OCR Text |
Show produced high NOx, staged combustion with two different residence times in the reducing zone, a low-NOx burner which used a long axial flame, and reburning using coal as the secondary fuel. The five conditions, abbreviated BASE, STCr@0.5S, STG@1.5S, AXIAL, and R E B U R N are described in Table 1. Since the sooting tendency of coal varies with rank, [7, 15] bituminous coals from Utah and from Illinois and a lignite from North Dakota were used for the laboratory study. The fuels, abbreviated UT, IL, and N D , are described in Table 2. All five combustion conditions were run with the UT coal, but the other two fuels were only burned at conditions BASE, STGr@0.5S and STGr@1.5S. During the laboratory furnace experiments, the exit temperature was measured using a ceramic-sheathed type B thermocouple. The oxygen, carbon dioxide, and N O x concentrations were recorded using continuous monitors. Stoichiometric calculations were performed using the target thermal input, the fuel ultimate analysis, and the target excess air to determine the furnace air flow meter setting. The air flow was held constant and the coal feeder speed was adjusted throughout the run to control the exit oxygen concentration. A water-cooled probe and a preheated 90 mm filter assembly were usedto collect most of the laboratory furnace samples. Either glass fiber (Whatman 934-AH) or 0.22 u m polyvinyldiene fluoride membrane (Millipore G V H P ) filters were used. Both the filter deposit and the material washed from the probe were weighed and analyzed. For some runs, samples were collected using an Andersen 1-acfm cascade impactor and preseparator as described elsewhere. [16, 20, 21] The largest fly particles settle to the bottom of the horizontal section of the laboratory furnace and can not be collected quantitatively. This is analogous to the bottom ash in coal-fired boilers. However, the lower gas velocity at the laboratory-scale results in more of the total ash remaining in the furnace than is typicall observed for full-scale pulverized coal combustion. Bulk samples of high-carbon fly ash collected from a commercial power plant were used as a source of large amounts of sample for method Char and Soot 4 September 4, 1998 |