Hydrogen 48.31
Carbon monoxide 35.93
Carbon dioxide 4.25
Nitrogen 8.75
Methane 1.05
Sulphureted hydrogen 1.20
Oxygen 0.51
------
100.00
The above is an analysis of water gas made from ordinary gas coke in a
Van Steenbergh generator.
The ratio of carbon monoxide and carbon dioxide present entirely
depends upon the temperature of the generator, and the kind of
carbonaceous matter employed. With a hard, dense anthracite coal, for
instance, it is quite possible to attain a temperature at which there
is practically no carbon dioxide produced, while with an ordinary form
of generator and a loose fuel like coke, a large proportion of carbon
dioxide is generally to be found.
The sulphureted hydrogen in the analysis quoted is, of course, due to
the high amount of sulphur to be found in the gas coke, and is
practically absent from water gas made with anthracite, while the
nitrogen is due to the method of manufacture, the coke being, in the
first instance, raised to incandescence by an air blast, which leaves
the generator and pipes full of a mixture of nitrogen and carbon
monoxide (producer gas), which is carried over by the first portions
of water gas into the holder. The water gas so made has no photometric
value, its constituents being perfectly non-luminous, and attempts to
use it as an illuminant have all taken the form of incandescent
burners, in which thin mantles or combs of highly refractory metallic
oxides have been heated to incandescence. In carbureted water gas this
gas is only used as the carrier of illuminating hydrocarbon gases,
made by decomposing various grades of hydrocarbon oils into permanent
gases by heat.
Many forms of generator have been used in the United States for the
production of water gas, which, after or during manufacture, is mixed
with the vapors and permanent gases obtained by cracking various
grades of paraffin oil, and "fixing" them by subjecting them to a high
temperature; and in considering the subject of enrichment of coal gas
by carbureted water gas, I shall be forced, by the limited time at my
disposal, to confine myself to the most successful of these processes,
or those which are already undergoing trial in this country.
In considering these methods, we find they can be divided into two
classes:
1. Continuous processes, in which the heat necessary to bring about
the interaction of the carbon and steam is obtained by performing the
operation in retorts externally heated in a furnace; and
2. Intermittent processes, in which carbon is first heated to
incandescence by an air blast, and then, the air blast being cut off,
superheated steam is blown in until the temperature is reduced to a
point at which the carbon begins to fail in its action, when the air
is again admitted to bring the fuel up to the required temperature,
the process consisting of alternate formation of producer gas with
rise of temperature, and of water gas with lowering of the
temperature.
Of the first class of generator, none, as far as I know, have as yet
been practically successful, the nearest approach to this system being
the "Meeze," in which fire clay retorts in an ordinary setting are
employed. In the center of each retort is a pipe leading nearly to the
rear end of the retort, and containing baffle plates. Through this a
jet of superheated steam and hydrocarbon vapor is injected, and the
mixture passes the length of the inner tube, and then back through the
retort itself--which is also fitted with baffle plates--to the front
of the retort, whence the fixed gases escape by the stand pipe to the
hydraulic main, and the rich gas thus formed is used either to enrich
coal gas or is mixed with water gas made in a separate generator. In
some forms the water gas is passed with the oil through the retort. In
such a process, the complete breaking down of some of the heavy
hydrocarbons takes place, and the superheated steam, acting on the
carbon so liberated, forms water gas which bears the lower
hydrocarbons formed with it; but inasmuch as oil is not an economical
source of carbon for the production of water gas, this would probably
make the cost of production higher than necessary. This system has
been extensively tried, and indeed used to a certain extent, but the
results have not been altogether satisfactory, one of the troubles
which have had to be contended with being choking of the retorts.
Of the intermittent processes, the one most in use in America is the
"Lowe," in which the coke or anthracite is heated to incandescence in
a generator lined with firebrick, by an air blast, the heated products
of combustion as they leave the generator and enter the superheaters
being supplied with more air, which causes the combustion of the
carbon monoxide present in the producer gas, and heats up the
firebrick "baffles" with which the superheater is filled. When the
necessary temperature of fuel and superheater has been reached, the
air blasts are cut off, and steam is blown through the generator,
forming water gas, which meets the enriching oil at the top of the
first superheater, called the 'carbureter,' and carries the vapors
with it through the main superheater, where the "fixing" of the
hydrocarbons takes place.
The chief advantage of this apparatus is that the enormous
superheating space enables a lower temperature to be used for the
"fixing." This does away, to a certain extent, with the too great
breaking down of the hydrocarbons, and consequent deposition of
carbon. This form of apparatus has just found its way to this country,
and I describe it as being the one most used in the States, and the
type upon which, practically, all water gas plant with superheaters
has been founded.
The Springer apparatus, which is under trial by one of the large gas
companies, differs from the Lowe merely in construction. In this
apparatus the superheater is directly above the generator; and there
is only one superheating chamber instead of two. The air blast is
admitted at the bottom, and the producer gases heat the superheater in
the usual way, and when the required temperature is reached, the steam
is blown in at the top of the generator, and is made to pass through
the incandescent fuel, the water gas being led from the bottom of the
apparatus to the top, where it enters at the summit of the
superheater, meets the oil, and passes down with it through the
chamber, the finished gas escaping at the middle of the apparatus.
This same idea of making the air blast pass up through the fuel, while
in the subsequent operation the steam passes down, is also to be found
in the Loomis plant, and is a distinct advantage, as the fuel is at
its hottest where the blast has entered, and, in order to keep down
the percentage of carbon dioxide, it is important that the fuel
through which the water gas last passes should be as hot as possible,
to insure its reduction to carbon monoxide.
The Flannery apparatus is again but a slight modification of the Lowe
plant, the chief difference being that, as the gas leaves the
generator, the oil is fed into it, and, with the gas, passes through a
D-shaped retort tube, which is arranged round three sides of the top
of the generator; and in this the oil is volatilized, and passes, with
the gas, to the bottom of the superheater, in which the vapors are
converted into permanent gases.
The Van Steenbergh plant, with which I have been experimenting for
some time, stands apart from all other forms of carbureted water gas
plant, in that the upper layer of the fuel itself forms the
superheater, and that no second part of any kind is needed for the
fixation of the hydrocarbons, an arrangement which reduces the
apparatus to the simplest form, and leaves no part which can choke or
get out of order, an advantage which will not be underrated by any one
who has had experience of these plants. While, however, this enormous
advantage is gained, there is also the drawback that the apparatus is
not fitted for use with crude oils of heavy specific gravity, such as
can be dealt with in the big external superheaters of the Lowe class
of water gas plant, but the lighter grades of oil must be used in it
for carbureting purposes.
I am not sure in my own mind that this, which appears at first a
disadvantage, is altogether one, as, in the first place, the lighter
grades of oil, if judged by the amount of carbureting power which they
have, are cheaper per candle power, added to the gas, than the crude
oils, while their use entirely does away with the formation of pitch
and carbon in the pipes and purifying apparatus--a factor of the
greatest importance to the gas manufacturer.
The fact that light oils give a higher carburation per gallon than
heavy crude oil is due to the fact that the latter have to be heated
to a higher temperature to convert them into permanent gas, and this
causes an over-cracking of the most valuable illuminating
constituents; and this trouble cannot be avoided, as, if a lower
temperature is employed, easily condensible vapors are the result,
which, by their condensation in the pipes, give rise to much trouble.
The simplicity of the apparatus is a factor which causes a great
saving of time and expense, as it reduces to a minimum the risk of
stoppages for repairs, while the initial cost of the apparatus is, of
course, low, and the expense of keeping in order practically _nil_.
When I first made the acquaintance of this form of plant, a few years
ago, the promoters were confident that nothing could be used in it
but American anthracite, of the kind they had been in the habit of
using in America, and a light naphtha of about 0.689 specific gravity,
known commercially as 76 deg Baume.
A few weeks' work with the apparatus, however, quickly showed that,
with a slightly increased blow, and a rather higher column of fuel,
gas coke could be used just as well as anthracite, and that by
increasing the column of fuel, a lower grade of oil could be employed;
so that during a considerable portion of the experimental work nothing
but gas coke from the Horseferry Road Works and a petroleum of a
specific gravity of about 0.709 were employed.
Having had control of the apparatus for several months, and, with the
aid of a reliable assistant, having checked everything that went in
and came out of the generator, I am in a position to state
authoritatively that, using ordinary gas coke and a petroleum of
specific gravity ranging from 0.689 to 0.709, 1,000 cubic feet of gas,
having an illuminating power of twenty-two candles, can be made with
an expenditure of 28 to 32 lb. of coke and 21/2 gallons of petroleum.
The most important factors, i.e., the quantity of petroleum and the
illuminating value of the gas, have also been checked and corroborated
by Mr. Heisch and Mr. Leicester Greville.
Total gas made = 8,700 cubic feet.
Time taken: Blowing. 1 hour.
Time taken: Making. 50 minutes.
Fuel used: Gas coke. 270 lb. = 31 lb. per 1,000 c.f.
Fuel used: Naphtha, sp. gr. 0.709. 34 gals. = 2.7 gals. per 1,000 c.f.
Illuminating power of gas = 21.9 candles.
I must admit that these results far exceeded my expectations, although
they only confirmed the figures claimed by the patentee; and there are
not wanting indications that, when worked on a large scale and
continuously, they might be even still further lowered, as it is
impossible to obtain the most economical results when making less than
10,000 cubic feet of the gas, as the proper temperature of the walls
of the generator are not obtained until after several makes; and it is
only after about 8,000 cubic feet of gas has been made that the best
conditions are fulfilled.
It will enable a sounder judgment to be formed of the working of the
process if the complete experimental figures for a make of gas be
taken.