Experimental Study on the Effect of Urea and Additive Injection for Controlling Nitrogen Oxides Emissions

Abstract

Selective noncatalytic reduction (SNCR) for nitrogen oxides (NOx) abatement, using urea (the NOxOUT process) as an N-agent was conducted experimentally on a multifunctional combustion facility. Results showed that the NOxOUT process resulted in 90.1% NO reduction at 1,273 K if the N-agent-to-NO mole ratio (NSR) varied between 1.5 and 2. Also, oxygen concentration of the inlet gas clearly impacted the process. NOx reduction was reduced by 17.1% when oxygen concentration was increased from 2.7 to 3.6%. If the concentration was increased further, decline of NOx reduction would become smooth. At the same time, ample residence time must be guaranteed, and a minimum value of 1.2 s was required for a thorough NOx reduction. The temperatures where 90% of the maximum reductions are achieved in low and high temperature zones are seen as the lower and upper limit temperature, respectively. The range between the lower and upper limit temperature is defined as a “temperature window.” Different reagents lead to different maximum reductions and the “temperature window” changes accordingly. Based on these findings, synergistic effect of additives was studied to broaden the narrow “temperature window” of SNCR. Sodium carbonate, ethanol, glycerol, and acetic acid, with a concentration of 200 ppm, respectively, were selected as additives, and the “temperature window” was broadened by 40–80 K. Different additives presented different characteristics. Sodium carbonate simultaneously broadened the “temperature window” toward low and high temperature zones and slightly compromised the maximum NOx reduction. The organic compounds mainly showed effect in low temperature zone and reduced the maximum NOx reduction by 11–14%. This investigation is performed from operating parameters to additive injection, which makes the effect of additive obvious. It is hoped that the conclusions of this work could further ascertain the NOxOUT process and make a contribution to the industrialization of the SNCR technology.