Page 12 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

P1: FJU Revised Pages
Encyclopedia of Physical Science and Technology EN002C-60 May 17, 2001 20:23

168 Biomass Utilization, Limits of

advantage of burning vegetable oils in a diesel engine is supply this amount of electricity is about the same as that
that the exhaust smells like cooking popcorn. However, required to supply food, housing, industry, and roadways
the energetics and economics of producing vegetable oils for a population of 100,000 people.
for use in diesel engines are negative. There are several factors that limit reliance on woody
Sunflower seeds with hulls have about 25.5% oil. The biomass. Some have proposed culturing fast-growing trees
average yield of sunflower seeds is 1560 kg/ha, and in in a plantation system located on prime land. These yields
terms of oil this amounts to 216 l of vegetable oil pro- of woody biomass would be higher than the average of
duced per hectare. This much oil has an energy value of 3 t/ha and with large amounts of fertilizers and freshwater
1.7 million kilocalories which appears promising. How- yields might be as high as 15 t/ha. However, this is un-
ever, the energy input to produce this yield of 1560 kg/ha realistic because this land is needed for food production.
is 2.8 million kcal. Therefore, 65% more fossil energy is Furthermore, such intensely managed systems require ad-
used to produce a liter of vegetable oil than the energy ditional fossil fuel inputs for heavy machinery, fertilizers,
potential of the sunflower oil. and pesticides, thereby diminishing the net energy avail-
A liter of vegetable oil sells for at least $2 whereas a able. In addition energy is not the highest priority use of
liter of gasoline at the pump today sells for 40c / per liter. forest wood, but rather for lumber for building and pulp.
There is no way that vegetable oil will be an economic The conversion of natural forests into plantations will
alternative to liquid fuels in the future. increase soil erosion and water runoff. Continuous soil
erosion and degradation will ultimately reduce the overall
productivity of the land. If natural forests are managed
E. Electricity
for maximal biomass energy production, loss of biodiver-
Although most biomass will continue to be used for cook- sity can be expected. However, despite serious limitations
ing and heating, it can be converted into electricity. With a of plantations, biomass production could be increased us-
small amount of nutrient fertilizer inputs, an average of 3 t ing agroforestry technologies designed to protect soil and
(dry) of woody biomass can be sustainably harvested per conserve biodiversity.
hectare per year, although this amount of woody biomass
has a gross energy yield of 13.5 million kilocalories (ther-
mal). The net yield, however, is lower because approx- IV. BIOMASS AND THE ENVIRONMENT
imately 33 l of diesel fuel per hectare is expended for
cutting and collecting wood for transport. This assumes The presence of biomass on the land protects not only
an 80-km roundtrip between the forest and the electric the land it covers, but also the natural interactions among
plant. The economic benefits of biomass are maximized all species that inhabit the ecosystem. Conversely, the re-
when the biomass is close to the processing plant. moval of biomass for all purposes, but most especially
In addition, a small amount of nitrogen fertilizer has to for energy production, threatens the integrity of the entire
be applied. For bolewood, 1 t contains about 15 kg of N. natural ecosystem.
Thus about 837,000 kcal is required for3tof bolewood.
The energy input : output ratio for the system is cal-
A. Soil Erosion
culated to be 1 : 6. The cost of producing a kilowatt of
electricity from woody biomass ranges from 7–10c / . This Once the biomass vegetation has been removed from the
is competitive with other electricity production systems land area and the land is exposed to wind and rainfall
that presently have an average cost of 6.9c / with a range of energy, erosion is a major threat. Land degradation by
5–13c / per kWh. Approximately 3 kcal of thermal energy soil erosion is of particular concern to agriculturists and
is expended to produce 1 kcal of electricity. foresters because the productivity of the soil is diminished.
Woody biomass could supply the nation with about 5 Too often soil erosion and the resulting degradation goes
quads of its total gross energy supply by the year 2050 unnoticed (note, 1 mm of soil weighs 15 t/ha). Soil refor-
with the use of approximately 112 million hectare (an mation is exceedingly slow. Under agricultural conditions,
area larger than the state of Texas). A city of 100,000 peo- approximately 500 years (range from 200 to 1000 years)
ple using the biomass from a sustainable forest (3 t/ha) are required to renew 2.5 cm (340 t) of topsoil. This soil
for fuel would require approximately 220,000 ha of forest formation rate is the equivalent of about 1 t/ha/yr. Forest
area, based on an average electrical demand of 1 billion soil re-formation is slower than in agriculture and is es-
kilowatthours (860 kcal = 1 kWh). More than 70% of the timated to take more than 1000 years to produce 2.5 cm
heat energy produced from burning biomass is lost in its of soil. The adverse effect of soil erosion is the gradual
conversion into electricity; this is similar to losses expe- loss of productivity and eventually the abandonment of
rienced in coal-fired plants. The forest area required to the land for crop production.