In boilers with pulverized firing systems, about 80% of the ash in coal being fired is carried as fly ash. The other about 20% get collected as bottom ash. During the combustion of coal, some portion of the hydrocarbon, mainly char, leaves the furnace as unburned particles. The amount of such unburned particles leaving the furnace depends on many factors like the coal property, the type of burning system, the resident time available in the furnace, the ash percentage in coal, the calorific value of coal, the fuel ratio, the operating conditions, etc. The existence of unburned carbon in ash decreases not only the combustion efficiency, but also the grade of fly ash for commercial sale.
Carbon loss is influenced by the following: (1) coal preparation and grinding, such as changes in ash and maceral content ; mean, standard deviation, and higher moments of the particle size distribution; moisture remaining in the pulverized coal, (2) properties of the pulverized coal and its char like heating value, char yield on pyrolysis, char structure, char reactivity, ash content and composition, and characteristics, and (3) adjustments of the burners and furnace such as air preheat, excess air, mixing, residence time, and furnace temperature.
Hottel and Stewert (1940) were the first to consider the interaction between furnace design and coal properties in the determination of carbon conversion, analyzing the effects of grind, reactivity, temperature, excess air, and residence time on unburned carbon loss.
With the estimated values of percentage combustibles in fly ash as well as bottom ash, the carbon loss can be calculated by using the formula given in BS_EN_12952, ASTM, PTC 4 and any other International Standards.
Boiler designers during the design stage have only proximate analysis, ultimate analysis and ash composition of coal. Carbon loss calculation involves calculating the carbon loss in fly ash and bottom ash. This article provides a tool for the designers and others to predict the percentage of combustibles in fly ash and bottom ash in a tangential fired boiler using proximate analysis of coal and the residence time in the boiler furnace. Based on combustibles in flyash and bottom ash, it is possible to compute the carbon loss in a boiler.