OCR Text |
Show vary residence time. A schematic diagram of the water-cooled collection probe is presented in Figure 4. At the entrance to the probe, the combustion products were rapidly quenched and diluted. Particle deposition within the probe was avoided by transpiring gas through a porous liner. The particles and gas from the collection probe were passed through an Andersen impactor for on-line aerodynamic size classification of the particles greater than 0.3 m (i.e., residual fly ash particles). The submicron particles, because of their small size, pass through the impactor. Consequently, the submicron particles can be isolated from the residual fly ash for independent characterization. The submicron particles can either be collected on a Fluropore 0.2 m filter for subsequent weight and chemical analysis or deposited on transmission electron microscope grids in an electrostatic precipitator for microscopy studies. Insight into the vaporization process can be obtained from weight and chemical analyses of the submicron particles since these are generated by vaporization and subsequent condensation of the ash (2,3,9). The bulk elemental composition of the submicron particles is determined by instrumental neutron activation analysis (INAA), atomic absorption (AA), and electron microprobe (EM) . Particle size distributions for the submicron particles were obtained from TEM micrographs by an automated analyzer. RESULTS Particle Temperatures The temperature histories of selected particles are shown in the top panels of Figures 5 and 6 for 90-105 um sized particles of a Montana lignite and an Illinois #6 coal burned in atmospheres of argon containing 35 percent, 50 percent and 100 percent oxygen and preheated to 1700 K. At early times, the intensity signals are influenced by the presence of a volatile cloud. The period during which the devolatilization occurs is marked by dashed curves in the upper and middle panels of Figures 5 and 6. The apparent temperature measured during this period is influenced by the presence of a volatile cloud surrounding the particles. The top solid curve is the temperature obtained from two-color pyrometry and will be anomalously high because of the 3-8 |