Methane can be made produced from waste of poop or rotting plants. 
It can be used to make electricity  or just used for heating and cooking.
Rabbit poop flamethrowerEdit
50% Rabbit poop and 50% water can be put into a barrel with a smaller barrel facing down inside it. Methane gas is produced, and this causes the smaller barrel to rise up.
By using methane gas and saltpeter made from dried and ground up rabbit poop, a flamethrower can be made.
Doomsday Preppers showed a guy doing this, on Season 4 episode 2.