Energy & Fuels 1997, 11, 965-971
965
Analysis of Carbon Loss from a Pulverized Coal-Fired Boiler Peter M. Walsh† Energy and Environmental Research Corporation, c/o Sandia National Laboratories, Livermore, California 94551-0969 Received December 9, 1996X
Despite the large body of excellent work that has been done on the quantitative description of char combustion, our ability to specify the conditions to be met in order to maintain the carbon loss from pulverized coal-fired boilers below a desired value is still limited. The problems arise, in part, from the large number of fuel, combustion, and equipment characteristics that may influence carbon burnout. Measurements of the concentration and size distribution of carbon in fly ash from a 30 MW (electric) utility boiler have been used to test assumptions regarding the factors controlling carbon conversion. The principal explanation for unexpectedly high carbon losses under the conditions investigated was air leakage into the furnace and convection sections, which caused the furnace gas to be richer than expected from the flue gas analysis. Assuming that the inleaking air made no contribution to combustion, a calculation of carbon burnout, using the mean char combustion rate of Hurt and Mitchell [Twenty-Fourth Symposium (International) on Combustion, 1992, pp 1243-1250] and the char reactivity distribution of Hurt, Lunden, Brehob, and Maloney [Twenty-Sixth Symposium (International) on Combustion, 1996, pp 3169-3177], reproduced the carbon loss and size distribution of unburned particles and was within a factor of 2 of the Babcock & Wilcox correlation of carbon loss with stoichiometric ratio [Steam/Its Generation and Use, 37th ed., 1963, p 17-21]. According to the calculations, the distribution of char reactivities has a significant influence on the carbon loss and abundance of small particles in the unburned carbon size distribution.
Introduction The limitations on char conversion in pulverized coalfired furnaces were analyzed by Hottel and Stewart,1 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. One of the major contributions of Hottel and Stewart’s work was the demonstration that the rate of the chemical reaction between carbon and oxygen could place a significant limitation on carbon conversion and that the assumption of diffusion-controlled burning was not sufficiently accurate to explain unburned carbon losses under conditions found in industrial equipment. Hottel and Stewart noted that their correlation predicted losses lower than those observed under conditions favoring high carbon conversion, an effect that they suggested might be caused by incomplete mixing of char with the remaining oxygen or a decrease in combustion rate due to the presence of ash on the surfaces of partially burned particles. More than 50 years later, the prediction of carbon loss at high conversion is an active area of research, motivated by the need for greater fuel flexibility, improvement of efficiency, and minimization of solid waste from power generation. Among the papers presented at the Eighth International Conference on Coal Science,2 for example, were many that considered influences on carbon loss. The complexity †
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[email protected]. Abstract published in Advance ACS Abstracts, August 1, 1997. (1) Hottel, H. C.; Stewart, I. McC. Ind. Eng. Chem. 1940, 32, 719730. X
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of the problem is indicated by the variety of phenomena with which carbon loss has been associated.1-5 Formation of a fraction of char particles having reactivity lower than the average for a given coal is one of the causes of higher-than-expected unburned carbon in fly ash.6,7 Crelling et al.8 observed that the average char reactivity at 773 K decreased with increasing volume fraction of inertinite in mixtures of vitrain and durain lithotypes. A two-component reactivity distribution based on the work of Crelling et al. helped to explain the abundance of unburned carbon among small particles (
