TABLE 4-8 Range in Energy Requirements by Process Stage for Open-Pond Cultivation of Algae to Produce Biodiesel (Plus Coproducts)


Production Stage

Energy Requirement
(MJ/MJ Biodiesel)


Nutrients (fertilizer + CO2)

0.2-1.6

Cultivation

0.04-3.14

Separation

0.01-0.26

Extraction

0.19-0.51

Conversion

0.03-0.22


SOURCE: Liu et al. (2011). Reprinted with permission from Elsevier.

from several studies (Lardon et al., 2009; Clarens et al., 2010; Jorquera et al., 2010; Sander and Murthy, 2010; Stephenson et al., 2010; Campbell et al., 2011).

The high end of the range of energy requirements for nutrients corresponds to the use of virgin fertilizers and industrial CO2 in production. Cultivation consumes energy primarily for mixing and pumping water. Murphy and Allen (2011) suggested that energy needs for water management could be substantially higher than current LCA studies indicate. Extraction is generally assumed to be via hexane solvent and conversion to biodiesel via transesterification.

The treatment of the energy credits for coproducts is critical in the energy balance of algal biofuels. For production of bio-electricity, the energy credit per megajoule of biodiesel ranges from 0.3-1.3 megajoule/megajoules biodiesel (Lardon et al., 2009; Stephenson et al., 2010; Campbell et al., 2011). Sander and Murthy (2010) assume that residual biomass from algae production substitutes for corn used in ethanol production, yielding an energy credit for the coproduct of 10.7 megajoules/megajoule biodiesel.

4.4.3 LCA Issues Related to Algal-Lipid Processing

Pathways described in Chapter 3 describe two different ways of converting crude algal lipids to liquid fuels. Both yield fuels suitable for use in diesel applications. The majority of the published LCAs assumed the production of FAME diesels, which are less desirable than “drop-in” fuels because of FAME’s incompatibility with existing infrastructure for petroleum-based fuels. Given the lack of LCA work on green diesel products from algae, differences in energy use between FAME and green diesel are addressed by analogy with conventional diesel processing. A number of studies of conventional diesel processing that have been reviewed allow comparisons to be made (Kalnes et al., 2009).

Comparison between seed oil-derived diesel fuels treated by esterification and by hydrotreating show that there is little difference in either the energy return or carbon emissions. These studies start with the same raw oils, meaning that the differences only reflect the processing to finished fuel. Figure 4-8 shows that these are nearly identical and, therefore, life-cycle impacts will be similar between esterification processes and hydrotreating.

4.4.4 Opportunities for Mitigation

Keeping other factors constant, increasing the productivity of algal growth drives down energy use for cultivation and harvesting. This said, if achieving higher productivity involves major process changes, such as using photobioreactors instead of raceways, the



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