Development of a model for calcination mechanism in a vertical shaft kiln.

Abstract

Limestone exists in large quantities throughout the world in many forms and various lime stones differ considerably in their chemical composition and physical structure. Limestone is an important raw material for many branches of industry. The greater part of limestone is decomposed to lime (CaO) and (CO2) in many different kinds of reactors before final utilization. This process is called calcination The calcination of limestone (CaCO3) is a highly endothermic reaction requiring 3.16MJ of heat input. Kiln is an oven, furnace or heated enclosure used for processing a substance by burning, firing or drying. A basic model will be developed using the mass and heat balances for the gas species and the solid phases. It will include chemical reaction enthalpies for the conversion of fuel and limestone. Heat and mass transfer between phases will be modelled as convection. The model will predict temperatures, conversion degree, mass flow rate, gas concentrations, profiles of gas and solid phases as a function of vertical position. Also, the pressure drop inside the kiln and gas velocity are predicted by the model. The validation shows that, the model has captured the essential phenomena sufficiently detailed for predicting temperature in the kiln as a function of the vertical position. The mathematical model of vertical shaft lime kiln is used to illustrate the effects of different operating conditions on the kiln operation. The parameters studied include fuel ratio, air excess number, lime throughput, limestone size and reactivity, kind of fuel, coke size and reactivity. The applications show how the model can give insight for the kiln performance under different operating conditions.