Page 316 - Synthetic Fuels Handbook

302 CHAPTER TEN

The basic chemical phenylpropane units of lignin are bonded together by a set of linkages
to form a very complex matrix. This matrix comprises a variety of functional groups (such
as hydroxyl, methoxyl, and carbonyl groups) which impart a high polarity to the lignin
macromolecule (Hashem et al., 2007).
The amount of extractable constituents is an important parameter which directly affects
the heating value of wood (or, for that matter, any biomass). A high content of extract-
able constituents makes it desirable as fuel. The extractable constituents usually have low
molecular weight and are soluble in neutral solvents. For example, terpenes, lignans and
other aromatics, fats, waxes, fatty acids and alcohols, turpentines, tannins, and flavonoids
are categorized as extractable constituents. The contents of extractives vary among wood
species, geographical site, and season and usually represent from between 3 and 10 percent
by weight of wood.
Wood cut in the spring and summer contains more water than that cut in the early part of
the winter. A cord (8 ft long, 4 ft wide, and 4 ft high) of hard wood, such as ash or maple, is
about equal in heating value to 1 ton of bituminous coal; soft woods, such as pine and pop-
lar, have less than half this amount. Wood burns with a long flame and makes comparatively
little smoke; but its calorific intensity is low, averaging from 3000 to 4000 cal/kg of air-
dried wood. It is, however, easily kindled, the fire quickly reaches its maximum intensity,
and a relatively small quantity of ash is formed. Wood is too expensive for industrial use,
except in a few special cases, where freedom from dirt and smoke is necessary.
Of other cellulose materials, shavings, sawdust, and straw are used for fuel in some
places. They are bulky and difficult to handle, while their heat value, which depends on the
amount of moisture they contain, is seldom more than from one-third to one-half that of
subbituminous coal. Waste matter such as spent tan-bark and bagasse (crushed sugar cane),
and the pulp from sugar beets is sometimes used for fuel for evaporation for steam, but
owing to the large amount of moisture they contain, the heat value is very low.
When considering the type of wood for use as firewood, several characteristics are
important. These include heat value, ease of splitting, weight per unit volume, ease of
starting, amount of smoking, and coaling qualities. Moisture content of the wood, number
of knots and pitch content affect these characteristics of the more common woods used as
firewood.
All woods dried to the same moisture content contain approximately the same heat
value per pound—from 8000 to 9500 Btu for fully dried wood and 5500 to 8500 Btu for
air-seasoned wood. However, the heat content of any fire depends on wood density, resin,
ash, and moisture. A general rule for estimating heat value of firewood is: one cord of well-
seasoned hardwood (weighing approximately 2 tons) burned in an airtight, draft-controlled
wood stove with a 55 to 65 percent efficiency is equivalent to approximately 175 gal of #2
fuel oil or 225 therms of natural gas consumed in normal furnaces having 65 to 75 percent
efficiencies.
Forest biomass or agricultural residues are almost completely comprised of lignocel-
lulosic molecules (wood), a structural matrix that gives the tree or plant strength and form.
This type of biomass is a prime feedstock for combustion, and indeed remains a major
source of energy for the world today (FAO, 2005). The thermochemical platform utilizes
pyrolysis and gasification processes to recover heat energy as well as the gaseous compo-
nents of wood, known as synthesis gas or syngas. Syngas can then be refined into synthetic
fuels, including Fischer-Tropsch, methanol, and ethanol, through the process of catalytic
conversion.
The wood biomass comprises stem wood from ordinary forestry, dedicated (short-rotation)
forestry, as well as various residues and wood wastes. However, two core types of bio-
mass raw material are distinguished: (a) woody and (b) herbaceous (Table 10.1). Currently
woody material accounts for about 50 percent of total world bioenergy potential. Another
20 percent is straw-like feedstock, obtained as a by-product from agriculture.