OCR Text |
Show convertion into fuel NOx) is affected by the concentration of 02 (i.e. air ratio). Though the influence of 02 concentration over the generation of fuel NOx is quite significant up to 3% of O2 concentration, the rate of conversion does not change too much even if the concentration of 02 is increased further staying at about 30 to 40%. It is regarded, therefore, that fuel NOx can be reduced substantially by controlling 02 concentration. Generally, combustion of boiler is made in the oxidized ambient of excessive air and when the air ratio is lowered, the generation of thermal NOx and fuel NOx can be reduced. If the air ratio is lowered too much, however, incomplete combustion is resulted causing the trouble of increased soot, carbon monoxide, unburned hydrocarbon and others. An example of the relation between 02 concentration in exhaust gas and NOx and soot concentration is as shown in Fig-9. NOx concentration reaches the peak at 4 to 5% of 02 concentration (air ration 1.2 to 1.3) and decreases as 02 concentration goes down. In addition, the rate of soot increase is high in the area of higher rate of reduction of NOx concentration, which indicates that 02 concentration exerts a close influence over the soot and NOx concentration but in reverse direction. This means that close care shall be paid to NOx reduction by low air ratio operation for heavy oil which apt to produce much soot and that NOx reduction is easier for the gas of good combustion quality. It shall also be noted that NOx reduction by low air ratio operation brings about the 8. - 13 - Standard ejection -* v»lv« (prtiiurc «pr«yli Standard ejection valvi (ateaa spraylsg) Standard ejection val* (preaeure •praying) Standard ejection vslv (ateaa spraying) 1 2 3 4 5 6 0j concentration(Z) (Note) taark indicates Bacharach smoke number. Fig .9 Relationship between 02 concentration sad DOi.'soot concentration (froa "Suppression of NOx generation" and "Thermal Control and Pollution Vol. 25, No. 5" by Sholehi Tsujl) |