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Show TYPE-0 TYPE-1 external c^ C recirculation i j T > p internal c~ redrcubllon TYPE-2 1 \ \ exlemal ^recirculation TYPE-3 4 •^TrS2^- ^--^>- ~"5? - C C" Memo! recirculation FIGURE 1: F L A M E CLASSIFICATION S Y S T EM secure good quality predictions of different flame types. To this end. the flame computations are compared with the measured data generated in a number of experiments. The data include measurements in swirling flames of thermal input in the range 0.9- 3.4 M W (Smart et al., 1989; Dugue* et al., 1991). G E N E R A L CLASSIFICATION O F S W I R U N G F L A M ES A range of well-defined flame types relevant to pulverized coal burner design can be identified. Figure 1 shows the various characteristic mixing patterns that result in the flame classification system (sec Smart and Weber, 1989). Each of these basic flame types produces significantly different N O x emission levels when firing the same coal. A typc-0 flame, produced without or with low inlet swirl, is a long jet-flame stabilized at the fuel injector or downstream. It is used in corner-fired boilers and cement-kilns. When a single, 2 M W type-0 flame of a highly volatile (34% daf) Scotts Brench coal containing 1.63% N (daf) is fired in the IFRF furnace, typical N O x emissions are in the range 470-520 ppm (0% O2) with burnout of circa 9 8 % (Weber et al.. 1987). When a sufficiently high degree of swirl is imparted on the combustion air. an internal recirculation zone (IRZ) is formed. The formation of such a flow pattern in the vicinity of the fuel injector results in stable combustion and rapid heat release. The pulverized fuel is rapidly entrained into swirling combustion air resulting in a short, intense, type-2 flame which ignites in the close vicinity of the coal injector. Coal dcvolatilization takes place on the IRZ boundary, in an oxygen-rich zone, and the N O x emissions for Scotts Brench coal are high, being in the range 900-1000 ppm (0% 02) with char burnout above 99.6% (Weber et al., 1987). Typc-2 flames arc encountered in wall-fired boilers equipped with conventional (high N O x ) burners. A typc-1 flame is a combination of typc-2 and type-0 flame; the fuel jet penetrates either partially or fully through the internal recirculation zone. By promoting the coal devolatilization inside the oxygen depleted IRZ, a substantial reduction of nitrogen oxides emission can be achieved. A typical 2 M W , type-1 flame of Scotts Brench coal produces 200-400 ppm N O x (0% 0 2) with char burnout above 99.5% (Weber el al., 1987; Smart and Weber, 1987). A number of industrial low-NOx burners designed to coal and or secondary V igniter ^ final mixing primary air • coal -V-a-e © IRZ boundary FIGURE 2: THE AERODYNAMICALLY AIR STAGED BURNER (TOP) AND PARTICLE TRAJECTORIES IN THE NEAR BURNER ZONE (BOTTOM) retrofit existing boilers utilize the principle of internal air staging to produce type-1 flames. Type-3 flames are intense, swirl stabilized with two strong internal recirculation zones; one is located inside the burner quarl while the second zone is formed downstream in the furnace. The NOx emissions of such flames are high, being typically in the range 600-840 ppm at 0 % O 2 (Weber et al., 1987). These flames are produced with a very high inlet swirl and a high furnace confinement and therefore rarely appear in industrial situations. FLUID FLOW IN THE NEAR BURNER ZONE Measurements in industrial swirling flames of high- and medium-volatile bituminous coals show that N O x production and destruction take place in a region close to the burner. Char burnout as high as 8 0 % and N O x concentrations representative of the flue gas concentrations levels are typically measured within two quarl (tile) diameters downstream of the burner outlet In our previous publications and more recently in a study of Abbas et al. (1992), the paramount influence of quarl zone aerodynamics on emission of nitrogen oxides was demonstrated. It is essential that computations of near-field aerodynamics of swirl burners provide a good knowledge of not only the size and shape of the IRZ but also the velocity profiles. These are required to describe the interaction between non-swirling particle-laden jet stream, the swirling combustion-air, and the IRZ. The flow-field resulting from such a complex interaction determines the trajectories of coal particles. 11-11 |