| OCR Text |
Show 7 complexity of the boiler environment and the wide variety of boiler operating conditions and fuels examined here, the residual carbon samples, as a class of materials, exhibit consistently low combustion reactivities relative to laboratory-generated chars in the early-to-intermediate stages of combustion. It is also noteworthy that the reactivity of the lllinois No.6 residual carbon sample is very similar to that found for the laboratory-generated chars after the deactivation phenomenon. Possible Reactivity Loss Mechanisms: 1. Carbon Accessibility The morphology, macroporosity, and general appearance of partially reacted chars can provide useful information regarding the accessibility of the carbon in a given particle. An evaluation of the carbonaceous surface area available for reaction must consider the nature of the particle surface, and whether "wetting" of this surface by mineral matter occurs. Figure 4 contains SEM micrographs comparing Illinois No.6 char particles at 47 msec (at the onset of heterogeneous oxidation) with particles at 117 msec (after particle reactivity has dropped significantly). The fluid history of these particles after 47 msec is quite evident. This sample contains, almost exclusively, carbon-rich particles, many of which exhibit features such as large cavities and rounded protuberances that indicate fluidity and vesicle formation during devolatilization. As shown in Fig. 4, lacy structures that are quite macroporous have developed after 117 msec of combustion as the coherent outer surface erodes and the inner structures are exposed. From these micrographs, it is obvious that loss of macroporosity is not a cause of reactivity loss for lllinois No.6. In addition to this evidence from entrained flow reactor sampling, the captive particle imaging work discussed previously verifies that the reactivity loss, which takes place between the second and third images, occurs without dramatic changes in the particle morphology or size. Another way in which the available combustible surface area of a particle can be reduced is by fonnation of an ash barrier. The possibility of ash acting to retard the rate of char combustion was postulated in the early work of Hottel and Stewart [Hottel and Stewart, 1940]. Mineral matter can affect carbon burnout by (1) undergoing fusion and surface-wetting that partially or totally encapsulates a portion of the char carbon, (2) catalyzing the carbon-oxygen reaction, (3) acting as a porous diffusion barrier, or (4) displacing carbon in char particles thus leading to low global reactivities at high conversion. These effects were evaluated for laboratory generated chars, residual carbon extracts, and during captive particle imaging. Microscopic examination of boiler residues and laboratory-generated chars from the combustion of bituminous coals does not reveal evidence that a significant fraction of the residual-carbon material is encapsulated by fused ash. Indeed, most of the carbon in the residual-carbon extracts is found |
