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

摘要

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 + O2_ = 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.