Thermal conversion of algal biomass and its derivatives to fuels and petrochemicals
Authors:   , Mustapha Sherif Ishola 
Published: August 2021

Thermal conversion processes have gained increased attention since they can be applied to whole microalgae (not lipids alone) resulting in higher biofuel yield with potential for production of other high-value products. The major challenges of microalgal thermal conversion are the high level of nitrogen and oxygen content present in the product stream, as well as high acidity which makes the bio-oil unstable and unfit for use as transportation fuels directly. Transportation fuels are expected to be low in oxygen and acid content for stability and also have low nitrogen content to meet environmental emission standards for combustion. Nutrient stress as a tool for enhancement of yields and quality of bio-oils produced from thermal conversion of microalgae has not received sufficient attention. This study investigated the conversion of Scenedesmus obliquus microalgae via three different thermal conversion processes which include pyrolysis, hydrothermal liquefaction and hydrothermal gasification. Scenedesmus obliquus microalgae were grown under nutrient stressed and unstressed conditions. To better understand the effect of nutrient stressing on the process, pyrolysis experiments were conducted on unstressed S. obliquus microalgae biomass (N3), nutrient- stressed S. obliquus microalgae biomass (N1) and its residual algae biomass after lipid extraction (R-N1) at different temperatures (400 °C to 700 °C) and the results compared. Detailed biomass characterization which includes proximate analysis, ultimate analysis, biochemical analysis, Fourier-transform infrared spectroscopy (FTIR) analysis, and thermogravimetric analysis (TGA/DSC) were carried out on the microalgae biomass (N1, R-N1 and N3) to provide useful information about the combustion behaviour of the biomass during pyrolysis. The biomass characterization results indicated that nutrient-stressed condition altered the microalgae biomass composition and empirical formula for N1, R- N1, and N3 microalgae biomass were CH2.00N0.07O0.71, CH2.36N0.08O0.75, and CH2.35N0.14O0.71, respectively. The maximum bio-oil yield for N1 (46.37 wt%) and R-N1 (34.85 wt%) were obtained at 500 °C, while the highest yield of bio-oil for N3 (41.94 wt%) was obtained at 600 °C. Also, the proportion of nitrogen compounds in N3 bio-oil (47.4 %) was significantly higher than that obtained in the nutrient stressed microalgae biomass (N1) bio-oil (5.92%) at pyrolysis temperature of 500 °C. Thus, nutrient stressed approach is considered more promising to produce a higher yield and good-quality pyrolytic bio-oil from microalgae biomass. A predictive model was developed based on artificial neural network (ANN) and can serve as a framework for the prediction of bio-oil yield from the pyrolysis of microalgae biomass. Finding better heterogeneous catalysts that can enhance the quality of microalgal bio-oils to meet transportation fuels standards is seen as a major advance toward developing efficient and sustainable thermal conversion processes. In this study, pyrolysis of nutrient- stressed Scenedesmus obliquus microalgae over various supported metal M/Fe3O4-HZSM- 5 catalysts (M = Zr, W, Co and Mo) was investigated. The synthesized catalysts were characterized by X-ray diffraction spectroscopy (XRD), thermogravimetric analysis (TGA), high-resolution scanning electron microscopy and energy dispersive spectroscopy (HRSEM/EDS). The catalyst: biomass ratio and temperature influence on pyrolysis product yield was also investigated. Between these, Co/Fe3O4-HZSM-5 catalyst showed better activity in enhancing the bio-oil quality and yield; it had the lowest nitrogen content (4.77 wt%) and highest bio-oil yield (17.73 wt %) as well as highest HHV (40.78 MJ/kg) which is almost similar to that of crude petroleum. The results showed that all the supported metal catalysts during pyrolysis promote aromatization and acid ketonization of bio-oils. The total amounts of acids present in pyrolytic bio-oil significantly decreased from 26.68% (non-catalytic) to between 0.58 – 9.68% (catalytic). Also, production of 2-pentanone was observed to increase from ~10% (non-catalytic) to 27.36 – 53.90% (catalytic). In terms of energy recovery, Co/Fe3O4-HZSM-5 had about 40% energy recovery, which was the highest while the least performing catalyst was W/Fe3O4-HZSM-5 with 24.18% energy recovery in bio-oil. Overall, Co/Fe3O4-HZSM-5 was the most effective catalyst in enhancing the quality of pyrolytic bio-oil produced from nutrient stressed Scenedesmus obliquus microalgae with properties close to that of petroleum crude. Hydrothermal liquefaction (HTL) of nutrient-stressed microalgae (Scenedesmus obliquus) (N1) with and without the use of Zr/HZSM-5 catalyst was investigated under temperature conditions ranging from 250 – 350 °C. The Zr/HZSM-5 catalyst was synthesized using wet impregnation technique and characterization was conducted on the synthesized catalyst for its crystalline nature, morphology and thermal stability using X- ray diffractometer (XRD), High-resolution scanning electron microscopy (HRSEM) and thermogravimetric analysis/differential scanning calorimetry (TGA/DSC). The HTL experiments were also conducted on the unstressed microalgae (N3) for comparison. Under the stressed condition, the protein content of the microalgae was reduced from 42.35% to 22.08% while the carbohydrate and lipid contents were increased from 25.36% to 42.55% and 17.16% to 21.62% respectively. The maximum HTL bio-oil yield of 52.80 wt% and 24.27 wt% were found for N1 and N3 respectively at 350 °C with addition of Zr/HZSM-5 catalyst. Higher denitrogenation and deoxygenation was achieved with N1 compared to N3. At high temperature of 350 °C, the most abundant fatty acid in N1 was found to be cis- vaccenic acid (omega-7- fatty acid), and this could be explored for possibility of extracting products of great value from the bio-oil for applications other than biofuels. Mainly, the use of Zr/HZSM-5 catalyst on nutrient-stressed S. obliquus microalgae resulted in enhanced bio-oil yield and characteristics which compared well with petroleum crude. The potential of using whole algae, lipid and residual algae of S. obliquus microalgae as feedstocks for production of high-quality hydrogen and methane-rich gas via hydrothermal gasification technique was also examined. The effect of operating parameters such as temperature, pressure and biomass concentration on the yield and composition of gaseous products using whole algae, lipid, and lipid extracted algae (LEA) as feedstocks was examined. The results showed that reaction pressure had minimal impact while temperature, biomass concentration and feedstock composition had significant effects on the composition of gaseous products. It was also found that low temperature (400 oC) and biomass concentration of 40 wt% favoured the production of methane-rich gas. In contrast, high temperature (700 oC) and low biomass concentration (10 wt%) favoured hydrogen- rich gas production in all the three feedstock considered. The highest mole fraction achieved for CH4 was 53.45 mole%, 61.70 mole% and 52.20 mole% which corresponded to CH4 yield of 31.14 mmol/g, 56.90 mmol/g and 30.15 mmol/g for whole algae, lipid and LEA respectively. For H2 rich gas production, the highest mole fraction achieved were 55.77 mole%, 52.29 mole% and 55.34 mole% which corresponded to H2 yield of 75.44 mmol/g, 105.51 mmol/g and 73.49 mmol/g for whole algae, lipid and LEA respectively. The ranking order for the yield and lower heating value (LHV) of the product gas from the HTG process was lipid > whole algae > LEA. This study has shown that hydrogen-rich and methane-rich gas can be produced from the hydrothermal gasification of microalgae as a function of the reaction conditions and feedstock composition. Also, the suitability of nutrient stressed approach and use of catalysts to enhance the quality of bio-oil produced from thermal conversion of microalgae biomass was established.