Pollution control systems using urea and ammonia for the control of emissions from the flue gas of fossil fuel burning facilities

Abstract

In many situations where the maximum NOx reduction capabilities of conventional SCR systems are not necessary, and alternative SNCR systems may not economically provide desired NOx reduction, a staged SCR/SNCR system is believed to be the most appropriate solution available. By means of the present invention, a system has been developed for delivering NOx reduction agents from a common urea feedstock, when it is appropriate to use combined NOx treatment stages (i.e. SNCR within the boiler followed by in-duct SCR and/or catalyzed air preheater elements). For the SNCR stage, the urea from the common feedstock is diluted prior to injection into the boiler and, for the SCR stage, the urea from the common feedstock is used to produce ammonia using hydrolysis. Urea solution which is not converted to ammonia in the hydrolyzer, is used for dilution of the solution being delivered to the SNCR system. Broadly, the invention herein is applicable to many instances in air pollution control wherein the facility in question requires both gaseous ammonia and urea solution to assist in the control of contaminants at discrete locations in one or more flue gas streams. In such instances the urea solution and gaseous ammonia will both use a common feedstock provided from a common urea source.