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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 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.

[Illustration: 686 Horse-power Babcock & Wilcox Boiler and Superheater
in Course of Erection at the Quincy, Mass., Station of the Bay State
Street Railway Co.]

LIQUID FUELS AND THEIR COMBUSTION

Petroleum is practically the only liquid fuel sufficiently abundant and
cheap to be used for the generation of steam. It possesses many
advantages over coal and is extensively used in many localities.

There are three kinds of petroleum in use, namely those yielding on
distillation: 1st, paraffin; 2nd, asphalt; 3rd, olefine. To the first
group belong the oils of the Appalachian Range and the Middle West of
the United States. These are a dark brown in color with a greenish
tinge. Upon their distillation such a variety of valuable