OCR Text |
Show 15 ed at a gas exit temperature of 900 C showed no attack on the tube, while the sample collected at 1200 C did pull off some of the tube scale from the surface of the tube as it was removed. Figures 30a and 30b show a piece of this scale. The EDAX analyses of Figure 30b, shown in Figures 30c, 30dt and 30e, indicate chromium and nickel present but very little iron, leading to the speculation that the iron in the alloy is attacked and withdrawn to the molten slag mass. Figure 31 shows another example of tube scale removed with the deposit, again with most of the iron missing from the alloy. This sample was also collected at a gas exit temperature of 1200°C. A further increase of gas exit temperature to 1400 C is accompanied by an increase in the thickness of the scale removed by the deposit. Figure 32 shows a 100-200 micron thick layer of tube scale attached to the slag mass. An even more severe attack at 1400 C is seen in Figure 33, where the tube scale consists of only individual grains of presumably chromium and nickel with no matrix binding them together. It should be reiterated that, while the severe attack of the deposited slag on the chrome alloy tube is most interesting, it represents thermal conditions outside boiler practice. 5.0 Summary and Conclusions A detailed combustion characterization study is underway at the MIT Combustion Research Facility (CRF) on industrial type (turbulent diffusion) flames obtained with coal-water slurries (CVS). Combustion trials carried out with a 70/30 coal-water slurry provided by the Occidental Research Corporation have high-lighted a number of technical problems associated with the combustion of this substitute fuel and potential means for their solution or amelioration. o Ignition and Flame Stability Before combustion of coal-water slurries can be initiated, the water contained in the fuel must be vaporized and the coal heated to ignition temperature. The experiments have confirmed that the water contained in the fuel, although requiring a small fraction of the heat content of the fuel for vaporization, cremates a potentially serious ignition and flame stability problem. The combustion tests have shown that CVS ignition and flame stability are highly sensitive to the quality of atomization, to the pattern of gas flow near the burner (the "near-field" aerodynamics), to heat extraction and to air preheat. High levels of combustion air swirl were found to greatly enhance the stability of CVS flames. Increased swirl causes more intense recirculation of hot combustion products to the flame root, resulting in a high convective feedback of heat to the fuel spray entering the combustion chamber. Further research is needed to quantify the ignition and stability characteristics of CVS flames. The development of a more fundamental understanding of the flame stabilizing mechanism is of scientific interest, and is expected to permit a more systematic approach to burner design and selection of flame input conditions for combustion of this fuel. One approach towards quantification of flame stability consists of carrying out |