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The interaction of sodium, sulfur, and silica at conditions typical in a pulverized coal furnace was investigated by using both model mixtures and a synthetic coal. The model mixtures consisted of selected inorganic constituents that were well mixed in proportions typically found in low-rank coal. The synthetic coal consisted of a furfuryl alcohol polymer with appropriate amounts of sodium, sulfur, and silica to duplicate the characteristics of low-rank coal. The model mixtures and synthetic coal were burned in a laminar flow (drop-tube) furnace at 900, 1100, 1300, and 1500 °C and residence times of 0.1,0.5,1.5, and 2.4 s. The resulting char and fly ash particles were quickly quenched, collected, and analyzed with a scanning electron microscope (SEM) to determine size and composition. Results indicated that the formation of sodium silicates is favored by higher temperatures and longer residence times. Thermodynamic calculations and the model mixture studies indicated above 1100 °C there is little interference in the formation of sodium silicates by sodium sulfates. In the synthetic coal studies, sodium sulfate particles were detected on the surface of the larger sodium silicate fly ash particles formed at lower temperatures. The size and prevalence of the sodium sulfate particles decreased as temperature was increased. Fly ash particle formation was characterized by fragmentation followed by coalescence. Fragmentation was more prevalent at higher temperatures and smaller fly ash particles were formed. Larger particles were formed at lower temperatures, indicating more complete coalescence with some cenosphere formation.

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The Clean Air Act Amendments (CAAA) of 1990 are expected to impact both conventional and advanced power systems. Pressurized fluidized-bed combustion (PFBC) and other coal combined-cycle processes combined with hig h-temperature cleanup devices are environmentally friendly and economically attractive. Though PFBCs are beneficial because they can decrease the emission of sulfur and NO, species, and advanced combined-cycle systems that incorporate high-temperature cleanup devices are beneficial because they can enhance efficiency, the impact of these technologies on the emission of trsce metals and certain organic compounds needs to be assessed. This paper compares 1) the Tidd PFBC demonstration plant with conventional pulverized coal-and cyclone-fired systems and 2) the Tidd plant advanced particulate filter with the pedormance of conventional electrostatic precipitators and baghouses. We also attempt to extrapolate the comparison to other advanced systems. Except for mercury, the PFBC at Tidd released less trace metals into the flue gas stream than the adjacent conventional pulverized-coal combustor, using the same coal. Similar to conventional power systems, hazardous air pollution emissions from advanced systems appear to be lower than the trigger level of 1990 CAAA, which requires maximum achievable control technologies.