(570f) Synergistic Mixed Functional Group Coupling for Biocrude Enhancement and Radical-Assisted Char Suppression during Hydrothermal Liquefaction

Hydrothermal Liquefaction (HTL) has emerged as a promising technology for converting organic waste into carbon-rich biocrude [1]. In the wastewater treatment industry, the addition of H₂O₂ to sewage sludge has shown benefits that may have applications for the HTL process, such as an increase in the solubility of organics and an overall decrease in particle size [2]. When tested, the addition of H₂O₂ to sewage sludge hydrothermal liquefaction (SS-HTL) increased biocrude carbon recovery by 48.7% while decreasing undesired char formation by 67%. To better understand the improvements seen in this process, experiments were conducted varying injection time for the H₂O₂ allowing for different mechanisms for sludge/peroxide interactions to be evaluated, with extensive and advanced ex situ molecular characterization.

As shown in figure 1, H₂O₂ injections were performed before heating began (t = -24 and -1 hours), directly when heating began (t=0 hours), and at multiple times during heat-up corresponding to temperatures of 30, 100, and 300 ℃ (t = 0.11, 0.19, and 0.55 hours, respectively). Pretreatment of sludge with H₂O₂ was found to be ineffective, suggesting that the reactive oxygen species formed from the H₂O₂ addition need to be active during the process for the yield enhancement to occur. Injections performed at reaction temperature also showed negligible increase in yield, which was consistent with independent analysis of H₂O₂ decomposition to water and molecular oxygen (half-life 3 seconds at 300 [3]) and control experiments with molecular oxygen addition to SS-HTL.

Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) was then performed on the biocrudes comparing H₂O₂ addition to SS-HTL to the baseline case. Over 12,000 unique species were identified with their relative abundances quantified and compared against the non-H₂O₂ case. The addition of H_­2­O₂ resulted in decreased relative abundance of compounds with six or more oxygen, correlating to an increase in the number of radical reactions with polyoxygenated carbohydrates. Highly nitrogenated compounds (N4+) were also found to be more common when H₂O₂ addition occurred, suggesting the prevalence of carbonyl-mediated Maillard reactions.

Sources:

1. LeClerc, H. O.; Tompsett, G. A.; Paulsen, A. D.; McKenna, A. M.; Niles, S. F.; Reddy, C. M.; Nelson, R. K.; Cheng, F.; Teixeira, A. R.; Timko, M. T. Hydroxyapatite Catalyzed Hydrothermal Liquefaction Transforms Food Waste from an Environmental Liability to Renewable Fuel. iScience 2022, 25 (9), 104916.
2. AOPs: Lin, H.; Li, X.; Yang, Y.; Wang, D.; Zhang, H.; Yang, M. Advanced Oxidation Processes (AOPs)-Based Sludge Conditioning for Enhanced Sludge Dewatering and Micropollutants Removal: A Critical Review. Sci. Total Environ. 2022, 807, 150673.
3. H₂O₂ Decomp: Lin, C. C.; Smith, F. R.; Ichikawa, N.; Baba, T.; Itow, M. Decomposition of Hydrogen Peroxide in Aqueous Solutions at Elevated Temperatures. J. Chem. Kinet. 1991, 23 (11), 971–987.