Direct and Two-Stage Hydrothermal Liquefaction of Chicken Manure: Impact of Reaction Parameters on Biocrude Oil Upgradation

Hydrothermal liquefaction (HTL) provides advantages to traditional methods (e.g., landfilling and composting) to convert wet biomass waste (e.g., agricultural residue) into energy-rich biocrude oil (bio-oil) and nutrient-rich aqueous phase. The challenge associated with these feedstocks lies in their high N and O contents. These heteroatoms are fixed into bio-oil produced from direct-HTL, making it infeasible as a drop-in fuel due to the low calorific value. To address these challenges for chicken manure, this study evaluated two thermochemical conversion approaches: (1) direct-HTL and (2) a two-stage process of hydrothermal carbonization (HTC) followed by HTL. The second scenario aimed to extract most N and oxygenates into the aqueous phase, producing C-rich bio-oil from the second stage (i.e., HTL). Experiments were conducted at different temperatures (160–350 °C), reaction times (30–60 min), and feedstock pHs (4–9). Acidic conditions were achieved by adding acetic acid as a catalyst, whereas the natural feedstock pH of chicken manure was around 9. Experiments revealed that the bio-oil properties improved from two-stage processing of acidic feedstock with pH = 4–5 with HTC conducted at 190 °C for 30 min and HTL at 300 °C for 30 min. Due to reduced O and N contents, the higher heating value of bio-oil increased from 32–33 (direct-HTL) to 37–38 MJ/kg (two-stage). Nonetheless, the overall C recovery in the bio-oil decreased from ∼35 to ∼20% compared with direct-HTL. This shows a trade-off between removing as many heteroatoms as possible and maximizing C recovery in bio-oil. A mechanistic study revealed the underlying degradation mechanisms (dehydration, decarboxylation, and denitrogenation) in direct-HTL and two-stage along with inhibition of Maillard reaction under the acidic environment.