Effect of Fuel Characteristics on the Thermal Processes in an Iron Ore Sintering Bed

Transcription

1 Effect of Fuel Characteristics on the Thermal Processes in an Iron Ore Sintering Bed Won YANG, Kwangheok YANG and Sangmin CHOI Recently, coke in an iron ore sintering process is replaced in part by powdered anthracite; less expensive fuel. In this study, influence of the different fuel characteristics on the thermal condition in the sintering bed has been investigated using a mathematical model. Numerical simulation along with experiments in lab-scale sintering pot has been performed. The mathematical model is based on the assumption that the sintering bed can be treated as homogeneous medium, through which a reacting flow passes. Temperature distribution and flue gas composition are predicted for various kinds of solid fuel and various particle sizes of anthracite. The simulation results show that propagation of combustion zone is faster in the case of using coke than the case of using anthracite, which shows that the coke is more appropriate in the sintering process than the anthracite. However, the results also show that the reactivity of the anthracite can be improved by decreasing the size of fuel particles. Key Words: Thermal processes, Combustion, Iron ore sintering bed, Coke, Anthracite, Fuel characteristics 1. Introduction Sintering is an agglomeration of small metal or ceramic particles through surface melting. An iron ore sintering process is applied to produce large particles (>5mm) of iron ore with appropriate metallurgical properties required in a blast furnace. Raw mix of iron ores and other additives including a small amount of coke and limestone form a bed on a traveling grate as shown in Fig. 1(a). Air is supplied to the bed by a down draft suction fan. The combustion commences at the top of the bed by a hot gas jet from ignition burners for approximately a few minutes after being fed, and propagates into the bed with sintering near the combustion front. Below the combustion front, the combustion gas evaporates moisture in the solid particles, while condensation occurs below the evaporation zone (Fig. 1(b)). These processes progress slowly through a traveling bed of 1m-length for 3~6 minutes. Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Corresponding author: smchoi@kaist.ac.kr From the view of bed combustion of the solid fuel, the iron ore sintering process has some important features: a relatively uniform solid bed, constant traveling speed and no interaction with the gas plenum. These features make it simpler to model it as an unsteady 1-dimensional solid fuel bed. Some model studies have been carried out for prediction of combustion conditions. (1-6) Muchi and Higuchi (1) and Hosotani et al. (2) constructed an unsteady 1-dimensional model, focusing on the coke combustion. Cumming and Thurlby (3) and Pattison et al. (4) considered bed structural changes by employing the shrinkage factor. Nath et al. (5) predicted the melting and molten zone, and Ramos et al. (6) predicted bed structural change more systematically by using the discrete element method. While physical changes of the solid particles were carefully considered in the previous studies, coke combustion and heat transfer should be considered in detail for the prediction of macroscopic combustion behavior in the sintering bed. In addition, previous models treated the solid material as a single solid phase, which makes it difficult to consider fuel characteristics and reactivity.

2 PSEUDO-PARTICLES +WATER +COKE, LIME etc AIR CHARGING STACK FAN HEARTH BED EP REROLLING MIXING ORE BIN COG BURNER SINTERING BED WIND BOXES RETURN FINE SCREENING COLD CRUSHING YARD SINTERED ORE HOT CRUSHING COOLING SINTERED ZONE COMBUSTION ZONE RAW MIX ZONE COOLING SINTERING COKE COMBUSTION MOISTURE EVAPORATION MOISTURE CONDENSATION HEARTH BED BLAST FURNACE Fig.1 Iron ore sintering process COMBUSTION GAS This study presents an unsteady 1-dimensional model for numerical analysis of an iron ore sintering bed, based on the solid fuel combustion modeling. (7) This model is aimed at investigating effects of different fuel characteristics on the combustion condition in the iron ore sintering bed. Simulation results are shown with sintering pot test results and parametric studies are performed for some operating parameters, focused on the solid fuel characteristics. 2. Numerical Approach In the mathematical model, the sintering bed is assumed as a homogeneous continuum of the solid and gas phases. Solid material is treated as multiple solid phases, which makes it possible to consider different fuel characteristics. For a 1-dimensional system of the y-coordinate, the governing equations can be represented as a form of partial differential equation, which is shown in Eq. (1). ( ρ fvφ) ( ρφv) + = ( Γeff φ ) + Sφ (1) t y Mass, energy, component and chemical species conservation equation for each phase are constructed and details of the model are represented in the previous research. (7) The heat and mass interactions of the phases, as well as the chemical and geometrical changes in each phase, are incorporated into the source term. Combustion of solid fuels is typically classified into three types: moisture evaporation, pyrolysis and char combustion. The moisture evaporation should be considered in two ways: boiling and diffusion below the boiling temperature. The reaction rate of char combustion is determined from the kinetic rate, diffusion rate and internal mass transport in the ash layer of the particle. (8) It can be expressed as Eq. (2) AvW s s charcg, idp Rci, = (2) 1 ( kζ ) + 1 k + 1 k r m eff Gaseous reactions occur within the bed by the combustible products of pyrolysis and char combustion released from the fuel particles. The reaction scheme should be chosen according to the reactants and the reaction environment. The loss of mass and true volume in the solid particle by combustion results in volumetric reduction or generation of internal pores, or both. The fraction to the internal pores is expressed by a factor, f ip, for each solid component. From this factor and the combustion rate, the particle size and the internal porosity are determined. The changes in the particle size are described by well-known models: the shell progressive (SP) and ash segregation (AS) models (8). Describing the internal porosity requires a transport equation since the internal pores move along with the advection of the solid phase, expressed as below. (7) f ε vε M& + = f + & ε (3) V, I ip, I s ip, I comb, i ip, i ip, loss, I t y i ρi Dominant modes of heat transfer are conduction in the solid and gas phases, radiation in the solid phase and convection. The conduction is solved as a diffusion of energy once the conductivity is calculated. The convection in the packed bed is expressed as the model of Wakao and Kaguei (9). The radiation in the solid phase is solved using the two flux method proposed by Shin and Choi. (1) 3. Experiments Sintering Pot Fig. 2 shows the schematic diagram of the sintering pot rig, of which bed has a diameter of 25 mm and height of 6 mm. R-type thermocouples are installed along the y-direction of the bed: y = 49mm, y = 3mm (center), and y = 11mm. Flue gas temperature is measured by K-type thermocouple installed in wind box. Hearth ores of 1 mm diameter are installed at the bottom of the pot for improving gas permeability. Assembling pseudo particles for the experiment

3 25mm 11mm T/C (y=49mm) 6mm y 19mm 19mm T/C (y=3mm) Hood T/C (y=11mm) 11mm Hearth ore > 1mm Height : 3mm Pot Sinter Air Anemometer R-type T/C AD Converter Digital Manometer PC Grate Bar T/C Pressure Gauge Damper Valve Suction Blower Wind Box Suction Pump Dust Collector Gas Preheater Fig.2 Sintering pot test rig Gas Analyzer followed the procedure shown in Fig. 1. Iron ores, coke, limestone and other additives are mixed with water in the rolling chamber, which produces various sizes of pseudo particles. Iron ores consist of many kinds of ores such as hematite, magnetite, and limonite produced in various places. After selected amount, which is set to be 35 kg in this experiment, of raw material is fed into the pot, the bed is exposed to the LPG burner for 9~12 sec and then ignited. Air inlet velocity, flow rates and flue gas compositions including O 2, CO 2, and CO are measured continuously. Mass conservation of the gas is checked by measuring inlet velocities and volumetric gas flow rate in the outlet. 4. Results and Discussion 4.1 Quantification of the results Propagation speed of combustion in an iron ore sintering bed is one of the key parameter for determining productivity of the process. Quantitative parameters are defined for characterizing the combustion propagation. Flame front speed (FFS) is introduced by previous researchers (7),(1) for expressing propagation speed of combustion zone in the solid fuel bed. FFS is defined as the distance between two specific points over the time consumed for propagation. In this study, the distance is defined as the distance between the location of y = 49 mm (the upper part of the bed) and that of y = 11 mm (the lower part of the bed), where R-type thermocouples were installed. The propagation time is defined as the time consumed for reaching K between the two points. The FFS is mainly influenced by char combustion rate and parameters involving in heat transfer and thermal diffusion within the bed. Sintering time is well known in the field of ironmaking process since it determines the productivity of the whole sintering process. It is defined as the elapsed time until the flue gas temperature indicates maximum, which means that char combustion is completed. It is inversely proportional to the FFS; the sintering time decreases when the propagation of the combustion in the bed is fast. The above two parameters cannot describe the whole combustion conditions in the sintering bed, since they consider only propagation of the combustion zone, not the characteristics of the combustion zone itself. Therefore, in addition to the two parameters, maximum temperature as a function of time is introduced for describing the characteristics of combustion zone. Higher maximum temperature influences quality of the products in a positive way. 4.2 Model verification Table 1 shows the operating and calculation parameters of reference case for model verification. Three phases of material are considered: iron ore, coke and limestone, each of which has a specific diameter and composition. Porosity is considered to be changeable with the bed height during the process. Coke in solid material is ignited by hot gas during 9 seconds. Air suction rate is set to be second order of polynomial as a function of time, which is derived from the measurement result of flue gas. Coke combustion is assumed to follow two steps: conversion of the carbon to CO by surface reaction followed by CO combustion. Fig.3 shows the solid temperature profiles and gas

4 compositions within the iron ore sintering bed. As coke combustion zone progresses downward, solid temperature at a certain point begins to rise and sustains for the period where the fuel component remains. After reaching the peak, the temperature gradually drops to the room temperature, by balancing the heat generation and the convection cooling from the inlet air. The process is shown in a repeated manner from the upper part (y=49mm) to the lower part (y=11mm) of the bed. The simulation results were compared with the estimated experimental data, from the lab-scale sintering pot tests. Fig. 3(b) shows the flue gas composition. After ignition, no external heat source is provided and coke combustion begins. Although the air supply is increasing, CO 2, O 2 and CO concentration remains constant, which implies that the coke combustion rate also increases during the process. After combustion zone reaches to the bottom and coke combustion is completed, CO 2 and CO concentration fall to zero rapidly. The simulation results show general agreement with the experimental results, although there are some differences in the temperature profiles at y = 49 mm and gas compositions after seconds. Table 1 Major input conditions and model parameters for the iron ore sintering bed Bed height 7m Type Air # of cells 57 Oxidizer T 3K tmax (sec) 16 vave 5m/s Δt (sec) 1 Type Hot gas Size 2.mm a Ignition T K 3.2mm b vave 4m/s Moisture 7% Pyrolysis - Solid Coke Material 3.8% A 2.3 Char Iron ore c 83.2% E 22 Limestone c 13.% Gaseous εo reaction CO+O2 CO2 a: Fuel, Limestone, b : Iron ore, c : including moisture 2 16 Measured Computed y = 49 mm y = 3 mm y = 11 mm Concentration (Vol. %) ,, : Measured,, : Computed O 2 CO 2 CO (b) Gas compositions Fig.3 Comparison of the simulation results with sintering pot test results 4.3 Effect of fuel reactivity Coke is a solid fuel derived from coke process, which convert coking coal to coke by carbonization. It has high reactivity and high cost compared with other types of coal. For this reason, coke in iron ore sintering process is being replaced in part with powdered anthracite of a low-cost fuel. However, its difference in reactivity can cause an undesirable effect on productivity and quality of sintered ore. The study described in this section is aimed to obtain basic results in the process of setting adequate operating conditions for using anthracite as a solid fuel. The anthracite used in this study is from China, which is employed in commercial iron ore sintering process in Korea. Table 2 shows the composition and heating value of coke and anthracite. The anthracite employed in this study contains ~3 % volatiles. Coke contains more fixed carbon and less ash content than the anthracite, which makes the heating value of coke is higher than that of the anthracite. Ratio of the fixed carbon in the coke and that in the anthracite has almost the same value as the ratio of the heating value of coke and that of anthracite. Table 2 Results of elemental and proximate analysis for coke and anthracite Temperature (K) (a) Temperature profiles Coke Anthracite C H Elemental O Analysis (%) N S 4 1 Proximate Volatile analysis Fixed carbon (%, dry base) Ash Heating value (kcal/kg)

5 Fig.4 presents the results of thermo-gravimetric analysis for coke and anthracite. There is a slight mass reduction of anthracite in inert environment, which shows the release of volatile content in anthracite. In the oxidation environment, TG curves show evidently that mass reduction of the coke by char combustion is faster than that of anthracite. From the DTA curves for the two kinds of the fuel, meaningful difference of char reactivity can be found. The reactivity of coke is higher than that of anthracite. That difference can affect the combustion conditions in the sintering bed. Fig.5 shows simulation results of temperature distribution within the sintering bed for the cases of using coke and anthracite. For the case of using highly reactive fuel (coke), front of the combustion zone reaches to the bottom more quickly, which implies that the propagation of combustion is faster. The result is shown more clearly in Fig.6, which shows FFS and sintering time for the cases of using coke and anthracite. For using coke, the value of FFS is larger and sintering time is smaller than the values of the case of using anthracite. It shows that the coke has advantages as fuel of sintering process from the view of productivity, which is proportional to the propagation of combustion zone. W/W 1..8 carrier gas : Ar DTA TG carrier gas : Air time(min) anth coke Fig.4 Results of thermogravimetric analysis for coke and anthracite -d(w/w )/dt 5 (b) anthracite Fig.5 Temperature distribution for coke and anthracite Flame front speed (cm/min) Coke1 Flame front speed Sintering time Anth1 Fig.6 FFS and sintering time for Coke1 and Anth1 Fig.7 shows the maximum temperature for coke and anthracite. It exhibits that the maximum temperature for highly reactive fuel (coke) rises and reaches to peak more quickly, which implies that the temperature in the upper part of the bed is higher in case of using coke than the case of using anthracite. And the maximum temperature for the case of using coke begins to fall down to zero earlier than the case of using anthracite, which shows that the combustion process is completed earlier for using coke. These results are clearly exhibited in Fig. 8, which shows the mass reduction curves of the fuels derived from the calculation results. The trend of mass reduction in the sintering pot shows similar manner with that of the results of thermo-gravimetric analysis. Sintering time (sec) 5 (a) coke

6 Maximum temeperature (K) Coke Anthracite temperature during 1~4 sec shows significant difference for each case. The temperature rises more quickly for small diameter of the anthracite, which implies that the solid temperature in the upper part of the bed can maintain high temperature during the period. This shows that anthracite has possibility to be suitable in the sintering process by controlling the particle size. Mass of Carbon (kg) Fig. 7 Maximum temperature in the sintering bed for using coke and anthracite Mass-Coke1 Mass-Anth Time(sec) d(mass)/dt-coke1 d(mass)/dt-anth1 Fig.8 Weight decrease of coke and anthracite d(mass of Carbon)/dt (kg/s) 5 (a) Diameter of anthracite : 1.2mm 5 (b) Diameter of anthracite : 2.mm Fig.9 Temperature distribution for various sizes of anthracite 4.3 Effect of particle diameter of the fuel Diameter of fuel determines the surface area and influences the combustion rate. For small diameter of the fuel, combustion rate is increased due to the large surface area. Fig. 9 shows the temperature distribution within the sintering bed for various diameters of anthracite. For the smaller particles size, combustion front reaches to the bottom earlier, which means propagation speed of combustion zone is increased. The result is shown in Fig. 1 more clearly. Sintering time can be reduced by ~1 seconds by changing the particle sizes from 2. mm to 1.2 mm. It is resulted from increase in the combustion rate for setting the particle size smaller. That is well shown in Fig. 11, which shows the weight decrease rate is faster for smaller particle size of anthracite. This implies that the combustion rate of the solid fuel can be controlled by setting particle sizes, in spite of the difference of the intrinsic reactivity of the fuel. Fig. 12 shows the maximum temperature in the sintering bed for various sizes of anthracite. Maximum Flame front speed (cm/min) Anth1.2 Anth1.4 Anth1.6 Flame front speed Sintering time Anth1.8 Anth2. Fig.1 FFS and sintering time for various particle sizes of the anthracite Sintering time (sec)

7 Maximum temperature (K) Weight of the anthracite (kg) =1.2mm =1.6mm =2.mm Fig.11 Maximum temperature in the sintering bed for various sizes of anthracite 1..8 increasing Weight =1.2mm Weight =1.6mm Weight =2.mm Time(sec) dw/dt =1.2mm dw/dt =1.6mm dw/dt =2.mm Fig.12 Weight decrease for various sizes of anthracite 5. Conclusion Effects of fuel characteristics on an iron ore sintering process are investigated by numerical approach. The model is 1-D unsteady and treats solid material as multiple solid phases, which make it possible to consider different particle sizes and reactivity. The mathematical model is validated by comparing the computed data with the result of sintering pot test data. Simulation is performed for the cases of using coke and anthracite as a solid fuel. Flame front speed, sintering time and maximum temperature are introduced for characterizing combustion characteristics in the bed. Coke is shown to be more suitable for the process because of its high reactivity compared with anthracite used in this study. However, computational results for various sizes of the anthracite show that the reactivity of anthracite can be improved by decreasing the particle size. Acknowledgements This study is supported by POSCO. The authors are also supported by the Combustion Engineering Research Center (CERC). References (1) Muchi, I. and Higuchi, J., Theoretical Analysis on the Operation of Sintering, Tetsu-to-Hagane 197; Vol. 56 (197), p (2) Hosotani, Y., Fujimoto, M., Konno, N., Okada, T. and Shibata, J., Influence of Oxygen and Humidity of Inlet Gas on the Sintering Reaction, Tetsu-to-Hagane, Vol. 83 (1997), p (3) Cumming, M.J. and Thurlby, J.A., Developments in Modelling and Simulation of Iron Ore Sintering, Ironmak. Steelmak., Vol. 17 (199), p (4) Patisson, F., Bellot, J.P., Ablitzer, D., Marli, re E., Dulcy, C. and Steiler, J.M., Mathematical Modelling of Iron Ore Sintering Process, Ironmak. Steelmak., Vol. 18 (1991), p (5) Nath, N.K., Da Silva, A.J. and Chakraborti, N., Dynamic Process Modelling of Iron Ore Sintering, Steel Res., Vol. 68 (1997), p (6) Ramos, M.V., Kasai, E., Kano, J. and Nakamura, T., Numerical Simulation Model of the Iron Ore Sintering Process Directly Describing the Agglomeration Phenomenon of Granules in the Packed Bed, ISIJ Int., Vol. 4 (2), p (7) Yang, W., Ryu, C. Choi, S., Choi, E., Lee, D. and Huh, W., Modeling of Combustion and Heat Transfer in an Iron Ore Sintering Bed with Considerations of Multiple Solid Phases, ISIJ Int., Vol. 44, No. 3 (24), p (8) Hobbs, M.L., Radulovic, P.T. and Smoot, L.D., Combustion and Gasification of Coals in Fixed Beds. Prog. Energ. Combust., Vol. 19 (1993), p (9) Wakao, N. and Kaguei, S., Heat and Mass Transfer in Packed Beds: Gordon and Breach Science Publishers, (1982) (1) Shin, D.H. and Choi., S., The Combustion of Simulated Waste Particles in a Fixed Bed, Combust. Flame, Vol.121 (2), p