OCR Text |
Show INTRODUCTION Recently, international attention has Increased dramatically to the role of NOx in forest damage, lake acidification, oxidant formation, and impaired atmospheric visibility. In the United States, NOx emissions have been regulated for new utility boilers since the passage of the Federal Clean Air Act and the establishment of the New Source Performance Standards (NSPS) of 1971 . Under pending clean air legislation being considered by Congress, the utility industry is being faced with the possibility of having to reduce NOx and S02 emissions from its pre-NSPS units. Since these pollutant reductions must be accomplished on a retrofit basis, under a diverse range of system constraints, it is desirable to have a flexible and cost effective technology for achieving any mandated reductions. Reduced NOx formation from fuel-bound and atmospheric nitrogen is achieved, in general, through lower oxygen levels and lower peak flame temperatures. Exploiting these phenomena is the basis for NOx control through combustion modifications in which favorable conditions are created through changes in furnace design and operation. Such methods, pioneered on oil and gas units in the 1970s, however, face a more difficult challenge in the application to pulverized coal-fired units. This results from the higher nitrogen content of the fuel and from the greater complexity of solid fuel combustion, which brings with it an Increased potential for adversely affecting combustion efficiency, furnace wall deposits, and corrosion of heat transfer surfaces. Combustion modifications (i.e.low-NOx burners, staged combustion via over-fire air, or reburning) generally afford the least capital intensive approach. Due to the relatively higher fuel nitrogen content, however, the effectiveness of combustion modification approaches for NOx reduction on coal-fired boilers is generally limited to 15% to 50% (EPRI, 1989). At the other end of the spectrum, selective catalytic reduction (SCR) of NOx with ammonia offers the potential for the highest NOx removal, potentially as high as 900;0, but is also capital intensive (Robie et aI., 1989). Intermediate between combustion modifications and SCR are selective non-catalytic reduction (SNCR) processes. These processes react NOx with a compound such as ammonia (Hurst, 1985), urea (Muzio and Arand, 1976), and cyanuric acid (Perry, 1988) in a temperature regime of 800-11 OO°C. Although increased NOx reductions can be achieved with SCR and SNCR, problems can arise when ammonia emissions react with S03 in the flue gas to form ammonium sulfates which preCipitate onto downstream equipment, causing pluggage and corrosion (Maulbetsch et aI., 1986). A potential means for addressing this problem when encountered with SNCR is to use a lime-urea hydrate. In essence, the technology represents a synergistic combination of two technologies that have already been independently demonstrated at full scale. Calcium sorbent injection has been demonstrated to achieve nominal 500;0 reductions in S02 on several coal-fired utility boilers (Nolan et. aI., 1990; England et aI., 1990; Towle et aI., 1990). For the current proposed -2- |