A careful estimate by a Broadway Chemist of the contents or
constituents of a ton of coal presents some interesting facts, not
familiar certainly to unscientific minds. It is found that, besides
gas, a ton of ordinary gas coal will yield 3,500 pounds of coke, twenty
gallons of ammonia water and 140 pounds of coal tar. Now, destructive
distillation of this amount of coal tar gives about seventy pounds of
pitch, seventeen pounds of creosote, fourteen pounds of heavy oils,
about nine and a half pounds of naphtha yellow, six and one-third
pounds of naphthaline, four and three-fourth pounds of alizarine,
two and a fourth pounds of solvent naphtha, one and a fifth pound of
aniline, seventy-nine hundredths of a pound of toludine, forty-six
hundredths of a pound of anthracine, and nine-tenths of a pound of
toluches—from the last-named substance being obtained the new product,
saccharine, said to be 230 times as sweet as the best cane sugar.
From an engineer’s standpoint the main constituents of all coal are
carbon and hydrogen; in the natural state of coal these two are
united and solid; their respective characters and modes of entering
into combustion, are however essentially different. The hydrogen is
convertable into heat only in the gaseous state; the carbon, on the
contrary, is combustible only in the solid condition. It must be borne
in mind that neither is combustible while they are united.
There are, however, other elements existing in coal in its natural
state, and new ones are formed during burning or combustion as will be
noted in the succeeding paragraphs.
For raising steam the process of combustion consists in disentangling,
letting loose or evolving the different elements locked up in coal; the
power employed in accomplishing this is _heat_. The chemical results of
this consumption of the fuels may be divided into four stages or parts.
First stage, application of existing heat to disengage the constituent
gases of the fuel. In coals this is principally mixed carbon and
hydrogen.
Second stage, application or employment of existing heat to separate
the carbon from the hydrogen.
Third stage, further employment of existing heat to increase the
temperature of the two combustibles, carbon and hydrogen, until they
reach the heat necessary for combination with the air. If this heat is
not obtained, chemical union does not take place and the combustion is
imperfect.
Fourth and last stage, the union of the oxygen of the air with the
carbon and hydrogen of the furnace in their proper proportions,
when intense heat is generated and light is also given off from the
ignited carbon. The temperature of the products of combustion at this
final stage depend upon the quantity of air in dilution. Sir H. Davy
estimates this heat as greater than the white heat of metals.
In the first stages heat is absorbed, but is given out in the last.
When the chemical atoms of heat are not united in their proper
proportions, then carbonic oxide, mixed carbon and hydrogen, and other
combustible gases escape invisibly, with a corresponding loss of heat
from the fuel.
When the proper union takes place, then only steam, carbonic acid and
nitrogen, all of which are incombustible, escape.
The principal products, therefore, of perfect combustion are: steam,
invisible and incombustible; carbonic acid, invisible and incombustible.
The products of imperfect combustion are: carbonic oxide, invisible but
combustible; smoke, partly invisible and partly incombustible.
Steam is formed from the hydrogen gas given out by the coals combining
with its equivalent of oxygen from the air. Smoke is formed from
the hydrogen and carbon which have not received their respective
equivalents of oxygen from the air, and thus pass off unconsumed. The
color of the smoke depends upon the carbon passing off in its dark,
powdery state.
The heat lost is not dependent upon the amount of carbon alone, but
also upon the invisible but combustible gases, hydrogen and carbonic
oxide; so that while the color may indicate the amount of carbon in the
smoke, it does not indicate the amount of the heat lost; hence, the
smokeless locomotive burning coke may lose more heat in this way than
that arising from the imperfect burning of coal under the stationary
engine boiler.
A practical and familiar instance of imperfect combustion is exhibited
when a lamp smokes and the unconsumed carbon is deposited all about in
the form of soot. When the evolving or disengagement of the carbon is
reduced by lowering the wick to meet the supply of oxygen, the carbon
is all consumed and the smoke ceases. What takes place in a lamp also
occurs in a furnace, so that the proper supply of air is a primary
thing, relating to economy, both as regards its quantity and its mode
of admission to a fire.
The economical generation of heat is one thing, the use made of
that heat afterwards is another. Combustion may be perfect, but the
absorption of heat by a boiler may be inferior.
The chief agents operating in the furnace are carbon, hydrogen and
oxygen, and their union in certain proportions produces other bodies,
as water or steam, carbonic acid, besides others of less practical
importance.
OXYGEN is an invisible gas, has no smell, and remains permanently in
receptacles, unchanged by time. It can be obtained in an experimental
quantity by heating the chlorate of potash, and collecting the gas
given off in a bladder or jar. It is a trifle heavier than common
air, _i.e._, 1.106 times and a cubic foot at 32° temperature weighs
1.428 ounces. It is one of the most abundant bodies in nature, and is
combined with many others in a great variety of ways.
CARBON is one of the most interesting elementary substances in
nature. It is combustible and forms the base of charcoal, and enters
largely into mineral coal. It is a mineral capable of being reduced to
a feathery powder, and is found in many different forms. It is obtained
by various processes: from oil lamps as lamp-black; from coal as coke,
and from wood as charcoal; the mineral particles of carbon in a state
of combustion render flame luminous from either gas, oil or candles.
Carbon unites with iron to form steel, and with hydrogen to form the
common street gas. Carbon is considered as the next most abundant body
in nature to oxygen. In the furnace the carbon of the fuel unites
with the oxygen of the air to produce heat; if the supply of air is
correctly regulated, there will be perfect combustion, but if the
supply of air be deficient, combustion will be imperfect.
HYDROGEN is an invisible gas, and the lightest known body in the world,
being many times lighter than oxygen. It is combustible and gives out
much heat. In our gas establishments it is made in large quantities and
combined with carbon for illuminating streets, shops and dwellings. It
is the source of all common flame. When united with sulphur in coal
mines it becomes explosive. By passing a current of steam through a hot
iron tube partly filled with filings, hydrogen gas is given off and
burns with a pale yellow flame.
The more hydrogen, therefore, there is in the fuel, the greater in
general is its heating power. But it must be borne in mind that the
element of hydrogen is, nevertheless, to a greater or less degree
neutralized by the other element, oxygen, when it is present as a
constituent of the fuel; since the affinity of hydrogen for oxygen is
superior to that of carbon, and the oxygen saturated with hydrogen is
converted into steam and rises in this form from the fuel bed without
producing heat. Thus it is that the more oxygen there is in the fuel
the less is its power for developing heat by combustion.
NITROGEN is also an elementary body. It neither supports life nor
combustion; it is lighter than air and has no taste or smell. One cubic
foot at 32° temperature weighs a trifle less than one ounce.
SULPHUR is also an elementary body, of a yellow color, brittle, does
not dissolve in water, is easily melted, and inflammable. It is also
called brimstone or _burnstone_, from its great combustibility. It
burns with a blue flame, and with a peculiar, suffocating odor.
CARBONIC ACID GAS is formed by the burning of sixteen parts of oxygen
and six parts of carbon. Its specific gravity is 1.529; it is fatal to
life, and it also extinguishes fire.
CARBONIC OXIDE is a colorless, transparent, combustible gas, which
burns with a pale blue flame, as may be seen at times on opening a
locomotive fire-box door. Its presence in a furnace is evidence of
imperfect combustion from a deficient supply of air, as it indicates
that only eight parts of oxygen instead of sixteen parts have united
with six parts of carbon.