Energy - Environmental Science - page 319

Science: Burning Solid Biomass
Plant materials and animal wastes can be burned to
provide heat or electricity or converted into gaseous
or liquid biofuels.
Biomass consists of plant materials (such as wood and
agricultural waste) and animal wastes that can be
burned directly as a solid fuel or converted into gaseous
or liquid biofuels (Figure 13-39). Biomass is an indirect
form of solar energy because it consists of combustible
organic compounds produced by photosynthesis.
Most biomass is burned directly for heating, cook-
ing, and industrial processes or indirectly to drive tur-
bines and produce electricity. Burning wood and ani-
mal manure for heating and cooking supplies 11% of
the world's energy and 30% of the energy used in de-
veloping countries.
Almost 70% of the people living in developing
countries heat their homes and cook their food by burn-

Figure 13-40 Natural biomass capital: making fuel briquettes
from cow dung in India. The scarcity of fuelwood causes people
to collect and burn such dung. However, this practice deprives
the soil of an important source of plant nutrients from dung
decomposition. Solid Biomass Fuels

Wood logs and pellets
Charcoal
Agricultural waste
(stalks and other plant debris)
Timbering wastes
(branches, treetops, and wood chips)
Animal wastes (dung)
Aquatic plants (kelp and water hyacinths)
Urban wastes (paper, cardboard,
and other combustible materials)

ing wood or charcoal (derived from wood). However,
2.7 billion people in these countries cannot find—or are
too poor to buy—enough fuelwood to meet their needs.
One way to produce biomass fuel is to plant, har-
vest, and burn large numbers of fast-growing trees
(such as cottonwoods, poplars, and sycamores), shrubs,
perennial grasses (such as switchgrass), and water hy-
acinths in biomass plantations.
In agricultural areas, crop residues (such as sugar-
cane residues, rice husks, cotton stalks, and coconut
shells) and animal manure can be collected and burned
or converted into biofuels (Figure 13-40). Some ecolo-
gists argue that it makes more sense to use animal ma-
nure as a fertilizer and crop residues to feed livestock,
retard soil erosion, and fertilize the soil.
Figure 13-41 (p. 320) lists the general advantages
and disadvantages of burning solid biomass as a fuel.
Burning biomass produces CO 2 . However, if the rate of
use of biomass does not exceed the rate at which it is
replenished by new plant growth (which takes up
CO 2 ), no net increase in CO 2 emissions occurs. Never-
theless, repeated cycles of growing and harvesting bio-
mass plantations can deplete the soil of key nutrients.
According to a 2004 study by the World Wide
Fund for Nature and the European Biomass Industry
Association, burning biomass currently provides
about 1% of the power needs of developed countries
but could provide 15% by 2020.

Conversion to gaseous
and liquid biofuels

Direct burning

Gaseous Biofuels Liquid Biofuels

Synthetic natural gas
(biogas)
Wood gas

Ethanol
Methanol
Gasohol
Biodiesel

x

H OW W OULD Y OU V OTE ? Should we greatly increase our
dependence on burning solid biomass to provide heat and
produce electricity? Cast your vote online at http://biology
.brookscole.com/miller11.

Figure 13-39 Natural capital: principal types of biomass fuel.