Reduction characteristics of NOX emissions by optimal urea injection rate in a LPG steam boiler with urea-SCR reactor

The aim of this study is to develop the urea-SCR reactor aftertreatment system for reducing NO_X emissions in a LPG steam boiler. The urea-SCR system utilizes ammonia converted from 17% aqueous urea solution injected in front of the SCR catalyst as a reducing agent for reducing NO_X emissions. In this study, NO_X emissions and exhaust gas temperatures according to boiler load were measured and compared with before and after urea injection in a LPG steam boiler with urea-SCR reactor. Also, the equivalence ratio, urea injection rate, ammonia slip and NO_X conversion rate relative to boiler load were investigated. The equivalence ratio is different by changing the fuel consumption rate or intake air rate. In this experiment, the average equivalence ratio is calculated by changing only fuel consumption rate while the intake air rate is constantly fixed at 25,957.11 cm³/sec. The average equivalence ratios are 1.38, 1.11, 0.81 and 0.57 when boiler loads are 100, 80, 60 and 40%. In this experiment, NO_X conversion rates are calculated by 89, 85, 77 and 79% in cases of the boiler loads of 100, 80, 60 and 40%. NO_X conversion rate is raised with increasing urea injection rate, and NH₃ slip is boosted at the same time. Consequently, the optimal urea injection rate for the respective boiler load should be determined by considering NO_X conversion rate and NH₃ slip simultaneously. In this study, the urea flow rates of 230, 235, 233 and 231 mg/min are injected at the boiler loads of 100, 80, 60 and 40%, and the final NH₃ slips are measured by 8.48, 5.58, 11.97 and 11.34 ppm at the same conditions.