Recuperator for waste heat recovery from rotary kilns

The energy balance of a rotary kiln used for calcination of dolomite in a magnesium production company identified the kiln shell (26.35% of the input energy) and exhaust gases (18.95%) as the major sources of heat losses. To decrease the heat loss, a heat exchanger that forms an annular duct over the calcination zone of the kiln is used to preheat combustion air. The exchanger uses both the convective and radiant heat loss from the mantle, prevents overheating, does not require air tightness, and could be implemented over rotary kilns with the similar surface temperature distribution. A mathematical model that defines the geometry of the heat exchanger so as the heat transfer from the kiln to the combustion air to be equal to the heat dissipated from the bare kiln is presented. The exchanger decreases fuel consumption of the kiln for 12.00%, and increases its energy and exergy efficiency for 7.35% and 3.81%, respectively. To obtain a better performance the airflow and geometry of the exchanger should be arranged to achieve the smallest possible temperature difference between the kiln surface and the preheating air, whose amount should always be kept at the optimal value for the used fuel.

► Mass, energy, and exergy balances of a rotary kiln are determined.
► Construction of a recuperator formed as an annulus over a rotary kiln is given.
► Mathematical model of the recuperator is made.
► Inclusion of recuperator in the kiln system decreases fuel consumption for 12.00%.
► The airflow and geometry of the recuperator influence exergy efficiency.