CHAPTER 7. EFFECTS OF CATALYSTS ON HTL PRODUCT YIELDS
7.1 CATALYST EFFECTS ON BIO-CRUDE OIL YIELD
To investigate the catalyst effects on bio-crude oil yields, 5 wt% of different catalysts were loaded into the reactor and mixed with algal slurry before the reaction. In this study, all of the catalysts were used as received, which indicates there is no pre-reduction for the metal catalysts.
A previous study implied that the pre-reduction of metal catalysts before HTL was not necessary because the catalysts would be exposed to the oxidizing hydrothermal environment when in use, and the pre-reduction has no significant effects on the deoxygenation of fatty acids (Fu et al., 2010).
Hydrogenation is widely used in the conventional petroleum upgrading process to remove oxygen content from the fuel. In the HTL process the oxygen atom is removed as CO2 rather than H2O via decarboxylation; therefore, no H2 was added in our experiments. In fact, H2
addition was not required for promoting decarboxylation under hydrothermal conditions (Fu et al., 2010; Fu et al., 2011a). On the other hand, although the addition of H2 could improve the H/C ratio of bio-crude oil produced from algae, the bio-crude oil yield with the presence of metal
oil yield (Duan and Savage, 2011b). Since large variables of the addition of H2 exist in literature, no H2 addition was applied in this study because the objective of this chapter is to conduct preliminary evaluation of different catalysts on the HTL of microalgae.
7.1.1 Effects of catalysts on bio-crude oil yield of Chlorella
Figure 7-1 shows the bio-crude oil yield from the HTL of C. pyrenoidosa at 240°C with 30 minutes retention time. Through the student’s t-test it was found that compared with bio-crude oil yield from an uncatalyzed test, the bio-crude oil yield was significantly increased from 31.1%
to 41.0% when sodium hydroxide was applied. However, all of the other catalysts did not show significant effects on the bio-crude oil yield under this temperature level.
Figure 7-1 Catalyst effects on bio-crude oil yield from Chlorella at 240°C with 30 min retention time
Note that the effects of the catalysts on HTL were usually investigated at a high temperature (350°C) in literature (Biller et al., 2011; Duan and Savage, 2011b), the reason we conducted the HTL experiment at the mild temperature level is to study whether the effects of the catalysts can be observed at low temperatures, under which the formation of bio-crude oil already starts to occur. From discussion in CHAPTER 6, it was found the lowest temperature for the bio-crude oil formation should be in the range of 200-240°C, therefore, we conducted a series
of tests at 240°C to investigate the effects of different catalysts. However, it was found the effects of catalysts addition at this mild temperature were not significant. In fact, at 240°C the formation of bio-crude oil could not be completed in 120 minutes (Figure 6-3), and it is expected that some unreacted algal biomass could remain after 30 minutes of the HTL process. The unreacted algal biomass could mix with the catalysts and even deposit on the surface of a catalyst so that the surface area of the catalysts decreases, and clogs the porous structure so that the catalysts are deactivated.
When similar HTL experiments were conducted at a higher reaction temperature level (280°C) with the same retention time, all of the catalysts increased the bio-crude oil yield. Figure 7-2 shows the bio-crude oil yield from C. pyrenoidosa with different catalysts at 280°C. This finding is consistent with the literature which shows under an inert condition, different metal catalysts could promote the formation of bio-crude oil (Duan and Savage, 2011b). It should be pointed out that although significant differences of bio-crude oil yield were achieved at 280°C, the difference between the highest yield (50.0% with 5% Raney nickel) and the lowest yield (39.0%, uncatalyzed) was only about 10% based on the dry matter of the feedstock. The bio-crude oil yields obtained under this condition are comparable with previous studies.
The bio-crude oil yields with the presence of Pd/Al2O3 and Pd/C were 46.6±1.6% and 42.6±0.7%, respectively. Although the difference between these two values was not pronounced, through statistical analysis it was found there was significant difference between these two series of experimental results. On the other hand, the bio-crude oil yield was 45.8±2.7% and 45.2±2.4%
with additions of Pt/Al2O3 and Pt/C, respectively. But no significant difference was found between the two series of data. To investigate the effects of the catalyst dosage on bio-crude oil yield, 30% of Pt/Al2O3 based on the total dry mass of feedstock was applied into the test of HTL of Chlorella at 280°C with 30 minutes retention time. The bio-crude oil yield was 41.8%, which was close to the bio-crude oil yield obtained with the addition of 5% catalyst. Therefore, it can be concluded at this temperature level, the effect of catalyst dosage was not substantial.
7.1.2 Effects of catalysts on bio-crude oil yield of Spirulina
The results of the effects of the catalysts on bio-crude oil yield produced from HTL of S.
platensis are shown in Figure 7-3 and Figure 7-4. Similar to the results of C. pyrenoidosa, at 240°C although the bio-crude oil yield obtained with the presence of several catalysts showed significant difference compared with the uncatalyzed test results, the difference between the highest oil yield (32.7±1.86% with 5% Na2CO3) and lowest oil yield (24.3±1.2% with 5% NaOH) were less than 10%.
Figure 7-3 Catalyst effects on bio-crude oil yield from Spirulina at 240°C with 30min retention time
At 280°C, except for Pt/C, addition of all the other catalysts increased the bio-crude oil yield. But again the difference between the highest oil yield (40.4%±1.9% with 5% Na2CO3) and the lowest oil yield (33.9±2.4%, uncatalyzed) was less than 10%.
Figure 7-4 Catalyst effects on bio-crude oil yield from Spirulina at 280°C with 30 min retention time
7.2 CATALYST EFFECTS ON BOILING POINT DISTRIBUTION OF BIO-CRUDE