STUDY OF THE EXPLOSIONS WHICH TAKE PLACE IN THE RECOVERY BOILERS AND LD CONVERTERS DURING THE PAUSES BETWEEN CHARGES

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1 METAL 6-556, Hradec nad Moravicí STUDY OF THE EXPLOSIONS WHICH TAKE PLACE IN THE RECOVERY BOILERS AND LD CONVERTERS DURING THE PAUSES BETWEEN CHARGES Alexandru Ivanescu, Antoaneta Ene, Lilica Ivanescu, Cosmin Catana, Miltiade Istrate Dunarea de Jos University of Galati, Domneasca, 8 Galati, Romania, address: ivanescu@yahooca; aene@ugalro Abstract The paper presents a research made with the occasion of some explosions which took place in converters and recovery boilers at Iron and Steel Works of Galati, Romania This research has led to a series of hypotheses which permitted, with a rigorous analysis, to take some technical solutions in order to avoid these undesirable events INTRODUCTION During the period 5 5, explosions were produced in recovery boilers of converters at LD steel plants, having variable intensities, manifested from platforms shocks with specific sound effects to movement or bending of the pipes panels Of these, three explosions had great intensities, generating the damage of the safety belts, mobile holder and failure key chute, the detachment of the pipes panels on considerable distances (-6 m) etc, consequences that determined the taking out of work of the recovery boilers IMPORTANT STATISTICAL DATA a) Distribution of explosions on converters is the following: converter explosions % converter explosions % converter explosions 666 % appearance probability The drawn conclusion is that the appearance probability of these explosions is approximately equal for the three converters b) Distribution of explosions on the seven months working is given below: Month No Frequency f, % - April - May - June 66 - July 66 - August - September - October 6 TOTAL: In May, which was a rainy month, the explosions frequency was maxim: % This indicator is very important because it puts in evidence the contribution of the materials humidity used in elaboration to the producing of the explosions in the converter or in the recovery boiler

2 METAL 6-556, Hradec nad Moravicí c) Distribution of explosions on the operating moments of the charge is given below: Operating moment No Frequency f, % Oxygen blowing Converter at test Evacuation of charge 66 Glazing End of charge 66 6 Orifice remaking TOTAL: It can be observed that the maxim frequency of 66% is accomplished at the operated moment marked with, at the end of the steel charge evacuation from converter and the introducing of the materials necessary for the refractory lining glazing (lime and dolomite) In Table are presented the technological parameters of process at the charges with explosions and the controller observations regarding the producing of the explosive phenomena in the recovery boiler or converter These witnesses elucidate some hypotheses regarding the mechanisms of the explosions producing in recovery boilers of LD converters OBJECTIVES OF THE STUDY The main objective of the study represents the establishment of the causes which have produced the explosions in recovery boilers of LD converters [-] In the frame of this objective, the study has to answer to the following: the causes which have generated the explosions; the mechanism of the producing of the explosive gaseous mixtures; the operating and technical solutions to avoid the producing of the explosive gaseous mixtures during oxygen blowing and expressly during the period between the charges PARAMETERS OF COMBUSTIBILITY AND EXPLOSION OF THE GASEOUS PRODUCTS In Table are presented the main parameters of combustibility and explosion of the gaseous products possible existing on the converter-exhauster path [8] Table Parameters of combustibility and explosion of the gaseous products possible existing in the system No Gas Relativ e density Ignition temperatur e Inferior limit of explosion (LIE) Superior limit of explosion (LSE) Temperatu re class Group of explosi on Hydrogen (H ) 56 T II C Carbon oxide (CO) Acetylene (CH CH) Nitrogen peroxide (N O) T II A 5 T II C EXPERIMENTAL RESEARCHES In order to elucidate the explosive phenomena which took place in the recovery boilers gas samples were taken in the operating moments similar to those in which the explosions have been produced, in the period 5-5 []

3 METAL 6-556, Hradec nad Moravicí Table Technological parameters of the charges with explosions N o Charge no before explosion Dat e Moment of charge Oxygen min 6 Nm Explosion hour Conver ter start end Exhaust hour Material for slag Materials for glazing end of charge Lime, CaF, :5 :58 : :5 6 Dolom ite 6 Converter at test : : : :5 Dolom ite Lime, Dolo mite, Durati on, min Coke addition at carbureting Scrap iron t light Number of charges Observations 8 During the blast of 6 Nm a strong booming was heard in the converter Boiler damage Explosion during exhaust Converter at test : 6:5 6: 6: Violent splashing Converter at test 5:8 5:6 5: 5: Without glazing addition Return TC cold charge 5 Fe 8% Converter at test : : : : - Stopping in dolomite Charge exhaust :5 : : : 5 Witho ut glazin g Stopping with dolomite bricks Charge exhaust 5:5 :5 5: 5: Without glazing Slag evacuation Converter at glazing : : :8 : t light Charge exhaust :5 : :5 : Charge exhaust :5 : :5 : - 6 Charge exhaust : : :5 : - - Charge exhaust : : : : End of charge :5 :5 : :8 Dolom ite Converter at glazing Waiting of transfer-car for evacuation 66 Interrupting of blast Boiler damage 86 FeMn Waiting for the boiler 8 5 light 8 Boiler damage FeMn; FeSi; 6 FeV Overflowing minutes : : : : 6 - Without glazing Without glazing : : :6 : Converter at test :55 : :5 : Slag evacuation min Holes adjusting Hole recovery :5 : : :56 - Without glazing addition Glazing :6 : : : - Without glazing addition Standing min at the end of the blast

4 METAL 6-556, Hradec nad Moravicí Charge exhaust 8:5 8: 8: 8: FeMn refining Glazing 5: : :5 : Charge exhaust 8: 8:8 8: 8: Boiler damage Charge exhaust :8 :6 : : FeMn Glazing :5 :8 : : Swinging converter min Explosion in converter and boiler Charge exhaust : :5 : : Swinging converter min End of charge : : : : Expanded slag Swinging converter min Converter at test :5 :8 : : Converter at test : : : : Strong reactions in converter before evacuation Charge exhaust : :8 : : Charge exhaust : :8 : : - Without glazing addition - 66 Explosion at the end of evacuation Charge exhaust :5 : : : During the spear lowering for glazing an explosion at the boiler was produced Table Chemical analyses of the gas samples taken from the recovery boiler in the period 5-85 No Day Boiler level Converter Glazing Chemical analysis Sample Observations Addition CO CO O H CH C n H m NO Duratio n - - m - minutes - % % % % % % % m - - -, 5,6,,, Hole remaking 5 + m - - -b,,,8,, Liquid iron load 5 + m 5 lime -,,,,6,, -,,,5,,, -, 8,,,, Middle of the glazing

5 METAL 6-556, Hradec nad Moravicí 5 + m 6 lime m 6 dolomite m dolomite m 6 lime m m 8 lime m lime -,,,,,, -,, 5,,,, -,,5,,,,8 -,,6 8,,,,6-5,, 6,,8,, -6,,,,8,, -, 8,,,,8 5-,,,,8, - 5-,,8 6,,, - 5-,,,,, - 6-, 8,, - 6-,8,, - 6-, 6,, - 6-,,, ,, - -,, - Charge at test -,,, - 8-,,,, - Charge evacuation 8-, 8,, - Charge evacuation -, 6,, - -, 8,, - -, 8,, - -, 8,, - -,,, - -,, - Charge at test 5

6 METAL 6-556, Hradec nad Moravicí For this, holes with mm (between pipes) were made in the boiler from the converter no, at the levels + m and + m, through which copper pipes with 8 mm inside the radiation tower (approx 5 m from the pipes panel) The aspiration of the gases was made with the aid of a car aspirator prepared for this purpose The gas samples were taken in glass tubes provided with airtight stopcocks in the following situations: at the evacuation of charge; at the glazing of converter masonry; at the liquid iron charging; converter at test; orifice remaking The chemical analyses of the gases taken from the recovery boiler were performed with MSI Multiwam Drägel apparatus and Varian Chromatograph at MITTAL STEEL Galati, Romania and the results are presented in Tables (period -85) and (day 5), respectively Table Chemical analyses of the gas samples taken from the recovering boiler (level + m) of converter no, in the day 5 Glazing Chemical analysis Sampl No Durati CO Addition e CO O on H CH N Observations - minut es - % % % % % % Scrap iron load Liquid iron load lime Glazing with lime and slag without nitrogen blast lime Glazing with lime and slag without nitrogen blast; charge no 8 5 lime Glazing with lime and slag without nitrogen blast 6 approx t liquid slag Glazing with nitrogen blast over viscous slag without lime or dolomite addition Glazing with nitrogen blast over viscous slag without lime or dolomite addition Glazing with nitrogen blast over viscous slag without lime or dolomite addition 5 Interpretation of the results The chemical analyses of the gas samples taken in the pause between the charges put in evidence the existence of hydrogen in variable concentrations during the periods of evacuation or glazing, with maxims representing a serious warning (6% compared to LIE for H which is %) [] In Figures and is presented the variation of hydrogen content during the period of the glazing of the refractory lining of the converter no, on 5 and 5, respectively In conclusion, during the glazing hydrogen is released from the reactions of glazing materials with the liquid slag in a percent which can exceed the inferior limit of explosion (LIE) Having a very small density, hydrogen ascends in the converter and arrives in the radiation tower of the recovery boiler where it finds optimal conditions to form explosive mixtures, especially in the zone between + m and + m It should be mentioned 6

7 METAL 6-556, Hradec nad Moravicí the fact that the reaching of the maxim hydrogen content is achieved in a very short time and could randomly generate explosions as a function of the existing conditions, the greater the hydrogen content, the stronger the explosions Figure The H content variation in the glazing period on 5 Figure The H and O content variation during glazing on 5 5 The mechanisms of the explosions that could take place in the recovery boilers of the LD converters in the period between charges a) The explosion generated by hydrogen The grade of water vapors dissociation in normal conditions of temperature and pressure is reduced It increases and becomes important as the temperature increases and especially in the presence of some catalysts such as slag or liquid steel The inferior limit of explosion for hydrogen is relatively small and this level can be reached in fractions of seconds during the charge evacuation or glazing of the converters refractory lining The liquid slag determines

8 METAL 6-556, Hradec nad Moravicí the dissociation by shock of water from the humid addition materials (lime or sintering dolomite) The hydrogen release during glazing depends on the slag quantity and fluidity, humidity and quantity of addition materials Hydrogen can also appear during charge evacuation due to the humidity of the deoxidizing and alloying materials, as happened some years ago in an accident at converter no 8 in OLD steel plant at MITTAL STEEL of Galati, Romania, where the falling of a quantity of humid sludge in the converter generated three successive explosions At the contact between the humid sludge and the liquid slag a thermal shock is realized under whose action the water dissociation generates hydrogen which is aspirated by the exhauster and in the recovery boiler it meets the aspired air in exhauster forming inflammable gaseous mixtures It is not excluded that, besides hydrogen, another combustion and oxidant gases participate to the forming of the explosive mixture b) The explosion generated by carbon oxide The carbon oxide has the characteristic that it forms bags (accumulations of CO) in the peripheral zones of the radiation tower, where the limit layer of velocity is great, especially when the exhauster rotation speed is small These bags of CO can explode when the local content of CO exceeds the value of 5% of LIE and the oxygen content is greater than 5%, as can be seen in Figure In the pause between the charges the CO content is small and the O content is great so that the formed gaseous mixture is out of the explosion zone The inferior limit of explosion (LIE) Failure zone Failure zone Danger zone Warning zone The superior limit of explosion (LSE) Attention zone Figure The explosion diagram for CO c) The explosion generated by acetylene and nitrogen peroxide 8

9 METAL 6-556, Hradec nad Moravicí The acetylene is a combustion gas that can be obtained in the circumstances that water is poured over carbide and this is possible to realize in the recovery boilers of the LD converters Carbide can appear in the dust depositions on the converter walls as a result of some reducing processes with CO of some powders of manganese, chrome, titan etc oxides, which play the role of catalysts Accepting the idea that the water can accidentally arrive in the boiler, either from the hydraulic closing of the failure chute or from the fissuring of the boiler pipes, in this way we achieve the conditions of acetylene generation which mixing with air forms an explosive atmosphere The nitrogen peroxide N O can be formed during the glazing of the converters refractory lining when the nitrogen jet acts by shock upon the sintering dolomite introduced in order to increase the MgO content of the glazing layer The reactions which take place during glazing are: liquid slag H O H + O ; N O MgO shock + + MgO+ N O jet The reaction is reversible (MgO behaves as a catalyst) and unrolls function of local temperature obtained at the nitrogen jet impact with the introduced material for the enrichment of liquid slag with MgO At a temperature of 5 o C the formed nitrogen peroxide, together with hydrogen, generates a detonator gaseous mixture In order to avoid the peroxide formation, the glazing maxim duration of 5 minutes must be respected, so that the local temperature should be greater than 5 o C 6 SOLUTIONS OF EXPLOSION PREVENTING AND CONCLUSIONS On each working shift the control of the additive materials humidity used for steel making and for glazing of the LD converter refractory masonry must be performed Appropriate quantities of liquid slag and addition materials (lime, sintering dolomite) must be established for glazing so that the maxim hydrogen content in the gases which arrive in the recovery boilers should be less than % The maxim glazing duration must be of 5 minutes The dust and slag depositions must be periodically eliminated on the gases evacuation path so that the value of the differential pressure should be maintained to a value as close as the projected one The recovery boilers must be equipped with gas analyzers placed at the levels +m or +m in order to monitor the evolution of these gases and especially of the hydrogen so that the inferior limit of explosion (% LIE for H ) should not be exceeded BIBLIOGRAPHY OPREA, F, IVANESCU, A etc Theory of the metallurgical processes, Bucharest: Editura Didactica si Pedagogica, 8 TRIPSA, I, KRAFT, N Steel elaboration in oxygen converters, Bucharest: Editura Tehnica, 6 WINNACKER, K Inorganic chemical technology, Bucharest: Editura Tehnica, 6 BRABIE, V, etc Steel elaboration and casting technology, Bucharest: Editura Didactica si Pedagogica, 5 MARINESCU, M, etc Burning Units, Bucharest: Editura Tehnica, 8 6 BASTKNECHT, W Explosionen, Berlin, Heidelberg, New York: Springer Verlag, 8 DRAGOMIR, I Theory of the iron and steel processes, Bucharest: Editura Didactica si Pedagogica, 86 8 Contract ICEM SA Bucharest, Analysis of the burned gases on the evacuation path from the converter to the exhauster, Contract INSEMEX Petrosani, The event analysis on the burned gases path at OLD plant of Iron and Steel Works of Galati, 5