Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 204

Figure 11.11. Maximum theoretical algal production. (a) Average daily biomass productivity as a function
of lipid content; (b) Curves of constant annual oil yield as a function of lipid content and
average biomass productivity, where 1000 gal/acre/y = 1531.9 L/ha/y. Vertical line in (b)
represents data derived from (Weyer et al ., 2010). The gray box represents a rough envelope
of current production capabilities. The remainder of the data is adapted from Cooney et al .
(2011), reprinted with permission from Elsevier, Inc.

and ash content. Assuming that the facility would grow algae 365 days a year (another optimistic
assumption!), Figure 11.11a depicts the average maximum biomass yield per day, which ranges
from 143 g/m 2 /d (lipids

100%) to 63 g/m 2 /d (lipids

0%). In these
extreme cases, there are no proteins or carbohydrates produced, so the limits are not physically
realistic, but in between those limits, all components (lipids, proteins carbohydrates, and ash)
are represented. Interestingly enough, note that there is a tradeoff between lipid production and
biomass yield. We will have more to say about this later. In an economic sense, whether or not a
given number for lipid or biomass production is “good” depends on the value of the lipids and
other co-products from the biomass, as well as the cost of oil extraction and refinement, and any
other processing of co-products that is needed.
Assuming that 100% of the oil can be recovered from the biomass, the oil yield is the product
of the biomass yield and the lipid fraction (i.e., the lipid content on a percentage basis divided
by 100). Figure 11.11b shows a family of curves representing annual oil yield that relates lipid
content to biomass productivity. Weyer et al .'s (2010) annual oil yield of 4350-5700 gal/acre/y at a
fixed 50% lipid content corresponds to an average biomass yield of about 21-27 g/m 2 /d in Cooney
et al .'s (2011) figure, shown as a vertical gray line. The gray box in Figure 11.11b represents
an envelope that encompasses a generous estimate of current capabilities on a commercial scale.
Some reviews are able to critically evaluate the overly optimistic assumptions that are used in
projections of algal oil output (Li et al. , 2008), based on understanding of the growth process.
Others simply report the bloated figures without much thought. These studies on the absolute
upper limit that is physically possible can help assess claims that seem too good to be true.
Most algae can grow autotrophically (using photosynthesis and organic compounds to create
food, also called phototrophically), heterotrophically (using complex organic compounds such
as sugar for food without the need for sunlight), or mixotrophically (a combination of growth
regimes). Contamination is a concern for heterotrophic growth since opportunistic bacteria also
grow well on organic carbon. The type of growth regime can significantly affect the lipid (oil)
composition. Under heterotrophic growth in the dark, the lipid content of Tetraselmis shifted
dramatically toward lower carbon number and increased saturation. The abundance of palmitic
acid (C16:0) increased from ∼ 20% in mixotrophic/phototrophic growth to 79% for heterotrophic
growth. The growth rate was somewhat slower, with a doubling time of 22 h for phototrophic and
25 h for heterotrophic growth (Day and Tsavalos, 1996). Figure 11.12(a) shows the effect of pho-
totrophic versus heterotrophic growth regime for Dunaliella tertiolecta (Tang et al. , 2011). Less

= 0%, ash = = 100%, ash =