Biomass and Waste Pyrolysis: A Critical Review of Reaction Pathways, Reactor Engineering, Product Quality, Catalytic Upgrading, and Industrial Scale-Up

Abstract

Pyrolysis is a thermochemical conversion route in which carbonaceous feedstocks are heated under oxygen-deficient or oxygen-free conditions to form a controllable mixture of solid, liquid, and gaseous products. This review critically synthesizes the engineering principles governing biomass and waste pyrolysis, with emphasis on reaction mechanisms, feedstock properties, operating parameters, reactor configurations, product formation, catalytic upgrading, environmental performance, and scale-up challenges. The literature shows that pyrolysis performance is not determined by temperature alone; heating rate, particle size, vapor residence time, pressure, mineral matter, moisture, reactor hydrodynamics, and secondary reactions jointly control product yield and quality. Slow pyrolysis generally favors carbon-rich solid products, whereas fast and intermediate pyrolysis can increase condensable vapors and bio-oil production when heat and vapor residence time are appropriately controlled. Fluidized-bed, circulating-bed, auger, ablative, and fixed-bed reactors each provide different compromises between heat transfer, solids handling, residence-time control, product recovery, and capital cost. Catalytic pyrolysis can reduce oxygenates and increase hydrocarbons or selected chemicals, but catalyst deactivation, coke deposition, contamination, regeneration, and product selectivity remain major barriers. Particular attention is given to oil-palm residues such as empty fruit bunches, palm kernel shells, mesocarp fiber, fronds, and trunks because they are abundant in Indonesia and provide a strong opportunity for decentralized thermochemical valorization. The review concludes that future development should move from yield maximization toward integrated process design, product quality, heat integration, emissions control, techno-economic assessment, life-cycle assessment, and digital process optimization.