Wet air oxidation of HTL aqueous waste

Abstract

Hydrothermal Liquefaction (HTL) provides a pathway to reducing wet waste volumes and producing biofuels such as SAF or sustainable diesel. However, 20–30% of the carbon in wet-waste feed is converted to low molecular weight, water-soluble organic compounds resulting in an aqueous product requiring treatment before disposal. Extraction of fuels or chemicals from the aqueous product is an appealing but challenging route due to the low concentrations and the wide variety of the organic compounds. Herein, we report on wet air oxidation treatment of the HTL aqueous stream to reduce its complexity and increase the concentration of acetic acid in the aqueous product stream.Catalyst screening was carried out at small scale followed by batch reactor studies of selected catalysts. Oxidized products were analyzed to determine the impact of wet air oxidation on chemical oxygen demand (COD), nitrogen content, oxygen consumption and intermediate by-product formation. Batch reactor studies found that metal catalysts including WO3 and ZrO2 increased the concentration of acetic acid in the product stream by at least 10–20% compared to non-catalytic wet air oxidation. Under increasingly intense reaction conditions, COD was further reduced and acetic acid concentration increased. For example, using WO3 at 225 °C increased the concentration of acetic acid in the product stream by 72%, along with significant increases in formic acid. A continuous flow trickle bed reactor with a CeZr catalyst at temperatures ≥200 °C reduced the COD of the aqueous stream by 50% and increased acetic acid concentration by 40% (200 h run).