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BYU is completing the first phase of work on the effect of alkali and alkaline earth material on selective catalytic reduction (SCR) NOx control systems. This work is being sponsored in part as part of the international collaboration with Tekes and is being performed in conjunction with Fortum and EPRI. BYU's role has thus far been the synthesis of data from diverse sources and reconciliation of these with a chemical mechanism. SCR systems are rarely used in the US today but are anticipated to become very widely used over the next few years as the second phase of the 1990 Clean Air Act Amendment is implemented. SCR systems are nearly the only control strategies available that will achieve the 80-90% reductions in NOx required for many power plants to comply with the second phase requirements of the Clean Air Act. While over 1000 combinations of materials have been tested for their catalytic performance, essentially all commercial systems are currently based on vanadia on titania (commonly anatase), typically with tungsta as a modifier that expands the effective temperature window, resists poisoning, and limits SO3 production. Ammonia is injected over this catalyst, and the catalyst promotes the formation of N2 by reduction of the NOx and oxidation of ammonia. Vanadia is the most active catalyst, with its catalytic activity being associated with its Brønsted acidity. Some reports suggest vanadia forms a sulfate when placed in SO2 laden streams (as is consistent with a thermodynamic evaluation) which is even more active that the oxide.
The introduction of alkali and alkaline earth metals such as potassium or calcium (common constituents in biomass fuels) or sodium and calcium (common constituents in low-rank coals) on the surface leads to formation of basic compounds (typically alkali and alkaline earth sulfates in SO2-laden systems), which decrease the surface acidity and therefore its catalytic effectiveness. This chemical poisoning mechanism is specific to biomass and low-rank coals and would be in addition to pore pluggage, fouling, etc. that might occur in all particle-laden flows.
There is scant direct field evidence that is publicly available that can be unambiguously associated with this poisoning. However, the anecdotal evidence of the deactivation rate is overwhelming. Several tests in Europe and the US by direct participants in this investigation as well as institutions not directly involved have documented deactivation that is consistent with this mechanism. Furthermore, there are no catalyst suppliers in the world that are willing to offer performance guarantees on either biomass or low-rank coal systems. There are a few controlled experiments from commercial- and sub-commercial-scale systems. The interpretation of these data, in particular those with which we have been intimately involved and for which we have samples and detailed analyses, is consistent with the above postulated mechanism and inconsistent with several earlier reported mechanisms. However, there have been no definitive experiments at commercial scale that quantitatively indicate the extent of this chemical poisoning by alkali vs. pore pluggage, etc. Such tests are being planned as part of the second phase of this investigation.
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