174
5. Corrosive biocrude oil (tanks, pipes, and reactors must be made of stainless steel
to prevent corrosion caused by subcritical or supercritical water).
6. Lack of use for by-products such as aqueous phase and solid residue.
7. Negative overall energy balance due to liquid pumping and heat losses.
8. Unclear understanding of the kinetics of catalytic reaction.
9. Unstable and nonrecyclable heterogeneous catalyst in a continuous mode of
operation.
Direction on the SOR for microalgae HTL is subdivided into four aspects: microalgae, residence time, heating rate, and separation. These aspects are expected to
address the challenges identified above to establish a commercial biorefinery based
on microalgae HTL in the foreseeable future.
Microalgae. Microalgae feedstock is fed into the HTL reactor as a dilute solution. Thus, significant operational cost is incurred in the form of pumping energy
use. The optimum microalgae-to-water ratio remains unclear since recent investigations on microalgae feedstocks showed contradicting results. As an example, a high
microalgal load (>10 wt% microalgae solution) requires high water heating energy,
but the water content is not sufficient to dissolve and stabilize the algal cells [26,
59]. Recent research has proposed microalgae forms such as pulverized dried samples [58, 59], intact microalgae cells in the form of paste [25, 38, 39, 41, 151], or
freeze-dried pulverized cells mixed with deionized water [31, 39, 60, 72]. However,
these forms can inadvertently alter fuel yield, fuel composition, or extractability of
some macroelements. The paste microalgae form appears to precisely track the fate
and transport of nutrients during the HTL and upgrading process, and it seems to
illustrate a more realistic scenario within the context of commercial- scale HTL
plant. Further research on these forms is warranted.
Residence Time. Residence time is defined as the period during which the maximum temperature is maintained for HTL, not accounting for the heating and cooling
periods. Currently, residence time for microalgae HTL is still too long. To achieve
high biocrude oil yield, reaction must take place at high temperature and low residence time. Most studies in literature applied 60 min. Up until recently, Kumar and
colleagues invented a method called flash hydrolysis, a type of HTL with reaction
time in the seconds range [119]. The technology significantly lowers operational
and energy costs. Still, the development of heterogeneous catalyst that is stable,
recyclable, and suitable to fast-heating condition presents an opportunity for future
research.
Heating Rate. The effects of heating rate on biocrude oil yield are still unclear.
Only a few studies provide information on this parameter [31, 36, 38, 39, 41]. Some
research suggests that increasing heating (e.g., 100 °C/min) rate has a positive correlation on biocrude oil formation [31, 39, 41, 38]. Design and optimization research
of heat transfer system for large-scale reactors which solve microalgae’s low heat
transfer coefficient and variabilities of reaction enthalpies must be performed.
Separation. Almost all studies in literature utilized organic solvents in separating
the HTL products from each other. These solvents include dichloromethane, chloroform, acetone, THF, or hexane. There is no information available on the potential
E. P. Resurreccion and S. Kumar
5. Corrosive biocrude oil (tanks, pipes, and reactors must be made of stainless steel
to prevent corrosion caused by subcritical or supercritical water).
6. Lack of use for by-products such as aqueous phase and solid residue.
7. Negative overall energy balance due to liquid pumping and heat losses.
8. Unclear understanding of the kinetics of catalytic reaction.
9. Unstable and nonrecyclable heterogeneous catalyst in a continuous mode of
operation.
Direction on the SOR for microalgae HTL is subdivided into four aspects: microalgae, residence time, heating rate, and separation. These aspects are expected to
address the challenges identified above to establish a commercial biorefinery based
on microalgae HTL in the foreseeable future.
Microalgae. Microalgae feedstock is fed into the HTL reactor as a dilute solution. Thus, significant operational cost is incurred in the form of pumping energy
use. The optimum microalgae-to-water ratio remains unclear since recent investigations on microalgae feedstocks showed contradicting results. As an example, a high
microalgal load (>10 wt% microalgae solution) requires high water heating energy,
but the water content is not sufficient to dissolve and stabilize the algal cells [26,
59]. Recent research has proposed microalgae forms such as pulverized dried samples [58, 59], intact microalgae cells in the form of paste [25, 38, 39, 41, 151], or
freeze-dried pulverized cells mixed with deionized water [31, 39, 60, 72]. However,
these forms can inadvertently alter fuel yield, fuel composition, or extractability of
some macroelements. The paste microalgae form appears to precisely track the fate
and transport of nutrients during the HTL and upgrading process, and it seems to
illustrate a more realistic scenario within the context of commercial- scale HTL
plant. Further research on these forms is warranted.
Residence Time. Residence time is defined as the period during which the maximum temperature is maintained for HTL, not accounting for the heating and cooling
periods. Currently, residence time for microalgae HTL is still too long. To achieve
high biocrude oil yield, reaction must take place at high temperature and low residence time. Most studies in literature applied 60 min. Up until recently, Kumar and
colleagues invented a method called flash hydrolysis, a type of HTL with reaction
time in the seconds range [119]. The technology significantly lowers operational
and energy costs. Still, the development of heterogeneous catalyst that is stable,
recyclable, and suitable to fast-heating condition presents an opportunity for future
research.
Heating Rate. The effects of heating rate on biocrude oil yield are still unclear.
Only a few studies provide information on this parameter [31, 36, 38, 39, 41]. Some
research suggests that increasing heating (e.g., 100 °C/min) rate has a positive correlation on biocrude oil formation [31, 39, 41, 38]. Design and optimization research
of heat transfer system for large-scale reactors which solve microalgae’s low heat
transfer coefficient and variabilities of reaction enthalpies must be performed.
Separation. Almost all studies in literature utilized organic solvents in separating
the HTL products from each other. These solvents include dichloromethane, chloroform, acetone, THF, or hexane. There is no information available on the potential
E. P. Resurreccion and S. Kumar