| OCR Text |
Show - 10- NOx formation from this mechanism increases exponentially with peak flame temperatures and most low NOx burners are designed to minimize these peak temperatures as much as practical. Hydrogen has a higher adiabatic flame temperature than natural gas and thus has a higher potential for thermal NOx production. Prompt NOx is formed via several mechanisms, one of which depends on hydrocarbon radicals: CH + N2 -+ HCN + N C + N2 -+ CN + N 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 hydrogen is initially added to the 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. Based on these tests, it appears that the reduction in prompt NOx |
