Environmental Engineering Reference
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transportation, are carbon-neutral, since the carbon dioxide released through combustion in
a stove or power plant is recaptured through photosynthesis to build new biomass.
Extracting Energy from Biomass
Biomass can be divided into two broad categories: traditional and modern. Traditional
biomass includes wood, charcoal, agricultural residues and animal dung used for cooking
and heating, mostly in poorer communities. Modern biomass encompasses biomass crops,
biogas, wastes, forest and food industry residues for electricity and heat generation,
together with fuels (such as bioethanol and biodiesel) for transportation (IEA 2012a ) .
Biomass may take solid, liquid, or gas form, and there is a wide variance in terms of
energy content; from fuel wood that contains about 15 GJ of energy per tonne, to sunflower
oil at 38 GJ per ton, not much less than petroleum (see Table 4.5 ). Biomass products may
be divided into four main categories:
1. Solid biomass , sold as wood logs, chips, pellets, or briquettes, is usually derived
from crops such as willow, poplar, eucalyptus and miscanthus that can be planted
and harvested in short rotation. 19
2. Wastes such as sewage, sludge or slurry; solid domestic and commercial waste;
and residues from agriculture and industries, such as timber, straw, paper pulp
and fruit stones.
3. Biogas , a mixture of carbon dioxide and methane that is produced when bacteria
digest certain low-fibre biomass.
4. Liquid biofuels , such as bioethanol or biodiesel, which are produced by
fermenting sugar and refining vegetable oils.
Table 4.5.
The energy content of various fuels
Fuel
GJ/ton
Fresh grass
4
Green wood (50-60% humidity)
6-7
Municipal solid wastes
9
Air dried wood (30%-20% humidity) 14-18
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