Heat transfer processes in Rotary kilns
Creators: name: Barr, Peter Vernon
Publisher: name: University of British Columbia
Year: 2010

description: An experimental investigation of rotary kiln heat transfer processes was carried out and a unified heat transfer model developed to describe the individual processes and their interaction. A 0.406 m ID by 5.5m refractory lined pilot kiln firing natural gas was utilized for a series of 23 heat transfer trials. Limestone, petroleum coke and two Ottawa sands were heated using a wide range of firing rates while rotation rate, kiln inclination, kiln loading and bed depth were held nearly constant. The bed material was in the rolling mode for all trials. Measurements were made to obtain the net heat transfer rates for the bed material, the freeboard gas, the refractory wall and, unique to the study, the heat flux at the inside wall surface as a function of circumferential position. High rates of net heat input to the bed material, Q[sub b], which occurred very near to the kiln entrance, were found to decline quickly with axial distance and, for an inert bed, the ratio of Q[sub b], to Q[sub ss], the rate of energy loss through the kiln wall, tended toward the ratio of the exposed bed surface area to the exposed wall surface area. At the onset of the limestone calcination reaction Q[sub b], increased sharply without a corresponding increase in Q[sub ss]. Over the fully instrumented portion of the kiln, which extended from 1.32m to 5.0m, the rate of heat transfer from the covered wall to the bed, Q[sub cw →cb], was &lt; 30% of the rate to exposed bed surface from the freeboard. With an inert bed the net exchange Q[sub cw →cb], was found to decline with axial distance and negative values were encountered beyond the kiln mid-point. The onset of bed calcination reversed this trend and positive values of Q[sub cw →cb], were always recorded in the calcination zone. The temperature of the bed material and the inside wall surface were found to be closely-coupled and the actual temperature difference could not be determined due to the limitations of the measuring technique. The onset of bed calcination was always characterized by significant increases in both the net heat input rate to the bed material and the amount of temperature cycling at the inside wall surface. A zone-type real gas model was developed for the radiative heat exchanges in the freeboard and the results presented in the form of radiative heat transfer coefficients. For the pilot kiln, firing at 10% excess air, the coefficient for radiative heat transfer from the freeboard gas to the exposed wall or bed surfaces was calculated to range from 2 15 →55 W/m²K for gas temperatures from 800 →1800 K. Model predictions for a prototype kiln of 4 m ID indicated an increase in the gas to surface radiation by a factor ~ 3. The coefficients for radiative exchange among the freeboard surfaces in the pilot kiln were significantly larger than for the gas to surface exchange while, in the prototype, they were of comparable magnitude. For the inert bed trials the convective component of the freeboard surface heat flux was calculated by subtracting the calculated radiative contribution from the net surface flux. Convection to the exposed bed surface was found to be enhanced relative to the exposed wall surface although less than reported previously. Coefficients for heat transfer between the covered wall and covered bed were shown to be significantly reduced in a refractory lined kiln relative to an unlined metal drum. A finite difference model of the refractory and bed material, incorporating the derived heat transfer coefficients, was verified using the pilot kiln data and extended to examine the relationship existing among the heat transfer processes at any kiln cross-section. Both the close-coupling of the pilot kiln bed and wall temperatures and the high rate of net bed heat input occurring near the kiln entrance and in the presence of a calcining bed were explained by the unified model. Model predictions were obtained for a 4m ID prototype kiln and aspects of kiln thermal performance identified which have important repercussions for the operation of rotary kilns.