Algal Biofuels Production using Hydrothermal Liqufaction of Microalgae and Hydrotreating of Biocrude Oil over Algal Hydrochar-Based Catalysts
Date
2021-06-02
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Degree Level
Doctoral
Abstract
Due to the environmental concerns related to CO2 emissions and the finite supply of energy from non-renewable fossil fuels, more attention has been paid to renewable energy. Of the candidate biomass (as sustainable energy source), microalgae has been considered one of the most promising alternatives for biofuel production, due to its high growth rate and the high CO2 capture ability compared to other biomasses. Eco-friendly transportation fuel such as biofuel produced from algal biocrude oil upgradation is considered a promising alternative due to its environmentally favorable and superior properties such as low sulfur content, non-toxicity and better lubricating efficiency. The overall objective of this research was to aid the development of commercially feasible technology for the production of sustainable fuels from microalgae. The study plan for this research was divided into four sub-objectives or phases.
In first phase, production and characterization of biocrude oil and hydrochar obtained from hydrothermal liquefaction of microalgae using methanol-water in a batch reactor system was investigated. The effects of methanol to water mass ratios at critical conditions were investigated to determine the maximum biocrude oil production. The comparatively higher yield of biocrude oil (47 wt.%) obtained at a methanol-water mass ratio of 0.75:0.25 also contained a higher amount of ester components resulting in higher biocrude oil quality. Response surface methodology was applied to study the effects of temperature (222-322°C), and reaction time (10-60min) at a constant pressure of 11.5MPa for methanol-water and biomass-solvent ratios of 0.75:0.25 and 1:5, respectively. The optimum yield of biocrude oil (57.8 wt.%) and the highest energy recovery (85.3%) was obtained at 272°C and a reaction time of 35 min. Subcritical conditions (temperature of 222°C, pressure of 11.5MPa) resulted in the highest hydrochar yield (19.5 wt.%).
Suitable utilization of the hydrochar obtained from the hydrothermal liquefaction (HTL) process could improve the overall economics of algal biofuel production. As hydrochar shows a low porosity, chemical activation becomes necessary to improve its physico-chemical properties. Hence, in second phase, a systematic approach was employed to study the effects of different activation factors such as temperature (T), impregnation ratio (mass ratios of KOH and hydrochar) (R), nitrogen flow rate (F), and different chemical activators during the chemical activation process on the characteristics of activated carbon obtained from hydrothermal algal-derived hydrochar. Based on the optimum condition of T=675 ℃, R=1.5 and F=267 cm3/min and using potassium carbonate as a chemical agent, the highest BET surface area of 2638 m2/g was obtained, which also revealed micropore and mesopore volumes of 0.68 and 1.02 cm3/g, respectively, with 79 wt.% of carbon content and a yield of 63.1 wt.%.
Since algal biocrude oil obtained from HTL process explained in the first phase had a high amount of oxygenated compounds (14.5 wt.%), it cannot be used directly as a transportation fuel and requires further processing to remove heteroatoms. Therefore, in third phase. hydrodeoxygenation (HDO) was used to upgrade the HTL biocrude oil. The most significant challenge for HDO of biocrude oil is developing a cost effective catalyst with high activity. Hence, in the third phase, a novel heterogeneous catalyst using activated algal-derived hydrochar as a support was developed. In this regard, for the first time different impregnation (incipient or co-impregnation) and reduction methods were used to synthesize the carbide phase of activated algal-derived hydrochar-supported NiMo to study their effects on catalyst characteristics, as well as their application for hydrodeoxygenation of algal biocrude oil to produce value added hydrocarbons. The NiMo carbide synthesized through co-impregnation and carbothermal reduction processes showed high activity for oxygen removal due to its higher acidity and active phase (Mo2C) as well as providing active hydrogen for HDO reactions. At reaction conditions of T=400 ℃, t=2.75 h and 10 wt.% catalyst loading, a minimum oxygen content of 0.9 wt.% due to removal of 94 wt.% oxygen from algal biocrude oil using NiMoC catalysts was achieved.
In fourth phase, techno-economic analysis (TEA) and life cycle analysis (LCA) of algal biofuels production in a two-stage process were investigated. Aspen plus simulation and SimaPro software were used to analyze process economics and greenhouse gas (GHG) emissions. The minimum fuel selling price (MFSP) for two stages of algal biofuels production was $8.8/gal to balance total production cost. For this study, the discounted cash flow rate of return (DCFROR) was 23%, greater than the internal discount rate, which means the project was profitable. Thus, the proposed two-stage HTL and catalytic HDO provides a feasible and profitable technology for the production of high quality algal biofuels. The effects of process conditions for biofuels production on the GHG emissions performance were estimated at -1.13 g CO2-eq/MJ, which is much lower than petroleum-based fuels GHG emissions of 91 g CO2-eq/MJ.
Description
Keywords
Algal biofuels, Hydrothermal liquefaction, Hydrodeoxygenation, Algal hydrochar-based catalysts
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Chemical and Biological Engineering
Program
Chemical Engineering