Biomedical Engineering Reference
In-Depth Information
to conversion to biodiesel, using a natural gas powered dryer. Razon and Tan (2011)
attributed 48% of their energy requirement (310 MJ per tonne biodiesel) to drying,
following production of 1 kg FAME and 1.5 m
3
biogas using a raceway pond. Drying
of the biomass is only feasible where natural resources (e.g., solar drying) can be
used while preventing lipid oxidation (Lardon et al., 2009).
Furthermore, the requirement of cell disruption for product recovery has also
been considered. Razon and Tan (2011) demonstrated that the energy input to the
bead mill for disruption of
Haematococcus pluvialis
formed the greatest contribution
to the process energy required (>30%). Stephenson et al. (2010) estimated cell dis-
ruption by high-pressure homogenization to account for approximately 5 GJ tonne
−1
biodiesel formed and 320 kg CO
2
eq tonne
−1
biodiesel in GWP. The need for cell
disruption is a function of both the flowsheet selected and the algal species.
In a comparison of the hypothetical wet and dry processing routes for the trans-
esterification to biodiesel, the benefit of developing an effective wet processing route
for which a discrete cell disruption step and rigorous drying are not essential is
clearly demonstrated (Lardon et al., 2009). The energy requirement was reduced to
70% to 75% of the dry processing route while the energy recoverable through further
processing of the oil cake increased by 67% to 115%. Overall, the additional energy
recoverable was 0.6 to 1.1 MJ per 1 MJ biodiesel. Direct esterification in the presence
of water has been demonstrated under analytical conditions (Griffiths et al., 2010)
and requires further optimization for large-scale use.
9.3.6 i
MpaCt
oF
s
peCies
s
eleCtion
Richardson (2011) has demonstrated the importance of the selection of algal species
on the environmental impact of the biodiesel process. Factors to consider include
the specie's ability to scavenge light and CO
2
, growth rate, lipid content, cell size,
cell wall strength, digestibility, ability to settle without flocculation, as well as the
nitrogen content of the biomass. The combination of these influences the volume of
the process, the relative recovery of biodiesel to biogas, the selection of downstream
processing equipment, and the need to pretreat the algae prior to fermentation or
digestion.
9.3.7 t
he
b
ioreFinery
During the production of algal biodiesel, an algal cake remains and can form a
feedstock for further product formation. Potential commodity by-products include
an algal biomass suitable for animal feed, fermentation of the carbohydrate portion
of the algal biomass to ethanol (Sandler and Murthy, 2010), or anaerobic digestion
of the algal cake to biogas. Here we consider the co-production of algal biogas to
demonstrate the multi-product approach (Campbell et al., 2010; Stephenson et al.,
2010; Richardson et al., 2012). Stephenson et al. (2010) investigated two modes of cell
disruption and assumed that only disrupted cells were digested. While the anaero-
bic digestion of
Spirulina platensis
was similar in the presence and absence of cell
disruption, disruption was essential for
Scenedesmus,
which also exhibited signifi-
cant resistance to disruption. Collet et al. (2010) demonstrated anaerobic digestion
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