Analysis of Microalgal Biorefineries for Bioenergy from an Environmental and Economic Perspective Focus on Algal Biodiesel - Biotechnological Applications of Microalgae - page 132

The effect of process variables and process selection on the costing is discussed
in Section 9.4.2, while the potential contribution of products other than biodiesel is
discussed in Section 9.4.3.
9.4.2 K ey p roCess C oMponents C ontributinG to C ost
It is well recognized that the production phase is most significantly affected by its
energy requirements. This is most marked for photobioreactors and contributes
significantly to costs. Norsker et al. (2011) explored these interactions. In the hori-
zontal tubular reactor, liquid circulation contributes to both capital costs through the
pump required and to energy costs. By reducing the linear velocity from 0.5 m s −1 to
the minimum value predicted of 0.3 m s −1 , a reduction in cost per kilogram of 25%
was achieved. Similarly, the aeration of the flat-plate reactor (or vertical tubular reac-
tor) contributes significantly to capital and operating costs. In the former, a reduction
in the aeration rate from 1 vvm (volume per volume per minute) to 0.3 vvm resulted
in a 48% reduction in cost. The contributions of mixing and mass transfer in the flat-
plate and tubular reactors were on the order of 52% and 30%, respectively.
Productivity significantly influences the cost of production. This is well demon-
strated by the increased productivity with increasing light intensity, as correlated
by Williams and Laurens (2010). Areal productivity increased from some 10 g
m −2 d −1 at an irradiance of 15 MJ m −2 d −1 to 30 g m −2 d −1 on doubling the irradiance
to 30 MJ m −2 d −1 . The impact of improved productivity under improved irradiance
is illustrated by comparing the cost for production in Eindhoven (the Netherlands)
and Bonaire (Dutch Antilles). In the three cases analyzed (Norsker et al., 2011), the
cost decreased by 40% to 45% under conditions of increased illumination (sum-
mer high of 7,000 and 5,000 W-h m −2 d −1 , respectively; winter low of 4,500 and
<1,000 W-h m −2 d −1 , respectively, in Bonaire and Eindhoven).
The provision of CO 2 can significantly impact the costing, based on whether
the CO 2 is provided “free” as a by-product of an adjacent process or requiring its
purchase as a compressed gas (Williams and Laurens, 2010). Stephenson et al.
(2010) further considered the compression energy, with associated costs, based on
the CO 2 concentration in the gas stream.
Williams and Laurens (2010) noted that their energy costs (typical of the US envi-
ronment) were some sixfold higher than those estimated by an analysis conducted in
British Columbia, Canada, where hydroelectric power was used. This highlights the
potential for the use of renewable energy resources in conjunction with algal production.
9.4.3 p otential e arninGs FroM b y -p roduCts
Potential by-products from the production of algal biodiesel include the capture
of carbon, the generation of biogas through anaerobic digestion, the generation of
bioethanol through fermentation of carbohydrates, the production of feeds for aqua-
culture, the production of animal feeds rich in protein and carbohydrates (~60%
protein), and the production of a high-grade, protein-rich material (~90% protein).
In the case of fermentation or digestion of the algal cell debris, the recycling of N and
P media components can be used to decrease media costs.