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Author: Subject: Function of coal in gunpowder
chornedsnorkack
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[*] posted on 28-10-2012 at 00:16
Function of coal in gunpowder


What is the role of coal in gunpowder?

Heat source? But brimstone burns, too.

Carbon/nitre mixtures with no or not enough brimstone burn rapidly - coal and oxidizer produce energy - but do not explode. They are known, and used, as priming powders.

But what happens to mixtures which have brimstone and nitre in optimum (stoichiometric) amounts, but no or not enough coal?
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HellstormOP
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[*] posted on 28-10-2012 at 02:25


Brimstone makes gunpowder more sensitive to ignition.

There are types of gunpowders purely composed of coal and potassium nitrate, e.g. 75/25. They are said to burn even faster than mixtures with brimstone. The coal needs to be very finely ground, though.

Brimstone doesn't yield as much energy during combustion as coal, as far as I know.
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[*] posted on 28-10-2012 at 03:57


Indeed, if one trusts wikipedia:

heat of combustion for carbon (in the form of graphite) is 32808 kJ/kg (or 393.7 kJ/mol)
and for sulfur: 9163 kJ/kg (or 293.82 kJ/mol)

If one assumes that essentially all of the heat of reaction for black powder comes from the combustion of these two fuels in their common 15/10 ratio, then only 15% of the total energy yield is contributed by the sulfur.

There exists a correlation between the ignition temperature and the melting point of the compounds in pyrotechnic mixtures and an inverse correlation between the melting point and the burning rate. Given the low melting point of sulfur, that would lead to the prediction that sulfur-less black powders react more slowly and have an increased ignition temperature compared to the sulfur-containing variants.




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HellstormOP
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[*] posted on 28-10-2012 at 07:56


Well, actually coal-based black powder doesn't need to melt at all - it simply burns off because of the enormous surface of the coal powder. According to my sources (a German pyrotechnics website that is currently down) a powder purely based on coal burns faster than one with sulfur, as long as the coal is ground finely enough. I guess, that you only need sulfur as a burning rate enhancer when the coal isn't ground finely enough - with very fine coal, the negative effects of sulfur (lower energy) should outweigh the advantages.
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cyanureeves
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[*] posted on 28-10-2012 at 13:14


or brimstone for that matter. doesnt potassium nitrate burn with just charcoal or coal alone? brimstone would be sulfur wouldnt it?just like chlorate burns fine with charcoal instead of sugar. i soaked a rag with dirty chlorate solution and it burns bright as flash powder almost.
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[*] posted on 28-10-2012 at 15:41


Probably, the very reason that a low melting point enhances ignition temp and burn rate is that in a molten state, the reactants are in much more intimate contact than solid particles could ever be.

The melting point of nitre is also not very high, at 334 deg C, so even in sulfur-less black powder, the melting of this will probably have a large effect on the burn rate.

Possibly, BP made using for instance lithium nitrate or zirconium nitrate is faster even.





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White Yeti
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[*] posted on 29-10-2012 at 08:03


When carbon burns in the presence potassium nitrate, it will produce carbon dioxide, a gas, which will contribute to the overall increase in pressure inside the chamber.
10 KNO3 + 3 S + 8 C → 2 K2CO3 + 3 K2SO4 + 6 CO2 + 5 N2

According to this formula, the sulphur is converted into sulphate, which does not contribute to the overall power of the explosion. You may notice that some of the carbon is converted to carbonate upon ignition, but notice how much is converted into carbon dioxide. Also, due to the high temperatures involved, I wouldn't be surprised of the carbonate was decomposed into carbon dioxide and potassium oxide; such is not the case with sulphate.

This is all in addition to what has already been stated about the energetics of carbon and sulphur combustion.




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watson.fawkes
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[*] posted on 29-10-2012 at 09:25


I would recommend that all the posters in this thread read some reference material on the subject rather than speculating. For example, Russell's Chemistry of Fireworks has a quite good chapter on black powder. Amongst other things, he talks about the importance of hydrogen liberated from charcoal as an intermediary in the form of H2S, as well as the importance of the oxides of nitrogen as gaseous intermediaries. Also, he states the main K products of combustion as the carbonate and the sulfide, not the sulfate, in addition to significant amounts of carbon monoxide (which is pretty much always present in any combustion process without an excess of oxygen).
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[*] posted on 29-10-2012 at 16:50


i just burned some potassium nitrate with charcoal and it burned just like gunpowder but i was left with a white substance that looked like ice cream and i mean alot of white stuff.when i burn potassium nitrate with charcoal and sulfur it just burns black but this time i was left with this white cream looking stuff and its bitter to the taste.my potassium nitrate is not pure white this time either so it probably contains some other fertilizer,but yes potassium nitrate burns fine without sulfur.
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[*] posted on 29-10-2012 at 17:10


Yes, references will definitely improve the quality of our posts, but information from different sources conflicts, so this can get very complicated very quickly even if information comes from authoritative sources. There is no set way in which gun powder has to deflagrate because of the relative complexity of the mixture and the reagents. As watson points out, charcoal is in fact a hydrated form of carbon, not pure carbon as many believe. Pure carbon is in fact very difficult to burn. Many factors will affect how gun powder will burn, grain size, purity of chemicals, humidity et-cetera. The only information that will remain consistent throughout every source is that if you mix dry charcoal powder with nitre and brimstone, it will go boom if held near an ignition source.
From the Artillerist's Manual page 14:
"Its use in gunpowder [sulphur] is to add consistency to the mixture, to make it less liable to absorb moisture, and to give a greater intensity to the flame."




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[*] posted on 29-10-2012 at 22:48


This page:
http://www.musketeer.ch/blackpowder/bp_menu.html
Has a good discussion on the topic. Showing that ignition temp is dependent on the charcoal production temp. Also a graph of energy vs diffrent proportions of coal/sulfur.




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[*] posted on 2-11-2012 at 17:09


I just found a relevant piece of information on the purpose of sulfur in black powder. Yes, I know it's not congruent with the subject, but I think we've already established the fact that carbon along with nitrate is the source of energy for this reaction. The purpose of sulfur, however, may not be as clear.
http://www.osti.gov/bridge/servlets/purl/5539136/5539136.pdf
page 22:
"The function of the sulfur in normal black powder is fourfold (Urbanski
1967, p, 333).

1. It facilitates an increase in the quantity of gases involved in the
explosion. K2SO4, K2S and CO2 are formed when sulfur is used. Without
sulfur, K2CO3 is a prominent reaction product.

2. Sulfur reduces the initial decomposition temperature. The following
table illustrates this point:[refer to table on page 22]

3. Sulfur intensifies the sensitivity of the mixture to impact.

4. Sulfur influences the type of reaction (explosion) products formed."

I would encourage anyone interested to read the full section. It has information on various carbon sources, their advantages and associated drawbacks.




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[*] posted on 27-11-2012 at 16:55


I've mixed coal, thoroughly crushed (but not flour consistency) with KClO4 and various other common stuff like dark Al. Mg turnings ,sulfur, and or sugar etc.

I found little use in coal as a fast blackpowder or flash substitute. It was difficult but I did find some steady burning mixes after trying at least 20 variations, despite its richness in complex energetic hydrocarbons (look at the amazing 100+ atom structure on wikipedia, only life could create such bonding- for those interested, a glance at chlorophyl types shows undeniable connection to coal molecules).

Charcoal worked much better, unexpectedly, but perhaps the very large molecules in coal mean basically that despite the huge energy potential in the bonds, there are so many new solids which could be created in reductive reactions but little gas.

This huge molecule probably weighs 1000x typical oxidizers, so it can be surprising how little oxidizer is needed and how adding more than 10% the weight of coal doesn't add much or can take away spark and burning effects(tried only with KClO4-using some conjecture in my reasoning here); weighing piles of each, a well mixed 10 coal to 1 KClO4 seemed best, and although Al and S created various additional effects, its best to start with 2 substances as it gets exponentially complex to understand after, and theoreticals aren't much use.

[Edited on 28-11-2012 by biomagir]




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[*] posted on 28-11-2012 at 03:53


Quote: Originally posted by biomagir  
richness in complex energetic hydrocarbons (look at the amazing 100+ atom structure on wikipedia, only life could create such bonding- for those interested, a glance at chlorophyl types shows undeniable connection to coal molecules).

i believe the structure you're referring to comes from lignin and not chlorophyll, the main structural non-polysaccharide component of wood. chlorophyll is only present in small amounts in dead trees.

coal vs. lignin:
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