Radiative heat transfer in pulverized-coal-fired boilers-Development of the absorptive/reflective character of initial ash deposits

Emission Fourier transform, infrare (FTIR) spectroscopy data provide in situ, time-resolved, spectralemissivity measurements for ash deposits generated from two U.S. Powder River Basin coals. The first 3h of deposit growth on a tube in cross flow in a pilot-scale furnace detail the development of surface emissivity with time. Measured emissivities vary significantly with wavelength, indicating the influence of the physical properties and chemical composition of the deposit. At long wavelength (>7 μm), emission features exhibit characteristics of silica, sulfates, and silicates. The spectral emissivity measured in this region approaches a steady value due to an increase in deposit thickness and the size of particles in the deposit. In contrast, deposits are not opaque at shorter wavelengths where the measured emissivity is influenced by the properties of the underlying metal surface. Theoretical predictions of the emsivity of a particulate layer were performed, and results are compared to the measured values. The theory adequately predicts the general features of spectral variation of the emissivity. The predicted trends in emissivity with particle size and deposit composition are also consistent with exerimental observations. Total (Planck-weighted) emissivities are calculated from the measured spectral values for the deposits at the tube temperatures. They increase with time from the clean tube value (0.2-0.3) to values typicals of deposits formed from western U.S. coal (0.45-0.55). Calculated total absorptivities are found to be lower than the correspoding emissivities.