Analysis of Microalgal Biorefineries for Bioenergy from an Environmental and Economic Perspective Focus on Algal Biodiesel - Biotechnological Applications of Microalgae - page 123

rates of nutrient-replete growth are combined with high lipid content of nitrogen
limitation to calculate productivity—such productivities are typically unobtainable.
These examples highlight the importance of data quality and the challenges found
owing to the absence of reliable production data. Clearly, sensitivity analysis forms
a key component of these studies. Such sensitivity analyses are presented by, among
others, Stephenson et al. (2010), Richardson et al. (2012b), Williams and Laurens
(2010), and Norsker et al. (2011).
9.3.2 a lGal b ioreaCtors
In most studies performed to consider the environmental burden of microalgal
biodiesel, the key contributors to the algal cultivation process have been identi-
fied across raceway ponds and closed photobioreactors. Across all reactor configu-
rations, the major contributions to environmental burden in terms of net energy
ratio, abiotic depletion, and GHGs were incurred from the energy requirement for
mass transfer and mixing in the reactor, as well as the energy requirement asso-
ciated with the provision of combined nitrogen for cell growth (Lardon et al.,
2009; Batan et al., 2010; Stephenson et al., 2010; Richardson et al., 2012b). These
requirements are sensitive to algal biomass concentration, lipid content, and algal
productivity. Algal productivity and concentration were the most influential (e.g.,
Stephenson et al., 2010; Razon and Tan, 2011), owing to their impact on system
volume, influencing both energy input for mixing and pumping, and the amounts of
nutrients required, as previously demonstrated in other microbial systems (Harding
et al., 2008; Harding et al., 2012).
Studies by Jorquera et al. (2010), Stephenson et al. (2010), and Richardson
et al. (2012b) compared selected types of reactors. Jorquera et al. (2010) compared
horizontal tubular reactors, flat-plate reactors, and raceway ponds using a basis
of 100 tons algal biomass per year. The closed photobioreactors provided higher
biomass concentrations, and higher volumetric and areal productivities than ponds.
Correlated with this, the raceway ponds had an increased land requirement. However,
their energetic requirements were significantly lower than the closed photobioreac-
tors. Under their operating conditions, the NER of the horizontal tubular reactor
illustrated that it was not feasible in terms of either oil or biomass production (NER
of 0.07 and 0.20, respectively). The NER of the flat-plate reactor was 54% of that
for the open raceway for oil production (NER of 1.65 and 3.05, respectively) and for
production of algal biomass (NER of 4.51 and 8.34, respectively).
Stephenson et al. (2010) compared the performance of the integrated algal
biodiesel process, from cradle to combustion, using the open raceway and tubular
airlift photobioreactor and a two-stage cultivation method to maximize lipid for-
mation under nitrogen starvation in the second stage. In their system, the reactor
was the dominant contributor to energy consumption. The design of the tubular
airlift photobioreactor required energy input an order of magnitude greater than
the raceway on an energy equivalence basis, despite its higher productivity. While
85% of the energy requirement of the tubular reactor was attributed to operation
and the remainder to reactor manufacture, the latter exceeded the total GWP of the
raceway system.