| OCR Text |
Show 6 o after a stable, natural gas assisted CVS flame was established and the furnace wall temperatures were at or near their steady state values, the natural gas flame was shut off. Initially, there were a few cases in which oxygen was also used to assist in the CVS light-up. Although the co-injection of oxygen with the slurry had the expected effect of promoting ignition it was found to be unnecessary and later was discontinued. As an additional point of interest, it should be mentioned that initially a highly dilute CVS slurry was pumped through the fuel gun and atomizer to wet their internal surfaces. It was found that passing the 70^ CVS fuel directly over these surfaces when the latter were dry (and hot) resulted sometimes in partial drying out of the fuel and plugging of the passages. 3.2 Visual Observations on CVS Flame Ignition and Stability The most immediate of the practical problems encountered in the initial phases of the CVS program centered on achieving good ignition, flame stability and satisfactory atomization quality (discussed in the following section). A CVS flame with a visually well defined front, stabilized close to the spray nozzle (within 1-2 nozzle diameters), and devoid of low frequency oscillations, was sought after. In the worst cases of these preliminary tests, the injected coal-water slurry was seen to burn on contact with the hot furnace walls rather than as a well defined flame. It was found that high levels of combustion air swirl (e.g. S • 1.8) were required to stabilize the flame. A high level of swirl causes a greater degree of recirculation of hot combustion products to the flame root, thereby increasing convective transfer of heat to the entering fuel. Relaxing the swirl level to a value characteristic of gently swirling oil flames (S - 0.3 - 0.6) resulted in complete 'loss'0of the flame with fuel burning on the hot combustion chamber walls. A 25 half angle divergent quarl was employed in the tests; the divergence of the nozzle has the beneficial effect of broadening the internal recirculation zone. The burner air exit velocity was varied by means of burner throat inserts from 25 - 60 m/s. Vithin this range of variation at a high level of swirl (S • 1.8) no great sensitivity of flame stability to this variable was observed. In general, increasing the exit velocity of the air jet leaving the burner has the effect of increasing the rate of entrainment of gases across the jet boundary. In the case of a confined jet these gases are the hot combustion products which are recirculated externally to the jet and across its boundaries into the "near field". This is beneficial from the point of view of energy feedback to fuel; however, higher air velocities have the detrimental effect of blowing the flame front back. Although not explored in detail in the scoping trials, it is expected that this variable will have an impact on the lower limit of swirl degree required for good flame stability, with higher exit velocities necessitating a higher minimum swirl degree for flame stabilization. Heat extraction at the root of the flame had a significant effect upon ignition and flame stability. The CRF burner quarl is water-cooled with bare-metal surfaces exposed to the fireside. Further benefits in flame |
