Influence of porosity of biochar and CO2 physisorption

https://doi.org/10.1016/j.ccst.2022.100059

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When biomass is pyrolyzed, dehydration and the release of volatile constituents from the carbon matrix form biochar pores and rudimentary pores (Bagreev et al., 2001). According to the pore size classification of the International Union of Pure and Applied Chemistry(IUPAC), pores with diameters greater than 50 nm are classified as macroporous, those with diameters between 2 and 50 nm as mesopores, and those with diameters less than 2 nm as micropores (Everett, 1972). Biochar’s CO₂ capture capacity depends on micropores less than 1 nm in diameters (Y. Li et al., 2022), because narrow micropores are close to the dynamic diameter of CO₂ molecule and have a stronger attraction to CO₂ due to overlapping adsorption forces and potential fields from neighboring pore walls (Guo et al., 2016). Macropores enhance CO₂ diffusion to reduce pressure drop, mesopores provide a passageway for mass transfer at the gas-solid interface, and micropores form the packing space for CO₂ adsorption (Shen and Fan, 2013). Physical adsorption can be enhanced by adjusting the pore distribution of biochar: containing lots of micropores, appropriate mesopores, and small macropores with high specific surface area to produce biochar with multi-stage porous structures (Chen et al., 2017). Angin (Angın, 2013) et al. found that micropores contributed less to adsorption capacity at high temperatures, and that pore volume and micropores of biochar decreased at elevated temperatures above 500 °C. It attributed to the fact that at high temperatures it is no longer micropores that dominate, but the phenomenon of pore widening and coalescence of adjacent pores, with a reduction in pore volume.

Specific surface area and microporosity are interrelated, where the generation of many small micropores will result in a large specific surface area. The larger surface area provides more active sites for CO₂ adsorption through physical adsorption. Brunauer-Emmett-Teller (BET) is the most common method used to calculate the surface area of porous materials based on nitrogen or argon adsorption isotherms. Huang et al. (Huang et al., 2015). pyrolyzed rice straw by microwaves to produce biochar for CO₂ adsorption. They reported that the specific surface area was the most important characteristic of biochar in CO₂ adsorption. Previous studies suggested that adsorbents with a large surface area could have a high CO₂ capture capacity. Mukherjee (Mukherjee et al., 2021) used coffee grounds to produce biochar and captured captures CO₂ at 30–90 °C at a constant CO₂ concentration. The maximum adsorption capacity of 2.8 mmol/g was obtained for a BET surface area of 539 m²/g of biochar. The adsorption kinetics of coffee grounds biochar followed a pseudo-first-order kinetic model and had a low activation energy, and it reflects the adsorption’s physical nature (Zhang et al., 2014). However, when the char surface area was large enough, the quantity of nitrogenous groups played a more important role in the adsorption of CO₂ on the biochar surface. The surface area of biochar rises with increasing pyrolysis temperature and residence time, which is owing to the production of volatiles that can increase pore volume (Ahmad et al., 2014). During modification of biochar using different compounds, the volume and surface area of the micropores increased initially with increasing the temperature, which was reduced as the temperature was raised to more than 800 °C (Xiong et al., 2013; Zhang et al., 2016). Francisco (Lopez-Tenllado et al., 2021) et al. compared the physical adsorption of carbon dioxide after modification of biochar with ball milling with unground biochar and found that ball milling led to an increase in pore volume and surface area of biochar. The adsorption capacity would be limited by the surface properties.

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