OCR Text |
Show Test Series No. 1: Runs No. 1 and No. 2 The objective of this test series was to obtain baseline data. As the experimental data given in Table 4 indicates, sulfur dioxide concentrations ranging from 1100 ppm to 1200 ppm were measured in the flue gas without the addition of sorbent, as the bed was operated at slightly different conditions. During these lists, freeboard combustion was clearly observed both visually through the sight glass and by the recorded temperature distributions along the fluidized-bed combustor axis. Yellow luminous flames as high as 1 1/2-inches were visible at the upper surface of the fluidized bed. The presence of freeboard combustion may be attributed to the escape of considerabl amounts of volatile matter through the bubbles to the bed surface where it burned as diffusion flames; this was the result of poor mixing and rather short gas residence time. Entrained solid particles collected from the primary and secondary cyclones contained substantial amounts of unburned carbon. Chemical analyses of these fine particles, as given in Table 5, indicated that the amounts of unburned carbon in the primary and secondary cyclone fines were as high as 57 and 69 percent of the captured fines respectively. These carbon particulates, "soot," may have formed from the combustion of volatiles in the freeboard. As can be seen from Table 4, a 94 percent carbon combustion efficiency was achieved without the recycling of fines. Nitric oxide emissions were low, as expected; this was due to a combination of the low combustion temperature and the fact that the scrap tires contain only 0.3 percent fuel bound nitrogen. Test Series No. 2: Runs No. 3 and No. 4 Greer (a local quarry) limestone was added to the bed to achiev various Ca/S molar ratios and observe the effect of Ca/S molar ratio on sulfur retention. At similar operating conditions, feedstock with Ca/S molar ratios of 2.3 and 2.9 was fed into the bed. With a feed stream Ca/S molar ratio of 2.9, 500 ppm S02 (which is equivalent to 0.91 lb S02/106 Btu input) was measured in the flue gases, which is lower than the current EPA standard for S02 emissions. However, at a Ca/S molar ratio of 2.3, the sulfur dioxide emissions could barely meet the EPA limit of 1.2 lb S02/106 Btu input. Sulfur retention by the limestone was lower in both tests than expected. This poor sulfur retention performance may be attributed to the short gas residence time and poor gas/solid mixing. The sulfur dioxide which formed in the freeboard region as a result of freeboard combustion of volatiles of course had no chance to react with the limestone in the bed. Provision for longer gas residence time and better gas/solid mixing is thus essential to the successful design and operation of a fluidized-bed combustion system for shredded scrap tires 11-24 |