A flameless ration heater, or FRH, is a water-activated exothermic chemical heater included with Meals, Ready-to-Eat (MREs), used to heat the food. US military specifications for the heater require that it be capable of raising the temperature of an eight-ounce entree by 100° F in twelve minutes, and that it display no visible flame.
The ration heater contains finely powdered iron and magnesium metals, 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 reaction proceeds.[1]
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Ration heaters generate heat in an electron-transfer process called an oxidation-reduction reaction. Water oxidizes magnesium metal, according to the following chemical reaction:
This reaction is analogous to iron being rusted by oxygen, and proceeds at about the same slow rate. On their own, the reaction between magnesium and water is too slow to generate usable heat.
To accelerate the reaction, the developers (see US Patents 4,017,414 and 4,264,362) mixed metallic iron particles and table salt (NaCl) with the magnesium particles.
Iron and magnesium metals, when suspended in an electrolyte (such as salt water), form a galvanic cell -- a "battery" -- that can generate electricity. (For an example of how to build such a battery, see http://www.miniscience.com/link/Airbattery.htm)
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 become thousands of tiny short-circuited batteries, which quickly burn out, producing heat in a process the patent holders call "Supercorroding Galvanic Cells".
The United States Department of Transportation (DOT) Federal Aviation Administration (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."[2] 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.
The tested configuration is not the standard configuration in true military and military style MREs. In US Military style MREs, there is no Styrofoam tray component, and the FRH heater pad is enclosed in a separate sealed bag, also some styles do not include a packet of water for activation. This different configuration, while still susceptible to a catastrophic accidental activation (e.g. piercing the entire MRE, and every pouch inside simultaneously) is much less likely to be accidentally activated. The MRE configuration would require two pouches (one containing the sealed FRH) to be accidentally ruptured, and/or to be improperly sealed to allow activation while the heater meal configuration requires only one package rupture or breach. Additionally, the MRE is wholly contained in an outer sealed pouch which may further reduce the hazards associated with transporting these meals, while the commercially tested sample was contained in only a cardboard box.
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