A flameless ration heater, or FRH, is a water-activated exothermic chemical heater included with Meals, Meals Ready-to-Eat (MREs), used to heat the food. Specifications for the flameless ration heater required that it be capable of raising the temperature of an 226.8 g entree by 56 °C in twelve minutes, and that it has no visible flame.

The flameless ration heater contains finely powdered magnesium metal, alloyed with a small amount of iron, and table salt. To activate the reaction, a small amount of water is added, and the boiling point of water is quickly

Flameless 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]

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 mixed metallic iron particles and table salt (NaCl) with the magnesium particles.[2]

Iron and magnesium metals, when suspended in an electrolyte (such as salt water), form a galvanic cell—a "battery"—that is able to generate electricity.

When water is added to a flameless 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".