Pyrolysis process of agricultural waste using CO₂ for waste management, energy recovery, and biochar fabrication

Highlights: • CO₂ reacts with VOCs enhancing syngas generation from pyrolysis of biomass. • CO₂ reduces tar formation by expediting thermal cracking of VOCs. • Properties of biochar can be easily modified using CO₂ as a pyrolysis agent. • A detailed mass balance for pyrolysis of red pepper stalk was provided. • Energy saving can be expected in pyrolysis of biomass using CO₂. - Abstract: This study focused on the mechanistic understanding of CO₂ in pyrolysis process of agricultural waste to achieve waste management, energy recovery, and biochar fabrication. In order to scrutinize the genuine role of CO₂ in the biomass pyrolysis, all pyrogenic products such as syngas, pyrolytic oil (i.e., tar), and biochar generated from pyrolysis of red pepper stalk in N₂ and CO₂ were characterized. Thermo-gravimetric analysis confirmed that during the thermolysis of red pepper stalk, the magnitude of exothermic reaction in CO₂ from 220 to 400 °C was substantially different from that in N₂, resulting in the different extents of carbonization. The physico-chemical properties of biochar produced in CO₂ were varied compared to biochar produced in N₂. For example, the surface area of biochar produced in CO₂ was increased from 32.46 to 109.15 m² g⁻¹. This study validates the role of CO₂ not only as expediting agent for the thermal cracking of volatile organic carbons (VOCs) but also as reacting agent with VOCs. This genuine influence of CO₂ in pyrolysis of red pepper stalk led to enhanced generation of syngas, which consequently reduced tar production because VOCs evolving from devolatilization of biomass served as substrates for syngas via reaction between CO₂ and VOCs. The enhanced generation of CO reached up to 3000 and 6000% at 600 and 690 °C, respectively, whereas 33.8% tar reduction in CO₂ was identified at 600 °C.