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The mixed feed shaft kiln is used in the lime industry in order to manufacture so-called medium to hard-burnt lime. This type of kiln is fired with lumpy coke particles. A mathematical model has been developed for the combustion under shaft kiln conditions which includes a counter current flow and an excess air number lower than one. The reaction with oxygen is dominated by mass transfer. The reaction with carbon dioxide (Boudouard reaction) is dominated by the chemical kinetics. To determine this, spherical coke particles with 30-40mm diameter were gasified with carbon dioxide and nitrogen mixtures in a tube furnace. The influence of the Boudouard reaction on the combustion time and the length of combustion zone in mixed feed shaft kilns are discussed as a function of excess air number, temperature and size. It is shown how the combustion zone is shifted to the bottom of kiln with decreasing excess air number.

This article presents a mathematical process model to simulate the limestone calcination in a single shaft kiln. The process model comprises a set of ordinary differential equations derived from the principle of mass and energy balance, coupled with a discrete particle model based on the shrinking core model. With the computed axial temperature profiles of the gas and solid, the heat loss through the wall as well as the pressure drop is determined. The simulation was done for a shaft kiln with a solid bed height of 13 m and a bed with 70 mm Sauter mean diameter. The width of the particle size distribution was changed to show the influence of the particle size distribution on the process. The results of this study can be transferred directly in the praxis for design, operation, regulation and optimization of the normal shaft kilns.