In the report the focus of HTL applications is on the use of micro-algae as substrate. Owing to their high growth rate and efficient CO2- fixing, algae are the focus of attention in relation to the production of propellants (Tsukahara and Sawayama 2005). There are only a few research works into the hydrothermal liquefaction of micro-algae however. Dote et al. (Dote et al. 1994) were among the first to apply HTL to Botryococcus braunii with and without catalyst (Na2CO3). At 300 °C bio-oil yields of 57 to 64 % were obtained. The quality of the oil roughly matched that of petroleum.
Dunaliella tertiolecta was treated by Minowa et al. (Minowa et al. 1995) under HTL conditions (300 °C, 10 MPa) likewise with and without Na2CO3 as catalyst. The average oil yield was 37 %. Altering the reaction parameters (temperature, time, Na2CO3 content) resulted in no significant increase in proportionate oil content. However, the chemical composition of the oil was heavily dependent on the reaction temperature. The gas phase mainly consisted of carbon dioxide. A further component of the reaction mixture was a tar-like substance which floated on the surface of the aqueous phase and so could be easily separated.
Yang et al. (Yang et al. 2011) investigated the liquefaction of Dunaliella salina with bifunctional Ni/REHY catalysts in ethanol and under H2 gas in moderate conditions (200 °C, 2 MPa, 60 min) in order to determine the influence of the catalyst on the bio-oil yield and composition.
Dunaliella tertiolecta and Botryococcus braunii were likewise hydrothermally treated by Sawayama and colleagues (Sawayama et al. 1999) and subsequently investigated in terms of energy balance sheet and CO2 reduction. They concluded from the results that micro-algae with higher lipid content are better suited than micro-algae with low lipid content.
Consequently, many research groups concentrated on the use of stocks with high lipid content. Lipid production is linked to stress situations, such as shortage of nitrates. This results in lower biomass productivity. The cultivation of such algae is usually more time-consuming and expensive, which is why Yu et al. see advantages in the use of algae with low lipid content in terms of large-scale industrial production (Yu et al. 2011).
The liquefaction of Spirulina has been the subject of several experiments (Jena et al. 2011a; T. Suzuki T. Matsui C. Ueda N. Ikenaga 2006; Ross et al. 2010; Huang et al. 2011).
The focus of the studies by Ross et al. (Ross et al. 2010) was on the use of alkali compounds and organic acids (formic acid, acetic acid) in the liquefaction of Spirulina and Chlorella vulgaris. Compared to the alkali catalysts, higher bio-oil yields were attained in the presence of the acids, and the yields increased as the temperature rose. The group likewise found higher bio-oil production in the case of Chlorella than with Spirulina algae which – similar to Sawayama previously (Sawayama et al. 1999) – led them to the conclusion that the bio-oil yield rises as the lipid content of the alga increases. Adding the organic acids caused the nitrogen content in the aqueous phase to decrease and the proportion of NH3 and HCN in the gas phase increased. The nitrogen content of the oil phase remained unchanged.
Suzuki et al. (T. Suzuki T. Matsui C. Ueda N. Ikenaga 2006) investigated the hydrothermal liquefaction of Spirulina in various organic solvents (tetralin, 1 methyl naphthalene, toluene) or water under hydrogen, nitrogen or carbon monoxide atmosphere in a temperature range of 300 to 425 °C. The
uncatalysed reaction delivered a conversion rate of more than 90 % and 60 % oil content. Adding Fe(CO)5 S as a catalyst increased the bio-oil yield from 52 % to 67 % at 350 °C with 60 minutes in tetralin. By comparison, hydrothermal treatment in water at 350 °C under hydrogen atmosphere without catalyst produced an oil yield of 78 %.
Since the reaction conditions in the liquefaction of biomass are similar to those in the hydrothermal liquefaction of coal, Ikenaga et al. (Ikenaga et al. 2001) conducted experiments in the combined liquefaction of various micro-algae (Chlorella, Spirulina and Littorale) with coal (Australian Yallourn brown coal and Illinois No. 6 coal) under H2 atmosphere in 1-methyl naphthalene at 350 to 400 °C for 60 min with various catalysts ((Fe[CO]5) S, ((Ru3[CO]12), (Mo[CO]6) S). All three catalysts were suitable for the combined liquefaction of micro-algae and coal. At 400 °C and with a surplus of sulphur relative to iron (S/Fe = 4), using 1:1-Chlorella and Yallourn coal a conversion rate of 99.8 % and 65.5 % oil yield could be attained. A similar trend was observed in the liquefaction of Littorale and Spirulina in the presence of iron-containing catalysts.
A summary of all publications relating to the liquefaction of micro-algae discovered to date is presented in Table 5.5.
In some experiments macro-algae were also used (Anastasakis and Ross 2011; Zhou et al. 2010). The results of those experiments are not considered here.
Drawing up mass and energy balances for hydrothermal liquefaction poses a major problem. Only a small number of research groups are investigating the process with regard to its efficiency (Minarick et al. 2011). Sawayama et al. (Sawayama et al. 1999) applied hydrothermal liquefaction to Botryococcus braunii and Dunaliella tertiolecta and compared the respective oil yields and their calorific values.
Botryococcus produced more oil with a lower calorific value compared to the micro-alga Dunaliella (Table 5.4). The researchers concluded from this that the energy inputs for the cultivation and separation of B. braunii must likewise be lower than for the comparative alga. Based on these calculations, B. braunii is more suitable for oil production than D. tertiolecta.
Table 5.4 Oil yield and energy consumption rate of oil production in hydrothermal liquefaction of micro-algae (Tsukahara and Sawayama 2005)
Oil yield (%)
Energy for HTL/
Energy of produced oil
Botryococcus braunii 64 0.15
Dunaliella tertiolecta 42 0.34
The elemental composition and calorific value of the bio-oil HTL obtained is presented in Table 5.7.
Table 5.5 Overview of the hydrothermal liquefaction of various micro-algae
Ester, glycerine, (Yang et al. 2011)
Chlorella
Micro-alga Reaction
Table 5.6 Overview of the elemental composition and calorific value of the bio-oil HTL
Alga Spirulina