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 in
the LCA (Lifecycle Analysis) than energy input from fossil fuel generated
heat.
Engineering the integration of solar thermal energy for HTL biooil
rather than electricity generation has not been widely reported
by other authors’. The objectives of this study were to investigate a
techno-economic analysis (TEA) of factors influencing the unification
of HTL and CSP parabolic troughs for the processing of
microalgae biomass into bio-oil. CSP parabolic troughs yield a temperature
of up to 400 C with oil as the heat transfer liquid (HTF);
the use of molten salts as a HTF can attain a temperature much
higher [39] whilst beneficial operational temperature requirements
for HTL occur within the range of 250–350 C [36,40]. A
thermodynamic assessment of parabolic troughs [41] with an economic
analysis using experimental field trials of microalgae productivity
justifies the potential viability of this technology
unification. Aspen plus and custom sizing equations have been
used to determine the economic viability of the process. Finally,
the influence of estimated parameters on the economic results
was assessed via sensitivity analysis.
2. Materials and methodology
2.1. Process overview
This sized CSP plant could process 200 kg of daily microalgae
biomass in 3 cycles. The schematic diagram of the solar-assisted
HTL plant is depicted in Fig. 1, which describes the integration of
solar CSP with the tubular HTL reactor aligned along the focal line
of the parabolic trough.
A land surface area of a 1 ha site could produce in the region of
180–200 kg biomass per day from a high rate microalgae pond,
additional waste biomass and recycled nutrients for secondary biomass
growth further supplements HTL microalgae feedstock. Considering
a 30% biomass to bio-oil conversion with a ratio of 20%
solids to water ratio in HTL, this would provide 1000 l feedstock
per day. The HTL reactant volume space replaces the heat transfer
fluid (HTF) as used in conventional electricity generating CSP
plants. Likewise, HTF molten salt is replaced by microalgae biomass
and water as the reactant components of HTL. Reactant
inputs and discharge on alternative ends of each linear row of parabolic
troughs function as semi-continuous batch processing. A proposed
diurnal thermal HTL capacity for 3 h either side of midday
permits 3 batch runs per day (Table 4). Estimation of CSP plant size
for processing of HTL feedstock is based on the 1000 l daily production
of HTL feedstock at 20% (w/v) microalgae – 160 l HTL reaction
core volume from 100 m of solar CSP parabolic troughs, with
226 m2 total solar aperture.