Optimizing co-hydrothermal carbonization of organic dairy manure and hemp stalks: a multi-criteria decision-making approach

The increasing demand for environmental sustainability practices highlights the importance of valorizing agricultural residues into efficient biofuels. This study focuses on the co-hydrothermal carbonization (co-HTC) of organic dairy manure (ODM) and hemp stalk (HS) residue to enhance the combustion performance of the hydrochar and assess liquid-phase valorization potential. HTC experiments were conducted at 180, 220, and 260 °C with varying HS:ODM ratios (1:0, 3:1, 1:1, 1:3, 0:1), and hydrochars were evaluated for physicochemical properties, combustion behavior, and emission factors estimation. A hybrid Analytic Hierarchy Process (AHP) – Entropy – TOPSIS multicriterial analysis integrated technical, environmental, and process-related indicators to rank the hydrochars.

Key findings indicate clear synergistic effects at low to moderate temperatures, where blending improved energy yield, combustion reactivity, and solid-phase carbon recovery. Although HY_180–0:1 achieved the highest overall performance score, its Alkali Index (AI) exceeded the high-risk fouling threshold (> 0.34 kg/GJ), as did other ODM-rich hydrochar, excluding them for practical combustion. In contrast, HY_{180–0.3:0.7} and HY_{180–0.7:0.3} emerged as the most combustion-viable hydrochars, combining high multi-criteria performance with acceptable AI levels.

This work highlights the novelty of systematically integrating multi-criteria decision-making with fouling risk assessment, ensuring both optimal performance and operational feasibility, supporting sustainable waste-to-energy pathways, and advancing circular economy practices in the agricultural sector. The findings demonstrate that co-HTC can strategically tailor hydrochar properties by leveraging complementary feedstock characteristics. By enabling renewable fuel production (SDG 7), advancing circular waste management (SDG 12), and mitigating emissions (SDG 13), this approach strengthens the role of hydrochar as a clean energy carrier.