STRUCTURAL AND COMPOSITIONAL TRANSFORMATIONS OF BIOMASS CHARS DURING COMBUSTION

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.