Effect of Incoming Mixing on Carbon Loss from a Pulverized Coal-Fired Burner

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 leakage 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 nm) 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.