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Overview of biochar performance for CO2 capture

“Owing to the unique structure and surface properties of biochar, it can act as an excellent adsorbent for the capture of several gases. In a study, Mohd et al. [35] reported that adsorption of toxic gases on biochar surface took place mainly through the physisorption process. The surface of biochar contains macro and micropores, which act as a storage place for gas molecules [35]. Table 1 shows the CO2 intake capacity of biochar at 1 bar atmospheric pressure and two different temperatures. It is clear from the table that chemically activated biochar prepared from Vine shoots were capable of adsorbing a higher amount of CO2 (6.08 mmol/g at 1 bar and 273 K) compared to physically activated biochar (4.07 mmol/g at 1 bar and 273 K) [36,37]. In another study, Ello et al. [37] prepared biochar and biochar activated with KOH at 1133 K for 1 h from Africa palm shells. They reported higher CO2 adsorption capacities (6.3 mmol/g at 273K and 4.4 mmol/g at 298 K and 1 bar, respectively). On the other hand, different CO2 intake capacities were also reported for chemically activated biochars from rice husk (3.71 mmol/g) [38], pine nutshell (5.0 mmol/g) [39], wheat flour (3.48 mmol/g) [40], vine shoots (2.46 mmol/g) [36], coconut shells (4.23 mmol/g) [11], Jujun grass (hydrochar, 4.9 mmol/g) [41], and Camellia Japonica (Hydrochar, 5.0 mmol/g) [41] at 298 K and 1 bar pressure. Moreover, single-step pyrolysis and activation of various biomasses to produce biochar and activated biochar were also reported by Serafin et al. [42]. They found that CO2 adsorption capacities of pomegranate peels, carrot peels, and fern leaves were 4.00, 4.18, and 4.12 mmol/g at 298 K, respectively, and 6.89, 5.64, and 4.52 mmol/g at 273 K, respectively, at 1 bar. Zhang et al. [43] produced amine functional group doped activated biochar from black locust. They reported a CO2 adsorption capacity of 5.05 mmol/g at 298 K and 1 bar. Similarly, Rouzitalab et al. [44] used urea to synthesize amine-functionalized activated biochar from the walnut shell in the presence of KOH, and they observed record CO2 adsorption capacity of 7.42 mmol/g at 298 K and 1 bar.”

Table 1. CO2 capture performances by top performance biochar produced from different biomasses and at different conditions. The surface area is based on Brunauer–Emmett–Teller (BET).”

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“However, CO2 adsorption capacity can significantly vary with the changing of the surface morphology of biochar, i.e., the surface area, micropore volume, and size, together with the effects of temperature and pressure [24,42]. For example, Deng et al. [39] reported that biochar having a pore size of 0.33–0.63 nm played an important role in the higher CO2 adsorption. It was also reported that the control of micropores had greater importance for adsorbing high CO2 compared to surface area and total pore volume [39,42]. Figure 2 shows the presence of functional groups and porous structures (mesoporosity and microporosity) of biochar materials. Metal oxyhydroxide biochar composites have also been used to increase the adsorption capacity of biochar. For example, Lahijani et al. [47] reported that Mg-loaded biochar showed a higher CO2 adsorption capacity (3.7 mmol/g) than that of raw biochar (3.2 mmol/g) at 298 K and 1 atm. This phenomenon can be explained by the fact that the incorporation of metals (i.e., Mg, Al, Ni, and Fe) onto the biochar surface will increase basic sites on the surface of biochar, which enhances the adsorption capacity of acidic CO2 [47].”

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