Combustion Behavior of Coke in Shaft Kilns with Hypostoichiometric Air Flow

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.