| OCR Text |
Show containing less than 1 percent potassium produced no deposits whatsoever on slagging probes, despite low ash-softening temperatures and low viscosity. Figure 14 illustrates the degree of slagging on the lower slagging probes and compares the potassium and iron levels in the coal ash, as well as softening temperatures for several fuels. Figure 15 presents the same information for the center slagging probes. Since slagging of heat transfer surfaces exposed to axial-symmetric flow depends upon the formation of an initial layer of very small particulates whose slagging potential appears to be related to the potassium level rather than the overall ash-softening temperatures or percent iron in the ash, it is evident that the specific gravity of particles plays a limited role in the deposition and retention of ash on the vertical surfaces in the unit. This further explains why there was little evidence of selective deposition of coarse,heavy molten particulate onto the slagging probes. The larger, heavier particulates which do impact on the slagging probes probably make contact at a very acute angle, and it is possible that the gravitational forces on the particle overcome the shear forces resisting reentrainment from the wall. Deposits formed in Zone D on the roof of the combustor, while firing the Eastern coal, were concentrated in the center of the combustor about 6 inches away from the wall. Inertial forces probably dominate the mode of transfer of fly ash to the refractory surface. SEM and EDAX analyses, summarized in Figure 16, indicate the deposits formed here were composed of sintered spheres about 40y's or less in size and rich in iron. The molten matrix holding the deposit together contained silica, alumina, iron, and potassium. Differential Thermal Analysis (DTA) revealed a minor endotherm at 2000°F which corresponds to the initial deformation temperature. The fluid temperature occurred at 2500°F, about 400 to 500 degrees above the local flue gas temperature. Selective deposition of iron-rich, heavy gravity fractions was very apparent in Zone D where the flue gas experienced a change in direction, leaving the heavier iron-rich particles to impact onto the furnace roof rather than remaining in the flow stream. Likewise, in Zone E where the gases are accelerated to speeds of 40 ft/s, approximately eight times that of the furnace, and 43 |
