Page 326 - Advances in Eco-Fuels for a Sustainable Environment

282 Advances in Eco-Fuels for a Sustainable Environment

The ability of water to act as a catalyst and a suitable reacting medium is due to its
ability to alter its characteristics under different conditions. During the hydrothermal
liquefaction process, the polarity of water and its dielectric constant will change as
reaction conditions approach the critical point of water [29]. The reduction in the
dielectric constant of water occurs due to the weakening of the intermolecular hydro-
gen bonds present between water molecules. As the supercritical condition is
approached, the dielectric constant of water reduces until the dielectric constant
becomes approximately 20, which is comparable to the mean dielectric constant
20.7 of acetone under a pressure of 1atm and a temperature of 25°C [30]. This change
in the dielectric property of water enables the hot compressed water to behave as a
nonpolar solvent [31, 32]. As a nonpolar solvent, the solubilization of macro mole-
cules and compounds released from the biomass matrix is enhanced [31, 32]. The sol-
ubilization of the organic molecules is followed by a range of complex reactions that
may include hydrolysis, retro-aldol condensation, isomerization, and dehydration
[33, 34], depending on the nature of the biomass. For instance, because digestate is
largely composed of carbohydrates (41.4wt% dry basis) [35], it is expected that
hydrothermal transformations will be dominated by the efficient cleavage of glyco-
sidic bonds existing between anhydro-glucose units present in the carbohydrate
molecules [36]. This suggests that hydrolytic reactions will play a crucial role during
the HTL of the digestate feedstock. It was also suggested that the nature of the prod-
ucts generated would vary with the pH value of the reacting mixture [36]. For instance,
it was reported that biocrude products generated during hydrothermal liquefaction
were mainly carboxylic acids or 5-(Hydroxymethyl)-furfural, depending on whether
the reacting medium is acidic or basic, respectively [37, 38].
In summary, this chapter will therefore investigate the appropriate processing con-
ditions for optimal biocrude yield using the digestate byproduct of an anaerobic diges-
tion process undertaken in our laboratory. Under the conditions for enhanced biocrude
production, the yields of the accompanying insoluble solids (biochar), the soluble
solids dissolved in the post-HTL water, and the gas phase products were also deter-
mined. A comparative assessment of the economics of digestate processing via the
proposed HTL based one-step pathway for digestate sterilization and useful post-
HTL water, biochar, and biocrude generation and the existing digestate processing
pathway for useful nutrient recovery in the solid fraction, prior to secondary water
treatment, will also be undertaken.

10.2 Materials and methods

10.2.1 Feedstock characterization
As stated earlier, a uniformly mixed high moisture digestate sample produced in our
lab [35] was utilized in this study. The digestate was produced from the anaerobic
digestion of a substrate mixture wet hydrolyzed dissolved air flotation sludge from
work described elsewhere in Ref. [39] and stockyard waste (from animal pens). Some
properties of the digestate utilized in the study are presented in Table 10.2.