Two-color transmittance measurements in a pulverized coal reactor

Abstract

A two-color extinction diagnostic technique has been investigated for the measurement of soot and other fine particulate in pulverized-coal flames. The measurements were made using two helium-neon lasers at 633 nm (red) and 543.5 nm (green) in a 0.2 MW, pulverized-coal, down-fired reactor. Parameters inves-tigated included axial position, swirl, and equivalence ratio. Under lean conditions, coal, char, and ash partiele transmittance was found to be insensitive to the wavelength, and transmittance increased with downstream axial distance showing char burnout. When soot was visually apparent in the reactor, the shorter wavelength (543.5 nm) was attenuated more than the longer wavelength (633 nm). Thus, the presence of soot and relative amounts of soot could be determined from the transmittance measurements. Soot was seen to form in the first 800 mm of the reactor, after which it reached a steady value. Soot was found to decrease with increasing swirl from 0.5 to 1.5, but the lowest amount of soot was found to be at zero swirl where previous measurements have shown the flame to be lifted. Near the exit, no soot was measured until the equivalence ratio in the flame reached a value of =1.1, after which increasing the equivalence ratio increased the soot volume fraction dramatically. An appropriate model for the soot and the coal, char, and ash was used to determine the soot volume fraction from the transmittance data and Mie theory. The highest measured value of soot was at =1.5; resulting in a soot volume fraction of 0.88 ×10 −6 . At higher equivalence ratios, the soot transmittance was below the detector noise level. Caleulations showed that at the sootiest conditions, approximately one-fifth of the volatiles were being converted to soot.