Hydrothermal processing of agricultural and food waste
Population growth, leads to high production of agricultural and food waste that challenges the sustainability food production systems. Hydrothermal liquefaction (HTL) is an attractive chemical process to generate energy and bio-based chemicals from high water content waste biomass. HTL offers opportunities for an energy efficient valorization of organic waste streams, such as sewage sludge, animal manure, food wastes, etc. The technology is based on accelerated hydrolysis and condensation reactions that are favorably influenced by the substantial changes in the properties of water (i.e., density, dielectric constant and ion dissociation constant) that occur near its critical point (Tc=374°C; Pc=22 MPa). Here, we elucidate the effect of adding acid and alkali to HTL of two waste feedstocks: carbohydrate-rich food waste and cattle manure. HTL reactions were conducted at 300 °C for 60 min, with and without the addition of acid or base to the media. We measured the quantity and characterized the quality of the three main reaction products: crude-oil, hydro-char and aqueous product. For both feedstocks, carbon recovery distributions had wide ranges among (1) crude-oil (26–61 wt %); (2) hydro-char (1–36 wt %); and (3) aqueous product (9–49 wt %). The results suggest that manure is more affected than food waste by the addition of acid to the media. For the aqueous product, the addition of acid increased the production of cyclic furan compounds and decreased the recovery of C1–4 carboxylic acids. GC–MS analysis of the crude-oil confirmed that dehydration reactions were enhanced by adding acid to the reaction media. FTIR spectroscopy coupled with principal component analysis (PCA) 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.