Highly efficient and economical nitrogen oxides controlled by an in-furnace urea solution pyrolysis coupled with SCR system for a coal-fired utility boiler

An in-furnace urea pyrolysis coupled with selective catalytic reduction (SCR) system was applied on a 125 MW tangentially coal-fired utility boiler in China. The coupling system was well-designed to distribute NO and NH3 more uniformly in the flue gas at the inlet of the SCR reactor. The experimental results showed that the homogeneous NO and NH3 concentrations distribution could result to better performance of the SCR reaction. A 15 wt% urea solution directly injected into the furnace at the same flow rate of 1.2 t/h in Cases 4–6, corresponded to the normalized stoichiometric ratio of 1.40, 1.47 and 1.35. The coupling system showed a high NO reduction efficiency of up to 85–89% and could control NO emission to as low as 40, 34 and 27 ppm, respectively, with NH3 slip less than 5 ppm. The calculated urea utilization efficiency was above 58% in the cases. The high utilization efficiency of the urea solution would significantly reduce the running cost of the system. There were only five injectors at the boiler's front wall, and one layer of total 90.3 m3 catalysts in the two SCR reactors, which would drastically reduce the initial investment. The effects of the urea solution flow rates were studied at 100  and 125 MW boiler loads; it was found that 1.2 t/h was the suitable flow rate of the urea solution, and the NO emission could be controlled lower than 40 ppm. The coupling system was stable, efficient and cost-efficient in industrial operation. © 2012 Curtin University of Technology and John Wiley & Sons, Ltd.