Metallurgy in the Carboniferous- Just how much hotter are fires?

The present day atmosphere is roughly 21% oxygen. Historically high-levels might have been as much as 35%, over 66% higher. This made the carboniferous a period of giant bugs and constant devastating wild-fires. How would this affect a civilization trying to develop? In particular, how much easier would it be to construct a forge/smelter for processing iron or steel? How much hotter will a forge/smelter burn if it's being fed with 66% more oxygen?

Cast Iron melts at around 2,100F (1150C). With a very naive calculation one would think a forge able to melt Cast-Iron on Earth today might burn at over 3000F, enough to melt low-carbon steel.

(Additional Edit: I had in mind a bloomery, similar to ones made over 2,000 years ago, which involve stacks of charcoal, as well as compressed air provided by a trompe. These forges burned hot in our 21% atmosphere, hot enough so that large ones could melt pig iron. What would happen if you used a similar one in 35%?)

• You generally don't want your forge too hot, then the material either burns or melts. for smelting it may be an advantage but for a forge it may very well be a disadvantage. Making a forge to work iron is not hard it is the smelting that is difficult. Part of that is heat but the other big part is getting rid of the impurities that melt along with the iron which took a lot longer to develop than generating the heat. . – John Mar 14 '20 at 4:25
• Modern smelter: son, fan harder. Carboniferous smelter: son, remember to set timer. – user6760 Mar 14 '20 at 10:55

2 Answers

One of the common misconceptions is that high concentrations of oxygen lead to hotter fires by default; it's actually fairer to say that high concentrations of oxygen lead to faster fires by default, but that in turn can create a hotter fire in certain circumstances.

Take a look at this oxygen enriched fire safety article which talks about this in great detail and you will note that on page 5 there is an example of them literally detonating a cotton shirt in an oxygen rich environment. Sure, in a case where you trap the oxygen and have lots of fuel ready to burn in an enclosed space, your fire will be hotter because so much more of the heat energy is released by the chemical reaction in fire all at once. And in this vein, the important point is this:

Fire, even in an oxygen rich environment, is still a triad of conditions;

1) Heat
2) Fuel
3) Oxygen

You need all three to create fire, and the heat of the fire will depend on the density of all these three components in proximity. The Carboniferous period ensured that the availability of the Oxygen is not the weak point in the triad, but you still need heat and fuel. I'd imagine that if you put a lot of dense wood, tightly packed, together in a single small area and set it alight, you could gather temperatures close to what you're describing for a short period, but like all things there is only so much fuel to be burned in this scenario and in achieving higher temperatures for your fire, you're burning your fuel faster. I suspect you'd be able to do it, but you'd need a lot of fuel, and people loading more wood onto your fire pretty much constantly.

Remember, energy is measurable, and we know how much heat energy can be released from wood through fire. If your fire is hotter, made possible by increased concentrations of oxygen, it's only because your wood is burning faster, meaning that a wood fire of (say) double the heat intensity will still need double the amount of fuel to maintain for the same period of time.

• Faster fires are hotter, though. If you increase the wattage of your fire, the temperature will also increase unless you also increase convection rates, which won't naturally happen. – ckersch Mar 13 '20 at 23:00
• One of the biggest impediments in metallurgy was (despite any amount of combustive material) the low temperature. This was improved through the use of bellows, the main function of those was to bring more oxygen to the fire. – Alexander Mar 13 '20 at 23:23

Just as a back of the envelope calculation: for cellulose, $$\text{C}_6\text{H}_{10}\text{O}_5+6\text{O}_2\rightarrow6\text{CO}_2+5\text{H}_2\text{O}$$, $$\Delta H=-2828\,\text{kJ}/\text{mol}$$. The molar specific heat for $$\text{CO}_2$$ is $$C_p=36.94\,\text{J}/\text{mol}$$, for $$\text{H}_2\text{O}$$ is $$C_p=37.47\,\text{J}/\text{mol}$$, and for $$\text{N}_2$$ is $$C_p=29.12\,\text{J}/\text{mol}$$. Assuming air is mole fraction $$f$$ of $$\text{O}_2$$ and $$1-f$$ of $$\text{N}_2$$ the molar heat capacity of the products of combustion counting $$\text{N}_2$$ is $$6\times36.94+5\times37.47+6\frac{(1-f)}f\times29.12=408.99+\frac{(1-f)}f\times174.72\,\text{J}/\text{mol}$$ So dividing the $$2828\times10^3\,\text{J}/\text{mol}$$ by this number we get $$\Delta T=2652°\text{C}$$ at $$f=0.21$$ vs. $$3650°\text{C}$$ at $$f=0.35$$.

Of course you're not going to get perfect combustion and heat capacities increase with temperature, but I think that the necessity of having to heat up extra $$\text{N}_2$$ among the products of combustion will reduce the fire temperature in any case on going from an $$\text{O}_2$$-rich to an $$\text{O}_2$$-lean atmosphere.