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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.
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Fly ash unburned prediction
The major portion of carbon loss in a boiler is from unburned carbon in fly ash. A method was developed by me after a large volume of data was subjected to analysis and validation. It is seen from the analysis and literatures that the fuel ratio i.e. the ratio of fixed carbon and volatile matter in coal has a very significant effect. The ash in coal is a burden for combustion and can cause large problems during and after combustion. Deposits and slagging in boiler furnaces using high amounts of medium slagging and slagging coals are common. After combustion they can foul the heat transfer surface in the convection region. So it is seen that log of ash % correlates well with fly ash combustibles. Coal calorific value indicates the heat value of the coal being fired hence has to be taken into account when we want to predict the fly ash combustibles. The calorific value of the coal in question divided by the calorific value of carbon gives meaning to the factor. This indicates the relative stage where the coal in question lies with respect to its ultimate transformation and also is an indirect indicator of the difficulty to ignite and burn. I would not like to call it as reactivity as the same has not been studied / understood much with respect to this ratio of coal. Inverse of residence time is another major factor which affects the fly ash combustibles. As boilers are operated within a close range of excess air and fineness of coal, these variables do not affect the unburned to any significant level.
A factor combining all the parameters is evolved which is used for fitting a curve with percentage combustibles in fly ash. The factor is defined as
The equation governing the curve fitted on a fourth order polynomial is
Y = -3E-06 X4 + 0.0004 X3 - 0.0161 X2 + 0.2969 X - 0.9438
with a ‘R square’ value of 0.8824.
As this predictive equation is only made for a pulverized coal tangentially fired boiler, this has to be verified for pulverized coal wall firing, down shot firing, opposed firing, etc. However, more than 50% of the pulverized coal fired boilers in the world are equipped with tangential firing system.
Bottom ash unburned prediction
The single most independent variable affecting the bottom ash combustibles is the plus 50 mesh size of pulverized coal. A plot of percentage bottom ash combustible plotted against percentage plus 50 particle sizes has a fourth order polynomial curve with an R2 value of 0.9412. The equation governing this fit is
Y1 = 0.0233X14 - 0.3925X13 + 1.9277X12 - 0.1593X1 + 0.2357
where, Y1 is percentage combustibles in bottom ash and X1 is plus 50 mesh particle percentage in the pulverized coal.
It is seen that this percentage plus 50 in the pulverized fuel should be retained below 2% to minimize the percentage combustible in bottom ash. This is generally recommended by boiler manufacturers.