Single Particle Experiments for the Investigation of Unburned Carbon in Flyash

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5 data are particularly useful for defining the point of near-extinction-it occurs when the particle diameter is still 850/0 of the initial diameter, and while the particle still contains dark material, typically elemental carbon, t under visible illumination. The extinction process is quite different for low rank coal and biomass chars. For these chars, the temperature during combustion remains uniformly high until well after ash begins to dominate the particle surface. One possible explanation for this difference in behavior is the high oxygen content of these low rank fuels. High levels of oxygen generally coincide with highly cross-linked carbonaceous structures that have limited ability to form crystalline phases [Womat et aI., 1993]. This topic will be explored in more detail in subsequent sections. In addition to this captive particle imaging work, experiments were also done using an optical technique to produce a set of single-particle-size, temperature, and emissive-factor measurements for three bituminous coals, one lignite, and two biomass chars at various residence times. In agreement with the imaging work discussed previously, the data for the bituminous coals indicate a gradual transition from a population of fully ignited char particles to a predominance of low­temperature particles and, fmally, to a predominance of inorganic-rich particles (with emissive factors < 0.3). Figure 2 shows the time evolution of the particle temperatures and sizes for a subset of this raw data with emissive factors greater than 0.3 (representing the carbon-rich particles and mixed particle types [Hurt, 1993]) for illinois No.6 coal. The temperatures in Fig. 2 are referenced (by subtracting) to the local gas temperature, which changes slightly along the reactor length, to facilitate comparison among data at different residence times. The temperature difference, as plotted in Fig. 2, is a convenient indicator of the reaction rate at any point. Also shown on Fig. 2 are the theoretical temperature limits for diffusion-limited burning and for inert particles, calculated for various assumptions (see caption). Figure 2 clearly shows a large temperature drop or near-extinction occurring between 72 ms (69% bulk conversion) and 117 ms (75% bulk conversion). At 117 ms and beyond, a large group of particles is observed at or near the gas temperature, with a smaller but significant group having temperatures from 50 to 300 K above that of the gas. A small number of particles (approximately 10 of 184, or 6%) are still seen at the high temperatures associated with the rapid combustion phase. Different particles undergo near-extinction and final burnout at different times due to differences in particle density, reactivity [Hurt, 1993], and initial ash content [Mitchell, 1990]. A t Most, but not all, of the black material observed in these highly-reacted samples is elemental carbon, the remainder being primarily iron-bearing phases, as determined by laser spark spectroscopy [Davis et al., 1993]. The phrase "elemental carbon" is used here to refer to organic carbon (which is impure and contains small amounts of nitrogen, sulfur, oxygen, etc.), as distinct from inorganic carbon (e.g. in carbonates).