Advances in laboratory tests for predicting coal-related combustion problems

Abstract

Problems in pulverized fuel combustion in steam-raising plant that can be attributed to the nature of the coal being fired include excessive slagging and fouling, poor char burnout, NOx emissions and flame stability. Coal performance in these areas can, to some extent, be related to the results from existing coal characterization tests but, particularly for coals from unfamiliar areas, these correlations may be unreliable and misleading. For example, attempts to relate slagging propensity to the chemical composition of the coal ash have met with only limited success. It now seems clear that empirical indices and viscosity measurements do not reflect the complex mineral interactions that occur during combustion. However, new microanalysis techniques, capable of characterizing the distribution of mineral matter in a pulverized fuel (PF), now appear likely to provide an improved assessment of the slagging propensity of a coal. Other factors, such as coal volatile yield and nitrogen release, are quite sensitive to the heating conditions. Laboratory tests which match PF combustion conditions much more closely than existing standard methods are now being developed. New approaches to predicting char burnout, by better identification of the small subset of the coal which is actually causing the problem, are also under investigation. While not a general panacea, the appropriate use of new tests is likely to offer engineers involved in coal procurement, who face an increasingly complex market, improved methods for evaluating coals before embarking on expensive pilot- or full-scale trials.