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The objective of this study was to elucidate the effect of adding acid and alkali to hydrothermal liquefaction (HTL) of two waste biomass feedstocks: manure digestate and carbohydrate-rich food waste. HTL reactions were conducted at 300 °C for 60 min, with and without the addition of acid or base. We measured the quantity and characterized the quality of the three main HTL products: oil, aqueous and hydro-char. For both feedstocks, carbon recovery distributions had wide ranges among (1) biocrude oil (26–61 wt %), (2) aqueous product (9–49 wt %) and (3) hydro-char (1–36 wt %). The addition of acid affected HTL reactions for manure more than for food waste. For the aqueous phase, the addition of acid decreased the recovery of C1–4 carboxylic acids and increased the production of cyclic furan compounds. GC–MS analysis of the biocrude oil suggested that dehydration reactions were enhanced by adding acid to the HTL media. FTIR spectroscopy coupled with principal component analysis showed that hydro-char samples cluster according to acid-modified and base-modified reactions, based on distinct chemical structures. This study clarifies the role of pH during HTL and its effect on chemical pathways and carbon distribution among products.

A contribution to biomass conversion into sugars and lignin using a compact reactor that is easy to scale-up was made. Wheat bran was continuously fractionated under supercritical water conditions.

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9This article summarizes the recent efforts in the High Pressure Processes Group labs at UVa 10 regarding the fundamentals of biomass hydrolysis in pressurized water medium. At extremely 11 low reaction times (0.02 s), cellulose was hydrolyzed in supercritical water (400ºC and 25 12 MPa) obtaining a sugars yield higher than 95% w·w -1 while 5-HMF yield was lower than 13 0.01% w·w -1 . When the reaction times was increased up to 1 s, the main product was 14 glycolaldehyde (60% w·w -1 ). Independently of the reaction time, the yield of 5-HMF was 15 always lower than 0.01% w·w -1 . In order to evaluate the reaction pathway and mechanism of 16 plant biomass in pressurized water, several parameters (temperature, pressure, reaction time 17 and reaction medium) were studied for different biomasses (cellulose, glucose, fructose and 18 wheat bran). It was considered that the reactions of glucose isomerization to fructose as well 19 as fructose dehydration to 5-HMF take place via proton or hydroxide anion association. So, 20 their concentration was taken into account as reagent concentration in the reaction 21 evaluations. It was found that the proton and hydroxide anion concentration in the medium 22 due water dissociation is the determining factor in the selectivity of the process. The reaction 23 of glucose isomerization to fructose and its further dehydration to produce 5-HMF are highly 24 dependent on ions concentration. By increasing pOH/pH, these reactions were minimized 25 allowing the control of 5-HMF production. At this condition, the retro-aldol condensation 26 pathway was enhanced instead of isomerization/dehydration pathway. 27 3 INTRODUCTION 28

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