Effects of Inorganics during Hydrothermal Liquefaction of Waste: A Comprehensive Study

Hydrothermal liquefaction (HTL) is gaining interest for the energy valorization of wet waste. While HTL performance is known to depend on biochemical composition, the role of inorganics remains poorly understood. This study evaluates the effects of the four most common metals (Na, K, Mg, and Ca) present as oxides, carbonates, phosphates, sulfates, and chlorides. Experimental results, supported by principal component analysis (PCA), revealed that inorganics significantly influence HTL performance, depending on both cation and anion type. More basic anions generally decreased solid production while favoring both biocrude and aqueous-phase yields, with carbonates performing better than oxides despite their lower basicity. Na and K enhanced these effects compared to Ca and Mg, while K and Ca led to higher HHVs and lower oxygen content in the biocrude than Na and Mg, respectively, indicating a specific role of the cations. Sodium and potassium carbonates performed best, increasing biocrude yield by 48% relative to the corresponding inorganic-free feedstock, while reducing solid production by 90%. CaCl₂ was the only compound reducing biocrude yield, while increasing solid residue by 90%. This study highlights the critical influence of inorganics on HTL performance and provides a foundation for deeper insights into the underlying mechanisms.