Laser, Paper, Vacuum

While this could be a great variant of Rock-Paper-Scissors, it’s actually a proposed experiment.

I learned in cub scouts that fire requires three things: fuel, heat, and oxygen. My question, in short, is what happens when oxygen is removed?

So say I’ve got a sheet of paper suspended in a near-perfect vacuum. I shine a very powerful laser at the paper. In a normal atmosphere, the paper would instantly ignite from the radiant heat of the laser. What happens in the vacuum?

If the experiment is flawed (e.g., because paper includes oxygen in its molecular structure or something), can you suggest a similar experiment that lacks that flaw?

Your answer is “pyrolosis”.

It basically disintegrates into gas.

Cody’s Lab on YouTube did a series of demonstrations of this - here’s one:

This is fantastic–thanks, both of you!

Wonderful! This is definitely, “Jasmine’s Thread of the Day”! LOL

Small pedantic nitpick, which I’m only doing in case others are looking this up: it’s “pyrolysis,” with two "y"s. Still, really cool, and thanks for the new word!

In my feeble self-defense, several times now I’ve caught myself posting without adequate edits and put my foot in my mouth. At least I didn’t make a complete hash of it this time.

I love picking up and learning assorted tidbits, so being able to give back a little gives me a warm fuzzy feeling inside. Or that’s just the laser trained on my chest.

We actually did an experiment on these lines in Cubs. We took two identical glass bottles, put identical sheets of paper in each one, and lit them. The only difference was that we put the cap on one of the bottles.

The paper in the uncapped bottle burnt completely.

The fire in the capped bottle quickly went out, as all the oxygen was consumed. The paper in that bottle was charred, but not destroyed.

In Firefighting we are taught the fire tetrahedron heat, fuel, oxidizer (O2 not actually required but usually it is O2), and the chemical reaction. The reason why we have 4 is we have tools and training to disrupt any of the 4 to stop the fire.

In your example, in atmosphere the paper does not burn directly, it turns into a flammable gas (as pointed out above), and it is that flammable gas that burns, the heat of that combustion causes more of the solid paper to become gas. In your example you exclude the oxidizer and no fire. Used of certain extinguishers such as Halon 1211 hinder the oxygen and fuel from combining in the heat of the fire, perhaps it’s better though of as the corners of the triangle, without the corners connecting there is no triangle even if the sides are there.

Actually, you don’t need oxygen gas. You can have the oxygen in chemical form as an oxidizer.

Alternatively, you can have an oxidizer that doesn’t even contain oxygen. Things have been made to burn in chlorine or fluorine atmospheres

Here’s someone doing your experiment. Interestingly, when they ignite gunpowder (which contains its own oxidizer) with a laser in vacuum it burns, but doesn’t continue burning, for reasons they give

Note that the thermite, properly treated, does burn in vacuum.

In that case, you were relying on the combustion of the paper to provide ongoing heat input, so not much happens in the vacuum bottle. In the OP’s case, ongoing heat is provided by the laser, which results in pyrolosis.

In ninth grade science class, we filled a test tube with wood stir sticks, inserted a stopper with a hose coming out of it, and heated the test tube with a burner. With no air in the test tube, the heat broke the wood down, vaporizing volatile compounds and sending them out the hose; by passing the hose through a beaker of water, you could cool/condense the mixture into a flammable liquid; back in the test tube, if you carried out the heating long enough, you were left with mostly carbon along with incombustible ash.

In other words, pyrolysis.

And if you don’t have any gas laying around, just wood, you can direct that into your engine and drive you car with it.

Close but some caveats.

Around 50% of the weight (dry basis) will turn into gas, about half of it will form char (carbon) and a few percent will form ash.

Here is the proximate analysis of a paper cup, to back the above up. Proximate analysis is the same as what the op is asking, you take something and heat it up avoiding oxygen.

Sure. You’ll just need some kind of heat source to drive the pyrolysis. Maybe a gasoline-fired burner? :grinning:

Or a laser. :slight_smile:

Though in practice, you would be burning some of the wood in open air to produce the heat for pyrolysis of the rest of the wood.

I’ve never understood the concept of adding “ongoing chemical reaction” to the list of what a fire needs. The reaction isn’t what a fire needs; it’s what it is, and the way you disrupt it is by removing one or more of the other three.

I would annoy my father-in-law, who worked as an engineer in the petrochemical industry, by bringing up Purple K. It’s basically potassium bicarbonate, and when used can be an awful pain in the backside to clean up.

From my (admittedly fairly hazy memory), potassium ions (in particular, though sodium was historically used) have a strong affinity for radicals and present a lower-energy path for reaction. Sodium/potassium bicarbonate are endothermic when breaking down, and release carbon dioxide as as a function of that breakdown, but that’s kind of a secondary function.

You can stop an oil well fire with an explosion. Sort of like blowing out an candle. In short, if you can cut off heat transfer between combustion products (radiant heat or convection heat) and the combustion reactants, you disrupt the “ongoing chemical reaction”.

I seem to recall some fellow from Texas who gained a measure of fame from doing exactly that.

Let’s see–what was his name? Ralph? Rudolph? Raymond? It’s on the tip of my tongue. . . . .