Numerical Model for Heat Transfer Processes in Dry Ash Conveyors

Abstract

The dry handling of bottom ash from coal-fired power plants has become more and more important in recent years, e.g. due to a lack of water availability at the location of power plants, or for environmental reasons. Thereby it is crucial that a sufficient cooling of the bottom ash can be ensured by the dry cooling air. Within this work, a numerical model for the assessment of heat transfer processes in dry ash conveyors is developed and implemented into Wolfram Mathematica. The model uses a newly introduced representative geometric quantity for the ash particle geometry. Moreover, in addition to the ash, the cooling air is considered as an own phase, for which a temperature solution is obtained. A numerical example, considering geometrical and operational data of an existing facility, shows that the main heat transfer between the ash and the cooling air takes place in the ash hopper, whereby convective heat transfer from ash to cooling air outweighs the effects from coke combustion and radiation from the boiler outlet area. The convective heat transfer in the ash hopper predominantly depends on the geometrical appearance, i.e. size and shape, of the particles, as well as on the grain density, and on the falling time/velocity. Conservatism of the calculation approach is indicated based on comparison of computed temperatures with measured data and literature values. The derived model can be used in future designs and projections of dry ash handling systems.