Selective Catalytic NOx Reduction Process and Control System

A process for reducing the concentration of nitrogen oxides in combustion gases from a plurality of combustors, each of which has an associated selective catalytic NOx reduction catalyst effective for utilizing gasified urea, the process comprising:

• a. heating aqueous urea at conditions of temperature and pressure for a time effective to gasify the urea and water by mixing the aqueous urea with a heated gas stream in amounts relative to the amounts of urea and water to produce a first gasified reductant stream comprising a predetermined mass and concentration of ammonia;
• b. monitoring the demand for urea at each of the plurality of combustors;
• c. feeding the first gasified reductant stream to each combustor at a rate sufficient to supply gasified reductant to supply the monitored demand to the selective catalytic NOx reduction catalyst associated with each combustor;
• d. determining an amount of carrier gas needed to achieve a predetermined degree of mixing of the gasified reagent with combustion gases generated at each combustor;
• e. based on this determination, admixing a determined amount of carrier gas with the first gasified reductant stream to provide a catalyst feed stream associated with each combustor;
• and
• f. at each combustor, introducing an associated catalyst feed stream by means of an ammonia injection grid to the effluent upstream of the catalyst under conditions effective to reduce the concentration of NOx in the combustion gases from each combustor.