Low-temperature Oxidation Mechanism Of Methanol In Active Thermo-atmosphere

The energy crisis and environment problem caused by the overuse of petroleum fuel have become a restriction to human's development. It's urgently to seek new technologies to reduce the energy consuming and emissions. Controlled Auto-Ignition (CAI) Combustion technology is considered as a high efficiency and low emissions combustion process. However, it's more difficult to control CAI ignition timing and combustion process compare with traditional spark and compress ignition engines. The main reason is that the CAI combustion is more affected by chemical kinetics. Exhaust gas recirculation have significant effects on the temperature scale of the gas blend and reactive substances concentration in the gas mixture in the compress progress, and it proved an effective technology to control the HCCI combustion indirectly. Therefore, it's essential to study how the reactive substances and fraction concentration in the gas mixture change during the combustion for explore the HCCI combustion strategy.In this paper, high speed sampling techniques inside cylinder and DNPH -derivatizing method combine with High Performance Liquid Chromatography technology were used to study the aldehydes and ketones concentration changing process via experiments focused on low temperature methanol and gasoline combustion. Testing results show that there are several kinds of aldehydes and ketones in the hot exhaust of methanol and gasoline combustion. The formaldehyde and acetaldehyde content is more than else, and due to the formaldehyde formation in the methanol combustion is different from gasoline, the formaldehyde in methanol combustion is more than gasoline. During the compression stroke, the formaldehyde concentration is decreased at first and then increased, it is believed that formaldehyde participates in the oxidation reaction. As the temperature and pressure of mixture in cylinder increased, plenty of fuel participates in the oxidation process result in regeneration of formaldehyde. Under the same conditions, the formaldehyde was detected in the compress procession of methanol more than fueled by gasoline, and provide more reactive substance in the pre-compression period of methanol than that in gasoline, and the aid-combustion effect is more obviously, that's one of the reason methanol'ignition time is more early than gasoline.Secondly, the chemical kinetic software package CHEMKIN 4.1 is used to study and analysis the existing methanol kinetic mechanism. The results show that the existing methanol kinetic mechanism can hardly depict the methanol concentration change in low temperature. In this paper, the rate constants of some key reaction in low temperature is adjusted, and the results show that the formaldehyde concentration experience at low temperature and the ignition time of optimized kinetic model is more satisfied with the experiment results.Finally, the optimized kinetic model has been applied to simulate methanol oxidization in order to study the effects of key substances in low-temperature oxidation. The results show that when the initial formaldehyde concentration changed from 0 ppm to 2 ppm, the ignition time advanced nearly 3°CA, however, when initial formaldehyde concentration changed from 2 ppm to 20 ppm the ignition time advanced 1°CA only. It illustrated that low concentration of formaldehyde has a significant effect on the low-temperature oxidation and the ignition time of methanol. The aid-oxidation effect of formaldehyde at low temperature oxidation process can be summarized as: 1mol formaldehyde consumption will lead to 4mol O₂ consumption and also can supply 2mol H₂O₂, HO₂, and CH₂O to the chamber. And these substances will involve in the low-temperature oxidation of fuel then generate double and redouble of the reactive substances, and that will accelerate the chain-reactions. When HO₂ participate the chain reaction, the initial chain-reactions rate of CH₃OH increase and the depletion of CH₃OH begin earlier, consequently, the start time of the accumulation of reactive substances advance, and the chemical reactant in chamber is more active, then, speed up the low-temperature oxidation of the fuel.