Quantitative measurements of HO2/H2O2 and intermediate species in low and intermediate temperature oxidation of dimethyl ether

Abstract

As two of the most important species that characterize hydrocarbon low temperature ignition, HO2 and H2O2 formation during dimethyl ether (DME) oxidation was quantified using the same experimental conditions, for the first time, in an atmospheric flow reactor at low and intermediate temperature range. Dual-Modulation Faraday Rotation Spectroscopy (DM-FRS) and Molecular Beam Mass Spectrometry (MBMS) were used to measure HO2 and H2O2 respectively. DME and other important intermediate species such as CH2O and CO are also measured by MBMS between 400 and 1150K at different fuel concentrations. Species profiles in the reactor were calculated by using both zero- and two-dimensional computations with different detailed kinetics for cross-validation and comparison with experimental results. The models predict adequately the low and intermediate oxidation temperature windows near 600 and 1000K, respectively. However, both models over-predicted the DME consumption as well as CO, HO2 and H2O2 formations at the low temperature oxidation window by more than a factor of four. Moreover, although the model predicted reasonably well the formation of CH2O and CO/CO2 at the intermediate temperature oxidation window, the concentration of H2O2 was also over-predicted, suggesting the large uncertainties existing in the DME low temperature chemistry and in H2O2 chemistry at intermediate temperature. Furthermore, to analyze the uncertainty of the low temperature chemistry, a branching ratio of QOOH decomposition to CH2O was derived using measured DME, CH2O and CO concentrations. The large difference between the modeled and measured branching ratios of QOOH decomposition suggests an underestimated QOOH decomposition rate to form CH2O in the current DME models.