L6. OXIDATION CHEMISTRY OF DIMETHYL ETHER  RELATED TO ITS PROPERTIES AS A DIESEL FUEL

E. W. Kaiser and T. J. Wallington

Ford Motor Company

Chemistry Department

Mail Drop 3083/SRL

Dearborn, MI 48121-2053

Recently, there has been substantial interest in the use of dimethyl ether (DME) as a potential diesel fuel in compression-ignition engines. This interest arises primarily because dimethyl ether has a very high cetane number (>55) and inherently low exhaust particulate emissions. In contrast, other ethers [e. g. methyl tertiary butyl ether (MTBE) and tert-amyl methyl ether (TAME)] are used as octane enhancers in gasoline. A high cetane number indicates that the fuel is very susceptible to compression ignition and suitable for use in diesel engines. A low cetane (and, therefore, high octane) fuel is resistant to compression ignition in an engine and is used to suppress knock in spark-ignition engines. Based on these observations, understanding the chemical mechanism which causes DME to ignite easily during compression heating in an engine is of substantial practical interest.

Experiments investigating the atmospheric chemistry of DME have identified an unexpected reaction path for methoxymethyl radicals (CH₃OCH₂) during the oxidation of DME at low pressure. For pressures above 100 Torr, the primary product is the methoxymethylperoxy radical

CH₃OCH₂ + O₂ + M = CH₃OCH₂O₂ + M (1a).

For pressures below 5 Torr, the reaction forms primarily formaldehyde and OH radicals because the excited peroxy intermediate is not collisionally stabilized quickly enough to prevent rearrangement

CH₃OCH₂ + O₂ = [CH₃OCH₂O₂]^* = 2CH₂O + OH (1b).

Such a reaction channel is not expected to occur for MTBE or TAME. We have postulated that reaction (1b) may be the cause of the efficient compression ignition of DME, because it provides the very reactive OH radical as a chain carrier during the period that the chain branching intermediates are formed. This would speed up the overall reaction. However, this process is efficient only for low pressures at ambient temperature. Compression ignition takes place at pressures of several atmospheres. Thus, establishing that this process becomes favored at higher pressures when the temperature is increased is important in verifying this possible ignition mechanism. Such an enhancement at higher pressure upon increasing the temperature has been observed in the reaction C₂H₅ + O₂ = [C₂H₅O₂]^* = C₂H₄ + HO₂, which involves the rearrangement and decomposition of an excited peroxy intermediate as in the case of reaction (1b).

To increase understanding of the combustion chemistry of DME at higher temperature, we have undertaken a study of the laminar flame chemistry of DME at atmospheric pressure using a McKenna flat-flame burner. The composition of both rich and lean flames have been examined using sampling by an uncooled quartz probe and analyzing the gas by both FTIR spectroscopy and gas chromatography. Profiles of DME, O₂, CO, CO₂, CH₄, C₂H₆, C₂H₄, and CH₂O have been measured as a function of height above the burner. These species profiles will be presented along with computer simulations obtained using the HCT flame code developed at Lawrence Livermore National Laboratory.