Page 331 - Synthetic Fuels Handbook

FUELS FROM WOOD 317

place concurrently and consecutively. Gaseous emissions are predominantly a product of
pyrolitic cracking of the fuel. If flames are present, fire temperatures are high, and more
oxygen is available from thermally induced convection.
In some of the most efficient burners, the temperature of the smoke is raised to a much
higher temperature where the smoke will itself burn. This may result in significant reduction
of smoke hazards while also providing additional heat from the process. By using a catalytic
converter, the temperature for obtaining cleaner smoke can be reduced. Some U.S. jurisdic-
tions prohibit sale or installation of stoves that do not incorporate catalytic converters.
Most of the nonhousehold fuel wood consumption in developing countries is in com-
mercial and industrial activities such as crop drying, tea processing, and tobacco curing,
as well as the brick and ceramic industries. Fuel wood consumption by these sectors is
smaller than that in households; nevertheless, it cannot be overlooked, as it can constitute
10 to 20 percent of fuel wood use, as seen in some Asian countries. In Africa, it is esti-
mated that consumption of wood fuels in industry accounted for about 9.5 percent of the
total in 1994. In developed countries, fuel wood uses for electricity and heat generation at
industrial sites or in municipal district heating facilities are rapidly rising as a substitute
for fossil fuels.
The most commonly known solid fuel produced from wood is charcoal, but there are
other sources such as coconut shells and crop residues.
Charcoal is produced in kilns by a process called pyrolysis, that is, breaking down the
chemical structure of wood under high temperature in the absence of air. During the pro-
cess, first the water is driven from the wood (drying), and then the pyrolysis starts when the
temperature in the kiln is high enough. When the pyrolysis is complete, the kiln gradually
cools down, after which the charcoal can be removed from the kiln. Because some of the
wood is burned to drive off the water, dry wood produces better charcoal at a higher effi-
ciency. Typically, approximately two-thirds of the energy is lost in the process, but charcoal
has advantages over fuel wood like stoves with higher efficiency, higher convenience, and
easier distribution.
The oldest and probably still the most widely used method for charcoal production is the
earth kiln. Two varieties exist, the earth pit kiln and the earth mound kiln. An earth pit kiln
is constructed by first digging a small pit in the ground. Then the wood is placed in the pit
and lit from the bottom, after which the pit is first covered with green leaves or metal sheets
and then with earth to prevent complete burning of the wood. The earth mound kiln is built
by covering a mound or pile of wood on the ground with earth. The mound is preferred over
the pit where the soil is rocky, hard or shallow, or the water table is close to the surface.
Mounds can also be built over a long period, by stacking gathered wood in position and
allowing it to dry before covering and burning.
Earth kilns can be made at minimal cost, and are often used near wood resources,
since they can be made entirely from local materials. Earth kilns can be made in any
size, with the duration of the process ranging from 3 days to 2 months. Gross varia-
tions in the quality of the charcoal can occur, because in one batch some of the wood is
burned and some of the wood is only partly carbonized. Efficiencies are generally low,
around 10 to 20 percent by weight and 20 to 40 percent in energy terms. The efficiency
and the quality varies depending on the construction of the kiln (e.g., walls can be lined
with rocks or bricks and external chimneys can be used), and the monitoring of the
carbonization process.
Several other types of charcoal kilns have been developed, which generally have higher
efficiencies but also require higher investments than the earth kiln. Two often-used types
are fixed kilns made of mud, clay, bricks, and portable steel kilns. Fixed kilns usually have
a beehive shape. Smaller beehives are usually made of mud and are not very durable. Larger
beehives are made of bricks and have external chimneys. Beehive kilns have an opening for
loading the wood and unloading the charcoal, which is closed after loading.