OCR Text |
Show HNCO + M = NH + CO + M HNCO + H = NH2 + CO HNCO + NH = NH2 + NCO HNCO + NH2 = NH3 + NCO HNCO + OH = NC + H60 HNCO + ° = NH + C 2 Siebers and Caton [4-6] have provided additional information about the temperature and concentration sensitivity of the process. One of their papers [6] dealt with a comparative study of NO removal by cyanuric acid and ammonia injection. Experiments were carried out in a heated quartz flow reactor, using a simulated Diesel en9ine exhaust mixture. Relative to NO, molar ratios of cyanur1c acid and ammonia were held at 1.4 and 2.3, respectively. Maximum NO reductions were greater than 90% for either process. NO reduction by thermal De-NOx was shown to be most effective at low oxygen concentrations and in the absence of CO and H20. Contrary to the thermal De-NOx results, lack of these species reduced the RAPRENOx efficiency from 95% at about 1200 K to 40% at 1350 K. Another favorable aspect of cyanuric acid injection was the wider temperature window between the onset of NO reduction and the point of highest removal efficiency~ But, the experimental studies of Siebers and Caton and the numerical modeling of Miller and Bowman suggest that under certain conditions, NO may convert to N20 rather than N2 by reacting with NCO. Wicke et ale [7] have argued that NO could be reduced by cyanuric acid, only in oxygen free exhaust streams. Their work also shows that surface reactions are critical to NO reduction. While prevaporized form of cyanuric acid was used in all the aforementioned bench-scale studies, solid phase injection was chosen in pilot-scale investigations (8-10]. Chen and coworkers [8,9] demonstrated hi9h NO reduct10ns when cyanuric acid treatment was combined w1th staged combustion in their 15 kW tunnel furnace. Greater reductions were also obtained at higher initial NO levels [8]. In a related study involving a 250,000 BTUjhr test facility, Perry [10] showed that effective NOx reduction with cyanuric acid powder occurs at about 300 K above its gaseous injection temperature. He also reported no effect by S02 on the NOx removing efficiency. This paper examines the performance of cyanuric acid as a NOx reducing agent in an industrial-scale boiler. More specifically, effects of excess oxygen concentration, injection temperature, and particle size distribution are investigated. One of the unique features in this investigation is that typical heating loads (-20 MBTUjhr) are about 2 orders of magnitude higher than any previously reported pilot-scale testing. -2- |