OCR Text |
Show level, a region of internal recirculation is generated on and around the flow axis local to the burner. Therefore, swirl is a very effective way. of controlling the mixing rate between: - the coal laden primary air stream, - the secondary combustion air stream, - the combustion products. A range of well defined mixing patterns relevant to pulverised coal burner design can be achieved using swirling combustion air in association with optimised burner geometric and kinematic parameters such as quarl confinement, injector blockage ratio and gas velocity and momentum ratios. The various characteristic mixing patterns which result in the IFRF flame classification system are shown in Figure 3. Each of these basic flame forms would be expected to produce significantly different NOx emissions levels when firing the same coal. Of the flame types illustrated in Figure 3, the initial experiments concentrated on Type 2 flames combined with external air staging [22, 23]. These initial studies generated qualitative information for externally staged flames on the effect of burner design on aerodynamics and mixing and, subsequently, on NOx formation. These burner types have been applied in industry as first-generation low-NOx burners, see Figure 4. However, a quantitative assessment of mixing rates between fuel, air and recirculated gases in the IRZ was only partially realised. Consequently, a research programme was initiated to establish quantitative characteristics of the mixing processes in swirled flow systems relevant to burner designs [19-21]. In these fundamental studies, a wide range of geometric and kinematic arrangements were studied under isothermal conditions. The flow field in the near burner zone was defined using a variety of analytical techniques including smoke wires, hot wire anemometry and laser doppler anemometry. Because of this fundamental experimental programme, considerable insight into the mixing phenomena and methods of modifying the mixing process were gained which enabled the design of internally air staged burners. This paper describes the design and evaluation of two internally air staged burners [24, 25]. 3.1 Burner A - Aerodynamically Air Staged Burner A schematic of the Aerodynamically Air Staged Burner (AASB) is shown in Figures 5 and 6. The basis for the burner design was the exploitation of the chemical environment that exists within the IRZ of this swirl type burner. The IRZ plays important role in the combustion 5 |