Analysis of Microalgal Biorefineries for Bioenergy from an Environmental and Economic Perspective Focus on Algal Biodiesel - Biotechnological Applications of Microalgae - page 133

Williams and Laurens (2010) considered the scenarios of biogas formation,
preparation of animal feed, and preparation of high-grade, protein-rich material.
They highlighted the uncertainty and process specificity of costing these options and
noted the requirement for improved knowledge in this area. The 60% protein feed
was valued at US$750 tonne −1 based on FAO (Food and Agriculture Organization)
statistics. Relating it to the value of soya meal containing 45% protein, a value of
US$500 tonne −1 was proposed. In comparison, the high-grade, protein-rich material
was valued at US$900 tonne −1 . Biodiesel was estimated at a value of US$125 bbl −1 .
Using this estimate, positive scenarios were found at protein feed values of
US$350 tonne −1 and above, and protein-rich extracts at US$600 tonne −1 and above.
Under the conditions used, anaerobic digestion was not cost effective; however,
the high energy requirement to maintain the digester temperature was a result of the
temperate environment, and revised analysis is required for warmer climates where
the performance of biodigesters is well documented.
9.5 KEY FOCAL AREAS FOR IMPROVING ENVIRONMENTAL
AND ECONOMIC SUSTAINABILITY
The motivation to overcome the challenges with respect to environmental and
economic sustainability of microalgal culture for biodiesel production and other
renewable products stems from the significant advantages of the microalgal sys-
tem as a biomass source. These include the potential to use nonarable land for
microalgal cultivation, the homogeneity of the biomass formed and the ability to
process all components, the much-improved oil production per unit area (15 to
300 times greater), the higher growth rate, and the photosynthetic efficiency of
microalgae compared with terrestrial plants (up to tenfold increase) (Chisti, 2007;
Schenk, 2008; Rodolfi et al., 2009). Freshwater, seawater, brines, and wastewater
are all potential water sources for algal growth (Vasudevan and Briggs, 2008).
CO 2 uptake by the autotrophic algae enables CO 2 cycling through uptake for bio-
mass generation and release on fuel combustion. Furthermore, the multiple energy
forms attainable from microalgae span liquid fuels, and heat and electricity genera-
tion, enabling ongoing support of existing technologies while developing a reduced
carbon economy. On attaining an energy economy in which dependence on carbon
combustion is reduced, the technology lessons learned through achieving environ-
mental and economically sustainable algal biomass will readily be transferred to
the production of carbon-based commodities with simultaneous carbon sequestra-
tion or cycling.
The NER and LCA studies conducted to date have highlighted the great sensi-
tivity of the GWP, fossil fuel requirements, and NER on the productivity of algal
biomass and of algal oil attainable in the algal cultivation process. While maxi-
mum specific growth rates and lipid content are partly defined through the algal
species selected, culture conditions may be used to enhance these through improved
light supply, mass transfer, and mixing. The energy requirement of the bioreactor to
achieve mixing and mass transfer is a major contributor to the energy requirement
of the integrated algal process, as is the CO 2 provision to the reactor. Based on the
volumetric concentrations attainable, pumping energies can be defined.