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Show 11 injected at the probe tip to produce rapid cooling of the sample. The sampled particles together with the quench water were drawn through the probe and passed through a filter on which the solids were collected. It is unlikely that the observed agglomeration could take place while sampling; on the contrary, the use of room temperature water for quenching at the probe tip is more likely to have caused fracturing of the collected particles due to large thermal stresses. Figure 20 shows that some of the particles from the coal-water slurry flame with less heat extraction (Flame 1) are spherical, typically 200 urn in diameter. (As indicated in Table 3 the coal used in the slurry was of a standard grind, with 85/6 of the particles having a diameter less than 75 ^n and 100^ less than 300 ^ m ) . In addition there are a number of particles which are more cylindrical in appearance, ranging in length from 700 pm to 1200 /id, and in diameter from 100 ^m to 200 pm. The process of atomization is generally characterized by the formation of a liquid jet or sheet which then breaks down into cylinders or ligaments and, finally, into droplets. The char particle shown in Figure 21 looks very much as if it could have been a ligament from which 3 or 4 drops were in the process of forming. It appears that the water was driven off before the atomization process was complete leaving a more or less intact ligament composed of individual coal particles bonded together by a sticky matrix. The scanning electron micrographs of solids samples taken from the second coal-water slurry flame (Figures 23-25) show that the char particles found in this flame are all roughly spherical and in the range sized 100 - 200/im in diameter which again indicates that agglomeration of individual particles was occuring. The reasons for the agglomeration observed in the SEM photographs are not fully understood. It is possible that as the slurry is sprayed into the combustion chamber, the coal particles contained in the individual droplets undergo radiative heating, to the point where they begin to pyro-lyze, swell and exude tars before the water is completely evaporated. The tars may serve to bond the swelling coal particles together and the pressure exerted by the gas inclusions in the particles would then blow the coal particle agglomerate into a hollow sphere (cenosphere). The chemical additives, which comprise, approximately ^% of the fuel, can also possibly play a role in bonding individual coal particles together during their initial stages of heating. 4.4 Characterization of Ash Deposits Deposit Collection The deposits analyzed to date in the MIT coal-water slurry combustion studies have all been collected on an uncooled Hastelloy X tube which is located at the exit of the combustion chamber ("5 m from the burner), and which is used for the purpose of sampling the furnace exhaust gases for a continuous on-line analysis. Since the tube is uncooled, and the combustion gases are continually withdrawn through it, the temperature of the deposit collection surface can be expected to be essentially the same as the temperature of the bulk combustion gases, ranging from 900 to 1400 C, |
