Hydrothermal liquefaction of marine macroalgae.
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Date
2021
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Abstract
The biofuel industry has experienced substantial growth during the past decade due to the extreme demands placed on the fossil fuel industry and the limited availability of fossil fuels. Biofuels are seen as a renewable source of energy while reducing the effects on the environment significantly. Renewable biofuels are made through the use or conversion of biomass such as algae and lignocellulosic biomass. Biomass is seen as a viable alternative to produce biofuel as it is readily available, and has a relatively low cost. Marine macroalgae (seaweed) may be considered as a feedstock for biofuel production due to their low cost, fast growth rate, and they do not cause land-use and fuel-vs-food conflicts. Hydrothermal liquefaction is a thermochemical process that utilises water as a reaction medium under high pressures and temperatures to produce bio-oils from biomass. Hydrothermal liquefaction is different from most other conversion techniques as it uses a wet feedstock and does not require an energy-consuming drying step.
In this work, hydrothermal liquefaction of marine macroalgae for the production of bio-oil was studied at various reaction conditions. The effect of the mass of seaweed, temperature, pressure, solids loading and reaction time were examined. A kinetic model of dissolution was developed and regressed against the experimental temporal data to obtain the kinetics of dissolution. A measured quantity of marine macroalgae and water were placed within the Parr reaction vessel and exposed to high temperatures and pressures for a set time. The resulting solution was filtered, to separate the algae from the liquid (water and bio-oil solution), and mixed with dichloromethane, to selectively separate the bio-oil from the water. The dichloromethane mixture was transferred to the rotary evaporator and the dichloromethane was evaporated to ensure only the bio-oil remained. The bio-oil was measured and transferred to the GC/MS for a more in-depth compositional analysis.
Bio-oil was formed for every variation of the process variables and every run conducted. The highest bio-oil yield obtained was for the 10g 10wt% run at the high reaction conditions (250°C and 4000 KPa) and a time of 30 minutes, with a bio-oil yield of 34.67%. This was for the highest manipulation of every process variable. The lowest bio-oil yield (not including the induction period) was obtained for the 6g 10wt% run at the low reaction conditions (200°C and 1500 KPa) and a time of 5 minutes, with a bio-oil yield of 18.14%. The bio-oil yield formed during the induction period ranged from 0.11% to 26.58%. A higher mass loading was observed to provide a higher dissolution and a higher bio-oil yield (ranging from 29.59% to 34.67% for a mass loading of 10g)). Higher temperatures and pressures were also found to increase the mass dissolution and bio-oil yield obtained. The higher solids loading of 10wt% observed a larger bio-oil yield (ranging from 27.96% to 32.62%) than a solids loading of 5wt% (ranging from 22.81% to 26.53%). The bio-oil yield was found to increase for an increase in the reaction time for every variation of the process variable.
The assessment of the quality of bio-oil through GC/MS analysis determined that the main compounds formed during the hydrothermal liquefaction process were hexanedioic acid (adipic acid), cyclopentene, hexadecenoic acid, phenol, butanone, ethanone, tetrapentacontane, furancarboxaldehyde, cyclohexane, and hexanedioic acid- bis (2-ethyhexyl) ester. A kinetic model was applied to the data obtained to determine the kinetic parameters of dissolution. The dynamic model was identified with the aid of MATLAB programming software. The kinetic models for the conversion of solids to bio-oil and the conversion of solids to the aqueous product have the same formula. The simplified model is expressed by the mass fraction of the solid biomass multiplied by the kinetic rate constant and then multiplied again by the exponential of the negation of the inhibition constant over the mass fraction of the solid biomass. Utilising both the non-linear least squares regression and the ode15s variable-step, variable-order solver, the kinetic reaction rates were determined to be 0.0059 g/g/s (𝑘1) for the conversion from solids biomass to bio-oil and 0.0103 g/g/s (𝑘2) for the conversion from solid biomass to the aqueous-phase product. The inhibition constants (𝑘3 and 𝑘4) were determined to be the same at a value of 4.44e-14.
The overall results of this work validate that the hydrothermal liquefaction of marine algae produces an adequate amount of bio-oil that may be further processed to produce biofuel. It was observed that higher process conditions resulted in higher bio-oil yields being obtained and that a kinetic model may be determined for the mass dissolution from the algae and bio-oil yield formed. The maximum yield of 34.67% obtained in this work was amongst the higher yield results for research in this section, while utilizing lower temperatures and a slightly higher reaction time, thereby requiring a lower amount of energy. The results of this work imply that enough bio-oil is formed from the hydrothermal liquefaction of marine macroalgae to allow for scale-up of the process to produce a cleaner biofuel fuel that may alleviate the demands placed on fossil fuel
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Masters Degree. University of KwaZulu-Natal, Durban.