1.1. Energy potential of agricultural residues
Biomass is the third largest primary energy resource in
the world, after coal and oil [1]. In all its forms, biomass
currently provides about 1250 million TOE which is about
14% of the world’s annual energy consumption [2]. Biomass
is a major source of energy in developing countries, where it
provides 35% of all the energy requirements. In developed
countries, biomass energy use is also substantial. In the
USA, for example, biomass contributes about 70 million
TOE whereas in the European Union, biomass contributes
between 20 and 40 million TOE to the annual energy
consumption [2]. The use of biomass to provide partial
substitution of fossil fuels, has an additional importance as
concerns global warming since biomass combustion has the
potential to be CO2 neutral. This is particularly the case with
regard to agricultural residues or energy plants, which are
periodically planted and harvested. During their growth,
these plants have removed CO2 from the atmosphere for
photosynthesis which is released again during combustion.
Biomass materials with high energy potential include
agricultural residues such as straw, bagasse, coffee husks
and rice husks as well as residues from forest-related
activities such as wood chips, sawdust and bark. Residues
from forest-related activities (excluding wood fuel) account
for 65% of the biomass energy potential whereas 33%
comes from residues of agricultural crops [3]. Table 1
shows the quantities of the residues (straw, stalk and shell)
generated from agricultural crops in 1983 [3]. These quantities
were estimated based on the crop production and the
residue-to-product ratios. Over three billion tonnes of agricultural
residues were generated world-wide. The production
of straw (from cereals like wheat, barley and oat) was
distributed fairly uniformly world-wide, both in developing
and developed countries, whereas the production of rice
husks, bagasse and coffee husks is predominantly found in
the developing countries. Rice husks and straw are the most
important agricultural residues in quantity, amounting to
43% of the total residues. Over 97% of the rice husks is
produced in the developing countries. In 1997, China
alone generated some 54 million tonnes of rice husks [4].
Similarly, in 1987–1988, out of the 65 million tonnes of
sugar produced from sugar cane world-wide around 90%
came from the developing countries [5]. Sugar cane residues