Structural and compositional transformations of biomass chars during combustion

In an investigation of the physical and chemical transformations of biomass chars during combustion, we have subjected two chars, produced from 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 structural and compositional transformations of the biomass chars. samples are extracted from the reactor at different residence times and subjected to a variety of analytical techniques: elemental analysis, scanning electron microscopy, energy-dispersive x-ray spectroscopy, x-ray diffraction analysis, and high-resolution transmission electron microscopy. The results point to several changes in both the organic and inorganic constituents of the chars. The early stages of conversion are characterized by devolatilization, which leads to the removal of amorphous material and the release of oxygen- and hydrogen-rich gases. After 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 transformation of inorganic constituents from amorphous phases to crystalline forms. Some short-range order appears in the carbon-rich portions of the chars as combustion proceeds, but the high levels of oxygen originally present in these chars foster cross-linking, which limits the extent of order ultimately attained. The transformations of the biomass chars are compared with those of coal chars, and the implications of these observations—with respect to reactivity and ash behavior—are discussed.