HYBRID DRYING OF GRANULAR MATERIALS BY COMBINED RADIATIVE AND CONVECTIVE HEATING

ABSTRACT

As one of the basic steps of the R&D on hybrid drying by combined radiative and convective heating, drying experiments of a stagnant horizontal wet granular layer heated simultaneously by infrared lamps and hot air were conducted throughout a whole period of the drying. An opaque model, in which the incident radiation is assumed to be absorbed only on the surface of the layer, was introduced to explain experimental trends. Then, the effects of combined heating on drying characteristics of three kinds of samples, a silica sand, a brick and a brown coal were discussed by comparing the experimental data with the calculated results.

The calculated results were found to be in a close agreement with the experimental data obtained on the time behaviors of the drying rate and the temperature distribution in the- layer during not only the preheating and the constant rate periods but also the falling rate period. In the case of the coal layer, however, the falling drying rate different from that in ordinary convective drying had to be considered since it appeared that the radiative heating had an effect to enhance the falling drying rate of the coal owing to some interactions of the coal and water.