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 4s. or 5s. 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—

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 (H₂O) and carbon (C), and on the right the products of the chemical action, carbon monoxide (CO), carbonic acid gas (CO₂), and hydrogen (H₂).

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—

and

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:—

(2) Lowe process:—

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 (CO₂) 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.”

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:—

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.

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 2s. 6d. per 1000 cubic feet, and working for 10 hours, the cost per day would be

per horse-power hour.

If, on the other hand, it were supplied with poor gas generated from anthracite costing 25s. a ton, it would burn about 1¼ lbs. per horse-power hour, costing 16d., 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:—

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.