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Show PAPER NO. 39 REDUCTION OF NITROGEN OXIDES FROM POST-COMBUSTION GASES UTILIZING MOLECULAR RADICAL SPECIES ~ John Boyle, Armistead Russell, Shi-Chune Yao Carnegie Mellon University James Ekmann, Mahendra Mathur ) JUL.Y) fv.- Department of Energy, Pittsburgh Energy Technology Center INTRODUCTION AND BACKGROUND Nitrogen oxides (NOx) are known to be primary precursors to acid rain and the '/ fonnation of urban and regional ozone. The reduction or elimination of these pollutants, found primarily in combustion emissions, is required in the newtlean Air Act, as well as by local regulations. It is also prudent to look beyond the current legislation in the development of new technologies which will satisfy increasingly stringent control requirements. Fuels containing high levels of nitrogen, such as coal, are particularly susceptible to high NOx emissions. Coal is, however, a very abundant and inexpensive fuel. A continually increasing demand for electric power coupled with the uncertainties and expense of alternate generation techniques makes it clear that the combustion of coal and other fossil fuels will be necessary well into the next century. NOx reduction from combustion applications is primarily a problem of NO removal. Nitric oxide (NO) accounts for more than 95% of the NOx emitted. Current emission control technologies are varied. Pre-combustion treatment attempts to remove fuel-bound nitrogen have been partially successful. Thennal-NOx, caused by the oxidation of atmospheric nitrogen in the combustion gases, is the source of a large portion of the NO. Burner modifications, water or steam injection, and other combustion alterations which lower the peak flame temperature and reduce exposure to oxygen have been successful in eliminating moderate portions of NO fonned in this way. Post-combustion treatment has been used with varied effectiveness to chemically reduce NOx downstream of the combustion chamber. Such flue gas cleanup devices include non-selective catalytic reduction (NSCR), selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR). SNCR and SCR both depend on radicals of ammonia (NH3) to react with and remove NO in the stream. Injection of urea, cyanic acid and, in general, NHi containing species has also been proposed. In particular, the radical amidogen (NH2) is considered the primary reducing agent. SNCR depends on having an optimal temperature of the post-combustion stream to fonn amidogen. This highly coupled reaction sequence is extremely temperature dependent Coupled with the problem of ammonia breakthrough, SNCR is capable of providing only about 50% NOx reduction. SCR utilizes a catalyst to promote NO removal by injected ammonia. The result is a more efficient process which is less susceptible to ammonia slip but which is far more expensive. Catalyst supply or regeneration and the associated problem of toxic catalyst disposal create additional expense and environmental concerns. An investigation of NOx reduction techniques reveals a need for new solutions which will be very effective and inexpensive. Investigation and utilization of the relevant radical chemistry will provide new, more effective and efficient NOx reduction techniques. Molecular radical species, which are highly reactive, dominate the flue gas chemistry. Molecular radical reduction employs the external control of radical populations to most effectively control emissions. 1 |