INFLUENCE OF COAL RANK AND PRETREATMENT ON RESIDUAL ASH PARTICLE-SIZE

The influence of coal treatment (cleaning, sizing, and grind) on the g eneration of ash in the 0.5-8 mum size range was studied in a laborato ry-scale flow reactor. The first objective of the work was to use devi ations between the data and the basic breakup model of residual ash fo rmation as a means of identifying non-ideal or second order effects th at are active in this environmentally sensitive size range. The second objective was to identify how coal treatment affected the formation o f this ash. The raw coals received only the treatment that would be ap propriate for utility boiler firing, i.e., grinding in an industrial p ulverizer to a broad size range centered at 30-50 mum. Initial experim ents showed that many more fine particles were generated than would be predicted by the breakup model. In particular, a persistent mode at 1 -2 mum appeared in most of the data. This mode was undiminished in coa ls cleaned of excluded mineral matter, which showed that the source of these particles was not small exclusions liberated during grinding. W hen the coals were subjected to aerodynamic sizing, the smaller cuts g enerated this 1-2 mum ash, while it was largely absent when the larger cuts were burned. This suggests that the formation is dominated by a mechanism based on the initial external surface area of the char, e.g. , burnout of small inclusions at the inception of char combustion, or microfragmentation if it only occurs during the initial stages of char combustion. Larger coal particles tended to produce more aerosol due to the vaporization/condensation mechanism. These particles also tende d to have very large breakup numbers, which suggests that fragmentatio n tends to produce similar sized fragments, independent of coal starti ng size. Comparison of the behavior of fine coals prepared by sizing v s. those prepared by grinding showed that aerosol yields (D < 0.3 mum) were decreased for the fine sized coals. This was apparently due to t he concentration of fine excluded minerals in this cut during the sizi ng process. Excluded minerals have previously been shown to suppress a erosol yields.