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.”
Fig. 43.—Dowson Gas-producing Plant.
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:—
| Carbon monoxide | 29·4 |
| Carbon dioxide | 5·9 |
| Hydrogen | 17·6 |
| Nitrogen | 47·1 |
| —— | |
| 100 |
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.