Results & Discussion

Influence of predictor variables on bio-oil's yield and HHV

The analysis observed that certain predictor variables caused higher variance in response variables compared to other predictor variables. The correlation between the predictor and response variables and the variance explained by each predictor variable were estimated using regression. The correlation values presented in Table 2 and Table 3 are R² values. The R² value is a statistical metric used to evaluate the percentage of variation caused in the response variables by the predictor variables. This value indicates the degree of fit in the regression model. Tables represent the significance of the relationship between the response and predictor variables through corresponding P-values. The predictor variables causing significant variance for bio-oil yield and bio-oil HHV are not completely similar, indicating that different factors will impact bio-oil yield and its HHV. For instance, bio-oil yield depends more on the pyrolysis residence time, algal biomass ash, hydrogen, and lipid content (Table 2). However, experimental parameters do not have much impact on HHV and depend on the algae's biochemical composition (carbohydrate, lipid, and protein) (Table 3). A detailed discussion on the impact of the top three predictor variables on bio-oil yield and its HHV has been mentioned in the subsequent sections.

Bio-oil yield depends on pyrolysis residence time and algae hydrogen and ash content

Four out of sixteen predictor variables caused significant variance in bio-oil yield and correlated with bio-oil yield. From my analysis, I observed that one of the proximate analysis parameters, algal biomass's “ash” content, explained 22% of the total variance in the bio-oil yield. The bio-oil yield was observed to decrease (Figure 3A), with a correlation coefficient of -0.47. It was observed that the bio-oil yield decreases significantly with increased ash content of algal biomass. The ash content is also referred to as the "non-convertible component" of bio-oils, which means that during the pyrolysis process, unlike volatile compounds, it is not converted to hydrocarbons (Chen et al., 2017). Thus, biomass with higher ash content generally has less volatile matter content, resulting in a lower bio-oil yield. However, algal biomass, which has a higher ash content than other biomasses, should not be considered a bottleneck in the bio-oil production industry. A recent study observed that bio-oil produced from biomass with a higher ash content has a higher phenolic monomer content, which enhances its miscibility with commercial diesel (Kim et al., 2021). More research is needed to evaluate the tradeoff between the reduced bio-oil yield and the miscibility with commercial diesel to confirm the role of ash in bio-oil production.

The pyrolysis process parameters can also impact the yield of bio-oil. The analysis also observed that bio-oil yield decreases with an increase in residence time (Figure 3B). With the increase in residence time, the carbonization of biomass produces a higher amount of non-condensable low molecular weight gasses; these gasses do not condense easily and therefore decrease the bio-oil yield (Gao et al., 2016). Apart from the ash, the hydrogen content of algae also influences bio-oil production. The bio-oil yield increased with higher hydrogen concentrations (Figure 3C). Higher hydrogen concentrations in materials help them act as a hydrogen donor. These hydrogen donors form H and OH radicals, which combine with certain compounds, such as polyisoprene and poly chain scission, present in the algal biomass to form compounds that stabilize the bio-oil produced during the pyrolysis process (Su et al., 2022). The higher lipid content of algae positively influences the bio-oil yield. This was obvious because lipid molecules are fatty acids that undergo decarboxylation during the pyrolysis process and produce hydrocarbons (Baskar et al., 2019). Thus, it can be concluded that algae having a higher lipid content and a lower ash content that is pyrolyzed with a short residence time can maximize the bio-oil yield.

The HHV of bio-oil depends on the lipid, carbohydrate, and protein content of algal biomass

The HHV of bio-oil was dependent on the biochemical composition of algal biomass. It was also observed to have a dependency on pyrolysis residence time. Algal biomass with a higher carbohydrate content yielded bio-oil with a higher HHV (Figure 4A). This is because the presence of polysaccharides (a type of carbohydrate) reduces the proportion of lighter components of hydrocarbons in the bio-oil. Having lesser lighter hydrocarbon and more proportion of heavier hydrocarbon increases the HHV of the oil (Yang et al., 2019). However, higher protein content in the algal biomass decreases the calorific value of the produced bio-oil (Figure 4B). A higher protein concentration represents a higher amount of nitrogen in the biomass. Based on the equation developed to calculate the HHV, nitrogen can negatively influence the HHV of oil (Channiwala & Parikh, 2002). Similarly, higher lipid content in the algal biomass positively influences the calorific value of the bio-oil (Figure 4C). Because having a higher content of lipids indicates a more amount of fatty acids get converted into hydrocarbons, enhancing the HHV of the produced oil. In short, bio-oil produced from algae having higher lipid and carbohydrate content and lower protein content will have a larger HHV.

Yield and HHV of bio-oil are dependent on the algal type and the growth habitat

My analysis observed that the bio-oil produced from algal biomass could have different yields and HHV depending on the type of algal biomass. A pyrolysis process using microalgae as biomass yields more bio-oil than the pyrolysis process using macroalgae as the feedstock (Figure 5A). The median yield and the HHV of bio-oil produced from microalgae have higher calorific values than those of macroalgae (Figure 5B). The lipid concentration in microalgae is higher as compared to that of macroalgae. Thus, having a higher lipid content makes the microalgae produce more quantity of bio-oil with higher calorific value.

Algal growth habitat can also influence bio-oil production through pyrolysis. The analysis observed that algae grown in freshwater have higher bio-oil yields than those grown in marine ecosystems (with saltwater) (Figure 6A). Algae grown in freshwater have lower ash content than marine algae, and it was reported that higher ash content decreases the bio-oil yield. However, regarding the calorific value of the produced bio-oil, both algae show comparable HHV (Figure 6B).

Conclusion and future recommendations

Thus, based on the findings, freshwater green microalgae such as Chlorella, Scenedesmus, Synedra, and Achnanthidium can be potential candidates for enhancing the quantity and quality of bio-oil production. With the appropriate application of molecular biotechnology, the traits associated with these species can be incorporated into the macroalgae to scale up bio-oil production. Nonetheless, controlling pyrolysis parameters such as residence time will assist bio-oil-producing industries in maximizing product yield. More research on experimental parameters is required to establish a clear relationship between the bio-oil and other experimental parameters.