Energy and exergy assessments of a lime shaft kiln

Abstract

Calcium oxide (CaO), commonly known as lime or quicklime, is an energy intensive product. In order to produce lime, vertical shaft kilns are widely used in the lime industry. The objective of this work is to analyze the energy and exergy consumption of the calcination process in vertical shaft kilns, in order to identify the factors affecting fuel consumption. Data on energy and exergy consumption and losses throughout the calcination process are given for two shaft kilns. In the process, the energy efficiency is found to be higher than the exergy efficiency, e.g. 71.6% and 40.8% for the energy and exergy efficiency of one of the kilns. Results also showed that the most irreversible processes taking place in the kiln are the exergy destruction due to fuel combustion and the exergy destruction due to internal heat and momentum transfer both accounting for about 40% of the efficiency loss. Moreover, the main exergy loss through the boundaries of the kiln is the exergy loss with the exhaust gases contributing with more than 10% of the efficiency loss. Improvements on both the energy and the exergy efficiencies can be achieved through a better control of the operational parameters of the kiln (ratio limestone/fuel supply, excess of combustion air, size and size distribution of the limestone fed to the kiln and exit temperature of quicklime flow). The present study proposes a tool for the analysis of energy and exergy utilization of the calcination process in limekilns, and also provides some energy conservation measures.