October 15, 2008

Computer simulation as a NOx reduction design tool

Dr. Gui-su Liu and Dr. Brian S. Higgins

A utility evaluated various methods of obtaining a NO_x reduction of at least 30%, as required by upcoming regulations for its boiler, which originally produced 0.54 lb of NO_x/million Btu at 410 MW full load. Nalco Mobotec engineers performed a computational fluid dynamics (CFD) simulation of the boiler to first understand the boiler’s combustion process and then determine the most economical method to achieve the required NO_x reduction.

Nalco Mobotec’s Rotamix selective noncatalytic reduction (SNCR) system injects urea into the furnace, using a boosted air delivery system to transport urea deep into the furnace flue gas cross-flow. The key to an economical installation is to minimize the number of injection ports while obtaining a good concentration of urea in the areas where NO_x is concentrated.

The Riley rear-wall-fired boiler was originally designed to burn oil, so when it was converted to fire eastern bituminous coal, there was insufficient residence time for coal combustion. The unit routinely has upper furnace post-combustion, superheater tube temperature alarms, high CO, and overall poor combustion. In the past, 28 Babcock & Wilcox low-NO_x burners were installed on the rear wall with seven two-chamber overfire air (OFA) ports on the front wall. Due to poor mixing between the OFA and the flue gas, deep staging for further NO_x reduction has been limited by poor post-combustion, leading to high upper furnace temperatures.

Nalco Mobotec engineers began their analysis with a simulation of the existing boiler operation to develop a better understanding of its operation. Initially, the original boiler was simulated for a better understanding of its operation—the first step in developing an optimal NO_x reduction solution. The engineers selected FLUENT software, a CFD package from ANSYS Inc., because it is capable of accurately modeling thermal NO_x, prompt-NO_x, fuel-NO_x formation, as well as SNCR NO_x reduction.