Thermochemical conversion of anaerobic digestate into light transportation- biofuel over HZSM-5 catalyst

Abstract

In this study, anaerobic digestate (AD), a by-product of biogas production, was thermochemically converted into gasoline-like biofuel through pyrolysis and catalytic upgrading using HZSM-5 zeolite. Non-catalytic pyrolysis was first performed over a temperature range of 400–700 °C to determine optimal operating conditions. A maximum liquid bio-oil yield of 43.5 wt% was achieved at 550 °C. Catalytic pyrolysis under the same conditions reduced bio-oil yield (35.8 wt%) but significantly enhanced product quality. GC-MS analysis showed that catalytic upgrading with HZSM-5 effectively reduced oxygenated compounds by up to 75.6 % and nitrogen-containing species by 33 %, while increasing hydrocarbon content from 34.2 % to 55.3 %. The presence of HZSM-5 favored the formation of monoaromatic hydrocarbons and shifted the hydrocarbon distribution toward lighter gasoline-range compounds (C5–C10). Elemental analysis confirmed a higher heating value (HHV) improvement from 21.53 to 31.39 MJ/kg with catalytic treatment. Additionally, the produced biochar showed high levels of essential nutrients like phosphorus and potassium, indicating potential application as a soil amendment. The findings demonstrate that catalytic pyrolysis of AD over HZSM-5 offers a promising route for producing cleaner, high-energy biofuels and valuable by-products, contributing to sustainable waste valorization and renewable energy development.