_______________________________________________________
| | | | | | | |
| 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.
In designing furnaces for burning wood, the question resolves itself
into: 1st, the essential elements to give maximum capacity and
efficiency with this class of fuel; and 2nd, the construction which will
entail the least labor in handling and feeding the fuel and removing the
refuse after combustion.
Wood, as used commercially for steam generating purposes, is usually a
waste product from some industrial process. At the present time refuse
from lumber and sawmills forms by far the greater part of this class of
fuel. In such refuse the moisture may run as high as 60 per cent and the
composition of the fuel may vary over wide ranges during different
portions of the mill operation. The fuel consists of sawdust, "hogged"
wood and slabs, and the percentage of each of these constituents may
vary greatly. Hogged wood is mill refuse and logs that have been passed
through a "hogging machine" or macerator. This machine, through the
action of revolving knives, cuts or shreds the wood into a state in
which it may readily be handled as fuel.
Table 42 gives the moisture content and heat value of typical sawmill
refuse from various woods.