Alkaline-Promoted Hydrothermal Liquefaction of Polo Horse Manure for Biocrude Production

Horse manure (collected as pure feces) is a high-moisture, nutrient-dense organic waste stream with potential for valorization via hydrothermal liquefaction (HTL). However, catalyst selection is often reported without direct catalyst-class comparisons under identical conditions. In this study, manure collected in Thailand was characterized and converted via noncatalytic and catalytic HTL in a batch reactor at 300 °C for 40 min (feedstock/water = 1:10, w/w; heating rate 10 °C min^-1). Four catalysts (KOH, NaOH, H₃PO₄, NH₃·H₂O) were evaluated at 10 and 20 wt % (dry feed basis), with biocrude recovered by dichloromethane extraction and quantified gravimetrically (mean ± SD, n = 3). Noncatalytic HTL produced 15.4 wt % biocrude, whereas alkali promotion substantially increased yield: KOH produced 42.8 wt % (10 wt %) and 51.1 wt % (20 wt %), and NaOH produced 39.1 wt % (10 wt %) and 55.1 wt % (20 wt %), the highest yield obtained. In contrast, H₃PO₄ (10.7-11.3 wt %) and NH₃·H₂O (4.6-11.7 wt %) did not improve yields relative to the baseline. Chemical characterization via FTIR revealed mixed aliphatic and oxygenated functionalities in both biocrudes. Furthermore, GC-MS analysis of the highest-yield condition (20 wt % NaOH) showed a dominance of lipid-like ester species (75.85% peak area). Ultimate analysis indicated that CAT-HTL increased the estimated higher heating value (HHV) of the product. Overall, alkali promotion particularly 20 wt % NaOH was the most effective strategy among tested catalysts for maximizing biocrude yield from horse manure under fixed reaction severity.