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Show Introduction The control of NOx emissions from many utility boilers is required under the acid rain provisions of the 1990 Clean Air Act Amendments. While these provisions specify reduced emission factors achievable with combustion modifications, additional emission reductions may be necessary to comply with Title I of the Act to control ground-level ozone in some areas (e.g., Possiel, 1991). Such additional requirements would potentially be met with post-combustion, flue gas treatment controls like selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR). SCR and SNCR both use chemical reactions with ammonia (NH3) or other agents to reduce the amount of NOx emitted into the atmosphere. SNCR proces~es, such as Thennal DeNOx® (Lyon, 1975), inject cold NH3 into combustion flue gases and rely on a narrow temperature window of 1100-1370 K (Salimian and H-anson, 1980) to dissociate the NH3 for subsequent reduction reactions. SNCR generally achieves 40-60% NOx reduction in commercial applications. In contrast, SCR can achieve 80-90% NOx reduction by, introducing a catalyst grid into the ammonia injection process to aid dissociation (EPA, 1992a). However, the introduction of the catalyst increases the expense of the reduction process considerably (EPA, 1992b). In an effort to lessen the dependence on a temperature window and to avoid the expense of a catalyst, a new approach using plasma technology was developed for NOx reduction. In it, ammonia radicals were created with plasma generators and injected into combustion flue gases. By breaking down the NH3 externally, the reliance on the temperature window and catalyst were avoided. Two independent studies, one on a laboratory-scale combustor (Zhou, et.al., 1992) and the other on a larger test facility (Boyle, et. al., 1993), demonstrated the potential of ammonia radical injection to achieve high NOx reduction. Zhou et. al. at Carnegie Mellon found a maximum NOx reduction of 86% using an inductively coupled plasma system (ICP-16), while Boyle et. al. found 85-90% NOx reduction at the Pittssburgh Energy Technology Center using a DC plasma torch. Both experimenters found that NOx reduction increased with increased ammonia flow, decreased plasma power, and decreased excess air. While the radical injection technique for NOx control shows promise, several key questions remain concerning its performance and its place relative to other post-combustion control techniques. For example, since the radicals in this process are necessarily injected at elevated temperature, the high level of NOx reduction achieved could be the result of increased NH3 dissociation by thermal breakdown within the combustor as well as the result of the presence of externally produced radicals. These two effects, bulk thennal heating and radical production via 2 |
