In the steel industry, environmental concerns and the

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1 Rotry herth furnce dvnced design methods A computtionl fluid dynmic model ble to simulte industril heting furnces hs been used to help revmp the rotry herth furnce t TenrisDlmine pipe mill. The model is ble to evlute the fields for fluid dynmic, therml nd chemicl species inside the furnce, together with fine scle representtion of the heting process for the processed chrge. This furnce is equipped with TRGX flmeless regenertive burners nd TRX flmeless roof burners, so minimising pollutnt emissions nd mximising furnce efficiency through intense ir preheting. Authors: Alessndro Dell Rocc, Mssimilino Fntuzzi, Vlerio Bttgli nd Enrico Mlf Tenov LOI Itlimpinti nd Centro Sviluppo Mterili S.p.A. In the steel industry, environmentl concerns nd the rising cost of fossil fuels hs focused ttention on minimising pollution nd mximising heting furnce process efficiency. Additionlly, demnds to improve chrge temperture uniformity without incresing cpitl nd operting costs from cceptble limits in order to mintin competitive edge, re ever present. Thus, ny project to redesign reheting furnce must ccomplish ll those trgets. Mny ttempts to model the heting process inside furnce hve been mde over the yers, with different levels of detil nd ccurcy. Since it is well known tht rdition to the chrge is the min het trnsfer mechnism, simplified models re commonly used to nlyse performnce nd evlute the efficiency of reheting furnces. Wellestblished zonl methods rnge from single- or twodimensionl models for preliminry estimtion of chrge temperture, to three-dimensionl models ble to ccount for the edge nd skid effects. Nevertheless, they cnnot give indictions bout the locl combustion processes tht develop inside the furnce volume, nor cn they give n indiction of the expected emissions. From the experimentl side, existing mesurement methods unfortuntely cnnot give continuous mesurement of temperture inside furnce. Triling thermocouples or dt loggers cn be used only with test chrges to give n indiction of the heting process t few mesurement points inside the chrge, while opticl pyrometers or therml cmers give informtion only on the chrge surfce nd re hevily ffected by the presence of scle. Therefore, in recent yers two min developments hve been on-going thnks to continuous increse in computtionl power: ` On-line 3D control tools bsed on existing zonl model extended to simulte trnsient rditive het trnsfer ` 3D Computtionl Fluid Dynmic (CFD) simultions of the complete furnce, coupling the recting flow combustion models developed for the single burner, with rditive nd conjugted het trnsfer models for the chrge heting process This rticle is focused on the definition of CFD model ble to evlute the fields for fluid dynmic, therml nd chemicl species quntities inside furnce, together with fine scle representtion of the heting process of the chrge, nd thus ble to simulte the lrge rotry herth furnce t TenrisDlmine pipe mill. This furnce is equipped with TRGX flmeless regenertive burners nd TRX flmeless roof burners to minimise pollutnt emissions nd mximising furnce efficiency through intense ir preheting. Estblished CFD methodology ws integrted with the computtionl fluid dynmics pproch tht Tenov pplies for the design, verifiction nd continuous improvement of combustion systems. Using this successful combintion, complete multiscle methodology for furnce design ws developed, nd excellent results on industril instlltion nd opertion re evident. CFD METHODOLOGY The physicl nd numericl model currently dopted for industril burner design, verifiction nd improvement, hs been extensively vlidted ginst experimentl dt from CSM test rigs. A complete dtbse of CFD simultions for ll Tenov FlexyTech burners of different sizes nd t discrete turndown levels, is continuously populted with new dt from current R&D CFD ctivities between CSM nd Tenov. This cretes the fundmentl knowledge bse required for the successful ppliction of different combustion technologies, such s flmeless nd regenertive combustion. Regenertive burners, in prticulr, pose technologicl issues for ll reheting furnces with nrrow chmbers, such s those of rotry herth furnces. In order to hve the chrcteristic cycling behviour between the regenertion nd firing phses for ech burner unit, it is necessry to 112

2 FORMING PROCESSES CFD model setup Fluid Turbulence Chemistry Thermophysicl properties Idel gs k-ω with Sher Stress correction (Wilcox) Reynolds Stress model Finite Rte / Eddy Dissiption with A = 4 nd B = 0.5 Eddy Dissiption / Arrhenius rte of rection Westbrook & Dryer reduced kinetic mechnism Mixture: Density: idel gs Specific het, viscosity nd therml conductivity: idel gs mixing lw Mss diffusivity: kinetic theory r Fig 1 TRGX flme tempertures instll double-size burners. As consequence, flme length increses, so rising concerns bout the possible ingestion of prtilly rected mixture by the opposite burner, with detrimentl effects both on combustion efficiency nd regenertive bed mintennce costs. In order to evlute this effect nd design flmeless regenertive FlexyTech burners ble to cope with these issues, Tenov strted CFD design project. The results of this study nd those for the other burners equipping the rotry herth furnce, is briefly reviewed becuse they re t the very bse of the entire CFD pproch used for the complete furnce simultion. Design of the short flme version of the TRGX flmeless regenertive burner The TRGX burner ws relised in 2006 when Tenov integrted flmeless technology with regenertive combustion in order to obtin burner ble to gurntee pollutnt nd energy reduction. These burners represent the ltest design genertion nd cn work both in flme mode (for cold ignition) nd flmeless mode (to rech the best performnce in terms of NOx emissions). Thnks to coupled gs nd ir stging, working in flmeless mode, NOx emissions re reduced to 35-40ppm. The first development stges of the TRGX burner were crried out in coopertion with CSM by mens of n itertive procedure consisting of the design of the burner prototypes bsed on the experience gined in the engineering of the previous TSX (flmeless) nd TRG (regenertive) models, extensive CFD modelling optimising the burner design, lbortory furnce tests nd industril furnce tests. The selection of the physicl models hs been bsed on previous extensive vlidtion work dedicted to evlute the performnce of the different turbulence representtions for simulting high velocity round jets nd combustion schemes for nturl gs. Flmeless combustion is chieved through intensive Species: Specific het: corrected for dissocition (Pelders) Viscosity nd therml conductivity: kinetic theory Rdition Discrete Ordinte model Absorption coefficient: WSGGM* r Tble 1 Reference operting conditions for the CFD simultion *Weighted sum of Gry Gses Model is modelling technique used to clculte the rditive therml flux emitted by gses produced during the combustion of fuel recircultion of hot wste gses inside the furnce chmber, promoted by high momentum ir jets exiting from the burner diffuser. As consequence, the flme length is prticulrly high compred with trditionl regenertive burners. This effect is further exggerted s the regenertive burners instlled re of double size in order to ccount for their regenertion nd firing phses, s evidenced bove. The min concern in the project ws to mintin high levels of ir dilution while developing the rection zone inside more compct volume thn with the originl long flme from TRGX flmeless regenertive burners. The technicl solution ws found by imprting improved swirl to the combustion ir flow, while mintining the ir jet perimeter to fixed vlue in order to dilute it with hot wste gses t pproximtely the sme level s long flme TRGX burners. Additionlly, the number of fuel injection lnces were doubled in order to give more uniform fuel distribution round the swirling ir jet. The entire design nd verifiction ctivity ws crried out relying on the CFD models well consolidted during the continuous coopertionl ctivities between CSM nd the R&D deprtment t Tenov. The modelling set-up for the simultion of single nturl gs burner (see Tble 1) hs been dopted. In prticulr, two-step reduced mechnism hs been selected s the best compromise between ccurcy nd CPU time to simulte the nturl gs combustion process. The resulting performnce in terms of expected flme length inside the 6m long experimentl chmber t the CSM test rig re shown in Figure 1 where flme 113

3 ny concerns bout extension of flme length for their smller therml power output. CFD nlysis of TRX flmeless roof burner Zones 4, 5, 6 nd 7 of the TenrisDlmine furnce re equipped with TRX flmeless roof burners (see Figure 2). The development ctivities for this burner fmily hve resulted in lrge dtbse of numericl simultion results. Different turndown levels, ir excess vlues, ir preheting tempertures nd fuel stging rtios re simulted for nerly ll the vilble roof burner sizes. Zones 4, 5, 6 nd 7 re equipped entirely with TRX 4. r Fig 2 FlexyTech TRX 4 burner temperture iso-surfce coloured by CO 2 mole frction (kmol/kmol) for the trditionl long flme FlexyTech TRGX burner (top) nd the short compct flme FlexyTech TRGX burner (bottom) re represented. It hs to be highlighted how the rection zone, mrked by the iso-temperture surfce t 1,500 C, is properly confined inside the test chmber for the short flme version compred to the trditionl long flme TRGX flmeless regenertive burner. After this first development phse, lrge number of tests were crried out t CSM test fcilities in Dlmine in order to verify therml lod profiles on test furnce wlls nd emissions levels. The development progrm ws successful nd short flme TRGX 16 burners were successfully instlled in zone 1 nd zone 2 of the rotry herth furnce t TenrisDlmine, while long flme TRGX 14 were instlled in zone 3 without MULTI-SCALE APPROACH FOR THE CFD SIMULATION OF A COMPLETE FURNACE The physicl nd numericl model used for single burner design, improvement nd verifiction phses is the strting point for extending these CFD models to the complete furnce nd nlysing possible interction effects between the burners. The min problems in conducting this nlysis using CFD re: ` The lrge number of computtionl cells required to discretise the different geometry scles (burner ir nd fuel holes, furnce dimensions, chrge dimensions) ` The modelling of movement of the chrge inside the furnce To represent the burner elements with sufficient detil nd to obtin sufficient sptil resolution in the region of flme interctions, computtionl mesh with tens of million cells is required. Consequently, to reduce the computtionl time, procedure hs been developed to reduce the cell number. Tking into ccount tht more thn 50% of the cells re required for representing the shrp grdients round the burners, it is evident tht multiscle pproch for the whole furnce simultion is required (see Figure 3). The method developed nd vlidted for every furnce r Fig 3 Multiscle pproch for the CFD simultion of complete reheting furnce 114

4 FORMING PROCESSES Zone Burner model Burner type Number of burners Fuel flow rte, Nm 3 /h Air flow rte, Nm 3 /h 1 TRGX16 Regenertive flmeless ,154 2 TRGX16 Regenertive flmeless ,285 3 TRGX14 Regenertive flmeless ,593 4 TRX4 Flmeless TRX4 Flmeless TRX4 Flmeless TRX4 Flmeless r Tble 2 Burner detils simultion extrcts the profiles of the relevnt vribles t the burner outlet from the lrge dtbse of CFD simultions vilble nd pplies those vlues s fixed boundry condition ptches to the complete furnce model. To fcilitte the ppliction of this pproch in cses with multiple burners, dedicted softwre hs been developed tht llows the profile rottion/trnsltion in the position required by the complete furnce grid domin nd the interpoltion between vilble profiles for the simultions in the dtbse. Moreover, the representtion of chrge heting requires time-dependent clcultions due to the movement inside the furnce, nd consequently very high computtion time. To overcome this limittion, multi-scle pproch, including time, hs been dopted. A coupling between the CFD clcultion tht itertively performs the sttionry simultion of the recting flow inside the furnce (furnce fluid model) with the unstedy simultion of the conduction inside the chrge during its dvncement inside the furnce (chrge solid model). This is possible due to the different chrcteristic time scles of the two processes: the fluid dynmic time scle is in the order of tenths of seconds, while the scle of solid heting is in the order of seconds. Hence, the chrge surfce temperture cn be ssumed constnt in time during the fluid-dynmic clcultion, while the resulting het flux on the blooms cn be ssumed s fixed boundry condition during the simultion of chrge movement inside the furnce. The solid simultion returns the updted chrge surfce temperture to the fluid simultion in n itertive process tht stops when the difference between the surfce temperture in two successive itertions is less thn tolernce vlue. The chrge model, developed in the User-Defined-Function (UDF) nd mcros frmework of Ansys FLUENT, hs been presented in detil nd successfully compred with experimentl heting curves nd with simplified model in which the steel chrge movement is represented by continuous solid moving t constnt velocity (equivlent strip concept) nd the chrge. APPLICATION CASE: A 200TPH ROTARY HEARTH FURNACE The good results of vlidtion work nd the limited CPU time required, indicte tht the CFD furnce coupled model r Fig 4 Computtionl domin for the complete rotry herth furnce cn be pplied to the simultion of reheting furnce where the nlysis of fluid dynmic, flmes interction, detiled representtion of furnce wll nd steel chrge temperture nd het fluxes is necessry. The model hs, therefore, been pplied for the nlysis of the revmped configurtion of TenrisDlmine rotry herth furnce. The min gols of the new design were productivity (+35%) by incresing of the width of the furnce chmber to chrge lrger blooms (up to 5,300mm) nd the therml efficiency enhncement (+15%) by instlling regenertive combustion system. The computtionl domin nd burners used in the furnce re shown in Figure 4 nd Tbles 2, 3 nd 4. Ech single burner is considered in the following working conditions: Zones 1, 2 nd 3 re regenertive zones, nd s such only hlf of the burner is in firing mode, while the others re in regenertion mode using hot wste gses from the furnce in order to prehet their regenertor beds. A typicl firing pttern from the furnce control system ws dopted for the CFD simultion to represent the het distribution inside the furnce. While Tble 3 is relevnt to the whole furnce, Tble 4 is relevnt to the reference operting conditions of the zones for the CFD simultion. 115

5 Production,t/h Totl therml power, kw Fuel therml power, kw Air therml power, kw Consumption, Nm 3 /h ,550 67,110 26, r Tble 3 Reference operting conditions for the CFD simultion Zone Fuel therml input, kw Air therml input, kw Power split,% Zone turndown,% 1 6,612 3, ,523 9, ,900 7, ,940 2, ,210 1, , , r Tble 4 Reference operting conditions for ech zone r Fig 5 Polyhedrl clusteristion The velocity components, turbulence kinetics energy nd dissiption, temperture, species concentrtion profiles obtined t defined section (outlet) hve been pplied s boundry condition to represent the 253 burners instlled in the complete furnce. The boundry conditions of the complete furnce CFD model re the following: ` Furnce wlls: mixed (rdition nd convective het trnsfer) wll boundry condition with mteril conductivity nd thickness for lterl wll, roof, bottom nd tunnel ` Burners: velocity inlets with profile of velocity components, turbulence, temperture nd species concentrtions extrcted by single burner simultions ` Wste gses outlet: pressure outlet ` Chrging door: trnsprent window exchnging rdition het flux with outside mbient t 25 C ` Furnce wll holes: trnsprent windows exchnging rdition het flux with outside mbient t 25 C ` Dischrging door: trnsprent window exchnging rdition het flux with outside mbient t 25 C. Since the representtion of this very lrge reheting furnce, bsed on the grid sensibility nlysis performed for single zone nd on the previous vlidtion test for wlking hert furnce, requires in the order of 100 million cells, the lgorithm hs been implemented in the ltest version of Ansys FLUENT. This llows clustering of the tetrhedrl elements in polyhedrl element to be dopted. An exmple of the result of polyhedrl trnsformtion of the grid for the ner burner zone of the burner, together with the comprison of temperture profiles t two xil loctions is shown in Figure 5. The unstructured grid size in the cse of the rotry herth furnce is reduced by 75% for totl number of bout 25 million cells. Figure 6 shows the generl behviour of the furnce temperture nd velocity fields for plne t the level of lterl burner inlets. The interction of the burner jets with the flue gs strem producing the observed bending of the flmes in zones 1 to 3 is evident. The heting curves (see Figure 7) lso confirms tht for incresed productivity in the revmped furnce, it is necessry to mintin n extended soking zone to homogenise the chrge temperture nd comply with the temperture uniformity trgets for the bloom t the exit both in rdil (from the surfce to the centre ΔTmx = 18 C) nd in longitudinl directions (ΔTmx = 6 C). Simultion shows ΔTmx = 433 C nd ΔTmx = 23 C respectively, during the heting chrge heting long the furnce. A 3D view of heting process nd surfce temperture mps on the furnce chrge is shown in Figure 8, with the bloom surfce temperture mps t the exit of ech furnce zone. 116

6 FORMING PROCESSES A detiled nlysis of the het flux on the bloom (see Figure 9) identifies tht, s expected, the min contribution is rdition (red line) with the exception of in-out zone where the wll temperture is the lowest (700 C) nd the recircultion zones re generted by the presence of the furnce bffles nd ploughshre. On the contrry, in zone 3 the rdition het flux is unexpectedly negtive, indicting tht the bloom surfce temperture is higher thn t the wlls. The high vlue of the convective het flux indictes tht this nomly is due minly to the impingement of the side wll burner flmes on the bloom surfce. This is rther detiled mount of informtion compred with those vilble from the other models, s discussed erlier. Looking t furnce het blnce (see Figure 10) it is possible to evlute the efficiency of the furnce fter revmping. Therefore the increse of combustion ir temperture for the side burners due to the instlltion of regenertive system from 450 C to 890 1,120 C increses the efficiency from 45% to 67% nd the mximum production from 160t/h to 215t/h. From the CFD simultion, the different sources of het losses cn lso be evluted. A totl of 8.8% re therml losses from the furnce wlls (5.4%) nd wter cooled ploughshre (3.4%), respectively. The energy in the flue gses from the furnce exit is 63.6%, of which 68% is extrcted by regenertive burners nd recovered t high efficiency, while 32% is in the flue gses exiting form the furnce outlet nd it is recovered by the centrl recupertor t lower efficiency. Figure 11 shows the comprison between the zone temperture mesured fter the revmping performed by Tenov in August 2010, during opertion of the furnce in the condition similr to tht considered in the simultion. The tble shows tht the temperture selected for the furnce control system (the men vlue between the mesured one t the inlet nd the outlet of ech zone) is very close to the clculted verge temperture of the wll zones. r Fig 6 Temperture field nd velocity mgnitude field on the horizontl section plne t hlf furnce height CONCLUSIONS CFD modelling developed for single burner simultion hs been extended to the complete reheting furnce including the effect of chrge movement inside it. A multiscle pproch for representing the burners hs been pplied to reduce the computtionl grid without losing ccurcy, in conjunction with model of the steel chrge tht couples the stedy stte CFD clcultion of the recting flow inside the furnce with the unstedy simultion of the chrge during its dvncement in the furnce. The pproch hs been experimentlly vlidted in previous study. The good results of vlidtion work nd the evlution of required CPU time (5 12 dys) on cluster with 16 processors, demonstrted tht the CFD furncer Fig 7 Heting curves computed from the CFD simultion 117

7 FORMING PROCESSES r Fig 8 Surfce temperture mps for the chrge inside the furnce r Fig 10 Furnnce het blnce r Fig 9 Het flux for the upper point of single bloom during heting coupled model is comptible with the design workflow of n industril furnce. The ppliction of the developed CFD simultion pproch of n industril reheting furnce revmping design nd the comprison with industril opertion dt hs confirmed the potentil of the tool when the nlysis of fluid dynmic, flme interction, detiled representtion of furnce wll losses, steel chrge temperture nd het fluxes is necessry to support the furnce design. Moreover, the vilbility of the chrge surfce temperture, het fluxes nd locl concentrtion of combustion products, potentilly llow multi-physics methodology. CFD could be coupled with structurl nlysis tool to evlute the deformtion of the chrge due to therml grdient or scle growth to evlute the process yield. These re the principl direction for future further reserch between Tenov nd CSM. The reduction of the CPU time due to the current multiscle pproch llows us to envisge further development to cope with the use of lterntive gseous fuels such s coke over, blst furnce nd producer gses, or oxygenenriched combustion ir inside reheting furnces, thus llowing the possibility to evlute in the design phse ll the relevnt detils for the combustion process relised inside the furnce. MS ACKNOWLEDGMENTS We wish to thnk TenrisDlmine (A Cprer nd M Gllino) who mde vilble the reference dt t TenrisDlmine. r Fig 11 Furnce wlls temperture mp (on the inner refrctory surfce) 118 Alessndro Dell Rocc nd Mssimilino Fntuzzi re with Tenov LOI Itlimpinti, Itly, nd Vlerio Bttgli nd Enrico Mlf re with Centro Sviluppo Mterili S.p.A., Itly. Contct: mtti.cnovro@it.tenovgroup.com