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Vladimir Lifshits
Two front fired power boilers, a 250 MW CE boiler with sixteen burners
and a 75 MW Foster Wheeler boiler with six burners were retrofitted with
new COEN burners developed for utility applications. The burners were
engineered to meet low air pressure drop, low excess air and low NOx requirements.
At the design stage a computational fluid dynamic models representing
a portion of ductwork, the wind boxes and burners were set up to in order
to evaluate the uniformity of air distribution between the burners. When
firing natural gas the flame in both boilers was quite different compared
with the original burners. Low flame emissivity - high transparency caused
initial difficulties with high temperature in the super heater. In the
250 MW boiler with the burners arranged in a four by four array the problem
was overcome with fuel biasing between the levels. At the same time low
excess air operation, down to 0.3% O2 in the flue gas, with practically
no carbon monoxide emissions and NOx emissions of 330 ppm, corr. to 3%
O2 dry, at high fire were achieved. That also resulted in the increased
overall efficiency of the system and power savings on the combustion air
fans. In order to achieve low excess air, opacity and NOx when firing
residual oil, a series of tests was conducted with different atomizer
cap configurations. With the final components 12% opacity achieved, while
firing with the excess oxygen level as low as 1.0%. NOx emissions at high
fire were 320 ppm. Due to the low burner air pressure drop and high fuel
pressure available, the burners could operate much higher the design capacity.
Full boiler load could be easily carried by 14 burners. This, coupled
with the ability to operate simultaneously on gas and oil, efficient individual
burner air shut off dampers, and easiness of light off, greatly increased
operational flexibility of the boiler. The 75 MW boiler had only two levels
of burners and the boiler depth was 25% more than the depth of the 250
MW unit, while the distance for mixing over the upper burner level was
much shorter. With this geometry the ability to bias gas with overall
low excess air was reduced, yet good overall performance was achieved
The excess oxygen was well below 1.0% on both fuels and NOx emissions
were 270 ppm (corr. to 3% O2 dry) on gas as well as on oil.
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