OCR Text |
Show -8- The variation in NOx emissions with hydrogen content in the fuel mixture at a constant excess oxygen level shows some further unusual behavior that may provide an insight into the differences between the mechanisms responsible for the NOx emission from these two fuels. Figure 5 shows NOx data as the fraction of hydrogen is increased in a mixture with natural gas. The data were collected while firing with 2% excess oxygen and an air preheat temperature of 350 of. The NOx emissions first increase as the hydrogen content increases, however, the emissions eventually peak when the mixture contains about 70% hydrogen. At this point the NOx level is about 50% above the level emitted when firing natural gas. The emissions then decrease for higher hydrogen fractions in the mixture. At 100 mole percent hydrogen the NOx emissions, expressed as PPM (dry basis), are only slightly higher than the levels seen with 100 mole percent natural gas. -c 30 Q,) ~25 >< o ~ 20 M ot: 15 o ;; 10 0- n:. 5 o>< z 0 +---~---+--~----~--+---~---+--~----~~ o 20 40 60 80 100 Hydrogen, mole % (Balance Nat. Gas) Figure 5 NOx Variation with Hydrogen Content of Fuel Gas Since the burning of hydrogen results in fewer dry products of combustion, expressing the NOx emissions in terms of PPM on a dry basis does not truly represent the actual net emission of NOx. A better comparison would be in terms of pounds of NOx per |