Infrared optical constants of coal slags - Dependence on chemical composition

Abstract

The results of an experimental study of the infrared optical constants of coal slags are presented. The real and imaginary parts of the refractive index were determined from transmittance and reflectance measurements carried out on thin polished glassy slag wafers at room temperature from visible wavelengths to 13 jun. The dependence of the optical constants on slag chemical composition was investigated, and correlations were developed based on the measured data to allow estimation of the optical constants as a function of wavelength and composition. HE presence of mineral matter in coal has several conse- quences for radiative heat transfer in coal combustion systems. The fly-ash particles dispersed in the combustion gases may absorb, emit, and scatter radiation, with possibly signifi- cant effects on heat transfer.1'2 Also, the slag layers formed on the walls alter the wall radiative properties. To estimate the effects of either fly ash or slag layers (which may be semitransparent3) on radiative heat transfer, one needs data on the complex refractive index m = n — ik of the ash/slag at infrared wavelengths. For calculations of radiant emission by the fly ash, the imaginary part k of the refractive index is of particular importance. At present, the available data on the optical constants (i.e., n and k) are fairly sparse and uncertain, particularly on k. Although there have been several reported measurements of n and k for fly ash,4'9 in each case, experimen- tal problems were encountered that appear to have significantly affected the measured values. The earliest reported optical constants for fly ash by Willis4 were later shown by Gupta and Wall10 to be in error from the use of incorrect data-reduction procedures. Similarly, the experimental technique used by Volz5 to measure k for fly ash was later shown by Toon et al.11 to be inapplicable for particles in the size range of fly-ash particles as a result of neglect of scattering. Lowe et al.6 and Gupta and Wall8 measured k for fly ash in situ in an operating pulverized coal utility boiler. In both studies, however, the measured values were biased by the presence of up to 10% residual carbon in the ash at the measure- ment location.8 Co wen et al.7 found similar effects of residual carbon in their measurements of k on sampled submicron ash. Finally, Wyatt9 has reported optical constants for fly-ash par- ticles at a wavelength of 0.6328 jLim, measured using a single- particle light-scattering technique. A later sensitivity analysis of the technique by Marx,12 however, concluded that the method cannot be used reliably to measure k for values below 0.01 (which is greater than the values reported by Wyatt) although it can be used to measure n accurately. In light of the problems encountered in the studies to date, it is clear that there is a need for improved data on the optical constants of fly ash in the infrared. Unfortunately, consider-