An investigation of urea decomposition and selective non-catalytic removal of nitric oxide with urea

Abstract

The use of urea (NH2CONH2) to remove nitric oxide (NO) from exhaust streams was investigated using a laboratory laminar-flow reactor. The experiments used a number of gas compositions to simulate different combustion exhaust gases. The urea was injected into the gases as a urea-water solution. The decomposition processes of the urea-water solutions and urea powder were examined. For both the nitric oxide removal and the urea decomposition experiments, a Fourier transform infrared (FTIR) spectrometer was used to determine the concentrations of the product species. The products from the decomposition were examined every 50 K from 500 K to 800 K. The dominant products were ammonia (NH3), isocyanuric acid (HNCO) and carbon dioxide (CO2). In case of urea-water solution decomposition, for gas temperatures between 550 and 650 K, the highest concentrations were for NH3 and HNCO. On the other hand, the concentrations of CO2 were highest for gas temperatures of about 500 - 550 K. For temperatures above about 650 K, the amount of these three dominant prod-ucts slightly decreased as temperature increased. ivFor the nitric oxide removal (SNCR) experiments, the gas mixture was heated to temperatures between 800 K and 1350 K. Depending on the temperature, gas composition, residence time, and urea feed rate, removal levels of up to 95% were obtained. Other by-products such as N2O were detected and quantified. The effects of the urea/NO (beta) ratio were determined by varying the urea concentration for a constant NO con-centration of 330 ppm. The effects of the levels of oxygen (O2) in the exhaust gases and the residence time also were investigated. Increasing the urea/NO ratio and residence time resulted in higher NO removal and increased the temperature window of the nitric oxide removal.