Recent Developments in Combined Control of SO<sub>2</sub> and NO<sub>x</sub>

Control of sulfur dioxide (SO<sub>2</sub>) and nitrogen oxides (NO<sub>x</sub>) emissions has been required of many utility and industrial installations for some time. Legislation recently passed by Congress to revise the Clean Air Act will require controls at many more sites because both species have been implicated as precursors to acidic deposition, which can have long-range and even international impacts. In addition, NO<sub>x</sub> emissions have been linked to local ozone pollution problems and potential global climate effects. Available control technologies for these pollutants typically involve combustion modification techniques for NO<sub>x</sub> and some form of wet scrubbing (usually based on lime or limestone as a sorbent) for SO<sub>2</sub>. Recently, catalytic reduction for NO<sub>x</sub> control has achieved commercial status, and spray-dryer technology has led to the development of a wet/dry scrubber system for SO<sub>2</sub> that produces an easily handled dry waste. Research and development programs are actively working on a wide variety of alternative technologies, including a number of integrated processes that remove both SO<sub>2</sub> and NO<sub>x</sub> in a single system. Integration represents a particularly attractive approach from both a cost and operational point of view since the number of components is minimized, system operability is enhanced, and reliability is generally improved over that attained with a series of independent technologies. In addition, there may be advantages due to lower consumption of resources, reduced waste volumes or improved waste forms, and beneficial synergisms between the pollutants in the removal process. This paper reviews a number of integrated flue-gas cleanup (FGC) systems that have reached a significant stage of development, focusing on post-combustion processes that have been tested or are ready for testing at the pilot scale or larger. Other integrated approaches, such as slagging combustors, fluidized-bed combustion, gasification/combined-cycle systems, and various processes combining low-NO<sub>x</sub> burners with SO<sub>2</sub>-sorbent injection in the furnace, also offer advantages that should not be overlooked when evaluating alternatives for a specific application.