Modelling of combustion and calcination in a cement precalciner D.K. Fidaros, C.A. Baxevanou, C.D. Dritselis, N.S. Vlachos ∗ Department of Mechanical & Industrial Engineering University of Thessaly Athens Avenue, 38334 Volos, Greece Abstract A numerical study is presented of the reacting flow and transport processes taking place in a calciner for cement production. The CFD model is based on the solution of the Navier-Stokes equations for the gas flow and on Lagrangean dynamics for the discrete particles with turbulence incorporated via the k-ε model. Distributions of velocities, temperatures and concentrations of the reactants and products as well as the trajectories of particles and their interaction with the gases are presented, allowing assessment of a particular calciner design to be made. Key Words: CFD, coal combustion, calcination, calciner modeling, cement production ∗ Corresponding author: vlachos@mie.uth.gr Proceedings of the European Combustion Meeting 2007 1. Introduction Cement production involves raw-mix preheating and calcination, clinker formation and cooling to achieve the required crystallographic structure. After cooling, the clinker is mixed with plaster and other additives in grinding mills that consume a very large amount of energy. The raw-mix consists mainly of pulverized calcium carbonate and silicon dioxide. During its preheating the moisture evaporates and at 890 o C the endothermous calcination reaction begins, where CaCO 3 is converted into CaO and CO 2 . The required activation energy is provided by the fuel heat of combustion. Dry heating of raw-mix in vertical suspension preheaters is mostly used, where some calcination also takes place. Modern cement plants use an additional device, in which the raw-mix undergoes calcination to a high level (90-95%), Fig.1. Thus, the calcined raw-mix enters the rotary kiln at a higher temperature, reducing its energy demand. The placement of calcination outside the cement kiln results in better quality of CaO and energy savings. For example, in the Olympus plant of AGET Hercules in Greece calcination takes roughly 60% of the total heat, while 35% is spent for preheating and 5% for clinkering [1]. This 60:40 ratio is reversed when calcination takes place inside the rotary kiln. After heated to appropriate temperature, the raw- mix enters the calciner with the fuel and hot combustion air. The combustion heat causes calcination according to the reaction: mol KJ CO CaO CaCO K / 178 2 1160 3 + + ⎯ ⎯ → ⎯ The high fineness of raw-mix and good turbulent mixing produce faster coal combustion and raw-mix calcination with good efficiency at lower temperatures. Among the advantages of calcination devices (see Fidaros et al [2]) are: a) The addition of a burner in the calciner increases the kiln capacity, b) The reduction of thermal load of the kiln extends its operational life, c) The reduction of energy demand and the minimal calcination in the kiln reduce its exhaust gases and heat, losses, d) The combustion at medium–low temperatures Fig. 1 Schematic of the calciner (<1400 °C ) in the kiln reduces the production of Nox, e) The lower temperatures in the calciner allow the use of fuels with relatively low heat value, and f) The reduced kiln thermal load decreases the condensing of vapours (SO 3 , Na, K and Cl). Some disadvantages are: a) The lower temperatures of the exhaust gases may cause condensation of volatile alkalis, and b) The use of low heat fuels requires particular attention to avoid undesirable emissions of polluting and eroding gases. It is important to control calcination as it affects fuel consumption, emission of pollutants and the final quality of cement. Using CFD, calciners have been studied at different geometries and operating conditions. For example, Hu et al. [3] used a 3D Eulerian- Lagrangean model in a dual combustor and precalciner to predict the burnout and decomposition ratio during simultaneous injection of two types of coal and raw material. Iliuta et al. [4] investigated the influence of operating conditions on the level of calcination, burnout and NOx emissions of an in-line low NOx calciner. 2. Specific Objectives In the present work a numerical model and a parametric study is presented for the flow and transport processes taking place in an industrial calciner. The model is based on the solution of the Navier–Stokes