Coal contains small amounts of nitrogen. Combustion thus gives rise in part to oxides of nitrogen. In the DeNOx reactor, nitrogen oxide is removed from the flue gas produced by firing coal. For this purpose, ammonia is sprayed into the flue gas stream and the mixture is directed over catalysts. Catalysts promote desired chemical reactions without themselves reacting with other substances. In the power plant catalysts, ammonia and nitrogen oxides react with one another to produce nitrogen and water vapor, i.e., natural components of the air we breathe.