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Water for Energy and Fuel Production
water-soluble components. The liquefaction of eucalyptus by water resulted in high
residue production due to dehydration and polymerization, such as the formation of
aliphatic ester bonds. The paper regeneration wastewater gave low residue and high
yield of bio-oil compared to water. It appears that condensation reaction observed
during the liquefaction in water was inhibited because carboxylic acid formed during the liquefaction of eucalyptus was neutralized with the cations in the wastewater. In the temperature range of 150°C–350°C, process wastewater gave the best oil
yield compared to other two solvents. The optimum temperature for bio-oil yield was
300°C. This study indicated that solvent pH and other additives can affect the bio-oil
production of lignocellulosic materials.
Finally, high degree of polymerization (>10,000) and complex branching make
lignin difficult to decompose even at high temperatures. The studies described earlier show that the conversion of lignin containing lignocellulosic materials requires
catalysts to produce bio-oil instead of solids and gases. An extensive literature review
of various catalyst studies for the HTL process applied to lignocellulosic biomass is
given by Zhang [45].
5.3.3.3 algae
Ever since Glen Meier of Renewable Energy Group introduced the concept of producing fuel from algae, it has caught everybody’s attention [75–79,81–88]. Like biowaste,
algae do not compete with food materials for fuel. The HTL of algae has been given
some attention in the recent years. Minowa et al. [89] converted Dunaliella tertiolecta with a moisture content of 78.4% directly into 37% oil by the HTL process
operated at 300°C and 10  MPa. The oil had a viscosity of 150–330  MPa-s and a
calorific value of 36 kJ/g, numbers comparable to that of fuel oil. Dote et al. [90]
hydrothermally converted the artificially cultivated Botryococcus braunii Kützing
Berkeley strain. The strain contained about 50% hexane solubles. The HTL process of this strain resulted in the production of 57% petroleum-like bio-oil at 300°C.
Similar work was carried out for Microcystis viridis harvested from a lake.
Brown et al. [91] reported hydrothermal conversion of marine microalgae
Nannochloropsis sp. into bio-oil in the temperature range of 200°C–500°C and for
the residence time of 60 min. The highest bio-oil of 43 wt% was obtained at 350°C
with a heating value of 39  MJ/kg, a number comparable to petroleum crude oil.
The H/C and O/C ratios changed from 1.73 and 0.12 at 200°C to 1.04 and 0.05 at
500°C, respectively. The major components of bio-oil were phenol and its alkylated
derivatives, heterocyclic N-containing compounds, long-chain fatty acids, alkanes,
alkenes, derivatives of phytol, and cholesterol. Gases largely contained CO 2 and H 2 .
Metal catalysts had been used in microalgae liquefaction. Matsui et al. [92] investigated the liquefaction of Spirulina, a high protein algae in water at 300°C–425°C
using Fe(CO) 5 –S catalyst. Other metal catalysts used were Ru 3 (CO) 12 and Mo(CO) 6 .
Continuous culturing of the B. braunii Berkeley strain in the secondary treated
sewage was conducted and then liquefied by Sawayama et al. [93]. The liquefaction was carried out at 200°C, 300°C, and 340°C. The yield of the hexane-soluble
fraction was 97% compared to that in the feedstock algal cells. The heating value
of the liquefied oil obtained from this reaction was 49 MJ/kg and the viscosity was
64 MPa-s at 50°C.