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

Abstract

In this work, the conversion of CO2 into 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 CO2 conversion. The results revealed that the presence of both biochar and plasma significantly facilitate CO2 conversion. In comparison to thermal CO2 splitting, the plasmatron CO2 + C process dramatically enhanced the CO2 conversion from 0 to 27.1%. Walnut shell biochar prepared at relatively high pyrolysis temperatures favored CO2 conversion 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 O2 by biochar. The high electron density achieved in the plasmatron (1015 cm–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 CO2 activation 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 CO2 splitting and advantageously boost the biochar-involved reactions, respectively, by thermochemistry. The synergy of plasma-chemistry-dominated CO2 dissociation and the thermochemistry-dominated CO2 + C and O2 + C reactions accounts for the high CO2 conversion obtained in the plasmatron CO2 + C process. The immediate study provides a novel route for efficient CO2 conversion by coupling plasma chemistry and thermochemistry.

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