Re: How much impact does oxygen saturation have on combustion?

Rick:
Sorry I couldn't get to your question before the 4th of July, sharing the 
knowledge of flammibles with your students might be dangerous, but still 
kinda fun!

Oxygen availability is pretty darn important for combustion.  Other 
oxidizers, if readily available would also be important, but the air 
around us has about 21% O2 to begin with.  I'm about a mile above sea 
level, and 19% is all we regularly can expect up here.  

Before I address the forest fires specifically, talk with your students 
about various forms of combustion and conditions with higher or lower 
oxygen levels.  For instance: carburetion.  If the mix in my truck is "too 
lean", meaning not enough fuel, the engine stalls.  If it's "too rich," 
again, it's kaput.  On gasoline jerry cans, there is often a warning, 
aside from the potential to build up static, that gasoline cans should not 
be partially filled.  This is because gasoline can fill the vacancy with 
vapors.  Mixed with air, this is an explosive condition.  Lighting the 
actual fluid itself is next to impossible, but as I'm sure you can 
imagine, once a little heat has warmed the fluid enough to ignite some 
stray vapors, there's little chance to just blow it out. 

You and the class might also investigate the fire suppression methods 
available to you.  Don't rule out fire blankets and dry ice(pressurized 
CO2 canisters).  

Oxygen is vital for our metabolisms, too.  Sure we aerobic, respiring 
organisms need it to "burn" our calories, but if the atmospheric O2 levels 
were to increase by even 2% (or if you or I traveled to a different 
altitude), most of us would notice the difference overnight.  When 
visiting people at sea level, I find that I can run farther and faster, 
but with all the oxygen, I also experience a mean headache that lasted 
almost 2 days straight.  My CO2 balance was thrown off too.  Yup, 
amazingly enough, we need a little of it in order to keep our blood 
buffered and at the right pH.

It may or may not be relevant to your inquiry, but at the end of the 
precambrian, before larger organisms developed, there was a considerably 
greater concentration of O2 in the atmosphere.  The organisms that were 
making it didn't instantly adapt, nor did aerobes form overnight.  A great 
deal of the oxygen that was formed was tied up in oxidizing things like 
iron.  Some of it was mineralized, some dissolved into the ocean, and a 
lot was probably consumed by combustion.  Which brings me back around to 
your initial question.  

I don't believe that humans would be able to adapt to an explosively 
oxygen-rich atmosphere without several hundred generations' adaptation.  I 
personally also don't think that, with the earth and its systems in the 
present state of things, the atmosphere will ever change that drastically.

All fires need fuel, heat, and usually the favored oxidizer.  A match (the 
way they're made now, at least) doesn't have enough fuel to do anything 
more than a little pop.    

(oh, and please keep in mind that I'm speaking from the perspective of 
someone who enjoys working on the fuel end of things, not necessarily the 
oxidizer expert.  Promise me you won't smoke near someone's oxygen tank 
just to see what happens!)

Key terms to research or sic your students on: LEL/UEL (upper/lower 
explosive limit); oxidation of metallic sodium (or potassium); partial 
pressure of oxygen; carburetion; hyperventilation& metabolism; altitude 
sickness; fire suppression; deep-ocean carbonate precipitation

Dan Berger adds:
A letter to Chemical & Engineering News a few years back pointed out, in an 
instance where the New Jersey Turnpike was closed down because of a liquid 
oxygen spill, by pointing out that oxygen-saturated asphalt is a high explosive.

Fuel-air explosives mix large quantities of inflammable fuel intimately with air 
(carburation on a large scale), generating bang-for-weight second only to 
nuclear weapons.