Temporal and spatial variations in fly ash quality

Fly ash quality, both as the amount of petrographically distinguishable car bons and in chemistry, varies in both time and space. Temporal variations a re a function of a number of variables. Variables can include variations in the coal blend organic petrography, mineralogy, and chemistry; variations in the pulverization of the coal, both as a function of the coal's Hardgrov e grindability index and as a function of the maintenance and settings of t he pulverizers; and variations in the operating conditions of the boiler, i ncluding changes in the pollution control system. Spatial variation, as an instantaneous measure of fly ash characteristics, should not involve changes in the first two sets of variables listed above. Spatial variations are a function of the gas flow within the boiler and du cts, certain flow conditions leading to a tendency for segregation of the l ess-dense carbons in one portion of the gas stream. Caution must be applied in sampling fly ash. Samples from a single bin, or series of bins, may not be representative of the whole fly ash, providing a biased view of the nature of the material. Further, it is generally not po ssible to be certain about variation until the analysis of the ash is compl ete. (C) 2001 Elsevier Science B.V. All rights reserved.