The value of a fuel is indicated by the num ber of heat units it contains. This heating value is expressed in British thermal units (generally abbreviated to B. T. U.). This unit is the amount of heat required to raise one pound of water one degree Fahrenheit from a starting point of 60°.
Coke is an artificial fuel obtained from bituminous or semi-bituminous coals by driv ing off their hydro-carbon constituents by means of heat. As the by-products of gas re torts, commercially known as gas-house coke, it has a low fuel value under direct firing methods; but it will make a very hot fire under forced draught. It contains about•60 per cent carbon and 1 to 2 per cent hydrogen.
Briquettes are made by pressing into blocks of varying shapes and sizes fine coal and coal dust mixed with tar, asphalt, pitch, molasses residues and similar materials as binders. At first prohibitory on account of manufacturing cost they have come more and more into favor as coal has advanced in price. In the United States alone, in 1917, there were 13 plants en gaged in making briquettes. They used 402,336 tons of fuel material which otherwise would have been valueless, and turned out 406,856 tons of briquetted fuel, valued at $2,233 an average value of $5.48 per ton. The calorific value of briquettes is from 13,700 to 14,600 British thermal units per pound.
Thermal Units.— The number of British thermal units (B.T.U.) per pound of fuel in some of the more common kinds of coal arc as follows: Pennsylvania anthracite, 13,100 to 14,200; Pennsylvania Connellsville coal, 13,640; Pennsylvania semi-bituminous and cannel coal, 13,150; Pennsylvania brown coal or lignite, 12,300; Kentucky bituminous, 14,400; Kentucky cannel, 15,200; Kentucky lignite, 9,300; Indiana bituminous, 14,160; Indiana cannel, 13,100; Vir ginia bituminous, 13,100; Virginia Pocahontas, 14,800; West Virginia bituminous, 13,740; southwestern Illinois, 12,800; Missouri bitu minous, 13,550; Ohio bituminous, 12,300; Mary land Cumberland, 12,200; Arkansas lignite, 9,200; Colorado lignite, 13,560; Texas lignite, 13,000; Washington lignite, 11,550; peat7 air dried, 7,400; peat, kiln-dried, 10,200 to 12,200; coke (average), 14,700. Many other examples
are given in Bulletin 23 of the United States Bureau of Mines. Coals which have a high moisture content or a high percentage of ash develop their best fuel results when used in a gas producer. The calorific value of any sample of coal may be roughly determined by the 0 formula 14,600 C 56,000 (H — )-F 4,000 S.
where the letters stand for the molecular weights of those elements — the result being in British thermal units per pound.
Wood, considered as fuel, is divided into two classes hard wood and soft wood. The heating value of different kinds of wood for a given weight is practically the same; or, in other words, one pound of hickory is not worth any more as fuel than one pound of pine, as the chemical composition of different kinds of wood is nearly the same. Wood, however, is usually purchased by the cord,. and the hard woods are very much heavier per cord than the soft woods. Hickory and hard maple weigh about 4,500 pounds per cord, equal to 1,800 pounds of coal; white oak weighs 3,850 pounds, equal to 1,540 pounds of coal; birch and the red and black oaks weigh 3,250 pounds, equal to 1,300 pounds of coal; poplar, chestnut and elm weigh about 2,350 pounds per cord, equal to 940 pounds of coal; pine weighs about 2,000 pounds per cord, equal to 800 pounds of coal. The average chemical composition of air-dried wood shows 37.5 per cent of carbon; 4.5 per cent of hydrogen; 30.75 per cent of oxygen; and 1.5 per cent of ash, with a moisture content ranging from 15 to 25 per cent. Its fuel value is about 5,800 British thermal units per pound. For some purposes wood is kiln-dried, increas ing its fuel value to 7,800 British thermal units per pound; but such wood, if kept for any length of time, absorbs from the air the moisture driven out, and falls again into the class of ordinary air-dried wood. Selected resinous wood, such as pine knots, give as high a fuel value as 10,800 British thermal units per pound. In estimating relative fuel values it is customary to reckon one pound of coal as equivalent to two and one-half pounds of wood.