VALUE OF COAL AS A FUEL. \f 
less and are items to be considered in the choice of a coal. With the 
ordinary furnace equipment there may be a considerable loss of effi- 
ciency and capacity through a large percentage of ash. It has been 
found that with some kinds of equipment, as the ash increases there is 
a decided drop in both efficiency and capacity. In some experiments 
made to determine the influence of excessive amounts of ash, coal 
containing as high as 40 per cent would generate no steam when fired 
on a chain grate, and therefore the efficiency and capacity of the plant 
would be zero. a Such coal would not only be worthless, but involve 
a direct expense, due to the cost of handling it. Whether the result 
would be similar with equipment other than a chain grate has not 
yet been determined. However, coals so high in ash that they are 
unsuited to boiler furnaces can be utilized in gas producers. 
The volatile part of coal as shown by the analysis may in some 
coals be all combustible, but it generally contains some inert matter. 
This varies in different coal deposits and makes it impossible to deter- 
mine the heating value of the coal from the proximate analysis alone. 
Moreover, not all coals having the same proportion of volatile matter 
behave alike in the furnace. It is important to know both the chem- 
ical composition and the British thermal units in order to determine 
the value of one coal as compared with another for the same purpose. 
Of two coals of different character, the one which contains the 
higher proportion of fixed carbon is most easily burned so as to 
give the maximum efficiency. However, if the coal containing the 
higher volatile matter is properly burned in a suitably designed 
furnace, it may be made equally efficient. 
Sulphur may be present in the free state, or, as is more commonly the 
case, in combination with iron or other elements. Other impurities 
with sulphur often form a clinker which shuts out the air and in- 
creases the labor of handling the furnaces. It is possible, however, 
to burn coals containing up to 5 per cent of sulphur without partic- 
ular difficulty from clinkers. A little steam introduced under the 
grate will relieve much of the trouble. Clinker may be due to other 
causes than sulphur, as any constituents of the ash which are easily 
fusible may produce it. There is need of further investigation to 
determine the influence of sulphur and the elements which com- 
prise the ash on furnace fires and combustion. 
The size of the coal influences the capacity of any given equip- 
ment, owing to its effect on the draft. With a poor draft fine coal 
can not be burned in sufficient quantities to maintain the rated capac- 
ity. If thin fires are resorted to, the efficiency is usually lowered as a 
result of an excessive supply of air through holes in the fire. As a 
"Abbott, W. L., Some characteristics of coal as affecting performance with steam 
boilers, a paper read before the Western Society of Engineers, Chicago, 111. 
24G73— Bull. 339—08 2 
