Pyrometric measurement of dust-laden gas temperature
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I.
INTRODUCTION
R A D I A T I O N pyrometers have been used in the metallurgical industry for many years. The most frequent applications have been to the measurement of the temperature of a target surface. If the sight path to that surface is through a nonradiating gas and there is no interference from radiation reflected from the surroundings, the measurement is simple and reliable. A single sighting of the pyrometer on the target, while measuring the target's true temperature by an independent means, permits setting an emissivity factor. Thereafter, a unique relation between the millivolt output of the pyrometer and the target's temperature pertains, and the pyrometer's calibration curve applies directly. Continuous monitoring and/or the use of the signal for process control become possible. To measure the temperature of a gas, be it clean or dust-laden, is, in general, more involved. An assumption that the above procedure can be applied uncritically usually leads to errors or outright misleading results, tl] It is necessary to formulate a sound model of the radiation exchange within the gas space in order for the radiation actually sensed by the pyrometer to be interpreted into a correct temperature. I f the measurement is to be continuous, it is necessary to monitor continuously the changes in the parameters characterizing the radiation exchange. This means that an on-line calculation is generally required. Prior to micro- or mainframe computers being available on-site, this was a formidable obstacle. This, however, is no longer so, and formulating a proper radiation model remains the outstanding problem. In this paper, we report on the experience of applying narrow-band infrared pyrometry to the temperature measurement of particle-laden gases in industrial-scale electrical utility boilers. Judicious signal interpretation was required, and the solutions developed have some general significance for metallurgical applications. R. MARR, Research Engineer, is with the Waterloo Centre for Process Development and is affiliated with the Depar~nent of Chemical Engineering, University of Waterloo. J.R. WYNNYCKYJ, Professor of Chemical Engineering and Extractive Metallurgy, is with the Department of Chemical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1. E. RHODES, formerly Professor and Chairman of the Department of Chemical Engineering, University of Waterloo, is in private consulting practice in Toronto, ON, Canada. Manuscript submitted May 2, 1989. METALLURGICAL TRANSACTIONS B
It turns out that for boiler service, a narrow-band or spectral pyrometer has distinct advantages over the total radiation pyrometer or a broad-band infrared pyrometer. Furthermore, the spectral band in which this pyrometer is sensitive is best chosen so that the former is in a region where the major radiating gases do not emit or absorb.t2~ Then, the signal derives solely from the radiating particles suspended in the gas. II.
THEORY
The radiant energy problem of concern is shown in Figure 1. The pyrometer receives radi
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