An Analysis of Coal Particle Temperature Measurements with Two-color Optical Pyrometers

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Abstract
Two-color optical pyrometers have been used to measure the temperature of reacting pulverized coal particles. An analysis of such measurements was performed to determine the effect of several possible conditions on the measured temperature. The conditions investigated were the use of a single photomultiplier to alternately measure the radiant emission at the two selected wavelengths, the presence of soot, light extinction, the choice of wavelengths used to compute the two-color temperature, and non-uniform particle clouds. A computer model of a one-dimensional coal particle cloud was written for this analysis. Results of calculations showed that artificially high temperatures can result if a pyrometer with a single photodetector is used to measure temperatures in a rapidly fluctuating flame. Emission by soot in the coal particle cloud caused unrealistically high temperature measurements. Light absorption by soot lowered the two-color temperature, but not enough to compensate for the rise in observed temperature caused by soot emission. When the wavelengths used are in the visible spectrum. the hotter particles are weighted much more heavily than when the wavelengths are in the infrared region. The use of Weins Law, a valid approximation to Planck's Law in the visible spectrum, causes substantial error for longer wavelengths. Finally, the two-color temperature of a non-isothermal cloud was weighted most heavily by the hotter particles, depending upon the wavelengths used for the measurement. From this analysis important questions have been raised as to the validity of such measurements under transient conditions and during devolatilization.