OCR Text |
Show 1 ( 1. Introduction It is preferred r.o reduce NOx emissions from combustion sources by combustion modification techniques as far as such techniques are applicable. To attain this target, various low NOx burners, which use such methods as flue gas self-recirculation, two stage air combustion, biased combustion and steam or water injection, etc., have been developed and/or in practical use with considerable results. Although these burners have different grades of effectiveness, it seems that their effects are not remarkable because of such problems as soot formation, increase in loss of power and heat, etc.. At Hitachi Zosen, it has been thought that the large amount of control of NOx emission should be attained by controlling not only the formation of thermal NO but also of prompt NO which will be formed rapidly in the fuel-rich hydrocarbon flames. Thus Hitachi Zosen have develped a new low NOx combustion method (that is,'two-stage fuel system) to meet the above-merit i oned requirements and put it into practical use. This paper presents an outline of the method. c 2. Formation and Control ut NOx : As is well known, the formation mechanism of NO with combustion can be classified into the following. "Thermal NO" is formed by reaction between nitrogen and oxygen in air under the high temperature condition due to combustion. "Prompt NO" is formed by reaction between nitrogen and HC-radicals, etc. in the fuel-rich hydrocarbon flames. "Fuel NO" is formed by oxidation of nitrogen compounds in the fuel. The controlling methods of NO to be formed by above reactions are generally as follows : (1) Thermal NO : Those are effective to lower combustion temperature, reduce oxygen concentration and shorten residence time in high temperature zone. In view of a large temperature-dependance, it is most effective to lower combustion temperature. 1 0 -1 |