ANALYSIS OF CARBON LOSS FROM A PULVERIZED COAL-FIRED BOILER

Despite the large body of excellent work that has been done on the qua ntitative description of char combustion, our ability to specify the c onditions to be met in order to maintain the carbon loss from pulveriz ed coal-fired boilers below a desired value is still limited. The prob lems arise, in part, from the large number of fuel, combustion, and eq uipment characteristics that may influence carbon burnout. Measurement s 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 exp lanation for unexpectedly high carbon lasses under the conditions inve stigated was air leakage into the furnace and convection sections, whi ch caused the furnace gas to be richer than expected from the flue gas analysis. Assuming that the inleaking air made no contribution to com bustion, a calculation of carbon burnout, using the mean char combusti on rate of Hurt and Mitchell [Twenty-Fourth Symposium (International) on Combustion, 1992, pp 1243-1250] and the char reactivity distributio n of Hurt, Lunden, Brehob, and Maloney [Twenty-Sixth Symposium (intern ational) on. Combustion, 1996, pp 3169-3177], reproduced the carbon lo ss and size distribution of unburned particles and was within a factor of 2 of the Babcock & Wilcox correlation of carbon loss with stoichio metric ratio [Steam/Its Generation and Use, 37th ed., 1963, p 17-21]. According to the calculations, the distribution of char reactivities h as a significant influence on the carbon loss and abundance of small p articles in the unburned carbon size distribution.