In HTL, water is an important reactant and catalyst, and thus the biomass can be directly converted without an energy-consuming drying step, as in the case of py­rolysis (Bridgwater, 2004). As hot compressed liquid wa­ter approaches its thermodynamic critical point (Tc = 373.95 °C, Pc = 22.064 MPa), its dielectric constant decreases due to a decrease in hydrogen bonding be­tween water molecules (Figure 10.11). At these condi­tions, water is still in a liquid state, and has a range of exotic properties very different from liquid water at room temperature. Among them is increased solubility of hydrophobic organic compounds, such as free fatty acids (Holliday et al., 1997). Subcritical water can also sustain acid and base ions simultaneously and promotes radical-driven chemistry. These properties make subcrit­ical water an excellent medium for fast, homogeneous and efficient conversions of algal organics to biocrude. But this technology is not without challenges—the solu­bility of some salts in the reacting medium decreases significantly leading to excess precipitate in the system. Salts present in the HTL process are typically subdi­vided into two categories: Type I and Type II. Type 1 salts, such as NaCl, still exhibit relatively high solubility at subcritical conditions. Type 2 salts such as Na2SO4, on the other hand, have very limited solubility at these con­ditions (Hodes, 2004). If Type II salts are present in the

FIGURE 10.11 The critical point of water. (For color version of this figure, the reader is referred to the online version of this book.)

reaction medium, the decreased solubility can lead to what’s known as "shock precipitate" which can adsorb onto the walls of processing equipment causing fouling and eventually blockage. Technologies designed to remove or reduce salts from the production stream are currently being evaluated (Marrone, 2004).

HYDROTHERMAL CATALYTIC
LIQUEFACTION

The principal role of HTL is to fractionate organic macromolecules into simpler molecular units that can then be further upgraded to produce specific liquid fuels. The HTL environment promotes the hydrolytic cleavage of ester linkages in lipids, peptide linkages in proteins, and glycosidic ether linkages in carbohydrates. The speed and efficiency of these cleavage reactions can be improved by the addition of catalysts to the reaction medium. Catalysts are generally classified as homoge­neous and heterogeneous. In chemistry, homogeneous catalysis is a sequence of reactions that occur when a catalyst is codissolved in the same phase as the reac­tants. The most reported homogeneous catalyst for HTL processing of microalgae is Na2CO3 (Tekin, 2013; Zhang et al., 2013). While it has been reported that the addition of Na2CO3 to the HTL process increases the overall biocrude yield from microalgae, others have re­ported that Na2CO3 negatively impacts yields derived from lipids or proteins, but improves yields of precur­sors derived from carbohydrates (Biller et al. 2011). The effects of other homogeneous catalysts (e. g. KOH, HCOOH, and CH3COOH) on HTL of microalgae have been examined and ordered according to effectiveness Na2CO3 > CH3COOH > KOH > HCOOH. For HTL
processing of microalgae, heterogeneous catalysts may provide a more attractive option than homogeneous cat­alysts because heterogeneous catalysts can be more easily separated from the reaction products. Further, the yields of HTL biocrude using heterogeneous catalyst have been reported to be as high as 71% (Zhang et al., 2013).