In an investigation of the physical and chemical transformations of bi
omass chars during combustion, we have subjected two chars, produced f
rom the pyrolysis of pine and switchgrass, to combustion at 1600 K in
a laminar flow reactor. In order to obtain time-resolved data on the s
tructural and compositional transformations of the biomass chars, samp
les are extracted form the reactor at different residence times and su
bjected to a variety of analytical techniques: elemental analysis, sca
nning electron microscopy, energy-dispersive x-ray spectroscopy, x-ray
diffraction analysis, and high-resolution transmission electron micro
scopy. The results point to several changes in both the organic and in
organic constituents of the chars. The early stages of conversion are
characterized by devolatilization, which leads to the removal of amorp
hous material and the release of oxygen- and hydrogen-rich gases. Afte
r devolatilization, combustion is accompanied by: vaporization of some
metals (particularly Na and K), surface migration and coalescence of
inorganic material, and the incorporation of metals (particularly Ca)
into silicate structures. The latest stages of combustion reveal the t
ransformation of inorganic constituents from amorphous phases to cryst
alline forms. some short-range order appears in the carbon-rich portio
ns of the chars as combustion proceeds, but the high levels of oxygen
originally present in these chars foster cross-linking, which limits t
he extent of order ultimately attained. The transformations of the bio
mass chars are compared with those of coal chars, and the implications
of these observations-with respect to reactivity and ash behavior-are
discussed.