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Experiments were performed in a pilot scale rotary kiln with coal and coke particles to study their mean residence time, residence time distribution, bed depth profile and time spent at the bed surface. The influence of filling ratio on residence time was studied with a uniform bed depth in the kiln. Residence time distribution and bed depth profile measurements were performed in a kiln without end constriction, at different rotational speeds and different solid inputs. The residence time fraction corresponding to the passage of the particles at the bed surface was measured with a photographic study of the movement of a coloured particle. Various equations were tested to represent the experimental results. The equations of Kramers and Croockewit (1952) and Ronco (1965) were adequate to calculate mean residence times, while only the former could represent correctly the bed depth profile. An equation is proposed which turns out to be quite accurate in predicting the fraction of the residence time spent in the upper layer of the bed.

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In order to simulate coal pyrolysis in a rotary kiln in the steady-state regime, a mathematical model has been developed which calculates the temperature profiles in the charge, the gas, and the furnace walls, together with the gas composition and the degree of removal of volatile species. The model takes into account the principal physicochemical and thermal phenomena involved, including the complex movements of the charge; the gas flow; heat transfer between the charge, the gas phase, and the furnace walls; drying and pyrolysis of the coal; the cracking of tars; the combustion of volatile species; and the combustion and extinction of the coke. The data necessary for the model were obtained by specific experiments or from the literature. The model has been validated by comparing its predictions to measurements performed on an industrial rotary kiln. The model has been used to study the influence of operating parameters such as the furnace rotation speed, in order to optimize the process. It is shown how a modification to the extinction zone leads to an increase in coke yield of 0.75 pct.

A mathematical model is presented which describes the pyrolysis of a single grain of coal and is designed to be incorporated into an overall model simulating the rotary kiln coal pyrolysis process. The grain model takes into account the principal physical phenomena occurring during the conversion of coal to coke, namely, heat transfer toward and within the grain, drying of the coal, and the evolution of volatile species. Particular care has been taken in the determination of the thermophysical and kinetic parameters necessary for the model. Thus, the drying kinetics for Lorraine coal were measured by thermogravimetry. The kinetics of pyrolysis were determined by both thermogravimetry and gasphase chromatography, in order to separately monitor the evolution of the nine gaseous species considered. The true specific heat and the thermal conductivity of the solid were also mesured as a function of temperature. The numerical model, based on the finite-volume method, calculates the temperature, composition, and mass flow rates for the different gases evolved at each point in the grain at any instant of time. The model was, finally, validated by comparing the calculated and measured values of the overall conversion of the pyrolysis reaction and the temperature at the center of the grain.

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