MILLENNIUM STEEL 2013 112 Rotary hearth furnace advanced design methods A computational fluid dynamic model able to simulate industrial heating furnaces has been used to help revamp the rotary hearth furnace at TenarisDalmine pipe mill. The model is able to evaluate the fields for fluid dynamic, thermal and chemical species inside the furnace, together with a fine scale representation of the heating process for the processed charge. This furnace is equipped with TRGX flameless regenerative burners and TRX flameless roof burners, so minimising pollutant emissions and maximising furnace efficiency through intense air preheating. I n the steel industry, environmental concerns and the rising cost of fossil fuels has focused attention on minimising pollution and maximising heating furnace process efficiency. Additionally, demands to improve charge temperature uniformity without increasing capital and operating costs from acceptable limits in order to maintain a competitive edge, are ever present. Thus, any project to redesign a reheating furnace must accomplish all those targets. Many attempts to model the heating process inside a furnace have been made over the years, with different levels of detail and accuracy. Since it is well known that radiation to the charge is the main heat transfer mechanism, simplified models are commonly used to analyse performance and evaluate the efficiency of reheating furnaces. Well- established zonal methods range from single- or two- dimensional models for preliminary estimation of charge temperature, to three-dimensional models able to account for the edge and skid effects. Nevertheless, they cannot give indications about the local combustion processes that develop inside the furnace volume, nor can they give an indication of the expected emissions. From the experimental side, existing measurement methods unfortunately cannot give continuous measurement of temperature inside a furnace. Trailing thermocouples or data loggers can be used only with test charges to give an indication of the heating process at a few measurement points inside the charge, while optical pyrometers or thermal cameras give information only on the charge surface and are heavily affected by the presence of scale. Therefore, in recent years two main developments have been on-going thanks to a continuous increase in computational power: ` On-line 3D control tools based on existing zonal model extended to simulate transient radiative heat transfer ` 3D Computational Fluid Dynamic (CFD) simulations of the complete furnace, coupling the reacting flow Authors: Alessandro Della Rocca, Massimiliano Fantuzzi, Valerio Battaglia and Enrico Malfa Tenova LOI Italimpianti and Centro Sviluppo Materiali S.p.A. combustion models developed for the single burner, with radiative and conjugated heat transfer models for the charge heating process This article is focused on the definition of a CFD model able to evaluate the fields for fluid dynamic, thermal and chemical species quantities inside a furnace, together with fine scale representation of the heating process of the charge, and thus able to simulate the large rotary hearth furnace at TenarisDalmine pipe mill. This furnace is equipped with TRGX flameless regenerative burners and TRX flameless roof burners to minimise pollutant emissions and maximising furnace efficiency through intense air preheating. Established CFD methodology was integrated with the computational fluid dynamics approach that Tenova applies for the design, verification and continuous improvement of combustion systems. Using this successful combination, a complete multiscale methodology for furnace design was developed, and excellent results on industrial installation and operation are evident. CFD METHODOLOGY The physical and numerical model currently adopted for industrial burner design, verification and improvement, has been extensively validated against experimental data from CSM test rigs. A complete database of CFD simulations for all Tenova FlexyTech® burners of different sizes and at discrete turndown levels, is continuously populated with new data from current R&D CFD activities between CSM and Tenova. This creates the fundamental knowledge base required for the successful application of different combustion technologies, such as flameless and regenerative combustion. Regenerative burners, in particular, pose technological issues for all reheating furnaces with narrow chambers, such as those of rotary hearth furnaces. In order to have the characteristic cycling behaviour between the regeneration and firing phases for each burner unit, it is necessary to
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Rotary hearth furnace advanced design methodsA computational fluid dynamic model able to simulate industrial heating furnaces has been used to help revamp the rotary hearth furnace at TenarisDalmine pipe mill. The model is able to evaluate the fields for fluid dynamic, thermal and chemical species inside the furnace, together with a fine scale representation of the heating process for the processed charge. This furnace is equipped with TRGX flameless regenerative burners and TRX flameless roof burners, so minimising pollutant emissions and maximising furnace efficiency through intense air preheating.
In the steel industry, environmental concerns and the rising cost of fossil fuels has focused attention on
minimising pollution and maximising heating furnace process efficiency. Additionally, demands to improve charge temperature uniformity without increasing capital and operating costs from acceptable limits in order to maintain a competitive edge, are ever present. Thus, any project to redesign a reheating furnace must accomplish all those targets.
Many attempts to model the heating process inside a furnace have been made over the years, with different levels of detail and accuracy. Since it is well known that radiation to the charge is the main heat transfer mechanism, simplified models are commonly used to analyse performance and evaluate the efficiency of reheating furnaces. Well-established zonal methods range from single- or two-dimensional models for preliminary estimation of charge temperature, to three-dimensional models able to account for the edge and skid effects. Nevertheless, they cannot give indications about the local combustion processes that develop inside the furnace volume, nor can they give an indication of the expected emissions.
From the experimental side, existing measurement methods unfortunately cannot give continuous measurement of temperature inside a furnace. Trailing thermocouples or data loggers can be used only with test charges to give an indication of the heating process at a few measurement points inside the charge, while optical pyrometers or thermal cameras give information only on the charge surface and are heavily affected by the presence of scale. Therefore, in recent years two main developments have been on-going thanks to a continuous increase in computational power:
` On-line 3D control tools based on existing zonal model extended to simulate transient radiative heat transfer
` 3D Computational Fluid Dynamic (CFD) simulations of the complete furnace, coupling the reacting flow
Authors: Alessandro Della Rocca, Massimiliano Fantuzzi, Valerio Battaglia and Enrico Malfa Tenova LOI Italimpianti and Centro Sviluppo Materiali S.p.A.
combustion models developed for the single burner, with radiative and conjugated heat transfer models for the charge heating process
This article is focused on the definition of a CFD model able to evaluate the fields for fluid dynamic, thermal and chemical species quantities inside a furnace, together with fine scale representation of the heating process of the charge, and thus able to simulate the large rotary hearth furnace at TenarisDalmine pipe mill. This furnace is equipped with TRGX flameless regenerative burners and TRX flameless roof burners to minimise pollutant emissions and maximising furnace efficiency through intense air preheating.
Established CFD methodology was integrated with the computational fluid dynamics approach that Tenova applies for the design, verification and continuous improvement of combustion systems. Using this successful combination, a complete multiscale methodology for furnace design was developed, and excellent results on industrial installation and operation are evident.
CFD METHODOLOGY The physical and numerical model currently adopted for industrial burner design, verification and improvement, has been extensively validated against experimental data from CSM test rigs. A complete database of CFD simulations for all Tenova FlexyTech® burners of different sizes and at discrete turndown levels, is continuously populated with new data from current R&D CFD activities between CSM and Tenova. This creates the fundamental knowledge base required for the successful application of different combustion technologies, such as flameless and regenerative combustion.
Regenerative burners, in particular, pose technological issues for all reheating furnaces with narrow chambers, such as those of rotary hearth furnaces. In order to have the characteristic cycling behaviour between the regeneration and firing phases for each burner unit, it is necessary to
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recirculation of hot waste gases inside the furnace chamber, promoted by high momentum air jets exiting from the burner diffuser. As a consequence, the flame length is particularly high compared with traditional regenerative burners. This effect is further exaggerated as the regenerative burners installed are of double size in order to account for their regeneration and firing phases, as evidenced above.
The main concern in the project was to maintain high levels of air dilution while developing the reaction zone inside a more compact volume than with the original long flame from TRGX flameless regenerative burners.
The technical solution was found by imparting improved swirl to the combustion air flow, while maintaining the air jet perimeter to a fixed value in order to dilute it with hot waste gases at approximately the same level as long flame TRGX burners. Additionally, the number of fuel injection lances were doubled in order to give a more uniform fuel distribution around the swirling air jet. The entire design and verification activity was carried out relying on the CFD models well consolidated during the continuous cooperational activities between CSM and the R&D department at Tenova.
The modelling set-up for the simulation of a single natural gas burner (see Table 1) has been adopted. In particular, a two-step reduced mechanism has been selected as the best compromise between accuracy and CPU time to simulate the natural gas combustion process.
The resulting performance in terms of expected flame length inside the 6m long experimental chamber at the CSM test rig are shown in Figure 1 where flame
install double-size burners. As a consequence, flame length increases, so raising concerns about the possible ingestion of partially reacted mixture by the opposite burner, with detrimental effects both on combustion efficiency and regenerative bed maintenance costs.
In order to evaluate this effect and design flameless regenerative FlexyTech® burners able to cope with these issues, Tenova started a CFD design project. The results of this study and those for the other burners equipping the rotary hearth furnace, is briefly reviewed because they are at the very base of the entire CFD approach used for the complete furnace simulation.
Design of the short flame version of the TRGX flameless regenerative burner The TRGX burner was realised in 2006 when Tenova integrated flameless technology with regenerative combustion in order to obtain a burner able to guarantee pollutant and energy reduction. These burners represent the latest design generation and can work both in flame mode (for cold ignition) and flameless mode (to reach the best performance in terms of NOx emissions). Thanks to coupled gas and air staging, working in flameless mode, NOx emissions are reduced to 35-40ppm.
The first development stages of the TRGX burner were carried out in cooperation with CSM by means of an iterative procedure consisting of the design of the burner prototypes based on the experience gained in the engineering of the previous TSX (flameless) and TRG (regenerative) models, extensive CFD modelling optimising the burner design, laboratory furnace tests and industrial furnace tests. The selection of the physical models has been based on previous extensive validation work dedicated to evaluate the performance of the different turbulence representations for simulating high velocity round jets and combustion schemes for natural gas.
Flameless combustion is achieved through intensive
CFD model setupFluid Ideal gasTurbulence k-ω with Shear Stress correction (Wilcox) Reynolds Stress modelChemistry Finite Rate / Eddy Dissipation with A = 4 and B = 0.5 Eddy Dissipation / Arrhenius rate of reaction Westbrook & Dryer reduced kinetic mechanismThermophysical Mixture: properties Density: ideal gas Specific heat, viscosity and thermal conductivity: ideal gas mixing law Mass diffusivity: kinetic theory Species: Specific heat: corrected for dissociation (Pelders) Viscosity and thermal conductivity: kinetic theoryRadiation Discrete Ordinate model Absorption coefficient: WSGGM*
r Table 1 Reference operating conditions for the CFD simulation
*Weighted sum of Gray Gases Model is a modelling technique used to calculate the radiative thermal flux emitted by gases produced during the combustion of a fuel
r Fig 1 TRGX flame temperatures
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temperature iso-surface coloured by CO2 mole fraction (kmol/kmol) for the traditional long flame FlexyTech® TRGX burner (top) and the short compact flame FlexyTech® TRGX burner (bottom) are represented. It has to be highlighted how the reaction zone, marked by the iso-temperature surface at 1,500°C, is properly confined inside the test chamber for the short flame version compared to the traditional long flame TRGX flameless regenerative burner.
After this first development phase, a large number of tests were carried out at CSM test facilities in Dalmine in order to verify thermal load profiles on test furnace walls and emissions levels.
The development program was successful and short flame TRGX 16 burners were successfully installed in zone 1 and zone 2 of the rotary hearth furnace at TenarisDalmine, while long flame TRGX 14 were installed in zone 3 without
any concerns about extension of flame length for their smaller thermal power output.
CFD analysis of TRX flameless roof burner Zones 4, 5, 6 and 7 of the TenarisDalmine furnace are equipped with TRX flameless roof burners (see Figure 2). The development activities for this burner family have resulted in a large database of numerical simulation results. Different turndown levels, air excess values, air preheating temperatures and fuel staging ratios are simulated for nearly all the available roof burner sizes. Zones 4, 5, 6 and 7 are equipped entirely with TRX 4.
MULTI-SCALE APPROACH FOR THE CFD SIMULATION OF A COMPLETE FURNACE The physical and numerical model used for single burner design, improvement and verification phases is the starting point for extending these CFD models to the complete furnace and analysing possible interaction effects between the burners.
The main problems in conducting this analysis using CFD are:` The large number of computational cells required to
discretise the different geometry scales (burner air and fuel holes, furnace dimensions, charge dimensions)
` The modelling of movement of the charge inside the furnace
To represent the burner elements with sufficient detail and to obtain sufficient spatial resolution in the region of flame interactions, computational mesh with tens of million cells is required. Consequently, to reduce the computational time, a procedure has been developed to reduce the cell number. Taking into account that more than 50% of the cells are required for representing the sharp gradients around the burners, it is evident that a multiscale approach for the whole furnace simulation is required (see Figure 3).
The method developed and validated for every furnace
r Fig 2 FlexyTech® TRX 4 burner
r Fig 3 Multiscale approach for the CFD simulation of a complete reheating furnace
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can be applied to the simulation of reheating furnace where the analysis of fluid dynamic, flames interaction, detailed representation of furnace wall and steel charge temperature and heat fluxes is necessary. The model has, therefore, been applied for the analysis of the revamped configuration of TenarisDalmine rotary hearth furnace. The main goals of the new design were productivity (+35%) by increasing of the width of the furnace chamber to charge larger blooms (up to 5,300mm) and the thermal efficiency enhancement (+15%) by installing a regenerative combustion system. The computational domain and burners used in the furnace are shown in Figure 4 and Tables 2, 3 and 4.
Each single burner is considered in the following working conditions:
Zones 1, 2 and 3 are regenerative zones, and as such only half of the burner is in firing mode, while the others are in regeneration mode using hot waste gases from the furnace in order to preheat their regenerator beds. A typical firing pattern from the furnace control system was adopted for the CFD simulation to represent the heat distribution inside the furnace.
While Table 3 is relevant to the whole furnace, Table 4 is relevant to the reference operating conditions of the zones for the CFD simulation.
simulation extracts the profiles of the relevant variables at the burner outlet from the large database of CFD simulations available and applies those values as fixed boundary condition patches to the complete furnace model. To facilitate the application of this approach in cases with multiple burners, dedicated software has been developed that allows the profile rotation/translation in the position required by the complete furnace grid domain and the interpolation between available profiles for the simulations in the database.
APPLICATION CASE: A 200TPH ROTARY HEARTH FURNACEThe good results of validation work and the limited CPU time required, indicate that the CFD furnace coupled model
r Fig 4 Computational domain for the complete rotary hearth furnace
Zone Burner model Burner type Number of burners Fuel flow rate, Nm3/h Air flow rate, Nm3/h 1 TRGX16 Regenerative flameless 13 95 1,1542 TRGX16 Regenerative flameless 22 188 2,2853 TRGX14 Regenerative flameless 22 200 2,5934 TRX4 Flameless 48 19 2255 TRX4 Flameless 52 14 1676 TRX4 Flameless 52 8 967 TRX4 Flameless 44 8 101
Figure 6 shows the general behaviour of the furnace temperature and velocity fields for a plane at the level of lateral burner inlets. The interaction of the burner jets with the flue gas stream producing the observed bending of the flames in zones 1 to 3 is evident.
The heating curves (see Figure 7) also confirms that for increased productivity in the revamped furnace, it is necessary to maintain an extended soaking zone to homogenise the charge temperature and comply with the temperature uniformity targets for the bloom at the exit both in radial (from the surface to the centre ΔTmax = 18°C) and in longitudinal directions (ΔTmax = 6°C). Simulation shows ΔTmax = 433°C and ΔTmax = 23°C respectively, during the heating charge heating along the furnace.
A 3D view of heating process and surface temperature maps on the furnace charge is shown in Figure 8, with the bloom surface temperature maps at the exit of each furnace zone.
The velocity components, turbulence kinetics energy and dissipation, temperature, species concentration profiles obtained at a defined section (outlet) have been applied as boundary condition to represent the 253 burners installed in the complete furnace. The boundary conditions of the complete furnace CFD model are the following:
` Furnace walls: mixed (radiation and convective heat transfer) wall boundary condition with material conductivity and thickness for lateral wall, roof, bottom and tunnel
` Burners: velocity inlets with profile of velocity components, turbulence, temperature and species concentrations extracted by single burner simulations
radiation heat flux with outside ambient at 25°C` Furnace wall holes: transparent windows exchanging
radiation heat flux with outside ambient at 25°C ` Discharging door: transparent window exchanging
radiation heat flux with outside ambient at 25°C.
Since the representation of this very large reheating furnace, based on the grid sensibility analysis performed for single zone and on the previous validation test for a walking heart furnace, requires in the order of 100 million
Production,t/h Total thermal power, kW Fuel thermal power, kW Air thermal power, kW Consumption, Nm3/h187 93,550 67,110 26,440 36.8
r Table 3 Reference operating conditions for the CFD simulation
r Table 4 Reference operating conditions for each zone
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A detailed analysis of the heat flux on the bloom (see Figure 9) identifies that, as expected, the main contribution is radiation (red line) with the exception of in-out zone where the wall temperature is the lowest (700°C) and the recirculation zones are generated by the presence of the furnace baffles and ploughshare. On the contrary, in zone 3 the radiation heat flux is unexpectedly negative, indicating that the bloom surface temperature is higher than at the walls. The high value of the convective heat flux indicates that this anomaly is due mainly to the impingement of the side wall burner flames on the bloom surface. This is a rather detailed amount of information compared with those available from the other models, as discussed earlier.
Looking at furnace heat balance (see Figure 10) it is possible to evaluate the efficiency of the furnace after revamping.
Therefore the increase of combustion air temperature for the side burners due to the installation of regenerative system from 450°C to 890–1,120°C increases the efficiency from 45% to 67% and the maximum production from 160t/h to 215t/h. From the CFD simulation, the different sources of heat losses can also be evaluated. A total of 8.8% are thermal losses from the furnace walls (5.4%) and water cooled ploughshare (3.4%), respectively. The energy in the flue gases from the furnace exit is 63.6%, of which 68% is extracted by regenerative burners and recovered at high efficiency, while 32% is in the flue gases exiting form the furnace outlet and it is recovered by the central recuperator at lower efficiency.
Figure 11 shows the comparison between the zone temperature measured after the revamping performed by Tenova in August 2010, during operation of the furnace in the condition similar to that considered in the simulation. The table shows that the temperature selected for the furnace control system (the mean value between the measured one at the inlet and the outlet of each zone) is very close to the calculated average temperature of the wall zones.
CONCLUSIONSCFD modelling developed for single burner simulation has been extended to the complete reheating furnace including the effect of charge movement inside it. A multiscale approach for representing the burners has been applied to reduce the computational grid without losing accuracy, in conjunction with a model of the steel charge that couples the steady state CFD calculation of the reacting flow inside the furnace with the unsteady simulation of the charge during its advancement in the furnace. The approach has been experimentally validated in a previous study.
The good results of validation work and the evaluation of required CPU time (5–12 days) on a cluster with 16 processors, demonstrated that the CFD furnace-
r Fig 7 Heating curves computed from the CFD simulation
r Fig 6 Temperature field and velocity magnitude field on the horizontal section plane at half furnace height
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coupled model is compatible with the design workflow of an industrial furnace.
The application of the developed CFD simulation approach of an industrial reheating furnace revamping design and the comparison with industrial operation data has confirmed the potential of the tool when the analysis of fluid dynamic, flame interaction, detailed representation of furnace wall losses, steel charge temperature and heat fluxes is necessary to support the furnace design.
Moreover, the availability of the charge surface temperature, heat fluxes and local concentration of combustion products, potentially allow a multi-physics methodology. CFD could be coupled with a structural analysis tool to evaluate the deformation of the charge due to thermal gradient or scale growth to evaluate the process yield. These are the principal direction for future further research between Tenova and CSM.
The reduction of the CPU time due to the current multi-scale approach allows us to envisage further development to cope with the use of alternative gaseous fuels such as coke over, blast furnace and producer gases, or oxygen-enriched combustion air inside reheating furnaces, thus allowing the possibility to evaluate in the design phase all the relevant details for the combustion process realised inside the furnace. MS
ACKNOWLEDGMENTSWe wish to thank TenarisDalmine (A Caprera and M Galliano) who made available the reference data at TenarisDalmine.
Alessandro Della Rocca and Massimiliano Fantuzzi are with Tenova LOI Italimpianti, Italy, and Valerio Battaglia and Enrico Malfa are with Centro Sviluppo Materiali S.p.A., Italy.
