DEM/CFD simulations of a generic oxy-fuel kiln for lime production

Abstract

Shaft kilns are the common type of reactor for the production of quicklime. Limestone (CaCO3) particles of cm-size are converted to quicklime (CaO) with the energy provided for calcination by the conversion of a typically gaseous fossil fuel. The CO2 emissions of lime production are severe due to the calcination reaction and the CO2 from combustion. Therefore, methods which pave the way for CO2 capture, such as the Oxy-Fuel technology, are of great interest for the lime industry. This motivates the current contribution, which examines the Oxy-Fuel operation of a single-shaft kiln fired by methane and the results are compared to conventional air operation. A generic, down-scaled version of a kiln has been examined. The height of the kiln is 2.5 m and its diameter is 0.64 m. It contains 23,000 limestone particles with four size classes approximating a polydisperse distribution. The particles are assumed to be spherical. The thermal input of the kiln has been fixed to 80 kW. The evaluation is based on 3D DEM/CFD simulations of the kiln. For simplicity, to avoid the combustion simulation, the exhaust gas produced by the burners of the system is precomputed by Cantera and added to the computational domain as a source term. The source terms are added in two different ways: homogeneously distributed over the cross section at a certain kiln height and spatially distributed at four different locations in a cross section to mimic the presence of burner lances, which are used in industrial kilns.The study shows that the average calcination degree is around 5 % lower for Oxy-Fuel operation compared to the conventional air-fired mode since the increased CO2 concentration hinders the reaction. Further, it is evident that considering a spatially resolved fuel inflow results in an improved degree of product calcination in the order of 5 %, implying that the positioning of burner lances is crucial for the design of industrial kilns.