OCR Text |
Show tailoring Itself to a site-specific application. Should the SOiNOx ratio of the flue gas change due to coal switching, for example, similar emissions reductions could be maintained by simply altering the urea/CaO ratio of the sorbent. The presence of the urea in the lime-urea hydrate did not degrade the S02 removal performance of the calcium, and in fact Increased S02 removals at a given Ca/S ratio. Having the urea incorporated into the lime-urea hydrate resulted in higher NOx removals than attained with a physical mixture of Ca(OH2) and pulverized urea. The reason for this enhancement in NOx removal is currently not known. Bench-scale hydration experiments were also performed to assess the impact of hydrating with a urea solution. In general, the time required for the hydration reaction to proceed to completion increased by a factor of two as the urea concentration in the water of hydration was increased to saturation. This increased hydration time requirement, however, could be accommodated within commercial hydrators (Babu, 1989). Examination of the physical properties of the resulting hydrates also demonstrated little effect from the urea. Over 99 percent of the urea present in the hydrating solution was accounted for in the solid lime-urea hydrate product. APPLICATION ISSU ES Ammonium Sulfates Formation During combustion, fuel sulfur compounds are oxidized to S02 and a small percentage (0.5% - 4.0%) is further oxidized to S03' Although the resulting concentrations are usually quite small (10 - 50 ppm), the S03 is still a concern. One of the problems confronting the use of a SNCR NOx removal process in coal combustion applications, is the reaction between residual ammonia byproducts and S03 to form ammonium bisulfates: NH3 + S03 + H20 ~ NH4S04 which is a corrosive liquid above 197°F. The residual NH3 in the flue gas is the result of unreacted urea that has decomposed. In the presence of excess NH3, ammonium bisulfate will further react to form solid ammonium sulfate: which can deposit and plug the air heater or any final heat recovery equipment. The temperature dependence of ammonium sulfate/bisulfate formation as a function of NH3 and S03 concentration is shown in Figure 4. For a given NH3 and S03 concentration, the diagonal lines indicate the temperature at which ammonium sulfate or bisulfate begin to form. Above that temperature, decomposition to NH3 -8- ;' |