The Locomotive - Lighthouse Survival Blog

1901.] THE LOCOMOTIVE. 107
the results are almost as good with poor as with good grades of coal; but, of course, the
better the coal, the less the quantity of it required. By passing the fuel into the fur-
nace by means of a stream of air the elements of combustion are brought under perfect
and convenient control, and one great advantage of the automatic stoker is attained, in
that there is no opening of fire doors. That the combustion may take place under
ideal conditions is evident from the fact that powdered coal has been burned with the
proportion of 12 pounds of air per pound of coal, which is precisely the theoretical
chemical requirement. We have also records of continuous tests showing 18 per cent,
of carbonic acid gas from flue-gas analysis of a powdered-fuel boiler. With conditions
such as these, or approaching them, increased evaporation may be expected, and is in
fact obtained, over that from the same fuel burned on grates with a necessarily large
excess of air. With powdered fuel there are no clinkers; the ash is apparently as fine
as the powdered coal, and it may be removed through pipes.
Assurance is given that lignite will work satisfactorily when pulverized, although
there are no authenticated records at hand confirming it. We have seen the fact
demonstrated, that very poor coal works almost as well in this process as better coal,
when the conditions are adjusted as they should be. There seems to be no difficultv in
igniting the powdered fuel, and while it is convenient to retain the ordinary grates
upon which to start a wood fire, as a preliminary to the dust firing, they are not
absolutely necessary and might be removed ; but where they are retained in the boiler,
the change back to grate firing may be easily and quickly accomplished, if for any
reason it becomes necessary. As to reliability, one experimenter informs us that he has
operated a stationary boiler with powdered fuel, continuously, day and night, for four
months, without any difficulty.
If we look for the disadvantages of the system, two come to the front, and both
appear possible to overcome. First, there is the cost of grinding the coal ; but this may
be safely figured at 25 cents per ton or less, although several early experiments were
terminated on account of the expense of this part of the process. With one type of
grinder now in use, one horse-power is said to be sufficient to grind 100 pounds of coal
per hour. The fineness of grinding differs among. the different systems, and ranges
from 200 mesh to impalpable powder. Great difficulty was formerly found in grinding
moist coal, but this has apparently been overcome. Second, after the completion of the
combustion the ash is left floating in the gases, and it must be given time to settle, or
it will pass out of the stack as an annoying product. Careful examination of this matter
appears to indicate that with the usual flame-way supplied by the ordinary cylindrical,
return-tube boiler, a sufficient distance is provided in which the dust will settle before
going into the tubes. Probably the change of direction of the gases at the back end of
the boiler contributes to this result, because in a boiler of this kind there appears to be
no more accumulation of dust in the tubes than in a grate-fired boiler, and there appears
to be no evidence of dust about the stack. It is believed that there need be no diffi-
culty from the ash in this type of boiler; but what the experience with locomotive or
marine firing may be is yet to be learned. It has been tried in both of these services,
but thus far its complete success has not been demonstrated in either.
The fundamental principles for the successful use of powdered coal appear to be:
(1) a combustion chamber maintained at a high temperature, which requires a fire-brick
arch to prevent the flame from impinging at once against the heating surfaces; (2) the
powdered fuel must be thoroughly mixed with the entering air, so that the air will
surround the particles of coal, and the fuel must be delivered in an uninterrupted stream ;
and (3) the particles of fuel must be maintained suspended in the gases until they are