Environmental Engineering Reference
In-Depth Information
soybean oil can account for up to 75% of the final cost per gallon of biodiesel. This
has resulted in crops being sold as fuel crops, reducing the food supply and leading
to an increase in food prices around the world.
To help with this issue, many oil-bearing non-edible plants have been investi-
gated for the production of biodiesel. These are mainly tree species that can grow
in harsh environments, such as
Jatropha curcas, Pongamia pinnata, Castor, Mohva,
Neem, Sal
,etc.
Jatropha curcas
has the most significant potential due to its char-
acteristics and growth requirements [16, 18]. It requires very little fertilizer and
water (as little as 25 cm a year), is pest resistant, and can survive in poor soil
conditions such as stony, gravelly, sandy or saline soils. Most important, it is fast
growing, and can bloom and produce fruit throughout the year with a high seed
yield. Optimized production has been found to yield an average of more than 99%
of Jatropha biodiesel [19], which has comparable fuel properties to that of diesel
from petroleum. It is expected that some varieties of Jatropha can produce as much
as 1,600 gal of diesel fuel per acre-year compared to the wild variety that produces
about 200 gal/acre-year [20]. Jatropha trees can capture four tons of carbon dioxide
per acre and the fuel emits negligible greenhouse gases.
There is a growing interest in using algae as a feedstock for biodiesel production
within the United States. Algae have become an appealing feedstock due to their
aquatic environment providing them an abundant supply of water, CO
2
, and other
nutrients. This results in a photosynthetic efficiency that is significantly higher than
the average land based plants [21]. However, the power required to use artificial
lighting to grow an aquatic species, such as microalgae, for the production of a
biofuel would greatly reduce the overall efficiency of the process [22]. As the algae
convert carbohydrates into triglycerides, the reproduction rate slows down so that
the higher oil storing strains of algae reproduce at a much slower rate than lower oil
storing strains [23]. This was shown by the Department of Energy's (DOE) Aquatic
Species Program, which found the overall yield to decrease as the algae's oil storage
increased.
Recently, Vasudevan and Briggs [21] summarized research on biodiesel pro-
duction in a review article. According to them, a crude analysis of the quantum
efficiency of photosynthesis can be done without getting into the details of the
Calvin cycle; rather simply by looking at the photon energy required to carry out
the overall reaction, and the energy of the products. In general, eight photons must
be absorbed to split 1 CO
2
and2H
2
O molecules, yielding one base carbohydrate
(CH
2
O), one O
2
molecule, and one H
2
O (which, interestingly, is not made of the
same atoms as either of the two input H
2
O molecules.)
With the average energy of “Photosynthetically Available Radiation” (PAR) pho-
tons being roughly 217 kJ, and a single carbohydrate (CH
2
O) having an energy
content taken to be one-sixth that of glucose ((CH
2
O)
6
), or 467 kJ/mole, we can cal-
culate a rough maximum efficiency of 26.9% for converting captured solar energy
into stored chemical energy. With PAR accounting for 43% of incident sunlight
on earth's surface [24], the quantum limit (based on eight photons captured per
CH
2
O produced) on photosynthetic efficiency works out to roughly 11.6%. In real-
ity, most plants fall well below this theoretical limit, with global averages estimated
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