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In
its simplest terms, coal burns when it unites with oxygen and gives out heat.
But, for an aspiring steam locomotive engineer, a much more detailed
understanding of the process can be helpful in the efficient practicing of his
or her craft. To begin, the oxygen provided to the burning process comes
from the air, which is a mixture of roughly 23% oxygen and 77% nitrogen.
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Coal, on the other hand, has a number
of substances included besides the critical base component – carbon. A
typical “dirty” coal might have these (table to the right) approximate
ingredients by weight.
When the coal is burned, the hydrogen
partly unites with the oxygen, causing water vapor or steam. Other
hydrogen portions unite with the carbon to create hydrocarbon vapors. It
is these vapors, when burned that create the bright flame in the firebox. |
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Sulphur |
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1.0% |
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Nitrogen |
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1.0% |
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Hydrogen |
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5.5% |
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Ash |
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7.0% |
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Oxygen |
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10.5% |
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Carbon |
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75.0% |
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The unburned
hydrocarbon vapor escapes up the smokestack, which often occurs when an engine
has just been fired up. The result is the distinctive yellowish smoke so
often seen on the steaming bays early in the day. When the engine is up to
steam and the firebox temperature reaches about 2,500 degrees Fahrenheit, these
hydrocarbons unite with the oxygen in the air, creating a smokeless flame. If
there is not enough air, unburned hydrocarbons escape, causing black smoke out
the stack. One cause of this restricted airflow could be clinkers, or chunks of
material caused when the Sulphur in the coal burns, separates and sticks to the
firebox grate.
Photo by Hank Bloch
 Through the
burning process the small amount of nitrogen in the coal escapes and passes up
the stack, taking some heat with it. The 77% nitrogen in the air, however,
has a more dramatic effect on the steaming process. One pound of coal
takes about 12 pounds of air to burn, and of those 12 pounds, 9 are comprised of
nitrogen. This nitrogen does not burn or aid in the process.
Instead, because it has to be heated along with the oxygen to the prescribed
temperature, it acts to reduce the rate of combustion and causes a great deal of
heat loss. For the steam engineer, this means care should be taken that
only the necessary amount of air be admitted to the firebox, and the surest way
to judge is to watch the smokestack. When coal is placed in a clean,
smokeless fire the hydrocarbons quickly convert to coal gas and combine with the
oxygen of the air being pulled in from the grate vents and firebox door opening.
Complete combustion occurs when the outside of the coal is heated and then
slowly distilled until the entire mass glows red-hot. This mass is mostly
carbon, as the other chemicals have been driven off. This carbon unites
with oxygen in the air being provided through the grates, and a flame forms.
If the air is not sufficient, some of the gases in the coal do not get burned.
As noted earlier, they exit through the smokestack as thick, black smoke.
When seeing this, an engineer needs to add some air. On the prototype this
meant opening the firehole door and the dampers.
As is often the
case, however, there is more to the story. In the British railroad manual
used for this article, the author offers this stern advice for prototype
engineers. The italics for emphasis are his. “A warning must be given here that
the extra air drawn in through the firehole door must not be excessive, as it
all requires heating up to the temperature of the firebox gases, and if it is
not heated up, it lowers the temperature of the gases passing through the tubes.
The water in the boiler is heated by these hot gases, and the heat given to the
water depends on the gases being much hotter than the water, otherwise no heat
can pass. A safe rule is to allow sufficient air to be admitted through
the firehole door to allow of the smoke being completely or almost completely,
consumed, but no more.”
Continuing the
discussion of the proper mix of air and the coal fire, the author goes on to add
some instruction on how to deal with an engine’s fire during a long wait in the
station, or anytime when the generation of steam needs to be reduced. “. .
. the firehole door must not be opened to cool the boiler and firebox.
This is bad practice, and leads to leaky tubes and broken stays; further, heat
is wasted unnecessarily. The proper thing to do, is to cut off the supply
of that gas necessary for coal to burn, viz., the oxygen of the air, by
closing the firehole door and one or more of the dampers.”
Lastly, to ensure
proper combustion and airflow, the management of the coal in the firebox is
important. “The fire should at all times be as thin as possible, without
being allowed to burn into holes, and thicker at the sides than the middle.
This allows free access of air through the grate, and the slight thickening at
the sides, prevents any gases creeping up the sides, and being drawn off through
the tubes unburnt. A glowing fire at the center of the grate will consume
any smoke formed.”
Taking all these elements and
combining them for a well run and efficient trip around the layout seems a
pretty good definition of the joys of live steaming, and it’s hard to improve
upon the words above as the objective for every live steamer. Here’s
hoping you have “a glowing fire at the center of the grate” on your next trip.
Based on How
the Locomotive Works and Why, a railroad manual produced for the Nigerian
rail system |
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