(362b) Multiscale Models for Characterizing the Pore Structure of Biochars

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. The environmental performance of biochars depends on their ability to absorb, 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 to 100 microns.  Since biochars consist of a mixture of amorphous and crystalline phases, they usually exhibit two subpopulations of micropores: randomly distributed pores and domains of orderly arranged pores. The latter pores are the slits formed between the graphitic-like layers of aromatic carbon clusters that are turbostratically arranged in nanometer-size crystallites. Such structures have been confirmed with NMR and XRD measurements.

We report here the development of a new class of multiscale discrete models that can accurately characterize the complicated pore structure of biochars.  The first step of our modeling approach is to generate a “nanoscale” model of the pore structure on a three-dimensional computational grid.  Pores of various shapes and sizes are distributed on the grid in a random or orderly fashion to match the experimentally determined micro- and mesoporosities, as well as any available information about the shape of the pores or the size of crystalline domains. The generated porous solids are then eroded using rules that simulate a gas-solid, non-catalytic reaction that takes place in the kinetic control regime where the entire surface area attributed to micro- and mesopores is completely accessible to the reactant.  In the case of very small slit pores, the model can be adjusted to consider that reaction takes place only at the pore mouths.

If the reaction takes place at high temperatures, however, strong diffusional resistances will appear first in the micropores and subsequently in the mesopores. As a result, progressively larger fractions of the micropore and mesopore structure will become inaccessible to the gaseous reactant.  At sufficiently high temperatures, reaction 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.  To model reaction in the diffusion control regime, we generate a “macroscale” model of the pore structure on a cubic computational grid.  This model matches experimental macroporosity measurements and information obtained from SEM or optical microscopy images about the size and spatial arrangement of the large macropore cavities.

Brute-force computations with pores ranging from 0.5 nm to 10 μm or more would require arrays with more than 8×10¹⁵ computational cells. To overcome this limitation, we use grids with “porous” cells. Since the gaseous reactants can now diffuse into the cells, we have both external and internal (bulk) reaction: External reaction occurs on surfaces fully exposed to the gaseous reactant.  Internal (bulk) reaction occurs on the surface of the pores that have been penetrated by the reactant.

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. 

To validate our models, we compared simulation results with experimental data from the combustion of various biochars at several reaction temperatures.  Chars produced from different biomass feedstocks exhibited different reactivity patterns in the kinetic control regime. These results were consistent with the presence of two pore subpopulations with relative volumes that strongly depend on the feedstock. In general, the first subpopulation consists of orderly arranged submicropores, while the second is formed by randomly distributed micropores.  Biochars from different feedstocks, however, 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.