A flameless ration heater (FRH) is a form of self-heating food packaging included in U.S. military Meal, Ready-to-Eat (MRE) rations (since the early 1990s) or similar rations, capable of raising the

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

of an

entrée An entrée (, ; ) in modern French table service and that of much of the English-speaking world (apart from the United States and parts of Canada) is a dish served before the main course of a meal. Outside North America, it is generally synonym ...

(main course) by in twelve minutes, which has no visible flame. The ration heater contains finely powdered

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...

metal, alloyed with a small amount of

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...

and table salt. To activate the reaction, a small amount of water is added, and the boiling point of water is quickly reached as the

exothermic reaction In thermochemistry, an exothermic reaction is a "reaction for which the overall standard enthalpy change Δ''H''⚬ is negative." Exothermic reactions usually release heat. The term is often confused with exergonic reaction, which IUPAC defines ...

proceeds.

Chemical reaction

Ration heaters generate heat in an electron-transfer process called an oxidation-reduction reaction. Water oxidizes magnesium metal, according to the following chemical reaction: : Mg + 2H₂O → Mg(OH)₂ + H₂ heat (q) This reaction is analogous to iron being rusted by oxygen, and proceeds at about the same slow rate, which is too slow to generate usable heat. To accelerate the reaction, metallic iron particles and table salt (NaCl) are mixed with the magnesium particles. Iron and magnesium metals, when suspended in an

electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon ...

, form a

galvanic cell A galvanic cell or voltaic cell, named after the scientists Luigi Galvani and Alessandro Volta, respectively, is an electrochemical cell in which an electric current is generated from spontaneous Oxidation-Reduction reactions. A common apparatus ...

that can generate electricity. When water is added to a ration heater, it dissolves the salt to form a salt-water electrolyte, thereby turning each particle of magnesium and iron into a tiny battery. Because the magnesium and iron particles are in contact, they essentially become thousands of tiny short-circuited batteries which quickly burn out, producing heat in a process the patent holders call "supercorroding galvanic cells". One brand of self-heating rations uses 7.5 grams of a powdered magnesium-iron alloy, consisting of 95% magnesium and 5% iron by weight, 0.5 grams of salt, in addition to an inner filler and anti-foaming agent. Upon adding of water, this mixture can raise the temperature of a meal packet by in about 10 minutes, releasing approximately of heat energy at about 80 watts.

Confined space hazard

The United States Department of Transportation (DOT)

Federal Aviation Administration The Federal Aviation Administration (FAA) is the largest transportation agency of the U.S. government and regulates all aspects of civil aviation in the country as well as over surrounding international waters. Its powers include air traffic ...

(FAA) conducted testing and released a report which in summary states "... the release of hydrogen gas from these flameless ration heaters is of a sufficient quantity to pose a potential hazard on board a passenger aircraft." This testing was performed on commercial grade 'heater meals' which consisted of an unenclosed flameless heat pouch, a bag of salt water, a styrofoam saucer/tray and a meal in a sealed, microwavable/boilable bowl.

References

{{Packaging Military food of the United States Heaters Packaging Military equipment introduced in the 1990s

Chemical reaction

Confined space hazard

See also

References