Hydrothermal coliquefaction of anaerobic digestate with polyphenolic extracts from agricultural byproducts producing nearly nitrogen-free biocrude oil†

We studied the potential of producing biocrude with ultralow nitrogen content via hydrothermal coliquefaction (co-HTL) of sewage sludge digestate and various polyphenolic extracts from apple pomace, olive pomace, spent coffee grounds, and sweet orange peels. We investigated the combined effects of the polyphenol profile, reaction temperature (280–370 °C), and feedstock pH (3–11) on product formation and element migration and speciation including the responsible reaction mechanisms and their kinetics and thermodynamics. In general, high contents of epicatechin, quercetin, caffeic acid, ferulic acid, and gallic acid in the feedstock mixture caused a significant reduction in the N-content of biocrude by trapping cellulose-derived α-dicarbonyls/α-hydroxycarbonyls (i.e., key precursors for N-fixing reactions in biocrude) and converting them into solid and aqueous N-polyheterocycles and amidated O-polyheterocycles via several mechanisms, e.g., electrophilic aromatic substitution, nucleophilic addition, Paal–Knorr furan synthesis, and amination. Coupled with more acidic conditions and higher reaction temperatures, the rate of carbonyl trapping was improved as the activation energy decreased and the nitrogen distribution into hydrochar was enhanced via endothermic amidation of the –COOH group on hydrochar. This was followed by the promoted depolymerization of lignocellulose into more-stable biocrude constituents and the improved deoxygenation of biocrude via dehydration, minimizing carbon loss. Thus, co-HTL of acidic (pH 3) mixtures of digestate and polyphenolic extracts at 370 °C for 60 min produced biocrude with an acceptable mass yield of around 30% and desirable elemental content compatible with upgrading in oil refineries, i.e., C: 72.0–73.8%, H: 9.1–10.4%, N: 0.04–0.27%, S: 0.01–0.03%, and O: 16.3–18.3%.