Oxides of nitrogen formed in high-intensity methanol combustion

Abstract

The oxidation of methanol in a jet-stirred reactor has been studied to characterize the mechanisms of methanol combustion and nitric oxide formation in a high-intensity, turbulent environment. Fuel/air equivalence ratios of 0.7 to 1.4 were investigated at a constant temperature of 1870 K and mean reactor residence times of ca. 2.5 msec. Total NO x was measured along with major cornbustion products, CO, CO 2 , H 2 O, O 2 , CH 4 , and C 2 H 2 . Emission spectroscopy was used to determine oxygen atom concentration as well as relative levels of electronically excited radical species, OH • , CH • , and C 2 • . Comparative measurements were repeated with methane as the fuel under similar reactor conditions. Measured NO x for methanol was 8.5 ppm maximum at equivalence ratio 1.10. The ratio of methanol-NO x to methane-NO x varied between one-half for fuel-lean combustion to one-third for equivalence ratios at or above stoichiometric. Molecular O 2 , atomic-O and calculated OH were essentially the same for both fuels. At fuel-rich conditions, methanol demonstrated higher carbon monoxide. Concentrations of CH 4 and C 2 H 2 were markedly higher with methane fuel, as were CH • and C 2 • while OH • was slightly higher. Evidence indicated the persistence of the C−O bond in the methanol molecule during oxidation. The NO x , O-atom, and O 2 concentrations indicated that the extended Zeldovich mechanism was insufficient to explain the difference in observed NO x for methanol and methane. It was hypothesized that hydrocarbon fragments played an important role in NO x formation in the stoichiometric and rich mixtures; it was concluded that low methanol-NO x was due to low hydrocarbon levels.