Hint: People burn this
The answer to this puzzle is:
1. GARBAGE
2. GRASS
3. GASOLINE
4. FOREST
5. FUEL
6. TWIG
7. ETHANOL
8. SAVANNA
9. CANDLE
10. PROPANE
11. LETTER
2. GRASS
3. GASOLINE
4. FOREST
5. FUEL
6. TWIG
7. ETHANOL
8. SAVANNA
9. CANDLE
10. PROPANE
11. LETTER