OCR Text |
Show SPECIFIC DESIGN GOALS At the outset of the project, the following specific design goals were identified as necessary in fulfilling product requirements: • The ability to obtain under 25 ppm NOx, when used in conjunction with a reasonable quantity of recirculated flue gas, on all gas fired industrial package boilers without loss of efficiency or operational difficulties. • The ability to obtain under .1 # /MMBTU NOx on all gas fired commercial package boilers without the aid of recirculated flue gas. • Restricting combustion driven rumble and vibration to an absolute minimum, in conformance with noise acceptance levels as determined by all plant engineering and operating personnel. • The ability to obtain a minimum turndown level of 10-to-1 on gas firing with single burner operation. • The ability to "fine tune" the flame shape during burner operation to suit plant, regulatory or operating requirements (i.e.-superheat temperature, CO emissions) without effecting NOx emissions. • Avoidance of sidewall and rearwall flame impingement on furnaces with SHRR's of well over 100,000 BTU/HRFf3. • A high level of register aerodynamic efficiency. • The ability to obtain low CO emission levels when operating with excess air levels of under 15%. In addition to the above named design goals, 4 it was desired to maintain a repeatability which would allow for startup times of one week or less under most circumstances. DESIGN FEATURES The OAF burner utilizes two separate combustion air zones. The primary air stream utilizes an ·isokinetic· spinner to establish a strong recirculation zone at the core of the flame, directly downstream of the burner throat. Although the isokinetic spinner is designed to yield an extremely high local swirl number (for maximized flame stability), it's relatively small diameter ensures a negligible increase in overall swirl number or flame width. Secondary combustion air enters the furnace via an annulus zone located between the spinner periphery and the inner burner throat.· An adjustable register blade louver assembly is used to adjust secondary air swirl to help achieve the optimum flame shape for the specific furnace geometry and job specifications. In addition, the OAF register assembly is equipped with a core/annulus biasing damper to further fine tune overall aerodynamic swirl and resultant flame shape. A secondary use of the register blade louver assembly is to ensure adequate combustion air turbulence at turndown conditions in excess of 10-to-1. For these applications, louver position can be automatically adjusted as a function of firing rate. To ensure balanced air distribution, the entrance to the core and annulus sections are shrouded with perforated plate, and windbox velocities are designed at conservative levels. In addition, computational fluid dynamics modeling is utilized to simulate windbox air flowsS, with modeled results used to optimize the arrangement of windbox baffles with regards to both pressure drop and air distribution. |