Gaseous fuels may be divided into four classes: natural gas, producer gas, water gas and coal gas.
Natural gas exists already formed in the earth, and is obtained by boring tube wells, similar to petroleum wells. Its essential heat producing constituents are methane (CH4) and hydrogen. It is the cheapest and most efficient of all fuels, when properly burned; but it requires a large amount of air for its combustion, and special burners must be used.
Producer gas is made by forcing air through a bed of incandescent coal or coke, in specially constructed furnaces. Its essential heat constituent is carbon monoxide (CO), of which it contains about 28 to 30 per cent. But it also contains about 63 per cent of nitrogen from the air, and some carbon dioxide, which dilute the gas very much, and reduce its calorific intensity greatly. It is extensively used for fuel, because of its cheapness, cleanliness, and the regularity of the temperature obtained.
In converting carbon to carbon monoxide, about one-third of the heat value of the carbon is set free, thus heating the gas very hot. If it is at once led, through short flues, into the combustion chamber and burned with air, a much higher temperature is obtained, than if it is permitted to cool before burning. In modern gas producers, this waste of heat is largely avoided by introducing steam into the incandescent coal, together with the air; the steam dissociates into hydrogen and oxygen, and the latter gas combines with the carbon, forming more carbon monoxide. These gases, mixing with the producer gas, increase its calorific intensity.
In the Siemens gas producer * (Fig. 17), the coal is introduced at (E), falls upon the step grate (B, B), and is brought to incandescence by air entering through the openings while steam is injected from the pipe (C), and the gas formed escapes through (A, A). The ashes fall through the grate (G) into the pit, which is kept closed
except when cleaning. A more modern producer (Taylor's) is shown in Fig. 18. The coal rests on a bed of ashes (A, A), and air is forced through the blast pipe (F), raising the fuel to incandescence. The gas formed passes out by the pipe (E). The grate (G) is made to revolve by the crank at (B), and the ashes fall over the edge of the grate at (H). The bed of ashes is kept about 3 feet deep on the revolving bottom at all times. Steam from the pipe (D) is introduced with the air through the blast pipe, which is provided with a hood to disseminate them through the fuel. In all producer gas plants, the regenerative heating system is used.
The Siemens regenerative furnace is a type of this style of heating. This furnace is represented in its simplest form in :Fig. 19. The material to be heated is placed on the hearth of the furnace (A). There are four passages, B, C, D, and E, filled with loosely
piled fire-brick called the "checker work." On their way to the chimney, the hot gases from the furnace pass through and heat two checker works, e.g. (B) and (C). 'When they are sufficiently heated, the flow of furnace gases is turned into (D) and (E), through which they pass to the chimney. Then fuel gas is conducted through the hot passage (B), to the furnace (A), where it mixes
with air which has been heated by passing through (C). The temperature of (A) is thus much higher than if the air and gas arrived at (A) cold. While (B) and (C) are being thus cooled, (D) and (E), are being heated by the furnace gases, and after a time, the dampers are turned, and the gas made to pass through (E), and the air through (D), while the combustion products pass through (B) and (C) to the chimney. Hence the process is an alternating one, the checker works on one side being heated, while those on the other are giving up their heat to the gas and air respectively. Since the interstices between the bricks of the checker work frequently become clogged with ashes and soot, the combustion gases are sometimes passed through flues containing narrow tubes, through which the gas and air are passing to the furnace, in a direction opposite to that taken by the fire gases. The waste gases from blast furnaces contain over 30 per cent of carbon monoxide and about 63 per cent of nitrogen. These gases are largely employed near the furnaces for heating purposes. Water gas is sometimes usee1 as a fuel, but oftener as a constituent of illuminating gas. It is made by blowing steam over incandescent anthracite coal or coke, and is a mixture of about
45 per cent each of carbon monoxide and hydrogen, with small amounts of nitrogen, oxygen, and carbon dioxide. :For the best results, the temperature must not fall below 1000° C.; above this point, the reaction is:
But at lower temperatures, the following takes place:
smoke or soot. Its calorific value is about 3000 C. per cubic meter. One kilo of coke produces about 1.13 cubic meters of water gas, but anthracite gives a better yield ..
The fuel is brought to incandescence by a blast of air, and during this part of the process the heat generally goes to waste. When it is white hot, the air is cut off, and the steam is turned on; decomposition occurs, according to the first reaction above. As soon as the temperature falls below 1000° C., the steam is cut off and the air blast turned on till the coal is again white hot. Thus alternate blowings of air and steam are carried on. The generator gas produced by the air blast is sometimes saved and used, but in making illuminating gas it goes to waste. For illuminating gas, this water gas is "enriched" with naphtha.
Coal gas is made by distilling bituminous coal in retorts. It contains hydrogen and marsh gas in large quantities, - about 40 per cent of each, - besides small amounts of carbon monoxide, carbon dioxide, nitrogen, oxygen, and hydrocarbons of the CnH2n and CnH2n-2 series, which impart illuminating properties. It has a limited use in domestic stoves and as a source of power in gas engines.
The average composition of the various fuel gases is shown in
the following table * :-
When burned. with 20 per cent excess of air, and assuming that the escaping gases have a temperature of 500°F., 1000 cubic feet of gas will evaporate the following number of pounds of water, at from 6O° F. to 212° F. :-
Organic Chemistry for the industry
Inorganic Chemistry for the industry