Page 324 - Advances in Eco-Fuels for a Sustainable Environment

280 Advances in Eco-Fuels for a Sustainable Environment

hydrophobicity of biocrude and the surface properties of biochar, such as porosity and
particle size of the product streams [11]. The comparative simplicity of the proposed
one-step digestate processing technology relative to the existing digestate processing
technology is elucidated in Fig. 10.1.
Also, as stated earlier, the utilization of the HTL process enables not just digestate
sterilization but also the generation of several product streams from the digestate
byproduct. These product streams include the energy-dense and hydrophobic biocrude,
post-HTL water containing soluble salts, and an insoluble solid residue called biochar
[12]. A small mass of a gas phase product is also generated [12]. This gas product is
largely composed of unwanted CO 2 gas [12]. The energy-dense hydrophobic product
could directly serve as a liquid fuel and also possibly serve as a replacement for petro-
leum crude if upgraded further via hydrogenation because it is essentially composed of
a mixture of useful biochemicals [12]. Although the post-HTL water (>99% wt water)
may contain some useful soluble compounds such as phosphate salts, the typical low
concentration of these soluble compounds in the post-HTL water does not make their
recovery economically feasible [13]. This is largely due to the fact that techniques for
the recovery of such dissolved salts from water will require the incorporation of some
sort of costly dewatering step, such as the drying operation to reduce water content.
Researchers have therefore suggested and subsequently demonstrated the feasibility
of utilizing the post-HTL water as a feedstock for secondary H 2 production via micro-
bial electrolysis or as a nutrient source for microalgae cultivation [14–16]. Alterna-
tively, the post-HTL water may be recycled in continuous systems to the HTL
reactor to further increase biocrude yield [17].
The solid HTL biochar product is another useful product that may be utilized in soil
remediation, carbon sequestration, and as adsorbents for the removal of impurities
such as phenol, heavy metals, and dye from wastewaters [18]. The gas phase product
is usually vented due to its typically small mass and the low concentration of any valu-
able gaseous components [12, 19]. It is therefore crucial at this juncture to provide
clarity with respect to the negative environment outcomes that may arise from the
venting of a gaseous stream that typically contains 95% (v/v) of CO 2 gas [19], relative
to the negative environmental outcomes that may occur from emissions associated
with the direct utilization of digestate as a fertilizer.
The negative environmental outcomes associated with the venting of the gaseous
product stream from the HTL-digestate process are solely due to the recognition that
its major component, CO 2 , is a greenhouse gas. It is important to clarify that in the con-
text of the present study, the possible negative environmental effects related to gaseous
emissions from the direct application of possible aqueous NH 3 containing digestate to
agricultural soils as a cheap fertilizer will outweigh the associated negative environmen-
tal effects of gaseous CO 2 emissions from the HTL digestate processing operation. This
is because although a mass of CO 2 is produced during the HTL process, the application
of digestate on agricultural soils may lead to the generation of NO and N 2 Ogases [20]
due to the action of nitrifying bacteria on any residual aqueous NH 3 that may be present
in the digestate [21]. These NO and N 2 O gases cannot be generated during HTL pro-
cesses because the oxidation reactions that occur are mainly decarbonylation (genera-
tion of CO), decarboxylation (generation of CO 2 ), and dehydration (removal of H 2 O)