Effect of Fe impregnation on CO2-assisted pyrolysis of hazelnut shell

Abstract

To find an economical and efficient way for mitigating CO2 emission, this study utilized CO2 as a reaction medium and investigated its influence on biomass pyrolysis. To this end, CO2-assisted pyrolysis of hazelnut shell (HS) and Fe-loaded HS (HSFe) was conducted using a thermogravimetric analysis (TGA) unit and a fixed-bed reactor. The temperature-programmed TGA tests demonstrated that there was no occurrence of the heterogeneous reaction between the solid char and CO2 at temperatures below 700 °C. However, the gaseous effluents evolved from the temperature-programmed pyrolysis of HS and HSFe in a fixed-bed reactor proved that CO2 played a mechanistic role in enhancing CO generation via the gas phase reactions (GPRs) between CO2 and volatile organic compounds (VOCs). Through the quantitative analysis of gas evolution rates, the CO production and the CO2 consumption in the GPRs always followed a stoichiometric relationship of about 2:1. A possible mechanism of the GPRs between CO2 and VOCs was proposed. Besides, Fe addition reduced the reaction temperatures of the GPRs between CO2 and VOCs by 120–160 °C, and significantly enhanced the reaction rate of these GPRs. As a result, the enhanced CO generation and the net CO2 consumption of HS loaded with 5 wt% Fe (HS5%Fe) in CO2 atmosphere were more than twice those of raw HS at temperatures above 500 °C. The XRD results revealed that Fe3O4 was the major chemical form of Fe species in HS5%Fe chars generated at temperatures ≥ 500 °C in CO2 atmosphere, which played a catalytic role in the GPRs between CO2 and VOCs.