Enhancing carbon dioxide uptake in biochar derived from husk biomasses: Optimizing biomass particle size and steam activation conditions

Abstract

This research focuses on developing activated biochars for CO2 adsorption, evaluating the impact of particle size and steam activation conditions on almond shells (AS), pistachio shells (PS), and nut shells (NS), three crops that are grown worldwide. A literature review was carried out on the characteristic parameters that a biomass must have to produce a biochar of an acceptable quality to capture CO2. Initially, a physicochemical characterization of the selected biomasses was conducted, revealing high levels of volatiles (78–84 wt%), carbon (41–53 wt%), and inherent metals (Ca, K, Mg and Na). This process involved pyrolysis and activation under pre-established conditions, followed by CO2 adsorption analysis using thermogravimetry. Interestingly, intermediate-sized biochars (ranging from 2 to 1.4 mm) exhibited higher CO2 uptake. Once the optimal particle size was determined, steam activation conditions were further optimized by varying temperature. In this regard, these conditions were established at 900 °C for 30 min, at a steam flow rate of 0.15 mL/min and a pressure of 1 bar. This study highlighted biomass heterogeneity, resulting in distinct biochar morphologies (microporous PS activated biochars, and mesoporous NS and AS biochars) and higher carbon content compared to raw materials. However, the O/C and H/C ratios fell as severity rose. On researching steam activation holding times at a constant temperature of 900 °C, similar trends were observed, with high carbon content in the biochars. Finally, based on CO2 adsorption results, 2.81, 2.12 and 1.76 mmol/g for PS, AS and NS activated biochars, respectively, optimal shorter activation times (15 min) were selected for AS and NS biochars.