Torrefaction and Pyrolysis of Lignocellulosic and Organo-Mineral Pellets: A Comparative Study of Green Coconut Husk and Sewage Sludge

Abstract Thermochemical conversion is a promising route for the valorization of residual biomass, enabling energy recovery and the production of solid, liquid, and gaseous products. However, direct comparisons between torrefaction and pyrolysis of chemically distinct pelletized residues remain limited, obscuring the influence of feedstock composition on mass and energy partitioning. Here, green coconut husk and sewage sludge pellets were subjected to torrefaction and pyrolysis in the same batch-operated reactor to determine how lignocellulosic and organo-mineral feedstocks respond to increasing thermal severity. Torrefaction was performed at 250 °C, while pyrolysis was conducted at 400 and 600 °C under a nitrogen atmosphere, using 1.0 kg of pellets per process. Green coconut husk pellets showed higher volatile matter and fixed carbon contents than sewage sludge pellets (50.61 ± 4.84% and 30.30 ± 3.92% versus 38.44 ± 3.94% and 7.35 ± 1.14%, respectively), whereas sewage sludge pellets presented higher ash, nitrogen, and sulfur contents (45.85 ± 4.42%, 2.88 ± 0.94%, and 0.70 ± 0.12%, respectively). Increasing thermal severity reduced solid yield and increased volatile product formation. At 600 °C, solid yield decreased to 29.44 ± 1.12% for green coconut husk and 37.12 ± 3.32% for sewage sludge, while non-condensable gas yield reached 28.66 ± 1.85% and 20.27 ± 2.15%, respectively. The solid HHV reached 26.06 ± 1.99 MJ·kg–1 for green coconut husk at 400 °C and 18.34 ± 2.26 MJ·kg–1 for sewage sludge at 600 °C. At 600 °C, H2 content reached 36.12 ± 2.33% and 39.22 ± 3.03% for green coconut husk and sewage sludge, respectively, with H2/CO ratios of 2.03 and 2.47, respectively. Torrefaction showed higher solid energy yield at 250 °C, reaching 74.25% for green coconut husk and 74.17% for sewage sludge, indicating better energy retention in the solid fraction. In contrast, pyrolysis promoted broader energy redistribution into condensable liquids and combustible gases. Overall, torrefaction favored solid-phase energy retention, whereas pyrolysis promoted a broader distribution of recovered energy among solid, liquid, and gaseous products.