Fuels

Fuels

Fuels are substances which, when burned with air, evolve heat
with sufficient rapidity and in sufficient quantity to be employed for domestic or industrial purposes.
'There are three classes of fuel: solid, liquid, and gaseous. In the majority of these the essential constituent is carbon, but in many of them hydrogen is also an important ingredient. In rare cases sulphur, phosphorus, silicon, or manganese may take part in the combustion; but for the purposes for which fuel is ordinarily used these constituents are deleterious. Oxygen is sometimes regarded as advantageous, but not always. Nitrogen may cause a direct loss of calorific power, owing to its dilution of the combustible gases, but in most solid fuels the percentage of nitrogen is so small that its effect is negligible.

SOLID FUELS

The solid fuels are wood and other matter containing cellulose,
peat, lignite or brown coal, bituminous coal, anthracite, charcoal, and coke.

Wood consists of cellulose (C₆H_IO0₅), resins, lignine, various
inorganic salts, and water. The quantity of water present has
great effect on the heating value and ranges from 25 to 50 per
cent in green wood, and' from 10 to 20 percent in air-dried wood.
Wood cut in the spring and summer contains more water than that cut in the early part of the winter. A cord of hard wood, such as ash or maple, is about equal in heating value to one ton of bituminous coal; soft woods, such as pine and poplar, have less than half this amount. Wood burns with a long flame and makes comparatively little smoke; but its calorific intensity is low, averaging from 3000 to 4000 C. per kilo of air-dried wood. It is, however, easily kindled, the fire quickly reaches its maximum intensity, and a relatively small quantity of ash is formed. Wood is too expensive for industrial use, except in a few special cases, where freedom from dirt and smoke is necessary.
Of other cellulose materials, shavings, sawdust, and straw are
used for fuel in some places. They are bulky and difficult to handle, while their heat value, which depends on the amount of moisture they contain, is seldom more than from one-third to one-half that of good coal. Such waste matter as spent tan-bark and begasse (crushed sugar cane), and the pulp from sugar beets is sometimes used for fuel for evaporation 01' for steam, but owing to the large amount of moisture they contain, the heat value is very low.
   Peat is the product of slow decay of mosses, especially Sphagnaceoe, tinder water. It is of little importance in this country, but is extensively used in parts of Europe where it is found. Since it contains a large amount of water and inorganic matter, its calorific power is not high, averaging from :3000 to ,1000 C. per kilo. It is dug from the bogs and dried in the air, sometimes being heavily compressed to reduce its bulk As thus prepared, it contains from 15 to 20 per cent of moisture and from 8 to 12 per cent ash. It is used considerably as a packing material, owing to its soft and spongy consistency.
  Lignite or brown coal is intermediary between peat and bituminous coal. It was probably formed from swamp plants which decomposed under water, and is geologically of more recent formation than true coal. It is dark brown or black in color, and its texture is fibrous, earthy, or sometimes vitreous. It usually contains from 15 to 20 per cent of moisture, a large quantity of ash, and often a considerable amount of sulphur. It burns freely with a long flame, producing much smoke, and its calorific power varies from 4000 to 5500 C. It is extensively used for heating steam boilers and evaporating pan~, and for domestic fires.
  Bituminous coal is the most, important of all fuels. There is a great variety in the kinds of coal classed under this name, but they differ chiefly in the amount of volatile matter, which ranges from 20 to 50 per cent. They were all formed from similar sources, the varieties having resulted from pressure and frol11 exposure to heat. The specific gravity varies from 1.25 to 1.7.They are classified according to their behavior when burning, as fat, caking, and non-caking. Fat coals usually have a dull lustre, are very rich in volatile matter, sometimes containing as much as 50 per cent, and burn with a long, smoky flame, sometimes caking in the fire. Non-caking coals are those which burn freely, with little smoke, and do not cake. The caking coals burn with a smoky flame and fuse or sinter together.
The formation of coal is probably due to a slow decomposition of cellulose matter, under fresh water, by which marsh gas (CH4) and carbon dioxide (CO2) were eliminated. The composition of a typical coal, as shown by the analysis of good samples, may be representedby the symbol (C₂₆H₂₀0₂) and assuming this, the change of cellulose may be represented by the equation:

Various changes were afterwards brought about by the heat and pressure within the earth's strata, and the character of the coals modified in many cases. Thus, more or less of the volatile constituents were removed, and the coal itself compressed to a very hard, compact mass. When this process went to the extreme, nearly the whole of the volatile constituents were expelled, and the resulting product is the hard coal known as anthracite.