with PTI JetBOxes.

Since then the sidewall burners have also been replaced and post-combustion injectors removed. This has resulted in a significant reduction in power-off time, from 38secs per heat in 2007 to less than 7secs in 2009. Energy

authors: Olivier Gervais and Jaroslav BrhelArcelorMittal Differdange and PTI Europe s.r.o.

r Fig 1 Furnace configuration prior to revamp

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Description unit Before after Diff.Tapping weight t 145.6 147.6 +2.0Power input MW 102.2 104.8 + 2.6Tap-to-tap time min 58.6 54.3 - 4.3Power-on time min 37.6 36.4 -1.2Power off – regular min 21.1 17.9 - 3.2Maintenance delays min 7.1 4.5 - 2.6Electrical consumption kWh/t 440 431 - 9Natural gas Nm3/t 11.2 11.3 + 0.1Oxygen consumption Nm3/t 36.0 35.5 - 0.5Electrode consumption kg/t 1.54 1.22 - 0.32Carbon bulk kg/t 9.5 8.5 - 1.0Carbon injected kg/t 8.9 8.5 - 0.4Scrap consumption kg/t 1135 1135 0 Productivity t/h 149 162 + 13

Description unit Before after Diff.Tapping weight t 147.6 150.3 + 2.7Power input MW 104.8 100 - 4.8Tap-to-tap time min 54.3 51.8 -2.5Power-on time min 36.4 35.9 -0.5Power off – regular min 17.9 15.9 -2.0Maintenance delays min 4.5 2.5 -2.0Electrical consumption kWh/t 431 398 -33Natural gas consumption Nm3/t 11.3 12.9 +1.6Oxygen consumption Nm3/t 35.5 40 +4.5Electrode consumption kg/t 1.22 1.05 -0.17Carbon bulk kg/t 8.5 6.5 -2.0Carbon injected kg/t 8.5 7.5 -1.0Scrap consumption kg/t 1135 1125 -10 Productivity t/h 162 174 +12

r Table 1 Operating parameters summary before and after burners changed (annual data)

r Table 2 Operating parameters summary before and after scrap preheating burners (annual data)

consumption and power-on time were also positively affected (see Figures 2, 3 & Table 1).

The life of the burner and the boxes has also been extended (the original 2007/8 JetBOxes are still working). This longevity is linked to their robustness and the quarterly review and repair approach used in the AMD workshop.

uppEr ShELL rEvampThe original spray-cooled furnace upper shell design had several limitations, the most significant of which was insufficient cooling, which often limited the electrical power input and triggered long power-off times. Also, to avoid water leakages, the EAF power was regularly stopped on each heat (using temperature measurement on the shell walls). As a result, melting and foaming was not optimised, which significantly increased maintenance costs.

To address these issues, in 2008 AMD replaced the spray cooled shell with a pipe-cooled panel design. This enabled better cooling, better scrap packing and higher average power input and, in combination with the PTI JetBOx system, provided good reliable equipment and a starting point for further process optimisation.

prOcESS OpTimiSaTiOnAlthough reliable equipment is important, the method of operation also has an important effect on overall plant performance. Therefore, significant effort has been made in optimising the process to improve furnace performance. One major process change is a significant increase in scrap preheating using burners.

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preheating practice before and after revamping Figure 4 shows the operation pattern before and after the changes. Previously, productivity was optimised by reducing downtime. In this mode, when the first basket is being melted in furnace 1 (F1), furnace 2 (F2) was prepared. Then, as soon as the F1 1st basket is molten, the electrode is switched over to F2 to melt its 1st basket. During this time the F1 2nd basket is charged, and so on; alternating between shells. This twin shell operating mode enables turnaround times to be ‘hidden’, except for moving the electrode arm and changing the electrode. However, although productivity is optimised by this method, energy consumption is not, since liquid steel is always waiting in one shell.

Following modifications the two baskets in F1 are melted consecutively rather than alternatively and within the shell that is not being electrically heated, thus there is now time to preheat the first basket in F2 with the burner equipment. In this mode energy losses are reduced but additional downtime is required to charge the second basket.

The new practice includes regular scrap preheating after charging prior to the introduction of electrical power using all six PTI burners (4 x JetBOx plus 2 in the wall). Each burner is fired at 6MW capacity to the scrap charge of 110t. The 17min preheating period of all six burners delivers about 10MW/hr of chemical energy power to the first charge. The burners are fired stoichiometrically so all the energy delivered comes from natural gas burnt with pure oxygen. The other shell operates with electrical power in superheating and refining stage at the same time.

Long-term practice shows that efficiency of energy

r Fig 3 Power-on time and energy consumption after burners were changed

r Fig 2 Power-off time due to burner maintenance – seconds/heat

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introduction by PTI burners matches the efficiency of electrical energy introduction. It is important to note that this practice brings significant cost savings because:` Natural gas energy is substantially cheaper than

electrical power` Electrodes are not consumed during preheating by

burners ` Furnace stress due to arc operation is much lower

which results in improved life of water-cooled panels, electrodes clamps and all other wearing parts

` The arc starts to operate on hot scrap which provides more rapid stabilisation and more consistent electrical power input.

Key operating improvements by preheating are summarised in Table 2 (yearly values).

carBOn inJEcTiOn A high-power DC furnace with a very long arc is highly sensitive to foaming slag quality. Slag foaming is affected by many factors such as slag chemical composition and temperature, and hence viscosity. The method of carbon injection and efficiency is one of the key influences on foamy slag maintenance. Carbon injection is traditionally performed by several sidewall injection lances usually integrated into burner boxes as originally done at AMD, or installed separately. Another option is for carbon injection by burners that produce a shrouded flame and use supersonic oxygen. This type of burner, the PTI QuadJetTM burner, has been installed at AMD.

QuaDJET OpEraTiOnThe QuadJet incorporates all the functions of a supersonic oxygen and carbon injector and can be operated up to 6MW as a burner and with 2,200Nm3/h O2 as a supersonic oxygen injector with up to 120kg/min of carbon injection (see Figure 5). Carbon can be injected in burner or supersonic mode with equal efficiency and without loss of efficiency of the supersonic stream.

The objective of the QuadJet burner is to improve the carbon particle penetration under the slag, thus increasing efficiency and providing better control of slag FeO content. Oxygen and gas profiles were modified in order to keep an approximately equal amount of oxygen as before the Quad installation, but distributed differently. Oxygen

r Fig 5 PTI QuadJet burner and carbon stream in the flame

r Table 3 Average slag composition before and after QuadJet carbon injection

% in slag Fe total mnO SiO2 caO al2O3 mgO cr2O3 FeOBefore injection 32.3 6.6 9.6 18.4 7.8 5.2 3.3 41.0After injection 28.1 6.8 12.5 21.8 8.8 6.8 2.3 36.0

Delta -4.2 +0.2 +2.9 +3.4 +1.0 +1.6 -1.0 -5.0

r Fig 4 EAF operation patterns (B indicates when the burners are working, PC when there is post-combustion, RF when refining

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injection starts approximately three minutes earlier than without the QuadJets and during the final five minutes of superheating, the two burners inject carbon and oxygen simultaneously, while the two JetBOxes are fired at minimal power. The majority of the carbon is injected during these last five minutes to reduce FeO in the slag. The injected carbon quantity was 550kg on average, and injected oxygen was 400Nm3. Slag samples were analysed before and after QuadJet carbon injection and the average data from 30 heats are shown in Table 3.

The results show that the Fe content of the slag can be reduced significantly in a relatively short time by efficient carbon injection, even with simultaneous oxygen injection. It is important to manage the balance between injected carbon and oxygen to ensure the necessary carbon excess which is required in order to reduce FeO.

These results provided several process improvements:

` Rapid adjustment of slag composition to reduce high

FeO and increase viscosity for good foaming ability` Yield improvement due to lower FeO.

Longer term data are required to quantify yield impact and potentially lower refractory consumption.

cOncLuSiOnSThe original twin shell furnace system at AMD had higher electrical energy consumption than desired. Modifications to equipment and the process enabled significant improvements in furnace reliability, efficiency and productivity, and the plant was able to reach operating and cost efficiency levels comparable to the most modern furnaces. Excellent operating reliability gave the operators confidence and provided a good outlook for continuous improvements. MS

Olivier Gervais is meltshop manager at ArcelorMittal Differdange, Luxembourg. Jaroslav Brhel is director at PTI Europe s.r.o., Prague, Czech Republic.