Blue Flamethrowers (Hypothetical physics/chemistry thingy)

[Rater202]

Hypothetically speaking, how to make them? I've always been bigger into biology than physics and chemistry, so my knowledge of these things is limited.

Here's a narrative that serves as a frame for the specific kind of things I'm asking about because my brain is wired for stories: I'm an aspiring Evil Overlord and I want my faceless death army to really style over the faces of our enemies in my conquest of the world. Gotta play up that intimidation factor.

So I figure, what if I equip my siege troopers with flamethrowers that throw blue flame?

How do I go about that?

Caveat: I am talking real-life military-style "shoot a stream of burning liquid fuel many yards away where it can bounce or flow or stick as needed to burn down an enemy fortification" flamethrowers, not Hollywood/Videogame style "puff of flaming gas a few feet out o set the other guy on fire" style. Gas flamethrowers are almost useless for tactical purposes and turning flamethrowers on infantry is both inefficient and pointlessly cruel.

So "using a higher burning gas" that naturally burns blue when it's undergoing complete combustion isn't an option.

My first thought was to mix some kind of sulfide into the fuel, but the only information I can find on what temperature sulfur burns at are that it ignites at about 200 degrees celsius. I cannot for the life of me find out how hot sulfur fires actually are once they're started, so for all I know mixing sulfides with gasoline or diesel fuel in quantities enough to affect the color of the flame might actually make the resulting fires less hot and therefore, less effective.

TL/DR: Effective means of coloring fire for purposes of style and intimidation.

[factotum]

Well, chemically, if you want a blue flame then you want to use a copper compound--any will work, not just the sulphate. (Sulphur itself would actually give a bright yellow flame). Given how hot the flame is, I imagine that adding powdered copper compounds into it might work, if you can devise a means of doing that--maybe mix them with the fluid you're burning? Only issue there is that I imagine the powder will settle over time, so you need some means to agitate the fluid to mix the powder back in properly before igniting it.

[Yora]

Really? I think sulfur would be the way to go. That's the classic blue fire material. Though you can't really do a flamethrower with a powder. You'd have to get it into some kind of thin sludge with a liquid that doesn't douse the fire but also doesn't burn with a bright flame that would wash out the the blue light.

As a nice side effect, you get a very strong gunpowder smell. Human smell is extremely sensitive to even tiny amounts of sulfur-dioxide, and I can't imagine the stench from a sulfur fire. The worst firecracker small you'd ever encounter.

[factotum]

Pure sulphur burns blue, sorry, I think I had it mixed up with sodium. However, I don't think its compounds will colour a flame in the same way as the copper compounds I suggested would:

https://en.wikipedia.org/wiki/Pyrotechnic_colorant

[Rogar Demonblud]

So you need a (preferably liquid) sulphur compound. Any suggestions?

[Peelee]

So you need a (preferably liquid) sulphur compound. Any suggestions?

Simple. Eggs.

[georgie_leech]

Simple. Eggs.

I mean, it *would* smell a bit like brimstone (hellfire anyone?) if they were rotten...

[Brother Oni]

I mean, it *would* smell a bit like brimstone (hellfire anyone?) if they were rotten...

Brimstone normally refers to the smell of burning sulphur - when sulphur is burnt in a standard Earth atmosphere, it typically forms sulphur dioxide because of the oxygen and relative lack of hydrogen.

Sulphur dioxide has an incredible sharp acrid smell, because it dissolves in the moisture on your wet mucus membranes, forming sulphuric acid (it cleared my sinuses for around 3 days). Other than forming acid on any wet surfaces (nose, mouth, eyes, etc) and acting as an aphyxiant, sulphur dioxide is relatively harmless. It's odour detection threshold is high compared to hydrogen sulphide, around 0.67 to 4.75 ppm.

Hydrogen sulphide is the typical 'rotten eggs' smell and doesn't normally form by burning sulphur, barring shenanigans like burning it in a hydrogen atmosphere. Hydrogen sulphide is both flammable and far more toxic than sulphur dioxide, hence why we can smell at very low concentrations compared to sulphur dioxide, between 0.008 to 0.13 ppm. In comparison, chlorine gas is detectable at around 0.2 ppm (its primary method of action as a chemical weapon is much the same as sulphur dioxide, although it's also corrosive in its own right).

With regard to the blue flame weaponry, I'd follow factotum's suggestion of adding a copper compound. You'd need a lot of sulphur to counter the natural colour of the flamethrower fuel mixture.

Edit: I'm also happy to see the number of old school British folk here who still spell it 'sulphur' rather than 'sulfur'.

[Rogar Demonblud]

Not British. I just read too much Victorian literature in school.

[Peelee]

And I just wanted to integrate with the cool kids. Everyone was doing it!

[Thomas Cardew]

I know that lead also burns blue. I wonder what concentrations you'd need to include in either diesel or gasoline significantly affect the color.

Edit: https://www.youtube.com/watch?v=D7_rpEejTKk Huh. FWIW he burns low lead aviation fuel in a see through motor at the end and it is a predominantly blue flame. Granted that's inside the engine, but I imagine in high concentrations it'd work just fine for coloring your fictional flamethrowers. Bonus evil overlord points for being extra toxic!

[DavidSh]

You'll also need to suppress somehow the yellow glow from the soot. A natural gas flame from a clean kitchen stove or a Bunsen burner is blue, and not smoky, while a candle flame, and all pictures I've seen of actual military flamethrowers, is more yellow, and can put out smoke. I'm told this has to do with how well the fuel is mixed with the oxygen required to burn it. Maybe adding alcohol would help.

[Yora]

Normal flamethrowers simply use combustion engine fuel. The same stuff movies use make "explosions". So nice orange-red flames with lots of black smoke.

[factotum]

Granted that's inside the engine

*Any* fuel, leaded or otherwise, will burn blue inside an internal combustion engine that's running properly--in fact, that's how you used to be able to check your engine *is* running properly with the likes of a Colortune spark plug. Just in open air, where you're not getting as thorough a mixing of the fuel and the air, you'll tend to burn yellow due to carbon being left after the main ignition. This is why I suggested a copper compound of some sort, because that will unquestionably turn the flames blue no matter how well or otherwise the combustion is going.

[Fat Rooster]

The main consideration is that any solids in the flame will give you yellow black body radiation, so we want to avoid them. Unfortunately, that black body radiation is also extremely useful at transferring heat from the combusting regions to the parts that are not combusting yet, so maintaining combustion without it can be tricky. Some pre-combustion could assist with that.
I guess Trisulphane could work, but liquid elemental sulphur might also just work. You can melt sulphur in a kettle, so it isn't particularly far out there. Unlike carbon, sulphur boils far before it gets hot enough to emit significant visible wavelength black body radiation, leaving you with just the interesting spectral features from gas phase combustion.
If you want something a little less random, some sort of accelerated methanol combustion might be interesting. Methanol can be thought of as CO with a couple of hydrogens strapped on, so it doesn't soot up the same way most hydrocarbons do no matter how you abuse it (why it is considered for fuel cells). I can't think of a good accelerant though, as we need something high energy that oxidises to a gas as well as boiling at low temperature. Maybe we are back to sulphur.

Edit: Monoxide, not dioxide.

[DavidSh]

If you want something a little less random, some sort of accelerated methanol combustion might be interesting. Methanol can be thought of as CO with a couple of hydrogens strapped on, so it doesn't soot up the same way most hydrocarbons do no matter how you abuse it (why it is considered for fuel cells). I can't think of a good accelerant though, as we need something high energy that oxidises to a gas as well as boiling at low temperature. Maybe we are back to sulphur.

Edit: Monoxide, not dioxide.

Maybe hydrazine would work as an accelerant. If your evil overlord doesn't worry about the toxicity. At one time, hydrazine-methanol blends were considered for rocket fuel.

[Bohandas]

Edit: I'm also happy to see the number of old school British folk here who still spell it 'sulphur' rather than 'sulfur'.

They're not necessarily British or even Commonwealth. Given that this a fantasy media and RPG forum they could easily be steampunk enthusiasts or Ren Faire people

[Fat Rooster]

I am British, and unlike some of the colonists am willing to accept that when something significant becomes convention worldwide it is worth going along with (cough, metric, cough )*. If it was just one word we might get a way with it, but I see no point in trying to use sulfur for the element and then having wikipedia force the 'ph' on me every time I want to link to a compound. It is part of a whole system of chemistry naming and it just looks silly to me to try to pretend the element is a special case for the sake of stubborn-ness. One off words like 'colour' are fine because it is rare that you will be unable to find what I am talking about just because of it, but it can cause real problems if you are searching papers for references to sulphur compounds and somebody as used an f.

Hydrazine could work as an accelerant, but it is also a monopropellant which might cause problems in this case. Liquid rocket fuel really wants to burn as fast as possible, but flame throwers don't. Fuel which burns substantially faster at higher temperature might be too unpredictable to be useful.


Yes I aware of the Irony given current events, and that I cannot speak for all Brits. Don't get me started.

[Peelee]

I am British, and unlike some of the colonists am willing to accept that when something significant becomes convention worldwide it is worth going along with (cough, metric, cough )*.

Oh, metric! Jeez, we should really talk about that some more. So, since you're British, there's probably a pub a mile or two up the road, let's get a pint and talk about how you, as a country, are apparently still unwilling to accept that when something significant becomes convention worldwide it is worth going along with (cough, consistent metric, cough )*.

[sktarq]

I'd still aim at the copper compounds. Mostly because they are already well known for burning blue when used as an adulterant to black powder vs in order to get a passable blue flame from the sulphur you'd need a quite high concentration of the stuff. Thus the KISS strategy would favor the copper and even your evil-overlord types will generally prefer things simple (especially because they are faster) when they can....they have enough on their plates with all the conquering, monologing to the would be heroes caught in the sewers, and setting up new skull, basalt, and axe based decor in whatever high office they have just taken over.

[Rater202]

Could have sworn that copper burned green.

[DavidSh]

I was just hoping that a lesser percentage of hydrazine than was proposed for rocket fuel might still boost the performance of methanol up to the level of diesel fuel or gasoline, sort of on a general principle of continuity. Not that I'd be willing to work with hydrazine myself, or methanol. Then dope with a copper salt for color.

[georgie_leech]

Oh, metric! Jeez, we should really talk about that some more. So, since you're British, there's probably a pub a mile or two up the road, let's get a pint and talk about how you, as a country, are apparently still unwilling to accept that when something significant becomes convention worldwide it is worth going along with (cough, consistent metric, cough )*.

Let he who is without sin cast the first stone and all that.

[sktarq]

Could have sworn that copper burned green.

A fair number of copper compounds are green and metallic copper turns green in normal atmospheric conditions which may be why you associate copper with green but the flame in order to get there is blue. I re-recommend the above linked wiki page for pyrotechnic colourants.

[Fat Rooster]

Oh, metric! Jeez, we should really talk about that some more. So, since you're British, there's probably a pub a mile or two up the road, let's get a pint and talk about how you, as a country, are apparently still unwilling to accept that when something significant becomes convention worldwide it is worth going along with (cough, consistent metric, cough )*.

lol, yeah I'll give you that. Though in our defence, pints are only really still used for beer to the point that the unit is a synonym, and mobile engineering is rarely done on kilometre scale, so those two shouldn't cause the same sort of 'Mars probe burned up' issues that lb kg, or cm inch can and have. Nobody is going to need two sets of spanners, one measured in miles and the other km. Hours aren't even SI, so you can argue that km/h is still 'wrong', and we should be using m/s*! I guess you could get a problem where a range was quoted in km but expected in miles, but I'm unaware of it ever happening. The Gimli Glider was another lb kg mix up rather than that. You would literally have to not know what country you were in for the miles thing to cause major confusion, and in that situation you probably have a larger problem in that you would be driving on the wrong side of the road!

Yeah, you may be right in the km thing... And probably the whole cars being the other way around (seriously, how did that even happen?), but you ain't shrinking my beer!


* The main beauty of metric SI is not needing constants all over the place, so in a sense km/h doesn't really fit. When doing any scientific or engineering work m/s competes for the convention anyway.

The colour of copper flames is dependent on a number of factors, as different oxidation states will give different colours. That makes it bad for pyrotechnics, and it would probably produce a copper[I] blue flame when used in any sort of flame thrower. In strongly oxidising situations though it will burn green from the doubly oxidised copper[II], which is where the confusion comes from.

[Lord Torath]

Let he who is without sin cast the first stone and all that.

US-ite here (nearly everyone on two continents get twitchy when I call myself an "American"), and I'd love to cast a stone at the US. And at the Congress that inserted the word 'Voluntary" into the 1975 Metric Conversion Act.

Why yes, I am an engineer! What gave it away?

A bit more on-topic, what would you 'package' the powdered copper compounds in? It needs to be liquid, and probably flammable (at least it can't extinguish the burning copper), but that won't overpower the blue from the copper.

[factotum]

A bit more on-topic, what would you 'package' the powdered copper compounds in? It needs to be liquid, and probably flammable (at least it can't extinguish the burning copper), but that won't overpower the blue from the copper.

It's not really the copper compound which is doing the majority of the burning here--you can use any regular fuel for the flamethrower you like. That's why they use things like that in pyrotechnics, because a small amount dissolved in your fuel goes a long way.

[Rogar Demonblud]

Yeah, you may be right in the km thing... And probably the whole cars being the other way around (seriously, how did that even happen?), but you ain't shrinking my beer!

Wouldn't be. A half-liter is 16.9 ounces, as opposed to 16 for a pint. So you get just a bit more.

[factotum]

Wouldn't be. A half-liter is 16.9 ounces, as opposed to 16 for a pint. So you get just a bit more.

Fat Rooster is apparently British, and over here a pint is the Imperial one, which is 20 fluid ounces or 568ml. So, no, half a litre *would* be a downgrade from a pint here.

[Rogar Demonblud]

Ah. I wonder if that explains why so many of our drinks on this side of the Pond are 20 oz.