[Illustration: 3520 Horse-power Installation of Babcock & Wilcox Boilers at the Portland Railway, Light and Power Co., Portland, Ore. These Boilers are Equipped with Wood Refuse Extension Furnaces at the Front and Oil Burning Furnaces at the Mud Drum End] SOLID FUELS OTHER THAN COAL AND THEIR COMBUSTION Wood--Wood is vegetable tissue which has undergone no geological change. Usually the term is used to designate those compact substances familiarly known as tree trunks and limbs. When newly cut, wood contains moisture varying from 30 per cent to 50 per cent. When dried for a period of about a year in the atmosphere, the moisture content will be reduced to 18 or 20 per cent. TABLE 41 ULTIMATE ANALYSES AND CALORIFIC VALUES OF DRY WOOD (GOTTLIEB) _______________________________________________________ | | | | | | | | | Kind | | | | | | B. t. u.| | of | C | H | N | O | Ash | per | | Wood | | | | | | Pound | |________|_______|______|______|_______|______|_________| | | | | | | | | | Oak | 50.16 | 6.02 | 0.09 | 43.36 | 0.37 | 8316 | | Ash | 49.18 | 6.27 | 0.07 | 43.91 | 0.57 | 8480 | | Elm | 48.99 | 6.20 | 0.06 | 44.25 | 0.50 | 8510 | | Beech | 49.06 | 6.11 | 0.09 | 44.17 | 0.57 | 8391 | | Birch | 48.88 | 6.06 | 0.10 | 44.67 | 0.29 | 8586 | | Fir | 50.36 | 5.92 | 0.05 | 43.39 | 0.28 | 9063 | | Pine | 50.31 | 6.20 | 0.04 | 43.08 | 0.37 | 9153 | | Poplar | 49.37 | 6.21 | 0.96 | 41.60 | 1.86 | 7834[40]| | Willow | 49.96 | 5.96 | 0.96 | 39.56 | 3.37 | 7926[40]| |________|_______|______|______|_______|______|_________| Wood is usually classified as hard wood, including oak, maple, hickory, birch, walnut and beech; and soft wood, including pine, fir, spruce, elm, chestnut, poplar and willow. Contrary to general opinion, the heat value per pound of soft wood is slightly greater than the same value per pound of hard wood. Table 41 gives the chemical composition and the heat values of the common woods. Ordinarily the heating value of wood is considered equivalent to 0.4 that of bituminous coal. In considering the calorific value of wood as given in this table, it is to be remembered that while this value is based on air-dried wood, the moisture content is still about 20 per cent of the whole, and the heat produced in burning it will be diminished by this amount and by the heat required to evaporate the moisture and superheat it to the temperature of the gases. The heat so absorbed may be calculated by the formula giving the loss due to moisture in the fuel, and the net calorific value determined.