Like title says, what properties of these make them so different in flammability? Just watched a video where a little gas added to fire quickly makes it seem to explode, but something like diesel just burns. I also think of the explosion in Lebanon, it was burning and then suddenly it was like a mega bomb went off.
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This is all about the rate of reaction, in this case, oxidation. A very slow oxidation process is like rusting. A faster oxidation is like burning, how you described diesel fuel. Then you have rapid oxidation which can seem nearly instantaneous to the human eye. We call that an explosion. So, to answer your question, it all has to do with the rate the reaction proceeds, and that can be dictated not only by the chemicals you mentioned, but the available oxygen, the ratios of each, the temperatures and pressures. There can be a lot of variables and that’s why some situations that seem identical may actually have very different results.
It’s a combination of things, but in this specific example, most of the difference is due to different vapor pressure.
Vapor pressure is a measure of how much of a substance is present in the air above a liquid. In this case gasoline has a much higher vapor pressure than diesel, so you’ve got more fuel up in the air to burn/explode.
Gasoline and diesel are literally distilled from the same source (crude oil). Gasoline is the lower boiling stuff, diesel a little higher – so gas is easier to evaporate, hence the higher vapor pressure.
Gasoline is more refined fuel. Made to be highly flamable.
Diesel less refined and thicker and burns slowly.
The major difference between diesel and gasoline that makes for the results you saw is that gasoline is much more volatile than diesel, and so the gasoline fumes are readily produced, mix with ambient air and can rapidly burn. This is the key with gasoline, it needs an external source of oxygen to burn, and it needs to be mixed into that air at a specific range of ratios to burn effectively and quickly. In a car this is usually achieved by spraying a mix of gasoline into a combustion chamber full of compressed air, but if you dump gas onto a fire it will do much the same thing.
Diesel is more like oil, its molecules are more tightly associated and it doesn’t become a vapor as readily as gasoline. When you have a vapor phase of a fuel mixed with an oxidizer (in this case oxygen in the air) this paves the way for something called deflagration, which is to put it simply subsonic burning. This is in contrast to what you saw in Lebanon, which was the transition from a standard fire into a detonation.
Detonation’s key distinction from deflagration is that the combustion in a detonation is supersonic, generally as a result of the compound in question already being well mixed with or containing its own oxidizer. In Lebanon there was a large storehouse full of an oxidizer, and when that was sufficiently heated and mixed with fuel by the fire, the whole mass detonated in a single enormous reaction.
It’s also possible to use a fuel that would normally deflagrate, pressurize it in a strong vessel, and then uses the pressure built up to create a detonation; BLEVE’s a great example of this. BLEVE’s are also a great way to understand the need for a fuel to be well mixed with its oxidizer, and the underlying mechanism of detonation: A very rapid phase change from liquid or solid, to vapor.
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