OCR Text |
Show These nitrogen compounds can then react further to form NOx. This mechanism requires the presence of carbon-containing radicals which are not present when burning hydrogen. Thus, there would not be any NOx formed via this mechanism by the combustion of hydrogen. When firing natural gas the NOx emissions are a combination of thermal and prompt NOx. When some hydrogen is mixed with natural gas the NOx emissions increase due to the higher thermal NOx generated from the increased flame temperature that results from the added hydrogen. However, as the hydrogen content increases, the hydrocarbon content of the fuel is decreasing. This decrease in hydrocarbon concentration in the flame will eventually influence the prompt NOx formation rates, since the formation rate of prompt NOx is sensitive to the concentrations of C .and CH radicals. If thermal NOx emissions are controlled to relatively low levels and if the prompt NOx levels are. a significant enough portion of the overall NOx emissions, there will be a decline in total NOx when the hydrocarbon concentrations in the fuel become low enough. Constant Flame Temperature Test In order to test this prompt NOx hypothesis, a test was arranged to vary the hydrocarbon content of the fuel while maintaining a constant adiabatic flame temperature for each fuel mixture. Maintaining a constant adiabatic flame temperature should maintain a constant rate of thermal NOx, while varying the hydrocarbon content of the fuel gas should affect prompt NOx formation. This test was accomplished by adding varying amounts of hydrogen and nitrogen to natural gas. The hydrogen/nitrogen mixture that was added had the same adiabatic flame temperature as the natural gas. Thus, as more of this mixture was added to the fuel gas, the hydrocarbon content of the fuel declined, but the flame temperature remained constant. The constant flame temperature test was run on an AMR burner. The results are plotted in Figure 12. This is a plot of the NOx emissions for each mixture relative to the 13 |