Environmental Engineering Reference
In-Depth Information
and other brands (in 1889 there were 77 manufacturers)
were sold in the United States, and many of these designs
were also used in Australia, Latin America, and South
Africa.
and hemicellulose; extractive resins have up to 35 MJ/
kg. Differences in energy content among common spe-
cies range between 17.5-20 MJ/kg for most hardwoods
and 19-21 MJ/kg for softwoods. The energy density of
typical wood (19 MJ/kg) slightly surpasses that of cereal
straws (17-18 MJ/kg), and is about 80% of steam coal's
(22-24 MJ/kg) and 45% of crude oils'.
All these energy densities refer to absolutely dry phyto-
mass, but fresh mature wood averages 30% water for
hardwoods and 46% for softwoods, and even air-dried
wood (cut, stacked, sheltered, and dried for at least two
months) still contains about 15% moisture, as do many
straws. As a result, the net heating values of phytomass
vary widely, from around 20 MJ/kg for very dry to
about 15 MJ/kg for air-dried wood and ripe straws to as
little as 5-6 MJ/kg for fresh (green) wood, crop stalks,
and grasses. In contrast, charcoal is a high-quality fuel
with an energy density (29.7 MJ/kg) equal to that of
the best bituminous coal or twice the heat content of
air-dried wood. But preindustrial production of charcoal,
virtually sulfur-free and smokeless and hence suitable for
indoor use, was often enormously wasteful.
The simplest process involved primitive earth or pit
kilns in which the partial combustion of the heaped
wood provided the heat necessary to initiate carboniza-
tion (fig. 7.8). Ancient charcoal makers had their prefer-
ences. According to Theophrastus (third century
B
.
C
.
E
.),
very old trees were the worst, and smiths preferred char-
coal from fir. Regardless of the charge, both the quality
and the quantity of the final product were difficult to
control in earthen kilns. Charcoal yields were just 15%-
25% of dry wood charge by weight, and 1:5 was probably
the best approximation of the typical charcoal:wood ratio
during the preindustrial era. Assuming a net heat value of
14.5 MJ/kg of air-dried wood, the energy loss in tradi-
7.3 Phytomass Fuels and Metallurgy: Wood,
Charcoal, Crop Residues
Woody matter (trunks, branches, twigs, bark, and roots),
charcoal made from this phytomass, and crop residues
(mainly cereal straws, legume stalks, and tuber vines) fu-
eled the subsistence as well as the complexification of all
preindustrial societies. Quantitative information on the
use of these fuels in preindustrial households, metallurgy,
and artisanal manufacturing is limited (Biringuccio 1540;
Evelyn 1664; Buck 1930; Schott 1997; Sieferle 2001;
Perlin 2004). However, the basic subsistence needs—for
cooking and, in some societies, for space or water heating
or food drying—have changed little over time, and valu-
able insights can be gained from modern studies of en-
ergy use in rural areas of low-income countries, where
phytomass remains the dominant or even sole fuel (Earl
1973; NAS 1980; Hall, Barnard, and Moss 1982; Smil
1987; Vimal and Bhatt 1989; RWEDP 2000; Bailis
2004).
Despite the enormous diversity of woody phytomass,
wood itself has a relatively uniform composition (Shelton
and Shapiro 1976; Tillman 1978; Smil 1983). Typically,
about 43% of it is cellulose, 28%-35% hemicelluloses, and
the rest lignin; carbon makes up 49%-56%, oxygen 40%-
42%. Ash (incombustible matter) varies from less than
0.5% to about 2%. Densities range from less than 0.4 g/
cm
3
for poplar to nearly 0.7 g/cm
3
for white oak and
1.0 g/cm
3
for some eucalyptus. The energy content of
wood goes up with the proportion of lignin, which con-
tains 26.5 MJ/kg compared to 17.5 MJ/kg for cellulose
