Mechanistic influence of calcium-rich inorganics in pulp and paper mill sludge on the organics pyrolysis

The mechanisms by which calcium-rich inorganics within pulp and paper mill sludge (PPMS) influence the pyrolysis of its organic components remain largely unexplored. This study investigated the effects of inorganic constituents on the evolution of pyrolysis products from major organic components (cellulose, plastics, lipids, proteins, and ink) in PPMS through fixed-bed pyrolysis experiments. The results demonstrated that inorganics significantly promoted secondary tar reactions by improving heat transfer, providing catalytic sites, and prolonging the residence time of pyrolysis vapors. These effects markedly increased gas yields, particularly CO 2 (e.g., by 29.27–35.41 mL/g for cellulose), and decreased tar yields (e.g., by 10.27–11.38 wt% for cellulose), while also altering tar composition. Specifically, calcium-rich inorganics catalyzed the conversion of cellulose into small ketone molecules such as hydroxyacetone, reducing the proportions of anhydrosugars and furans. For plastics, PPMS ash promoted the β -scission of long-chain aliphatic hydrocarbons and the transformation of monocyclic hydrocarbons into polycyclic aromatic hydrocarbons while also increasing char formation. For lipids, calcium-rich inorganics effectively catalyzed the decarboxylative ketonization of palmitic acid to produce 2-heptadecanone. For proteins, PPMS ash suppressed NH 3 release and promoted nitrogen migration into nitrogen-containing heterocyclic compounds. For ink, the inherently high ash content limited the regulatory effect of additional ash. Overall, this study provides mechanistic insights into PPMS pyrolysis and supports its sustainable conversion.