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“Moisture when present to the extent of 2 per
cent. (the proportion present in coal-gas saturated at 20° C. and 760 mm.) in ethylene, reduces the illuminating power 3.6 per cent., or in coal-gas, 3:3 per cent. Of the inert, or noncombustible diluents therefore CO, is the most injurious and atmospheric air is the least harmful. Wurtz has determined the loss of light incidental to the addition of air to coal-gas, and he gives the following results:
“The addition of oxygen to gas, rich in hydrocarbon causes an increase in the illuminating power up to a certain point. The temperature of the flame is increased by burn. ing up the hydrogen of the hydrocarbons and rendering the carbon incandescent without diluting the flame with nitrogen to the extent which would have been necessary had air been used for the purpose. This effect is very noticeable in rich hydrocarbon gases of more than 50-candle power, but in a poor coal-gas of (say) 16 candles it is very difficult to get any increase in illuminating power, even a small percentage of oxygen overdoing the combustion.
“The effect of such gases as hydrogen and carbon mon. oxide is simply to dilute the flame, and by separating the molecules of the hydrocarbons to make them more difficult to decompose, while such bodies as CO2, nitrogen, air, and water vapour, not only dilute, but also cool the flame, as they do not add to the heat by any action of their own, and have to be heated up to the same temperature as the flame itself.”
THE ENRICHMENT OF COAL-GAS.
N places where the illuminating power of the gas has
nary common coal available in that particular district, it is necessary that the gas should be enriched by some means so as to enable it to reach the recognized standard, and up to a few years ago this enrichment was effected by mixing a certain proportion of cannel coal with the ordinary black or common coal; but, owing to various causes, cannel coal has lately so increased in price as to render its use almost prohibitory as an enriching agent, consequently gas companies have been compelled to find substitutes for cannel for the purpose of such enrichment, and these substitutes may be classed as carburetted water-gas,” “ oil-gas,” and the impregnation of the gas with the vapour of volatile hydrocarbons.
When steam acts upon carbon at a high temperature, the resulting action may be looked upon as yielding a mixture of equal volumes of hydrogen and carbon monoxide, both of which are inflammable but non-luminous gases. Owing to various reasons, however, this reaction does not occur in working on the “practical” scale, carbon dioxide to a certain extent being invariably present in the
"water-gas," as it is termed, which on an average has the following composition:
35:93 Carbon dioxide
1:05 Sulphuretted hydrogen.
The ratio of CO and Co, present depends entirely upon the temperature of the generator and the kind of carbonaceous material employed. If a hard anthracite coal is employed, then a temperature can be attained at which there is practically no CO, formed, but with an ordinary generator, and a loose fuel like coke, a large percentage of that gas is generally produced. The sulphuretted hydrogen is derived from the sulphur, which is always present in gas-coke, and this latter impurity is also avoided by the employment of anthracite.
The nitrogen is the result of heating the fuel by an air blast, which leaves the pipes full of producer gas (nitrogen and carbon monoxide), which is carried over into the holder by the first portions of water-gas. Water-gas so produced is non-luminous.
The various forms of generator employed for the manufacture of water-gas are very numerous, and in some of these the gas is rendered luminous by the carburetting of the non-luminous water-gas with the vapours of certain oils either after or during the process of manufacture. The most successful of these carburetting processes can be
1 Lewes, “ Journal of Gas-lighting,” vol. Isiii., p. 385.
divided into two classes :-(1) Continuous processes in which the heat necessary to bring about the inter-action of the carbon and steam is obtained by performing the operation in retorts externally heated in a furnace; (2) intermittent processes, in which the carbon is first raised to incandescence by an air blast. When the air blast is cut off, steam is admitted until the temperature falls too low for the action, when the air is again admitted. The process, therefore, consists of alternate blows and runs. Continuous processes have not as yet been a commercial
The best of the intermittent processes, and the one which is mostly used in America, and has made the greatest progress in this country, is the Lowe. In this apparatus the fuel (coke or anthracite) is heated to incandescence by an air blast in what is known as the generator, which is lined with fire-bricks, and the resulting producer gas passes on to vessels known as the super-heaters, more air being admitted to insure its combustion. By this means the super-heater is raised to a high temperature. When the fuel and super-heater are' at a sufficiently high temperature, the air blast is cut off, and steam blown through the generator, forming water-gas, and the latter then meets the oil necessary for its enrichment at the top of the first super-heater, which is known as the carburetter, and from thence passes on to the fixing chambers, where the hydrocarbons are rendered permanent gases. The chief advantage of this apparatus is that a low temperature can be used for fixing, owing to the large amount of surface for super-heating, which to a great extent does away
with the deposition of carbon.
Plate V. shows a water-gas plant as erected by Messrs. R. and J. Dempster, Limited, of Manchester, the working of which may be briefly described as follows. generator and carburetter, the generator being filled with anthracite, coal, or coke. The coke is lighted at the
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