| OCR Text |
Show and air flow velocity are to be attained so that there is a considerable reduction of N O x emission becomes remarkable as mentioned later. 3-4. Experimental results and discussion 3-4-1. Relation between furnace internal mixing characteristics and N O x emission Fig. 31 shows the relation between the furnace internal mixing characteristics and the N O x emission obtained by changing the position of the diffuser and burner. As the diffuser position is moved as indicated by arrow a, a share of the secondary air quantity which becomes the outer swirl flow increases because the outer circumferential wall of the swirl vane offers a conical form, and moreover, the mean axial air flow velocity drops. In this case, diameter of the vena contracta area enlarges and swirl expand starting position draws near the burner side because combustion air flow changes from solid line A (O) to broken line A (a). Consequently, the effect of the outer swirl flow becomes remarkable, fuel becomes atomized to the inner side of the outer swirl flow, and thus N O x emission decreases gradually. Besides, as the burner position is moved toward the furnace inner side as shown by arrow b, fuel becomes atomized in large quantities to the inner side of the outer swirl flow because fuel changes in position from fine line F (O) to broken line F (b). Consequently, multi-stage combustion takes place to suppress formation of NOx £ 14}. With adjus tment of only one of the diffuser positions and burner position, N O x formation decreases by about 25% or 40% at maximum respectively, and 150 Fig. 32. Effect of swirl number on N O x emission as fuel is atomized with the proper burner position into the combustion air flow with the proper swirl intensity and flow velocity as mentioned later, the effect based on principle of reducing N O x emission of the low N O x air register becomes remarkable, and the percentage of N O x reduction is as much as 60%. 3-4-2. Effect of swirl intensity on N O x emission Figc 32 shows the effect of the swirl number on N O x emission. Because swirl intensity increases as swirl number becomes large, an effect of the outer swirl flow is produced to suppress N O x formation. In boiler A in particular, its N O x reducing effect becomes remarkable because the burner position has been also adjusted. However, when swirl number becomes greater than about 1.0, the rich fuel region expands extremely although it depends on boiler type, and there might be a case of inducing a low-frequency pulsation combustion for which attention' must be paid. 11- 18 - |
