Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 206

with a shared evolutionary history were collected from the wild. This work suggests that such
communities may store significantly more solar energy as lipids than monocultures in photo-
bioreactors (Stockenreiter et al. , 2011). It would also be useful to know if naturally occurring
communities have other beneficial properties, such as better stability than monocultures.
Currently, open-pond design is based on deep familiarity and experience with such systems,
making it in some sense a black art. Is there a way to move toward the theoretical maximum in a
more scientific, cost-effective way?We know that in high-density cell cultures, the cells nearest to
the surface absorbmost or all of the available light (Chisti, 2007) due to self-shading. An engineer-
ing solution is to improve the vertical mixing in the system, so that more cells have access to light.
One design choice in pond construction is whether to use a rectangular cross-section, or an angled
cross-section with a wider top than bottom. Recent work at NASAGlenn Research Center showed
that the addition of passive mixing devices is more effective in providing access to light than the
cross-sectional geometry in dense suspensions. This knowledge permits the choice of channel
geometry to be based on other factors, such as ease of construction or maintenance. Other, more
sophisticated approaches are to examine in greater depth the stochastic effects of hydrodynamic
mixing on cell growth (Chait et al. , 2012). This requires better understanding of the time scales
that are relevant to industrial production. There is a great deal of research on the short time scales
associated with photosynthesis, such as the amount of time it takes a cell to absorb a photon, con-
vert it into food, and be ready to absorb another one, which is on the order of milliseconds. There
are similarly small hydrodynamic time scales associated with turbulence, as well as larger ones
linked to pond traversal. In current designs, the primary locations for vertical mixing occur at the
paddlewheel and at the circular bends at either end of the pond, so that time scales associated with
the unmixed state are on the order of tens of minutes. Algae that are near the surface will be con-
tinuously in the light during that stage, while the those near the bottom may be in the dark. There
are also photoadaptation time scales that appear to be on the order of hours. Exposure to too much
light can lead to photoinhibition. There are growth time scales with doubling time on the order
of days. The complex, species-specific interplay among all of these factors with the biokinetics
is an area that is ripe for exploration and may provide a scientific basis for effective pond design.
See DOEAlgal Biotechnology Program Roadmap for a good description of all aspects of algae
growth for biofuel (DOE, 2010); for background on basic photosynthetic processes (Nelson and
Yocum, 2006); for studies on bioprospecting/screening, see Araujo et al. (2011), Doan et al .
(2011), Gouveia and Oliveira (2009), Griffiths and Harrison(2009), Lee et al. (2011), Rodolfi
et al. (2009); for screening for biodiesel and sewage remediation see Sydney et al. (2011); for
algae and CO 2 remediation see Ho et al. (2011).
11.5 MANUFACTURING STAGES
Extraction processes from vegetable oils, algae and greases release oil that is high in triglycerides
and fatty acids with hydrocarbon subunits which are primarily in the range of C16-C18. The goal
of biorefining is to convert this green crude (also called “green diesel”) into a less viscous fuel
with better cold-temperature properties.
Biodiesel is defined as a fuel that is made up of mono-alkyl esters of long-chain fatty acids,
which is derived from vegetable oils or animal fats. In petroleum fuel, straight-chain, saturated
hydrocarbons are called n -paraffins. For biodiesel, paraffinic subunits contained in fatty acids are
combined with alcohols to form esters that can be blended with conventional diesel and used in
vehicles (section 11.5.2). This form of biofuel is referred to as FattyAcid Methyl Esters (FAME),
or equivalently Fatty Acid Alkyl Esters (FAAE). Transesterification reduces the viscosity of the
vegetable oil feedstock to a range that is acceptable for biodiesel, as shown in Figure 11.13.
For renewable jet fuel, the freeze point and cold-temperature properties must be improved
further. To manufacture Hydrotreated Esters and Fatty Acids (HEFA) fuel, the fatty acids and
triglycerides in the crude oils undergo a different set of reactions to yield CO 2 ,H 2 O and
long-chained n -paraffins. This is followed by a second reaction (hydrocracking) that breaks