The Combustion, Heat Transfer, Pollutant Emission and Ash Deposition Characteristics of Concentrated Coal Water Slurries

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16 an in-flame heat transfer study under different sets of input and operating conditions. Based upon detailed near-field flame structure measurements the radiative and convective energy feedback to the entering cold fuel spray can be calculated as a function of major burner-combustor input variables. o Carbon Conversion Efficiency Carbon burn-out in the case of the CRF CVS flames, obtained under intense fuel-air mixing and high air preheat conditions was good (>99^), but considered to be highly sensitive to atomization quality. Scanning electron micrographs of particulate samples taken along the centerline of CVS flames indicate that a significant degree of agglomera­tion is occurring in the fuel spray exiting the burner. The degree of agglomeration is thought to be dependent upon the quality of atomization, to the rates of fuel heating and peak temperatures in the near field, the nature of the coal (e.g., highly swelling or nonswelling) and also possibly upon the nature of the additive used to stabilize the fuel, which may be acting as a bonding agent between individual coal particles. The occurrence of coal particle agglomeration in CVS flames is thought to have a major impact both on carbon conversion efficiency and on ash deposition processes, and consequently warrants further more detailed investigation. o NO Emissions Coal typically contains 1-2 wt/6 organic nitrogen which under certain combustion conditions can contribute towards an excessively high NO emis­sion. The experiments have shown that conditions favoring rapid CVS igni­tion, flame stability and a high carbon conversion efficiency - intense fuel-air mixing, high air preheat, low near-field heat extraction - also tend to aggravate the NO emission problem. In general the selection of the best or 'optimal' CVS flame input conditions will reflect a comprise in these conflicting requirements. Distributed air introduction (staging) is considered to be potentially an effective means for alleviating this combination of problems and will be examined in detail in future studies. o Ash Deposition Studies of ash deposit samples by scanning electron microscopy and energy dispersive x-ray spectrometry have indicated that iron is a key actor in depressing ash melting temperature. In the presence of carbon in the deposit its fluxing action is even more strongly felt, thus underlining the importance of achieving good fuel burn-out. The nature of the ash deposits formed upon surfaces in the furnace can be significantly affected by the amount of heat extracted from the flame (relative to the fuel thermal input). Ash deposits in the experimental furnace tended to be more densely packed/bonded when the heat extracted from the flame was low enough (35/6 of the input) to yield exit gas tempera-