Design requirements and performance optimization of waste heat recovery systems for rotary kilns

Abstract

Heat loss from the shell of a rotary kiln accounts for a certain proportion of total energy consumption. In order to reduce heat loss, a practical heat recovery system with nine heat exchangers is proposed to preheat water in this paper. We first propose a mathematic model to analyze the shell temperatures and heat loss rates of several regions on rotary kilns. Integration of theoretical analyses and experimental measurements yields the temperatures and heat transfer rates of heat exchangers, i.e. the design requirements of the nine heat exchangers in the practical system. Secondly, an optimization model is formed to describe the relation between design parameters, i.e. heat transfer area and mass flow rate of each heat exchanger, and system requirements without introducing any intermediate temperatures. With the aid of the Lagrange multiplier method, the optimal design parameters are obtained. Finally, an optimization case of the practical system is studied. The results show that the heat recovery system should meet the requirements of chemical reactions in rotary kilns and mechanical characteristic of shell. The required total heat transfer area of the system after optimization is reduced by 15.6% compared to the value before optimization. As the total mass flow rate increase and inlet temperature decreases, the required heat transfer area decreases while the mass flow distribution ratios remains unchanged.