For the purpose of this discussion, the reference pathway assumes that microalgae are cultivated in saline water in an open raceway pond. Algae are harvested and lysed to release lipids, which are collected for further processing into green diesel1 (also called renewable diesel), a drop-in hydrocarbon fuel (Figure 3-3).
Under the reference pathway, lipid-producing species are selected, and recovered lipids are converted by known chemical processes to yield hydrocarbon fuels. The chemical structures of these hydrocarbon fuels are oxygen-free and appropriate for use in aviation and as on-road fuels. Lipid recovery in the most often described processes requires that the cells be destroyed (lysed) and cell membranes ruptured to release intracellular oils. The algal triacylglycerol can be processed in several ways. Similarly, the remaining cellular mass can have different uses. Two of the uses being considered are recycling back to algae cultivation or selling as a coproduct. For the reference case, the biomass is treated anaerobically to produce biogas for power generation, with the effluent being returned to the culture to provide needed nutrients. According to Davis et al. (2011), this treatment recovers almost all of the biomass phosphate and much of the nitrogen (N) used during cultivation.
The reference pathway is further amplified in Figure 3-4, which shows the details of the processing steps. For this and subsequent figures, several conventions have been adopted. Yellow diamonds show the inputs to the system and orange the outputs. Green process steps represent those associated with algae cultivation, light blue with lipid collection, and
FIGURE 3-3 Reference pathway: Open raceway pond producing green diesel, a drop-in hydrocarbon fuel.
1 Green diesel is a product of hydrotreating of triglycerides.