In-situ pyrolysis of biomass for N-biochar

“In-situ pyrolysis of N-containing functional groups and biomass are the most promising way to prepare N-rich biochar. It is required to mix the N-containing functional groups and biomass before pyrolysis through the methods such as the impregnation method, gaseous activation, mixing, etc. Impregnation is normally used to chemically modify the adsorbents. For example, the amine is typically dissolved in a polar solvent such as methanol or ethanol and subsequently mixed with the porous carbon (Chen et al., 2014). Madzaki et al. compared biochar from raw sawdust without the modification to the amine-modified biochar through the amine impregnation with monoethanolamine (MEAs) (Madzaki and KarimGhani, 2016). It was shown that the impregnation reduced the BET surface area of biochar attributed to the pore filling effects of the nitrogen. In addition, the impregnation method did not enhance basic functionalities linked to the surface of the biochar. Thus, the modified biochar showed less CO2 adsorption compared to the biochar without modification, although it had a higher N-content but lower BET surface area.

To overcome these disadvantages, ammonia treatment was explored to introduce the N-functionalities to porous carbon. Generally, gaseous activation was proved to increase porosity between 700 and 800°C, which can be attributed to high reaction rates of steam with carbon (Gergova and Eser, 1996). In addition, ammonia is used as a modification method. For example, Zhang et al. reported a cotton stalk-derived biochar prepared by pyrolysis and modification by gaseous NH3 (Zhang et al., 2014). The surface area of CA-char was the highest at 627.15 m2 g−1, and its CO2 adsorption capacity was 2.18 mmol g−1 at 20°C when N content was 1.52%. Using the method, CN 800 was obtained from soybean straw by modification of CO2–NH3 mixture at 800°C, processing CO2 adsorption of 88.89 mg g−1 (Zhang et al., 2016).

Another challenge that lies in the activation technologies is the significant mass loss during activation. Mixing between solids is also one efficient way to introduce the nitrogen groups to avoid mass loss. Our group has also proposed a one-step method to obtain N-rich biochar derived from anaerobic digestion digestate used as a CO2 adsorbent (Qiao et al., 2020). The N-rich biochar was obtained by pyrolysis of a mixture of the biomass and urea, with the CO2 capacity up to 1.22 mmol g−1. In this work, the low BET surface area limited the CO2 capacity, which may be linked to blocked pores. However, both the porous property and functionalities are important to CO2 capture.Peiyu et al. used a “mixed molten salt” method to receive hierarchical N-rich activated carbons (N-ACs) derived from chitosan with a molten salt template of LiCl-ZnCl2 (Wang et al., 2020). This simple one-step carbonization method is also advantageous in terms of avoiding significant mass loss. The CO2 adsorption capacity of N-ACs was 7.9/5.6 mmol•g–1 at 0°C/25°C under 1 bar pressure, respectively.

Generally, the process of biochar production used for CO2 capture is conducted by two steps including carbonization and activation, which means harsh and multiple processes are used. But if the biochar is prepared through a single-step process, its costs could be decreased.”

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