Biochar is charcoal generated for intentional soil amendment by pyrolyzing sustainable biomass feedstocks. Properly “engineered” charcoals can increase the water holding and cation exchange capacities of soils, improving the ability of plants to survive under drought conditions and reducing fertilizer runoff into watersheds. Fertilizer runoff has become a serious problem, because as much as 70% of fertilizer applied to crop fields is leached into the groundwater or lost to streams and rivers, eventually leading to large hypoxic 'dead' zones in the world’s oceans (including the Gulf of Mexico). The environmental performance of biochars depends on their ability to adsorb, retain and release water and nutrients. These biochar properties are controlled by their porosity and surface chemistry, which can vary widely depending on the composition of the biomass feedstocks and on the pyrolysis conditions employed during biochar production.
Biochars have a complicated pore structure consisting of multiple interconnected networks of micropores, mesopores and macropores that span multiple length scales: from sub-nanometer micropores to macropores with sizes of the order of 10 microns. Such pore structures cannot be characterized by a single analytical technique. Comprehensive studies will use a combination of analytical techniques to bridge the vastly different length scales: adsorption of multiple gases (like nitrogen, carbon dioxide and water) for the micropores, mercury porosimetry for the mesopores and sectioning with optical microscopy and 3-D reconstruction techniques for the macropores.
We report here the development of a novel approach to probe the multiscale pore structure of biochars. This approach uses thermogravimetric measurements of the transient reactivity of biochars in air over a wide range of temperatures. At low temperatures, combustion proceeds in the regime of kinetic control and the entire surface area attributed to micropores is completely accessible to the reactant. As the temperature rises, the reaction regimes shifts to diffusion control and strong diffusional resistances start appearing first in the micropores and subsequently in the mesopores. Thus, larger and larger fractions of the micropore and mesopore structure will become inaccessible to oxygen as the temperature is raised. At sufficiently high temperatures, combustion will take place only on the micropore and mesopores “mouths,” where they open up into the large macropore cavities identified in SEM or optical microscopy images.
The novelty of our approach lies in the use of sophisticated discrete models that can accurately describe the temporal evolution of the surface area of solid reactants with complicated pore structure. The new models are significant extensions of the continuous [1-2] and discrete models  that have appeared in the literature. Simulations start by generating solids with the desired pore structure on three-dimensional computational grids. Pores of various shapes and sizes are distributed on the grid in a random or orderly fashion to match the experimentally determined micro, meso- and macroporosities, as well as any available information about the shape of the pores. The generated porous solids are then eroded using rules that simulate non-catalytic reactions between a gas and a solid reactant. Access of the gas reactant into pores of progressively increasing sizes is restricted to model pore diffusional limitations and parallel simulations are carried out to handle the multiple scales of this problem. Multiple realizations of a solid with the same pore structural properties are generated and reacted to estimate the average pore surface and to establish confidence intervals. For a given pore structure, this process will generate a family of curves that give the evolution of pore surface area with conversion as the intraparticle diffusional resistances increase. A comparison of these surface evolution patterns to the char reactivity patterns measured at different temperatures can provide important information about the complex pore structure of carbonaceous materials [1-4].
Simulation results were compared with experimental data from the combustion of different biochars to validate this technique. Chars produced from different biomass feedstocks (apple wood or corn stover) exhibited different reactivity patterns in the kinetic control regime. The simulations showed that the reactivity patterns of corn stover chars are consistent with a random distribution of the micropores. On the other hand, apple wood chars exhibited reactivity patterns that indicate the presence of a subpopulation of orderly distributed micropores. These micropores must be the slit pores formed between the graphitic-like layers of aromatic carbon clusters that are turbostratically arranged in nanometer-size crystallites, a structure that has been confirmed with NMR and XRD measurements. The same corn stover and apple wood chars exhibited similar reactivity patterns when combusted with oxygen in the regime of strong diffusional limitations, when oxygen cannot penetrate deeply into the micro- and mesopores and the reaction takes place at the pore “mouths” where they open up into the large macropore cavities. These patterns were consistent with a macropore structure that consists of many large cavities separated by walls of similar thickness.
These results demonstrate how TGA data and pore structure models can be used to gain significant insights into the pore structure of biochars. The new approach has the potential to overcome some of the major difficulties encountered in characterizing the complex pore structure of biochars and other carbonaceous solids.
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