Simulation of Fluid Flow, Combustion, and Heat Transfer in a Coal-Fired Cement Kiln

Abstract

This paper presents the details of a mathematical model for fluid flow, combustion, and heat transfer in cement kilns. The model is based on the finite-volume technique for the solution of the gas-phase equations. The k-ε model is used to represent the turbulence effects. A two-step kinetics scheme is employed to simulate the gas-phase combustion. The Lagrangian formulation is employed for the particle phase, and the coupling between the particle and gas phases is handled by introducing appropriate source terms in the gas-phase equations. The particle reaction model includes coal devolatilization and char oxidation. The radiation heat transfer in the kiln is modeled using the six-flux model. The feed bed is represented as a solid region that is moving in the axial direction with a uniform velocity, which is calculated from the feed rate, feed density, and the fill area. The effect of chemical reactions in the feed bed is modeled by including temperature-dependent heat release/absorption rates in the energy equation. The model has been applied to a cement kiln in operation. The predicted results agree with the observations and experience for the kiln.

Modeling of Rotary Cement Kilns: Applications to Reduction in Energy Consumption

“…The chemical reactions occurring in the bed are driven by energy supplied by the freeboard and kiln walls. Although heat transfer occurs by all three mechanisms, radiation is a dominant mode of heat transfer. , There was a considerable work on heat transfer in rotary kilns. − Gorog et al . have reported a comparison of results with and without considering contributions of radiative transfer over axial distances.…”

“…Heat transfer from the freeboard to the solid bed needs to be captured accurately, since this drives the chemical reactions in the bed. For operating conditions prevailing in cement kilns, radiation dominates the heat transfer process. ,, Key parameters appearing in heat transfer models are emissivity of gas, absorption coefficient of gas, and conductivity of gas and solid. Estimation of these properties is not straightforward.…”

“…The velocity profiles of air are highly nonuniform, especially near the kiln burners. Comprehensive 3D computational fluid dynamics (CFD) models have been published in the literature to capture these complex flows in the freeboard region. , However, transport processes and reactions occurring in the freeboard region encompass a wide range of length and time scales, and therefore, developing CFD models to simulate the entire kiln becomes computationally prohibitively expensive. The motivation of the present work is to develop a computational model, which would require orders of magnitude lower computational resources and still provide a realistic simulation for industrial cement kilns.…”

“…Modeling of the heating phase of the coal particle is fairly straightforward. Modeling of coal devolatilization is also fairly well-established. , Most widely used approaches use correlations of volatile yield with particle temperature or define devolatilization rates using single- or two-step Arrhenius schemes. , Modeling char combustion is, however, not straightforward. Char combustion is controlled by a complex coupling between transport phenomena around and within the particle.…”

“…It is also evident that, in order to develop a reliable computational model, it is required to incorporate all the above-mentioned processes occurring in cement kilns. None of the previously published models for cement kilns have done this. ,, We make such an attempt here. The motivation of the present work was to predict flow of solids, heat transfer (between solids, gas, and kiln walls), melting of solids, coating formation, and clinker reactions simultaneously in a single framework.…”