CHAPTER XII

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.

[Ill.u.s.tration: FIG. 77.--A complete Dowson producer-gas plant.]

[Ill.u.s.tration: 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 pa.s.sing 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 quant.i.ty 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 sc.r.a.pe 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 princ.i.p.al 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).

[Ill.u.s.tration: 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 quant.i.ty of air which is injected is likely to be diminished in quant.i.ty while the quant.i.ty 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.

[Ill.u.s.tration: 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 quant.i.ty 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.

[Ill.u.s.tration: 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 cla.s.s 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.

[Ill.u.s.tration: 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 quant.i.ty 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.

[Ill.u.s.tration: 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 s.p.a.ces 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 ma.s.s 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.

[Ill.u.s.tration: FIG. 84.--Moss or fiber purifier.]

Physical purification thus begun is completed by pa.s.sing 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.

[Ill.u.s.tration: 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 quant.i.ty 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 circ.u.mstances the bell acts as a pressure-regulator, a.s.sures a sufficient h.o.m.ogeneity 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.

[Ill.u.s.tration: 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 pa.s.sage 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 sc.r.a.ped during operation, in order to avoid the production of a flame which may escape from the apparatus.

The scrubber is simply a column without c.o.ke, and is provided with an interior sprinkler. The c.o.ke 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.

Pa.s.sing 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 pa.s.ses 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.