Wood is the most used of all fuels. All woods when perfectly dry consist of nearly 99% of combustible material and about 1% of inorganic matter which remains as ash when the wood is burned. Air-dry wood contains about 25% of water, and in green wood it may be as much as 50%. This water reduces the fuel value not only as taking the place of combustible substances but also as using up the heat necessary for its evaporation. Hence the economy of well-seasoned fire-wood. The value of different fuels may be conveniently compared when stated in terms of the amount of water which a unit weight will evaporate. Thus, green wood is found to yield enough heat to convert about twice its weight of water at 100 C. into steam; air-dry wood about three and a half times; and perfectly dry wood over four times its own weight. So far as chemical composition is concerned soft woods should yield on burning about the same amount of heat as hard woods of equal dryness. In practice, however, considerable differences are found, depending in part upon the ease with which complete combustion may take place, as shown by the amount of smoke, and in part upon compactness of structure, and so forth. Wood as being a flaming fuel is especially well adapted for heating surfaces of large extent, as in the boilers of steam-engines. The small amount and the soft crumbly nature of its ash give wood a further advantage over peat and coal.

Charcoal - Plant Fuels

Charcoal burns without flame or smoke, and has over twice the heating power of wood, or as much as the average coal. It is produced mostly by smothered combustion of billets of wood, commonly arranged in conical piles, and covered with earth. When wood is subjected to dry distillation creosote, wood-alcohol, and other volatile compounds pass into the condenser, leaving charcoal in the retort. The charcoal produced at the highest temperature yields most heat when burned, and is therefore of most use in metallurgy; that produced at as low a temperature as possible is the most inflammable and thus the most suitable for mixing with niter and sulphur to make gunpowder.