GENERAL RESEARCH Modeling of Rotary Cement Kilns: Applications to Reduction in Energy Consumption Kaustubh S. Mujumdar, Amit Arora, and Vivek V. Ranade* Industrial Flow Modeling Group, National Chemical Laboratory, Pune 411008, India Rotary cement kilns are used for converting calcineous raw meal into cement clinkers. In this paper, we discuss and evaluate possible ways of reducing energy consumption in rotary cement kilns. A comprehensive one-dimensional model was developed to simulate complex processes occurring in rotary cement kilns. A modeling strategy comprising three submodels, viz. a model for simulating the variation of bed height in the kiln, a model for simulating reactions and heat transfer in the bed region, and a model for simulating coal combustion and heat transfer in the freeboard region, was developed. Melting and formation of coating within the kiln were accounted for. Combustion of coal in the freeboard region was modeled by accounting for devolatilization, finite-rate gas-phase combustion, and char reaction. The simulated results were validated with the available data from three industrial kilns. The model was then used to understand the influence of various design and operating parameters on kiln performance. Several ways of reducing energy consumption in kilns were then computationally investigated. The model was also used to propose and to evaluate a practical solution of using a secondary shell to reduce energy consumption in rotary cement kilns. Simulation results indicate that varying kiln operating variables, viz. solid flow rate or RPM, can result only in small changes in kiln energetics. Use of a secondary shell over the kiln and energy recovery by passing air through the annular gap between the two appears to be a promising way to achieve significant energy savings. The developed model and the presented results will be useful for enhancing the performance of rotary cement kilns. 1. Introduction Rotary cement kilns are used for producing cement clinkers from calcineous raw meal. A schematic of the cement-making process is shown in Figure 1. Typically, cyclone preheaters, a kiln, and a cooler are the main components of cement manufacturing. The raw meal (a mixture of predetermined proportions of limestone, silica, and small quantities of alumina and iron oxide) is first preheated in a series of cyclone preheaters via hot gases coming from a kiln and cooler. The last of the preheaters acts as a calciner, where ∼60-80% calcination takes place. The partly calcined charge from the calciner is fed slowly to a rotary kiln. The energy required for endothermic clinker reactions is provided by combusting a suitable fuel in the kiln. Since most of the cement kilns in India are equipped with pulverized coal burners, such kilns are considered in this work. The pulverized coal along with the preheated air (secondary air) is fed to the kiln in a countercurrent mode with respect to the solids. In the initial region of the kiln (from the solid entrance), the remaining calcination is completed. Other solid- solid reactions also occur as the solids move forward in the kiln. Rotary kilns are designed and operated in such a way as to ensure uniform mixing and heating of the solid bed. One of the key reactions in clinker formation occurs in the melt phase. The hot clinkers and part of the melt are discharged from the kiln to the clinker cooler. Clinker coolers are used to preheat secondary air supplied to the kiln. Hot gases exiting the kiln are fed to the calciner/preheaters to recover energy. Operation of rotary cement kilns is very energy intensive. A typical rotary cement kiln producing ∼3000 TPD of clinkers requires an energy input of ∼1000 kJ/(kg of clinker) (in terms of coal fed to the kiln). Considering the volumes involved in manufacturing cement, it is worthwhile to explore ways to reduce the energy consumption per unit weight of product. The energy consumption in cement kilns is attributed to several processes occurring within the kiln, namely, those for raising the temperature of the solids to the reaction temperature, for endothermic clinkerization reactions, and for melting of solids. Apart from these, energy is lost by the kiln to the environment and other connected equipment. The energy of hot solids and hot gases leaving the cement kiln is recovered (at least partially) in the clinker cooler and the calciner/preheaters. The energy lost to the surroundings from the kiln surface * Corresponding author e-mail: vv.ranade@ncl.res.in. Figure 1. Schematic of clinker production. 2315 Ind. Eng. Chem. Res. 2006, 45, 2315-2330 10.1021/ie050617v CCC: $33.50 © 2006 American Chemical Society Published on Web 03/01/2006