The effect of soot on pyrometric measurements of coal particle temperature

Abstract

Multiple wavelength pyrometers have been used by many investigators for the measurement of temperature of solid phase suspensions in flames. It is known that this temperature may differ from that of the surrounding gas by hundreds of degrees due to transient heating and chemical reactions. Recent observations of burning coal particles indicate that during the devolatilization phase, a highly luminous mantle can exist about the particles due to soot which is formed in the boundary layer or wake. This paper analyzes the affect uniform soot clouds about the coal particles have on the temperature inferred from radiant emission measurements of burning coal dust suspensions. The emissive power from the nonisothermal coal/soot system is computed relative to the power emitted by the coal particles alone as a function of the soot properties, the coal particle size, the optical thickness of the suspension, and differences in temperature. The soot greatly increases the effective temperature if it is more than 200K above the coal surface temperature and can, under some circumstances, lead to an inferred temperature greater than either the soot or coal particle temperature. For soot volume fractions below about 10 −7 the error created by the soot is usually less than 100K. It is concluded that measurements in the infrared are preferable to the visible when soot is evident if the soot is more than 200K above the coal particle temperature. To improve confidence in the particle temperature indicated, it is recommended that a two-color measurement be made in each of two widely separated spectral regions and that the results be compared.