An experimental study of convective heat transfer from hot air to the solid charge and walls in a non-fired rotary kiln is reported. Ottawa sand was heated by passing it counter-current to a flow of preheated air in a 2.5 m × 0.19 m I.D. rotary kiln. Axial temperature profiles of gas, wall and solids were measured. Local and average convective heat transfer coefficients from gas to solids and from gas to wall were determined assuming plug flow of gas and solids. Solid feed rates to 1750 kg/m2 h and air rates to 3300 kg/m2 h were investigated at rotational speeds to 6 r/min, holdup ratios to 17% and gas temperatures from 350–590 K. The gas/solids convective coefficient was found to depend on the gas through-put and to a lesser extent on solids holdup and rotational speed. Over the range tested, the angle of kiln inclination, solids throughput and particle size showed no significant effect on heat transfer. Gas/wall coefficients were about a factor of ten below gas/solid coefficients. Heat transfer results are compared to the limited data available in the literature, and to commonly used equations. Correlations of the experimental data on gas/solids, and gas/wall coefficients are presented; data from the literature on the wall/solids heat transfer coefficient are summarized.



On presente une etude experimentale sur le transfert thermique convectif de l'air chaud a la charge solide et aux parois dans un four rotatif sans brǔlures. On a chauffe du sable Ottawa en le faisant passer a contre-courant dans un flux d'air pre-chauffe, dans un four rotatif de 2.5 metres par 0.19 metre de diametre interieur. On a mesure les profils des temperatures axiales du gaz, des parois et de la charge solide. On a determine, en supposant l'existence d'un ecoulement piston du gaz et des solides, les coefficients de transfert thermique convectif local et moyen du gaz a la charge solide et du gaz aux parois. On a etudie des vitesses d'alimentation en solides allant jusqu'a 1750 kg/m2 h et des vitesses de l'air atteignant 3300 kg/m2 h et ce, a des vitesses de rotation allant jusqu'a 6 tours a la minute, pour des rapports de retention de 17% et des temperatures comprises entre 350 et 590 K. On a trouve que le coefficient de transfert thermique convectif du gaz a la charge solide dependait du debit du gaz et, a un degre moindre, de la retention des solides et de la vitesse de rotation. On a constate qu'a l'echelle des experiences faites, l'angle d'inclinaison du four, le debit et les dimensions des particules solides n'avaient aucun effet important sur la transfert de chaleur. Les coefficients de transfert thermique convectif du gaz aux parois avaient une valeur dix fois moindre que celle des coefficients de transfert du gaz aux matieres solides. On a compare les resultats relatifs aux transfert de chaleur avec les renseignements restreints qu'on trouve dans la litterature technique et avec les equations usuellement employees. On presente des correlations entre les resultats experimentaux obtenus, pour les transferts de chaleur entre gaz et solides et gaz et parois; on resume aussi les donnees parues dans la litterature technique relativement au coefficient de transfert de chaleur des parois aux solides.

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Convective heat transfer in a rotary kiln

Radiative heat transfer between a nongray freeboard gas and the interior surfaces of a rotary kiln has been studied by evaluating the fundamental radiative exchange integrals using numerical methods. Direct gas-to-surface exchange, reflection of the gas radiation by the kiln wall, and kiln wall-to-solids exchange have been considered. Graphical representations of the results have been developed which facilitate the determination of the gas mean beamlength and the total heat flux to the wall and to the solids. These charts can be used to account for both kiln size and solids fill ratio as well as composition and temperature of the gas. Calculations using these charts and an equimolar CO2−H2O mixture at 1110 K indicate that gas-to-surface exchange is a very localized phenomenon. Radiation to a surface element from gas more than half a kiln diameter away is quite small and, as a result, even large axial gas temperature gradients have a negligible effect on total heat flux. Results are also presented which show that the radiant energy either reflected or emitted by a surface element is limited to regions less than 0.75 kiln diameters away. The radiative exchange integrals have been used, together with a modified reflection method, to develop a model for the net heat flux to the solids and to the kiln wall from a nongray gas. This model is compared to a simple resistive network/gray-gas model and it is shown that substantial errors may be incurred by the use of the simple models.

Radiative heat transfer in rotary kilns

A model for rotary kiln heat transfer, which accounts for the interaction of all the transport paths and processes, is presented in a three-part series. In this first paper, the pilot kiln facility is described, and the significant results from the heat-transfer trials are identified. Limestone, Ottawa sand, and pctroleum coke were heated using a range of firing rates, while other operating variables were held nearly constant. Measurements were made to obtain the net rates of heat transfer for the bed material, freeboard gas, refractory wall, and, unique to the study, the radial heat flux at the inside refractory surface as a function of circumferential position. High rates of net heat input to the bed material, occurring very near the feed end, were found to decline quickly with distance, and for an inert bed, leveled out at a value well below the rate of loss through the kiln wall. The onset of an endothermic bed reaction resulted in sharp increases in both the temperature cycling at the inside refractory surface and the net heat input to the bed, but no corresponding jump in the kiln wall heat loss. The temperatures of the bed material and inside refractory surface always were coupled closely, even in the presence of bed reaction. Regenerative heat transfer from the covered wall to the contacting bed material was not a major component of the net input to the bed, and for the inert bed, negative regeneration was en-countered beyond the kiln midpoint.

A heat-transfer model for the rotary kiln: Part I. pilot kiln trials

Some aspects of heat transfer in rolling mode rotating drums operated at low to medium temperatures

Six algorithms following single particle trajectories are used to predict the axial dispersion in rolling or slumping flow in a continuous rotary kiln. Models incorporating different physical phenomena show that axial dispersion is affected by the Froude number, L/D ratio, solid fill level, and rolling or slumping layer thickness. The main cause of axial dispersion is segregation of the nonuniform particle size, density and shape. In a rolling bed with uniform particles the rolling layer thickness and the time of roll have to be accounted for in the prediction of the dispersion. The Peclet numbers computed for sold particles with uniform physical properties are of the order of 104. The sold segregated motion may lead to Peclet numbers of the order of 10 – 103, a much larger dispersion. The main obstacle for a-priori prediction of the axial dispersion is lack of a reliable relation between the segregated roll or slump distance and the variation in particles properties, as well as the change in segregation as the flow properties of kparticles are changed during calcination.

Axial dispersion of solid particles in a continuous rotary kiln

A mathematical model has been developed to predict the operating behavior of an SL/RN direct reduction kiln from a knowledge of the main process variables. The model is based on steady state principles and is capable of quantitatively describing the different chemical reactions in the kiln such as reduction, Boudouard reaction, coal devolatilization, combustion in the freeboard together with the mass and heat flows. Results of the model, which are in the form of axial temperature profiles in the freeboard gas, solids bed and wall, and axial concentration profiles in the gas and solids, are in good agreement with measurements made on a 200 ton per day pilot plant at the Steel Company of Canada. The model has been used both to investigate the influence of process variables on the kiln performance and to elucidate operating features of the process. The model has shown that highly reactive coals such as subbituminous and lignite, and highly reducible pellets are most suitable for the SL/RN process. It has also demonstrated that the most important process variable from the standpoint of control is the air profile in the kiln; and consequently that heat transfer is the rate controlling step. The model has also been employed to examine scale-up of the SL/RN process by calculating the operating behavior of a large commercial kiln. It has shown that the large kiln should run cooler than the pilot-size unit so that accretion problems may be significantly reduced.

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V. Venkateswaran

Papers

Mathematical model of the SL/RN direct reduction process