Heat Transfer in Rotary Kilns
by Greg Palmer, B.E. Ph.D. and Tony Howes, B.E. Ph.D.
Palmer Technologies Pty Ltd, Brisbane, Australia

Abstract
Rotary kilns are one of the most widely used pieces of processing equipment. They are used for drying or calcining a variety of products including sand, aggregates, limestone and food products. With an ever increasing focus on reducing greenhouse gas emissions, the continued or increased use of rotating kilns can only be achieved by reducing the thermal and electrical energy consumption used in these processes. Heat transfer in kilns is very complex, with radiation, convection and conduction all contributing to energy transfer between the gas, the feed and the vessel wall. A fluid bed calciner or dryer achieves rapid drying by the large heat transfer coefficient obtained through the high air volume being circulated. The penalty is the increase in electrical energy required to circulate this high air volume. Rotary kilns on the other hand have poor heat transfer coefficients, hence higher thermal energy demand, due to the need for larger devices and thus more opportunity for heat to be lost.

In most rotary kiln operations the chemical reactions in the bed require high temperature, for example cement kilns will require temperatures of approximately 1500C. The energy to raise the temperature and drive endothermic reactions is from the combustion of a range of fuels such as natural gas, coal and more and more alternative fuels. Heat transfer from the gas to the bed is complex and occurs from the gas to the bed surface and kiln wall to bed surface via conduction, convection and radiation.

A number of rotary kiln models have been proposed over the years and recent computational fluid dynamic models can be developed but all have their limitations. Most assume isothermal conditions through the bed at any axial position. However, practically it is known that such conditions do not exist as product variations are known to occur, for example in lime kilns where fine particles are calcined and larger particles are only partly calcined. The bed motion regime, either cascading, rolling or slumping depends on the rotational speed of the kiln, the percentage fill and the feed physical properties.

The work carried out by Palmer Technologies is aimed at understanding and improving the heat transfer in a rotary kiln and to provide a systematic basis for the efficient design of new kilns.