Effect of the burner position on an austenitizing process in a walking-beam type reheating furnace

Applied Thermal Engineering 153 (2019) 633–645

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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Research Paper

Effect of the burner position on an austenitizing process in a walking-beam type reheating furnace

T

Alex M. García, Andrés F. Colorado, Julián E. Obando, Carlos E. Arrieta, Andrés A. Amell⁎ Grupo de Ciencia y Tecnología del Gas y Uso Racional de la Energía – GASURE, Facultad de Ingeniería, Universidad de Antioquia – UdeA, Calle 67 N° 53 – 108, Medellín, Colombia

HIGHLIGHTS

position has a great effect on efficiency and heating characteristics. • Burner varied up to 18 percentage points by changing the burner position. • Efficiency air leaks were responsible for the lower efficiencies and billet heating problems. • Cold • The most appropriate burner configuration depends on the geometry of the furnace. ARTICLE INFO

ABSTRACT

Keywords: Self-recuperative burner Reheating furnace CFD Billet heating Heat treatment Austenitizing process

An analysis of the effect of burner location on the performance of a walking-beam type reheating furnace for an austenitizing process is presented in this work. Four configurations were evaluated, where the main difference was the position of four high-speed self-recuperative burners. The analysis was done through computational fluid dynamics (CFD) simulations, using a set of models suitable, and previously validated, to consider combustion, heat transfer, and billet heating, all in a 3D steady-state calculation. The self-recuperative burners were modeled by programming a custom user-defined-function (UDF) for the specific burner. This UDF calculates air preheating temperature in each burner as a function of air mass flow rate and the flue gas temperature entering the burner recuperator. The efficiency of the heating process, the billet heating characteristics, and the heat transfer rate to the billets for the different configurations were analyzed and compared. The results show that position and type of burners have a great effect on the furnace performance. The entrance of cold air through the furnace openings was responsible for the lower efficiencies and some billet heating problems observed. The configuration with the burners staggered on the sidewalls presented the best results in terms of energy efficiency and the billet heating characteristics required for an austenitizing process (heating rate, austenitizing temperature, holding time, and temperature uniformity).

1. Introduction The production processes for manufacturing steel are both energyintensive and one of the biggest contributors to CO2 emissions. Therefore, research and innovation in steel production and processing are necessary to meet the future demands of the environmental regulations, improve energy efficiency, reduce fuel consumption and pollutant emissions while strengthening the competitiveness of the steel industry [1,2]. The optimization of the processes in this sector is of high importance for achieving goals related to the reduction of emissions of greenhouse gases as well as the reduction of production costs, especially in times of high energy price volatility. According to the statistical data

⁎

of International Energy Agency (IEA) published in 2015, the iron and steel industry accounted for 18% of the world's total industry final energy consumption [3]. A recent review by He and Wang in 2017 presents a list of energy-efficiency technologies and practices applicable to the steel industry. The paper also includes case studies around the world and information of energy savings and cost when available. The review of He and Wang points out a technical potential to reduce the steel and iron industry total energy consumption by approximately 20% by applying the best available technology (BAT) [1]. A common steel thermal treatment is the austenitizing. Austenitizing is the heating of iron (or another iron-based metal such as steel) to a temperature at which it changes crystal structure from ferrite

Corresponding author. E-mail address: [email protected] (A.A. Amell).

https://doi.org/10.1016/j.applthermaleng.2019.02.116 Received 10 December 2018; Received in revised form 15 February 2019; Accepted 24 February 2019 Available online 28 February 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.

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to austenite. Austenitizing refers to heating into the austenite phase field, during which the austenite structure is formed. Austenite is the high-temperature, face-centered cubic form of iron, stable at intermediate temperatures on the iron-carbon binary phase diagram. The austenitizing heat treatment almost always involves heating, but austenite formation also occurs during direct processing from very high temperature [4]. Austenitizing of steel is carried out in reheating furnaces. According to Mayr et al., in steel manufacturing plants, one of the largest consumers of energy is the reheating furnace [5]. Reheating furnaces elevate the temperature of steel slabs or billets with hot combustion gases to the desired level which is needed to process the steel in subsequent processes like rolling mills. A reheating furnace has to produce heated slabs which satisfy two slab requirements – target temperature and temperature uniformity [6]. The former characteristics are directly related to austenite mean grain size and austenite grain size distribution, which are two important parameters to be controlled when specific mechanical properties are searched for steels [7]. Furthermore, a uniform temperature distribution is desired inside the billets with optimized operating conditions regarding fuel consumption, residence time, NOx and CO2 emissions, etc. [8]. The control variables associated with the reheating process and the steel quality are the heating rate (K/s), the austenitizing temperature, and holding time [9]. Process in steel industry represent around to 20% of the total industrial final energy consumption [1,10–13]. The high energy demand combined with poor efficiency has made steel processing a primary target for advanced model-based analysis and control. Increasing the energy efficiency of steel processing plants by the restructuring of processes, applying advanced control strategies and optimization techniques will directly impact the overall energy consumption and related CO2 emissions. Among the BAT for the reheating furnace, we distinguish two technologies classified as high-efficiency burners: The regenerative burner and the recuperative burner [14]. Regenerative and recuperative burners optimize energy efficiency by incorporating heat exchanger surfaces to capture and use the waste heat from the hot flue gas from the combustion process [15]. Typically, regenerative devices consist of two burners with separate control valves, which are connected to the furnace and alternately heat the combustion air entering the furnace. The system works by guiding the exhaust gases from the furnace into a body which contains refractory material such as aluminum oxide. The exhaust gas heats up the aluminum oxide media and the heat energy from the exhaust is recovered and stored. When the media is fully heated, the direction of the air entering the burner is reversed and the burner with hot media starts firing [15]. The system operates in cycles and uses a control valve to reverse the flow direction periodically. Heat regenerators can preheat the combustion air up to 1000 °C and achieve energy savings of up to 48% based on a natural gas furnace with 10% excess air and without any heat recovery system [16–20]. Likewise, recuperative burners are equipped with a heat exchanger that preheats the combustion air using the available energy in the exhaust gases. By recovering heat from the combustion products, the combustion gases are emitted at lower temperatures and the combustion air is preheated using the exhaust gases sensible heat. Burners equipped with a recuperative heat exchanger are able to preheat air up to a maximum of 750 °C [2], which lead to energy savings of up to 30% based on a natural gas furnace with 10% excess air and without heat recovery [14,15,21]. Tangjitsitcharoen et al. analyzed the implementation of regenerative and recuperative burners for various reheating furnaces (variable capacity 40, 70 and 150 tons/h) of the Thai steel industry. Based on considerations of net present value and payback period, they determined that regenerative burners (above the recuperative ones) are more suitable for all reheating furnaces. Even though regenerative burners offer shorter payback periods, their initial investment always surpassed the recuperative burners of the same capacity by approximately 1.5 times [14].

Computational fluid dynamics (CFD) is a useful tool for energy optimization of combustion heating systems. It significantly reduces the need to run costly experimental tests before making modifications to the system. The literature reports numerous papers addressing the simulation of walking-beam type reheating furnaces. Venturino and Rubini [22] proposed a methodology, which decouples the combustion reactions and heat transfer inside the furnace from the heating of the slabs. The first part consists of a steady state simulation in and the second simulation is a transient run based on the finite difference method. This method proposes an iterative calculation, where the results of the heat flow from the combustion reactions and furnace walls to the slabs are used as boundary conditions to calculate the heating rate of the slabs. Zhang et al. [23] proposed a method for modeling the entire process in a single simulation. Their method involves using a simplified version of the geometry of the slabs, by modeling them as a single and continuous plate located on the furnace floor. Hsieh et al. [24], recently used this concept. They modeled the heating process in the walking-beam type reheating furnace as a steady state process; the steel slabs were modeled as a thin sheet of high viscosity fluid with the same thermophysical properties of the materials thermally treated. Thus, the transport of mass through the furnace and its corresponding heat transfer rate was achieved by using this laminar flow at a constant speed and without wall-shear stress. This strategy provides a low computational cost method of easy implementation with a good level of agreement with corresponding experimental evaluations [5,25]. Similarly, for the modeling of the complete system without the need of simplifying the slab geometry, two approaches were identified. Hana et al. [26] modeled the system as an unsteady state case. The movement of the slabs considered the heat transfer storage during each time step, by passing the total energy contained in the previous step to the next position in the furnace. Casal et al. [27] modeled a walking-beam type reheating furnace in steady state. They modified the conservation equation for energy transport in the slab zone by adding a source term that takes into account the transport of energy in each position due to the output of a slab of higher temperature and the arrival of another of lower temperature. The present work evidences that the location of self-recuperative burners in the furnace has a significant effect on the heating efficiency and uniform heating of the load. This paper addresses furnace design and energy efficiency issues by analyzing the aerodynamics and thermochemistry of the combustion chamber using CFD analysis. The aim of this work is not to present a novel simulation setup. The set of models used was taken from previous works in literature. The objective is to numerically investigate and optimize the location of high-speed selfrecuperative burners and their effect on the austenitizing process, using a simulation setup which has already shown good results. The work outcome indicates the best geometric configuration to guarantee the quality of the thermal treatment in terms of heating rates, austenitizing temperature and holding time. 2. Furnace configuration and operating conditions The analysis made in this work is based on a walking-beam type reheating furnace, used for thermal treatment of low carbon steel pieces. The furnace is located in the city of Bogotá, Colombia, located at 2600 m.a.s.l. where the mean ambient temperature and atmospheric pressure are 297 K and 74.7 kPa (74 kPa/101.3 kPa = 0.73), respectively. The austenitizing treatment includes a heating process that raises the steel piece temperature from the ambient temperature up to a holding temperature above 1100 K. In this study, the furnace is equipped with four nozzle mixing high-speed self-recuperative burners. Fig. 1 shows a diagram of the four geometric furnace configurations evaluated. The furnace dimensions differ slightly between each evaluated condition. In general, the furnace has an internal height and width of 0.7 m and 4.15 m, respectively. The billets travel a distance of 3.8 m along the furnace. In cases C1 and C2, the burners are located on 634

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0.2 m 0.7 m

0.36 m

0.15 m

C2

C1 Stack Billets

y

x z

C3

C4

Billet outlet

Billet inlet

Burner x4

Fig. 1. Schematics of four geometric furnace configurations.

0.14 m × 0.20 m (see billet outlet in Fig. 1). The furnace stack is located on the roof at 0.37 m from the furnace inlet and 1.3 m from the left wall. It has a square section of 45 cm × 50 cm and a length of 62 cm. Since in the operation of a selfrecuperative burner a large part of the flue gases exits the furnace through the burner recuperators, the stack was completely closed, so that the remaining gases leave the furnace through the inlet and outlet openings. All the furnace walls are insulated with a ceramic blanket with an average thickness of 25 cm, except the chimney and front wall that has a thickness of 6 cm and 11.5 cm, respectively. The furnace floor is made of refractory bricks with a thickness of 35 cm. Table 1 shows the thermal properties of these materials at 1273 K, which is the temperature expected of the furnace internal walls during the austenitizing process. These properties were taken from the data-sheets provided by the manufacturers of the isolation of the furnace that was taken as reference for the work.

Table 1 Thermal properties of the insulation materials at 1273 K.

Density Specific heat Thermal conductivity Emissivity

3

[kg/m ] [J/kg K] [W/m K] [ −]

Ceramic fiber

Refractory brick

128 1130 0.25 0.68

2230 960 1.66 0.75

the back wall, opposite the billet movement direction. The separation between the burners axis is 1 m and the burner closest to the right wall is 0.65 m from it. In order to avoid billets overheating, given due to the location of the flames directly over the billets, the back wall supporting the burners was located 0.36 m away from the last billet position. Thus, in cases C1 and C2 the furnace is slightly longer. The difference between cases C1 and C2 is that the latter volume is reduced on the opposite side to the burners. Volume reduction requires only increasing insulation thickness, extending it 0.86 m on the side of the billets inlet. For C3 and C4, the burners are located on the sidewalls in crossflow to the movement of the billet. The first burner is at 1 m from the billets inlet. The separation between the burners axis is reduced from 1 m to 0.8 m. In case C4, all four burners are on the left side, hence their axes are parallel, while, in case C3, the burners are located on both sides or staggered. In all cases, the burners are located 0.4 m above the furnace floor and 0.3 m above the billet top surface. The four burners operate at a fixed equivalence ratio of 0.87 (the ratio of the actual fuel/air ratio to the stoichiometric fuel/air ratio). The fuel used is natural gas, whose composition by volume is: 82.8% CH4, 10.0% C2H6, 3.6% C3H8, 1.2% C+3, 1.9% CO2, and 0.5% N2. The high heating value (HHV) of the fuel at standard conditions is 41,150 kJ/m3 and the stoichiometric air-to-fuel ratio is 10.45 by volume. To study only the effect of the different furnace configurations, all cases were evaluated with the furnace operating at a total power of 680 kW (based on the HHV), equally distributed in the four burners. The furnace inlet is an opening of 0.15 m × 4.15 m. The steel enters the furnace in billet form with a thickness of 12 mm and a length of 3.35 m. The displacement of the billets along the furnace is given by a walking beam mechanism supporting the billets at a height of 85 mm above the furnace floor and 0.25 m from the right wall. The beams have a width of 15 mm and are not refrigerated, thus skin marks were neglected. The billet speed of displacement depends on the production rate requirements, so the average billet temperature at furnace outlet was around 1220 K. At the end of the furnace, the billets fall on a roller system that takes them out of the furnace for an opening of

3. Numerical model A double precision solver based on the pressure was used for the CFD simulations, using the commercial software ANSYS® - Fluent® version 18.0. The fluid in the furnace is the mixture of gases resulting from the combustion species of natural gas. This gas mixture is modeled as a steady state incompressible Newtonian fluid. The viscous model is based on the Reynolds-averaged Navier-Stokes (RANS) equations. Turbulence was modeled with the Realizable k-ε model [28] with the Enhanced Wall Treatment method [29] for the calculation of the flow in the viscous region of the boundary layer in the vicinity of the walls, taking into account the thermal effects. The ideal gas law modeled gas density variations. Couple and PRESTO! algorithms were applied for pressure-velocity coupling and pressure interpolation [29], respectively. The Steady Diffusion Flamelet [30] modeled species transport and combustion. For modeling the chemical kinetics, The UC San Diego reaction mechanism for natural gas was used [31]. This mechanism has 72 species and 321 reactions and last updated in 2016. The specific heat (Cs) of the gas mixture is calculated with the mixing law. The viscosity (µ) and thermal conductivity (λs) were set functions of temperature using the kinetic theory of gases for a typical flue gas composition. The transfer of radiation was modeled with the Discrete Ordinates (DO) model [32], along with the weighted-sum-of-gray-gases. The same coefficients set by Smith et al. [33] for absorption coefficients of the gas mixture were used in this model. This set of models have shown to accurately describe the thermo-physical phenomena that occur inside a walking-beam type reheating furnace [8,9,25,26,34–37]. The wall heat 635

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60

losses by radiation and convection were calculated by setting a natural convection coefficient of 6 W/m2 K and an external emissivity of 0.75. The reference temperature for walls radiation and convection losses was 297 K.

λs [W/m K]

50

3.1. Heat transfer of billets The heating of the billets during their pass through the furnace was modeled using the method proposed by Hsieh et al. [24]. Their method is easy to implement with a good accuracy-to-computational cost relation. The method assumes billets to be a continuous sheet of metal that crosses the furnace at a constant velocity. This sheet was modeled as a high viscosity laminar fluid (106 kg/m s) with the thermophysical properties of the billet material. In this manner, the billets heating is modeled in the steady state. Heat reaches the billets surfaces by convection and radiation. Due to the billets high viscosity, there is no movement of layers inside the billets, thus in the material interior, the heat transfers only by conduction, while in the direction of flow, the bulk motion of this high viscosity fluid describes the heat and mass transfer due to the displacement of the billets from one position to another, as they move along the furnace. The billet material was a half-carbon steel (0.56C - 0.58Mn - 1.43Si 0.47Cr [wt%]), whose density is 7850 kg/m3. The specific heat, thermal conductivity, and emissivity are highly dependent on temperature; hence, for accurately describing these variations, these properties were defined as a function of temperature using piecewise polynomials. Fig. 2 shows the specific heat and the thermal conductivity as a function of the temperature. The abrupt change of the properties of the steel occurs at the temperature of the structure phase change from the bodycentered cubic structure to a face-centered cubic structure. In these figures, the continuous lines correspond to the polynomials used in the simulations. The black points are the experimental points used to adjust the polynomials [38]. The values measured by Sadiq et al. [39] for structural steel with a non-oxidized surface finish were used for the billet surface emissivity.

20 0 200

600

1000

1400

Temperature [K] 1700

Cs [J/kg K]

1400 1100 800 500 200 200

600

1000

1400

Temperature [K] 1.0

ε [-]

0.8 0.6 0.4 0.2

The self-recuperative burners used in this study are commercial high-speed burners manufactured by Eclipse, Inc. The burner reference is TJSR0100. This burner incorporates a heat exchanger (recuperator) in its body used to preheat air by recovering sensible heat from the flue gases before they exit the furnace. Fig. 3 shows a schematic representation of the operating principle of the self-recuperative burners. The flue gases leave the furnace through the self-recuperative burners due to an under-pressure generated by an ejection system in each burner. In practice, this is controlled so around 80% of the flue gases leave furnace through the burners [40]. In the simulations, this 80% value was held constant by a mass flow outlet boundary condition. The air preheating temperature was calculated during the simulation using a user-defined function (UDF) based on Eq. (1). This temperature is a function of the air mass flow rate (mair ) and the flue gases temperature entering the recuperator (Tflue ). In Eq. (1),Tref , a, b, and c are constants equal to 298.15 K, 0.0059, 6.0889, and 1.6750, respectively. These constants were obtained for the eclipse TJSR0100 autorecuperative burner from the information of the combustion efficiency reported by the burner manufacturer [x], following the method described in a previous work [25]. In this equation, the temperatures and the air mass flow rate must be in K and kg/s, respectively. b m air (T flue

30 10

3.2. Self-recuperative burners

Tair = Tref + a e

40

Tref ) c

0.0 200

600

1000

1400

Temperature [K] Fig. 2. Thermal conductivity (λs), specific heat (Cs) [38], and emissivity (ε) [39] of billets as a function of temperature.

Air

Cold gases

Recuperator

80% of flue gases

Secondary air

Hot gases

Flue gases

Fuel Primary air

Fig. 3. Schematic representation of the principles of operation and flow distribution of a self-recuperative burner.

(1)

shows one of the meshes used to perform the numerical simulations. The furnace walls were not included in the simulation, instead, they are modeled by heat conduction through them, considering their thickness and the data in Table 1. The meshes were built using the ICEM CFD software. The mesh consists of hexahedral structured elements, which

3.3. Mesh The domain of interest is the volume inside the furnace where the flue gases can flow, plus the volume corresponding to the billets. Fig. 4 636

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(d)

Billet outlet Furnace outlet

(a) Hot gases outlet Primary air inlet Fuel inlet

(b)

Secondary air inlet

(c)

Furnace inlet

Billet inlet

(e)

Fig. 4. Example of a mesh used to perform numerical simulations (Case C4). (a) Full furnace. (b) Burner detail. (c) Furnace inlet. (d) Furnace outlet. (e) Furnace interior.

are more appropriate for simulations of combustion systems [36]. Since the furnace does not have any geometric symmetry, it is necessary to consider the entire furnace for the simulations. The computational domain has around 880,000 elements. The maximum equiangle skewness was less than 0.58 and the maximum aspect ratio was 27.9, where the highest values correspond to the elements in the billet zone, which are long and narrow. For all burners, the air inlets were simplified from a discrete number of holes to continuous rings. The ring area is equivalent to the combined area of the existing holes, thus securing similar flow velocity. In order to perform mesh independence analysis, the methodology presented in [41] was used. Three meshes were tested; the first one had approximately 400,000; the second 880,000, and the third 2,000,000 elements. Based on this methodology, it was determined that the mesh with 880,000 elements produced acceptable results, comparable in accuracy to the 2,000,000 element mesh but at significantly lower computational cost.

exit. Table 2 shows the flow conditions necessary to guarantee this mean temperature at the exit. As a first result, a large difference in the billet mass flow for the evaluated configurations was observed. The billet mass flow rate varied from a minimum and maximum of 293 g/s and 491 g/s, for cases C4 and C3, respectively. This max/min difference corresponds to a variation of 67.6%. In addition, a substantial difference in the preheated air temperature is observed. Since the power input and the air-to-fuel ratio are kept constant, the air preheat temperatures for each burner depends only on the flue gas temperature that flows out through each burner. This temperature changes for each condition because of installing the self-recuperative burners in different locations. The mean preheated air temperature for C1, C2, C3, and C4 was 680.2 K, 686.8 K, 642.7 K, and 587.5 K, respectively. Fig. 5 shows temperature contours on the horizontal plane that cuts through the burner nozzles for the four cases evaluated in this work. The region of maximum temperature corresponds to the reaction zones, which extends from the burners nozzles up to about 1 m inside the chamber. The reaction zone temperature is higher for those cases when the air-preheated temperature is higher. The maximum temperature in the furnace was 2149 K, 2120 K, 2078 K, and 2029 K for C1, C2 C3, and C4, respectively. Fig. 5 additionally shows the streamlines inside of the furnace. Large regions of gas recirculation are observed in all cases. High recirculation zones are generated by high-speed burners. It is known that, when a gas jet is injected into a quiescent atmosphere, a

4. Results 4.1. Furnace performance As mentioned above, all configurations were evaluated at the same input power. However, the production (billet mass flow rate) was varied to achieve a billet mean temperatures of 1220 K at the furnace Table 2 Summary of inlet flows. ID

C1 C2 C3 C4 * ** †

Fuel inlet* [g/s]

3.3

Primary air inlet* [g/s]

25.6

Secondary air inlet* [g/s]

Billet mass flow [g/s]†

35.4

294 375 491 293

Flow for each burner. From right to left for cases C1 and C2, as shown in Fig. 1. Results from simulation. 637

Preheated air temperature [K]† Burner 1**

Burner 2

Burner 3

Burner 4

620 637 659 582

663 668 560 583

715 734 726 569

724 709 626 616

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C1

C2

C3

C4

x

z Fig. 5. Temperature contours in a horizontal plane at burner height [K].

burners were located far enough from that opening. Increasing the distance of burners to the furnace outlets/inlets is an effective strategy to avoid the cold air leakage. The results show that all cases have some cold air infiltration through the furnace billet inlet. The mass flow rate of cold air entering the chamber through the furnace inlet is 38 g/s, 81 g/s, 90 g/s and 181 g/s for C1, C2, C3, and C4, respectively. A characteristic of the furnace studied in these cases, is that its inlet area is larger than its exit, consequently facilitating the entry of cold air in response to the flow patterns promoted by the different burners locations. For C3 and C4, the streamlines in Fig. 5 shows that the gases in the chamber rotate clockwise, which causes cold air infiltration mainly on the left side of the furnace (inlet zone). This air leaking affected the performance of the burners located on this side, which explains the lower air preheating temperature in burners 2 and 4 for case C3 and for all the burners in case C4. The largest cold air infiltration takes place when the burners are located on the left sidewall. In case C4, the amount of air entering the furnace is doubles of that in Case 3. Moreover, in C4, a single recirculation zone is generated. The cold air that enters the furnace, in this case, is affecting the burners to a lesser extent from the one closest to the entrance of the furnace to the furthest, so the preheating temperature of the air, and therefore the impulse is smaller as it approaches the billet inlet. This behavior helps to reinforce the flow pattern obtained in case C4. Fig. 7 shows bar charts for the energy balance and distribution of the four cases. Fig. 7 shows energy inputs with striped bars and energy outputs with continuous bars. The energy contributed by the fuel (base on the HHV) and energy available in the preheated air are energy (energy recovered) inputs to the system. The heat transferred and stored within the billets material is the useful energy; the latent and

mass flow of the surrounding gases is induced to flow into the jet due to the conservation of the linear momentum. In other words, the jet centerline velocity decays inversely proportional to its downstream distance from the nozzle while simultaneously increasing its mass. Greater jet impulse produce larger suction or entrainments of surrounding gases, which increases the recirculation inside the furnace. For a given mass flow rate, the impulse is a function of the discharge speed from the nozzle, and the latter is a function of the preheated air temperature. However, the streamline patterns suggest that the location and size of these recirculation zones are, as expected, strongly dependent on the location of the burners. Fig. 5 evidences that for all cases there is important infiltration of cold air from the surroundings to the furnace. The temperature contours show low temperature (blue tones) regions corresponding to these leakage of cold air. These infiltrations are undesired and occur through the furnace openings at the billets entrance and exit. Air leakage is a consequence of the injection of high-speed jets that produce high entrainment of gases up to the point where the local pressure in the chamber is lower than the atmospheric pressure. Fig. 6 shows the temperature contours on a vertical plane aligned with the center of the furnace exit. This figure shows cold air leakage near the billets exit when all four burners were positioned on one side in counter-flow to the billets movement (C1 and C2). The amount of cold air that flows into the furnace through this opening is 870 g/s and 920 g/s for C1 and C2, respectively. This cold air inflow affects the performance of the burner located nearest the furnace exit (Burner 1), decreasing the temperature at which the hot gases enter the burner recuperator. This result explains the lower air preheating temperature observed in those burners. As shown in Fig. 6, for C3 and C4 there is no cold air leakage through the furnace exit; this is explained because in those cases the 638

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C1

C3

C2

C4

y z

Fig. 6. Temperature contours in a vertical plane at furnace outlet [K].

rate, so billet mean temperature reaches the outlet goal temperature, but its efficiency lowers. Although the total mass flow of cold air entering the furnace is a little lower in C1 (908 g/s) compared to C2 (1001 g/s), C2 exhibits higher efficiency than C1. This is because C2 considers a reduction of the furnace volume at the furnace inlet. As a result, the temperature in this region decreases because the hot gases injected by the nozzles cannot reach it easily. This region of lower temperature at the furnace inlet reduces both wall and opening losses. Sensible heat losses are reduced as well, because of the temperature of flue gases leaving the furnaces is lower (1107 K for C1 and 1025 K for C2). The greatest losses due to sensible heat are presented in case C4. Due to the way in which the flow patterns behave in this case (Fig. 4), the flue gases that do not leave the furnace through the burners, leave the furnace at a much higher temperature, 1046 K compared to 630 K in case C3. It was found that the heat recovered by the self-recuperating burners was the highest for C2 (14.4%), followed by C1 (14.2%). For case C3 and C4 the heat recovery represents 12.8% and 10.6%, respectively. The highest energy recovered in cases C1 and C2 is explained by the availability of gases at higher temperature passing the recuperators, this in comparison to C3 and C4 when air infiltration reduced this temperature. From these results, it is observed how a considerable increase in the efficiency of a reheating system can be obtained by changing the burner configuration. Wang et al. [42] showed a higher heating efficiency was obtained by decreasing the number of burners in a 35 m-long walkingbeam type reheating furnace. In addition, they obtained a more uniform temperature distribution inside the furnace, which improved the heating of the slabs. They decreased the side number of burners from 13 to 6, keeping the thermal input constant, and concluded that with less burners the slabs obtain a more uniform heating environment, due to the higher combustion intensity per burner and the higher impulse of flue gases.

Fig. 7. Energy balance of the four configurations based on thermal input (680 kW = 100%).

sensible energy of flue gases, radiation and convection heat loss through the walls, finally the energy that leaves the furnace directly by radiation through the openings are accounted as wasted output energy. The percentages in Fig. 7 were calculated based on the power provided by the fuel (680 kW), so the sum of the energy outputs minus the percentage of energy recovered in the recuperators add to 100%. Because the thermal input is held constant, the efficiency of each configuration directly relates to the mass flow of steel billets passing the combustion chamber. Table 2 shows that C3 configuration allows the largest production rate while securing the desired thermal treatment temperature at the furnace exit. Therefore, it gives the highest efficiency, with a value of 45.2%. The useful efficiency for C1, C2 and C4 were 27.0%, 34.5% and 27.0%, respectively. These lower efficiencies are caused by the infiltrations of cold air that become an additional thermal load to the system, since it is expected that greater entrances of cold air lead to greater flue gas losses. For cases C1 and C2, a large part of the cold air enters through the hottest zone of the furnace, which significantly cools down the billets in the region close to the furnace exit. To compensate for this, it is necessary to reduce billet mass flow

4.2. Billet heating characteristics The austenitizing temperature or hardening temperature is determined by the carbon content of the steel that is being treated. This temperature is chosen to obtain maximum hardness while maintaining a fine grain structure that guarantees both a good tensile strength (TS) and a good fracture strength (FT). In general, for hypo-eutectoid steels (carbon content lower than 0.77%) the austenitizing temperature must be at least TAc3 + 50 (K or °C) [43]. Where TAc3 is the temperature that changes steel grain structure from ferrite + pearlite to austenite. For the steel that was taken as reference in the present work, TAc3 is equal to 820 °C [44]. However, Lai et al. [45] showed that mid/low carbon steels exhibit a greater resistance to fracture if the austenitizing process reaches 1200 °C instead of 870 °C. Higher austenitizing temperatures avoid the formation of twinnedlarge reticular martensite plates, which improves FT without significantly affecting the TS, despite the larger grain size obtained. However, very high temperatures cause some degree of damage during 639

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the cooling process. Both TS and FT are reduced if the temperature is very high, which is commonly known as “overheating” [46]. For very high temperature, the cooling rate is not enough to avoid the precipitation of sulfides at the borders of the austenite grains, reducing the toughness. The temperature at which “overheating” occurs depends on the steel composition, in addition to the cooling rate. In this work, we evaluated the different furnace configurations at an austenitizing temperature of 947 °C (1220 K). Fig. 8 shows the heating curves of billets obtained with the four evaluated configurations. For each configuration, we present temperature data collected on 6 points along the billet as it travels through the furnace. We selected three equidistant points to the right of the central line and three to the left. Fig. 8 also includes the billet mean temperature. Even though the mean temperature at the outlet for all configurations is the same around 1220 K, notice the temperatures are not homogeneous throughout the billet. C1 and C2 show considerable differences between minimum and maximum temperatures of the billet at the outlet. Furthermore, temperature profiles for C1 and C2 show a temperature drop near the furnace exit. The cold air infiltrations explain this behavior. The maximum and minimum temperature for C1 is 1324 K and 1063 K, respectively. For C2 these temperatures are 1296 K and 1083 K. Although the maximum temperatures for these two cases are high, there are no risks of “overheating” conditions. However, the minimum temperatures are below the TAc3 + 50 (1143 K), which negatively affects the austenitizing process. At these temperature levels, it is not possible to guarantee the entire sample reached the austenitic phase. For C1 and C2, around 60 cm of the billet was below TAc3 + 50. To avoid this, it would be necessary to heat up the billets to a higher mean temperature, which for a same thermal input implies a lower billet mass flow rate (production) and therefore a lower useful efficiency of the austenitizing process. C3 and C4 do not show underheating below TAc3 + 50. In these two cases cold air was not infiltrated through furnace outlet. For case C3 the maximum and minimum temperatures are 1259 K and 1161 K, respectively. For case C4 these temperatures are 1236 K and 1170 K, respectively. The holding time is defined as the time from when the entire billet is at the austenitizing temperature until it leaves the furnace [43]. For

structural low alloy steels, a holding time of 0.5 min per millimeter of metal thickness is suggested. In this work the billets have a thickness of 12 mm, so a minimum holding time of 6 min must be guaranteed. The holding time obtained with the four configurations evaluated was 47.0 min, 30.7 min, 9.0 min, and 29.4 min, for C1, C2, C3, C4, respectively. For all cases, the holding time was greater than the minimum recommended. However, a prolonged holding time also negatively affects the tensile strength obtained after cooling, due to the increase in retained austenite. Huang et al. [47] examined the retention of austenite in a steel 0.lC-5Mn [wt%] after a simple thermal treatment, that consisted of a heating, holding, and cooling. They found that the amount of retained austenite increases, until reaching a peak, when increasing the holding time. C1 presents the longest holding time. The position of the burners in C1 causes a higher temperature in the front wall of the furnace (the wall opposite to the burners), which greatly increases the heat transfer by radiation in the first section of the furnace. This feature causes the billets to heat up at a higher speed in that section, so it reaches the austenitizing temperature just after the billet have traveled a short distance (1 m inside the furnace). One way to improve this is to move the front wall toward the furnace interior, which is evaluated in configuration C2. In this configuration, moving the front wall by 83 cm towards the interior of the furnace reduced the holding time by 17 min. For most applications, the heating rate (K/s) from ambient temperature to the austenitizing temperature is less important than other factors such as austenitizing temperature, temperature uniformity and holding time [43]. However, this is an important variable. In the first place, holding time directly relates to the heating rate. At faster heating rates, the entire billet is at the austenitizing temperature earlier, so the time until it leaves the furnace is greater. Second, the heating rate at which the austenite phase is reached has a great influence on the grain size distribution. Danon et al. [7] determined the smaller the heating rate, the smaller and more homogeneous the grain size will be. They also reported a critical value for the heating rate. Above this rate, the steel structure will no longer be homogeneous, forming isolated coarse grains. Furthermore, they found this critical value to decrease when increasing austenitizing temperature [7]. 640

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Fig. 9 shows the heating rates as the billet passes the furnace. The heating rates were calculated from the mean temperature and the six temperatures alongside the billet center. The results indicate C1 presents the highest heating rates. With a maximum value of 2.34 K/s, obtained near the furnace inlet (opposite the cold air infiltration). For cases C2, C3, and C4 the maximum heating rates were 1.72 K/s, 1.25 K/ s and 1.51 K/s, respectively. High heating rates cause large temperature difference inside the billet (lack of uniformity). The thermo-physical properties of the material limit the rate of heat diffusion to the material interior. This temperature difference is one of the major issues in the industrial operation of thermal treatments since uneven temperature distribution causes thermal stresses and increases the risk of deformation or cracking [43]. Fig. 10(a) shows the differences between max and min temperatures ( Tmax ) that were registered as the billet travels through the furnace. For all cases, the maximum Tmax is reached within the first half of the furnace. The maximum deltas of temperature Tmax were 470.6 K, 464.5 K, 454.0 K, and 359.8 K for cases C1, C2, C4, and C3, respectively. The point of maximum Tmax is located slightly after the point where the maximum heating rate occurs. After this point, Tmax begins to decrease as the billet approaches thermal equilibrium. For cases C1 and C2, Tmax increases as the billet approaches the furnace outlet. This behavior is explained by the cold air infiltration at the outlet for these configurations. This is shown in Fig. 9 as negative heating rates. The temperature distribution of the sample at the furnace outlet is shown in Fig. 10(b). For cases C1 and C2 the billets are cooler to the right side with a temperature difference from the hotter side of 261.0 K and 212 K, respectively. On the other hand, for cases C3 and C4 the billets are cooler to the left, with a temperature difference from the hotter side of 98.8 K and 66.7 K, respectively. A similar result was obtained by Mayr et al. [5], who simulated a pusher type reheating furnace for steel billet. The simulation revealed an asymmetric temperature distribution in the billets along their length. Which comes from the asymmetric flow in the furnace, where cold flue gas is drawn back into the furnace, and the billets gets cooler on one side. During the heating process, heat is transferred to the billets by radiation and convection. The net energy transferred by radiation is equal

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to the difference between the incident radiation absorbed by the billets and the radiation emitted again by the surface of the billets. Fig. 11 shows the average heat fluxes on the billets along the furnace length. In general, radiation is the main heat transfer mechanism. For C1, radiation represents 90.2% of the energy that is transferred to the billets. Fig. 12 shows average walls and gas temperature along the furnace, the 641

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reduction in volume. This low-temperature region affects radiation more than convection due to the dependence of radiation on the fourth power of the absolute temperature. Additionally, gas radiation, which represents a significant percentage of the total radiation flux on the billets in this type of furnaces [20], is a function of the gas volume length. Thus, having less volume of gases in case C2, the radiation flux is less. Fig. 11 shows the cold air infiltrations have a significant impact on the heat transfer by convection at the end of the furnace. The negative values reported in Fig. 11(b) for C1 and C2 indicates that at the end of the furnace the heat is transferred from the billets to the gases. For cases C3 and C4 radiation is 85.8% and 77.8% of the total energy transferred to the billets, respectively. All works found in the literature on walking-beam reheating furnaces report that radiation represents between 90% and 98% of the total energy transferred to the billets (or slabs) [24,26,48–52]. The difference with respect to the present work is mainly due to the furnace size. The bigger the furnace the greater the radiation. This because to a greater surface area of walls, and a greater volume of gases. Temperature also has an effect, so that low temperature zones due to cold air inlet reduced total radiation flux. It is observed from Fig. 11 that the heat fluxes declines rapidly from the maximum for all configuration. However, the curve obtained for C3 exhibits a flatter behavior than those obtained for C1, C2, and C4. This is attributed to the greater billet mass flow in case C3, resulting in a lower billet temperature along the furnace, as seen in Fig. 8. Therefore, the higher temperature difference between the billets, combustion gases, and furnace walls is maintained during greater distance along the furnace. The authors discussed this behavior in more detail in a previous work [25]. In this one, the same reheating furnace configuration with self-recuperative burners was compared under different billet mass flow rates. As was shown by Danon et al. [53], in parallel configuration the heat transfer is increased compared to staggered configuration, due to, first, the formation of a longer zone of gases with better properties for the radiation. In addition, parallel configuration produces higher gas velocities along the furnace, increasing the convection heat transfer. The latter because the staggered configuration reduces gas momentum because of the stronger stream interactions [53]. In the present work, the parallel configuration (case C4) did not get the highest heat transfer due to the problems associated with the entry of cold air and the lower billet mass flow necessary to satisfy the billet temperature required.

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As already mentioned, billet mass flow rate has an effect on the billet heating characteristics, such as the holding time and the heating rate. Consequently, to complement the previous analysis, additional numerical simulations were carried out for the four furnace configurations under identical production conditions. In this case, the billet mass flow rate was kept constant at 0.491 kg/s for all simulations. This value corresponds to the billet mass flow rate obtained previously for case C3. Additionally, comparing with Table 2, it corresponds to an increment in the billet mass flow rate of 67%, 78.5% and 67.5% for C1, C2, and C4, respectively. Fig. 13 shows the heating curves of the billet for the four configurations under identical thermal input (680 kW) and billet mass flow rate conditions. As expected, the mean billet temperature at the furnace exit is considerably lower than that obtained for C3. These temperatures are 1077.8 K, 1122.5 K and 1069.6 K for C1, C2, and C4, respectively. The temperature levels obtained for each configuration are related to the heating process efficiency. However, for this billet mass flow rate condition, the efficiency no longer corresponds to that reported in Fig. 7. The efficiency was found to be 38.5%, 40.6% and 38.1% for C1, C2, and C4, respectively. These results represent an increment of 11.5 percentage points for C1. This behavior can be explained by the increase of the heat flux towards the billets with respect the previous analysis, as the mass flow rate increases, the mean temperature

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trends of which explain the high heating rates observed for C1 near the billets inlet. For C2, radiation accounts for 87.0% of the total energy transferred to the billets. This percentage is lower because of the region of low temperature at the beginning of the furnace, caused by the 642

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This is because the radiation emitted by the billet, which increases with the billet-surface temperature to the fourth-power, and more energy is required to achieve the same temperature increase. For the configurations that consider the location of the burners on the rear face of the furnace (C1 and C2), the heating issues described in Section 4.2 are again obtained. These two configurations exhibit high heating rates, 3.61 K/s for C1 and 2.02 K/s for C2. Which lead to high ΔTmax, with a maximum of 521 K and 490 K for C1 and C2, respectively. On the other hand, the holding times obtained were 21.5 min for C1 and 20.1 min for C2. The holding time was reduced, despite the high heating rates, because increasing billet mass flow rate lowers the residence time. Good heating characteristics were obtained for C4, with a maximum heating rate of 1.99 K/s, a maximum ΔTmax of 409 K and a holding time of 12.4 min. However, this configuration continues presenting a low efficiency due to the excessive leaks of cold air through the furnace inlet.

difference between the billets and the furnace walls and combustion gases along the furnace also increases. For C1 the billet maximum temperature was 1243.3 K and occurred on the billet left side. In spite of the high temperature, the whole billet never reached the austenitization temperature (TAc3 + 50 K), therefore, for this case a holding time is not defined. This behavior is caused by the air infiltrations through the furnace openings. Similarly, C4 reached a maximum temperature of 1108.4 K. On the other hand, C2 reached a maximum temperature of 1241.9 K and the. For this configuration, the whole billet did reach the austenitization temperature. In fact, the austenitization temperature was reached 11.4 min before the billet exits the furnace. However, given the air leakage into the furnace, when the billet leaves the furnace around 30% of its length has cooled down below austenitization temperature, which cannot be allowed for the austenitizing process. Fig. 13 shows the heating curves for all configurations. This curves are less slope than those presented in Fig. 8. The corresponding heating rates for this simulation strategy vary from a maximum heating rate of 2.34 K/s to 1.91 K/s, for C1; from 1.72 K/s to 1.68 K/s, for C2; and from 1.51 K/s to 1.17 K/s for C4. As a result of these lower heating rates, lower temperature differences along the billet (ΔTmax) are also obtained. For C1, C2 and C4 the maximum ΔTmax obtained was 442 K, 422 K, and 344 K, respectively. In order to carry out the austenitizing process (quick cooling after heating), the whole billet must reach a temperature above the austenitization temperature at the furnace exit. To achieve this with the different configurations while keeping the billet mass flow rate exit constant, the thermal input must be varied in each case. Fig. 14 shows the heating curves of the billet for the four configurations operating at a billet mass flow rate of 0.491 kg/s and the thermal input required to ensure that the billet mean temperature at the furnace exit is 1220 K. The thermal input required in each case depends on the efficiency of the heating process for each configuration. The efficiencies were now 31.5%, 37.5% and 30.9% for cases C1, C2, and C4, respectively. The efficiency reduction with respect to the previous condition (thermal input of 680 kW and billet mass flow rate of 0.491 kg/s) is a consequence of increasing billet temperature at the exit. The higher the temperature, the higher the relative energy cost of the heating process.

5. Conclusions This study analyzed the effect of burner location on the billet heating for an austenitizing process in a walking-beam type reheating furnace. Four configurations were evaluated where the main difference was the position of four high-speed self-recuperative burners. This was done through CFD simulations, using a set of models suitable to consider combustion, heat transfer, and billet heating, all in a 3D calculation in steady state. The performance of the self-recuperative burners was modeled by programming a custom user-defined-function (UDF), which calculates air preheating temperature as a function of air mass flow rate and the flue gas temperature entering the burner recuperator. In addition to the billet heating analysis, the efficiency of the heating process and the heat transfer rate to the billets were also analyzed. The following conclusions can be drawn from this work.

• The position and type of burners have a great effect on the performance of a walking-beam type reheating furnace, in terms of efficiency and, product quality of the heating for an austenitizing process. The most appropriate burner configuration depends on the

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•

•

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•

•

geometry of the furnace since overall system performance depends on how the burners relate to the furnace chamber. The configurations with the burners located in the rear wall of the furnace, in front of the billet inlet produce the entrance of cold air through the opening for the billet outlet. This cold air inlet decreases billet temperature, so it is necessary to decrease the billet mass flow rate (production) to guarantee the required mean temperature at furnace outlet. A lower production rate is directly related to a lower process efficiency. Locating the burners on the sidewalls, far from the furnace outlet, reduces the amount of air infiltrating the furnace. However, in the parallel configuration (C4) the flow pattern inside the furnace induces the entry of a large amount of cold air through the opening for the billet inlet. This drastically affects the efficiency, due to a greater flue gas losses and also a decrease in the energy recovered in the self-recuperative burners. The maximum useful efficiency was achieved with the staggered (C3) configuration with a value of 45%. The cooling of the billet due to the cold air entry in cases C1 and C2 causes a lower billet temperature and worsens homogeneity at the furnace outlet. Additionally, air infiltrations might reduce billet temperature below the temperature recommended for an effective austenitizing process. When the burners are located on the rear face of the furnace, higher heating rates are obtained. This causes greater billet temperature differences during heating and a longer holding time. The longer holding time leads to a lower tensile strength and a larger and heterogeneous grain size. The heating rate and holding time can be reduced a little by reducing the volume in the furnace inlet. The configuration with the burners located in parallel on the sidewall did not present good results in terms of the energy efficiency and the billet heating characteristics, despite the fact that a previous work has shown that with this configuration heat transfer is better than with the staggered configuration. This shows the importance of a good burners-to-furnace-geometry relationship for a given heating process. The configuration with the burners staggered on the sidewalls presented the best results in terms of energy efficiency and the billet

•

heating characteristics required for an austenitizing process. The heat transfer to the billets varied for the different configurations. When the burners were located in the rear wall of the furnace, a greater radiation heat flux is obtained. The higher wall and gas temperatures at the furnace inlet (wall opposite to the burners) explain the larger fluxes. The configuration with burners located in parallel on the sidewall promotes an increase in the heat transfer rate by convection compared to radiation. The parallel configuration avoids the destruction of momentum due to strongest streams interactions presented with staggered configuration.

Acknowledgments The authors acknowledge COLCIENCIAS and its support through “Fondo Nacional de Financiamiento para la Ciencia, la Tecnología y la Innovación Francisco José de Caldas” for funding “Red de Investigación e Innovación en Combustión Avanzada de Uso Industrial Fase II INCOMBUSTION”, within the project, “Aumento de producción y optimización del consumo de gas natural por la recuperación de calor residual en línea de tratamiento térmico de una empresa del sector industrial colombiano con la implementación y adaptación de nuevas tecnologías de combustión,” was developed. The authors also acknowledge Universidad de Antioquia UdeA for the valuable economic contribution toward the development of this research through the ‘Sostenibilidad’ program. References [1] K. He, L. Wang, A review of energy use and energy-efficient technologies for the iron and steel industry, Renew. Sustain. Energy Rev. 70 (Apr) (2017) 1022–1039. [2] E. Worrell, L. Price, N. Martin, Energy efficiency and carbon dioxide emissions reduction opportunities in the US iron and steel sector, Energy 26 (5) (2001) 513–536. [3] IEA, Energy and climate change, World Energy Outlook Spec. Rep. (2015) 1–200. [4] J.G. Speer, R.J. Gaster, Austenitizing in steels, ASM Handbook, Vol 4A: Steel Heat Treating Fundamentals and Processes, 2013, p. 8. [5] B. Mayr, R. Prieler, M. Demuth, L. Moderer, C. Hochenauer, CFD analysis of a pusher type reheating furnace and the billet heating characteristic, Appl. Therm. Eng. 115 (2017) 986–994. [6] S.H. Han, D. Chang, Optimum residence time analysis for a walking beam type

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