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

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 1150 K 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 1000 K, 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.