This hand warmer from Zippo uses a platinum-catalyzed glass fiber burner to produce heat from lighter fluid without a flame for up to 24 hours. Except for the burners which have to be replaced (perhaps every 24 hours; I'm not able to tell from the website) the whole thing is reusable. It's even claimed to be low odor so you won't scare off your quarry when hunting.
It's my belief that if God hadn't wanted us to stay warm while hunting deer he wouldn't have put deer stands in flammable trees. A forest in flames will cause the deer to run—right into the busy highway. It's a convoluted system, perhaps, but lighter fluid is cheaper than bullets.