Pyrolysis of pulp and paper mill sludge: Mechanistic effects of process conditions and feldspar catalysis on product formation and biochar carbon permanence

We investigated the pyrolysis of pulp and paper mill sludge (PPMS) as a carbon dioxide removal strategy by examining the effects of feedstock moisture (0 and 30 wt%), temperature (400–800 °C), and ex-situ catalysis using feldspar minerals on product yields, carbon and nitrogen recovery, chemical speciation, and biochar carbon stability. Product distribution was temperature-dependent: biochar was favored at low temperatures, biocrude at intermediate temperatures, and syngas at high temperatures. Without catalysis, wet PPMS produced more biochar and CO₂-rich syngas but less biocrude and aqueous coproducts compared to dry PPMS. While feldspar catalysts had negligible effects on biochar yield due to the ex-situ configuration, Na-feldspar—with greater Lewis acidity than K-feldspar—significantly enhanced biocrude yield and increased methane and hydrogen content in the syngas, particularly from wet PPMS. These improvements originated from the activated CO₂ methanation, water–gas shift, and hydrocarbon-forming gas-phase reactions, e.g., dimerization and telomerization. Nitrogen distribution varied with temperature: it was retained in biochar at low temperatures and migrated to the aqueous phase as amines and pyrroles at higher temperatures. Shrinking core kinetic modeling of biochar oxidation, supported by elemental analysis, solid-state ¹ ³C NMR, and reflectance microscopy, revealed the formation of inertinitic, graphitic-like carbon structures. Biochars produced at higher temperatures exhibited increased oxidation resistance, with extrapolated half-lives from 0.25 to 1 and from 1 to 5 million years for wet-derived and dry-derived samples, confirming geological-scale carbon stability. These findings underscore the potential of optimized pyrolysis conditions for producing stable biochar while enhancing the energy value of co-produced biocrude and syngas.