TNT equivalent of a dust explosion

Quinn_Inuit · Oct 15, 2018

My characters have some dust and enemies they'd like to get off their hands, and they're the type of people who like using one problem to solve another. I tried doing some research on this, but I couldn't find the lower bound heat information that gave me results that made sense. Here's my math so far:

Assume a ~~spherical cow~~ library with an air volume of 3500 cubic meters. Actual square meters = 750.
The air quality is poor, with a PM 2.5 of 150 micrograms/cubic meter comprising primarily cellulose debris from the books. If I'm doing the mass right, the contribution of this component is negligible, but I wanted to get my assumptions out there.
Unknown: weight of book dust. Sawdust weighs 210 kg / cubic meter, but that's tightly packed. I'm assuming that, at the density of your average dust layer, we're looking at minimum a ten-fold decrease in density.
Assumption: the library is poorly maintained, so there's a layer of half a centimeter of dust on most surfaces.
Surface area: there are lots of stacks, but you need space for tables and aisles. I'm assuming 1600 sq. m. of actual dust-containing surface, with maybe a quarter-centimeter thick layer averaged over all affected surfaces.
The library air isn't entirely evacuated. Assume maybe 1000 cubic meters of air gets sucked out, as well as a quarter of the dust.

If I'm doing the math right there, we're looking at about 1 cubic meter of loosely-packed dust, so call it no more than 21 kg. Probably half that, realistically. There's also an upper combustible limit at play, which I can't find anywhere. Too much and there's not enough O2 for the Earth-shattering kaboom. The minimum is around 500g/cu.m.

Once we figure out the mass of dust there, pack it all in a small space and throw in a lit match. How big a boom do we get? That's the other part that's giving me trouble. You've got to figure that only 0.2-20% of the dust actually combusts.

I'm tempted to just assume that I get about 1 kg of dust to actually go boom. That simplifies the math a little and probably does the trick. Unfortunately, I just can't get reasonable numbers for the explosive potential of cellulose dust. My math keeps coming up at way more than the equivalent in TNT, and I know that's wrong.

Any suggestions? Rough approximations are fine.

WeaselFire · Oct 16, 2018

Normal dust doesn't combust.

Jeff

ironmikezero · Oct 16, 2018

Jeff is right; that sort of dust isn't like flour, for example. If your story must have an explosion, you'll need to add another flammable element. How about a gas leak?

jclarkdawe · Oct 16, 2018

Normal dust is relatively inert. However, you're assuming a high level of cellulose in the dust, I'm presuming from decomposition of the books. Having worked in a large library, with many old books, I'm not sure I'd accept that as probable. I think the composition of the dust would be close to normal household dust.

Dust explosions depend upon a primary and a secondary ignition. And the strength of the explosion and likelihood is directly related to the flammability of the dust. In other words, coal dust explodes with a minimal primary ignition compared to sawdust. Sawdust can explode, but requires a lot of force in the primary ignition. You can get that by the sawdust being compressed and decomposing, generating a lot of heat, which reaches the combustion point. A coal explosion, on the other hand, can be set off from a spark.

I doubt that a match would set off cellulose, which I'm guessing would be near to sawdust in flammability. (Among other things here, if it were very explosive, terrorists would be using it.)

Now you could get a good effect out of ordinary dust in the following manner. Compact in the bottom of a barrel or other solid container. Set off a primary explosion at the bottom of the barrel. This will cause the dust to rise into the air in a tight cloud. This is followed by a secondary explosion in the cloud, causing the dust to rapidly disperse through the air. The flammable parts of the dust will ignite, causing a rapidly expanding cloud of burning particles. This will create a lot of structural damage, although I'm not sure how fatal it would be to people trapped in it.

I would think booby-trapping the library stacks would be more likely to be fatal than what you're proposing with a lot less force. And a lot more probable. Library stacks can be quite high, and the force of the falling force of books rather nasty.

Jim Clark-Dawe

blacbird · Oct 16, 2018

Not only does normal dust not combust, to get an explosion even from something like flour dust (as in an agricultural mill), it needs to be mixed in the air, not "closely packed" or scattered on surfaces.

caw

Cyia · Oct 16, 2018

Could your boom-dust be in a place where fertilizer is manufactured, or maybe someone is using the library for more than book storage?

Quinn_Inuit · Oct 16, 2018

jclarkdawe said:
Normal dust is relatively inert. However, you're assuming a high level of cellulose in the dust, I'm presuming from decomposition of the books. Having worked in a large library, with many old books, I'm not sure I'd accept that as probable. I think the composition of the dust would be close to normal household dust.

Dust explosions depend upon a primary and a secondary ignition. And the strength of the explosion and likelihood is directly related to the flammability of the dust. In other words, coal dust explodes with a minimal primary ignition compared to sawdust. Sawdust can explode, but requires a lot of force in the primary ignition. You can get that by the sawdust being compressed and decomposing, generating a lot of heat, which reaches the combustion point. A coal explosion, on the other hand, can be set off from a spark.

I doubt that a match would set off cellulose, which I'm guessing would be near to sawdust in flammability. (Among other things here, if it were very explosive, terrorists would be using it.)

Now you could get a good effect out of ordinary dust in the following manner. Compact in the bottom of a barrel or other solid container. Set off a primary explosion at the bottom of the barrel. This will cause the dust to rise into the air in a tight cloud. This is followed by a secondary explosion in the cloud, causing the dust to rapidly disperse through the air. The flammable parts of the dust will ignite, causing a rapidly expanding cloud of burning particles. This will create a lot of structural damage, although I'm not sure how fatal it would be to people trapped in it.

I would think booby-trapping the library stacks would be more likely to be fatal than what you're proposing with a lot less force. And a lot more probable. Library stacks can be quite high, and the force of the falling force of books rather nasty.

Jim Clark-Dawe

This was an extremely helpful post. Thank you for answering my questions and providing such useful information.

First, are you sure about sawdust being less flammable than coal? I mean, that seems reasonable, but this site says the minimum ignition energy of coal dust is > 1000 megajoules, but this site says the minimum ignition energy of sawdust is 7-250 megajoules. It also says a 1mm layer is dangerous and the industrial safety limit is 3mm, which can cause flammable clouds up to 5m if ignited. Of course, that's pure, fine sawdust, and I'm talking about a normal dust/paper dust mixture, so I'm assuming my mixture is much less dangerous per unit mass.

For the record, they'll have access to a powerful ignition source, roughly the equivalent of half a stick of dynamite, so I think they can get the explosion going regardless. That question is just for my own edification.

Second, I respect your experience working in the library, but I'm genuinely surprised that you don't think bits of paper pulp would comprise a larger portion of the total dust in the facility. I'll do some more research on this topic. I'm open to suggestions on that front, but you're under no obligation to provide any. I consider your experience to be prima facie proof of your claim, it's just that I find it so surprising I want to confirm it.

Update: there is surprisingly little on the internet about this topic. However, this article seems to imply that a large percentage of the dust in the library of Trinity College (Dublin) is "decaying leather book covers, floating paper fibers, the building itself, and clothing worn by visitors." (And coal dust, which is discussed heavily in the preceding sentence.) So wood from the stacks and perhaps the walls (depending on building composition) and paper fibers are likely to be major components of the dust.

Third, the barrel of compressed dust idea was interesting, and I think it captures well for anyone else who happens to read this thread in the future the mechanism by which a dust explosion forms. Do you think you could get the same effect if you were to vacuum a kilo or two of the dust mixture out of the air and dump the contents of the vacuum bag on the ground?

I like the idea of booby-trapping the stacks, but I don't think that will work in my case. I'm going to keep that in my back pocket for future projects, though.

Quinn_Inuit · Oct 16, 2018

Cyia said:
Could your boom-dust be in a place where fertilizer is manufactured, or maybe someone is using the library for more than book storage?

Hmmm...I might be able to rework that section into something that takes place near the facility's waste disposal areas. Interesting suggestion, thank you. Sewer sludge dust is also highly combustible.

Quinn_Inuit · Oct 16, 2018

blacbird said:
Not only does normal dust not combust, to get an explosion even from something like flour dust (as in an agricultural mill), it needs to be mixed in the air, not "closely packed" or scattered on surfaces.

caw

That's correct. All combustible dusts have minimum and maximum explosives concentrations in the air. That's why I was discussing concentrations in terms of grams per cubic meter above.

That said, a layer of dust on the floor can very easily turn into a cloud, as I'm sure you know. That's why OSHA apparently requires classification (and concomitant safety practices) for any location where there's regularly a sawdust layer of 3mm or more.

bwebs · Oct 17, 2018

For an explosion even the perfect cloud of dust also needs to be pressurized or it'll just burn.

Quinn_Inuit · Oct 17, 2018

bwebs said:
For an explosion even the perfect cloud of dust also needs to be pressurized or it'll just burn.

That's a fair point. OK, assume they dump the contents of that vacuum into a sturdy trashcan and toss the dynamite in after it. Boom?

jclarkdawe · Oct 17, 2018

I think you're thinking of the scenario in this video -- https://www.youtube.com/watch?v=70fZqHsEdMo

Unfortunately, we tend to use the words "explosion" and "explosive force" interchangeably and we really shouldn't. What we're seeing in the video when the entire mill goes up in flames is closer to a flash-over than an explosion. Although the events are similar, they're not the same and a flash-over is much slower (although still measured in the thousands of seconds). Mainly you want to look at blast waves, the Friedlander waveform, and shock front. An explosion has a much more intense blast wave than a flash-over.

A flash-over occurs when you have super-heated air in an enclosed area. In this super-heated air you have floating combustibles such as sawdust. When oxygen is suddenly let into this area, the combustibles burst into flame, causing a sudden expansion of the gases, generating heat, fire, and a shock front. It will knock you over, but lacks a lot of the force that a blast wave contains. In other words, a blast wave from an explosion is a lot more powerful than a blast wave from an explosive force event. Also in an explosion, the blast wave carries debris through the air which hit objects with high speed.

The system I described and what you start to describe with putting the material into a trash can is the same approach. You send a cloud of debris up into the air which places a lot of combustible material in dust form into the air. This material, if a secondary explosion occurs, will cause the combustibles to ignite. You'll produce a blast wave, but one with significantly less force than an explosion. (In addition, all the shelving in the library will block this blast wave rather effectively.) You won't have a force strong enough to kill very far from the source. In other words, your killing force will not be significantly larger than what you would get with your half stick of dynamite.

What I remember of the dust at Boston Public Library was mainly concrete dust. Trinity Library would have a lot of books a lot older than any normal library. But when books are shelved, they're rather jammed into the shelving, which is often metal. There's just not a lot of surface area for the books to decompose in. Now understand that all dust has combustibles in it. In certain situations this will ignite, like a flash-over. But normally you can't light dust on fire.

Sawdust comes in a lot of different sizes and moisture contents. With time, sawdust tends to attract moisture, increasing its moisture content. But in a fine, dry state, it is highly flammable. But to get an explosion of much strength, you've got to have some sudden spark, and then get some chain reactions going. Going back to the exploding barrel idea, you fill it with dust (and sawdust would be a good source, although not as good as coal dust). You set a primary explosion following by a secondary or chain reaction explosion. The first explosion will send the dust into the air, will ignite some of it, but won't spread it around much. The second explosion spreads fire through a wide area, but the blast wave isn't that strong once you move away from what is providing the second explosion.

Jim Clark-Dawe

WeaselFire · Oct 19, 2018

Put a meth lab in the basement.

Jeff

Bolero · Oct 20, 2018

I haven't tried it but there were some students I knew once who spent time out in the garden chucking milk powder in the air and setting it off with a bit of burning paper. But again this was fine dry powder in the air - and they were all standing round it and not getting knocked off their feet. More whoosh than boom.
You could experiment with the contents of your vacuum cleaner bag - though I'd do it outside......
If you had a cylinder of oxygen that would get it going nicely.
Propane cylinder in the basement - bearing in mind it's heavy so if you have the gas slowly leaking out it tends to collect downwards - there are regs about not having a cylinder within so many meters of an open drain cover.

Regarding layers of sawdust - are the safety regs about layers of sawdust themselves being something likely to go boom, or is it that a layer of sawdust of thickness x if disturbed and made airborne is then at sufficient concentration in that area to be explosive?

Quinn_Inuit · Oct 20, 2018

WeaselFire said:
Put a meth lab in the basement.

Jeff

I suppose that would provide a good ignition source.

Quinn_Inuit · Oct 20, 2018

Bolero said:
I haven't tried it but there were some students I knew once who spent time out in the garden chucking milk powder in the air and setting it off with a bit of burning paper. But again this was fine dry powder in the air - and they were all standing round it and not getting knocked off their feet. More whoosh than boom.

That makes sense. There's also the question of how much milk powder they're using per whoosh. Oh, and what the combustion energy provided by the milk powder is. Some of the different powders have wildly varying amounts of energy they give up in combustion.

Bolero said:
You could experiment with the contents of your vacuum cleaner bag - though I'd do it outside......

I think the HOA (and my wife) would disapprove.

Bolero said:
If you had a cylinder of oxygen that would get it going nicely.
Propane cylinder in the basement - bearing in mind it's heavy so if you have the gas slowly leaking out it tends to collect downwards - there are regs about not having a cylinder within so many meters of an open drain cover.

Interesting! I didn't know that. I'm used to thinking of methane as lighter than air, so I forgot how much heavier some of the other 'ane gasses got.

Bolero said:
Regarding layers of sawdust - are the safety regs about layers of sawdust themselves being something likely to go boom, or is it that a layer of sawdust of thickness x if disturbed and made airborne is then at sufficient concentration in that area to be explosive?

The latter. Basically, once you get a 3mm layer, if it gets kicked up somehow you have a problem. On the ground, it's fine and will burn normally.

Quinn_Inuit · Oct 20, 2018

jclarkdawe said:
I think you're thinking of the scenario in this video -- https://www.youtube.com/watch?v=70fZqHsEdMo

Unfortunately, we tend to use the words "explosion" and "explosive force" interchangeably and we really shouldn't. What we're seeing in the video when the entire mill goes up in flames is closer to a flash-over than an explosion. Although the events are similar, they're not the same and a flash-over is much slower (although still measured in the thousands of seconds). Mainly you want to look at blast waves, the Friedlander waveform, and shock front. An explosion has a much more intense blast wave than a flash-over.

A flash-over occurs when you have super-heated air in an enclosed area. In this super-heated air you have floating combustibles such as sawdust. When oxygen is suddenly let into this area, the combustibles burst into flame, causing a sudden expansion of the gases, generating heat, fire, and a shock front. It will knock you over, but lacks a lot of the force that a blast wave contains. In other words, a blast wave from an explosion is a lot more powerful than a blast wave from an explosive force event. Also in an explosion, the blast wave carries debris through the air which hit objects with high speed.

You're right that people do often use the words interchangeably, but the video is pretty specific about it being an explosion, or technically a rapid series of them. Also, the video shows the mill operating normally prior to the explosion, and a flash-over (as you stated) requires super-heated air. IIRC, a warning sign of a flashover is tiny things randomly sparking into flame as the temperature reaches the ignition point of small bits of wood or scraps of cloth. That's pretty different from the mill situation in the video, where everything is normal until a cloud of sawdust comes into contact with a flame.

jclarkdawe said:
The system I described and what you start to describe with putting the material into a trash can is the same approach. You send a cloud of debris up into the air which places a lot of combustible material in dust form into the air. This material, if a secondary explosion occurs, will cause the combustibles to ignite. You'll produce a blast wave, but one with significantly less force than an explosion. (In addition, all the shelving in the library will block this blast wave rather effectively.) You won't have a force strong enough to kill very far from the source. In other words, your killing force will not be significantly larger than what you would get with your half stick of dynamite.

What I remember of the dust at Boston Public Library was mainly concrete dust. Trinity Library would have a lot of books a lot older than any normal library. But when books are shelved, they're rather jammed into the shelving, which is often metal. There's just not a lot of surface area for the books to decompose in. Now understand that all dust has combustibles in it. In certain situations this will ignite, like a flash-over. But normally you can't light dust on fire.

Oh, sorry, I should've specified that this library is centuries old and has wooden shelves. And I agree that the dust in normal concentrations should be barely combustible. It's the artificial concentration of the dust here that takes us into potential danger territory.

jclarkdawe said:
Sawdust comes in a lot of different sizes and moisture contents. With time, sawdust tends to attract moisture, increasing its moisture content. But in a fine, dry state, it is highly flammable. But to get an explosion of much strength, you've got to have some sudden spark, and then get some chain reactions going. Going back to the exploding barrel idea, you fill it with dust (and sawdust would be a good source, although not as good as coal dust). You set a primary explosion following by a secondary or chain reaction explosion. The first explosion will send the dust into the air, will ignite some of it, but won't spread it around much. The second explosion spreads fire through a wide area, but the blast wave isn't that strong once you move away from what is providing the second explosion.

Jim Clark-Dawe

That barrel idea really is a solid one, thank you.

I'm curious, what do you suppose would happen if you took a handful of fresh sawdust and threw it into a portable, miniature oven, like the kind a Middle Eastern street vendor from two hundred years ago would've used to bake pita? Based on that one part of the video about 2/3 of the way through, I'd say a sheet of flame, but do you think I'm reading/viewing it correctly?

jclarkdawe · Oct 21, 2018

Quinn_Inuit said:
You're right that people do often use the words interchangeably, but the video is pretty specific about it being an explosion, or technically a rapid series of them. Also, the video shows the mill operating normally prior to the explosion, and a flash-over (as you stated) requires super-heated air. IIRC, a warning sign of a flashover is tiny things randomly sparking into flame as the temperature reaches the ignition point of small bits of wood or scraps of cloth. That's pretty different from the mill situation in the video, where everything is normal until a cloud of sawdust comes into contact with a flame.

The video was the reason I mention the difference between an explosion and explosive force. Any time you get a rapid expansion of gases, you get a force wave from the expansion. This is what Bolero is describing as a "poof." Depending upon how rapid the expansion of gases, the stronger the force wave. In an explosion, this expansion of gases is practically instantly and the force wave, if within the blast zone, is strong enough to kill you. A flash-over is a slower, although still very rapid, expansion of gases. The force wave is still strong, and will knock you on your ass, but deaths from flash-overs are from the heat and fire, not the force of the blase wave. As a volunteer firefighter, I've been in a flash-over, and seen the the difference between a flash-over and an explosion.

Watching the video, what it appears to be showing is something moving with explosive force, but not an explosion. Note the depiction of the guy flipping an electrical switch and getting caught in the expanding gases. He's going to have massive burns, probably is going to die, but the force of the blast does not knock him over. In an explosion, if he was that close, he'd be at least down on the ground. There would be a good possibility of him suffering fatal traumatic injury to the chest from the blast wave.

A flash-over has enough strength to blow out glass windows, and will knock over some but not all furniture. An explosion has enough strength to knock all of the furniture all over the place. In the case of a flash-over, walls will effectively divert the explosive force of the shock wave. In an explosion, walls will also divert the shock wave, but are more likely to fail. I think in the case of a library, with book shelves going all the way to the ceiling, a dust explosion would not have the strength to knock over the shelving, and the force would effectively be diverted.

Understand that any type of explosive shock wave is unpredictable as to both its force and direction. Even experts can screw this up and use either too little or too much in planned explosions. A kilo of plastic explosive should kill everybody in a room, but a solid table leg was able to divert the force enough that Adolf Hitler was not killed. But I don't think a dust explosive would have the force to kill people immediately. It might knock them over, might distract them, but killing them immediately doesn't seem likely. Major burns could render the attackers unable to continue.

Quinn_Inuit said:
Oh, sorry, I should've specified that this library is centuries old and has wooden shelves. And I agree that the dust in normal concentrations should be barely combustible. It's the artificial concentration of the dust here that takes us into potential danger territory.

How dusty was the inside of King Tut's tomb? Remember that sawdust is manufactured in the process of working with wood. I'm thinking there's a limit to how dusty a place would get.

Quinn_Inuit said:
That barrel idea really is a solid one, thank you.

I'm curious, what do you suppose would happen if you took a handful of fresh sawdust and threw it into a portable, miniature oven, like the kind a Middle Eastern street vendor from two hundred years ago would've used to bake pita? Based on that one part of the video about 2/3 of the way through, I'd say a sheet of flame, but do you think I'm reading/viewing it correctly?

I think you'd get an explosive cloud probably about ten feet in diameter, maybe able to knock people down, and maybe resulting in significant burns.

My guess is that although dust explosions can do significant damage to buildings, it is more from the subsequent fire. Deaths are more likely to be from fire injuries and smoke inhalation rather than traumatic chest crushing from explosive waves. This is opposed to explosions designed to kill people, where the explosive force is strong enough to crush the chest, and shrapnel for added effect. I could see a dust explosion as being more likely to destroy the library than stop attackers. However, there would be a confusion factor, and a lot of people don't know a whole lot about how explosions work.

Jim Clark-Dawe

Bolero · Oct 24, 2018

Reminds me of the difference between gunpowder - which is actually a fast burn and high explosive like TNT which is an explosion. If you sprinkle gunpowder on a fire it just burns. To get anything like a blast wave (or fire a gun) you have to compress/confine the gunpowder - as in a muzzle loading musket you ram a wad down on the powder charge and contain the burn until the force of the gas expansion is enough to shove the wad and musket ball out the barrel.

In terms of the library blowing up I'm now wondering how much noise there would be - thinking about the thunderflashes used to stun people - as in they are wandering around doing a "what the ***" and not able to hear much.

TNT equivalent of a dust explosion

Quinn_Inuit

Not a real eskimo

WeaselFire

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ironmikezero

practical experience, FTW

jclarkdawe

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blacbird

Cyia

Rewriting My Destiny

Quinn_Inuit

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Quinn_Inuit

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Quinn_Inuit

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bwebs

Quinn_Inuit

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jclarkdawe

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WeaselFire

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Bolero

Quinn_Inuit

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Quinn_Inuit

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Quinn_Inuit

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jclarkdawe

Feeling lucky, Query?

Bolero