Fact Sheet 4.22. Energy By-Products from Food and Fiber Wastes
Use and Potential
Biomass waste products arising from traditional agricultural and forest production,
and processing contributes significantly to the energy mix (Hall, 1991; Pingoud
et al., 1999; Hall et al., 2000). Resources include rice husks,
corn cobs, cereal straw, bagasse, wood process residues (bark, sawdust, off-cuts),
black liquor, nut shells, animal manures, and municipal solid wastes. A wide
range of energy conversion routes exist to produce electricity, heat, or fuels
(landfill gas, biogas, producer gas, ethanol, methanol, briquetted pellets,
charcoal, etc.). Currently, a significant proportion of biomass waste at food
and fiber processing plants is either burned to waste or dumped into landfills
for disposal. Where this dumping entails a disposal cost, use of the resource
for energy production may be economically feasible and reduce the possibility
of local air and water pollution.
Monitoring and Verification
The resource is widely distributed, highly variable, and difficult to assess
with any degree of accuracy. It tends to have a low energy density and high
moisture content. In countries with extensive forest industries-such as Sweden,
Finland, and Austria-energy from woody biomass by-products can be monitored
relatively easily and can supply up to 30 percent of the country's primary energy.
In developing countries with dispersed rural populations, use of biomass wastes
for cooking and heating can only be estimated.
Removals
Agricultural and forest production wastes are traditionally left in the field
after harvest to decompose; hence, these wastes return organic matter and nutrients
to the soil and carbon to the atmosphere. Sustainable production methods would
need to be carefully evaluated for each site and soil type if this biomass were
to be removed from the site along with the traditional food and fiber products.
Conversely, removal of these waste by-products (straw, logging slash, etc.)
may have benefits-such as ease of cultivation and replanting, disease control,
and avoiding methane production during natural decomposition. Because of the
importance of organic residues to the maintenance of soil quality, it has been
suggested that only about 50 percent of agricultural residues can be removed
from fields without affecting future crop productivity (Sampson et al.,
1993).
Scientific Uncertainties
Waste-to-energy plants that produce biofuels or generate heat and electricity
may create local emission problems relating to heavy metals, dioxins, and so
forth, depending on their design and operation, as well as the nature of the
biomass. For example, considerable debate continues about the benefits of incineration
of municipal solid waste and sewage sludge (e.g., Aumonier, 1996) versus disposal
to properly designed landfills after increased diversion of the organic component
to composting facilities or anaerobic digestors (e.g., Finnveden and Ekvall,
1998). If GHG emissions reduction is the primary objective, incineration with
energy recovery may be preferred. When other economic and environmental factors
are also considered, there may be no general solution.
Collection and transport of biomass by-products such as cereal straw and forest
residues to a central processing plant are energy-intensive. Full life-cycle
analyses need to be undertaken and energy ratios evaluated.
Time Scales
Many commercial waste-to-energy plants are already being operated successfully.
Bagasse is used for on-site co-generation and electricity exports in Australia,
South Africa, and Hawaii. Cereal straw is used for district heating in Denmark,
Germany, and the United Kingdom. Wood processing residues are used in the United
States, Australasia, and northern Europe. Landfill gas plants are widely distributed
in most developed countries. Community-scale biogas plants are gaining in popularity
in Denmark, India, and China. Several power plants that burn chicken litter
are operating in the United Kingdom. Development of further plants depends on
local energy prices and government strategies for waste avoidance and carbon
trading.
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