FIGURE 3-16 Water consumption by photosynthesis in the production of algal biomass for fuels.
NOTES: In the case of a stable culture that continuously secretes products for collection, there are no residual biomass carbons. In contrast, water was consumed fixing the carbons in both the residual biomass and the fuel component in the case where algal cells are destroyed to collect lipids or to be processed to fuel.
carbon average molecular weight of the fuel component and the fuel component mass ratio in the dry biomass. Using a closed photobioreactor to make ethanol from cyanobacteria consumes less than 1 liter of water per liter of ethanol for photosynthesis. As discussed in the previous chapter, water consumption through photosynthesis for lipid-forming algae is at least three times higher.
Thermochemical pathways for processing biomass to fuels have garnered interest as the focus shifts away from production of alcohols and esters and toward production of “drop-in” hydrocarbon fuels (Figure 3-18; Huber and Dale, 2009). Figure 3-19 shows the detail input and output of a thermochemical pathway—pyrolysis. Anaerobic heating of virtually any biomass causes thermal degradation that begins to fractionate the biomass into gaseous, liquid, and solid components (Mohan et al., 2006). Subsequently, liquid fractions can be upgraded by hydrotreating to yield a hydrocarbon fuel. The final fuel products are compatible with existing petroleum refinery infrastructure. An advantage of thermochemical technologies is that they are largely feedstock agnostic and can accept any type of biomass, including biomass of aquatic microalgal and macroalgal species. Pyrolysis is the only process discussed that easily accepts macroalgal species.
Lipid-producing microalgae are not required for fuel production in this pathway. Algal strains or mixed cultures are selected for their high biomass productivity and ability to fix