Single Particle Experiments for the Investigation of Unburned Carbon in Flyash

Page 8

8 in distinct, carbon-rich particles and not in intimate association with the bulk of the mineral matter. In addition, during captive-particle imaging, near-extinction is observed even for particles containing very small amounts of mineral matter (as inferred from the size of the final ash particle). For these low-ash particles, the near-extinction is clearly not caused by the formation of an ash­fIlm diffusion barrier. Naturally occurring, well-dispersed alkaline and alkaline-earth metals that are common in lower rank coals can accelerate the rate of carbon oxidation under some conditions. Therefore loss or fouling of such catalytic components could lead to a loss of reaction rate. However, catalysis is generally not a large effect for bituminous coals, nor for any coals, regardless of rank, at the temperatures typical of pulverized-coal flames. Captive-particle-imaging experiments have shown that ash content does not affect reactivity significantly in the early stages of combustion for bituminous coals. At low to intermediate extents of burnout, the ash is not present in large enough quantities to act as a diffusion barrier nor to displace carbon in the particle matrix, both of which can lead to low global reactivities. Its role in the latter stages of burnout is, however, more complex. Clearly, ash reduces burning rates in the very late stages of combustion by displacing carbon surfaces and leading to low global or apparent reactivities. These low reactivities lead, in turn, to low particle temperatures and long times required to completely oxidize carbon embedded in ash-rich particles. The slow ash decarburization process occurs only in the very latest stages of combustion, however, and is unlikely to playa major role in determining total burnout in commercial combustors. Indeed, most of the residual carbon observed in boiler fly ash samples is not in intimate association with ash, but is present in distinct, carbon-rich particles. In addition to this surface effect, inorganic matter adds mass to the fly ash stream and thus "dilutes" the residual carbon present and reduces LOI for a given carbon conversion. By this mechanism, a high ash content in the parent coal promotes fly ash marketability, but does not affect carbon conversion or efficiency. Possible Reactivity Loss Mechanisms: 2. Maceral Effects Reflected light microscopy has also been used to examine the petrology, morphology, and ordering of Illinois #6 coal chars at low-to-intermediate conversion[Hurt et aI., 1993a] and lllinois #6 residual carbon extracts [Hurt et ai. , 1994]. Qualitative observations were made as well as selected quantitative measurements, including mean random vitrinite char reflectance and the distribution of char particle morphological types using the classification scheme proposed by Bailey and coworkers[Bailey et aI. , 1990]. Table 1 summarizes the results for a series of Illinois #6 chars