Action of oxygen and sodium carbonate in the urea-SNCR process

Abstract

Experimental researches are focused on the effects of O 2 concentration and sodium carbonate on Selective Non-Catalytic Reduction (SNCR) performance in a tube reactor, and plug flow reactor model and perfectly stirred reactor model in CHEMKIN are adopted to simulate the reactions processes. It is found that there is a conversion temperature point (CTP), on the two sides of which oxygen performs different effect. Below CTP, which is 1273 K in our experiments, higher NO reduction efficiency can be gained with higher oxygen concentration because more O 2 results in more radicals to drive the reduction chain reactions by speeding up the reactions O 2 + H = O + OH and H 2O + O = 2OH. At 1473 K without oxygen, 60% of NO reduction efficiency can be achieved and a 15 ppm Na 2CO 3 addition improves it to 90%. In this case the reaction H 2O + H = OH + H 2 becomes fast enough to provide the radical OH without the aid of O 2 to produce NH 2 which reduces NO. And H 2 is the byproduct of this reaction. Na 2CO 3 addition shifts the optimal temperature of SNCR 50 K towards lower temperature and more NO is removed in the temperature window. The reactions NaO + H 2O = NaOH + OH and NaOH + O 2 = NaO 2 + OH and NaOH + M = Na + M + OH offer new pathways to produce OH radical, which results in more OH and more NH 2 to reduce NO. The promotion effect of Na 2CO 3 is significant when temperature is lower or O 2 concentration is lower, which means the radicals are insufficient.