COMBUSTION.
QUESTION 370. _What is meant by combustion?_
_Answer_. By combustion is meant the phenomenon ordinarily called
burning, as when a piece of wood or coal or a candle is burnt. In
reality combustion is a union of one of the “_chemical elements_,”
oxygen, of which the atmosphere is composed, with the elements which
constitute the fuel.
QUESTION 371. _What is meant by the term “chemical element?”_
_Answer_. The science of chemistry has demonstrated that nearly all
substances by which we are surrounded are composed of certain other
substances, which latter, as far as is now known, are not compounds,
and are therefore called _elementary substances, or chemical elements_.
Thus the air by which we are surrounded is composed of two gases,
called nitrogen and oxygen; water is composed of hydrogen and oxygen,
and coal chiefly of carbon and hydrogen. There are now over sixty of
these elementary substances known. From no one of them have chemists
been able to extract any material excepting the substance itself. These
elementary substances will combine with others so as to form what is
apparently a new material, but on weighing it it will be found that the
weight of the new material is greater than the original elementary
substance, showing that something was added to it which effected the
change.[87]
[87] “The New Chemistry,” by J. P. Cooke, Jr.
QUESTION 372. _To what fact is this combination or combustion of
elementary substances due?_
_Answer_. It is owing to the fact--the exact reason for which is
perhaps not yet understood fully--that the atoms of the elementary
substances of which fuel is composed, that is hydrogen and carbon, and
the atoms of oxygen, which forms part of the atmosphere by which we are
surrounded, attract each other with great energy when they are excited
into activity by the application of heat.
QUESTION 373. _What phenomenon always attends chemical combination of
substances?_
_Answer_. Such combination always gives out heat, whereas their
separation absorbs heat. It has further been proved by actual
experiment that the amount of heat liberated by the chemical union
of the same quantity or number of atoms of two or more substances is
always the same, and that when, by any cause, the atoms thus joined are
separated, exactly the same amount of heat is absorbed.[88]
[88] “The New Chemistry,” by J. P. Cooke, Jr.
QUESTION 374. _In what proportions do the elementary substances combine
with each other?_
_Answer_. It is a law of chemistry that each of the elementary
substances combines with the others in certain definite proportions
only. These proportions vary for the different elements, and have been
determined with great accuracy by chemists. Thus, eight parts by weight
of oxygen will combine with nitrogen and form atmospheric air, or the
same proportion of oxygen will combine with hydrogen and form water,
or with carbon and form carbonic acid, which is the deadly gas which
accumulates at the bottom of wells.
Now oxygen always combines with other substances in the proportion of
eight parts by weight, or by _some simple multiple of_ eight, that is 8
× 2 = 16 parts, or 8 × 3 = 24 parts, etc. Each of the other elementary
substances also has a certain fixed proportion in which it combines
with others, and this proportion, which is usually given by weight, is
represented by a number called its _chemical equivalent_. Thus 8 is
the chemical equivalent of oxygen. Carbon combines with other elements
in proportions of 6 and nitrogen in proportions of 14, so that 6 and
14 are the chemical equivalents of carbon and nitrogen. Now 8 parts by
weight of oxygen can be made to combine with 14 parts of nitrogen, or
8 × 2 = 16 parts of oxygen will combine with 14 of nitrogen, but it is
impossible to make, say 12 parts of oxygen combine with 14 parts of
nitrogen. We can combine 14 × 2 = 28 parts of nitrogen with 8 parts of
oxygen, but no chemical process can make say 10 or 20 parts of nitrogen
combine with 8 parts of oxygen. If 20 parts of nitrogen are mixed with
8 parts of oxygen, then the latter will combine with 14 parts of the
former, but 6 parts of nitrogen will be left, and chemical combination
will then cease.
The following table will give the chemical equivalents of the principal
elements which enter into the process of combustion of the fuel used in
locomotives:
Chemical
equivalent
by weight.
Oxygen 8
Nitrogen 14
Hydrogen 1
Carbon 6
Sulphur 16
QUESTION 375. _What effect do the proportions in which elements
are combined have upon the substances which are produced by the
combination?_
_Answer_. A change in the proportions in which the elements are
combined usually alters the entire nature of the substance, so far at
least as it affects our senses. For instance, oxygen unites chemically
with nitrogen in different proportions, forming five distinct
substances, each essentially different from the others, thus:
14 parts of Nitrogen with 8 of Oxygen forms Nitrous Oxide.
14 „ „ „ „ 16 „ „ „ Nitric Oxide.
14 „ „ „ „ 24 „ „ „ Hyponitrous Acid.
14 „ „ „ „ 32 „ „ „ Nitrous Acid.
14 „ „ „ „ 40 „ „ „ Nitric Acid.
We here find the elements of the air we breathe, by a mere change
in the _proportions_ in which they are united, forming distinct
substances, which differ from each other as much as _laughing gas_
(nitrous oxide) does from that most destructive agent _nitric acid_,
commonly called _aqua fortis_.[89]
[89] Combustion of Coal and the Prevention of Smoke, by C. Wye
Williams.
QUESTION 376. _What occurs when a fresh supply of bituminous coal is
thrown on a bright fire in the fire-box of a locomotive?_
_Answer_. The fresh coal is first heated by the fire, and if a
sufficient quantity is thrown in to prevent the immediate formation of
flame,[90] a volume of gas or vapor, usually of a dark yellow or brown
color, is given off. The quantity evolved will be greatest when the
coal is very small. This gas or vapor is commonly called smoke, but
it does not deposit soot and in reality is not true smoke. If a sheet
of white paper be held over the vapor as it escapes from the coal and
there is no flame, the sheet will become slowly coated with a sticky
matter of brown color difficult to remove, and having a strong tarry
or sulphurous smell; whereas if a sheet of paper is held over smoke
it will quickly be covered with black soot. The color and smell left
on the paper in the first case are due to the tarry matter, sulphur,
and other ingredients in the gas. Deprived of the coloring matters,
the vapor is a chemical mixture of 2 parts of hydrogen and 6 parts of
carbon, and is called carburetted hydrogen, and is nearly the same as
the colorless gas by which our houses are lighted.[91] A similar gas is
generated at the wick of a burning candle or lamp and is consumed in
the flame. Before the gas is expelled from the fresh coal the latter
must be heated to a temperature of about 1,200 degrees, so that if
100 pounds at a temperature of 50 degrees is put on the fire 23,000
units of heat will be absorbed to heat the coal.[92] Nor is this all,
as has been explained in answer to Question 38, when any substance is
vaporized a certain amount of heat apparently disappears, which has
been called the _heat of evaporation_ or _of gasification_. Average
bituminous coal contains about 80 per cent. of carbon, 5 per cent.
of hydrogen and 15 per cent. of other substances usually regarded as
impurities. When the coal is heated up to about 1,200 degrees, the 5
per cent. of hydrogen unites with three times its weight of carbon, and
thus 20 per cent. of the coal is converted into the gas described. In
this process a large amount of heat is absorbed or becomes latent, as
it does when water or any other substance is converted into vapor. It
will therefore be seen that the first effect of putting fresh coal on
the fire _is to cool the fire_. This fact has an important bearing on
the question of combustion and will be referred to hereafter.
[90] Usually if more than two or three shovels full are thrown in
there will be no immediate formation of flame.
[91] A Treatise on Steam Boilers, by Robert Wilson.
[92] The quantity of heat required to heat coal is only about
one-fifth that needed to heat the same weight of water to the same
temperature.
QUESTION 377. _How can the process of the combustion of the gas
generated from the coal be best explained?_
_Answer_. As this gas is substantially the same as ordinary
illuminating gas, the manner in which it burns can perhaps be made
clearer by examining the combustion of an ordinary gas-light. As stated
before, combustion is a chemical union of the oxygen which forms one
of the elements of the air with the hydrogen and carbon of the fuel,
which, in this case, form gas. It should be clearly kept in mind that
combustion is the result of this union, and that the oxygen is as
essential to combustion as coal or gas, and in fact is the fuel of
combustion just as much as coal or gas is. If we were to conduct a
pipe from the external air into a vessel filled with coal gas we could
_light the air_ and it would burn in the gas as the gas burns in the
air.
It will be noticed, however, that before either the gas or the air will
burn, they must be lighted. Air and gas, even if mixed together in the
same vessel, will not burn unless they are lighted. This can be done by
the flame of any burning material, or with a piece of metal heated to a
very high temperature, or by an electric spark. In other words it may
be said that the atoms of the two gases must be excited into activity
by the application of heat, that is, what is called an _igniting
temperature_ must be communicated to them before chemical combination
will begin. The chief feature which distinguishes combustion from
other chemical union is the circumstance that the heat generated during
the combination is sufficient to maintain an igniting temperature, and
the necessity of doing so in order to continue the process is of very
great importance in the combustion of coal in locomotive boilers, as
will be shown hereafter.
QUESTION 378. _How does an ordinary gas-light burn after it is lighted?_
_Answer_. Under ordinary conditions the hydrogen, which is the most
combustible of the two elements of which coal gas is formed, is the
first to burn. This part of the combustion forms the lower bluish part
of the flame. The combustion of the hydrogen thus separates it from
the carbon, which is then set free; and as carbon is never found in a
gaseous condition when uncombined with other substances, it at once
assumes the form of fine soot when the hydrogen is burned away from it.
This fine soot, or pulverized carbon, is, however, intensely heated by
the combustion of the hydrogen. Now carbon when heated to an igniting
temperature will, if brought into contact with a sufficient quantity
of oxygen, combine with it or be burned. Each particle of carbon thus
becomes a glowing centre of radiation, throwing out its luminous rays
in every direction. The sparks last, however, but an instant, for the
next moment they are consumed by the oxygen which is aroused to full
activity by the heat, and only a transparent gas rises from the flame.
But the same process continues; other particles succeed, which become
heated and ignited in their turn, and it is to this combustion of the
solid particles of carbon that the light which is given out by a
gas-burner or candle is due.[93]
[93] “The New Chemistry” by J. P. Cooke, Jr.
[Illustration:
_Fig. 210._
_Fig. 211._]
QUESTION 379. _Why does a gas-burner, candle or other flame sometimes
smoke?_
_Answer_. Because the supply of oxygen is then insufficient to consume
the particles of solid carbon which are set free and which then assume
the form of soot. This can be illustrated if we cut a hole in a card,
_d d_, fig. 210, so as to fit over an ordinary gas-burner, _b_. If we
then light the gas and place a glass chimney, _a a_, over the burner
and let it rest on the card, it will be found that the flame will at
once begin to smoke, because very little air can then come in contact
with the flame, and therefore when the fine particles of carbon are
set free by the combustion of the hydrogen, instead of being burned as
they would be if the air with its supply of oxygen were not excluded
from the flame by the chimney, they escape unconsumed in the form of
fine black powder or soot. If we raise the chimney up from the card,
as shown in fig. 211, so as to leave enough space between them at the
bottom of the chimney to permit air to enter so as to supply the flame
with oxygen, the smoke will instantly cease, as the particles of carbon
are then consumed. The same principle is illustrated in an ordinary
kerosene lamp. It is well known that without a chimney the flames of
nearly all such lamps smoke intolerably, whereas with a glass chimney
and the peculiarly formed deflector which surrounds the wick the light
burns without smoke unless the wick is turned up high. The effect of
the chimney is to produce a draft, which is thrown against the flame by
the deflector, and thus a sufficient supply of oxygen is furnished to
consume all the particles of carbon, whereas without the draft produced
by the chimney the supply of oxygen is insufficient to ignite all the
carbon, which then escapes in the form of smoke or soot.
[Illustration:
_Fig. 212._
_Fig. 213._]
It must not, however, be hastily assumed that if the flame does not
give out a bright light, therefore the combustion is not complete.
As has already been stated, the light of the gas flame is due to the
presence of burning particles of solid carbon, which is set free by
the combustion of the hydrogen with which it is combined. After it is
separated from the hydrogen it immediately assumes a solid form. If
the coal gas is mixed with a sufficient quantity of air before it
is burned, the oxygen in the latter will be in such intimate contact
with the former that the difference of affinity of oxygen for the
carbon and hydrogen does not come into play, and as there is enough
oxygen for all, the carbon is burnt before it is set free, and as
there are then no solid particles in the flame, there is no light.
This is illustrated by a “Bunsen burner,” fig. 212, which is much used
in chemical laboratories. It consists of a small tube or burner, _a_,
which is placed inside of another larger tube _b_. The latter has
holes, _c_, _c_, a little below the top of the small tube. The current
of gas escaping from the small tube produces what is called an _induced
current_ of air in the large tube. This air enters through the holes
_c_, _c_, and is mixed with the gas in the tube _b_, and the mixture
is burned at _d_. The flame from such a burner gives hardly any light,
but the heat is intense, as is shown if a metal wire is held in it for
a few seconds, as it will very soon glow with heat.
QUESTION 380. _What important difference is there in the structure of
the flame of a Bunsen burner and that of an ordinary gas-burner or
candle?_
_Answer._ The gas which escapes from the mouth _d_ of the pipe _b_,
fig. 212, is mixed with air, and therefore contains within itself the
elements which only need to combine to produce combustion; whereas with
an ordinary gas-burner or candle the air comes in contact with the
flame only from the outside, or on its surface. This is shown better
perhaps in the flame of an ordinary candle. The heat of such a flame
distils a gas from the melted tallow, which is similar in nature to
that which escapes from coal at a high temperature. Now by observing
the candle very closely it will be seen that at the bottom close to the
wick there is very little combustion, as the gas there first escapes
from the wick and is not heated to a sufficiently high temperature
to burn freely. A little above the lowermost part the flame is of a
pale bluish color, which is due to the combustion of the hydrogen.
Above that, where the carbon is set free, its particles glow with heat
imparted by the burning hydrogen and are then consumed by uniting with
the oxygen of the air. The combustion occurs only at the surface of the
flame, the inside being a mass of combustible gas which cannot burn
until it in turn comes in contact with the oxygen of the air. This can
be proved by inserting one end of a small tube, fig. 213 (a pipe stem
will do), which is open at both ends, into the flame. The combustible
gas will then escape at the other end and can easily be lighted with a
match.
It will be found that the flame from the Bunsen burner is much more
intense than that of an ordinary candle or gas-burner. The reason of
this is that combustion, as already stated, takes place through the
whole mass of its flame, whereas an ordinary flame burns only at its
surface. Common gas-jets are therefore arranged so that the flames
will be flat, thus exposing as much surface to the air as possible,
and, as explained in answer to Question 329, in describing the lamps
for head-lights, their burners are usually made with a circular
wick, through the centre of which a current of air circulates. This
arrangement exposes a larger surface of the flame to the air, and also
with the aid of a chimney furnishes an abundant supply for combustion.
In stationary boilers with long flues of a large sectional area the
flame will often extend for thirty feet, showing that while combustion
is going on only at the surface of the flame, it takes a long time to
complete the process. The same thing is shown if a gas-burner is made
with a single round hole. The flame will then be very long and liable
to smoke at the top.
QUESTION 381. _From the preceding considerations what may we infer to
be necessary in order to consume coal gas perfectly?_
_Answer._ In the first place, that there must exist a certain degree
of what chemists call “molecular activity,” which is produced by heat,
or what we have called the _igniting temperature_. The necessity of
this is sufficiently obvious with ordinary gas-burners, as they must
always be _lighted_ before they will burn. Now imagine that it was
required to burn gas which was issuing from a hundred jets, of every
variety of size, in a violent wind storm, or gusts of wind. Obviously
it would be necessary to keep a lighted torch all the time to relight
those which would be blown out. The gas in a locomotive fire-box is in
reality burnt in a storm of wind more violent than any natural one. It
is therefore necessary to be constantly ready to relight the streams
of gas which the faintest breath would extinguish, or those of larger
volume which have absorbed a great deal of heat and thus reduced the
temperature at the time and place of their birth, when they assumed the
gaseous form, as was explained in answer to Question 376. To relight
them with certainty it is necessary to keep a constant temperature
in the fire-box high enough to ignite the gas which escapes or is
distilled from the coal.
_Second._ That the chemical change in combustion consists simply in the
union of the elements burned with the oxygen of the air; and therefore,
to burn the gas perfectly, without smoke or waste, _enough_ air must be
furnished to supply all the oxygen which will combine with the fuel.
_Third._ That the air must be mixed with the gas, otherwise combustion
will occur only at the surface of the flame, and will therefore be so
slow that much of the gas will escape unconsumed.
It must be clearly kept in mind that no one or two of these
requirements alone, without the third, will burn coal perfectly.
What is needed is all three in combination. A very common error is
to suppose that passing smoke over a hot fire, or in other words,
maintaining an igniting temperature, will alone effect perfect
combustion; or that if a sufficient supply of air is admitted, without
an igniting temperature in the fire-box, the fuel will be burnt
completely. Neither of them will accomplish the object alone, and the
gas and air must at the same time be thoroughly mixed with the burning
gas in the fire-box.
QUESTION 382. _What substances are produced by the combustion of coal
gas?_
_Answer._ The hydrogen of coal gas unites during combustion with oxygen
in the proportion, as indicated by their chemical equivalents, of 1
part by weight of hydrogen with 8 parts of oxygen, the product of which
is water. Of course at the high temperature at which the gases combine
or burn the water is produced in the form of steam. That water or steam
is one of the products of combustion is shown every cold evening, when
the insides of shop show-windows are covered with moisture, which is
due to the steam that is given off by the burning gas-lights or lamps
inside, and is then condensed against the cold glass.
Carbon combines with oxygen in two proportions: first, 6 parts of
the former will unite with 8 of the latter, forming what is called
_carbonic oxide_; or 6 parts of carbon will combine with 16 parts of
oxygen, forming _carbonic acid gas_ or _carbonic dioxide_, as it is
called in some of the new books on chemistry. It is probable that the
former compound, that is carbonic oxide, is never or very rarely formed
in the flame of coal gas; but, as will be seen hereafter, is a very
common and wasteful product of the combustion of the solid portion of
the coal which is left after the gas is expelled from it. When there
is not enough oxygen for the perfect combustion of the carbon in the
flame, it smokes, and the carbon escapes in the form of soot. This, as
will be shown, may in a locomotive fire-box help to form carbonic oxide
after it leaves the flame.
QUESTION 383. _What remains in the coal after all the gas is expelled
by heat?_
_Answer._ What remains is ordinarily called coke, which, with the
exception of some incombustible substances, such as sand, ashes and
cinders, which the coal contains, is nearly pure carbon.
QUESTION 384. _What is the chemical process of the combustion of coke?_
_Answer._ The solid carbon of the coke when raised to an igniting
temperature; or, in other words, on being lighted, unites with the
oxygen in one of the two proportions already given; that is, if the
supply of oxygen is sufficient, 6 parts of the carbon of the coke
unite with 16 parts of oxygen, forming carbonic acid gas, or carbonic
dioxide. If, however, the layer of fuel on the grates is thick, or the
supply of air is comparatively small, there will not be enough oxygen
to supply 16 parts of the latter to each 6 parts of the carbon, so that
when that occurs, instead of combining in that proportion, and thus
forming carbonic dioxide, 8 parts of oxygen will unite with 6 parts
of carbon and form carbonic oxide. Now it should be carefully kept in
mind that the heat of combustion is due to the union, or, as it is
sometimes expressed, it is the clashing together of the molecules of
the two elements which unite. If, therefore, only half the quantity of
oxygen unites with 6 parts of carbon, evidently there will be less heat
evolved than there would be if twice that amount of oxygen combined
with the carbon. From carefully made experiments it was found that the
total heat of the combustion of one pound of carbon when converted into
_carbonic oxide_ was 4,400 units, whereas when it was converted into
_carbonic dioxide_ 14,500 units were given out. It will thus be seen
that it is extremely wasteful to burn coal without a sufficient supply
of air to produce carbonic dioxide. The danger of waste from this
cause is also increased by the fact that carbonic oxide is colorless
and odorless, and therefore its production is not apparent, especially
as most persons have the impression that when there is no smoke from
a fire combustion is then complete. It burns with a blue or yellowish
flame when air is admitted into the fire-box, and its presence can
often be detected by these phenomena when the furnace door is opened.
QUESTION 385. _How can the requisite quantity of air be supplied to the
fire in a locomotive fire-box?_
_Answer_. It is done in two ways: one is to keep but little coal on the
grates, or in the phraseology of firemen, to “carry a light fire.” The
other method is to admit fresh air above the fire. If the latter plan
is adopted when the supply of air through the grates is insufficient
for perfect combustion, the carbonic oxide will unite with the oxygen
of the air above the fire, and thus a second combustion will take
place, the product of which will be carbonic dioxide. It must be kept
in mind, however, that not only must there be enough air supplied to
the fire to consume the coke, but the gases which are distilled from
the coal must also be supplied with oxygen in order to effect their
perfect combustion. Even if enough air is admitted to consume the coke
perfectly, if the carbonic dioxide thus formed is mixed with large
quantities of smoke above the fire, the solid carbon or soot of the
smoke may then combine with the dioxide and thus form carbonic oxide,
if there is not enough fresh air present to furnish the requisite
oxygen for the carbon in the smoke. A very common error is to suppose
that smoke can be burned by passing it over or through a very hot fire.
The smoke may thus be made invisible, it is true, but it does not
therefore follow that it is perfectly consumed.
QUESTION 386. _Is it possible to admit too much air into the fire-box
of a locomotive?_
_Answer_. Yes; probably all the air that is admitted which is not
necessary for combustion, or, in other words, the oxygen of which
does not combine with the fuel, instead of increasing diminishes the
amount of water converted into steam. It does this in two ways; first,
by reducing the temperature of the gases in contact with the heating
surfaces, and second, by increasing the volume or quantity of the gases
which must pass through the tubes. Heat is transmitted through the
heating surface of a boiler in proportion to the _difference_ of the
temperature of the products of combustion on one side and the water on
the other.[94] Thus, if the temperature of the water on one side is 250
degrees, and the hot gases on the other is 500, there will be only half
as much heat transmitted to the water in a given time as there would be
if the gases had a temperature of 750 degrees. If the volume of gases
is doubled by the admission of too much air, then obviously in order
to pass through the tubes they must move at double the velocity, so
that not only is their temperature diminished, but the time they are
in contact with the heating surface is diminished in like proportion.
This is shown by the effect of opening the furnace door, or of allowing
the fire to burn away so that portions of the grate are left uncovered.
The volume of cold air which will in either of these cases enter the
fire-box will be so great that the pressure of the steam in the boiler
will begin to fall at once.
[94] This law is perhaps not absolutely correct, but is near enough
for our present illustration.
QUESTION 387. _What determines the amount of air which must be admitted
to the fire-box of a locomotive to effect perfect combustion?_
_Answer_. This depends chiefly upon the _rate of combustion_, that is,
the number of pounds of coal consumed per hour on each square foot of
grate surface. Of course if 100 pounds is burnt it will require twice
the supply of air that would be needed if only 50 pounds were burnt.
QUESTION 388. _How should the air be admitted so as to burn the coal
perfectly?_
_Answer_. In burning bituminous coal it has been shown that there are
two distinct bodies to be dealt with, the one coke, a _solid_, the
other coal gas, which is of course a _gaseous body_. The combustion
of each of these is necessarily a distinct process. If the requisite
quantity of air is supplied to the burning coke, or solid portion
of the coal, it will, as has been shown, be converted into carbonic
dioxide, and thus be perfectly consumed. If the supply of air is
insufficient, the product of the combustion will be carbonic oxide,
which is very wasteful. If, for example, there is a thick layer of
coke on the grate, the air will enter and unite with the lower layer
of coal and form carbonic dioxide, but as it rises there will not be
enough air to supply oxygen to the carbon, and another equivalent of
the latter will therefore combine with the carbonic dioxide and form
carbonic oxide. It is evident, though, that the thinner the fire, the
easier it is for air to pass through it, and consequently the greater
will be the quantity which will enter the fire-box. Nothing would seem
easier then than to regulate the thickness of the fire on the grates
so that just the needed amount of air would pass through it. If coke
alone was to be burned, undoubtedly very perfect combustion would be
(and has been) effected in this way, but if a charge of fresh coal, say
100 pounds, is thrown on the fire, the coal gas is very soon generated
and escapes into the fire-box. This gas needs an additional amount of
air for its combustion. It would seem that this could be supplied by
reducing the thickness of the fire still further, so that more air
would pass through it than was needed for the combustion of the coke
alone. If this was done then too much air would pass through the coke
after the gases had all escaped from the fresh coal and were burned.
Besides, the passage of the air would be the most restricted after the
fresh charge had been put on the fire, just at the time when the most
is needed. This difficulty might be overcome if a constant supply of
fresh coal just equal to that consumed were kept on the fire all the
time, and the thickness of fuel on the grates was then regulated so
as to admit just air enough for the combustion of the coke and also
that of the gases, the production of which would then be uniform. An
approximation to this method of feeding the fire is, in fact, what is
aimed at on most locomotives, and probably the best practical results
are produced by that method.
Two difficulties are, however, encountered in this method. In the first
place it is impossible to feed a fire continuously with a shovel. There
will be intervals between the charges which are thrown in, so that the
supply is not uniform, even if the charges do not consist of more than
a portion of a shovel-full at a time; and if the fire was fed in this
way as uniformly as possible it would then be necessary to open the
furnace door every time fresh coal was put on the fire, and so much
cold air would thus be admitted that more would be lost by lowering
the temperature of the boiler than would be gained by the improved
combustion.
Another difficulty also is encountered in this method of burning coal
in locomotives. In order to admit enough air through the fire it is
necessary to keep the latter so thin on the grates that the violent
draft produced by the blast lifts the coal from the grate-bars and
carries the lighter particles through the flues unconsumed. It is thus
extremely difficult to keep the grate uniformly covered with coal,
and if it is not, the air will enter in irregular and rapid streams
or masses through the uncovered parts, and at the very time when it
should be _there_ most restricted. Such a state of things at once bids
defiance to all regulation or control, so that it is found almost
uniformly that firemen of locomotives keep enough coal on the grates to
avoid the danger of “losing their fire,” as they express it; that is,
having all the burning coal drawn through the tubes by the blast. _Now,
on the control of the supply of air depends all that human skill can
do in effecting perfect combustion and economy_; and unless the supply
of fuel and the quantity on the bars can be regulated, it will be
impossible to control the admission of the air.[95]
[95] The Combustion of Coal, by C. Wye Williams.
Another method of feeding locomotive boilers is to pile up the coal in
the back part in a thick layer and slope it downward towards the front,
so that there is a comparatively thin fire in front. The mass piled
up at the door becomes converted into coke, and the production of gas
from the coal is more gradual and uniform than it is when only a small
quantity is thrown in at a time, and therefore a more uniform supply of
air is needed for its combustion. But it is apparent that very little
air can pass through the thick heap of coal at the back part of the
fire-box, and that therefore all, or nearly all the air which enters
it must come in through a comparatively small portion of the grate. It
will of course be difficult to admit the requisite quantity, for the
reasons already stated.
It is consequently apparent that it is practically impossible to admit
enough air through the grates to effect a constantly perfect combustion
of bituminous coal. It is, therefore, necessary to admit a portion of
the air above the fire. In doing this, however, in order to effect
perfect combustion the air thus admitted must be thoroughly mixed with
the gases, and in order to be able to enter into chemical combination,
or in other words, to burn, the gases must combine with the air at an
igniting temperature. If too much air is admitted, it will reduce the
temperature in the fire-box so much that the gases will not ignite;
or, if it is admitted in strong currents, the air and the gases will
flow side by side like the currents of two streams of water, the one
muddy and the other clear, which, as is well known, mingle very slowly.
Besides, if a hot stream of gas encounters a strong stream of cold air
and comes in contact with it only at its surface, the latter will be
cooled down below the igniting temperature; whereas if the two had been
intimately mixed in the right proportion, the whole mixture would have
been hot enough to burn. It is therefore of the utmost importance that
the air which is admitted above the fire should enter the fire-box in
many small jets. None of the openings for its admission should exceed
¹⁄₂ inch in diameter. With the violent draft in a locomotive fire-box
there is an extremely brief period of time for chemical combination
to take place after the gases are expelled from the coal and before
they are hurried into the tubes. As the chemical action between the
gases and the oxygen can only take place when the two are in intimate
contact, too much pains cannot be taken to distribute the currents of
admitted air and thus mix them with the combustible gases. In many
cases means are adopted to delay the air and the gases in the fire-box
so as to give them time for chemical combination or combustion before
entering the tubes.
QUESTION 389. _Does any combustion take place after the gases enter the
tubes?_
_Answer._ Very little; as the flames are extinguished soon after they
enter.
QUESTION 390. _Why are the flames extinguished in the tubes?_
_Answer._ They are then in contact with large quantities of
incombustible gas and beyond the reach of a supply of air; besides, the
temperature of the tubes which are surrounded with water is so low that
the flame is soon cooled down below an igniting temperature.
QUESTION 391. _What temperature is necessary to ignite coal gas or
produce flame?_
_Answer._ A temperature considerably hotter than red-hot iron is
needed, as can easily be shown by the fact that a gas-light can not be
ignited with a red-hot poker.
QUESTION 392. _Are there any parts of the fire-box where the
temperature is probably below the igniting point?_
_Answer._ Yes; along the sides and ends near the plates, which are
covered with water on the opposite side. At these points the coal is
usually “dead” or incandescent, as it remains at too low a temperature
to burn. For this reason, in some cases a space of from 8 to 12 inches
on each side and still more at the ends of the grates, is made of solid
plates, without any openings, and therefore called “_dead-grates_,”
so that no cold air can enter at those points. These plates are made
sloping downwards from the sides towards the centre of the fire-box,
so that the coal which falls on them and is thus coked, can easily be
raked towards the middle of the fire. This arrangement of dead plates
often improves the combustion and results in greater economy of fuel.
The reduction of the area of the openings between the grate-bars can
usually be compensated by making the bars narrower or the spaces
between them wider.
QUESTION 393. _What should be the condition of the coal when it is put
on the fire?_
_Answer._ It is true of the coal as well as of the gases that the
chemical action between it and the oxygen can only take place when
the two are in intimate contact, and therefore the rapidity and
completeness of combustion and intensity of heat will be increased by
increasing the number of points of contact, or by reducing the size of
the fuel. The coal should therefore be broken up, but not so small as
to fall between the grate-bars or be carried out of the fire-box by the
blast.
QUESTION 394. _What amount of air must be admitted to the fire to
effect perfect combustion?_
_Answer._ It was stated that average bituminous coal contains about 80
per cent. carbon, 5 per cent. of hydrogen and 15 per cent. of other
substances. As a large proportion of the latter are incombustible, we
will confine ourselves for the present to the consideration of the
combustion of the hydrogen and carbon alone.
The hydrogen, as has been explained, unites with oxygen in the
proportion by weight of 1 part of the former to 8 parts of the latter,
and the product of this union is water, or steam. As 36 parts of air
contain only 8 of oxygen, IN ORDER TO BURN THE HYDROGEN IT MUST BE
SUPPLIED WITH 36 TIMES ITS WEIGHT OF AIR.
In order to burn the carbon perfectly it must, as has been explained,
be converted into carbonic dioxide, which consists of 6 parts of carbon
and 16 of oxygen; and as air consists of 28 parts of nitrogen to every
8 of oxygen, we must furnish 72 parts of air to every 6 of carbon, or,
in other words, CARBON NEEDS 12 TIMES ITS WEIGHT OF AIR FOR ITS PERFECT
COMBUSTION.
Every pound of average bituminous coal therefore requires 1.8 lbs. of
air to burn its hydrogen, and 9.6 lbs. for the carbon, or 11.4 for
both. As a portion of the other substances of which coal is composed,
besides the oxygen and hydrogen, which others have been classed as
impurities, are combustible, there will be no material error if we
estimate the amount of air required for the combustion of bituminous
coal at 12 POUNDS PER POUND OF FUEL. As each cubic foot of air weighs
0.08072 lb., 12 pounds will be equal to
12
------- = 148.6 cubic feet of air,
0.08072
or for the sake of even figures and a quantity which can easily be
remembered, we will say 150 CUBIC FEET OF AIR ARE NEEDED FOR THE
COMBUSTION OF EACH POUND OF COAL. This is the theoretical quantity of
air which is needed for combustion. Now, unfortunately, the process of
combustion in the fire-boxes of locomotives is one in which any very
exact combination of the substances which unite is not possible with
the appliances which are now employed. If, therefore, we admitted the
exact amount of air given above, while some portions of the fire where
combustion was not very active might have more air than is needed,
other portions would have too little; and if the air is not very
thoroughly mixed, the flame and burning coal may be surrounded with the
products of combustion, which would exclude the air and thus reduce
its effect upon the fire. For this reason, besides the air required
to furnish the oxygen necessary for the complete combustion of the
fuel, it is also necessary to furnish an additional quantity of air
for the _dilution_ of the gaseous products of combustion, which would
otherwise prevent the free access of air to the fuel. The more minute
the division and the greater the velocity with which the air rushes
among the fuel, the smaller is the additional quantity of air required
for dilution. In locomotive boilers, although this quantity has not
been exactly ascertained, there is reason to believe that it may on
an average be estimated at about _one-half_ of the air required for
combustion.[96] We would therefore have as the quantity of air needed
for combustion
150
150 + --- = 225 cubic feet.
2
This estimate is roughly made, but it is the nearest approximation at
present attainable. It is probable that the supply of air required
for dilution varies considerably in different arrangements of the
fire-box and for different kinds of fuel, and it is possible that by
admitting the air for combustion in small enough jets, and deflecting
the currents of smoke and gases so as to cause them to mingle with the
air, the quantity required for dilution might be reduced below that
indicated by the above calculation. Undoubtedly all the air which is
admitted into the fire-box which does not combine with the chemical
elements of the fuel lessens the amount of steam generated in the
boiler, both with reference to time, that is to say per minute, and
to fuel, that is per pound of coal consumed. But with the present
locomotive boiler it is simply a choice of two evils. If no more air is
admitted than theory indicates to be needed for combustion, then, owing
to the imperfect means which are usually employed to cause the air and
fuel to combine, a portion of the latter will escape unconsumed; and if
_more_ air is admitted, the temperature of the products of combustion
is lowered and their volume increased, the evils of which have already
been pointed out. It therefore becomes a matter in which we are obliged
to consult experience and determine by experiment what amount of air
it is necessary to admit to the fuel to produce the most economical
results.
[96] Rankine.
QUESTION 395. _What proportion of the air should be admitted through
the grate, and how much above the fire?_
_Answer._ This, too, is a question which can probably be answered
best by consulting experience. The relative quantity of air required
above and below the fire depends very much on the nature of the fuel.
Coal which “runs together” or cakes very much or has a great deal of
clinker in it, doubtless, will need more air above the fire than other
coal which is said to be “dryer,” for the reason that it will be found
impossible to admit so much air through the caking coal in the grate as
through the other kind. An idea of the relative quantity which should
be admitted above and below the fire may be found if we know how much
air is needed to burn the solid carbon or coke which is left after the
gas is expelled from it, and how much for the gas itself. The gas which
is expelled from a pound of coal consists of about 0.05 lb. of hydrogen
and 0.15 lb. of carbon. Now, it has been shown that hydrogen requires
36 times its weight of air to burn it perfectly, so that 0.05 lb. would
need 0.05 × 36 = 1.8 lbs.; and carbon requires 12 times its weight of
air, so that for 0.15 lb. of carbon 0.15 × 12 = 1.8 lbs. is needed, so
that for both 3.6 lbs. of air is required for perfect combustion. As
has been shown, 12 lbs. is needed to consume the whole of the fuel, so
that 30 per cent. of the whole supply is required for the combustion
of the gas alone. If this is diluted in the same proportion as that
required for the combustion of the carbon, and it probably should be
even more so, we would have 30 per cent. of 225 = 67.5 cubic feet of
air required for the combustion of the gas. It is certain, however,
that the solid coke on the grates is not perfectly consumed, or, in
other words, converted into carbonic dioxide, especially when the layer
of it on the grates is very thick. When this is the case the air coming
in contact with the lower layer of coke forms carbonic dioxide, but as
it rises through the burning coke another equivalent of carbon unites
with the carbonic dioxide, and thus forms carbonic oxide. If, now,
enough air is admitted above the fire, this carbonic oxide will combine
with it, and, as has been explained before, a second combustion will
take place if there is time and opportunity for combination before the
gases enter the flues. It is therefore probable that more than 30 per
cent. of the whole supply of air should be admitted above the fire. It
is at any rate best to provide the means for admitting more, and also
appliances for regulating the supply, so that it can be governed as
experience may indicate to be best.
QUESTION 396. _Is it not possible by enlarging the grate to admit
enough air to the fire to produce perfect combustion?_
_Answer._ Yes; when no air is admitted above the fire, large grates are
found to produce the best combustion. But while it is true that the
same amount of heat will be produced by the union of each equivalent
of oxygen and fuel, yet if we can force _more_ air and fuel to unite
in the _same place_, a higher temperature is produced in that place,
just as a fire in a blacksmith’s forge is hotter because of the forced
blast than that in an ordinary stove, or a smelting furnace than a
parlor grate. If, then, we can concentrate the draft in the fire of a
locomotive, we secure a greater _intensity_ of combustion; and when the
air is urged against the solid carbon with considerable force, it comes
in contact with every point of its surface, and therefore less dilution
of the air is needed, and consequently the products of combustion have
a higher temperature; and, as has been explained, a larger proportion
of the heat is then transferred to the water than if the temperature is
lower and the volume greater.
Intensity of combustion also has the effect of maintaining an igniting
temperature; whereas, if the same amount of fuel is burned slowly, its
heat may not be high enough to ignite the gases as they are produced.
It is desirable, however, to have all the space that is possible in the
fire-box, so as to give room for the mixing of the gases; but with a
large fire-box and large grate a decided improvement and economy will
often result by diminishing the effective area of the grate by covering
a part of it with dead plates, but at the same time making provision
for the admission of air above the fire.
QUESTION 397. _What is meant by the “Total Heat of Combustion?”_
_Answer._ It is the number of units of heat given out by the combustion
of a given quantity (usually a pound) of fuel.
QUESTION 398. _How is this determined?_
_Answer._ The heat given out by the combustion of one pound of the
chemical elements of which coal is composed has been determined by
experiment, and from such data, knowing the substances of which fuel is
composed, we can determine the amount of heat which would be developed
if they were each perfectly consumed. Thus the total heat of combustion
of one pound of hydrogen is 62,032 units, and of the same quantity of
carbon 14,500 units.[97] Therefore, if a pound of coal contains 5 per
cent. of hydrogen, the heat given out by the combustion of that element
will be 62,032 × 0.05 = 3,101.60 units, and if it has 80 per cent. of
carbon, the combustion of the latter would develop 14,500 × 0.80 =
11,600 units, so that the total heat of the combustion of these two
elements would be 3,101.6 + 11,600 = 14,701.6 units. It was shown in
answer to Question 40 that it required 1,213.4 units of heat to convert
water at zero to steam of 100 pounds pressure. As steam is usually
generated from water at a temperature of about 60 degrees, the total
heat required to convert it into steam of 100 pounds pressure would
be 1,213.4 - 60 = 1,153.4 units. A pound of average bituminous coal,
therefore, contains heat enough to convert 12¹⁄₄ lbs. of water into
steam of 100 lbs. absolute pressure. Ordinarily only about half that
amount of water is evaporated in locomotive boilers per pound of fuel.
[97] The experiments which have been made to determine these amounts
do not agree exactly, but those given are thought to be the most
trustworthy.
QUESTION 399. _What are the chief causes of this waste of heat?_
_Answer._ It is due, _first_, to the waste of unburnt fuel in the
solid state. This occurs when fuel which is very fine falls through the
grates, or is carried through the tubes and out of the stack in the
form of cinders.[98]
[98] It should be remarked here that some and perhaps most of the
cinders which are carried out of the stack are not combustible but
are composed of the same materials that form clinkers on the grate.
_Second_, to the waste of unburnt fuel in the gaseous or smoky state.
The method of preventing this waste by a sufficient supply and proper
distribution of air has been explained in the answer to preceding
questions.
_Third_, to the waste or loss of heat in the hot gases which escape up
the chimney or smoke stack. The temperature of the fire in a locomotive
fire-box in a state of active combustion is probably from 3,000 to
4,000 degrees. This heat is in part radiated and conducted to the
heating surface of the fire-box, and it is found that more water is
evaporated by this portion of the heating surface in proportion to
its area than by any other in the boiler. The gases when they enter
the tubes transmit a portion of their heat to the surfaces with which
they are first in contact. The amount of heat thus transmitted, as
has been stated, is in proportion to the _difference_ in temperature
of the gases inside the tubes and that of the water outside. After
passing over the part of the tube with which the gases are first in
contact, they then arrive at another portion of the tube surface with
a diminished temperature, and the rate of conduction is therefore
diminished; so that each successive equal portion of the heating
surface transmits a less and less quantity of heat, until the hot air
at last leaves the heating surface and escapes up the chimney with a
certain remaining excess of temperature above that of the water in
the boiler, the heat corresponding to which excess is wasted.[99] It
is, therefore, desirable to extract as much heat as possible from the
gases before they escape from the tubes. Now it will be impossible to
heat the water outside of the tubes hotter than the gases inside. When
the temperature of the water is equal to that of the gases, no more
heat will be transmitted from one to the other. If the temperature of
the water is 350 degrees, that of the gases in the tubes will never
be any lower, but will escape into the smoke-box with not less than
that amount of heat. If, however, the cold water is introduced at the
front end of the tubes, so that the surface with which the gases are
last in contact has a temperature considerably lower than 350, then an
additional amount of heat will be transmitted before they escape. It
is, therefore, important that the cold feed-water should be admitted
near the front end of the boiler, so that the products of combustion
will be in contact with the coldest part of the heating surface last,
and thus give out as much of their heat as possible before they escape.
As a matter of fact, the gases escape at a much higher temperature.
Experiments made by the writer showed that the temperature in the
smoke-box of a locomotive when first starting was 270 degrees, and when
working at its maximum capacity on a steep grade and with a heavy train
it was as high as 675 degrees. The average temperature while running
was, in three trials on different parts of the road, as follows:
Average steam pressure, 98.8 lbs.; average temperature, 499.8 lbs.
Average steam pressure, 106 lbs.; average temperature, 535.1 lbs.
Average steam pressure, 112.2 lbs.; average temperature, 554 lbs.
[99] Rankine.
In making these experiments a record was made of the indications of
a pyrometer and of the steam gauge once every minute while the engine
was running. The distance run was 19 miles for the first experiment, 13
for the second and 6 for the third, with 30 loaded freight cars in the
train. The last experiment was made while the engine was working on a
heavy grade and very nearly up to its maximum capacity.
It will thus be seen that a great deal of heat is wasted by escaping up
the chimney.
_Fourth_, by external radiation from the boiler. This occurs chiefly
from the fact that it is not sufficiently well protected or covered
with non-conducting material. The practice, or rather the neglect, of
not covering the outside of the fire-box with lagging doubtless causes
a very considerable loss of heat by radiation and convection from the
hot boiler plates.
QUESTION 400. _What is the ordinary form of fire-box employed for
burning bituminous coal?_
_Answer._ It is that represented in plate II and figs. 41 and 44, and
is simply a rectangular box, and for that reason it is called a _plain_
fire-box. Sometimes provision is made for admitting air into such
fire-boxes through hollow or rather tubular stay-bolts, which are put
into the sides and front. In most cases, too, the fire-box door has
perforations for admitting air.
[Illustration: Fig. 214. Scale ³⁄₈ in. = 1 foot.]
QUESTION 401. _What other appliances are used for burning bituminous
coal?_
_Answer._ The most common appliance which is added to the plain
fire-box is what is called a _fire-brick arch_. This is shown in
fig. 214. _B C_ is the arch which, as its name implies, is formed
of fire-brick and extends backward and upward from a point in the
tube-sheet below the tubes. In order to be self-supporting it is
built in the form of an arch, the two sides of the fire-box acting as
abutments for its support. The engraving represents with sufficient
clearness the direction of the flames and smoke. These must take a more
circuitous “_run_,” as it is called, after leaving the fire, in order
to reach the tubes. Time is thus given for the gases to combine and
combustion to take place. The fire-brick becomes heated, and thus to
some extent prevents the gases from being cooled down below an igniting
temperature by contact with the cold surface of the fire-box before
combustion is complete. The fire-brick, however, soon burns out, and
must be replaced, but owing to its cheapness and the ease with which
it can be removed, this does not make a serious objection to its use.
Air is nearly always admitted above the fire when the brick arch is
used, either by tubular stay-bolts, _a_, _a_, _a_, or perforations in
the door, or both.
[Illustration: Fig. 215. Scale ³⁄₈ in. = 1 foot.]
In order to avoid the inconvenience and expense of replacing the
fire-brick arch, what is known as the “Jauriet water-table” has been
extensively used on some roads. This is the invention of Mr. C. F.
Jauriet and is represented in fig. 215, and consists of a flat “table,”
_B C_, formed of two boiler plates placed about 4¹⁄₂ in. apart, with
the space between filled with water. The two plates are stayed with
ordinary stay-bolts in the same way as the sides of the fire-box. The
form of the water-table is similar to that of the fire-brick, excepting
that it is not arched, this form not being necessary, as the plates
are riveted to the sides of the fire-box. Air is admitted above the
fire both by hollow stay-bolts and holes in the door, as shown at _A_.
Tubes, _f_, are put into the front and lower portion of the water-table
to allow the ashes and cinders, which would otherwise be deposited
above, to fall down on the grates.
[Illustration: Fig. 216.]
When air is admitted at the furnace door of an ordinary fire-box, it
is very apt to rush directly into the tubes without mingling with the
gases. It was found by some of the firemen on English railroads that by
placing a shovel over the top of the furnace door, the current of air
which entered could thus be deflected downward, and in this way smoke
could be almost entirely prevented. This led to the adoption of a hood
or deflector, _A_, fig. 216, which is made of sheet iron and is placed
over the fire-box door and is arranged with a lever, _B_, so that it
can be raised in order to be out of the way when coal is thrown on the
fire. It is suspended from a hook, _C_, from which it can easily be
detached and taken out for repairs. This is frequently necessary, as
the intense heat of the fire-box burns away the sheet iron of which it
is made very rapidly. It can be made of old boiler plate, so that the
expense of renewal is very slight. When this plan is used, a double
sliding door, shown in fig. 217, is commonly used with it. These doors
are opened by the levers _f d_; and _e g_, which are all connected
together. With these sliding doors the opening for the admission of air
can easily be regulated, and the opening through which the lever, _B_,
is attached to the deflector, _A_, can be arranged more conveniently
than with a swinging door. This plan has been employed by the Rogers
Locomotive Works.
[Illustration: Fig. 217.
Scale ³⁄₈ in. = 1 foot.]
Another plan of fire-box, which was designed and patented by Mr.
William Buchanan, Master Mechanic of the Hudson River Railroad, and
used extensively on that line, is shown in fig. 218. This consists
of a water-table, but it extends completely across the fire-box from
the tube sheet to the back-plate, thus dividing the fire-box into two
compartments, _M_ and _N_. In order to afford communication from the
lower one to the upper one a round hole, _D_, about 24 in. in diameter,
is put in the water-table in the position shown. It will thus be seen
that all the currents of gas, smoke and air must unite in passing
through this opening, and are thus brought into close contact with
each other. After they enter the upper chamber and before they enter
the tubes, there is room and time for combustion. The position of the
lower side of the table, it will be seen, is similar to that of the
deflector shown in fig. 216, so that it acts in somewhat the same way,
by directing the currents of air, which enter through the furnace door,
downward on the fire.
[Illustration: Fig. 218. Scale ³⁄₈ in. = 1 foot.]
QUESTION 402. _How do these different plans operate?_
_Answer._ They will all burn coal more perfectly, and therefore more
economically, if they are carefully and skillfully managed, than is
possible in ordinary plain fire-boxes; but it is probable that more
economy in the consumption of coal would result from the improvement of
the practice and knowledge of firemen than can be expected from the use
of any of the appliances described, if they are used without care, or
knowledge of the principles of combustion.
QUESTION 403. _In what respect does anthracite coal differ from
bituminous?_
_Answer._ It differs chiefly in the fact that it contains a much larger
proportion of carbon and less of hydrogen, and in the fact that it
consequently gives off very little or no coal gas. Its combustion is
therefore more simple than that of bituminous coal, as there is very
little else than solid carbon to burn.
QUESTION 404. _In what kind of a fire-box is anthracite usually burned?_
_Answer._ It is usually burned in a very long grate, and as the heat is
very intense, the grate-bars are usually made of iron tubes, through
which a current of water circulates, so as to prevent them from
melting.
QUESTION 405. _Is it important to admit air above an anthracite fire to
facilitate combustion?_
_Answer._ As there are no gases to be burned, it is not so important
as it is with bituminous coal, but if the layer of anthracite on the
grates is very thick, it will be impossible to get enough air through
the coal to convert all the carbon into carbonic dioxide, and the
carbon and oxygen will therefore unite so as to form carbonic oxide.
If air is admitted above the fire, as has already been explained,
another equivalent of oxygen will unite with the carbonic oxide, and
a second combustion will then take place above the fire, and the
carbonic oxide will thus be converted into carbonic dioxide. If, under
these circumstances, no air was admitted above the fire, the second
combustion would not occur, and all the heat produced thereby would be
lost.
QUESTION 406. _How can we determine the relative value of different
kinds of fuel for use in locomotives?_
_Answer._ This can only be determined satisfactorily by actual
experiment. The chemical composition, excepting so far as it indicates
the presence of deleterious substances, such as sulphur, ashes,
clinkers, etc., affords but little assistance in determining the value
of fuel. Nearly the same quantities of elements in different fuels may
arrange themselves, before and during combustion, so as to produce very
different series of compounds. It is true that the composition of coal
gives us some indication of its heat-producing _capacity_, but the
extent to which that capacity can be converted into actual steam in
locomotive boilers, depends to a very great extent upon the conditions
under which the fuel is burned. It should also be remembered that the
rapidity with which steam can be generated is a very important matter
in locomotive practice. Whether a heavy freight train can be taken up a
given grade, or a fast express make time, often depends upon the amount
of steam which can be generated by the fuel in each second of time that
the boiler is worked to its maximum capacity. Therefore any appliance
for improving combustion, which reduces the quantity of steam which can
be generated by the boiler in a given time, is quite sure to fall into
disuse or be abandoned. It is of course often necessary to adapt the
appliances for burning fuel to the fuel itself; and when a poor quality
of the latter must be used, more boiler capacity must be given than is
needed to do the same work with better fuel.
The table in the appendix will no doubt be valuable as indicating the
properties and relative value of several different kinds of fuel used
in this country. The table is copied from a report made to the Navy
Department of the United States by Professor Walter B. Johnson in
1844, and the conclusions are deduced from a series of very elaborate
experiments made for the Navy Department. This report furnishes the
most full and reliable data regarding the value of American fuel
thus far (1874) published; but it contains little or no information
concerning the fuels which are now used on railroads in our Western
States. The first eight specimens of coal given in the table are
anthracite; all the rest are bituminous coals.