Environmental Engineering Reference
In-Depth Information
These algae have chlorophylls
a
and
c
and several xanthophylls. The most common
form of reproduction in dinoflagellates is by cell division, but sexual reproduction
has also been observed.
a
lgal
B
iomass
Algal biomass contains three main components:
•
Carbohydrates
•
Protein
•
Natural oils
Biodiesel production applies exclusively to the natural oil fraction, the main product
of interest to us in this section. The bulk of natural oil made by oilseed crops is in
the form of triacylglycerols (TAGs), which consist of three long chains of fatty acids
attached to a glycerol backbone. The algae species of concern can produce up to
60% of their body weight in the form of TAGs. (Recall that the species of concern,
the oil producers, are
Ulva
,
Botryococcus braunii
,
Chlorella
,
Dunaliella tertiolecta
,
Gracilaria
,
Pleurochrysis carterae
, and
Sargassum.
) Thus, algae represent an alter-
native source of biodiesel, one that does not compete with the exiting oilseed market.
Algae can produce up to 300 times more oil per acre than conventional crops,
such as rapeseed, palms, soybean, or
Jatropha
(Christi, 2007). Moreover, algae has
a harvesting cycle of 1 to 10 days, permitting several harvests in a very short time
frame, a strategy quite different from that for yearly crops. Algae can also be grown
on land that is not suitable for other established crops, such as arid land, land with
excessively saline soil, and drought-stricken land. This minimizes the issue of taking
away pieces of land from the cultivation of food crops (Schenk et al., 2008). Algae
can grow 20 to 30 times faster than food crops (McDill, 2009).
Algae can be produced and harvested for biofuel using various technologies. These
include photobioreactors (plastic tubes full of nutrients exposed to sunlight), closed-
loop (not exposed to open air) systems, and open ponds. For the purposes of illustra-
tion (even though there are many objectors and dissenters to open-pond systems), the
open-pond configuration of algae farms is discussed here, because the open-pond
raceway system is a relatively low-cost system and is an easily understood process.
Open-Pond Algae Farms
Algae farms consist of open, shallow ponds in which some source of waste carbon
dioxide (CO
2
) can be bubbled into the ponds and captured by the algae. As shown
in
Figure 5.8
,
the ponds in an algae farm are “raceway” designs, in which the algae,
water, and nutrients circulate around a racetrack. Paddlewheels provide the flow. The
algae are thus kept suspended in water. Algae are circulated back up to the surface
on a regular frequency. The ponds are kept shallow because of the need to keep the
algae exposed to sunlight and the limited depth to which sunlight can penetrate the
pond water. The ponds operate continuously; that is, water and nutrients are con-
stantly fed to the pond while algae-containing water is removed at the other end.
Some kind of harvesting system is required to recover the algae, which contains
substantial amounts of natural oil.
