(716a) Resolving Controversies in the Surface Area Measurements of Biochar and Hydrochar

Waste biomass is an abundant and underutilized resource that results in greenhouse gas emissions from its decomposition and significant waste of valuable carbon when it has the potential to be thermochemically or biochemically converted into fuels or commodity chemicals. This waste biomass can serve as a sustainable feedstock to produce hydrochar and biochar materials. Hydrochar is produced through the thermochemical reaction of wet waste, without energy intense drying, at moderate temperatures (170 - 250°C) and autogenic pressures in a process coined hydrothermal carbonization (HTC). Biochar results from pyrolysis in the absence of oxygen to convert pre-dried wastes at high temperatures (400 - 800°C). Both types of char can be used as adsorbent materials, as a soil amendment, or as a biofuel. However, while hydrochar and biochar have clear similarities in their application and production method, their physicochemical structures differ significantly.

One key difference between hydrochar and biochar is surface area, which can be adjusted via post-synthesis activation. While biochar surface area has been more extensively characterized in literature [1], there is currently no optimized method for accurate measurement and analysis of hydrochar surface area. Since surface area is directly correlated with the material’s adsorption capacity and performance in certain applications, it is paramount that hydrochar surface area is accurately measured and standardized. Furthermore, enhanced knowledge of hydrochar surface area will allow for a greater understanding of hydrochar synthesis and activation to enable tunable reaction engineering.

Currently, the most common method for measuring hydrochar surface area is through N₂ physisorption at low temperature (77 K) prior to analysis via the Brunauer–Emmett–Teller (BET) method. Traditional BET theory was developed for use on nonporous materials with a uniform surface and across a standard pressure range that does not account for the presence of micropores. The BET method’s assumptions of a nonporous and uniform surface are inconsistent descriptors of the porous and functionalized surface of hydrochars. Therefore, hydrochar evaluated by N₂ physisorption often lead to unmeasurably small results inconsistent with the materials adsorptive performance in other media.

By utilizing the known similarities and differences between biochar and hydrochar, this study develops new methods to determine the surface area of hydrochar more accurately. Hydrochar was produced from glucose after HTC treatment at 180 °C for 12 hours in an autoclave reactor. A fraction of the hydrochar was pyrolyzed at varying temperatures ranging from 250 °C – 800 °C, to monitor the conversion to biochar. N₂ isotherm results on hydrochar output poor (negative) BET model fit C constants that indicate the resulting surface area are not physically meaningful. Due to the approximate size of hydrochar micropores (>2 nm) and the kinetic diameter of N2, this work compares the use of CO2 as an adsorbent gas in the determination of hydrochar surface area. However, the measured hydrochar surface area values were approximately 50 to 100 times larger when CO₂ was used as the adsorbate gas than N₂, with a valid model fit C constant. This can be seen in Table 1.

This study investigates hydrochar as well as pyrolyzed hydrochar/biochar, to develop and validate surface area methodology. This study develops an accurate procedure for measuring hydrochar surface area by utilizing CO₂ as the adsorbent gas at warmer temperatures (273 K), which has a higher quadrupole moment and smaller kinetic diameter than N₂, therefore interacting more with the abundant heteroatoms, and allowing for easier diffusion into the pores. This study also applies DFT to simulate the interactions between N₂ and CO₂ with hydrochar, thoroughly investigating both pore specific and pore non-specific surface area calculation theories and their physical meanings when applied to hydrochar. DFT aims to determine which combination of experimental methods and theory produces the most meaningful surface area values for hydrochar and evaluates the correlations between the methods and results. Our recommendations include using the Dubinin-Radushkevich theory and CO₂ at 273 K to measure a meaningful hydrochar surface area.

References:

(1)

Maziarka, P.; Wurzer, C.; Arauzo, P. J.; Dieguez-Alonso, A.; Mašek, O.; Ronsse, F. Do You BET on Routine? The Reliability of N2 Physisorption for the Quantitative Assessment of Biochar’s Surface Area. Chemical Engineering Journal 2021, 418, 129234. https://doi.org/10.1016/j.cej.2021.129234.