https://doi.org/10.1016/j.ccst.2021.100018
“Biochar from coffee grounds was reported to be treated by three methods as follows: (i) KSHC, dispersion of the biochar in an alcohol solution of 3-aminopropyltrimethoxysilane (APTES), (ii) KPHC, through the polycondensation of C6H5NH2, and (iii) KMHC, hydrothermal treatment with melamine solution of biochar at 160 °C for 24 h (Liu and Huang, 2018). KMHC led to the highest CO2 adsorption (117.51 mg/g) among three prepared adsorbents, possibly due to the presence of high microporosity, good N content (5.1 wt%) with active sites (pyrrolic nitrogen). These sorbents also showed surpassing renewability and cyclability.
Nguyen et al. prepared a biochar (AMBC, porous nitrogen-doped biochar) sorbent at lower pyrolysis and modification temperature with higher CO2 adsorption (Nguyen and Lee, 2016). The preparation method was also based on the gas modification but a little complex, with the following steps: (1) slow pyrolysis of chicken manure for 1 h at 450°C; (2) chemical treatment from HNO3 and anhydrous ammonia gas for 1 h at 450°C; (3) reaction with sodiumα-l-gulopyranuronate to form solid beads. The AMBC beads performed a high capacity of CO2 adsorption (10.15 mmol g−1 at 20°C) with a specific surface area (328.6 m2 g−1) and CO2/N2 selectivity (79.1). Another important performance was the stability in that the CO2 adsorption capacity kept 85% after ten regeneration cycles.
To simplify the preparation procedure, Serafin et al. used a one-step method of combining carbonization with activation to make use of Amazonian wastes (nutshells waste), including Andiroba (AD), Biochar Brazilian nut (BBN), Brazilian nutshell (BNS), Cascara Cupuassu (CasCup), COSC Cupuago (CosCup), Hueso Assai (HA), to produce biochar-based CO2 adsorbents (Serafin et al., 2021). The highest capacity of CO2 adsorption at 298 K with 3.67 mmol g−1 was investigated on Cascara Capuassau-derived biochar.
Riya et al. described pinewood-derived biochar via a two-stage activation process including sonication treatment and amine-activation (tetraethylenepentamine,TEPA) near room temperature (Chatterjee et al., 2018). The optimum capacity of CO2 capture was 2.79 mmol g−1 at 70 °C, 0.15 atm. The method of post-modification is related to high consumption of energy or cost because of high temperatures, corrosive materials, blocked pores.
From table 2, the highest capacity of CO2 capture is 10.15 mmol/g at 20°C for AMBC. Because of the existence of CO2 and N2 in flue gas from fossil-fuel power plants, CO2/N2 selectivity is a key factor for the development of biochar-based adsorbents. The highest CO2/N2 selectivity is also obtained using AMBC, with 79 at 20°C. However, the highest BET surface area is KNWS-xy with 4230 m2 g−1 and the highest N-content with 8.99% is from UCAD 1:1. Researchers have discussed the co-effect of BET surface area, N-functionalities on the CO2 adsorption (Wei et al., 2012, Gupta and Kua, 2017). During the preparation of AMBC, the chemical treatment of the biochar with HNO3 and NH3 enhanced the presence of amine groups on its surface, thus leading to the increased capacity of CO2 capture. Therefore, it is desirable to directly convert biomass into porous carbon with good CO2 capacities. In addition, the comparison between the sample before and after nitrogen enrichment proves that N-functionalities have a larger contribution to CO2 adsorption compared to BET surface area. In all the cases shown in table 2, CO2 adsorption has been improved after nitrogen is enriched, while in some cases like US0.5-EH 1:1-T2.5 (Chatterjee et al., 2018), BET surface area decreased after the introduction of nitrogen.”
