STUDIES ON SCALE-UP OF SWIRL-STABILIZED PULVERIZED-COAL BURNERS IN THE THERMAL INPUT RANGE 2.5-12 MW

Over the years, the effects of scale-up on the performance of pulveris ed-coal burners has been the subject of much controversy. This is due in part to the fact that not all physical and thermo-chemical processe s scale in the same way when the scale of the burner is changed, even if the geometric similarity of burners and the momentum ratios between fuel and air streams are maintained constant. The problem is made eve n more complex by differences in flame surface-to-volume ratios; by sc ale effects on the turbulent mixing process; and by differences in two -phase interactions between coal particles and the main flow field. Th e latter has an influence on particle distribution within the flame, a nd consequently it affects the location and environment of devolatilis ation; it also affects ignition behaviour, char burnout within the vol atile flame envelope, and consequently the entire thermochemical and a erodynamic structure of the flame. For reasons of economics or ease of in-flame diagnostics and flexibility of operation, burner developers tend to evaluate smaller-scale versions of the burners than will ultim ately be applied. This paper describes the results of a series of expe riments where a 12 MW(t) large-scale burner is compared with two 2.5 M W(t) geometrically similar burners designed around a constant-velocity and constant-residence-time scaling criterion. High- and low-NOx flam es were produced from burners and were extensively probed to determine in-flame thermo-chemical structures. Results show that complex differ ences exist between flames. The results detailed in the text are inter preted with reference to the effect of scale on turbulent mixing and t wo-phase interactions; they confirm that, regardless of the scaling cr iterion selected, similarity of the in-flame thermochemical features i s almost impossible to maintain as burner scale is changed.