Effect of Aging in Nitrogen and Air on the Properties of Biocrude Produced by Hydrothermal Liquefaction of Spirulina

Abstract

Hydrothermal liquefaction (HTL) allows direct conversion of wet biomass into biocrude oil, which has comparable energy content to petroleum. The stability of biocrude oil has significant effects on the downstream oil-refining and field applications. In this work, we investigated the physiochemical properties of HTL biocrude oil converted from Spirulina at different storage times and environment conditions: temperature (15 and 35 °C), headspace environment gas (air or N2), and storage duration (up to 12 weeks). A layer of “oxidation shell” was formed on top of the biocrude oil during the storage. The rigidity of the “oxidation shell” and the viscosity of the inner oil increased significantly when the temperature shifted from 15 to 35 °C. The total acid number of the biocrude oil decreased by 22.6–24% in an N2 environment but increased by 9.1–10.1% in air, regardless of the temperature, after long-term storage. Gas chromatography–mass spectrometry analysis revealed significant differences among chemical compositions in biocrude oil. Seven aging pathways were proposed for the physiochemical property transformation of algal biocrude oil during different storage scenarios. The formation of the “oxidation shell” was mainly due to the oxidation and polymerization of phenols and nitrogen-containing compounds, whereas the aging of the inner biocrude oil was caused by esterification and polymerization of esters, phenols, ketones, and nitrogen-containing compounds.

Viscosity Variation of Model Compounds during Hydrothermal Liquefaction under Subcritical Conditions of Water

“…The formation of high-molecular-weight organic compounds in solid products, which include 1-(2-furanyl)­ethanone, 2-ethyl-1 H -benzimidazole, 2-methylfuran, and 3-methyl-2-cyclopenten-1-one, during HTL between 250 and 300 °C contributes to the changes in the apparent viscosity of the reacting slurry. An increase in the production of high-molecular-weight compounds from 200 to 300 °C increases the apparent viscosity, while a decrease in the production of lower molecular weight compounds from 300 to 350 °C decreases the apparent viscosity of the slurry . The apparent viscosity of the reacting slurry increased by 0.58 Pa·s from 230 °C (70 bar) and peaked at 280 °C (114 bar), where solid formation was maximum.…”

“…An increase in the production of high-molecular-weight compounds from 200 to 300 °C increases the apparent viscosity, while a decrease in the production of lower molecular weight compounds from 300 to 350 °C decreases the apparent viscosity of the slurry. 32 The apparent viscosity of the reacting slurry increased by 0.58 Pa•s from 230 °C (70 bar) and peaked at 280 °C (114 bar), where solid formation was maximum. Above 280 °C and 114 bar, solids decomposed into the aqueous phase and recombined to form gaseous products, thereby decreasing the solid yield from 280 (114 bar) to 350 °C (200 bar) by 1%.…”

Evaluation of Storage Stability for Biocrude Derived from Hydrothermal Liquefaction of Microalgae

“…Various parameters such as water content, viscosity, and average molecular weight (AMW) were chosen to evaluate the storage stability of the pyrolysis bio-oil . However, some works focus on the storage stability of biocrude from the HTL process until recently, − while the continuous and pilot-scale HTL operating is rapidly developing. − Therefore, there is an urgent assessment demand for the practical application of biocrude, and a discussion about the storage stability of HTL biocrude should be given.…”

Factors Affecting the Sensitivity of Hydroxyl Group Content Analysis of Biocrude Products via Phosphitylation and 31P NMR Spectroscopy

“…The compositional complexity of biocrudes creates challenges for their refining or coprocessing with petroleum feedstocks. − Biocrudes also tend to be quite corrosive to metallic surfaces (especially at elevated temperatures), which adversely impacts their transportation, storage, and processing. − The corrosivity of biocrudes has been attributed to various protic oxygen containing molecular components, including alcohols, phenolics, carboxylic acids, and water. Such Brønsted acids are also known to catalytically accelerate dehydrative condensation − and oligomerization/polymerization reactions to form larger molecules and polymeric structures with reduced solubility, which lead to biocrude instability . In addition, condensation reactions may occur during biocrude aging, causing precipitation of deposits associated with clogging and fouling of process equipment. − An improved understanding of relationships between biocrude’s corrosivity or instability and its native hydroxyl group containing populations is needed to inform production best practices and aid further integration of biocrude into the energy industry.…”

“…Such Brønsted acids are also known to catalytically accelerate dehydrative condensation − and oligomerization/polymerization reactions to form larger molecules and polymeric structures with reduced solubility, which lead to biocrude instability . In addition, condensation reactions may occur during biocrude aging, causing precipitation of deposits associated with clogging and fouling of process equipment. − An improved understanding of relationships between biocrude’s corrosivity or instability and its native hydroxyl group containing populations is needed to inform production best practices and aid further integration of biocrude into the energy industry.…”