OCR Text |
Show 5 In Fig. 12, NOx emission at the firing with fresh air is s hown. With fresh air, it can be operated stably unti l 4% (dry) of remaining 0 2 ' The NOx emission is less than 70 ppm (02 = 0%) . 4-2 Flame Temperature and NOx Emission Fig. 13, shows relationship between NOx emission and theoretical flame temperature at firing rate of 2,800 kW when the condition of exhaust gas is changed. The remaining 0 2 is uncharged, approx. 3% (dry ) . Theoretical flame temperature and NOx emission have strong correlation of 20 ppm/100oe, and it is thought that NOx emission in case of the same remaining 0 2 is influenced by the flame temperature. For decreasing NOx, we thought that the decrease of flame temperature is effective. At 500 0 e of the exhaust gas and 14% 0 2' the flame temperature is 1,750 o e and NOx emission is 25 ppm (02 = 0%). As the firing is unstable below 1,750 oe of flame temperature, it is thought that limit of NOx emission is 25 ppm (02 = 0%) . 4-3 Method for Lower NOx Level Although we have achieved our development objective of which NOx emission is less than 50 ppm (02 = 0%) at most likely exhaust gas condition we examine further reduction of NOx by steam inj ection and two-staged combustion. 4-3-1 Steam Injection Fig. 14 and 15 show effect of steam injection on NOx emission. Steam injection quantity is shown as ratio of steam quantity = W (kg ) to the fuel gas weight of burner = Fb (kg); W/Fb . Fig. 14 shows the effect of steam injection into the burner without steam injection in gas turbine exhaust (W/Ft = 0) and Fig. 15 shows the same effect with steam injection in gas turbine exhaust (W/F t = 0.8). H20 content ratio of W/F t = 0.8 is equal to steam injection of W/Fb = 0.43 at burner load 100%. We can expect 20% of NOx reduction ratio with steam injection ratio W/Fb = 0.5 (kg/kg ) regardless of H20 content in the exhaust gas (W/Ft = 0.8) . 4-3-2 Two-Staged Combustion Fig. 16 shows the relation between the secondary air ratio to total amount of exhaust gas and the NOx reduction ratio at two-staged combustion. The secondary air is supplied to the place of 4.0 rn from the burner tile surface through the rear part of the test furnace. The burner firing rate is set for 2,800 kW, and the residual 0 2 is set for 2.0 to 2.5% (wet ) . The two-staged combustion is effective to reduce NOx emission. When the secondary air ratio is 15 to 20%, NOx emission is reduced by 32 to 42% as compared without secondary air. As is evident from the above-mentioned facts, it is supposed that the NOx emission can be reduced to 40 or 30 ppm or lower at 500°C of exhaust gas and 15% of 0 2 by stearn injection W/Fb = 0.5 or by using the two-staged combustion (ratio of second stage exhaust gas is 20 %) . |