Page 11 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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Encyclopedia of Physical Science and Technology EN002C-60 May 17, 2001 20:23

Biomass Utilization, Limits of 167

has undergone is the breakdown of the fibrous material it C. Pyrolysis
contains, making it a less effective agent for the control of
Air-dried wood or other biomass heated in the absence of
soil erosion.
oxygen can be converted into oil, gas, and other valuable
In contrast, when biomass is directly burned as a fuel,
fuels. The biomass feedstock, before it is fed to the pyrol-
both nitrogen and other nutrients are lost to the atmo-
ysis reactor, must be ground or shredded into smaller than
sphere. The nitrogen in the biogas slurry (for the 146 t/yr
14-mesh size units. Flash pyrolysis takes place at 500 C
◦
amounts) would amount to about 3.7 t/yr. This has an en-
and under high pressure (101 kPa). After processing the
ergy value of 77 million kcal and market value of $2293.
solid char is separated from the fluids produced in a cy-
Then if the nitrogen value and the gas value combined,
clone separator. The char is then used as a heating source
the return for such a system is approximately $2388. The
for the reactor.
nitrogen fertilizer value of the processed manure makes
Using dry municipal refuse, the resulting products from
it worthwhile as a biogas source rather than burning it as
a kilogram of biomass are water, 10%; char, 20% (en-
a primary fuel cakes. Based on this, each laborer would
ergy content is about 4500 kcal/kg); gas, 30% (energy
receive about 60c / per hour for his work. 3
content is 3570 kcal/m ); and oil, 40% (energy content
The total amount of manure produced annually in the
is 5950 kcal/kg). Other investigators have reported up to
United States is about one billion tons. It would be an
50% oil production. This gas and oil can be reprocessed,
achievement to manage to process even half of this in
cleaned, and utilized in internal combustion engines.
biodigesters. Due to low net yield of energy, as described,
The oil and gas yield from a rapid processing pyrolysis
even 500 million t of manure, with gas produced at 28% ef-
plant is about 37% or about 2.7 kcal return per kilocalo-
ficiency,wouldprovideenergyforapopulationof270mil-
rie invested. Since the plant analyzed in the study was
lion Americans of 0.0076 kW per person per year. This
processing city wastes, there was no energy or economic
represents only 0.0008% of present net energy use.
charge for biomass material. However, if tropical dry-
wood is used for pyrolysis about 5 kcal of wood is required
B. Gasification to produce 1 kcal of oil.
The gas from a gasifier-pyrolysis reactor can be fur-
Biomass wood with less than 50% moisture can be heated
ther processed to produce methanol. Methanol is useful
in the presence of air and gasified. The gas produced can be
as a liquid fuel in suitably adjusted internal combustion
used to run internal combustion engines and also used as a
engines.
gas fuel and for other purposes. When used in the internal
Employing pyrolysis in a suitably large plant to produce
combustion engine, the gas must be cleaned thoroughly
methanol would require at least 1250 t of dry biomass per
as the several chemical contaminates it contains corrode
day. Based on tropical dry-wood, about 32 kcal of wood
engines and reduce its efficiency.
is needed to produce 1 kcal of methanol (or 1 t of wood
A kilogram of air-dried biomass will produce approx-
yields 14 l of methanol). A more recent study reports that
imately 2000 kcal of clean gas which can generate about
1 t of wood yields 370 l of methanol. In either case, more
0.8 kWh of net power electricity. The low heating value of
than 150,000 ha of forest would be needed to supply one
the gas-air mixture in a gasoline engine results in derating
plant. Biomass generally is not available in such enormous
the engine by 30–40%. This problem can be overcome by
quantities from extensive forests and at acceptable prices.
supercharging the engine. Using the gas as a mixture in a
If methanol from biomass was used as a substitute for
diesel engine results in derating the engine by only 10%
oil (33 quads) in the United States, about 1000 million
because of its high excess in the gas-air ratio. However,
hectare of forest land per year would be needed to supply
the diesel engine will require a small amount of diesel fuel
the raw material. This land area is much greater than the
for ignition.
162 million ha of United States cropland now in produc-
Although gasifier units can be relatively simple for
tion. Although methanol production from biomass may be
small-scale operations designed, large-scale systems are
impracticalbecauseoftheenormoussizeoftheconversion
most efficient. Thus, about 11.4 kcal of woodfuel is re-
plants, it is significantly more efficient than ethanol pro-
quired to produce 1 kcal of gas. If the gas is cleaned, then
duction using corn based on energy output and economic
the net return is diminished. The input : output results in
use of cropland.
an energy return in terms of wood to gas of 1 : 0.09. The
equipment for cleaning the gas is expensive and uneco-
nomical for use in rural areas, especially in developing
D. Vegetable Oil
countries. In addition to using the produced gas for inter-
nal combustion engines, it may be utilized as feedstock Processed vegetable oils from sunflower, soybean, rape,
for various chemical products. and other plants can be used in diesel engines. One major