The points to which I specially called your attention in the first lecture,
and which it is necessary to recapitulate to-day, are these: (1) That coal
is distilled, or burned partly into gas, before it can be burned. (2) That
the gas, so given off, if mixed with carbonic acid, cannot be expected to
burn properly or completely. (3) That to burn the gas, a sufficient supply
of air must be introduced at a temperature not low enough to cool the gases
below their igniting point. (4) That in stoking a fire, a small amount
should be added at a time because of the heat required to warm and distill
the fresh coal. (5) That fresh coal should be put in front of or at the
bottom of a fire, so that the gas may be thoroughly heated by the
incandescent mass above and thus, if there be sufficient air, have a chance
of burning. A fire may be inverted, so that the draught proceeds through it
downward. This is the arrangement in several stoves, and in them, of
course, fresh coal is put at the top.
Two simple principles are at the root of all fire management: (1) Coal gas
must be at a certain temperature before it can burn; and (2) it must have a
sufficient supply of air. Very simple, very obvious, but also extremely
important, and frequently altogether ignored. In a common open fire they
are both ignored. Coal is put on the top of a glowing mass of charcoal, and
the gas distilled off is for a longtime much too cold for ignition, and
when it does catch fire it is too mixed with carbonic acid to burn
completely or steadily. In order to satisfy the first condition better, and
keep the gases at a higher temperature, Dr. Pridgin Teale arranges a
sloping fire-clay slab above his fire. On this the gases play, and its
temperature helps them to ignite. It also acts as a radiator, and is said
to be very efficient.
In a close stove and in many furnaces the second condition is violated;
there is an insufficient supply of air; fresh coal is put on, and the
feeding doors are shut. Gas is distilled off, but where is it to get any
air from? How on earth can it be expected to burn? Whether it be expected
or not, it certainly does not burn, and such a stove is nothing else than a
gas works, making crude gas, and wasting it--it is a soot and smoke
factory.
Most slow combustion stoves are apt to err in this way; you make the
combustion slow by cutting off air, and you run the risk of stopping the
combustion altogether. When you wish a stove to burn better, it is
customary to open a trap door below the fuel; this makes the red hot mass
glow more vigorously, but the oxygen will soon become CO_{2}, and be unable
to burn the gas.
The right way to check the ardor of a stove is not to shut off the air
supply and make it distill its gases unconsumed, but to admit so much air
above the fire that the draught is checked by the chimney ceasing to draw
so fiercely. You at the same time secure better ventilation; and if the
fire becomes visible to the room so much the better and more cheerful. But
if you open up the top of a stove like this, it becomes, to all intents and
purposes, an open fire. Quite so, and in many respects, therefore, an open
fire is an improvement on a close stove. An open fire has faults, and it
certainly wastes heat up the chimney. A close stove may have more
faults--it wastes less _heat_, but it is liable to waste _gas_ up the
chimney--not necessarily visible or smoky gas; it may waste it from coke or
anthracite, as CO.
You now easily perceive the principles on which so-called smoke consumers
are based. They are all special arrangements or appendages to a furnace for
permitting complete combustion by satisfying the two conditions which had
been violated in its original construction. But there is this difficulty
about the air supply to a furnace: the needful amount is variable if the
stoking be intermittent, and if you let in more than the needful amount,
you are unnecessarily wasting heat and cooling the boiler, or whatever it
is, by a draught of cold air.
Every time a fresh shovelful is thrown on, a great production of gas
occurs, and if it is to flame it must have a correspondingly great supply
of air. After a time, when the mass has become red hot, it can get nearly
enough air through the bars. But at first the evolution of gas actually
checks the draught. But remember that although no smoke is visible from a
glowing mass, it by no means follows that its combustion is perfect. On an
open fire it probably is perfect, but not necessarily in a close stove or
furnace. If you diminish the supply of air much (as by clogging your
furnace bars and keeping the doors shut), you will be merely distilling
carbonic oxide up the chimney--a poisonous gas, of which probably a
considerable quantity is frequently given off from close stoves.
Now let us look at some smoke consumers. The diagrams show those of Chubb,
Growthorpe, Ireland and Lowndes, and of Gregory. You see that they all
admit air at the "bridge" or back of the fire, and that this air is warmed
either by passing under or round the furnace, or in one case through hollow
fire bars. The regulation of the air supply is effected by hand, and it is
clear that some of these arrangements are liable to admit an unnecessary
supply of air, while others scarcely admit enough, especially when fresh
coal is put on. This is the difficulty with all these arrangements when
used with ordinary hand--i.e., intermittent--stoking. Two plans are open to
us to overcome the difficulty. Either the stoking and the air supply must
both be regular and continuous, or the air supply be made intermittent to
suit the stoking. The first method is carried out in any of the many forms
of mechanical stoker, of which this of Sinclair's is an admirable specimen.
Fresh fuel is perpetually being pushed on in front, and by alternate
movement of the fire bars the fire is kept in perpetual motion till the
ashes drop out at the back. To such an arrangement as this a steady air
supply can be adjusted, and if the boiler demand is constant there is no
need for smoke, and an inferior fuel may be used. The other plan is to vary
the air supply to suit the stoking. This is effected by Prideaux automatic
furnace doors, which have louvers to remain open for a certain time after
the doors are shut, and so to admit extra air immediately after coal has
been put on, the supply gradually decreasing as distillation ceases. The
worst of air admitted through chinks in the doors, or through partly open
doors, is that it is admitted cold, and scarcely gets thoroughly warm
before it is among the stuff it has to burn. Still this is not a fatal
objection, though a hot blast would be better. Nothing can be worse than
shoveling on a quantity of coal and shutting it up completely. Every
condition of combustion is thus violated, and the intended furnace is a
mere gas retort.
_Gas Producers_.--Suppose the conditions of combustion are purposely
violated; we at once have a gas producer. That is all gas producers are,
extra bad stoves or furnaces, not always much worse than things which
pretend to serve for combustion. Consider how ordinary gas is made. There
is a red-hot retort or cylinder plunged in a furnace. Into this tube you
shovel a quantity of coal, which flames vigorously as long as the door is
open, but when it is full you shut the door, thus cutting off the supply of
air and extinguishing the flame. Gas is now simply distilled, and passes
along pipes to be purified and stored. You perceive at once that the
difference between a gas retort and an ordinary furnace with closed doors
and half choked fire bars is not very great. Consumption of smoke! It is
not smoke consumers you really want, it is fuel consumers. You distill your
fuel instead of burning it, in fully one-half, might I not say nine-tenths,
of existing furnaces and close stoves. But in an ordinary gas retort the
heat required to distill the gas is furnished by an outside fire; this is
only necessary when you require lighting gas, with no admixture of carbonic
acid and as little carbonic oxide as possible. If you wish for heating gas,
you need no outside fire; a small fire at the bottom of a mass of coal will
serve to distill it, and you will have most of the carbon also converted
into gas. Here, for instance, is Siemens' gas producer. The mass of coal is
burning at the bottom, with a very limited supply of air. The carbonic acid
formed rises over the glowing coke, and takes up another atom of carbon to
form the combustible gas carbonic oxide. This and the hot nitrogen passing
over and through the coal above distill away its volatile constituents, and
the whole mass of gas leaves by the exit pipe. Some art is needed in
adjusting the path of the gases distilled from the fresh coal with
reference to the hot mass below. If they pass too readily, and at too low a
temperature, to the exit pipe, this is apt to get choked with tar and dense
hydrocarbons. If it is carried down near or through the hot fuel below, the
hydrocarbons are decomposed over much, and the quality of the gas becomes
poor. Moreover, it is not possible to make the gases pass freely through a
mass of hot coke; it is apt to get clogged. The best plan is to make the
hydrocarbon gas pass over and near a red-hot surface, so as to have its
heaviest hydrocarbons decomposed, but so as to leave all those which are
able to pass away as gas uninjured, for it is to the presence of these that
the gas will owe its richness as a combustible material, especially when
radiant heat is made use of.
The only inert and useless gas in an arrangement like this is the nitrogen
of the air, which being in large quantities does act as a serious diluent.
To diminish the proportion of nitrogen, steam is often injected as well as
air. The glowing coke can decompose the steam, forming carbonic oxide and
hydrogen, both combustible. But of course no extra energy can be gained by
the use of steam in this way; all the energy must come from the coke, the
steam being already a perfectly burned product; the use of steam is merely
to serve as a vehicle for converting the carbon into a convenient gaseous
equivalent. Moreover, steam injected into coke cannot keep up the
combustion; it would soon put the fire out unless air is introduced too.
Some air is necessary to keep up the combustion, and therefore some
nitrogen is unavoidable. But some steam is advisable in every gas producer,
unless pure oxygen could be used instead of air; or unless some substance
like quicklime, which holds its oxygen with less vigor than carbon does,
were mixed with the coke and used to maintain the heat necessary for
distillation. A well known gas producer for small scale use is Dowson's.
Steam is superheated in a coil of pipe, and blown through glowing
anthracite along with air. The gas which comes off consists of 20 per cent.
hydrogen, 30 per cent. carbonic oxide, 3 per cent. carbonic acid, and 47
per cent. nitrogen. It is a weak gas, but it serves for gas engines, and is
used, I believe, by Thompson, of Leeds, for firing glass and pottery in a
gas kiln. It is said to cost 4d. per 1,000 ft., and to be half as good as
coal gas.
For furnace work, where gas is needed in large quantities, it must be made
on the spot. And what I want to insist upon is this, that all
well-regulated furnaces are gas retorts and combustion chambers combined.
You may talk of burning coal, but you can't do it; you must distill it
first, and you may either waste the gas so formed or you may burn it
properly. The thing is to let in not too much air, but just air enough.
Look, for instance, at Minton's oven for firing pottery. Round the central
chamber are the coal hoppers, and from each of these gas is distilled,
passes into the central chamber, where the ware is stacked, and meeting
with an adjusted supply of air as it rises, it burns in a large flame,
which extends through the whole space and swathes the material to be
heated. It makes its exit by a central hole in the floor, and thence rises
by flues to a common opening above. When these ovens are in thorough
action, nothing visible escapes. The smoke from ordinary potters' ovens is
in Staffordshire a familiar nuisance. In the Siemens gas producer and
furnace, of which Mr. Frederick Siemens has been good enough to lend me
this diagram, the gas is not made so closely on the spot, the gas retort
and furnace being separated by a hundred yards or so in order to give the
required propelling force. But the principle is the same; the coal is first
distilled, then burnt. But to get high temperature, the air supply to the
furnace must be heated, and there must be no excess. If this is carried on
by means of otherwise waste heat we have the regenerative principle, so
admirably applied by the Brothers Siemens, where the waste heat of the
products of combustion is used to heat the incoming air and gas supply. The
reversing arrangement by which the temperature of such a furnace can be
gradually worked up from ordinary flame temperature to something near the
dissociation point of gases, far above the melting point of steel, is well
known, and has already been described in this place. Mr. Siemens has lent
me this beautiful model of the most recent form of his furnace, showing its
application to steel making and to glass working.
The most remarkable and, at first sight, astounding thing about this
furnace is, however, that it works solely by radiation. The flames do not
touch the material to be heated; they burn above it, and radiate their heat
down to it. This I regard as one of the most important discoveries in the
whole subject, viz., that to get the highest temperature and greatest
economy out of the combustion of coal, one must work directly by radiant
heat only, all other heat being utilized indirectly to warm the air and gas
supply, and thus to raise the flame to an intensely high temperature.
It is easy to show the effect of supplying a common gas flame with warm air
by holding it over a cylinder packed with wire gauze which has been made
red hot. A common burner held over such a hot air shaft burns far more
brightly and whitely. There is no question but that this is the plan to get
good illumination out of gas combustion; and many regenerative burners are
now in the market, all depending on this principle, and utilizing the waste
heat to make a high temperature flame. But although it is evidently the
right way to get light, it was by no means evidently the right way to get
heat. Yet so it turns out, not by warming solid objects or by dull warm
surfaces, but by the brilliant radiation of the hottest flame that can be
procured, will rooms be warmed in the future. And if one wants to boil a
kettle, it will be done, not by putting it into a non-luminous flame, and
so interfering with the combustion, but by holding it near to a freely
burning regenerated flame, and using the radiation only. Making toast is
the symbol of all the heating of the future, provided we regard Mr.
Siemens' view as well established.
The ideas are founded on something like the following considerations: Flame
cannot touch a cold surface, i.e., one below the temperature of combustion,
because by the contact it would be put out. Hence, between a flame and the
surface to be heated by it there always intervenes a comparatively cool
space, across which heat must pass by radiation. It is by radiation
ultimately, therefore, that all bodies get heated. This being so, it is
well to increase the radiating power of flame as much as possible. Now,
radiating power depends on two things: the presence of solid matter in the
flame in a fine state of subdivision, and the temperature to which it is
heated. Solid matter is most easily provided by burning a gas rich in dense
hydrocarbons, not a poor and non-luminous gas. To mix the gas with air so
as to destroy and burn up these hydrocarbons seems therefore to be a
retrograde step, useful undoubtedly in certain cases, as in the Bunsen
flame of the laboratory, but not the ideal method of combustion. The ideal
method looks to the use of a very rich gas, and the burning of it with a
maximum of luminosity. The hot products of combustion must give up their
heat by contact. It is for them that cross tubes in boilers are useful.
They have no combustion to be interfered with by cold contacts. The _flame_
only should be free.
The second condition of radiation was high temperature. What limits the
temperature of a flame? Dissociation or splitting up of a compound by heat.
So soon as the temperature reaches the dissociation point at which the
compound can no longer exist, combustion ceases. Anything short of this may
theoretically be obtained.
But Mr. Siemens believes, and adduces some evidence to prove, that the
dissociation point is not a constant and definite temperature for a given
compound; it depends entirely upon whether solid or foreign surfaces are
present or not. These it is which appear to be an efficient cause of
dissociation, and which, therefore, limit the temperature of flame. In the
absence of all solid contact, Mr. Siemens believes that dissociation, if it
occur at all, occurs at an enormously higher temperature, and that the
temperature of free flame can be raised to almost any extent. Whether this
be so or not, his radiating flames are most successful, and the fact that
large quantities of steel are now melted by mere flame radiation speaks
well for the correctness of the theory upon which his practice has been
based.
_Use of Small Coal_.--Meanwhile, we may just consider how we ought to deal
with solid fuel, whether for the purpose of making gas from it or for
burning it _in situ_. The question arises, In what form ought solid fuel to
be--ought it to be in lumps or in powder? Universal practice says lumps,
but some theoretical considerations would have suggested powder. Remember,
combustion is a chemical action, and when a chemist wishes to act on a
solid easily, he always pulverizes it as a first step.
Is it not possible that compacting small coal into lumps is a wrong
operation, and that we ought rather to think of breaking big coal down into
slack? The idea was suggested to me by Sir W. Thomson in a chance
conversation, and it struck me at once as a brilliant one. The amount of
coal wasted by being in the form of slack is very great. Thousands of tons
are never raised from the pits because the price is too low to pay for the
raising--in some places it is only 1s. 6d. a ton. Mr. McMillan calculates
that 130,000 tons of breeze, or powdered coke, is produced every year by
the Gas Light and Coke Company alone, and its price is 3s. a ton at the
works, or 5s. delivered.
The low price and refuse character of small coal is, of course, owing to
the fact that no ordinary furnace can burn it. But picture to yourself a
blast of hot air into which powdered coal is sifted from above like ground
coffee, or like chaff in a thrashing mill, and see how rapidly and
completely it might burn. Fine dust in a flour mill is so combustible as to
be explosive and dangerous, and Mr. Galloway has shown that many colliery
explosions are due not to the presence of gas so much as the presence of
fine coal-dust suspended in the air. If only fine enough, then such dust is
eminently combustible, and a blast containing it might become a veritable
sheet of flame. (Blow lycopodium through a flame.) Feed the coal into a
sort of coffee-mill, there let it be ground and carried forward by a blast
to the furnace where it is to be burned. If the thing would work at all,
almost any kind of refuse fuel could be burned--sawdust, tan, cinder heaps,
organic rubbish of all kinds. The only condition is that it be fine enough.
Attempts in this direction have been made by Mr. T.R. Crampton, by Messrs.
Whelpley and Storer, and by Mr. G.K. Stephenson; but a difficulty has
presented itself which seems at present to be insuperable, that the slag
fluxes the walls of the furnace, and at that high temperature destroys
them. If it be feasible to keep the flame out of contact with solid
surfaces, however, perhaps even this difficulty can be overcome.
Some success in blast burning of dust fuel has been attained in the more
commonplace method of the blacksmith's forge, and a boiler furnace is
arranged at Messrs. Donkin's works at Bermondsey on this principle. A
pressure of about half an inch of water is produced by a fan and used to
drive air through the bars into a chimney draw of another half-inch. The
fire bars are protected from the high temperatures by having blades which
dip into water, and so keep fairly cool. A totally different method of
burning dust fuel by smouldering is attained in M. Ferret's low temperature
furnace by exposing the fuel in a series of broad, shallow trays to a
gentle draught of air. The fuel is fed into the top of such a furnace, and
either by raking or by shaking it descends occasionally, stage by stage,
till it arrives at the bottom, where it is utterly inorganic and mere
refuse. A beautiful earthworm economy of the last dregs of combustible
matter in any kind of refuse can thus be attained. Such methods of
combustion as this, though valuable, are plainly of limited application;
but for the great bulk of fuel consumption some gas-making process must be
looked to. No crude combustion of solid fuel can give ultimate perfection.
Coal tar products, though not so expensive as they were some time back, are
still too valuable entirely to waste, and the importance of exceedingly
cheap and fertilizing manure in the reclamation of waste lands and the
improvement of soil is a question likely to become of most supreme
importance in this overcrowded island. Indeed, if we are to believe the
social philosophers, the naturally fertile lands of the earth may before
long become insufficient for the needs of the human race; and posterity may
then be largely dependent for their daily bread upon the fertilizing
essences of the stored-up plants of the carboniferous epoch, just as we are
largely dependent on the stored-up sunlight of that period for our light,
our warmth, and our power. They will not then burn crude coal, therefore.
They will carefully distill it--extract its valuable juices--and will
supply for combustion only its carbureted hydrogen and its carbon in some
gaseous or finely divided form.
Gaseous fuel is more manageable in every way than solid fuel, and is far
more easily and reliably conveyed from place to place. Dr. Siemens, you
remember, expected that coal would not even be raised, but turned into gas
in the pits, to rise by its own buoyancy to be burnt on the surface
wherever wanted. And not only will the useful products be first removed and
saved, its sulphur will be removed too; not because it is valuable, but
because its product of combustion is a poisonous nuisance. Depend upon it,
the cities of the future will not allow people to turn sulphurous acid
wholesale into the air, there to oxidize and become oil of vitriol. Even if
it entails a slight strain upon the purse they will, I hope, be wise enough
to prefer it to the more serious strain upon their lungs. We forbid sulphur
as much as possible in our lighting gas, because we find it is deleterious
in our rooms. But what is London but one huge room packed with over four
millions of inhabitants? The air of a city is limited, fearfully limited,
and we allow all this horrible stuff to be belched out of hundreds of
thousands of chimneys all day long.
Get up and see London at four or five in the morning, and compare it with
four or five in the afternoon; the contrast is painful. A city might be
delightful, but you make it loathsome; not only by smoke, indeed, but still
greatly by smoke. When no one is about, then the air is almost pure; have
it well fouled before you rise to enjoy it. Where no one lives, the breeze
of heaven still blows; where human life is thickest, there it is not fit to
live. Is it not an anomaly, is it not farcical? What term is strong enough
to stigmatize such suicidal folly? But we will not be in earnest, and our
rulers will talk, and our lives will go on and go out, and next century
will be soon upon us, and here is a reform gigantic, ready to our hands,
easy to accomplish, really easy to accomplish if the right heads and
vigorous means were devoted to it. Surely something will be done.
The following references may be found useful in seeking for more detailed
information: Report of the Smoke Abatement Committee for 1882, by Chandler
Roberts and D.K. Clark. "How to Use Gas," by F.T. Bond; Sanitary
Association, Gloucester. "Recovery of Volatile Constituents of Coal," by
T.B. Lightfoot; Journal Society of Arts, May, 1883. "Manufacture of Gas
from Oil," by H.E. Armstrong; Journal Society of Chemical Industry,
September, 1884. "Coking Coal," by H.E. Armstrong; Iron and Steel
Institute, 1885. "Modified Siemens Producer," by John Head; Iron and Steel
Institute, 1885. "Utilization of Dust Fuel," by W.G. McMillan; Journal
Society of Arts, April. 1886. "Gas Producers," by Rowan; Proc. Inst. C.E.,
January, 1886. "Regenerative Furnaces with Radiation," and "On Producers,"
by F. Siemens; Journal Soc. Chem. Industry, July, 1885, and November, 1885.
"Fireplace Construction," by Pridgin Teale; the _Builder_, February, 1886.
"On Dissociation Temperatures," by Frederick Siemens; Royal Institution,
May 7, 1886.
* * * * *
Near Colorados, in the Argentine Republic, a large bed of superior coal has
been opened, and to the west of the Province of Buenos Ayres extensive
borax deposits have been discovered.
* * * * *