| OCR Text |
Show Both the original and the retrofitted combustion system utilized a common windbox enclosing four burners, all at an equal elevation of approximately five feet above the boiler operating floor. All burners are designed to fire 7.5 GPM of No.6 fuel oil, with a specified 6.67-t0-1 turndown. Furnace pressure is specified at 4.85" w.c. at capacity and elevation, with maximum windbox pressure of 5.8" w.c. above furnace pressure. For compliance with EPA New Source Performance Standards, NOx emissions were specified at .3 Ib/MMBTU (HHV) based on firing #6 fuel oil with .35 percent bound Nitrogen by weight. In addition. total combustibles were limited to 100 PPMv maximum. based on excess oxygen levels of 1.2% at 40 to 100 percent of MCR, 2.0% at 25 to 40 percent of MCR. and 4.0% at 20 to 25 percent of MCR. Opacity was limited to less than 10 percent for all cases. SUMMARY Using inhouse correlations, base thermal and fuel NOx predictions were estimated as a function of adiabatic flame temperature. furnace heat release rates, and fuel bound nitrogen level. Staged air stoichiometry required to achieve specified NOx levels were then determined. Although there was little c~oncern in regards to meeting the specified NOx level of .3Ib/MMBTU. achieving a combustibles level of under 100 PPM at 7 percent excess air under staged conditions was another matter. A further stipulation imposed on the design of the combustion system was the decision to limit tertiary air entry to floor ports: The addition of front wall or overfire air ports was ruled out due to a reluctance to bend the closely spaced front wall tubes, while the single row-four burner windbox ruled out the use of side wall ports. Optimization of fuel burnout under the above stated conditions required a detailed investigation of the NOx port and burner design parameters. To study the effects of tertiary air ve-locity, port locations, and orientation of oil spray on fuel/air mixing and burnout. a commercially available computational fluid dynamics software code, Fluent, was utilized. A three dimensional base case was developed, which enabled initial discernment of gas flows and burnout patterns. Sensitivity cases, discussed later in this report, examined the relative effects of design variables on system performance. COMBUSTION MODIFICATIONS The existing burners were replaced with four Coen standard design, Dual Air Zone (OAZ) register burners. Each burner is rated at 66 MKB,and is deSigned for a combustion air differential pressure of 5.8 inches wc at design load. Burner throat size is 25 inches, with 15 inch deep refractory. To accommodate the larger OAZ registers, the front end of the existing windbox was removed and a windbox extension was welded in place. No modifications were made to the existing inlet and supply ducting. Figure 2: OAZ REGISTER ,.,......-.m • ...., ~T ... _S~ceFIt_ ....,.-~ |
