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Wet Oxidation for Organic Carbon Recovery from Hydrothermal Liquefaction Byproducts

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Abstract

Hydrothermal liquefaction (HTL) is a process for producing liquid bio-oil having high carbon density from wet biomass feedstocks, such as food waste, wastewater sludge, and livestock manure. The feedstock is subjected to temperatures between 250°C-400°C and pressures up to 30 MPa. HTL is regarded as a promising technology in the transition toward a sustainable and decarbonized future; it can offer a reliable strategy for reducing dependence on crude oil. However, a major challenge associated with HTL is the formation of an aqueous phase (AP), rich in carbon, nitrogen, phosphorous, and potassium. This byproduct is often discarded without recovering any useful products. My work focuses on the recovery of carbon from this AP using wet oxidation, which involves thermochemical processing with added oxygen at temperatures ranging from 200-500°C. Specifically, subcritical (WAO) and supercritical water oxidation (SCWO at T > 374°C and P > 220 bar) were studied, where WAO aimed to maximize acetic acid formation, and SCWO reduced the total chemical oxygen demand of the process water. Kinetic models for different classes of hydrocarbons present in the AP were developed to include a range of relevant oxidation and decomposition pathways. This study presents a modular approach to managing of organic waste, specifically dairy manure. HTL of manure digestate is modeled to obtain maximum bio-oil yield. Following HTL, wet oxidation of the HTL-AP is modeled to determine the amount and concentration of desired carbon product at different temperatures and residence times. The opportunity to make the value-chain net carbon-negative by employing biomethanation to convert CO2 to Renewable Natural Gas (RNG), is also explored. An economic analysis is conducted to study if market prices for recovered value-added chemicals (i.e., acetic acid) and prices and/or incentives for greener co-products (like RNG) can offset the high capital and operating costs for HTL and wet oxidation. Finally, to explore the potential of using different feedstock types, a preliminary comparison has been made for various organic wastes, based on the recoverable amount of acetic acid and its economic feasibility.

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122 pages

Supplemental file(s) description: Zip file containing all required scripts and a sample input spreadsheet.

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Date Issued

2023-08

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Keywords

bioenergy; circular bioeconomy; hydrothermal liquefaction; renewable energy; sustainability; waste valorization

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Committee Chair

Tester, Jefferson

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Lei, Xingen

Degree Discipline

Chemical Engineering

Degree Name

M.S., Chemical Engineering

Degree Level

Master of Science

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Government Document

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Attribution 4.0 International

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dissertation or thesis

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