|Abstract:||The objective of this study is to develop viable technologies to upgrade biocrude oil converted from wet biowaste via hydrothermal liquefaction (HTL). Three types of feedstocks (algae, swine manure, and food processing waste) were converted into biocrude oil via HTL for upgradation processes, which includes pretreating feedstocks, separation, esterification and neutralization of biocrude oil.
Previous studies have revealed that excessive ash content in mixed-culture algal biomass (AW) appeared to reduce the higher heating value (HHV) and hydrocarbon compositions in the biocrude oil. To resolve this issue, physical pretreatments on AW biomass were carried out to decrease the ash content and improve the biocrude oil quality. AW biomass with different ash content was converted into biocrude oil via HTL at 300°C for 1 h reaction time, which is the previously determined optimum condition for producing biocrude oil. As the ash content of AW biomass was decreased from 53.3 wt.% to 39.0-43.5 wt.% after screen pretreatments, the HHV of the biocrude oil was substantially improved from 27.5 MJ/kg to 32.3 MJ/kg and the amounts of the light oil (boiling point of 100-300°C) were increased from 31wt.% to 49 wt.%. In contrast, GC-MS analyses of pretreated algal biocrude oil and aqueous products demonstrate that the ash content promoted denitrogenation, catalyzed the formation of hydrocarbons, and mitigated the recalcitrant compounds in aqueous products under the HTL processes. In order to elucidate the role of the ash content under the HTL processes, model algae, Chlorella, with different amounts of representative ash content (egg shells in this study) were converted into biocrude oil at the same reaction condition (i.e., 300°C for 1 hr reaction time). Elemental and thermogravimetric analyses of the biocrude oil both show that when the ash content in the algal feedstock was below 40 wt.%, the HHV and boiling point distribution of the algal biocrude oil could be hardly changed. This result signifies the feasibility of using ash-rich biomass as an HTL feedstock and diminishes the necessity of multi-step pretreatments of ash-rich biomass for biofuel applications.
This study also demonstrates a proof-of-concept in the production of high quality renewable biofuel from wet biowaste via hydrothermal liquefaction (HTL). Distillation was employed to effectively separate the biocrude converted from swine manure (SW), food processing waste (FPW), and Spirulina platensis (SP) via HTL into different fractions. Distillation curves of different types of HTL biocrude oil were reported. Physicochemical characterizations, including density, viscosity, elemental test, chemical compositions, and acidity, were conducted on distillates separated from different feedstocks. SW-, FPW-, and SP-derived biocrude respectively contains 15 wt.% , 56 wt.%, and 15 wt.% distillates with heating values of 43-46 MJ/kg and alkanes with carbon numbers ranging from C8 to C18. Compared to the distillates from SW- and SP-derived biocrude oil, the distillates from FPW-derived biocrude demonstrate the closest density and energy content to petroleum diesel, though this type of distillates contained an excessively high acidity that need to be reduced from 35.3 mg KOH/g to ≤ 3 mg/g (the requirements suggested by the ASTM standard for a 10 vol.% biodiesel). Therefore, an orthogonal array design of esterification experiment was performed to optimize the reaction temperature (50-70°C), reaction time (0.5h-6h), catalysts concentration (0.5 wt.%-2 wt.%), and the molar ratio of FPW-distillates to methanol (1:5-1:15), for achieving the lowest acidity. Compared to other available methods to upgrade HTL biocrude oil, the integrative upgrading approach proposed by this study (distillation plus esterification/neutralization) demonstrates a competitive energy consumption ratio (0.03-0.06) to zeolite cracking (0.07), supercritical fluid (SCF) treatment (0.17), and hydrotreating (0.24) (assuming 50% heat is recovered from upgrading processes). Moreover, the reaction severity of the upgrading approach used in this study (with log Ro of 5.9-9.5) is much lower than those of zeolite cracking (with log Ro of 11.0), SCF treatment (with log Ro of 10.6), and hydrotreating (with log Ro of 11.3), without the consumption of high pressure hydrogen gas. Finally, the fuel specification analysis and engine test were conducted with the drop-in biodiesel, which was prepared with 10 vol.% (HTL10) and 20 vol.% (HTL20) upgraded distillates and 80-90 vol.% petroleum diesel. According to the fuel specification analysis, HTL10 and HTL20 exhibited a qualified Cetane number (>40 min), lubricity (<520 µm), and oxidation stability (>6 hr), as well as a comparable viscosity (0.2%-19% lower) and net heat of combustion (3%-4% lower) to those of petroleum diesel. Further, diesel engine tests demonstrated that HTL10 can lead to a superior power output (8% higher) and lower emissions of NOx (3-7%), CO (1-44%), CO2 (1-4%), and unburned hydrocarbons (10-21%). The present study showcases an energy-efficient and technically cohesive approach to produce renewable high-quality drop-in biofuels for demanding transport applications.