(749d) Characterizing the Pore Structure of Biochars Using Multiscale Models and Reactivity Data

Characterizing the Pore Structure of
Biochars Using Multiscale Models and
Reactivity Data

Ashton Gooding¹, Pauline Markenscoff ²
and Kyriacos Zygourakis¹

1.    
Department
of Chemical and Biomolecular Engineering, Rice University, 6100 Main Street, Houston,
TX 77005, USA

2.    
Department
of Electrical and Computer Engineering, University of Houston, Houston, TX
77005, USA

Biochar
is charcoal generated for intentional soil amendment by pyrolyzing sustainable
biomass feedstocks.  Properly
engineered (or "designer") biochars can provide significant agricultural
benefits by increasing the water holding and cation exchange capacities of
soils, and by 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 pore
structure and surface chemistry, which can vary widely depending on the
composition of the biomass feedstocks and on the pyrolysis conditions employed
during biochar production[1].

Biochars have complex
pore structures 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. Instead, a
combination of time-consuming analytical techniques must be used 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.

At low temperatures,
combustion of biochars takes place in the regime of kinetic control and the
entire surface area attributed to micropores (or even the sub-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 sub-micropores,
then in the micropores, the mesopores and, eventually, in the macropores.  As a result, larger and larger fractions
of the micropore and mesopore structure will become inaccessible to oxygen as
the combustion temperature is raised. 

In a recent
publication[2], we have shown how experimental
reactivity data can be used to probe and characterize the pore structure of biochars
and other solid reactants. The nanoscale model developed in that study used
discrete simulations to describe the reactivity of solids consisting of
multiple components and having a mixture of ordered and random pores whose size
ranges from a fraction of a nanometer to several nanometers. Since biochars
consist of a mixture of amorphous and crystalline phases, the model considered
two subpopulations of micropores: randomly distributed pores and domains of
orderly arranged pores. The latter pores were the slits formed between the
graphitic-like layers of aromatic carbon clusters that are turbostratically
arranged in nanometer-size crystallites in biochars. Such structures have been
confirmed with NMR and XRD measurements. 
Simulation results were then used to analyze and interpret reactivity
data obtained by burning biochars in air at low temperatures where the
reactions occur primarily on the surface of micropores.

When 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 mesopore mouths, where they open up
into the large macropore cavities. Eventually, the overall reaction rate will
be determined by the temporal evolution of the surface of large macropores identified
in SEM or optical microscopy images.

The two panels of
Figure 1 show the reactivity patterns obtained for apple wood and corn stover
biochars during the transition from the kinetic to the diffusion control regime.
As the combustion temperature increases above 350 C, the normalized reaction
rates for both chars exhibit pronounced maxima for conversions between 20-40%
as strong diffusional limitations appear in the smallest pores and control of
the reaction rates shifts to the mesopores. At even higher temperatures, the
reactivity patterns for both chars become similar with a plateau at
intermediate conversions and a sharp decline as the reaction approaches
completion.

To model reaction in
the diffusion control regime, we developed a hybrid model that uses (a) the
continuous diffusion-reaction PDEs to describe the mass and energy balances in
the microporous and mesoporous solid and (b) discrete
(erosion) simulations to describe the growth and eventual coalescence of the
large macropores.  Models of the macropore
structure of biochars were generated on cubic computational grids to match
experimental macroporosity measurements and information obtained from SEM or
optical microscopy images providing information about the size and spatial
arrangement of the large macropore cavities. The computational cells representing
solid biochar were assumed to be porous and the gas reactant (oxygen) was
allowed to diffuse in them.  The
reaction rate in every cell was controlled by the local reactant concentration
computed by solving the transient mass and energy balance PDEs and the transient
reactivity pattern was calculated using the nanoscale model of Zygourakis et al.[2]  Reaction
could also take place of the exterior of a computational cell if that cell had
one or more faces exposed to macropores.

Multiple realizations
of a solid with the same macropore structural properties were generated and
reacted to obtain the reactivity patterns during the transition from the regime
of kinetic control to the regime of diffusion control.  Simulation results revealed that the
reactivity patterns in the transition regime were influenced both by the volume
ratio of the crystalline over the amorphous phase and by the size distribution
of the random pores of the amorphous phase.  In all cases, however, similar
reactivity patterns were observed when the temperatures were high enough so
that reaction took place only on the mesopore mouths opening up into the large
macropore cavities identified in SEM images. At that point, the temporal
evolution of the surface of large macropores controlled the overall reaction
rate. The combustion rates measured at high temperatures changed relatively
little for intermediate conversions (20-60%) and dropped sharply as reaction
approached completion (see, for example, the reactivity patterns of Figure 1
for combustion at 500 or 550 C). 
These results are indicative of macropore structures consisting of large
cylindrical cavities separated by walls of similar thickness, an observation
that is consistent with the SEM images of our biochars.

To validate our
model, we compared simulation results with experimental data from the
combustion of several biochars over a range of reaction temperatures.  Biochars from apple wood, slash pine,
eucalyptus and corn stover were produced in a thermogravimetric analyzer using
different heating rates (1 and 10 C/min), various final pyrolysis temperatures
(400-600 C) and different durations of heat treatment at the maximum
temperature. These results will be analyzed to demonstrate that reactivity
patterns measured at different temperatures can provide important information
about the pore structure and, in particular, the accessibility of the interconnected
pore networks of biochars.