The autoignition temperature or kindling point of a substance is the lowest temperature at which it spontaneously ignites in normal atmosphere without an external source of ignition, such as a flame or spark. This temperature is required to supply the activation energy needed for combustion.
In your question you are considering only the effect of the temeprature on the reaction. If this were the only effect, you would be right. But the ignition is the contribution also of the dispersion of energy, therefore increasing the pressure, it changes the diffusion, thermal conductivity and so on. Moreover a complex chemical system include many processes, sconsidering also the activation of internal degrees of freedom, that can be affected by the pressure.
I believe the AIT should decrease with increasing pressure for hydrocarbon gases/vapours or their mixtures with the oxidant, but the decrease should become negligible for liquid HC fuels.
One reference for the pressure dependence is given below:
Caron et al., Pressure dependence of the auto-ignition temperature of methane/air mixtures March 1999 · Journal of Hazardous Materials 65(3):233-244
Thanks for your reply. Still I'm Confused. Let me put put my question in different way.
I am injecting methanol in to a chamber at high pressure (say 22 bar) and simultaneously, I am heating the chamber at the rate of 10 degree Celsius. At what temperature does methanol ignite?
Hi Saagar, the problem as you stated is similar to what happens in a diesel engine cylinder. It may be fair to assume that the methanol is injected as fine droplets and they will be fully evaporated as you heat up the chamber either by external heating duty or by compression. At atmospheric pressure, the AIT of methanol vapor in air falls in the range 433-470°C. At 22 bar, it will be lower. How much lower, I don't know, but you should be able to measure it on your test apparatus.
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