Carbon-negative syngas production via CO2-assisted catalytic pyrolysis of lignocellulosic yard waste: A case study on horse chestnut shells

Abstract

Yard trimmings, a major component of municipal solid waste, are abundant and carbon-neutral feedstocks used for renewable-energy production. However, their complex lignocellulosic structures limit the efficiency of energy conversion via conventional biological methods. This study presents a thermochemical strategy for syngas generation from yard trimmings. Among these, horse chestnut shells (HCSs) are one of the representative types of yard trimmings produced in large quantities worldwide from ornamental trees. Given their abundance and sustainability potential, HCSs were selected as the model biomass for this study. Lab-scale pyrolysis was conducted from 100 to 640 °C at a constant heating rate of 10 °C min−1 under a CO2 atmosphere, which enhanced the process sustainability by promoting gas-phase reactions with HCS-derived volatiles. In this setting, CO2 acted as a mild oxidant, converting volatile biocrude components into CO and facilitating syngas production. The incorporation of a 5 wt% Ni-based catalyst accelerated these CO2-assisted reactions, increasing the syngas yield (CO/H2 ratio: 3.7) by 2.51-fold compared to that of conventional catalytic pyrolysis under N2. Net CO2 emissions were reduced to −87.55 mg g−1 HCSs, indicating effective CO2 utilisation during the process. These results establish CO2-assisted catalytic pyrolysis as an effective method of valorizing lignocellulosic waste while contributing to CO2 mitigation.