ELI5• Why you cannot use methanol or even ammonia inbpower plants instead of water. water takes more energy to boil and thus requires more coal burnt. You can condense the methanol by passing it through a waterbody of some sort so it doesn’t get lost in the atmosphere like steam does?
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1) The heat of vaporization isn’t wasted energy. Most steam engines rely on the condensor to create a significant vacuum with regards to ambient pressure, in order to produce full power. That is, to remove spent low pressure steam from the last stage. Therfore work is performed by the latent heat of vaporization contained in the cool, low pressure steam by means of transferring that heat into the condensor walls, thus dramatically reducing the volume of the steam.
2) Anhydrous ammonia is pretty toxic. It’s still used as a refrigerant in some large industrial freezers. (Which are in a sense, steam engines that operate in reverse.) Although new systems generally avoid it’s use because nontoxic refrigerants (like CO2) require far fewer safety measures and less regulatory compliance concerns.
There have been a good number of incidents over the decades where ammonia leaks from refrigerant systems or chemical plants have resulted in deaths. Anhydrous ammonia can be extremely dangerous because high volume leaks can cause people exposed to become confused then lose consciousness in a matter of seconds. Then, asphyxiatiate and die faster than emergency services can get there.
I remember reading about one case where a freezer had a crack in a refrigerant line that slowly grew so workers had just became accustomed to the smell or smell blind. The cracked refrigerant line suddenly broke, one worker in the freezer collapsed, the other worker outside saw him on the floor, then rushed in and was overcome almost immediately, bringing the death toll to 2.
Ammonia may be a good choice for heat engines that operate in space past the orbit of jupiter, or on the surface of mars for example. Specifically, small lightweight nuclear powerplants. Toxicity is a minor concern in space, it has a very low freezing point and the moderate increase in performance means weight savings. Such equipment would operate only at mildly elevated temperatures of a few hundred C in the reactor core and would take advantage of very cold, subzero temperatures on the condensor side to produce high efficiency. The low core temperature would mean thinner lighter weight materials could be used.
3) Methanol has a tendency to pyrolize at high temperature forming a number of other products like dimethyl ether, aromatic hydrocarbons, tar, and formaldehyde.
So while it could be used, the fact that the max operating temperature and thus max pressure is limited to reasonably below the pyrolysis temp, would mean less efficiency than H2O.
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