| OCR Text |
Show increases because local oxygen partial pressure of the combustion zone rises if the excess air ratio is higher. On the other hand, flame temperature is lowered by cooling effect of excess air to bring about a lowering of N O x formation. As the result of this, the process of mixing fuel with combustion air is characterized by deviation from the stoichiometric air ratio of the N O x increase trend (rate) against excess air ratio and the excess air ratio which indicates the maximum NOx value. That is, since mixing of fuel with combustion air is comparatively good (is made to be a rapid mixing) by the conventional combustion method, local oxygen partial pressure becomes higher as diffusion velocity of oxygen to the combustion zone quickens, and consequently, N O x increase tread shows a large increase rate and is obviously influenced by the excess air ratio as showing the maximum NOJC value in the normal operating range. With a low N O x air register, however, diffusion velocity of oxygen to the combustion zone becomes slower and local oxygen partial pressure drops because mixing of fuel with combustion air is suppressed (made to be de-layed mixing) in the initial stage of combustion. Besides, since the flame temperature level comes down, N O x emission lowers and its increase rate lessens, and thus the maximum N O x value is less likely to appear. 3-4-4. Boiler efficiency Fig. 35 shows the relation between the excess air ratio and the boiler efficiency in the case of the flue and smoke tube boiler. Boiler efficiency in steam injection is the value obtained by regarding enthalpy of that injected steam as heat release. Boiler efficiency rises as excess air ratio decreases further, Symbol X O a% • • Combustion method Conventional air register Low NOx air register Low NOx air register with 20ka/H 40kg/H steam Injection • 60kg/H 1.0 1.1 1.2 Excess Air Ratio 1.3 1.4 Fig. 35, Relation between excess air ratio and boiler -efficiency (boiler A) but percentage of its rise goes up by about 0. 5 ~ 0.8% if the oxygen concentration is lowered by 1%. Boiler efficiency of the conventional combustion method is about 87.5%. This value is high, but as this low N O x air register was used, boiler efficiency rose by 1.5% because the discharge of unburnt substance could be suppressed and consequently oxygen concentration could be lowered by about 2%. Incidentally, similar results appeared in boiler B, and about 2% rise of boiler efficiency was noticed in the normal operation. Besides, as steam injection takes place, boiler efficiency worsens by nearly quotient of steam injection quantity and evaporation of boiler under the identical condition of excess 11- 20 |
