Classification of Technology:
Air Pollution Control Type of Industry:
Petroleum Refining Subject:
Reduction of NOx in Boiler Exhaust Gas
1. Purpose
To reduce the concentration of nitrogen oxides ( NOx ) in boiler exhaust gas from 240ppm to 170ppm in accordance with the discharge regulation of the Air Pollution Control Law.
2. Applied Technology and the Time
Among the techniques used for improving combustion conditions in furnace, the exhaust gas recirculating equipment for circulating the exhaust gas was adopted to control NOx discharge. ( June, 1974 )
3. Principle
In the exhaust gas recirculating method, there are two methods in which a part of the combustion exhaust gas is recirculated consisting of 1) mixing directly in the combustion zone and 2) mixing in the combustion air. We are using method 2). The oxygen concentration in the air diluted with exhaust gas is low and it causes a drop in maximum flame temperature which, in turn, delays the reaction of fuel and oxygen or, in other words, delays the combustion and lowers the maximum temperature of the flame. Generation of NOx is therefore controlled by thus created uniform temperature distribution of the flame.
This equipment was designed to enable the use of recirculated air up to 30% of the combustion air in maximum load of the boiler.
Process flow is shown on the next page.
4. Effect and Achievement
NOx Concentration ( ppm ) of Exhaust Gas
Denigration Percentage ( % )
During Normal Combustion
140
-
During Recirculation of Exhaust Gas
(Amount of Recirculation 15% )
100
28.6
5. What to do next ?
Since the amount of recirculation of the exhaust gas will also be determined by the generation of dust and by the stability of combustion, in addition to the structure of the combustion chamber, it will be necessary to obtain the optimum amount of the recirculation to control NOx.
