Improving the Quality of Alumina-Containing Sinter Using Water-Cooled Furnace Shell

This article deals with the use of computer modeling to develop technical solutions to ensure better quality of alumina-containing sinter. The simulation accounted for the influence of the feed materials on the thermal processes in the furnace. The energy balance (including thermal conductivity, heat convection and radiant heat exchange) was solved assuming steady state. A good correlation was observed for the actual and calculated temperatures of the solids and gases, with less than 15% discrepancy. Using the model of the furnace investigated the possibility of lowering the temperature of sintering by removing heat from the outside of the furnace shell. To reduce the sintering temperature to 1000 °C length of the refractory lined steel is 5 m, the height of the lining should not exceed 0.06 m, the required rate of cold water 54.7 m/h


Introduction
Rotary kiln are widely used in metallurgy for thermal processing of mineral raw materials and semi-products.They are employed in the production of alumina form low-grade bauxite or nepheline ores in the main stage of raw material (alumina mixture) preparation.Rotary kilns are used most frequently in the production of aluminum containing sinter.
Earlier [1] was the built computer model of the furnace of sinter.Simulation results obtained good agreement with real data measurements performed on an industrial furnace.Using this model, we carry out the calculation of sintering temperature reduction at the outlet of the furnace by removing heat from the outer shell furnace sintering.

Determination of the main flows in the rotary kiln
To build an adequate computer model of a rotary kiln, the main processes that take place in the kiln must be firstly identified and the appropriate methodology chosen.
The theoretical models used to develop a computer model presented in the article [1].Therefore, we will not describe them in detail, we give only a sketch of heat flow in the furnace.
The mathematical apparatus for the thermal processes in the furnace is based on heat exchange equations [2][3][4][5][6].The conditions of raw material heating in a rotary kiln are defined by complicated heat transfer processes.
Rotary kiln used to obtain sinter employ the counter-flow principle.The raw mixture is fed to its cold header, while the air-fuel mixture is blown through the hot header.Hot gases move towards the materials and heat them.
The heat of fuel combustion is transferred to both the material and the furnace lining.As the furnace revolves, the material is constantly moving horizontally and axially.The heat, accumulated by the lining, is then transferred to the material.Thus, the lining acts as a heat accumulator.Fig. 1 shows the cross-section scheme of a rotary kiln with existing energy flows.-passing of heat flux through radiation and thermal conductivity, between kiln lining and material.Q rad.gaz-lin.and Q conv.gaz-lin.-passing of heat flux through radiation and convection, between gas phase and kiln lining.Q rad.gaz-mat.and Q conv.gaz-mat.-passing of heat flux through radiation and convection between gas phase and heated by the material.Q cond.lin.and Q cond.body.-passing of heat flux through lining and metallic kiln body through thermal conductivity.Q loss -loss of heat through kiln body.

Simulation coal combustion, removals of volatile components
The combustion model chosen was the eddy dissipation model (EDM).The EDM model is designed to characterize turbulent flame, pre-stirred mixtures and diffusion flame, and is reputed in project engineering of combustion chambers for gas turbines, to predict temperature gradients inside the flame tube, and the corresponding thermal loads.
Several types of one-step reactions were accounted for: evolution of volatile compounds form coal, oxidation of the volatiles, oxidation of the carbon residue.
The discreet transfer method (DTM) was used to consider radiative heat transfer.To characterize the rate of the CH 4 +2O 2 =CO 2 +2H 2 O reaction of hydrocarbon turbulent-flow combustion, a model was used that assumes the rate of a gas-phase reaction to be limited by the time of reaching molecular-level mixing of the reactants [7].
Heterogeneous reactions of volatiles' pyrolysis and carbon combustion, which conforms to the brutto-reaction C+O 2 =CO 2 on the surface of porous coke particles' proceed according to diffusion-kinetic dependencies.

Description of parameters and input data for model
The shell, lining and free space of the furnace, and the material layer are defined as separate geometrical objects with certain relations between them.External aerial environment as a separate domain holding the other domains of the furnace was created to enable the most complete heat transfer.The starting air temperature was 10 °C, the transverse velocity -2 m/s.
The furnace outer metal shell and the brick lining represent two 25 m-long hollow interposed cylinders, with thickness of the shell being 50 mm, and thickness of the brick lining -300 mm.
According to results of laboratory researches it has been established that the condition of getting of the sinter of high quality is cooling it from the sintering temperature to 1000ºC.One possible way is to cool the outer section of the furnace shell.Using the brick lining has a low thermal conductivity (0.8 W/m•K) can not remove the required amount of heat to reduce the temperature of the material.In the case of reducing the thickness of the lining is likely to accelerate the destruction of the lining material due to increased thermal stresses in the brick.Therefore, the calculation was applied to heat-resistant alloy having high thermal conductivity of 30.2 W/m•K and a low coefficient of abrasion.
The length of the section of the lining of heat-resistant steel is 5 m (site 59 -65 m).The height of the lining is chosen to obtain the required temperature at the output sinter.In order to create conditions simulating the cooling water, outside the outer shell furnace was installed an additional domain, through which the water flow rate is set at 50 m 3 /h with an initial temperature of 20 °C.
To perform calculations used tetrahedral cell grid with an average of 100,000 cells for each domain.The inlet input boundary conditions for the "material" domain are material introduction rate -0.014 m/s and starting temperature 1100°С.The gauge pressure at domain outlet is set to 101325 Pa (free output).
The inlet boundary conditions for the "furnace free space" domain are represented in Table I The positions of coal and dust nozzles in the furnace and their geometry are represented in Fig. 2. At the domain outlet the gauge pressure was set to -75Pa (base pressure 101325 Pa).
The task was solved in stationary formulation, i.e. thermal gradients and other thermal values were calculated under constant boundary conditions.The solid phase (material, coal dust, coke and carbon black particles) were used in lagrangian representation, while the gas mixture (O 2 , N 2 , H 2 O, CH 4 , CO 2 ) was used in eulerian representation.
The flow turbulence effects were introduced using the RNG k-e model.This model has shown high stability and accuracy in various scientific and engineering tasks.
The convergence criterion was reaching a value's mean-square discrepancy the level of 10 -4 or lower, together with finding the integral characteristics; the criterion is reached after approx.600 iterations.The iteration management was automatic.

Simulation results and Discussion
The computed thermal gradients along the furnace of materials is shown in Fig. s 3,4.Using the EDM model provided good results, the combustion of coal dust fuel gave a temperature of 1450-1500°C in the sintering zone.The achieved temperature level is in accordance with literature data [8].Coal fuel starts igniting at the distance of 6 m from the hot furnace bevel, the temperature of the flame body is 1670°C.
The temperature of the material is increasing along the movement direction, reaches its maximum in the sintering zone and is then decreasing in the cooling zone.In fig. 3 shows the results showing the temperature distribution of the material at different heights lining.Analyzing the data, we can conclude that to achieve a temperature of 1000 °C the height of the lining should not exceed -0.06 m.
With a further selection of the height of the lining found that the requirements for temperature sintering, the rate of its cooling run at the height of the lining -0.05 m, flow rate -54.7 m 3 /h the final water temperature is 65 °C.Further heated water can be used for industrial consumption, for example for heating of premises.Analysis of the data shows that the distribution of temperature sintering of the material on the site has not changed significantly.The length of the site, where the temperature varies 1200-1250-1200 °C is 14 m, the difference from baseline (15 m) is less than 1.5 m in these circumstances it will not lead to a deterioration in the quality of the final product.

Summary
The computer simulation of thermal performance of tubular rotating furnace for batch sintering, taking into account thermal effects of chemical reactions, those occur in the layer during the heating process, has been developed.Using a computer model examined the external water-cooled furnace shell.To reduce the temperature sinter to 1000 °C length of the refractory lining of steel is 5 m, the height of the lining should not exceed -0.06 m, the required rate of cold water -54.7 m 3 /h The implementation of this decision will generate the optimal phase composition of sinter.

Fig. 1 .
Fig. 1.Cross-section scheme of a rotary kiln with existing energy flows.

Fig. 3 .
Fig. 3.The temperature distribution of the material along the length of the furnace at different heights lining.