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

Experimental researches are focused on the effects of O2 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 O2 results in more radicals to drive the reduction chain reactions by speeding up the reactions O2 + H = O + OH and H2O + O = 2OH. At 1473 K without oxygen, 60% of NO reduction efficiency can be achieved and a 15 ppm Na2CO3 addition improves it to 90%. In this case the reaction H2O + H = OH + H2 becomes fast enough to provide the radical OH without the aid of O2 to produce NH2 which reduces NO. And H2 is the byproduct of this reaction. Na2CO3 addition shifts the optimal temperature of SNCR 50 K towards lower temperature and more NO is removed in the temperature window. The reactions NaO + H2O = NaOH + OH and NaOH + O2 = NaO2 + OH and NaOH + M = Na + M + OH offer new pathways to produce OH radical, which results in more OH and more NH2 to reduce NO. The promotion effect of Na2CO3 is significant when temperature is lower or O2 concentration is lower, which means the radicals are insufficient.