Page 321 - Synthetic Fuels Handbook

FUELS FROM WOOD 307

Logs and wood chips can come from forest thinnings, coppicing, tree-surgery, and prun-
ing operations—even whole trees can be chipped to provide fuel. Woodchips can also be
produced from fast-growing varieties of willow and poplar which are cultivated by short
rotation coppicing specifically to produce energy. On the other hand, wood pellets are
made from highly compressed dry wood shavings and sawdust, usually from sawmilling
and joinery operations.
After drying and resizing (in which the wood is cut and split in sizes that are easy to
transport), the main processes for the use of wood as fuel are:
1. Carbonization: the process of burning wood or biomass in the absence of air breaks it
down into liquids, gases, and charcoal.
2. Gasification: this is the process in which solid biomass fuels (e.g., wood, charcoal) are
broken down by the use of heat to produce a combustible gas, known as producer gas.
3. Densification: to overcome the bulky nature, low thermal efficiency, and smoke emis-
sion of wood and agriculture residues, these can be processed to produce smokeless
briquettes with or without binders.
4. Liquid fuel production: for example ethanol production by alcohol by hydrolysis or
anaerobic digestion; liquid fuels can be produced from several biomass types such as
wood pulp residues (black liquor), sugarcane, and cassava.
5. Combustion: biomass fuels can also be used for power generation and/or heat produc-
tion by direct combustion, either in its primary form or after one or more of the above
mentioned transformation processes.
Generally, biomass fuels such as wood are consumed by direct combustion in their
primary form after drying and resizing, or in the form of charcoal. New conversion tech-
nologies such as gasification and anaerobic digestion have been developed to provide alter-
natives and to match biomass resources with modern end-use devices. However, as of yet
these technologies are not widely used, so also in the near future wood energy conversion
will consist mainly of resizing, drying, and charcoal production.
Nevertheless, in recent years there has been increasing use of wood and other biomass
for energy conversion in modern applications, such as combined heat and power generation
(CHP) and cogeneration.
Wood fuels consist of three main commodities: fuel wood, charcoal, and black liquor.
Fuel wood and charcoal are traditional forest products derived from the forest, trees outside
forests, wood-processing industries, and recycled wooden products from society. Black
liquors are by-products of the pulp and paper industry (Fig. 10.1).

10.4.1 Gaseous Fuels

Wood can be used to make both liquid and gaseous fuels. When wood is heated in the
absence of air, or with a reduced air supply it is possible to produce a liquid fuel which can
be used in a similar way to conventional oil fuels. It can be used to run internal combustion
engines in vehicles or generators. The gas produced from wood is a mixture of hydrogen
and carbon monoxide which is similar to the coal gas which was made before the arrival of
natural gas from the North Sea. This wood gas can be used in internal combustion engines
or in gas turbines which can be used to power generators. Although the liquid fuels are
rarely produced from wood at present, wood gas is important in other countries for produc-
ing electricity in more remote areas.
Thus, gasification technology is an attractive route for the production of fuel gases
from biomass. By gasification, solid biomass is converted into a combustible gas mixture