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Hydrothermal Processes in Subcritical Water
Different microalgae are not the same in producing oil through liquefaction.
Recent literature have shown a wide variety of performances in the production rate
of bio-oil and its quality [75–79,81–94]. The performance of HTL of microalgae has
been improved by the use of a catalyst such as Na 2 CO 3 among others [75–79,81–94].
Zhou et al. [79] examined the HTL of marine macroalgae Enteromorpha prolifera
in the temperature range of 220°C–320°C for 30 min and in the presence of 5 wt%
Na 2 CO 3 catalyst. The highest bio-oil yield of 23 wt% with higher heating value of
28–30  MJ/kg at 300°C was obtained. These numbers are smaller than what are
reported for microalgae [81]. The bio-oil contained ketones, aldehydes, phenols,
alkenes, fatty acids, esters, aromatics, and nitrogen-containing heterocyclic compounds. Acetic acid was the main component of water-soluble components.
Vardon et al. [77,78] studied the HTL of Scenedesmus (raw and defatted) and
Spirulina algal biomass at 300°C and 10–12 MPa pressure and compared the performance with that of Illinois shale oil and bio-oil produced by dry pyrolysis (at 450°C).
Both wet and dry pyrolyses gave energy-dense bio-oil (35–37 MJ/kg) that approached
shale oil (41  MJ/kg). Bio-oil yields (24%–45%) and physicochemical characteristics were highly influenced by the conversion route and feedstock selection. Sharp
differences were observed for the mean bio-oil molecular weight (dry pyrolysis:
280–360  Da; HTL: 700–1330  Da) and the percentage of low-boiling compounds
(bp  < 400°C) (dry pyrolysis: 62%–66%; HTL: 45%–54%). For wet algal biomass
containing 80% moisture, the energy consumption ratio (ECR) for HTL (0.44–0.63)
was more favorable than that for dry pyrolysis (0.92–1.24). In another study, Vardon
et al. [77,78] showed that Spirulina algal biomass gave 32.6% biocrude as opposed to
9.4% for digested sludge under the same reaction conditions as mentioned earlier and
for 30 min residence time. While swine manure, digested sludge, and Spirulina algae
gave biocrudes of similar heating value (32–34.7 MJ/kg), they differ substantially in
their detailed chemistry. The molecular weights tracked with obdurate carbohydrate
content followed the order: Spirulina < swine manure < digested sludge.
Duan and Savage [81] were the first to evaluate the effects of various hydroprocessing catalysts on HTL of microalgae Nannochloropsis sp. The experiments were
performed at 350°C with Pd/C, Pt/C, Ru/C, Ni/SiO 2 –alpha-Al 2 O 3 , CoMo/l 3 -Al 2 O 3
(sulfide), and zeolite catalysts. In the absence of hydrogen, all catalysts gave higher
yields of bio-oil, but the elemental compositions and heating value of bio-oil (about
38 MJ/kg) were insensitive to the nature of the catalyst used. Gases contained H 2 ,
CO 2 , CH 4 , and lesser amounts of C 2 H 4 and C 2 H 6 . Ru and Ni catalysts produced nitrogen. The H/C and O/C ratios of the products were about 1.7 and 0.09, respectively.
While the presence of hydrogen and higher pressure suppressed the gas formation,
the bio-oil yield and its characteristics did not significantly change.
Generally, high lipid content of algal mass limits its conversion to bio-oil by the HTL
process. Yu et al. [83] examined the HTL process for low-lipid microalgae Chlorella
pyrenoidosa and found that at 280°C and 120 min reaction time, the bio-oil yield of
39.4% was obtained. The bio-oil yield, water solubles, and gases strongly depended on
the temperature and reaction time. Biller and Ross [84] correlated the performances of
various types of algal biomass in the HTL process by correlating the bio-oil yield with
the biochemical content of the biomass. They examined microalgae Chlorella vulgaris,
Nannochloropsis oculata, and Porphyridium cruentum, and cyanobacteria Spirulina