Page 318 - Synthetic Fuels Handbook

304 CHAPTER TEN

Residual wood is the material that is a refuse without objective value within a specific
context, otherwise it constitutes a material at the end of its usefulness. Thus, a number of
woody materials can be included in the group of residues and waste: thinning and log-
ging residues from forest industry (tops, branches, and small-size stems), demolition wood
and railway sleepers, fiberboard residues, cutter shavings, and plywood residues. Residual
woody material is believed to be a very promising bioenergy resource, since it is available
at much lower or negligible cost compared to wood logs and short-rotation forestry.
However, the availability of residual woody biomass depends on the primary wood
yield and typically accounts for 25 to 45 percent of all harvested wood on average. The
heterogeneous composition of the residual and waste woody biomass (content of moisture,
impurities, etc.) might sometimes preclude its application for biomass-to-liquids produc-
tion. Hence, preliminary treatment of the residual and waste woody biomass may be neces-
sary, in order to make it appropriate for processing to liquid fuels (Calis et al., 2002; van
Loo and Koppejan, 2003; US DOE, 1996).
Fireplaces and wood stoves, popular aesthetic accessories of the recent past, are rapidly
gaining prominence as primary or supplemental heat sources for homes. The rising costs,
and in some instances, actual shortages of conventional domestic-heating energies have led
to greatly increased utilization of wood as a heating fuel.

10.3 ENERGY FROM WOOD

Combustion remains the most common way of converting biomass into energy. It is well under-
stood, relatively straightforward and commercially available, and can be regarded as a proven
technology. However, the desire to burn uncommon fuels, improve efficiencies, cut costs, and
decrease emission levels results in new technologies being continuously developed.
On the other hand, wood as a feedstock for, one of nature’s most common methods of
storing solar energy, is a renewable energy source. It is a relatively clean, efficient, safe
energy source having low sulfur content and is generally found throughout the country. Its
primary products of combustion are carbon dioxide, water vapor, and ash. The ash content
is low—only 1 to 2 percent by weight—and that which does remain can be used as a worth-
while soil conditioner.
A wood fire is easy to start and produces a large quantity of heat in a short time as well as
adding a cheerful atmosphere to the home. An ample air supply to the wood fire is important
to ensure complete burning or combustible gases. Wood fires are ideal where heat is required
only occasionally, for warming a living area on cool days or for supplying extra heat in
extremely cold weather. When considering wood as a primary heat source, several factors
must be carefully weighed to ensure satisfactory results and acceptable deficiencies.
Generally, hardwoods which provide long-burning fires contain the greatest total heat-
ing value per unit of volume. Softwoods which give a fast burning, cracking blaze are less
dense and contain less total heating value per unit of volume. However, the amount and
types of wood fuel used vary considerably between regions, mainly due to different local
situations and conditions.
Charcoal continues to be used as an important industrial source of energy. For example,
in Brazil, some 6 million metric tons of charcoal is produced every year for use in heavy
industry, such as steel and alloy production. The industrial demand for charcoal in the last
few years has led to new, more efficient, and large-scale technologies, mainly aimed at
improving charcoal yield and quality.
On the other hand, the production and consumption of black liquor (Table 10.2), which
is a by-product of pulp and paper production, are concentrated in developed countries with
large paper industries. In the pulp and paper industry, black liquors are widely used for heat