Synergistic Effects and Pseudo-Component Behavior in Ternary Co-Pyrolysis of Low-Rank Coal with Pectin- and Lignin-Rich Agricultural Residues

This study investigates the ternary co-pyrolysis of Soma lignite (SL), a low-rank Turkish coal with high ash content, with two agricultural residues: pectin-rich sugar beet pulp (SBP) and lignin-rich peanut shell (PS). The primary objective is to clarify how biomass structure and blend composition control synergistic interactions, and how co-pyrolysis can upgrade the fuel properties of a low-quality coal while valorizing agro-industrial waste. Four SL:SBP:PS blends (80:10:10, 60:20:20, 40:30:30, and 20:40:40 wt.%) were tested by non-isothermal thermogravimetric analysis at 10 ${}^{\circ }$C min${}^{-1}$ under nitrogen. Differential thermogravimetric curves were deconvolved into four pseudo-components representing pectin/hemicellulose, cellulose, lignin/early coal, and main coal/mineral fractions. Mass-based deviation indices ($\Delta W$) and rate-based deviations ($\Psi$) from the additive prediction were calculated in three temperature regions to detect synergy and antagonism. The results demonstrate that interactions are strongly composition-dependent. The 40:30:30 blend exhibits the most pronounced synergistic enhancement, with average $\Delta W$ values of approximately −0.94 wt.% and −1.05 wt.% in the 350–500 ${}^{\circ }$C and 500–650 ${}^{\circ }$C ranges, respectively, while the 60:20:20 blend shows antagonistic behavior across all regions. For the 40:30:30 blend, the calculated higher heating value increases from 11.21 to 14.74 MJ kg${}^{-1}$, reflecting a gradual upgrading of the feed-mixture composition by biomass loading. Overall, the findings indicate that combining a pectin-rich, fast-devolatilising biomass with a lignin-rich, slower-decomposing biomass at an intermediate coal loading can shift mass loss to lower temperatures. This combination also produces measurable non-additive behaviour within the experimental noise level. In addition, it improves several feed-mixture indicators that are relevant to sustainable energy recovery from lignite-dominated regions.