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Does anyone else find this a little odd?  What is the atmosphere of Rimworld composed of, fluorine?

(I don't know whether or not to report this as a bug...)

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It is not a bug, as it's intended behavior.

I'm not sure how much I agree with it, since we all know that jet fuel can't melt steel beams, but you also can't mine steel right out of raw rock, either, so it's just one of those things you accept.

Well, maybe a modder can model how steel buildings react to fire. It's a bit beyond me, to be honest.

Perhaps we should rename it magnesium instead of steel?

(Ooooo, magnesium miniguns.  Watch what happens when *those* overheat!)

Quote from: Shurp on July 29, 2016, 08:44:44 PM
Perhaps we should rename it magnesium instead of steel?

(Ooooo, magnesium miniguns.  Watch what happens when *those* overheat!)

Yes, magnesium is flammable, but it is actually safe and very, very, very hard to ignite when in large structures. (e.g. car parts)

Did you ever did chemistry? Steel DOES burn.

Quote from: Shurp on July 29, 2016, 08:44:44 PM
Perhaps we should rename it magnesium instead of steel?

(Ooooo, magnesium miniguns.  Watch what happens when *those* overheat!)

I like your sense of humor.

Quote from: Mutineer on July 30, 2016, 01:22:09 AM
Did you ever did chemistry? Steel DOES burn.

My iron skillet on the range as never caught fire.  Sure, you can ignite steel wool fairly easily, but this was a steel *wall* on fire.  Yowza.

Quote from: Shurp on July 30, 2016, 09:23:27 AM

Quote from: Mutineer on July 30, 2016, 01:22:09 AM
Did you ever did chemistry? Steel DOES burn.

My iron skillet on the range as never caught fire.  Sure, you can ignite steel wool fairly easily, but this was a steel *wall* on fire.  Yowza.

When it comes down to it, it is all for better game play. You can always mod that if you really need it changed.

Well, it's just unexpected.  We know wood walls are going to burn and sandstone walls aren't.  On ice sheet play I often use my initial steel for walls, and all my doors are steel.  It never occurred to me that they would be flammable. 

I assume it is a coding issue -- that there is no way to keep windmills flammable yet make steel walls fireproof.

Quote from: Mutineer on July 30, 2016, 01:22:09 AM
Did you ever did chemistry? Steel DOES burn.

Did you ever do grammar?

Quote from: DariusWolfe on July 29, 2016, 07:27:01 PM
It is not a bug, as it's intended behavior.

I'm not sure how much I agree with it, since we all know that jet fuel can't melt steel beams, but you also can't mine steel right out of raw rock, either, so it's just one of those things you accept.

What you are mining is compacted steel - debris leftover from prior civilizations, ship impacts, etc.

Besides, if steel didn't burn, wood and plasteel would be the only flammables. I think. Stone and uranium (??) don't burn. Not sure about silver, gold, or jade.

Steel burns all the time on Earth... course it's a slow burn that most folks just call rust  ;)

Personally, I just think of in-game steel burning as being cut through. Acetylene torch style.

I'm always fighting the urge to drag steel ore inside so it won't degrade(rust)

Quote from: Havan_IronOak on July 30, 2016, 02:07:11 PM
Steel burns all the time on Earth... course it's a slow burn that most folks just call rust  ;)

Personally, I just think of in-game steel burning as being cut through. Acetylene torch style.

I'm always fighting the urge to drag steel ore inside so it won't degrade(rust)

I'd love to see steel ore rust when not stored inside. If Tynan don't make it so, why don't modders do such things? Mods seems to be mostly for making the game easier, not more about hard choices and priorities sadly.

Um, steel rusting is just the iron in steel reacting with oxygen in the air to make iron oxide (rust), so there is no reason steel would not rust inside.

Unless you mean to say there is no oxygen inside...

I think the main debate here is not about whether or not steel can burn on extreme circumstances, but rather what the circumstances are in which steel could burn. Steel does burn at high temperatures, to be sure; it's where all the flakes of slag and the like come from in metal processing. The oxidization of steel, even at high temperatures, is exothermic.¹ So okay, steel can theoretically burn. The question, to me, becomes: is it easy to burn?

The answer in the article cited above implies, reasonably, that it is not. For something like an open flame in an open environment, with a large hunk of steel at the edge, it's reasonable to expect not, as the rest of the steel will conduct the heat away from the point of contact. So a single spark lighting an entire steel wall on fire seems ridiculous, as I'm sure all here would agree.

What, though, if the steel wall is surrounded by fire? Soil is a reasonable insulator, so the ground likely couldn't siphon enough heat away through conduction to keep all energy out. So if a steel wall is surrounded by perpetual fire, sure, that could work. Mercifully, most fires in Rimworld burn themselves out. (If they didn't, I'd probably lose every colony I ever made to bad wiring and flash storms. ;_; ) So now we're looking at the following: could surrounding fire produce enough heat to ignite the steel wall before depleting its own fuel source?

Steel has a reasonable thermal capacity of about a half joule per gram-degree Celsius.² Let's assume, for the sake of example, a 1 m thick by 2 m tall by 10 m long steel wall surrounded on all sides by fire. Steel also has a density of about 8 g/cm^3. This can tell us the total energy required to raise the temperature of the wall to 600 *C, roughly the temperature of a match flame.

The volume of the wall is 1 m * 2 m * 10 m = 20 m^3. We want that in cm^3, so we multiple by (100 cm)^3 / (1 m)^3 = 20 000 000 or 2x10^7 cm^3 . Okay, cool. We know that would weigh 8 gm / cm^3, so we're looking at 1.6x10^8 g of steel. Let's say it's a nice 20 *C outside, so we need to go up 580 *C. With the specific heat capacity of 0.5 J/g *C, we're looking at 1.6x10^8 g * 580 *C * 0.5 J/g *C = 5.22x10^10 J, or 5 220 mega joules.

Okay, that's a lot of energy. Cool. How much wood would it take to yield that much energy? (We're going to ignore, for now, energy lost to the surroundings instead of transferred directly to the wall, though such losses would be significant.) An example wood, red oak, has combustible energy around 10.4 MJ/kg under less-than-ideal circumstances.³ That's a good start. Okay, so without heat loss to the environment, we're looking at a bit shy of 502 kg of red oak. Per the OneOak Project, oaks can weigh up to 14 000 kg.⁴

On the surface, it seems like an easy assumption to make that a burning oak next to a 1x2x10 m steel wall could easily set it ablaze. But here's the catch; how does that energy get from the oak to the wall? Air is a very good insulator, so we can reasonably ignore conduction. That leaves us with two methods; radiation and convection. Let's begin with radiation.

For radiation, heat is radiated out in all directions. That's neat, but we're only interested in heat heading towards the steel wall. Heat going away from the wall, up, or down into the ground are all ignored for this purpose; what we're left with is what I'll approximate to about an eighth. In short, for an arbitrary number of radiation vectors, I'll assert that only an eighth of them hit our wall. (To visualize, this is about a 90* arc wide and tall.) Unfortunately, here is where I hit the limit of my knowledge; I'm not a physicist by training, and though I could approximate and say, "About an eight of the total burnt energy gets to the wall" I could very easily be wrong. If anyone else has better knowledge of thermodynamics, I'd very much appreciate your input. :)

In the absence of an expert opinion, I'm going to take up the tried and true traditions of the internet and speculate wildly. I'm going to speculate that, given a bunch of burning trees surrounding a steel wall, they would heat the wall quickly enough and thoroughly enough to cause it to burn. This is only an opinion; I'm still hoping a physicist will come in to save the day with legitimate calculations, I stand stand eternally ready to admit I was totally wrong.

Anyway, that's enough of a wall of text; I hope someone found it interesting. :) I know I sure did; my first thought when coming into this was, "Phah, steel can't burn! Even slagging it takes lots of energy! Now way could steel have a self-sustained exothermic reaction!" I sure showed me.

EDIT: Also, Mossy Piglet, I think it was more about how steel rusts far more quickly in the presence of water than it does in only the presence of air.⁵ While steel should eventually rust unless it is, as you suggest, in an oxygen-free environment, the presence of water does seem to dramatically increase the rate at which rust accumulates.

¹: http://www.popsci.com/diy/article/2007-10/burning-metal
²: http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MQ304A
³: https://en.wikipedia.org/wiki/Wood_fuel#Energy_content
⁴: https://sylva.org.uk/oneoak/tree_facts.php
⁵: https://van.physics.illinois.edu/qa/listing.php?id=485

mabe we should justg build out of bricaks

OK, for people who did not do chemistry..

Demonstrations - Combustion in Pure Oxygen - Burning Iron

https://www.google.co.nz/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&uact=8&ved=0ahUKEwjxl9y8hJzOAhUILpQKHUjGBrYQFgg2MAQ&url=https%3A%2F%2Fwww.angelo.edu%2Ffaculty%2Fkboudrea%2Fdemos%2Fburning_iron%2Fburning_iron.htm&usg=AFQjCNEEiliuTn-NWQJywbuPuDaowA7bRg&sig2=6pZV9tB48HidpXZ1uJzTbw&bvm=bv.128617741,d.dGo


We do not know concentration of oxygen on every planet., even terraformed. My chemistry teacher demonstrated burning of steel in oxigen in our class. It was 40 years ago when we still had education in schools...

On additional note, many materials you think as non burning can easy burn in very unseal conditions. Aluminium for example. The only reason it is not burning because it constantly create layer of oxide, which is extremely strong and has very high melting point. But if you prevent creation of this layer, it will self ignite in water. Adding a little mercury to aluminium can quickly burn it (do not do it in hope, mercury is extremely poisoned)

Again, Mutineer, I don't think anyone is arguing that steel cannot possibly burn. I believe the argument is more that the conditions in which steel burns, particularly in terms of both total heat involved and the rate of heat transfer, are unlikely to occur in an uncontrolled environment. I happen to disagree, as per my previous post, but it's important to not misrepresent an opponent's argument; if we want to find some truth, and educate ourselves and/or others, we've got to understand what they're trying to say in order to decide if it has merit.

Also, I'll note that in a pure oxygen environment, sugar is a bomb waiting to go off, but I don't recall noting explosions from my food stockpiles. (True, there is no sugar there yet, but I hope you take my point that a high-oxygen environment is unlikely.) We can also gather that Rimworld is likely not a high-oxygen planet based on the lack of oxygen toxicity in the colonists. OSHA advises against a great than 21% oxygen atmospheres in a workplace.[sip]1[/sup] However, I don't like that number, as it seems low. Instead I'm going to go with 28%, which has been suggested to cause some pretty significant damage if used long-term.²  Given that we do not note hyperoxia in our pawns, I'm going to go ahead and say that we're not likely to have more than 28% oxygen on any Rimworld planets.

So yeah, Mutineer, your points are accurate, but I'm note sure they're relevant, given that we're looking at open flames in an open field interacting with unknown but presumably less than 10% carbon steel likewise in an open environment. Stuff like steel and aluminum CAN burn, yes, but the question we're asking is "is it LIKELY to burn?"

¹: https://www.osha.gov/SLTC/etools/shipyard/shiprepair/confinedspace/oxygendeficient.html
²:  New, A (1 February 2006). "Oxygen: kill or cure? Prehospital hyperoxia in the COPD patient". Emergency Medicine Journal 23 (2): 144–146.

Quote from: newcadence on July 30, 2016, 04:23:23 PM
mabe we should just build out of bricks

Personally I'd prefer if you build out of straw or at worst sticks.

-- B.B. WOLF--
;)

So if we're going to assume that the Rimworld atmosphere is 90% oxygen meaning that plate steel will catch fire if a tree next to it is burning, how much more flammable will iron filings become?

Because I think it's time for steel fuel-air bombs.  Spray iron dust at your opponents and light it up!!!

See, Shurp, that's the kind of creative and Dorf-like thinking that I've come to know and love from the Rimworld community. "This feature seems odd, and presents a huge danger. How can we weaponize it?"

Quote from: milon on July 30, 2016, 01:20:39 PM
Besides, if steel didn't burn, wood and plasteel would be the only flammables. I think. Stone and uranium (??) don't burn. Not sure about silver, gold, or jade.

Is there a problem with that? Why is PlaSTEEL flammable? "Advanced spacer tech structural material." So something that you build a starship out of, but you're completely fucked if a fire starts because it won't be long before your ship turns into a fireball. Noice. Game design reason of "things should burn because burning" aside.

Not only sugar, grain for example. and not in pure oxygen, in usual condition.

<<Time is of the essence in salvaging wet feed and grain. Both will begin to heat and mold very quickly, leading to spoilage as well as the possibility of spontaneous combustion. >>

https://www.ag.ndsu.edu/flood/farm-ranch/salvaging-stored-wet-feed-and-grain

What is the end game to this thread exactly? (to the OP)

1. Trying to get Tynan to make steel non-flammable in game?
2. Get him to change the name of steel to "flammable steel-like substance"?
3. Just have fun speculating about things?

I would prefer the first of those three options.

Quote from: brcruchairman on July 30, 2016, 03:53:25 PM
[work by the quoted author; too long to be quoted here]

While your post is an interesting read and certainly well-thought-out, there is a major problem with it.


You assume that at 600 °C, steel burns. This, however, is based solely on the temperature of a burning match, which might be an appropriate figure for a wood fire, but tells us nothing about steel. I have found a source indicating that the temperature of ignition of iron is somewhat higher, at 930 °C in an atmosphere of pure oxygen.¹ At a lower partial pressure of oxygen, the temperature of ignition is likely to be higher.

Under the assumption that the temperature of a match is equal to that of burning oak (according to your findings, 600 °C), it becomes clear that it is impossible to heat steel to the required temperature simply by burning wood, since the second law of thermodynamics prevents the flow of heat from a colder body to a hotter one on its own. However, the properties of oak trees and matches may be sufficiently dissimilar to allow for sufficient temperatures under certain conditions.


As an additional factor to be considered, in the oxidation of steel, iron oxide forms. Unlike the combustion products in more usual fires (mainly CO₂ and H₂O for wood), iron oxide is not gaseous at these temperatures. Therefore, it remains at the surface and blocks additional oxygen from reaching the unoxidized steel unless it is removed somehow.

According to Wolfram|Alpha², the melting points of various iron oxides appear to be too high in order for them to flow away from the point of oxidation at the given temperature. Not only would this prevent further combustion, but as a layer of iron oxide would form while it was being heated (since the greater heat would accelerate the rusting reaction, according to the Arrhenius equation), combustion might not start at all. On the other hand, in another study³, it was found that if exposed to oxygen during the heating process, iron ignites at slightly above 1400 K (or about 1150 °C). This indicates that oxides form during the heating process and inhibit ignition, but that reaching the melting points of the oxides as given by Wolfram|Alpha is not necessary for ignition. It is possible that this is because the melting point is lower, which can be caused by impurities (for example, different oxides mixed with each other, as is likely to be the case here).


In conclusion, though the previous findings indicate that it is possible to heat steel to a temperature of 600 °C, this temperature is insufficient for the ignition of iron, even under optimal conditions. However, it might be possible to reach sufficiently high temperatures anyway. Further examination is warranted.

Again, I would like to stress that I do not mean to attack your work, but merely add to it. I hope that my post was as interesting to read as yours.

¹Grosse, A.V. and Conway, J.B., 1958. Combustion of metals in oxygen. Industrial & Engineering Chemistry, 50(4), pp.663-672.
²http://www.wolframalpha.com/input/?i=melting+points+of+Fe_2O_3,+Fe_3_O_4,+FeO (http://www.wolframalpha.com/input/?i=melting+points+of+Fe_2O_3,+Fe_3_O_4,+FeO)
³Nguyen, K. and Branch, M.C., 1987. Ignition Temperature of Bulk 6061 Aluminum, 302 Stainless Steel and 1018 Carbon Steel in Oxygen. Combustion Science and Technology, 53(4-6), pp.277-288.

Quote from: Mossy piglet on July 30, 2016, 03:39:48 PM
Um, steel rusting is just the iron in steel reacting with oxygen in the air to make iron oxide (rust), so there is no reason steel would not rust inside.

Unless you mean to say there is no oxygen inside...

And gameplay reasons, if you've heard of those.

Concerning "plasteel", I assume this is steel reinforced carbon fiber.  And everyone who has watched auto racing other than NASCAR knows that carbon fiber burns nicely once it gets going.  All that's left of the car when it's done is a puddle and the steel engine block.  (Note that the engine block doesn't burn, even though it's in the middle of the fire...)

And the goal of the thread?  I dunno, it just struck me as odd and funny.  Like brcruchairman said, why complain about something if we can find a way to weaponize it instead? :)

It seems that under most outdoor conditions in rimworld, steel igniting temperatures cannot be reached. (No citation, feel free to disprove me if possible) But, indoors, it is not difficult to reach temperatures up to 2000 celsius. (alpha 13 alpha 14 data is null)  For scale this is twenty times the boiling point of water, and I dare say that is enough to combust any reasonable combustible material. (Also, this seems to be the temperature cap, suggesting it could ve higher yet but not showing) So, maybe steel cannot burn outdoors, but inside under less than perfect conditions, it seems quite possible.

The issue with real steel fires is not so much temperature as vaporization.  When you heat up wood hot enough the surface molecules break free and can mix with the oxygen in the air.  The boiling point of iron is 2862'C (thanks google!) so even if you have it hot enough to burn you're only going to torch the surface.

(Steel wool burns so nicely because it's *all* surface)

Quote from: Shurp on July 31, 2016, 02:41:24 PM
The issue with real steel fires is not so much temperature as vaporization.  When you heat up wood hot enough the surface molecules break free and can mix with the oxygen in the air.  The boiling point of iron is 2862'C (thanks google!) so even if you have it hot enough to burn you're only going to torch the surface.

(Steel wool burns so nicely because it's *all* surface)

Wouldn't it be sufficient to merely melt the iron oxide, causing it to flow away and allow the (still solid) iron underneath to oxidize? There wouldn't be a flame, but there isn't one either when burning steel wool and according to your own post, that counts as burning. The temperatures would be quite a bit lower: 1360 °C in the case of iron(II)oxide and even lower if there are impurities.

Quote from: Nonmomentus Brain on July 31, 2016, 06:17:54 AM
While your post is an interesting read and certainly well-thought-out, there is a major problem with it. <SNIP>

First off, *applause* Well played, sir; your work was really well thought out, well-sourced, and basically awesome. That's what I was hoping for when asking for a physicist. :)

The points you make are completely valid; I hadn't even thought of surface area as a limiting factor, though it was, as Shurp pointed out, critical in one of my examples. If steel walls were made of fractals, we might be more interested in volume and total heat, but with surface area being a limiting factor (for, as you you point out, we can't expect the steel OR the iron-oxide to melt off) it does seem that a steel wall burning would happen for maybe a few seconds and then the surface would be charred inert.

Mossy Piglet brings up the notion that indoor fires may be a different story, thanks to the confined space. Wood-air mixtures have been reported to be around 1980 *C¹, so a wood stockpile in a room could conceivably heat the room enough to, as you suggest, melt the iron(II)oxide off and burn the next layer of steel underneath. However, in such an environment I strongly suspect that the oxygen in the room would quickly become depleted, possibly before sufficient temperatures were reached.

Let's look at that. For starters, it looks like fires will suffocate themselves at 15% oxygen or less.² Okay, cool. How much energy can be released consuming the ~6% difference in oxygen in a room? Fortunately, I managed to find a pretty neat article about this.³ It tells us that each kg of oxygen can release, through combustion, 2.751 MJ of energy. It also tells us that a good baseline for air density is 1.2 kg/m^3, which gives us 3.302 MJ/m^3 of air space.

For the purposes of this analysis, I'm going to assume there's excess wood reactant mass, and the limiting factor is oxygen in the air. I'm also going to assume a sealed room, strictly to simplify calculations by removing new oxygen entering the system and heat leaving it except through absorption into the walls.

I'm going to take an arbitrary 10 m wide, 10 m deep, and 2 m tall room, with 1 m thick steel walls. (It's a bit short for a room, but it lets us use my figures from the previous calculations, and I'm lazy.) To heat one wall of that room to 600 *C would take 5 220 MJ. To heat it to 1 200 *C would take twice that, 10 440 MJ. To heat four such walls would take four times that, or 41 760 MJ. This is a bit shy of the iron(II)oxide melting point, but let's call it good for the sake of estimation.

Cool, so heating the walls enough to (almost) melt off the iron(II)oxide would take 41 760 MJ. I'm ignoring the floors and ceilings, but let's call it good for now. How much energy can be released by the oxygen in that room? At my convenient 10x10x2 m dimensions, we can see that there's 200 m^3 of air in there, so multiplying by 3.302 MJ/m^3 gives us 660 MJ of energy. That's actually pretty close. [EDIT2: Original figure read 66 040 MJ, but was pointed out by Nonmomentus Brain to have a misplaced decimal point. Remainder of post left intact.]

What this tells me is that it MAY be possible to burn steel in Rimworld. If you add ventilation (such as by opening a door or collapsing the roof) it'll alter the details dramatically. As it is, the figures are too close for me to trust my slipshod math to give much of a conclusion either way. So my official stance on the topic of steel burning in indoor fires is: "I'unno."

Of course, all this does absolutely nothing to add to or change the work which Nonmomentus Brain did on exterior fires; in those cases I'd be inclined to believe that so much heat would be lost to the environment that, as Brain points out, the iron(II)oxide on the surface would remain in place and prevent further steel combustion.

I'd also like to thank Shurp and especially Nonmomentus Brain for their contributions to this discussion; it's pretty pleasing to me to discover new truths, or at the very least strongly documented trends, and it's like they say: "You don't use science to prove that you're right, you use science to become right." Thanks, Brain; you helped me change my opinion to a better one. :)

EDIT: Added source #3. D'oh!

¹: https://en.wikipedia.org/wiki/Adiabatic_flame_temperature#Common_flame_temperatures
²: https://www.fpgltd.co.uk/suppression-products/gas/hipoxic-oxygen-depletion/
³: http://cfbt-us.com/wordpress/?tag=oxygen-consumption-principle

OMG, you're right, we're looking at this all wrong.  The issue isn't the steel, it's the oxygen.  Steel is 8+ times denser than wood.  The volume of oxygen you need to bring in to keep that surface burning compared to an equal surface of wood is stupendous.  Unless you have it seriously compressed or your spraying pure oxygen at it you'll never keep it lit. 

And I imagine steel burns *hotter* than wood (ie: releases more energy per molecule burned), which makes it even harder to keep enough oxygen present to keep the fire going (higher temperature combustion products pushing away fresh oxygen)

Magnesium has less than a quarter of steel's density.  No wonder it goes up like a candle :)

Quote from: brcruchairman on July 31, 2016, 03:28:51 PM
First off, *applause* Well played, sir; your work was really well thought out, well-sourced, and basically awesome. That's what I was hoping for when asking for a physicist. :) [snip]

Your high praise makes me very happy! Though I am not yet one, I do, in fact, plan to become a physicist eventually.

I am writing most of this past midnight, so there might be a number of errors. Please check anything that seems peculiar and point it out to me.

You seem to have forgotten to provide a reference for your source number 3 – would it be possible for you to correct that? I'm interested in how they got that number.

Another problem regarding your calculations of the energy potential from the oxygen is that you seem to have multiplied the given value with the entirety of the air instead of just the oxygen content.
Because of these problems, I shall try and determine myself how much energy we can get out of the air.
Firstly, I will have to determine what burns in order to find how much oxygen is needed and how much energy is released. Cellulose seems to make up roughly 50% of wood¹. In order to simplify things, I will pretend that wood is pure cellulose. The molar heat of combustion of cellulose is about -2800 kJ/mol² (the value is negative because energy is released). In case it seems peculiar to have a molar value for a polysaccharide such as cellulose, it seemed so to me as well. The paper gives the formula as C₆H₁₀O₅, which appears to be the smallest unit in the structure. Regardless, we can work with this. It is easy to find that the combustion can be described by the following reaction equation:

C₆H₁₀O₅ + 6 O₂ --> 6 CO₂ + 5 H₂O

We can see that for each mol of cellulose (as the source number 2 seems to define it), 6 mol of oxygen are required. Therefore, for each mol of oxygen, 470 kJ of energy are released. Assuming oxygen is an ideal gas, one mol takes up 22.4 L of volume. If 20% of the air is oxygen, the amount of oxygen is given by 40 m³/22.4 m³/kmol, which is about 1800 mol. Multiplying this with the previously established value for the energy release per mol of oxygen, we get 1800 mol * 470 kJ/mol = 850 MJ of energy released if all the oxygen is used up.

Another way to get to this value is to take the mass of the air (240 kg, assuming 1.2 kg/m³ of density and 200 m³ of volume), multiply it by 20% to get an approximation of the oxygen mass and divide by the molar mass (32 g/mol for molecular oxygen). This gives us about 1500 mol of oxygen, which is close enough to the other value. The difference is probably due to rounding errors and the fact that there is a difference between mass percentage and volume percentage. Simply multiply by the 470 kJ/mol and we get a value of 705 MJ.

For comparison with your source, we can also calculate the energy released per kilogram of oxygen: 470 kJ/mol / 32 g/mol = 15 kJ/g or 15 MJ/kg. This greater than your source's value of 2.75 MJ/kg by a factor of more than 5 – again, I would be very interested to read that article.

Of course, what you have already noticed is that my value for the energy released is lower than yours by a factor of almost 80. I was quite confused by this until I checked your calculations as well – you appear to have dropped a decimal point when calculating the energy released from the 200 m³. It should be 660 MJ rather than 66000. As you can see, this is far less than the 41000 MJ required to heat the walls up.

Of course, these values are still too high, since the air itself must also be heated (we'll ignore the fuel).
Assuming 240 kg of air and a specific heat capacity of 1 kJ/kgK³, we need about 290 MJ of energy to heat the air by 1200 °C. If we go with the most optimistic result, that leaves about 560 MJ to heat the walls.

It is obvious that this is insufficient to heat the entire wall. However, because we are assuming a wood-air mixture, the energy will be released very rapidly due to the massive surface area. Since the assumption that the entire wall would have to be heated was based on the premise that the heat would be conducted away otherwise, we might relax this requirement since there might simply not be sufficient time in order for that to happen.
So, if we don't have to heat the entire wall, how much of the wall can we heat? The maximum depth up to which we can heat the steel by 1200 °C is given by 550 MJ/(1200 °C * 8 g/cm³*80 m²*0.5J/(g*°C)), in other words the energy we put into it divided by the product of the temperature change, the density of the material, the area and the specific heat capacity. This gives us a depth of about 1.4 mm. This may not seem like much, but due to the exothermic nature of combustion, the burning steel might keep the reaction going.

In addition, if the heating is sufficiently rapid to prevent the formation of an oxide layer before ignition, only about 900 °C of heating are required (see my previous post), which would allow the steel to be heated sufficiently to a depth of 1.9 mm.


¹Pettersen, R.C., 1984. The chemical composition of wood. The chemistry of solid wood, 207, pp.57-126.
²Colbert, J.C., Xiheng, H. and Kirklin, D.R., 1981. Enthalpy of combustion of microcrystalline cellulose. JOURNAL OF RESEARCH of the National Bureau of Standards, 86(6), pp.655-660.
³http://www.engineeringtoolbox.com/air-specific-heat-capacity-d_705.html (http://www.engineeringtoolbox.com/air-specific-heat-capacity-d_705.html)

---

Quote from: Shurp on July 31, 2016, 06:56:22 PM
OMG, you're right, we're looking at this all wrong.  The issue isn't the steel, it's the oxygen.  Steel is 8+ times denser than wood.  The volume of oxygen you need to bring in to keep that surface burning compared to an equal surface of wood is stupendous.  Unless you have it seriously compressed or your spraying pure oxygen at it you'll never keep it lit. 

And I imagine steel burns *hotter* than wood (ie: releases more energy per molecule burned), which makes it even harder to keep enough oxygen present to keep the fire going (higher temperature combustion products pushing away fresh oxygen)

I'll see if I can back your assumptions up with a few calculations. Again, I might make mistakes, so be careful and if something looks wrong, just check and ask.

I will look at the volume of the different substances that can be oxidized by the same amount of oxygen. While I am aware that I thereby fail to take surface conditions etc. into account, it should be sufficient to shed some light on whether the assumption in your first paragraph is true.

If we assume that when steel combusts, iron(II) oxide is formed (this is the oxide that would require the least amount of oxygen), then for each atom of iron, one atom of oxygen is required. Since in an atmosphere, only molecular oxygen O₂ and not atomic oxygen is present in any meaningful amount⁴, this means that each molecule of oxygen can oxidize two atoms of iron. As a reaction equation:
2 Fe + O₂ --> 2 FeO
The molar mass of iron is about 56 g/mol. This means that each mol of oxygen can oxidize 112 g of iron. The density of steel has previously been established as 8 g/cm³. A simple division shows that a mol of oxygen can oxidize 14 cm³ of iron.

As discussed above, each mole of oxygen can oxidize on sixth of C₆H₁₀O₅, the substance that makes up a large percentage of wood. Its molar mass is 162 g/mol. Therefore, one mol of oxygen can oxidize 27 g of cellulose. The density of wood varies widely, so I will use the density of cellulose instead (which is a good idea, since that's the substance for which I've actually calculated anything so far). It seems to be around 1.6 g/cm³.⁵
Using the same method as before, it can be calculated that each mole of oxygen can oxidize about 17 cm³ of cellulose. This is higher than the value for steel, which supports your assumption. However, the difference is quite small and cannot account for the difference in flammability observed.

In response to your second paragraph, I will examine how much energy is released by the combustion of cellulose and iron.
As can be seen above, the molar heat of combustion for cellulose is about -2800 kJ/mol.
The molar heat of combustion for iron is easily calculated if one has knowledge of the reaction equation (which is given above) and the heat of formation of every involved substance. Since the heat of formation is defined to be 0 for the most stable form of an element, the only substance that must be taken into account here is iron(II) oxide, the heat of combustion of which is -272 kJ/mol.⁶ Since the heat of combustion is given by the sum of the heats of formation of the products minus the sum of the heats of formation of the reactants and the heats of formation of the reactants are 0 in this case, the heat of combustion is -272 kJ/mol as well.

As you can see, this contradicts your assumption that steel "releases more energy per molecule burned" (technically, steel doesn't consist of molecules, actually). However, it is clear that this is not because you what you meant is wrong, but because your wording is. Since organic molecules consist of many atoms, they cannot be compared directly to the single metal atoms in this case.

Instead, we might look at the energy per volume, which is more relevant here.
Using the values given above, we can easily calculate that each of cellulose produces about 28 kJ per cm³.
We can equally easily calculate that the corresponding value for steel is 39 kJ per cm³.

Again, it can be seen that the assumption is confirmed, but the difference is not as great as expected. However, there is a significant difference in the temperature of combustion of the two materials. While wood burns at 600 °C (as has been established previously), pure iron ignites at temperatures beyond 930 °C (as previously established). This is not because of the heat released in combustion, but because of the activation energy required to start the reaction.

Furthermore, there's a problem with your assumption that the hot combustion product would "push away" fresh oxygen: the combustion products of steel, various iron oxides, aren't gaseous. In fact, I presume that the higher temperature would only serve to increase convective airflow, thereby bringing in more oxygen. I admit, I didn't think of that either while calculating the numbers above.

The lower density of magnesium is largely because of the lower mass of the magnesium atoms themselves. The actual difference in oxygen required is much lower than you might think.

I hope I could help.

⁴You are alive.
⁵Sun, C.C., 2005. True density of microcrystalline cellulose. Journal of pharmaceutical sciences, 94(10), pp.2132-2134.
⁶http://www.wolframalpha.com/input/?i=heat+of+formation+of+iron%28II%29+oxide (http://www.wolframalpha.com/input/?i=heat+of+formation+of+iron%28II%29+oxide)

---

Good night. I hope my sleepiness did not introduce too much error (yes, I know I should have waited until I was well-rested with something like this, but it was too much fun).

Nonmomentus Brain, you may be my new favorite person on the internet. I salute you, good sir. *salutes* Your work is more thoroughly researched, sourced, and clearly written than my own. It brings a tear to my acadamian eye. I'd also like to say that this may be my favorite internet discussion to date, if only for the fact that, even while being an active participant, I've changed my opinion and conclusion four times and counting. Now that's discourse! 

Regarding the source, d'oh! I've added it in an edit. It also appears that, while I played fast and loose with my terminology (e.g., I said "2.75 MJ / kg Oxygen" rather than the more accurate, "2.75 MJ / kg air") I believe our numbers end up agreeing; the value for pure oxygen in the article cited was 13.1 MJ/kg O2, which is very close to your calculated value of 15 MJ/kg O2. So again I salute you; you deduced a figure I had to look up, and did it very accurately. Well done!

Regarding my figures being off for the energy produced, again, good catch! I think I tried converting to kJ in my head and lost track of the units; your figure of 660 MJ appears to be accurate. I'll edit it in my above post for accuracy, but credit goes to you for pointing it out. :) Thank you!

The point regarding heating the air is also well-taken; convection will transfer heat well, but it isn't free, energy-wise. I also like your point on the topic of partial heating of the wall. Given that the article cited above (Hartin) specified that a fire would smother itself in a 2.4 x 3.7 x 2.4 m room in minutes, I think we can assume that the rate of heat transfer may be sufficient to overcome conduction through the steel wall, as you suggest.

However, I think we run into the problem, in that case, oxygen depletion. With 1800 mol of O2 in the room to begin with, we can expect around 1250 mol left by the time the wood fires have smothered. Unfortunately, I'm not sure at what oxygen percentage an iron fire would smother itself, so I'm going to assume total depletion for the sake of argument. (If a figure can be found for the minimum air oxygen percentage for an iron fire, I'd be very interested in that.)

So we've got 1250 mol O2 left to burn, literally. How much steel can that oxidize? I'm going to borrow your calculations for iron densities, and use one mol O2 for 14 cm^3 of iron. With 1250 mol O2, we get 17 500 cm^3 of iron, which if divided evenly among the wall's surface area of 2 m x 10 m x 4 walls=80 m^2=800 000 cm^2, we get a depth of 0.022 cm, or about a fifth of a milimeter. Of course, this assumes that the walls burn only AFTER the wood fuel, which is not a reasonable assumption. It is, however, a simple assumption, and it being late, I think I'll stick with it for now.

So for an indoor fire, it seems that the lack of oxygen really dooms our steel wall to a sad life of noncombustion. I was trying to find information on outdoor fires, particularly what sort of oxygen and air flow they can be expected to get, but the twenty seconds I spent looking turned up nothing, and I'm a bit too tired to do a more in-depth analysis at the moment. A job for another day, I think. Especially given that in such an example I'd also have to look at heat lost to the environment, and that's a whole can of worms I am far from qualified in dealing with. :p

In any case, for the closed room I'm comfortable saying there won't be a steel fire due to air constraints, though if the door were left open, that may change things. More research for us to do tomorrow, methinks. :) In any event, I'd like to again thank you, Shurp and Brain, for a really fun and enlightening conversation. I'll be hoping for more awesome posts in the coming days. :)

I don't deserve such a good opinion – all I did was expand on your great work! I won't deny that I appreciate it, though.

Thank you for giving the source! It was an interesting read. I hadn't heard about Thornton's rule before.

Now that we've ruled out ignition of the walls in a closed room, let's look at rooms with ventilation. I will assume that the required oxygen is brought into the room and the gaseous combustion product are expelled, but the insulation of the walls, ceiling and floor is still perfect.

Since the released energy is now, again, determined by the amount of wood rather than the amount of oxygen, it is necessary to determine how much wood we can store within this room. Since I know next to nothing about fluid dynamics, I will simply assume that if 50% of the volume is taken up by wood, that leaves enough space for the air to flow. This gives us 100 m³ of wood. If we assume red oak (which is the type of wood used in your first post, so we already have the released energy per kg), that gives us a density of 0.63 g/cm³.⁽¹⁾ Therefore, we have 63000 kg of wood, which is equivalent to about 650 GJ, according to the value of 10.3 MJ/kg given by you.

Of course, not all of this heat is used to heat the walls. Some of it is used to heat up the oxygen that enters the room, while some of it is lost because hot CO₂ and H₂O leave the room.
To find these values, it is necessary to know how much of those gasses must be heated. Assuming, again, that the wood is entirely composed of cellulose, the previously established reaction equation C₆H₁₀O₅ + 6 O₂ --> 6 CO₂ + 5 H₂O is useful again.
If the mass of the wood is 63000 kg and the molar mass of cellulose is 162 g/mol, that gives us 390 kmol of cellulose. With the factors from the reaction equation, it is easy to calculate that the molar amounts of oxygen and carbon dioxide are both 2300 kmol, while the molar amount of water vapor is 1900 kmol. One can already see that the amount of oxygen inside the room (1.8 kmol, according to my previous calculations) is little more than a rounding error here.
Knowing the molar heat capacities of oxygen, carbon dioxide and water vapor (29.4 J/mol*K, 37.1 J/mol*K and 33.6 J/mol*K, respectively)⁽²⁾ and assuming a temperature increase of 1200 °C, one can calculate that heating the oxygen takes 81 GJ of energy, while the heat loss caused by carbon dioxide is 102 GJ and that caused by the water is 77 GJ. The combined loss is 260 GJ, still far less than the energy released through the combustion of the wood.

However, we cannot filter the oxygen from the outside air and only pump that in. A large amount of (for this purpose) inert nitrogen will be brought in as well, and it well be heated as well. If, by molar amount, 20% of the air is oxygen and 80% nitrogen, that means that 9200 kmol of nitrogen will have to be heated as well. With the heat capacity of nitrogen being 29.1 J/mol*K⁽²⁾, this means that heating it to 1200 °C will need 320 GJ.

This leaves 70 GJ (or 70000 MJ) to heat the walls. According to our previous numbers, that should be more than enough to heat the entirety of the walls to the required temperature. When I, however, decided to calculate what the increase in temperature would be, I noticed that it was far lower than 1200 °C. This lead me to check your initial value for the energy required to heat one wall. I then noticed that you were off by a factor of 10 when converting from joules to megajoules, resulting in a value of 5220 MJ instead of the correct 52200 MJ. Fortunately, this error did not cause our conclusions to be wrong, since it only reinforces our stance that it is not possible. Furthermore, when I calculated the same value using Wolfram|Alpha, the result was a mere 41000 MJ⁽³⁾.

After having discovered this error, I decided that instead of calculating the temperature change of the entire wall, I would once again calculate the depth to which a temperature change of 1200 °C could be caused, using the same method as previously. The result was 20 cm, which should be sufficient to ignite the wall.

The next question to be answered is whether the exothermic oxidation of iron is sufficiently exothermic to be self-sustaining. In other words, does the combustion of a certain amount of iron provide sufficient energy to ignite at least the same amount of iron?

To answer this question, one first must find the processes that take place in the combustion of iron in our scenario. I assume that at first, a certain (oxide-free) volume of iron is at the required temperature. Then, it oxidizes, with the required oxygen being at the required temperature. The energy released in the oxidization melts the iron oxide to allow for the oxygen to reach the iron underneath, which is heated by the oxidization of the first volume. Furthermore, sufficient oxygen to oxidize the iron underneath is heated as well. Of course, I know that in reality, these are not discrete steps. It is, however, a useful simplification.

For my calculations, I assume that a volume of 1 cm³ oxidizes and must provide enough energy to melt the resulting iron oxide and heat the 1 cm³ of iron underneath as well as the required oxygen by 1200 °C. As has been shown in my previous post, the oxidization of 1 cm³ of iron into iron(II) oxide releases 39 kJ. This volume of iron has a mass of 8 g and, due to the molar mass of 56 g/mol, contains 0.14 mol of iron.
Due to the coefficients in the reaction equation 2 Fe + O₂ --> 2 FeO, 0.14 mol of iron(II) oxide are formed. The molar heat of fusion of iron(II) oxide is 24.1 kJ/mol⁴, which gives us 3.4 kJ of energy required to melt the iron(II) oxide.
The heating of the next volume requires 4.2 kJ of energy, as can be calculated from the temperature difference of 1200 °C, the molar amount of 0.14 mol and the molar heat capacity of 25.1 kJ/mol⁽²⁾. 0.14 mol of iron require 0.07 mol of oxygen for oxidization. This amount of oxygen requires 2.5 kJ to be heated.
Overall, of the 39 kJ released, 10 kJ are required to keep the reaction going; therefore, the net release of energy is 29 kJ/cm³. If all walls (80 m³ of steel) combust completely, this would mean a release of 2.3 TJ of energy, which seems too high, since 29 kJ/cm³ is only slightly higher than the 28 kJ/cm³ I have calculated for wood in my previous post and the volume of the steel is 80% of that of the wood. This would indicate that there would not be a factor 4 between the two values. However, in this post, I have used values for red oak instead of values for cellulose, as I have done in the previous post. I shall calculate the energy released by the combustion of 100 m³ of cellulose.

100 m³ of cellulose have, given the previously established density of 1.6 g/cm³, a mass of 160000 kg. Dividing by the molar mass of 162 g/mol, this gives us 1000 kmol of cellulose. Knowing that the heat of combustion of cellulose is -2800 kJ/mol, it can easily be calculated that the energy released in the combustion of 1000 kmol of cellulose is 2.8 TJ, which agrees with the value of energy received for the combustion of the steel.

This also shows that cellulose and wood are not the same material and should not be treated as such. It is necessary to carefully consider whether they are sufficiently similar to be seen as equivalent for any particular situation. However, I do not think that the previous results are to be considered wrong, since the amount of oxygen required to burn a certain amount of cellulose should be similar to that required to burn a certain amount of wood.

In conclusion, if the necessary temperatures can be reached, it seems that a storage room for wood can ignite steel if it catches fire and the ventilation systems are sufficient. Furthermore, it seems that fire can spread through steel objects, if sufficient oxygen is present.

¹http://www.wolframalpha.com/input/?i=density+of+red+oak (http://www.wolframalpha.com/input/?i=density+of+red+oak)
²http://www.wolframalpha.com/input/?i=molar+heat+capacity+of+O_2,+CO_2,+H_2O,+N_2,+Fe (http://www.wolframalpha.com/input/?i=molar+heat+capacity+of+O_2,+CO_2,+H_2O,+N_2,+Fe)
³http://www.wolframalpha.com/input/?i=%28specific+heat+capacity+of+iron%29*%28density+of+iron%29*%2820m^3%29*%28580+%C2%B0C%29&rawformassumption=%22UnitClash%22+-%3E+{%22%C2%B0C%22,+{%22DegreesCelsiusDifference%22}} (http://www.wolframalpha.com/input/?i=%28specific+heat+capacity+of+iron%29*%28density+of+iron%29*%2820m%5E3%29*%28580+%C2%B0C%29&rawformassumption=%22UnitClash%22+-%3E+%7B%22%C2%B0C%22,+%7B%22DegreesCelsiusDifference%22%7D%7D)
⁴http://www.wolframalpha.com/input/?i=molar+heat+of+fusion+of+iron%28II%29+oxide (http://www.wolframalpha.com/input/?i=molar+heat+of+fusion+of+iron%28II%29+oxide)

So are you guys basically saying that Superman "The Man of Steel" is also The Torch ???   

Wait, wait, wait.

Is this the old "jet fuel can't melt steel beams" discussion?
I could swear I already saw this.

Quote from: Wex on August 03, 2016, 08:34:36 AMWait, wait, wait.

Is this the old "jet fuel can't melt steel beams" discussion?
I could swear I already saw this.

I cracked that joke back on the first page, but alas, no one seemed to notice...

Quote from: Spectreofoz on August 03, 2016, 03:25:38 AM
So are you guys basically saying that Superman "The Man of Steel" is also The Torch ???

Yes. Provided that superman uses his own laser vision to heat himself to his own ignition point. Philosophical aside: could Superman be so tough that even he couldn't break himself?

Quote from: Wex on August 03, 2016, 08:34:36 AM
Wait, wait, wait.

Is this the old "jet fuel can't melt steel beams" discussion?
I could swear I already saw this.

Heh, yes, but unfortunately for the joke we're not looking for the melting point of steel, (which, incidentally, is not required for structural weakening) but rather the ignition point. That, plus Brain and I are being total science nerds right now and nibbling away on an interesting problem; 'aint no time for jokes when MATHS and CHEMISTRIES is callin'!

Quote from: Nonmomentus Brain on August 01, 2016, 08:06:42 PM
[snip] In conclusion, if the necessary temperatures can be reached, it seems that a storage room for wood can ignite steel if it catches fire and the ventilation systems are sufficient. Furthermore, it seems that fire can spread through steel objects, if sufficient oxygen is present. [snip]

Well, sir. *salutes* Your work, as always, is excellent. I was even going to chime in about enthalpy of fusion but upon rereading your post; you've already included it! So all that's left to do is shake your metaphorical hand and convey my heartfelt appreciation for a truly awesome conversation. :)

Unfortunately, this seems to be where my utility ends; the next step, a steel fire in an open environment, is very far beyond my training; having neither thermodynamics nor fire management training, I haven't the foggiest idea of how heat and energy would move in such an environment, and haven't the background to accurately interpret any literature I find on the subject. However, the knowledge we've gotten so far (specifically, the risk of a steel fire in a closed room is negligible, while a properly ventilated room acting as a blast furnace could harbor such fires) jives so well with what seems to be the case in reality that I am content. This has been a fantastic and very fun exercise in finding answers to on-the-surface silly problems. Randal Munroe would be proud. :)

I hope to see you in other threads sometime. :) In any event, one last time, Nonmomentus Brain, thank you; it's been a pleasure.

¹: http://broadneck.org/wohlfarth1/Energy%20and%20Specific%20Heat%20Honors.pdf bottom of p. 3

Quote from: Shurp on July 30, 2016, 12:06:55 PM
...
I assume it is a coding issue -- that there is no way to keep windmills flammable yet make steel walls fireproof.

Nope. Flammability is a mod-able stat for pretty much all "ThingDefs".
Windmill Flammability is set to 0.4
Wall is set to 1.0, but the steel Item is set to 0.2

So buildables you can customize the material for(like walls) use the stuff they are built out of for Flammability, while buildables that can't be customized use their own Flammability stat.

You can even see the Flammability stat in game via the Info popup.
Select the buildable you want to build, then click the "i" on the left side of the screen.
EDIT:
A note of clarification: In the info popup for items like steel or wood(that can be used to build stuff), there are two Flammability entries. The top most one is for the item itself and the other is for objects made of that item. Interestingly, steel as an Item is not flammable but imparts a 20% Flammability to things that are made from it. I assume this difference accounts for the small amounts other materials necessary to build it that are abstracted out. (like how building a steel bed only requires steel even though your colonists are clearly not just sleeping on a slab of steel)

Yes, for the steel bed this makes sense -- you've added a mattress, blankets, and pillows that are flammable.  But what are you putting on a steel wall? 

Maybe it's painted with the same rocket fuel as the Hindenburg?
https://en.wikipedia.org/wiki/Hindenburg_disaster#Incendiary_paint_hypothesis

[edit] Hmmm, this actually almost makes sense.  A plain steel wall would rust which would look annoying.  Colonists would be silly enough to paint it with something highly flammable :)

Quote from: Shurp on August 06, 2016, 03:01:27 PM
Yes, for the steel bed this makes sense -- you've added a mattress, blankets, and pillows that are flammable.  But what are you putting on a steel wall? 

Maybe it's painted with the same rocket fuel as the Hindenburg?
https://en.wikipedia.org/wiki/Hindenburg_disaster#Incendiary_paint_hypothesis

[edit] Hmmm, this actually almost makes sense.  A plain steel wall would rust which would look annoying.  Colonists would be silly enough to paint it with something highly flammable :)

I highly doubt a steel wall is just a big solid slab of steel.
It is likely a hollow frame with steel plates bolted onto it.
Flammability would depend on what that frame and the bolts(or whatever is use to latch or secure the plates) is made of. Because once those go its not really a wall anymore.
It should probably drop the steel as an item tho.
Maybe not recovering non-flammable materials from fire destroyed buildings is the game balance issue?

Well, if I'm putting up big plates of steel, I'll probably use a steel frame and steel bolts, right? :)

Quote from: Shurp on August 06, 2016, 04:47:29 PM
Well, if I'm putting up big plates of steel, I'll probably use a steel frame and steel bolts, right? :)

Yea, I would imagine so. But that doesn't apply so much for Stone, Silver, or Gold walls.
I mean you wouldn't make a gold wall on a gold frame with gold bolts. Stone walls would require mortar to bond the bricks together, ect...

Yet you don't have to worry about the materials that would be use to build those parts. You don't need mortar for stone walls and you are not using extra steel(or wood) for the frame of Gold or Silver walls. To say nothing of some players that would want to purposefully build the frame out of wood to save building costs.
Which implies the frame and such is abstracted out as "a flammable material that doesn't cost anything extra to construct the building". Hence why I suggested not recovering non-flammable materials from fire destroyed buildings is the source of the balance issue, instead of the steel structures themselves being flammable.

Well I build everything out of stone as soon as stonecutting is researched, so there's not much of a balance issue anyway.  I just thought it was hilarious that a lightning strike had set an abandoned steel building on fire, and I was trying to imagine what physics would cause this. I've decided that the builders used an advanced polymer paint to offer 1000-year protection against the elements... but one which burns so hot that it ignites the steel underneath.