| OCR Text |
Show application, these demonstrations established the ability of particulate collection devices to handle significantly higher particulate loadings (e.g. 70% - 1100/0). Urea injection, although currently not applied on coal fired boilers with appreciable sulfur content, has been demonstrated to achieve NOx removals of 45% - 800/0 on oil, gas and MSW fired Installations (Nylander et aI., 1989; Hofmann et al, 1989; Jones et aI., 1989). The focus of the current paper involves the hydration of lime with a saturated urea solution. Through this approach, the urea can reduce NOx emission levels on the order of 600/0, while also achieving nominal reductions of S02/S03' The lime initially incorporated within the sorbent serves to reduce local S03 concentrations so as to alleviate the potential fouling of the air heater due to ammonium sulfates formation. For a flue gas containing 750 ppmv NOx' the lime-urea hydrate would have a mass ratio of urea to lime of approximately 0.6. At this mass ratio, a sorbent injection rate corresponding to a N/NOx ratio of 1.0 would increase the particulate loading approximately 18% over that associated with a 10% ash coal. Thus, the particulate loadings are significantly less than that associated with furnace sorbent injection for S02 control. TECHNOLOGY DESCRIPTION A schematic of the hydration process is depicted in Figure 1. When applied in a manner so as to optimize NOx reductions, the lime-urea hydrate is prepared by hydrating quicklime with a saturated aqueous urea solution. The relative ratios of NOx and S02 removal can also be adjusted to site specific needs, however, by varying the concentration of the urea solution used in the hydration process. Thus, to increase S02 removals while maintaining a given level of NOx reductions, the aqueous urea concentration is reduced. The sorbent injection rate is correspondingly increased to maintain a constant N/NOx ratio, thereby increasing the relative Ca/S ratio. The decision of whether to prepare the sorbent on-site or off-site is generally dependent upon site specific factors which impact the cost of the sorbent preparation (i.e. lime and urea transportation distance and cost sorbent requirements, etc.). In general, it is only cost effective to hydrate on-site for those cases where the reduced cost of transporting the raw materials, and adding the water of hydration on-site, offset the cost associated with the installation of hydration equipment. Otherwise, the purchase of a commercially prepared sorbent, stored onsite, is economically preferred. The end result of the hydration process is a fine, dry, powder sorbent containing urea and calcium hydroxide. The lime-urea hydrate is effective in removing NOx from combustion gases when injected into the upper furnace region of a boiler. The urea reactions take place over a temperature range of 850°C to 1100°C (1560°F - 2000°F) producing primarily N2 and H20. Reactions between the lime and S02 occur over a broader temperature regime, typically between 800°C to 1200°C (1470°F - 2200°F). After injection, the spent calcium portion of the sorbent can be captured and -3- |
