Page 333 - Synthetic Fuels Handbook

FUELS FROM WOOD 319

For the domestic user, pellets offer the most user-friendly form of wood heating. In
Scandinavia, wood pellets are delivered by tanker and are pumped into storage silos, which
feed, automatically into the boiler.
Charcoal. Charcoal is the blackish residue consisting of impure carbon obtained by
removing water and other volatile constituents from animal and vegetation substances.
Charcoal is usually produced by heating wood, sugar, bone char, or other substances in
the absence of oxygen. The soft, brittle, lightweight, black, porous material resembles coal
and is 85 to 98 percent carbon with the remainder consisting of volatile chemicals and ash
(Table 10.3).

TABLE 10.3 Characteristics of Charcoal Briquettes
Briquettes
Lump charcoal Without filler With filler
Ash, % w/w 3–4 8 25
Moisture, % w/w 5 5 5
Carbon, % w/w Balance Balance Balance
Volatiles, % w/w 10–15 10–15 10–15
Binder, % w/w — 10 10
Calorific value, kJ/Kg 28,000 25,000 22,000
Btu/lb 12,050 10,750 9,500

The first part of the word is of obscure origin, but the first use of the term “coal” in
English was as a reference to charcoal. In this compound term, the prefix chare meant
turn with the literal meaning being to turn to coal. The independent use of char, mean-
ing to scorch, to reduce to carbon, is comparatively recent and must be a back-formation
from the earlier charcoal. It may be a use of the word charren or churn, meaning to turn,
that is, wood changed or turned to coal, or it may be from the French charbon. A person
who manufactured charcoal was formerly known as a collier (also as a wood collier).
The word collier was also used for those who mined or dealt in coal, and for the ships
that transported it.
Historically, production of wood charcoal in districts where there is an abundance of
wood dates back to a very remote period, and generally consists of piling billets of wood on
their ends so as to form a conical pile, openings being left at the bottom to admit air, with
a central shaft to serve as a flue. The whole pile is covered with turf or moistened clay. The
firing is begun at the bottom of the flue, and gradually spreads outward and upward. The
success of the operation depends upon the rate of the combustion. Generally, 100 parts of
wood yield about 25 parts by weight (60 parts by volume) of charcoal but this yield is very
dependent on the type of wood.
The production of charcoal (at its height employing hundreds of thousands, mainly in
Alpine and neighboring forests) was a major cause of deforestation, especially in Central
Europe. In England, many woods were managed as coppices, which were cut and to regrow
cyclically, so that a steady supply of charcoal would be available (in principle) forever;
complaints (as early as in seventeenth century England) about shortages may relate to the
results of temporary over-exploitation or the impossibility of increasing production. In fact,
the increasing scarcity of easily harvestable wood was a major factor that led the switch to
the fossil fuel equivalents, mainly coal and brown coal.
The modern process of carbonizing wood, either in small pieces or as sawdust in
cast iron retorts, is extensively practiced where wood is scarce, and also for the recovery
of valuable by-products (wood spirit, pyroligneous acid, wood tar), which the process