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Show American Flame Research Committee, 1995 Fall International Symposium Monterey, California, October 15-18, 1995 EFFECT OF INCOMPLETE MIXING ON CARBON LOSS FROM A PULVERIZED COAL-FIRED BOILER . Peter M. Walsh Energy and Environmental Research Corporation c/o Sandia National Laboratories Livermore, CA 94550, USA / z-/ Hottel and Stewart (1940), in a discussion of the relationship of fixed carbon conversion in pulverized coal-fired furnaces to fuel properties, furnace characteristics, and combustion conditions, noted difficulty in reproducing measurements made under conditions favoring low carbon loss. In spite of the large body of excellent work done on the quantitative description of char combustion, our ability to specify the conditions to be met in order to maintain carbon loss below a desired value is still restricted. Measurements of the concentration and size distribution of carbon in fly ash from a small electric utility boiler have been used to test assumptions regarding limitations on carbon conversion in pulverized coal firing. The observations were consistent with effects of air inleakage into the furnace and convective sections, causing the furnace gas to be richer than expected from the flue gas analysis, and resulting in stratification of gas composition and char particle concentration due to incomplete mixing of the air leaking into the furnace. Low air/fuel ratio in the furnace was not a sufficient explanation for the presence of carbon in fine (smaller than -90 f.lm) particles in the fly ash. Assuming uniform char reactivity and furnace temperature, incomplete burning of fine particles is expected only in the presence of an excess air distribution broad enough that a significant fraction of the furnace flow is fuel rich. INTRODUCTION Because fixed carbon is the slowest -burning of the organic combustible constituents of coal, its burnout is an important consideration in boiler design, fuel selection, preparation of fuel, and adjustment of operating conditions. The limitations on char conversion in pulverized coal-fired furnaces were analyzed by Hottel and Stewart (1940), who derived a dimensionless group describing the dependence of carbon loss on the effective rate coefficient for the carbon-oxygen reaction, the size distribution of char particles, residence time in the furnace, and excess oxygen. The correlation of unburned carbon from three pulverized coals having various particle size distributions, burned under a variety of conditions, is reproduced in Figure 1, showing good agreement with the measurements over a wide range of carbon |