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Fuel, Vol.263, 2020
Investigations on the emissivity of burning coal char particles: Influence of particle temperature and composition of reaction atmosphere
Radiative properties of burning char particles are one of the key parameters for heat transfer in pulverized fuel boilers. Among the parameters which are suspected to change emissivity of burning char surfaces, particle temperature and the reaction atmosphere (air-fired vs. oxy-fuel) are two factors needing investigation. An experimental campaign is presented which studies the effect of both factors in the spectral range from 1.25 mu m to 5.5 mu m. An in-flight spectrometer test rig measuring thermal radiation emitted by single coal particles was employed. Particle temperature was adjusted by varying the oxygen content in the oxy-fuel atmosphere between 15 and 40%. Average char particle temperatures increase from 1952 to 2582 K with increasing oxygen content. The emissivity drops from epsilon = 0.6 to epsilon = 0.26 (lambda = 2.4-5.5 mu m) and epsilon = 0.33 to epsilon = 0.24 (lambda = 1.25-2.25 mu m) with increasing temperature. The influence of the reaction atmosphere was investigated by performing experiments in a N-2/O-2 atmosphere and comparing to the results to data from an oxy-fuel atmosphere, both containing 20% of oxygen. The difference epsilon(oxy-fuel) - epsilon(N2/O2) is in the range of 0.1 and 0.2 for lambda = 1.25-2.25 mu m and lambda = 2.4-5.5 mu m. Effects of burnout and char structure are discussed to evaluate the results.