FIGURE 3-20 IH2 process schematic.
SOURCE: Adapted from Marker et al. (2010).
Many other processes for extracting fuel from microalgae are being discussed and investigated. Lack of published or available data on key sustainability metrics means that little can be said about the sustainability attributes of other potential pathways relative to pathways discussed earlier. In addition to the pyrolysis route described above, microalgal systems that use thermochemical transformation techniques to process whole algal cells are beginning to be tested. These systems appear to have energy requirements similar to other production techniques in which the cells are killed to harvest product. Clearly, thermochemical pathways can manufacture a range of fuel products. Whether the subsequent energy use in processing offers advantages from an energy or emissions perspective is unclear. These whole-biomass systems offer the advantage that cultivation is not limited to oleaginous species. Species can be grown at maximum carbon fixation rates to feed processes that retain high fractions of the fixed carbon in their final fuels. Examples of whole organism conversion technologies include (Gouveia, 2011; Hatcher, 2011):
• Fermentation of algal biomass to yield alcohols or hydrocarbons;
• Gasification and syngas conversion to alkanes, alcohols, or aromatics (through methanol and subsequent conversion);
• Gasification and syngas conversion to alcohols by conventional catalysis;
• Gasification and syngas conversion to alcohols by syngas fermentation;
• Anaerobic digestion to methane (making no liquid fuel); and
• Hydrous pyrolysis (Hatcher, 2011).
These techniques are not widely used and could be put into wider practice (Gouveia, 2011). High water content is not a desirable characteristic of feedstock for fuels. The