PRESSURE GAS-PRODUCERS
As we have already seen, producer-gas as a fuel for engines may be
generated in two kinds of apparatus, the one operating under pressure,
and the other by suction.
=Dowson Gas-Producers.=--The first pressure-generators were introduced
by Dowson of London and necessitated installations of quite a
complicated nature. Later improvements made by the designers contributed
much to the general employment of their system. Many installations
varying from 50 to 100 horsepower and more may be found in the United
Kingdom, all of them made by Dowson. Indeed, for a long time the name of
Dowson was coupled with producer-gas itself. The Dowson system
necessitates the utilization of anthracite or of comparatively hard
coal, such as that mined in Wales and Pennsylvania. Owing to the
necessity of employing this special quality of coal the Dowson system
and the systems that sprang from it were burdened with cooling, washing,
and purifying apparatus, which complicated the installations to such an
extent that they resembled gas works. The generator that took the place
of the retort was fed with air and steam, blown in under pressure,
necessitating the employment of a boiler. Furthermore, the production
of the gas under pressure necessitated the use of a gasometer for its
collection before it was supplied to the engine-cylinder. Such
Installations were evidently costly, and were, moreover, difficult to
maintain in proper working order. Nevertheless, there are many cases in
which they must be industrially employed.
[Illustration: FIG. 77.--A complete Dowson producer-gas plant.]
[Illustration: FIG. 78.--A Simplex producer-gas plant.]
Among these may be cited works in which producer-gas is employed as a
furnace fuel or as a soldering or roasting medium. Still other cases are
those in which the producer-gas must be piped to some distance from a
central generating installation to various engines, in the manner
rendered familiar in gas-lighting practice.
Most pressure gas-generators have been copied from the original type
invented by Dowson. These include a generator in which the gas is
produced; an injector fed by a boiler; a fan or a compressor by means of
which a mixture of steam and air is blown under the generator-furnace;
washing apparatus termed "scrubbers"; gas-purifying apparatus; and a
gas-holder (Fig. 77).
=Generators.=--The generator consists of a retort made of refractory
clay, vertically mounted, and cylindrical or conical in form. This
retort is protected on its exterior by a metal jacket with an
intermediate layer of sand which serves to reduce the heat lost by
radiation. The fuel is charged through the top of the retort, which is
provided with a double closure in order to prevent the entrance of air
during the charging operation. The generator rests on a grid arranged at
the base of the retort, upon which grid the ashes fall. The outlet of
the injector-pipe opens into the ash-pit, and this injector constantly
supplies a mixture of steam and air. The mixture is generally
superheated by passing it through a coil arranged in the fire-box of the
boiler, in the generator, or in the outlet for burnt gases. Sometimes
the air is subjected to a preliminary heating by recuperating in some
way the waste heat of the apparatus.
The chief features in the arrangement of generators which have received
the attention of manufacturers are the following: Good distribution of
the fuel in charging; easy descent of the fuel; reduction of the
destructive action of the clinkers on the walls; means for cleaning the
grate without interfering with the generation of gas; prevention of
leakage. Many devices have been employed to fulfil these requisites.
A perfect distribution of the fuel during charging is attained chiefly
by the form of the hopper, and of its gate, which is generally conical.
In most apparatus the gate opens toward the interior of the generator,
and the inclination of its walls causes a uniform scattering of the fuel
in the retort. It is all the more necessary to disperse the fuel in this
manner when the cross-section of the retort is small compared with its
height.
_The facility of the fuel's_ descent is dependent largely upon the
nature and the size of the coal employed. Porous coal gives better
results than dense and compact coal. It is therefore preferable to
employ screened coal free from dust in pieces each the size of a
hazel-nut. The various sections given to the interior, including as they
do cylindrical forms, truncated at the summit or the base, partially
truncated toward the base and the like, would lead to the conclusion
that this question is not of the importance which some writers would
have us believe. Still, it must be considered that if the fuel drops
slowly, its prolonged detention within the walls of the hopper and its
transformation into fusible slag may result in a disintegration of the
refractory lining of the furnace.
The quantity of steam injected, greater or less, according to the nature
of the fuel, renders it possible to obtain friable slags and
consequently to prevent grave injury to the retort. Red-ash coal is in
general fusible, containing as it does some iron. Its temperature of
fusion varies between 1,832 to 2,732 degrees F.
_Cleanliness_ is most important so far as the operation of the generator
is concerned. It should be possible to scrape the generator during
operation without changing the composition of the gas, when the
incandescent zone is chilled, or an excess of air is introduced, or the
steam-injector be momentarily thrown out of operation. Mechanical
cleaners with movable grates or revolving beds have the merit of causing
the ashes to drop without interfering with the operation of the
apparatus. The same meritorious feature is characteristic of ash-pits
having water-sealed joints.
Pressure gas-generators need not be as perfectly gas-tight as suction
apparatus. Leakage of gas, which is usually manifested by a
characteristic odor, results in a loss of consumption and renders the
air unfit to breathe.
A generator should be provided in its upper part with openings through
which a poker can easily be introduced in order to shake up the fuel and
to dislodge the clinkers which tend to form and which cause the
principal defects in operation, particularly with fuels that tend to
swell, cake, and adhere to the furnace walls when heated. Many
apparatus, moreover, are provided with lateral openings having mica
panes through which the progress of combustion can be observed (Fig.
79).
[Illustration: FIG. 79.--Fichet-Heurtey producer with rotating
bed-plate.]
=Air-Blast.=--The system by which air and steam are injected
necessitates the employment of a steam-boiler of 75 pounds pressure.
This method of blowing, which is rather complicated, has the
disadvantage of varying in feed with the pressure of the steam in the
boiler, which pressure is not easily maintained at a given number of
pounds per square inch. Moreover, when more or less resistance is
offered by the fuel in the generator the quantity of air which is
injected is likely to be diminished in quantity while the quantity of
steam remains the same. The result is a change in speed which follows
from the modification of the proportions of the two elements. For these
reasons some manufacturers have resorted of late years to the employment
of fans and blowers.
[Illustration: FIG. 80.--Koerting blower.]
=Blowers.=--The fans or blowers employed vary considerably in
arrangement. Most of them are based on the Koerting system (Fig. 80),
and comprise essentially (1) a tube through which the steam is supplied
under pressure, and (2) a cylindro-conical blast-pipe. The tube is
placed in the axis of the blast-pipe at its outer opening. As it escapes
under pressure the steam is caught in the blast-pipe and draws with it a
certain quantity of air, which can be regulated. It is important that
these injection blowers should operate in such a manner that the
pressure and the feed of air and steam can be controlled.
=Fans.=--Mechanical blowers have the advantage of dispensing with the
employment of steam under pressure and the consequent installation of a
boiler (Fig. 78). Driven by the engine itself or from some separate
source of power, these apparatus are easily placed in position, require
no great amount of attention, and utilize but little energy. They are
either of the centrifugal type or of the rotary type, exemplified in the
Root blower (Fig. 81). The latter system has the advantage of high
efficiency, and of enabling comparatively high pressures--19 to 27
inches of water--to be attained, which, however, are used only for
special fuels, such as lignite, peat, and the like. The air supplied by
the blower, before reaching the fire-box, is superheated, either before
or after it is charged with steam.
[Illustration: FIG. 81.--Root blower.]
=Compressors.=--In some installations air is supplied by compressor
under the high pressure of 70 to 90 pounds per square inch, and seem
well adapted to the production of a gas of good quality. Moreover,
neither tar nor ammoniacal waters are produced. The Gardie producer may
be considered typical of this class of apparatus (Fig. 82). The chief
feature of this producer is to be found in simple washing and purifying
apparatus. It may be well to state here that the compression of air at
high pressure occasions some complications, and a considerable
expenditure of power.
[Illustration: FIG. 82.--Gardie producer.]
=Exhausters.=--Some designers have invented devices which draw gas into
the generator whence it is supplied to the engines, these suction
apparatus being connected with the blowers or used separately. But with
the exception of a few special instances, such arrangements are not
widely used--at least not for the production of motive power alone.
Whatever may be the arrangement employed for the introduction of a
mixture of air and steam under the grate of the generator, the
blast-pipe as a general rule discharges toward the center of the
apparatus. Still, in large producers it becomes desirable to provide a
means for varying the quantity of air and steam within wide limits so as
to regulate the heat of the fire. For that reason several outlets are
symmetrically arranged below the fuel.
[Illustration: FIG. 83.--Sawdust purifier.]
=Washing and Purifying.=--In pressure producers the gas is generally
washed and purified with much more care than in suction apparatus. Given
a sufficient pressure, the gas can be driven through the different
apparatus and the spaces between the material which they contain without
any difficulty. The gases emerge from the generator highly heated, and
this heat is used either to warm the injection water or to generate the
steam fed to the furnace. The gases then enter the washing apparatus,
which most frequently consists of a succession of contrivances in which
the gas is washed either by causing it to bubble up through the water,
or by subjecting it to superficial friction against a sheet of water, or
by systematically circulating it in a mass of continuously besprinkled
inert material. The object of washing is to remove the dust contained in
the gas and to precipitate it in the form of a slime which can be
removed by flushing.
[Illustration: FIG. 84.--Moss or fiber purifier.]
Physical purification thus begun is completed by passing the gas through
a filtering bed consisting of fiber, sawdust, or moss (Figs. 83 and 84).
Chemical purification if it is necessary, is effected by means of
calcium hydrate, iron oxide, or, still better, by a mixture of lime and
iron sulphate. This filtering material must necessarily be renewed after
it is exhausted.
[Illustration: FIG. 85.--Combined gas-holder and washer.]
=Gas-Holder.=--The gas-holder is composed essentially of a tank and a
bell. Sometimes, for the purpose of simplifying the apparatus, the tank
is so arranged as to take the place of a washer or scrubber (Fig. 85).
The bell should be provided with mechanism which, when the bell is full,
automatically diminishes or stops the generation of gas. It is advisable
to provide the bell with a blow or flap valve opening toward the
interior. If, therefore, it should happen that the gas supply is cut off
while the engine still continues to run, the suction of the engine will
not draw the water from the tank of the gas-holder.
When engines are employed the horse-power of which does not exceed 50,
it is sometimes customary to use the water of the tank (placed at a
higher elevation than the engine) to cool the cylinder. In this manner
the cost of installing special reservoirs is saved. If such an
arrangement be employed, however, the quantity of water contained in the
tank should be at least double that ordinarily contained in reservoirs.
If this precaution be not observed, the water may become excessively
heated and expand the gas in the bell.
The volume of the bell of the gas-holder should preferably be
not less than about 3 cubic feet per effective horse-power of the
engine to be supplied. Under these circumstances the bell acts as a
pressure-regulator, assures a sufficient homogeneity of the remaining
gas, and renders it possible to supply the engine during the short
intervals in which it is necessary to stop the blast to poke the fire.
But if the engine consumes 60 to 80 cubic feet of producer-gas per
horse-power per hour, the bell must be very much larger in size if the
generation of gas is to be checked for some time.
It may be well to recall here that coal is not the only fuel which lends
itself to the generation of gas suitable for driving engines, but that
some generators are able to utilize lignite, peat, and the like. In
others, straw, wood, shavings and sawdust, tannery waste, and other
organic matter is burnt with an efficiency very much higher than that
which they would give in the fireboxes of steam-boilers.
[Illustration: FIG. 86.--Otto Deutz lignite-producer.]
=Lignite and Peat Producers.=--Lignite and peat generators (Fig. 86)
cannot operate on the suction principle because of the resistance
offered to the passage of gas by the layer of fuel. This resistance is
considerable and extremely variable. Consequently, lignite and peat
generators must operate on the pressure principle by utilizing a blast
of air or a steam injector, depending upon the amount of water contained
in the lignite. As a general rule a Root blower operating at a pressure
of 8 to 27 inches of water, depending upon the quality of the lignite,
is employed. These generators are not to be recommended for powers less
than 50 horse-power, for the cost of the apparatus becomes too great.
The best lignite is that which, after combustion, leaves a fine ash and
no agglomerated clinker. Lignite has the peculiarity of forming dust
which ignites very easily when air is admitted into the generator. For
this reason the generator should not be scraped during operation, in
order to avoid the production of a flame which may escape from the
apparatus.
The scrubber is simply a column without coke, and is provided with an
interior sprinkler. The coke is too rapidly clogged with tar. Much of
this tar is deposited in a chamber which precedes the gas-holder.
Several quarts of tar may be tapped from the chamber daily.
The gas-holder serves merely to regulate the production of gas. The
pipes leading to the engine should be cleaned several times each month,
in order to remove the thin layer of tar which is deposited within them.
There are many kinds of lignite, and the gas-generator should be
constructed to meet the peculiar requirements of the variety employed.
The layer of fuel should be such in thickness that the gas as it emerges
from the generator has a temperature of about 77 degrees F. This is the
temperature of the gas which leaves the scrubber in the case of
anthracite-generators. If the lignite contains much water, the greater
part is retained in the washer by the gas in the form of drops.
Sometimes the water drips through the grate of the generator.
Lignite-generators may also be operated with peat, and even with town
refuse, with slight modifications. The consumption per horse-power per
hour is 3.3 pounds of lignite containing 2,400 calories (9,424.9
B.T.U.). In order to generate the same power with a boiler and
steam-engine, 8.8 pounds would be required. An engine driven unloaded
with fuel furnished by a lignite-generator will consume 50 per cent. of
the weight of the fuel required at full load. This depends upon the
proportion of water contained in the lignite and on losses of heat by
radiation from the generator. In street-gas engines running without
load, the absorption is 20 per cent., in anthracite-generators 40 per
cent. of the consumption at full load.
Passing now to the utilization of wood, of which something has already
been said in Chapter XI, two entirely distinct processes are
successfully employed in apparatus of the Riche type, these processes
depending upon the form of the wood used--whether, in other words, the
wood be consumed in the form of sticks or blocks or in the form of
chips, sawdust, bark, and the like, all of them the wastes of factories
in which wood is used.
=Distilling-Producers.=--If the wood consists of logs, it is burnt in a
generator comprising a fire-box and a distilling retort. The fire-box is
charged with ordinary coal which serves to heat the retort to redness.
The wood is discharged through the top of the retort, and the gas,
produced by the distillation, escapes through the bottom and passes to
the washing apparatus. The base of the retort is heated to about 1,652
degrees F., while at the top this temperature is reduced to 752 degrees
F. The wood thus treated is transformed into charcoal, which is a
by-product of some value.
[Illustration: FIG. 87.--Riche distilling-producer.]
The lower part of this cast retort (Fig. 87) is lined with charcoal, the
residue of previous distillations. The wood which is introduced in the
upper part of the retort is distilled in the chamber. The retort is held
by its own weight in a socket on the foot, which socket is lined with a
special refractory cement, made of silicate, asbestos forming the joint.
The products of combustion, issuing from the furnace, pass by way of
the flue to the lower part of the casing, and raise the temperature of
the retort and the charcoal it contains to that of a cherry red (1,652
degrees F.). These products of combustion then float to the upper part
of the casing and heat the top of the retort to a temperature of about
752 degrees F., in which part the wood or the wooden waste to be
distilled is enclosed. Thence the products of combustion pass through a
horizontal flue, provided with a damper, into a collecting flue by which
they are led to the smoke-stack. The products of distillation formed in
the chamber, having no outlet at the top of the retort, must traverse
the zone filled with incandescent carbon. The condensible products are
conducted as permanent gases (carbonic-acid gas in the state of carbon
monoxide) and are collected in the receptacle, after having passed the
funnel and the bell of the purifying apparatus.
A gas-furnace is formed by grouping in a single mass of masonry a
certain number of elements of the kind just described. It is essential
that the retorts should be vertically placed, that they be made only of
cast metal and not of refractory clay, and, finally, that their diameter
be not much more than 10 inches, which size has been found most
expedient in practice. The gas collected in the bell or in one or more
of the receptacles passes into the gasometer and then into the service
pipes. If 2.2 pounds of wood be distilled by burning in the furnace 8/9
of a pound of coal of average quality or 2.2 pounds of wood (either
sawdust or waste), 24.5 to 28 cubic feet of gas will be generated
having a thermal value of 3,000 to 3,300 calories per cubic meter
(11,904 to 13,094 B.T.U. per 35.31 cubic feet), and a residue 44 pounds
of charcoal will be left.
In practice only the wood of commerce containing in the green state 20
to 40 per cent. of water, depending upon the variety, is used. Hornbeam
contains the least water (18 per cent.), while elmwood and spruce
contain the most (44 to 45 per cent.).
The blast apparatus of the generator being started, the gas is supplied
under pressure. By reason of its permanent composition and its richness,
it is an excellent substitute for street-gas in incandescent lighting, a
good furnace fuel reducing agent.
_Producers Using Wood Waste, Sawdust, and the Like._--If waste wood in
the form of shavings, sawdust, straw, bark, and the like, should be
employed, a still higher efficiency is obtained with self-reducing
generators of the Riche type.
_Combustion-Generators._--In combustion-generators (Fig. 88) the fuel is
burnt and not distilled. The generator comprises two distinct elements.
The first is the generator proper, in which the combustion takes place.
Upon it is placed a hopper or fuel supply box. The Second element is the
reducer, in which by an independent process the reduction of the
carbonic-acid gas, the dissociation of the steam, and the transformation
of the hydrocarbons takes place. The generator is provided at its base
with a grate having oblique bars in tiers, which grate is furnished with
a channel in which the water for the generation of hydrogen flows. On a
level with this grate, at the opposite side, is a flue communicating
with the reduction column of coke. The incandescent zone of the
generator should not extend above the level of the grate. Instead of
passing through the layers of fresh fuel and out by way of the top, the
gas generated flows directly into the reduction column where it heats
the coke to incandescence. The high temperature to which the coke is
subjected, coupled with the injection of air, effects useful reactions.
This additional air, however, is not used if the fuel is free from all
products of distillation.
[Illustration: FIG. 88.--Riche combustion-producer.]
Experience has shown that gas of 1,000 to 1100 calories per cubic meter
(3,968 to 4,365 B.T.U. per 35.31 cubic feet), which heat content is
necessary to develop one horse-power per hour, can be obtained with 3.96
pounds of wood in the form of shavings and sawdust containing 30 per
cent. of water. The corresponding quantity of coke consumed in the
reduction column is insignificant, and may be placed at about 0.112
pounds per horse-power per hour.
It has been proven in actual practice that, both in the distilling and
combustion types of apparatus, the wood, either in the green state or in
the form of saw-mill waste, may contain as much as 60 per cent. of
water. Either of the two systems can be operated under pressure with an
air-blast, in which case a gas-holder and bell must be employed. The gas
as it passes from the generator to the gas-holder is conducted through a
cooler and washer and through a moss filter, which removes traces of the
products that may have escaped the distillation.
=Inverted Combustion.=--With a few exceptions the pressure-generators
which have been described, as well as suction gas-producers which will
be later discussed, are fed with anthracite coal or with coke. They
cannot be operated with moderately soft or bituminous coal. For this
reason they limit the employment of producer-gas engines. Manufacturers
have long sought generators in which any fuel whatever can be consumed.
Among the producers which seem to overcome the objections cited to a
certain degree, are those which are based on the principle of inverted
combustion. These apparatus embody the ideas of Ebelmen, the products of
distillation being decomposed by passing them over layers of
incandescent fuel.
[Illustration: FIG. 89.--Deschamps inverted-combustion producer.]
Many writers place in the class of inverted combustion producers,
apparatus of the Riche, Thwaite, and Duff type, in which this idea is
also carried out. Riche employs an independent incandescent mass to
reduce the products of distillation of another mass. Thwaite employs two
vessels which serve alternately as distilling retorts and reducing
columns. Duff draws in the products of distillation for the purpose of
blowing them under the fire. All these generators can hardly be said to
be of the inverted combustion type.
[Illustration: FIG. 90.--Fange-Chavanon inverted-combustion producer.]
The generators of Deschamps (Fig. 89) and of Fange and Chavanon (Fig.
90), on the other hand, are producers in which the combustion is really
inverted, and which are worked continuously. The air enters at the upper
part of the retort, passes through the entire mass of fuel, carrying
with it the distilled volatile products, and when the mixture reaches
the incandescent zone, chemical reactions occur that result in the
production of a gas entirely free from tar and other impurities.