OCR Text |
Show 9 generated by partial combustion in our entrained flow reactor. These results show a modest increase in the concentration of inertinite-derived (non-fused) particle types as combustion progresses. The inertinite enrichment is consistent with the lower combustion reactivity and higher density typically observed for inertinite-derived chars [Wall et ai., 1992; Oka et ai., 1987; Crelling et ai., 1992; Hurt et ai., 1986; Bailey et al., 1990; Thomas et al., 1993; Morley and R.B., 1986], but this effect is far too small to explain the large decrease in reactivity that we observe. Additionally, as discussed by Hurt and coworkers [Hurt et ai., 1994], inert or non-fused particle types are not typically responsible for the bulk of unburned carbon in boiler flyash from the combustion of u.s. coals. There are a number of possible explanations for this apparently contradictory result [Hurt et al., 1994]. Table 1 Mean Random Reflectance and Char Particle Morphology for Laboratory-Generated Illinois #6 Samples Combustion Char Carbon % Vitrinite-t % Inertinite-tt % Otherttt Mean Random Residence Time Conversion Derived Particles Derived Particle Reflectance 47 msec 0% 93.2 6.6 0.2 6.4 +/- 0.3* 117 msec 76% 76.5 17.5 6.0 6.9 +/- 0.2* 234 msec 90% 55.6 24.8 19.6 7.5 +/- 0.3* t The sum of all but the fusinoid, inertoid, and solid morphology types in the Bailey classification scheme. t t The sum of fusinoid, inertoid, and solid morphology types in the Bailey et ale [Bailey et al., 1990] classification scheme. t t t Particle fragments whose identity could not be definitively established. * 90% confidence limits Possible Reactivity Loss Mechanisms: 3. Physicochemical Changes in the Organic Structure Much of the organic portion of coal char is composed of an imperfect arrangement of aromatic layers varying in size from approximately 5 A. to greater than 100 A.. The size and ordering of these layers can be related to the reactivity of the carbon present in three fundamental ways. First, as the layers increase in size, the amount of hydrogen and the ratio of reactive edge sites to unreactive interior sites decrease. Second, as the individual layers approach a more perfectly graphitic arrangement of aromatic units, active sites associated with defects, heteroatoIDS, or nonsp2- hybridized carbon are lost. Third, as the aromatic layers reach more compact stacking arrangements, the microscopically available surface area decreases. For fast oxidation reactions |