Environmental advantages of carbon-negative syngas production from the CO₂-mediated pyrolysis of reed

Notwithstanding a technical maturity of the biofuel production systems, the selective conversion of specific nutritional components in biomass, such as carbohydrates and lipids, poses challenges in achieving efficient carbon utilization. To address this issue, pyrolysis has emerged as a potential approach to maximizing carbon conservation by converting biomass into three pyrogenic products: syngas, biocrude, and biochar. In this study, reeds (Phragmites australis) were selected as the model biomass for pyrolysis due to their global abundance stemming from the rapid growth rate and strong adaptability to environmental stresses. To impart a sustainability to the pyrolysis system, carbon dioxide (CO₂) was introduced as a partial oxidant. CO₂ interacts homogeneously with thermally derived volatiles from reeds, leading to their conversion into carbon monoxide (CO). This redox reactivity of CO₂ to produce carbon-negative syngas is driven by electron density differences of carbon atom in CO₂ with functional groups of the volatiles. However, the production of carbon-negative syngas was observed at temperatures ≥ 550 °C. To further accelerate the redox reactivity of CO₂, the pyrolysis setup was modified by incorporating an external heat source and a nickel catalyst. The production of carbon-negative syngas was further optimized by systematically adjusting the CO₂ concentration and operating temperature. The carbon footprint reduction of the CO₂-mediated catalytic pyrolysis process was maximized under specific conditions (700 °C and 80 vol% CO₂). The environmental advantage of integrating CO₂ into the pyrolysis process was quantified as an annual suppression of 1.34 g CO₂ per gram of reeds.