OCR Text |
Show DISCUSSION OF RESULTS Low-Temperature Experiments Particle burning profiles were measured directly at two temperatures in terms of the fraction of the initial mass remaining versus time. This measurement was in turn converted to the fraction of burnable carbon remaining by subtracting the ash portion from the total. Because TG samples are slowly heated to the reaction temperature, devolatilization occurs over much longer times than is the case in an AFB. Combustion is distinguished from devolatilization by considering only that portion of the thermogram beyond the loss of the percent VM found in the proximate analysis. The combustion portion of the thermograms were converted to reaction rates on two different bases: 1) the fraction of the burnable carbon reacting per unit time, q (1/s), and 2) the mass of burnable carbon reacting per unit 2 external area per unit time, q (gC/cm s). These different rates are useful in distinguishing the particle burning mode; i.e., the relationship between particle density and particle diameter as the combustion proceeds. The two limiting modes are 1) "shrinking-core" combustion, where diameter changes and density remains constant and 2) "constant diameter" combustion, where density decreases and diameter is constant. TG runs were made with Kentucky No. 9 and AFB elutriate at 825 and 72S K. These data were compared with previous low-temperature data for Kentucky No. 9 and Ohio No! 6. The previous data consisted of other TG 11 12 experiments conducted by Frazier et al ' and tubular-reactor experiments 13 14 conducted by Daw and Krishnan. ' In the previous TG experiments, Frazier et al measured char combustion rates for Kentucky No. 9 coal, Ohio No. 6 coal and elutriate from the Babcock and Wilcox 6'x6' AFB with Ohio No. 6 feed. In the tubular-reactor experiments, Daw and Krishnan measured the combustion rate 10 |