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. To model the processes in the kilns the conversion behavior of the coke and anthracite particles must be known. This conversion results from the chemical reaction kinetics as well as from the heat and mass transfer which is strongly influenced by the operational parameters and material properties. 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 of different origin with 38mm diameter were gasified with carbon dioxide and nitrogen mixtures in a tube furnace. The influence of the origin of the coke and the diameter of the coke particles on the combustion time and the length of the combustion zone in mixed feed shaft kilns are discussed. It is shown how the combustion zone is shifted to the bottom of the kiln with the using of a coke which has lower reactivity.