Modeling of the Thermochemical Conversion of Biomass in Cement Rotary Kiln

Abstract

Because of the depletion of fossil fuels and because of its increasing cost, waste has been used as alternative fuels in cement rotary kilns for several years. In order to fulfil the requirements of environmental protection and quality of the final product, it is necessary to understand and quantify the different processes occurring in the kiln. The aim of our work is to develop a mathematical model of the processes occurring in the kiln. This model will rely on the coupling between a CFD model and homemade software. More precisely, the CFD model, which will be fully three-dimensional will account for the homogeneous processes taking place in the freeboard of the bed of material being processed. This bed of material will be at the center of the second model which will represent it as a 1D plug flow reactor. In the present work, we focus on this 1D model. We first give insights on the main assumptions on which the model relies, and information on the reaction pathway leading to the production of cement. Indeed, it is considered that the bed is composed of a mixture of CaCO3, MgCO3, Al2O3, SiO2, Fe2O3, MgO, CaO, C2S, C3A, C4AF and C3S undergoing thermochemical transformation. The bed under consideration is also composed of biomass (agricultural residues). During its transformation (pyrolysis, combustion of volatiles, combustion of the pyrolysis residue), this material contributes to thermal equilibrium of the reactor, by carrying the energy associated to its complete combustion. In this paper, the different equations that translate into mathematical formalism the processes of transport of the bed as well as mass and energy balance are also presented. The results show that the cement obtained complies with the requirements of Portland cements (73.06% of Silicates and 18.76% of Aluminates), the conversion of the biomass is total (100%), and the specific energy consumption is almost in conformity with the values of literature.