Propane undergoes combustion reactions in a similar fashion to other alkanes. In the presence of excess oxygen, propane burns to form water and carbon dioxide.
C3H8 + 5 O2 → 3 CO2 + 4 H2O + heat
propane + oxygen → carbon dioxide + water
When not enough oxygen is present for complete combustion, incomplete combustion occurs when propane burns and forms water, carbon monoxide, carbon dioxide, and carbon.
2 C3H8 + 7 O2 → 2 CO2 + 2 CO + 2 C + 8 H2O + heat
Propane + Oxygen → Carbon Dioxide + Carbon Monoxide + Carbon + Water
Unlike natural gas, propane is heavier than air (1.5 times as dense). In its raw state, propane sinks and pools at the floor. Liquid propane will flash to a vapor at atmospheric pressure and appears white due to moisture condensing from the air.
When properly combusted, propane produces about 50 MJ/kg. The gross heat of combustion of one normal cubic meter of propane is around 91 megajoule
Propane is nontoxic; however, when abused as an inhalant it poses a mild asphyxiation risk through oxygen deprivation. Commercial products contain hydrocarbons beyond propane, which may increase risk. Commonly stored under pressure at room temperature, propane and its mixtures expand and cool when released and may cause mild frostbite.
Propane combustion is much cleaner than gasoline combustion, though not as clean as natural gas combustion. The presence of C–C bonds, plus the multiple bonds of propylene and butylene, create organic exhausts besides carbon dioxide and water vapor during typical combustion. These bonds also cause propane to burn with a visible flame.
Greenhouse gas emissions factors for propane are 62.7 kg CO2/ mBTU or 1.55 kg of CO2 per litre or 73.7 kg/GJ.