Hell turns out to be a real, physical place. It’s a physical planet. I have most of the science figured out for this literal hades, except the heat engines used in here need a cold sink—a strong one like at the absorbed by the thermal decomposition of D-block oxides (CaO was considered but decomposition happens at such a high temperature I couldn't contain it).

Anyway, it's a real planet. Here is where my engines need to work:

It has a sun that you never see because of perpetual cloud cover. No oxygen, so no combustion engines. Ambient temperature is 420°C. The planet has everything earth does except life. There is abiogenic petroleum near the surface but no coal. The planet is dead so whatever they don’t have, like liquid water or oxygenated air, they make. The planet has every inorganic natural resource they need They Make water; (Water has to be made from methane and carbon dioxide by power from windmills driving a sabatier engine). They control their habitats. (Cold air is made by air compressors.) They travel (they made trains and steam tractors). They make oxygen (natural potassium chloride solution is electrolyzed to potassium chlorate to finally generate oxygen). No question, it is Hell.

But for the story, I can’t find the heat sink reaction. The only high energy endothermic reactions I can find are thermal decomposition of D-block oxides (like CaO). That’s this question. The only thing they needed for an engine was a heat sink to cool the condensation tanks. But a practical one, working at reasonable temperatures, I can’t find. Whatever it is, it goes through an endothermic chemical reaction absorbing almost half as much energy per kilogram as oxidizing anthracite releases. What is special about anthracite coal? Coal was what powered the Boulton-Watt steam engines for 200 years, and those engines were only 3% efficient. Too much info; bottom line is, engines will be 7% efficient on this planet so I only need half as much delta heat.

What decomposing metal oxide or other natural process could run their condensation tanks with a $\Delta \textbf{H}$ close to half as large as anthracite?

(An endothermic reaction is a chemical process which absorbs heat energy. Ammonium nitrate and water are an example, but that is not energetic enough to run large engines)

  • The physics are real here. Only real naturally occurring chemical reactants can answer the question. Everything naturally occurring and inorganic on earth occurs there.
  • there is no magic here. Physics is all the same.
  • No opinions or imagined chemical reactions.
  • Freon or refrigerants can’t do it for vehicles. How do you condense the freon?

The reaction cools and drives the condensation tank exact l’y like a steam locomotive. But here, they use the condensation stage of their steam engines to draw steam through turbines rather than drive wheels by pistons. (Yes, same concept as my horses).

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Commented Feb 24, 2022 at 6:54
  • $\begingroup$ @Vogon Poet You have wind power. You can generate electricity. Use electric locomotives to run train and use Ambri batteries to run vehicles. By the way, HELL is a painful place for punishment. It is not just a hot place. If there is no intense pain, it is not HELL. $\endgroup$
    – imtaar
    Commented Mar 16, 2022 at 9:00
  • $\begingroup$ Off topic from the question: How does one get oxygen from KCl? You can't turn KCl into KClO3 without oxygen. $\endgroup$
    – user86462
    Commented Mar 16, 2022 at 10:45
  • $\begingroup$ @SeanOConnor It's in solution, I added that in. Thanks $\endgroup$
    – Vogon Poet
    Commented Mar 16, 2022 at 13:30

2 Answers 2


Unfortunately, the most endothermic spontaneously occurring reactions that are likely to be found are things like the dissolving of ammonium nitrate in water, which consumes 321 J/g of Ammonium nitrate. Certainly there won't be any spontaneous endothermic chemical reaction which absorbs even anywhere near 1000 J/g, let alone 12,500 J/g as the OP desires, that being half of anthracite combustion's energy release of 25,000 J/g.

The main problem with ammonium nitrate is that its dissolution requires liquid water to start with, and that's going to be hard to come by in the OP's environment.

Considering that there is petroleum on this world, an internal combustion engine would be a better idea than a condensation steam engine. Such petroleum would likely be heavy, high-boiling-point, energy-dense compounds, since the lighter compounds would boil off readily at 420°C, meaning that the available fuel would be even more energy-dense than our automotive fuels.

If there is some particular need for temperatures low enough to condense water, an internal combustion engine driving the compressor of a refrigerative heat pump might be the best way to solve this problem, but it would make for a very inefficient engine, as historic condensation steam engines were not particularly efficient.

However, the lack of free oxygen is going to be a real problem. These people are pretty much out of luck, unless they could harness atomic fusion of Iron or heavier elements, which seems highly unlikely.

Even if the desired highly endothermic substance was available, the environmental temperature of a mere 420°C wouldn't boil water very fast compared with a historical steam engine, which used a coal fire with a temperature on the order of 1000°C+ to boil water.

Now, as a frame challenge, I propose a different solution. Instead of trying to condense water, why not boil something that boils at a temperature >420°C? Consider Sulphur. It is liquid at 420°C, and boils at 445°C. It would actually be easier to boil Sulphur in this environment than it is to boil water in a terrestrial environment. Not to mention that sulphur has a much lower specific heat than water, so it takes a lot less heat to get it hot enough to boil.

Even if for some reason brimstone engines are not suitable, there may be some other substance that is liquid at the required temperature and boils at a temperature not too much higher.

People probably have better things to do with liquid water than put it in an engine after all... like drinking it, and what would Hell be without Brimstone?

  • 1
    $\begingroup$ Just a note, the 1000°C fire box was doing much more work heating the tons of iron and the cold ambient air (97%) than it was in boiling water. Those engines would have needed much less coal and been far more efficient running through ambient temperatures of 400°C+... if they had had a condensate heat sink. $\endgroup$
    – Vogon Poet
    Commented Feb 24, 2022 at 14:50
  • 2
    $\begingroup$ @VogonPoet No, it wasn't. Look up the specific heat of water, as well as the latent heat of vaporization. Put that much heat into an equal mass of iron from the same starting temperature and you'll be close to melting the iron. Condensers do help the efficiency of steam engines, but not enough to make them worth hauling on a locomotive (even as late as the 1940s). $\endgroup$
    – Zeiss Ikon
    Commented Feb 24, 2022 at 15:00
  • $\begingroup$ Making quicklime is more than $ \text{NH}_3 \text{NO}_4 + \text{H}_2\text{O} $ - need to compress air to a 830C heat source, but $$ \text{CaCO}_3\to \text{CO}_2 + \text{CaO} = 178 \frac{\text{kJ}}{\text{mol}} \\ \frac{178 \frac{\text{kJ}}{\text{mol}}}{100.0869\frac{\text{g}}{\text{mol}}}\simeq 1.78 \frac{\text{kJ}}{\text{g}} $$ Is this right? Sounds like a pressurized water closed system might improve odds? Anyway I think there are more energetic reactions I hoped others may know of. $\endgroup$
    – Vogon Poet
    Commented Feb 24, 2022 at 16:03
  • $\begingroup$ @ZeissIkon - A Rankine cycle engine will be able to use condensate vacuum more efficiently than the Boulton-Watt reciprocating piston. Turbines don't care if you push or pull the steam, it's just simple mass flow. $\endgroup$
    – Vogon Poet
    Commented Feb 24, 2022 at 16:27
  • $\begingroup$ There are no spontaneous endothermic reactions with high heat absorption. than ammoniuim nitrate -- read up on entropy vs. energy driving reactions to understand why. $\endgroup$
    – Zeiss Ikon
    Commented Feb 24, 2022 at 16:58

The best compounds will be the ones that are under investigation for thermochemical heat storage. Around 400 to 500 deg C, you have the options of Ca(OH)2, potassium oxide/hydroxide, or metal hydrides. I'd add: magnesium hydroxide as well.

As [frame challenge] your scheme to get oxygen from potassium chloride is impossible (you need oxygen to make potassium chlorate, KCl contains no oxygen), you should consider potassium oxide, which actually will decompose to give oxygen. If it was present in the earth as potassium hydroxide, you'd get water out as well as some additional endothermicity.

Note that most oxides and hydroxides don't decompose at one exact temperature in real life, due to bad kinetics, etc, they generally only decompose at 50 to 100 degrees above the theoretical temperature at any appreciable rate (Source: former ceramics chemist, I used to do a lot of DTG and DSC and oxide prep).

This means KOH could easily persist a few metres below ground at 400, but being heated to 600 deg or so would decompose into 1/2 K + 1/4 O2 + 1/2 H2O.


Failing that, what the other fellow said; boil sulfur. Or aluminium.

EDIT: The Shomate Equation for K2O gives -330 kJ/mol at 750K (475ish deg C) as the heat of formation, so invert it and you have +330 kJ/mol, or roughly 3.5 kJ per gram, or 3.5 MJ/kg. Potassium hydroxide will likely be double or triple that, maybe more (I'm going to bed). Sleepy calc: K2O + H2O gives 2KOH, KOH has enthalpy of formation -424kJ/mol, adjust for additional mass and it's 5ish MJ/kg. Coal will give out less energy at 750 K than at room T, probably more like 20 (total guess). So 25%. And your people get to breathe. Metal hydrides very likely higher but I'm not checking.


Plus walking around on pH 14 soil...that's hell.

  • 1
    $\begingroup$ Thanks for the bounty: having woken up, I feel obliged to point out: KOH and K2O are solid. Their powder might be free flowing but not sure if it's still usable...it might be the thermodynamic solution but not the engineering one. $\endgroup$
    – user86462
    Commented Mar 16, 2022 at 19:20

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