Influence of coal properties and pretreatment on ash aerosol yield

Abstract

The formation of ash from a broad range of coals has been studied in a laboratory-scale flow reactor. The objectives of the work were to: (1) examine the factors governing the residual fly ash size distribution for a wide range of minimally processed coals; and (2) identify the influence of coal pretreatment (e.g. cleaning and sizing) on yields of ash aerosol relative to that from the minimally processed parent coals. The raw coals received only the pretreatment that would be appropriate for utility firing, i.e. pulverizing to a broad size distribution which corresponds to 70–80% −200 mesh. The results for the residual fly ash size distribution were correlated through the breakup model. In general, the number of ash particles produced from each coal particle increased for lower rank, western coals, with some notable exceptions. The size distribution predicted by the breakup model, after the breakup number was optimized, generally matched the measured ash size distributions well. Physical coal cleaning, via froth floatation, was applied to three of the coals. For two bituminous coals, the cleaned fractions yielded substantially more aerosol than the parent coals. The fraction of the parent coal that suppressed the formation of the aerosol was associated with the minerals removed in the first step of the cleaning process, i.e. the most easily removed excluded minerals. The various cleaned cuts of the western lignite showed essentially identical behaviour. Aerodynamic sizing yielded coals that produced significantly more aerosol than the parent coal. These results suggest that excluded minerals play a role in modifying the vaporization mechanism responsible for aerosol production.