OCR Text |
Show Soot aggregates have been photographed in the wake of single coal particles burning in a laminar flow laboratory furnace. [12, 13] [14] [15] In a previous study by the authors, [16] the fly ash produced from Utah coal and collected near the exit of a pilot-scale laboratory furnace was shown by electron microscopy to contain spheres of mineral ash, porous char particles and soot aggregates up to 50-100 urn long. That study also reported that submicron soot particles, but no char, were found in a filter sample collected from a power plant that was experiencing a "dark stack" while operating low-NOx burners. Observation of 100 um soot aggregates in the emissions from a South African power plant was orally reported at a recent conference. [17] Detection of soot aggregates at the exit of both laboratory and commercial coal combustors immediately lead to the question whether the soot represents a significant fraction of the unburned carbon, or if the soot was a trace component detectable only by electron microscopy. This paper describes methods that have been developed for quantitatively estimating the soot and char in a high-carbon coal fly ash sample. A method to prepare refined fractions of the soot and char has also been developed, and the preliminary results of experiments to characterize the composition, mechanical properties and thermal reactivity of the two fractions are reported. Previous laboratory data [16] on the unburned carbon in the fly ash from low-NOx combustion of Utah bituminous coal have been supplemented by recent experiments with Illinois bituminous coal and with North Dakota lignite. Methods Both laboratory-generated coal fly ash samples and samples of high-carbon fly ash from commercial power plants were used in this study. Laboratory fly ash samples were produced in a 30 kW, down-fired, U-shaped furnace with an inside diameter of 0.17 m and an overall length of 7.3 m which is described in more detail elsewhere. [18, 19] The combustion conditions used to produce fly ash samples consisted of a base case which Char and Soot 3 September 4, 1998 |