Biochar production from slow pyrolysis of biomass under CO2 atmosphere: A review on the effect of CO2 medium on biochar production, characterisation, and environmental applications

The replacement of inert gas by CO2 in pyrolysis has piqued a great deal of interest in recent years due to its numerous positive effects on end products. The reactive nature of CO2 at higher pyrolysis temperatures (above 700 °C) during pyrolysis improves pyrolytic gas production (H2, CO, and CH4) by increasing thermal cracking of tar, inhibiting secondary char formation, and enhancing char gasification, thereby reducing char yield. The existing review articles focus on the impact of CO2 on tar and gaseous products during pyrolysis, with little information on its impact on biochar production and properties via slow pyrolysis of various organic wastes. Therefore, it is worthwhile to investigate the effect of CO2 on biochar production during slow pyrolysis for industrial purposes, because CO2 from flue gas can be used as the pyrolysis gas environment, which can ultimately improve biochar production in terms of economic viability, environmental impact, and modified properties. The goal of this review is to conduct a thorough investigation of the effect of CO2 atmosphere on slow pyrolysis performance (400–700 °C), with a particular emphasis on biochar yield and properties for environmental and agricultural applications in comparison to inert atmosphere by highlighting the existing challenges and future opportunities. [Display omitted] •Impact of CO2 on biochar production via slow pyrolysis is deeply investigated.•Agro-environmental properties of biochar is reviewed.•CO2 increases biochar yield within the temperature 400 and 600 °C.•CO2 greatly improves biochar’s surface area, carbon and aromatic contents.•CO2 could be used to alter the agro-environmental properties of biochars.