indicates conversion of other cellular constituents (e.g., protein,
carbohydrate, algaenan) by HTL reaction processes [24,25].
Reaction times are a crucial factor for economical operation of
HTL, short reaction times will reduce bio-oil yield whereas long
reaction times may lead to higher gas and compromised bio-oil
yield [26]. 30 min is the most appropriate HTL reaction time for
Enteromorpha prolifera [27] and Dunaliella [20]. A hydrothermal liquefaction
techno-economic analysis of a modelled 2000 dry tonne
per day processing facility using defatted microalgae indicates that
66% external electricity should be supplied with an overall energy
efficiency of 56% [28]. The well to pump lifecycle comparison of
fossil energy use and greenhouse gas emissions were higher for
HTL bio-oil than lipid extracted renewable diesel production on
account of net heat energy input to establish the operational process
conditions, however HTL used 1.8-fold less biomass than the
lipid extracted renewable diesel production pathway [29]. The
amount of energy required to dry algal biomass to levels typical
of terrestrial crops for solvent based oil extraction would exceed
the energy content in the algal oil [30]. The energy required for
microalgae and biogas production from Nannochloropsis has been
calculated to be as much as 8–11 times more than the bio-gas
energy yield [31]. A trade-off between high algal oil yields and high
energy recovery via catalytic hydrothermal gasification of the
aqueous HTL solubles is amplified by using Escherichia coli grown
on aqueous HTL solubles for secondary HTL, boosting the oil yield
per unit of microalgae biomass and suggesting that recovery and
recycling of aqueous phase product from HTL is instrumental to
overall lifecycle economics [32].