Boosting the Conversion of CO2 with Biochar to Clean CO in an Atmospheric Plasmatron: A Synergy of Plasma Chemistry and Thermochemistry

In this work, the conversion of CO₂into O₂-free CO has been investigated in an atmospheric plasmatron via the reaction with biochar. The effects of the biochar source, pyrolysis temperature for biochar preparation, and gas-solid reaction patterns (fixed bed and fluidized bed) on the reaction performance were evaluated under different feed flow rates. The underlying mechanisms were explored using in situ optical emission spectroscopy focusing on understanding the role of plasma chemistry and thermochemistry in CO₂conversion. The results revealed that the presence of both biochar and plasma significantly facilitate CO₂conversion. In comparison to thermal CO₂splitting, the plasmatron CO₂+ C process dramatically enhanced the CO₂conversion from 0 to 27.1%. Walnut shell biochar prepared at relatively high pyrolysis temperatures favored CO₂conversion due to a high carbon content. A fixed bed surprisingly provided remarkably better performance than a fluidized bed for the CO₂+ C reaction, benefiting from a prompt consumption of the generated O₂by biochar. The high electron density achieved in the plasmatron (10¹⁵cm^-3) allows for a high processing capacity, and the moderate electron temperature (1.1-1.5 eV) with enhanced vibrational energy (6300-8200 K) obtained stimulates the most efficient CO₂activation routes through vibrational excitation. The relatively high rotational (gas) temperatures in the core plasma area (2100-2400 K) and in the gas-solid reaction region (<1573 K) detrimentally drive the reverse reactions of CO₂splitting and advantageously boost the biochar-involved reactions, respectively, by thermochemistry. The synergy of plasma-chemistry-dominated CO₂dissociation and the thermochemistry-dominated CO₂+ C and O₂+ C reactions accounts for the high CO₂conversion obtained in the plasmatron CO₂+ C process. The immediate study provides a novel route for efficient CO₂conversion by coupling plasma chemistry and thermochemistry.