154
extraction step reduces the overall bio-oil yield, and trade-off must be evaluated.
The biocrude undergoes upgrade to produce bio-oil. Catalytic upgrade of biocrude
oil is discussed in Sect. 8.
3 Mass, Energy, and Nutrient Balance in Microalgae HTL
Regardless of microalgae species, the mass balance of the HTL process share similar trends; that is, the main fractions are (1) biocrude (bio-oil if upgraded), (2)
organics in the aqueous phase, (3) solid residues, and (4) gaseous phase, typically
lower in concentrations. High-value chemical co-products may be delivered.
Biocrude, solid residues, and the gaseous phase are measured gravimetrically, while
the aqueous phase is determined by calculating the mass difference from the input
feedstock. Inadvertent overestimation of the organics in the water is commonly
achieved by incorporating the losses in the system.
Energy balance in HTL is presented in terms of two energy ratios: (1) energy
recovery (ER) and (2) energy consumption (EC). ER ratio is loosely defined as the
ratio of the energy embodies in main product, in this case biocrude, to the total input
energy required in HTL (E in /E out ) [36, 45, 46]. However, there are discrepancies
among researchers in calculating ER [47, 48]. Across various energy platforms, EC
calculations give rise to substantially different values making reporting ambiguous.
Some researchers strictly include biocrude bioenergy as E in , while some include coproduct energy in the form of bioelectricity and, to a lesser extent, the gaseous phase
energy. The ambiguity is easily resolved by employing energy return on investment
(EROI), which explicitly includes all energy products in the numerator and all
energy consumption in the denominator without premature crediting of co-products
[39, 48]. In terms of EC ratio, researchers are uniform in defining it as the ratio of
the HTL energy demand versus the energy embodied in the biocrude [39, 45, 49].
Comparatively, the heating value of a typical microalgae is around 20 MJ/kg [50],
while biocrude has a value of 43 MJ/kg [38, 51]. It was reported by Brown et al. that
the heating value of the gaseous phase is 4 MJ/kg [38] from the HTL of
Nannochloropsis sp. for 60 min at 350 °C. In perspective, the average lower heating
value for methane is 50 MJ/kg, for propane 46.4 MJ/kg, and for butane 45 MJ/kg
[52], implying that the gaseous phase contains low fraction of these gases. Below is
a typical mass and atomic balance (dry basis, mass fraction) of Spirulina platensis
undergoing HTL for 60 min at 350 °C [45] (Fig. 4).
A significant portion of nutrients is recovered in the aqueous medium coming out
of HTL which is subsequently recovered as nutrient-rich culture medium during
microalgae cultivation. Specifically, around 40–75% of N in aqueous phase is
recovered [36, 45]. HTL utilizes various catalysts which affect the percent nitrogen
recovered. In combination, both lipid yield and nutrients recycle are optimized to
render HTL economically viable method in converting microalgae to biofuel.
Integrated microalgal cultivation, HTL, and nutrient recycling are tenable approaches
from an energy and mass balance approach [31, 53, 54].
E. P. Resurreccion and S. Kumar
extraction step reduces the overall bio-oil yield, and trade-off must be evaluated.
The biocrude undergoes upgrade to produce bio-oil. Catalytic upgrade of biocrude
oil is discussed in Sect. 8.
3 Mass, Energy, and Nutrient Balance in Microalgae HTL
Regardless of microalgae species, the mass balance of the HTL process share similar trends; that is, the main fractions are (1) biocrude (bio-oil if upgraded), (2)
organics in the aqueous phase, (3) solid residues, and (4) gaseous phase, typically
lower in concentrations. High-value chemical co-products may be delivered.
Biocrude, solid residues, and the gaseous phase are measured gravimetrically, while
the aqueous phase is determined by calculating the mass difference from the input
feedstock. Inadvertent overestimation of the organics in the water is commonly
achieved by incorporating the losses in the system.
Energy balance in HTL is presented in terms of two energy ratios: (1) energy
recovery (ER) and (2) energy consumption (EC). ER ratio is loosely defined as the
ratio of the energy embodies in main product, in this case biocrude, to the total input
energy required in HTL (E in /E out ) [36, 45, 46]. However, there are discrepancies
among researchers in calculating ER [47, 48]. Across various energy platforms, EC
calculations give rise to substantially different values making reporting ambiguous.
Some researchers strictly include biocrude bioenergy as E in , while some include coproduct energy in the form of bioelectricity and, to a lesser extent, the gaseous phase
energy. The ambiguity is easily resolved by employing energy return on investment
(EROI), which explicitly includes all energy products in the numerator and all
energy consumption in the denominator without premature crediting of co-products
[39, 48]. In terms of EC ratio, researchers are uniform in defining it as the ratio of
the HTL energy demand versus the energy embodied in the biocrude [39, 45, 49].
Comparatively, the heating value of a typical microalgae is around 20 MJ/kg [50],
while biocrude has a value of 43 MJ/kg [38, 51]. It was reported by Brown et al. that
the heating value of the gaseous phase is 4 MJ/kg [38] from the HTL of
Nannochloropsis sp. for 60 min at 350 °C. In perspective, the average lower heating
value for methane is 50 MJ/kg, for propane 46.4 MJ/kg, and for butane 45 MJ/kg
[52], implying that the gaseous phase contains low fraction of these gases. Below is
a typical mass and atomic balance (dry basis, mass fraction) of Spirulina platensis
undergoing HTL for 60 min at 350 °C [45] (Fig. 4).
A significant portion of nutrients is recovered in the aqueous medium coming out
of HTL which is subsequently recovered as nutrient-rich culture medium during
microalgae cultivation. Specifically, around 40–75% of N in aqueous phase is
recovered [36, 45]. HTL utilizes various catalysts which affect the percent nitrogen
recovered. In combination, both lipid yield and nutrients recycle are optimized to
render HTL economically viable method in converting microalgae to biofuel.
Integrated microalgal cultivation, HTL, and nutrient recycling are tenable approaches
from an energy and mass balance approach [31, 53, 54].
E. P. Resurreccion and S. Kumar