| OCR Text |
Show -...-~.- . ,.: .' when an irregular particle tumbles in the view of the detector, alternately exposing different burning areas as it does so. For more spherical particles, the oscillation amplitudes are much smaller. 5.:1 Ignition The third and last example of the kind of detailed observation that is possible from pyrometry experiments is shown in Figure 1. It is seen in the figure that although the particle temperature is almost constant during the duration of combustion, the intensity shows a maxima. While the reason for the decline in intensity in the latter part of combustion is geometrical (namely, the particle is shrinking as it burns and therefore the detector sees a monotonically decreasing radiating area with time), the gradual rise in the intensity, at first, has a different explanation.Since the temperature is constant and the distance between the detector and the particle is almost constant during the complete combustion period, the cause of the rise can only be a gradual increase in the radiating area of the particle. The physical mechanism of this phenomena is that ignition of the particle takes place at a few 'hot-spots' on the surface. These spots then grow, finally engulfing the entire particle. A simple thermal theory of the growth of such spots has shown remarkable agreement between the observed rise times and those predicted by the theory [9]. This observation is quite important because it contradicts the common modelling assumption of uniform ignition of the whole particle. 6. Conclusions This paper attempts to show the usefulness of two-color optical pyrometry as a diagnostic tool in coal char combustion. The theoretical basis of the technique is outlined and results from typical experiments are analyzed. The power of the method in helping to understand the details of char combustion is shown through the examples of particle-particle variability, particle tumbling, and heterogenous Tlition on the particle surface. 10 |
