Integrated catalytic/non-catalytic process for selective reduction of nitrogen oxides

Claim 1

In a combined SCR-SNCR process for reducing the NO content of a gas stream effluent from a combustion system, said effluent containing combustion products, including NO, said process including contacting said gas stream in an SNCR zone with NH 3 to reduce part of the NO in said stream, and contacting the SNCR zone effluent in an SCR zone with NH 3 and a NO reduction catalysts to further reduce the NO content of said gas streams, the improvement comprising: (a) injecting NH 3 into said gas stream downstream of said SNCR zone and upstream of said SCR zone to provide a mixed gas-ammonia stream, the quantity of NH 3 injected being just sufficient to effect the NO reduction in the SCR zone; (b) providing sufficient catalyst in said SCR zone to reduce the NO content of the SCR zone effluent to a maximum value; (c) passing said gas stream with an amount of NO that exceeds said maximum value to said SCR zone; and (d) injecting NH 3 into said SNCR zone only when the NO content of the SCR zone effluent exceeds said maximum value.