Gasification of four different kinds of coal chars were carried out in a 1.0 ton/day fluidizedbed gasifier within a range of the following operating conditions; the gasification temperature from 1150 to 1320K, the total pressure from 0.6 to 1.9MPa, the superficial gas velocity from 0.5 to 0.8m/s, the steam to air and/or oxygen ratio from 0.8 to 2.5, and the fluidized bed height of 0.2, 0.4 and 0.6m. The experiments were focussed to examine the effects of the above operating variables, particularly the properties of fed char particles, on the rates of entrainment of fine particles from the freeboard and overflow of bed particles through the overflow pipe as well as on their microphotographic properties and carbon content.
Microphotographs of the entrained and overflow particles showed that distribution of fine ash particles and extent of dilatation of the residual char particles depended on the properties of fed char particles. The rate of entrainment exhibited a variation corresponding to the size distribution of fed char particles and the attrition characteristics of gasifying char particles. The weight fractions of entrained particles were about 0.2 to 0.5 of the fed char particles under the present operating conditions. Among the chars used so far, the maximum entrainment rate was observed for Blair Athol coal char due to its highest content of fine particles and possibly to its most fragile structure for rapid heating.
The fraction of entrained particles increased with the superficial gas velocity while that of overflow particles decreased. Also, increase of the fraction with the pressure became appreciable when the pressure exceeded 1MPa. The content of fine particles in the entrained particles increased with the carbon conversion and the fluidized bed height, which might be attributed to fine ash formation by attrition of ash layer on the surface of gasifying char particles.
