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and found that the yields of biocrudes from these species were 5–25 wt% higher than
the lipid content of the algae depending on the biochemical composition. The yields of
biocrudes follow the order: Lipids > proteins > carbohydrates.
Ross et al. [94] examined the effects of alkalis and organic acids on HTL of low lipid
content C. vulgaris and Spirulina algae at 300°C and 350°C, respectively. The effects
of the temperature and the catalyst types on the product yields and composition were
examined. The catalysts used were alkali, potassium hydroxide, sodium carbonate, and
the organic acids, acetic acid and formic acid. The yields of biocrudes were higher using
an organic acid catalyst and these crudes had a lower boiling point and improved the
flow properties. The higher heating value ranged from 33.4 to 39.9 MJ/kg. The biocrude
contained 70%–75% carbon, 10%–16% oxygen, and 4%–6% nitrogen.
Biller et al. [87] examined a range of microalgae and lipids extracted from the terrestrial oil seed for the HTL process at 350°C and 150–200 atm pressure in the presence of a variety of heterogeneous catalysts. The results showed that the HTL process
converted triglycerides to fatty acids and alkanes in the presence of certain heterogeneous catalysts. While heterogeneous catalysts increased biocrudes only slightly with
the use of heterogeneous catalysts, higher heating value and deoxygenation of the
products increased by up to 10% due to the presence of the heterogeneous catalysts.
5.3.4 hTu ProCeSS
The HTU process is a successful pilot-scale HTL process in which biomass reacts
with liquid water at an elevated temperature and pressure but under subcritical conditions. The reactor is operated under complex phase equilibria due to the simultaneous presence of water, supercritical carbon dioxide, and various alcohols along with
biocrude that contains 10%–13% oxygen. In subsequent upgrading, a large portion of
oxygen is removed as carbon dioxide.
In the HTU process, biomass chips (or other organic materials) are first digested
by water under pressure at about 200°C–250°C. The digested slurry is then passed
into a reactor that is generally operated at 300°C–350°C, 12–18  MPa, and a residence time of 5–20  min. The feed slurry contains about 25% of biomass such as
wood or other organic wastes. Once biomass is converted, the product biocrude that
is a mixture of light (oil) and heavy (solid) materials is separated. Light biocrude is
dehydrogenated and upgraded to premium diesel fuel or kerosene, or used as a refinery feedstock. Heavy biocrude is combusted along with coal to generate electricity.
The product biocrude has higher energy density than the feed biomass and it
contains alcohols, acids, and numerous other water-soluble components. The typical data of a pilot plant and the typical feedstock, the reaction conditions, and the
products of HTU process are given by Demirbas [3,4]. The process is simple and of
high efficiency.
5.4 hydrOthermal GasiFiCatiOn
The fast hydrolysis of organic molecules such as biomass at high temperature leads
to a rapid degradation of the polymeric structure of biomass [6–11,91]. A series
of consecutive reactions lead to the formation of gas whose composition depends