Methods for enlarging the useful temperature window for NOx reduction in combustion systems

Methods for reducing NOx emissions from stationary combustion systems having variable flue gas temperatures are disclosed. Use of an annonium salt of an organic acid enlarges the temperature window for effective selective noncatalytic NOx reduction thereby accounting for variable flue gas temperatures. Currently preferred ammonium salts of organic acids include ammonium formate, ammonium acetate, and ammonium oxalate. Mixtures of urea and either an ammonium salt of an organic acid or a metallic salt of an organic acid provide an even greater temperature window for NOx reduction. Currently preferred metallic salts of organic acids include Ca(COOH)2, Ca(CH3COO)2, Ca(C2H5COO)2, Mg(COOH)2, Mg(CH3COO)2, and Mg(C2H5COO)2.