Gas-Engines and Producer-Gas Plants Part 12

[Ill.u.s.tration: 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, pa.s.s 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 pa.s.s 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 pa.s.sed the funnel and the bell of the purifying apparatus.

A gas-furnace is formed by grouping in a single ma.s.s 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 pa.s.ses 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 subst.i.tute 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 c.o.ke. The incandescent zone of the generator should not extend above the level of the grate. Instead of pa.s.sing 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 c.o.ke to incandescence. The high temperature to which the c.o.ke 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.

[Ill.u.s.tration: 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 quant.i.ty of c.o.ke 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 pa.s.ses 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 c.o.ke. 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 pa.s.sing them over layers of incandescent fuel.

[Ill.u.s.tration: FIG. 89.--Deschamps inverted-combustion producer.]

Many writers place in the cla.s.s 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 ma.s.s to reduce the products of distillation of another ma.s.s. 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.

[Ill.u.s.tration: 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, pa.s.ses through the entire ma.s.s 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.

CHAPTER XIII

SUCTION GAS-PRODUCERS

The high cost and the complicated nature of the pressure gas-generators which have just been discussed have led manufacturers to attempt in some other way the generation of producer-gas intended for operating motors.

Several inventors, among whom we will mention Benier and A. Taylor (in France), made some praiseworthy although not immediately very successful attempts to simplify the manufacture of producer-gas.

=Advantages.=--In these systems the suction occasioned by the motor itself has taken the place of a forced draft, produced in the generator by an air-injector or a fan, so that the gas, instead of being stored under pressure in a gas-holder, is kept in the apparatus under a pressure below that of the atmosphere.

As the device for producing a draft by means of boiler pressure or of a fan, and the gas-holder, are dispensed with, the result is a saving, first in the cost of installation, consumption, and floor s.p.a.ce.

Furthermore, the cooler and washer are supplanted by a single scrubber.

Manufacturers have succeeded in devising apparatus remarkable for the simplicity of the processes employed and yielding economical results which would never be obtained with pressure-generators employing gas-holders and boilers, considering that the boiler alone calls for a consumption of from 15 to 30 per cent. of the total amount of coal used for making the gas.

The best results obtained by the author with pressure gas-producers have indicated a consumption of not much less than 1 to 1-1/4 pounds of anthracite per horse-power per hour at the motor, while with suction-generators, under similar conditions and with the same grade of fuel, he has repeatedly found a consumption of from 9/10 pounds per effective horse-power per hour. In either case, the gas obtained developed between 1,100 and 1,300 calories (4,365 and 5,158 B.T.U. per 35.31 cubic feet) if produced from anthracite yielding from 7,500 to 8,000 calories (29,763 to 31,746 B.T.U.) per 2.2 pounds.

The suction apparatus will also work very well with inferior coal containing up to 6 to 8 per cent. of volatile matter and from 8 to 10 per cent. of ash. This great advantage added to all the others explains the favorable reception which European manufacturers at once gave to suction-producers. The petroleum engine itself will find a serious compet.i.tor in the new system.

As regards the possibility of employing suction gas generators with respect to the somewhat peculiar properties of the fuel, it may be said at the outset that c.o.ke from gas works yielding from 6,000 to 6,500 calories (22,911 to 24,995 B.T.U.) and also charcoal are perfectly available.

One horse-power per hour is obtained with a consumption of 1.1 to 1.3 pounds of c.o.ke.

Blast-furnace c.o.ke may be used in case of need, but its employment is not to be recommended on account of the sulphides it contains, which sulphides, being carried along by the gas, are liable to form sulphuric acid with the steam, the corrosive action of which would soon destroy the cylinder and other important parts of the engine.

=Qualities of Fuel.=--Anthracite coal is, upon the whole, so far the best available fuel for generators. However, it should possess certain qualities which will now be briefly indicated.

In suction gas-generators, above all, it is important that no harmful resistance should be opposed to the pa.s.sage of the air and of the gas produced. It is therefore necessary to employ coal of a size that will answer the foregoing condition, without being too expensive.

The size of the pieces, to a certain extent, determines the price; and with coal of the same properties, pieces 1.1 to 2 inches may cost 1.4 of the price for the ordinary size of 0.59 to 0.98 inches, which is very well adapted for gas-generators. This is the size of a hazel-nut.

Moreover, it will be advisable to select the dryest coals, containing a minimum of volatile matter and having no tendency to c.o.ke or to cohere, in order that the volatilized products may not by distillation obstruct the interstices through which the gases must pa.s.s. For the same reason coal which breaks up and becomes pulverized under the action of the fire is not to be recommended. The coal should also be such as to avoid the formation of arches which would interfere with the proper settling of the fuel during its combustion. It may be stated as a rule that, with coal that does not cohere, the content of volatile matter should not exceed 5 to 8 per cent.

Coal which contains more than 10 to 15 per cent. of ash should not be used, for the reason that it chokes up and obstructs generators in which the dropping and discharge of the ashes is done automatically, a fact which should not pa.s.s unnoticed. The furnace cannot be cleaned safely with a fire of this kind, where combustion takes place in an enclosed s.p.a.ce, without hindering the production of gas. Here again a point may be raised very much in favor of suction gas-producers. In a good generator, the ash-pit can be cleaned and the fire stoked without interrupting the liberation of the gas drawn in and without appreciably impairing the quality of the gas. These considerations are of importance so far as the gas-generator itself is concerned. Other conditions which should be noticed affect the engine fed by the generator, the grade of coal used, and the purification of the gas obtained from it.

Unless special chemical cleaners and purifiers are employed, thereby complicating the plant, the coal utilized should yield as little tar as possible during distillation; for the tendency of the tar to choke up the pipes and to clog the valves is one of the chief causes of defective operation of producer-gas engines.

Tar changes the proper composition of the explosive mixture. When it catches fire in the cylinder it causes premature ignition, which is so dangerous in large engines.

From what has been said in the foregoing, it follows that, in the present state of the art, the satisfactory operation of gas-generators depends no longer on the use of pure anthracite, such as Pennsylvania coal in America and Welsh coal in England, containing an amount of carbon as high as 90 to 94 per cent. and having a thermal value of 33,529 B.T.U. On the contrary, good dry coal yielding from 29,763 to 31,746 B.T.U. is quite suitable for the generation of producer-gas.

A final, practical advantage which speaks in favor of a generator and motor plant as compared with a steam-engine, is the small amount of water required. Apart from the water used for cooling the engine, which may be used over and over again if cooled, any water, whether it forms scale or deposits, may be employed for cooling and was.h.i.+ng the gas in the scrubber.

According to the author's personal experience, an average of 3.3 gallons of water per effective horse-power per hour is sufficient for this purpose. This is about one-half of the amount required by a non-condensing slide-valve engine of from 15 to 30 horse-power. The difference in the consumption of water is quite important in city plants, where water is rather expensive as a rule.

=General Arrangement.=--A suction gas-generator plant of the character we have been discussing is shown in Fig. 91.

[Ill.u.s.tration: FIG. 91.--Engine and suction gas-producer.]

The apparatus _A_ is the generator proper, in which combustion takes place. The gas produced pa.s.ses into the apparatus _B_ through a series of tubes, to be conveyed to the washer _C_. In the apparatus _B_, which is the vaporizer, the water admitted at the top under atmospheric pressure is vaporized by contact with a series of tubes, heated by the gas coming from the generator. The steam, together with air, is drawn into the lower part of the generator to support combustion. This vaporizer is provided with an overflow for the outlet of the water which has not been vaporized. The producer-gas pipe which leads from the vaporizer to the washer has a branch _D_, for the temporary escape to the atmosphere of the gas produced before and after the operation of the engine. In the washer, as the drawing shows, the gas enters at the bottom and leaves at the top to pa.s.s to the gas expansion-chamber _E_ and thence to the motor. The gas thus pa.s.ses through the body of c.o.ke in the opposite direction to the wash water, which then flows to the waste-pipe. The c.o.ke and the water free the gas not only from the dust carried along, but from the ammonia and other impurities contained in the gas.

When firing the generator, a small hand ventilator _G_ is used for blowing in air to fan the fire. The gas obtained at first, being unsuitable for combustion, is allowed to escape through the branch _D_.

After injecting air for about 10 to 15 minutes, the engine can be started after closing the branch _D_. The suction of the engine itself will then gradually bring about the proper conditions for its regular running, and after a quarter of an hour or half an hour the gas is rich enough to run the engine under a full load.

The apparatus just described is the original type, upon which many improvements have been made for the purpose of securing a uniform gas production and of diminis.h.i.+ng the interval of time elapsing between the firing of the generator and the running of the engine under a full load.

Each of the elements of this apparatus--to wit, the generator, vaporizer, super-heater, and washer--have been modified and improved more or less successfully by the manufacturers; and in order that the reader may perceive the merits and the drawbacks of the various arrangements adopted, the most important ones will be separately discussed.

=Generator.=--With respect to the general arrangement of parts, generators may be divided into two cla.s.ses: