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

Abstract

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.

Topics & Concepts