Combustion of coal char agglomerates in a combustion gas environment

Abstract

Coal char agglomerates produced at the latter stage of coal-oil or coal-water slurry spray combustion are studied using artificially-made coal char agglomerates suspended on a fine quartz fiber in the post-combustion zone of a methane preximed flat flame. The size and mass of the burning agglomerate were measured after exposure to the hot environment for a short time. The apparent density of the agglomerate was calculated from the measured mass and size. The external appearance of the agglomerates was also observed using a scanning electron microscope (SEM). The char agglomerate in a hot combustion gas decreases in size until the carbon is burned up and only the ash remains. The apparent density continues to decrease as long as the carbon contained reacts and then increases till the size changes no more. The final apparent density is almost equal to or exceeds the initial density. The relation between the apparent density and the carbon mass reduction is linear, independently of the surrounding combustion gas conditions. These results mean that as the combustion of a char agglomerate progresses, the agglomerate comes to have porous structure composed of ash. Changes in coal type changes combustion characteristics. The ash content and ash melting point control the burning rate and carbon mass consumption rate due to generation of an ash layer round the char core. Howeever, the fuel ratio of the raw coal influences the porosity of the char agglomerate, which promotes carbon reaction with the environment.