GAS GENERATING PLANT
We have so far followed, step by step, the various improvements
undergone by internal furnace engines, from their first practical
realization thirty-seven years ago by M. Lenoir. This improvement has
continually tended, as one might naturally expect, to cheapen the cost
of the motors themselves, and lessen the cost of the fuel consumption
so that they might successfully compete against the steam engine,
over which they have many advantages. As a rule, coal gas is pretty
expensive, in some particular districts ruling as high as 4_s._ or
5_s._ per 1000 cubic feet. This price would be prohibitive were it
not for the extreme economy which has been obtained in gas engines by
such devices as compression and the lengthening out of the combustion.
Inventors have sought to still further lessen the cost of power by
replacing the expensive coal gas by other gases of lower illuminating
power, produced by special gas plants attached to the engine, and
which can be used in places where there is no coal gas laid on. Other
engines have been devised, as we have already shown, which get over
the difficulty by consuming petroleum oil or distillates of same, such
as benzoline and other light hydrocarbons.
Gas companies are, as a rule, heavily taxed, and if it were not for a
vast amount of idle capital buried in the streets in the form of gas
mains, they would probably supply gas at a very low price, especially
as the sale of bye-products practically covers the cost of production.
Some companies have even reduced the price of gas if used for driving
gas engines, and have therefore slightly decreased the cost of
production of power by this means.
In France, the home of the oil motor, they are severely handicapped
by having to pay double duties on all petroleum entering, first the
country itself, afterwards the towns.
Inventors have therefore sought a method of getting round these
difficulties by producing a cheap gas, which would answer the same
purpose as coal gas. First of all many manufacturers tried to cut
down the expense by erecting their own coal gas plants, so as to
be independent of arbitrary taxation. The system, although fairly
satisfactory, had many drawbacks, and further experiments were made
with a view to simplifying the process of production. The result
has been to give us _water gas_ and also _poor gases_, whose great
practical value we shall presently demonstrate.
In order to produce what is called water gas, the process essentially
consists of placing in contact with one another red-hot carbon and
superheated steam. The result is to form a mixture of gases according
to the following chemical equation—
3 H_{2}O + 2 C = CO + CO_{2} + 3 H_{2}.
For those not understanding the above it will be as well to explain,
that on the left-hand side of the equation are placed the compounds
brought into contact, water (H_{2}O) and carbon (C), and on the right
the products of the chemical action, carbon monoxide (CO), carbonic
acid gas (CO_{2}), and hydrogen (H_{2}).
The resulting water gas contains 60% of hydrogen and 20% of carbon
monoxide, both of which are combustible gases. The proportions of the
gases evolved can be varied at will by admitting more steam or using an
excess of carbon; by this process a richer gas can be obtained—
C + H_{2}O = CO + H_{2},
and
5 H_{2}O + 3 C = 2 CO_{2} + CO + 5 H_{2}.
With 18 parts of steam and 12 parts of carbon we can obtain by the
former equation 28 parts of carbon monoxide and 2 parts of hydrogen.
This forms an extremely calorific mixture.
There are several processes for preparing water gas which give good
practical results, and the gas produced has been used in America and
Germany for lighting towns on the incandescent gas-burner system.
The Strong and Lowe processes consist of a furnace lined with
fire-bricks, and in which is placed coal or coke. When this mass of
carbon has reached a state of bright incandescence by playing upon it a
stream of air, steam is admitted at high temperature, and is decomposed
by the carbon forming oxides and liberating hydrogen. The gaseous
products pass from the furnace to a reservoir.
When the chemical action ceases, due to cooling of the carbon, the
steam is shut off and the stream of air turned on till it becomes
incandescent once more. The process of admitting steam is then repeated.
About 2½ lbs. of coke are necessary to produce 20 cubic feet of
water gas.
Analysis of the gas reveals the following parts by volume of the
constituents.
(1) Water gas produced by the Strong process:—
Hydrogen 53 volumes
Carbon monoxide 35 ”
Hydrocarbons 4 ”
Other gases 8 ”
———
100 volumes
(2) Lowe process:—
Hydrogen 30 volumes
Carbon monoxide 28 ”
Hydrocarbons 34 ”
Other gases 8 ”
———
100 volumes
It is possible to entirely get rid of the cooling effect of the steam,
by using instead a jet of air which passes up through the carbon. First
of all carbon dioxide (CO_{2}) is formed near the bottom of the mass,
but this gas passing upwards through it is reduced to carbon monoxide
(CO) by the excess of carbon, and a mixture is obtained consisting of
34 parts by volume of carbon monoxide and 65 of nitrogen. This gas has
been named after its inventor, Siemens gas.
By proceeding for ten minutes with this air process, and then stopping
it and generating water gas, two different mixtures are obtained, which
can be combined together forming a gaseous product containing 10 parts
of hydrogen, 20 of carbon monoxide, and 50 of nitrogen. One kilogramme
of coal produces 4·5 cubic metres of this mixture.
Instead of performing the operations of producing Siemens gas, or
producer gas, as it is sometimes termed, and water gas alternately and
separately, it is possible to so arrange the furnace and apparatus,
that both are generated at the same time, and continuously instead of
intermittently. The combination of the two is called poor gas.
The invention of gas-producing plant is due to two Frenchmen,
Thomas and Laurens, who studied deeply the question of the economic
generation of poor gas, and constructed the first working plant.
These two inventors stood, however, in the same position relative to
the production of poor gas that Beau de Rochas had held relative to
the gas engine, and it was not till Siemens came forward, and showed
how they might most economically be generated, that poor gases were
generally adopted. Siemens adapted them especially to metallurgy and
the manufacture of glass. We shall now describe the most interesting
processes which have been brought out since the time of the appearance
of the Siemens plant.
_Dowson gas-producer_ (Fig. 43).—This process was the first to appear
after the Siemens process, and the gas produced by it is used in a
large number of manufacturing operations. It consists of a generator,
a boiler for producing superheated steam, an hydraulic box, the
scrubbers and the gasometer. The generator is simply a gas retort lined
internally with fire-bricks, and placed vertically in position. It will
be seen in front on the left-hand side of the illustration. The fuel
is usually anthracite coal, and is supported on a grate. It is fed in
through a hopper placed at the top of the generator. By an arrangement
of valves the anthracite enters without direct communication being ever
established between the interior of the generator and the exterior
atmosphere, which would result in explosions. The steam which is to
be decomposed by the heated coal is generated in the small boiler,
seen in front on the right, at a pressure of 50 lbs. per square inch,
and superheated in a spiral coil inside. The steam passes into the
retort through an injector, drawing a quantity of air along with it
whilst passing from a nozzle across an air space. The air enters the
generator along with the steam and causes the coal to burn, and the
steam is decomposed, forming a mixture of producer gas and water gas.
The quantity of gas produced is therefore regulated by the injector.
The gases generated by the combustion of the anthracite are conveyed
by a pipe into a flat hydraulic box seen behind the generator, and
divided into two parts and half filled with water. The gases are washed
by this water and then pass on to the scrubbers, where they are cooled
and washed by passing through a mass of coke moistened by fine streams
of water. To further cleanse them they are passed through saw-dust and
thence pass to the gasometer. A number of analyses made by M. Witz
show that Dowson gas consists on the average of 25% hydrogen, 16 to
25% carbon monoxide, and 50% of nitrogen. The heat of combustion of
one litre varies according to the quality of coal used, but averages
about 1400 calories. One kilogramme of anthracite will produce about
four cubic metres of Dowson gas, the cost being one-tenth of a penny
per cubic metre. It must not be forgotten that this gaseous mixture is
only a quarter as rich as coal gas, but it costs about one-tenth to
produce, and is therefore cheaper on the whole. At some future date
this type of apparatus may, to a great extent, replace the boiler of
the steam engine. That Mr. Emerson Dowson’s process has succeeded
beyond his most sanguine expectations goes without saying. His
apparatus is in use in every corner of the globe, and, to quote his own
words, “still better results can and will be obtained when an engine is
really designed to give the best effect with this gas.”
[Illustration: FIG. 43.—Dowson Gas-producing Plant.]
_Buire-Lencauchez gasogene._—The analysis of the gas produced on this
system shows that it contains 20 volumes of carbon dioxide, 115 of
carbon monoxide, 66 of hydrogen, and 178 of nitrogen; its percentage
composition is therefore the following:—
Carbon monoxide 29·4
Carbon dioxide 5·9
Hydrogen 17·6
Nitrogen 47·1
———
100
Theoretically one kilogramme of coal should develop 5·26 cubic
metres of gas, having a heat of combustion at 0° C., and atmospheric
pressure of 1360 calories. These figures enable us to calculate the
efficiency of a gas-producing plant and the value of the gas obtained.
Very good results have been obtained by gas generated by the latest
Lencauchez process, with improvements added by the firm of Buire of
Lyons, who construct the apparatus. The chief point to be noticed in
these plants is the suppression of the steam boiler, which requires
constant attention and stoking. The hearth of the generator is made of
refractory bricks surrounded by a layer of sand to keep in the heat.
The fuel enters through a hopper, which by means of a bascule and
counterpoise never allows any direct communication between the interior
of the retort and the surrounding atmosphere. The fuel is either coke
or anthracite, and is spread over a grate situated over an ash-pit.
This ash-pit forms an important part of the apparatus, for it is fed
with water which evaporates from the heat striking down on to it from
the incandescent coke. The steam generated by this novel process passes
together with air up through the heated mass of fuel, forming a mixture
of producer and water gases in the generator above. The supply of air
is regulated by a centrifugal fan driven by the gas engine which the
plant is supplying. The necessity of having to use this fan very often
more than destroys the advantage gained by the absence of a boiler.
The gases produced pass by a pipe into the scrubbers after first
surmounting the pressure of a water valve, which prevents them from
returning to the generator. The scrubbers are filled with coke, with a
continual stream of water flowing down over it. The gases in passing
up are therefore thoroughly cleansed, so much so that they are fit to
pass straight to the gasometer. When the gasometer is full and has
reached its top position, it acts on a lever connected by a wire rope
with a tap regulating the air supply of the generator. The centrifugal
fan ceases to act, and the coke in the generator soon cools down,
and the production of gas ceases. As the gasometer falls again the
process is re-started, but not before the coke or anthracite in the
generator has been re-lit automatically. The whole plant, therefore,
only produces gas in proportion to the demand made on it, which is a
necessary condition when driving gas engines. The coke is automatically
re-lit by a small jet at the side of the generator, and fed by gas from
the gasometer. A plant producing gas sufficient for 60 horse-power, or
about 200 cubic metres per hour, uses up about 100 litres of water for
vaporization, and about 500 for cooling the scrubbers. The water used
for cooling the cylinder of the engine which the plant is supplying
may be used for vaporizing purposes, and requires the addition of a
small pump. Matter et Cie. of Rouen uses the Buire-Lencauchez gasogene
for supplying their Simplex engine, which we have already described.
Altogether about 3000 horse-power have been supplied by them on this
system, and have given repeated proof of the value of this process of
generating poor gas, especially in France, where poor French coal,
which can be used, is cheaper than imported English anthracite.
_Gardie gas-producing plant._—This apparatus is characterized by the
use of high-pressure air at about 80 lbs. per square inch, mixed with
steam at the same pressure; this arrangement being more concentrated
only requires small plant. The generator is of peculiar construction,
without any grate, the coke being held up on shelves. The air and steam
enter the generator by a ring of twyers, and a small window is pierced
at the side through which the attendant can see if the proper degree
of incandescence is maintained. The fuel is poured in through a hopper
in a similar manner to other generators already described. The gas
is produced at a high temperature, and is made to heat a coil through
which the steam passes, which is thus economically superheated. After
accomplishing this duty the gases pass on to a scrubber, formed by two
concentric tubes of different heights, and travel on straight to the
gasometer. The air is compressed in a reservoir by a special pump, and
is heated by the waste gases from the cylinder of the motor. These
various heating arrangements, therefore, prevent as much as possible
any loss of waste heat.
Some of the operations described are novel and interesting, and the
gas produced is very rich, having a heat of combustion of over 1400
calories, and very little ammonia is produced. The result of this is
that only rudimentary scrubbers are required, other systems requiring
elaborate methods of cleansing the gases. The only drawback to the
apparatus is the reservoir of compressed air, which necessitates a pump
using up power.
_Taylor gas-producing plant_ (Fig. 44).—The future of gas engines,
especially large units, is intimately connected with production of poor
gases at low cost, for they would be far from economical if coal gas
were used at the price at which it is usually sold. For this reason
many inventors have attempted to devise apparatus which should produce
gases suitable for being used in a gas engine, by the decomposition
of steam by red-hot carbon. The Taylor system is one of the best of
those which have appeared in the last few years. It has been thoroughly
tested in practice. The plant consists of a generator and boiler, a
series of cleansing and washing towers, and a gasometer. The most
important feature is the automatic manner of getting rid of the coke
ash from the generator by a moving hearth, which enables it to be
cleaned out without stopping the production of gas. By this system
cheap coal can be used instead of anthracite, which is more expensive.
The steam boiler is placed on the generator, and is heated by the gases
coming from it. The steam passes first of all through a superheater
consisting of a number of tubes round which circulate the gases from
the generator. The high temperature of the mixture of steam and air
ensures a good efficiency. The gases produced in the generator pass
through vertical tubes, exposing them to a large cooling surface, where
they are chilled; they then pass through scrubbers lined with coke and
so into the gasometer.
[Illustration: FIG. 44.—Taylor Gas-producing Plant.]
The gaseous mixture, consisting of hydrogen, carbon monoxide, etc.,
produced in the generator has a heat of combustion of from 1400 to
1500 calories. The relative heating power compared with coal gas is
therefore two-sevenths. A motor consuming 700 litres of coal gas would
require 2500 litres of the poor gas produced by this system to develop
the same power, and 550 grammes of anthracite would be consumed in the
process.
The cost per horse-power hour varies according to the price of
materials. Supposing we have an eight horse-power motor consuming 25
cubic feet of coal gas per horse-power hour, at 2_s._ 6_d._ per 1000
cubic feet, and working for 10 hours, the cost per day would be
8 × 25 × 30 × 10_d._
———————————————————— = 5_s._ or ¾_d._
1000
per horse-power hour.
If, on the other hand, it were supplied with poor gas generated from
anthracite costing 25_s._ a ton, it would burn about 1¼ lbs. per
horse-power hour, costing 16_d._, which is about one-fifth the cost,
and using cheap coal this cost can be still further reduced. These
figures clearly show how much cheaper it is to burn poor gases than
coal gas.
_Bénier gasogene._—Since the 1889 Exhibition, where a Simplex engine
was to be seen working with poor gas generated on the Dowson system,
engineers and others have fully recognized the advantages of this cheap
motive power. In the last few years a very large number of these plants
have been erected, and the experience gained by practice has shown,
that in spite of the extreme cheapness there are serious faults to be
found with this class of apparatus.
The gases are produced under pressure and are of an exceedingly
poisonous nature, carbon monoxide being the same gas which is
principally evolved from burning charcoal. Great care has therefore to
be taken that there are no leaks, which might have fatal results if
inhaled by the persons attending to the plant. The chemical operation
of the production of the gas is a very delicate one, and requires
skilled attendants if constancy in the quality of the gas is required.
The Bénier gasogene has been designed to eradicate or circumvent these
grave drawbacks. It presents many novel features, the most interesting
of which is the device for absolutely ensuring no leaks. This is done
by generating the gas below the pressure of the atmosphere. This low
pressure is maintained throughout the plant right up to the valve
which admits the explosive mixture to the motor cylinder. The only
places in which the pressure is raised are surrounded by vacuum jackets
leading to the gas reservoir. The gas-producing plant can therefore be
installed anywhere, even in dwelling-houses or cellars, without running
any risk of poisoning the inmates.
The irregularity of the qualities of the gas produced by other
generators is due to the difficulty of always admitting the air and
steam in the same proportions to the generator. This difficulty is
got over in the Bénier gasogene, indirectly as the result of the low
pressure in the interior: the air and steam enter under atmospheric and
therefore constant pressure, and their proportions can be regulated to
a fine degree of accuracy by varying the size of the orifices through
which they are admitted. So regular is the production of the gas in
quality that diagrams taken from an engine at intervals of several
hours showed no appreciable difference. The functions of attendant can
be thoroughly fulfilled by an unskilled labourer or boy, the operation
of emptying a quantity of coke once every half-hour being sufficient
to keep the plant working steadily. This apparatus is also an economic
one, partly because of the invariable qualities of the gas produced,
but chiefly owing to air and steam being heated before entering the
hearth. A rotating grate is also provided, so that the generation
of gas is in no way interfered with by the clearing out of ash and
clinkers. A special engine is constructed for use with this plant,
which we shall describe in the next chapter.
_Taylor gas-producing plant modified by Wimand._—There exist a number
of systems of gas-producers which are not so well known as those which
we have already described, but which are none the less interesting.
The most important of these is a modification by Wimand of the Taylor
system, in which the boiler is discarded, and replaced by a jacket or
vessel surrounding the generating plant and heated by the waste gases.
The hot water trickles down over a column of coke, and meets at the
base the current of air passing towards the generator; the air is
therefore heated, and becomes saturated with water vapour. This device
can be applied to any generator.
_Kitson gas-producer._—A rotating hearth is provided, as in other
systems already described. The air is driven by a steam injector
through holes in it, and the steam is supplied from a spiral coil
situated in the fire-brick lining of the generator. A second tube
through which the steam passes acts as a superheater. The steam enters
a reservoir chamber at the side of the generator, from which it passes
out again to fulfil its functions. There is therefore no fan needful,
and the generating furnace is not liable to become fouled. The price of
the plant is moderate.
_Loomis gasogene._—This apparatus aspirates its air and draws it
completely through a layer of carbon from top to bottom. The generator
is open at the top, and instead of the usual grate at the bottom it is
provided with a cone-shaped base instead. The air drawn in combines
with the carbon during its downward journey, and then passes through
a cooling tower surrounded by a water jacket which acts as a boiler.
The heat extracted from the gas is therefore used to produce steam,
which drives an engine working the pump which aspirates the air. The
exhaust steam from this engine passes into the incandescent fuel and
is decomposed, forming hydrogen and carbon monoxide. The valves which
govern the supplies of air and steam are so arranged that producer gas
or water gas, or both at once, may be generated at will. One particular
plant is known to have worked unceasingly for two years without
stopping, but in this case it did not supply a gas engine, in fact, we
believe that this system has not been applied for driving motors at all
as yet.
_Wilson gas-producer._—The generator is broader than it is high, and it
is possible to use in it all sorts of cheap coal and coke. A window is
provided, so that the attendant can see if the fuel is sticking and not
settling down properly. The air is blown into the centre of the hearth
by a steam injector at a low pressure of about two inches of water or
less. Stoking is effected by turning spiral grate bars which rid the
fire of clinkers and ashes. The air is heated by waste gases from the
motor, a process which gives a slight gain in economy.
_Longsden process._—We have mentioned that water gas generated by
various processes has been used for lighting towns in America on the
incandescent system. The drawback of the system is the exceedingly
poisonous nature of carbon monoxide even when very dilute, and it is
all the more dangerous because it is odourless. Many accidents have in
consequence occurred, some of them terminating fatally. Mr. Longsden
has attempted to produce a gas entirely free from carbon monoxide, so
as to avoid this difficulty. He first attempted to rid water gas of the
monoxide, but not being able to find a cheap enough solvent for it he
tried other means. His present process consists of adding a sodium salt
to the carbon in the generator. The gas produced has then the following
composition:—
Hydrogen 62·2 volumes
Carbonic acid gas 26·4 ”
Carbon monoxide 1·2 ”
Hydrocarbons 6·5 ”
Nitrogen 2·2 ”
Oxygen 1·5 ”
—————
100 ”
This very rich gas, which might easily be ridded of its carbonic acid
by passing it through lime, is very suitable for supplying motive power
through the medium of the gas engine.
_Gayon and Métais process._—This process, which, as far as we know, has
not been put into practice, was intended by its inventors to diminish
the price of coal gas. Instead of allowing coke to be deposited in the
gas retorts, as it usually is, and selling it as a bye-product, their
intention was to use it there and then for the production of water
gas, which was to be mixed with the coal gas and increase its heat of
combustion, while diminishing its cost of production. One ton of coal
would produce 450 cubic metres of gas instead of 300; the idea has,
however, never been put into practical shape.