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Tan Bark —Tan bark, or spent tan, is the fibrous portion of bark remaining after use in the tanning industry. It is usually very high in its moisture content, a number of samples giving an average of 65 per cent or about two-thirds of the total weight of the fuel. The weight of the spent tan is about 2.13 times as great as the weight of the bark ground. In calorific value an average of 10 samples gives 9500 B. t. u. per pound dry. [43] The available heat per pound as fired, owing to the great percentage of moisture usually found, will be approximately 2700 B. t. u. Since the weight of the spent tan as fired is 2.13 as great as the weight of the bark as ground at the mill, one pound of ground bark produces an available heat of approximately 5700 B. t. u. Relative to bituminous coal, a ton of bark is equivalent to 0.4 ton of coal. An average chemical analysis of the bark is, carbon 51.8 per cent, hydrogen 6.04, oxygen 40.74, ash 1.42.
Tan bark is burned in isolated cases and in general the remarks on burning wet wood fuel apply to its combustion. The essential features are a large combustion space, large areas of heated brickwork radiating to the fuel bed, and draft sufficient for [Pg 211] high combustion rates. The ratings obtainable with this class of fuel will not be as high as with wet wood fuel, because of the heat value and the excessive moisture content. Mr. D. M. Meyers found in a series of experiments that an average of from 1.5 to 2.08 horse power could be developed per square foot of grate surface with horizontal return tubular boilers. This horse power would vary considerably with the method in which the spent tan was fired.
686 Horse-power Babcock & Wilcox Boiler and Superheater in Course of Erection at the Quincy, Mass., Station of the Bay State Street Railway Co.
FOOTNOTES
[ [40] ] B. t. u. calculated.
[ [41] ] Average of two samples.
[ [42] ] Assuming bagasse temperature = 80 degrees Fahrenheit and exit gas temperature = 500 degrees Fahrenheit.