Chemical Heating

Found on Amazon 

Heats Food Warming Pads | Full Pan Water Activated Disposable Food Warmers for Parties | Used w/ Chafing Dish, Aluminum Pans, Foil Trays, Buffet Kit, Kitchen Accessories (Pans NOT Included) | 6 Pack See less

They are expensive enough that for prepper use they would be wasteful but I might see if they make a smaller version suitable for warming a can of chili at a time.  Perhaps the heating packets could be used in smaller quantities to get can sized food to acceptable temperature.