Various parameters affect product yield of HTL derived bio-oil
including microalgae species, feed ratio of solids to liquid, reaction
temperature, holding time, heating rate, cooling rate, presence of
catalysts and effective product separation [35]. In recent years
HTL process development from batch to continuous feed has
occurred [36]. Continuous feed systems have advantages of higher
feedstock flows and lower process and retention times, lacking
uncertainties in heating and cooling rates common in batch run
experiments [37]. Development of a continuous feedstock process
requires thermal quenching to reduce temperature differentials,
ensure preservation of reactant products and optimise the viable
and scalable commercial integration into a CSP/HTL production
system. Thermal retention, multi-phase flow fluid mechanics and
feedback control optimisation within the core of the reaction pressure
vessel should be identified to define reaction process boundaries.
Microalgae biomass of concentration 10–20% (w/v) is
optimal for HTL boundaries of solids loading [38]. A 20% solid content
is estimated to be a reasonable trade-off between the capital
costs for the HTL system and the costs for dewatering. Higher biomass
solid input concentrations to HTL reduce the capital cost and
make product separation easier but also incur greater dewatering
costs [3]. Solar heat integration as the vector for biomass to biofuel
transformation does not jeopardise holistic energetic transformation
pathways resulting in a more favourable energy return inthe LCA (Lifecycle Analysis) than energy input from fossil fuel generated
heat.