Propane and natural gas have a lot in common. Both are used as fuels, are gaseous, can be stored in tanks and pumped through lines, and have similar safety hazards associated with them. To compare how they are both similar and different, take a look at a few properties in turn.
Propane is a simple hydrocarbon (a molecule made only of carbon and hydrogen). It is a chain of three carbon atoms, attached by single bonds. Three hydrogen atoms adorn each of the terminal carbon atoms, and two more are attached to the central carbon atom. The chemical formula is commonly written as C3H8, but to show the structural connectivity, it may also be written as CH3CH2CH3.
Natural gas is a mixture of gases, but is primarily methane. Small amounts of ethane, propane, hydrogen sulfide, and other gases are present in the natural source, but the gas is purified before use as a fuel so that “natural gas” is usually interpreted as “methane”. Methane is the smallest hydrocarbon, having only one carbon atom, surrounded by four hydrogen atoms. Its chemical formula is CH4.
Both gases are extremely flammable. They are fuels, and can only burn in the presence of oxygen or some other oxidizing chemical. While properly stored in a pressurized tank, the gases cannot ignite. If the tank is opened or leaking, the gas is then released into the air. If that tank was stored in a closet or a room, it can then form an explosive mixture in the air, needing only a spark to set it off.
Natural gas is lighter than air, so it fills a room easily. Pure methane is odorless, so to make people aware of its presence, gas companies add a small amount of a sulfur compound (called a mercaptan or thiol) to give it an unpleasant stench. The smell alerts people to its presence, and gives them the chance to exit the room carefully – and without generating any sparks by flipping switches or using anything electric.
Propane is slightly heavier than air, so while it can still fill a room, it has an additional hazard. It can pool in low places, and flow along the ground over a distance. If this flow reaches an ignition source, it can ignite and “flash” back to the source. (Think of how a trail of gunpowder works, and then imagine it traveling through the air in the blink of an eye.) If there is an explosive mixture (as in a confined space like a room or closet) at the source, it ignites just the same as if the ignition source had been in the room. Unlike methane, propane does have a scent. It is usually described as slightly sweet. Companies will typically add an unpleasant odor to propane, just the same. By association with the scent of natural gas, people then know when to be alert and vacate.
Since both gases burn readily, they are well suited as fuels. Being gaseous allows for easy transport by cylinder/tank or through pipes. Natural gas is typically delivered by pipe, while propane is typically seen tanked, but either method is applicable to each. Transport by tank requires that the gas be pressurized, such that if the tank is allowed to vent completely, the volume of gas that is released may be several thousand times the size of the tank itself.
The amount of energy released when the gases are burned (known as the heat of combustion) is a measure of how useful the gas is as a fuel. Propane has a higher heat of combustion, generating almost three times as much energy for every molecule of the gas that is burned. This makes sense if you look at the chemical structures – propane is almost three times as big, with many more chemical bonds to break than tiny methane. If you consider the amount of energy given off per kilogram (or pound) of gas burned, natural gas is slightly better. This creates a trade-off that helps to explain why propane is usually tanked while natural gas is piped. Since a tank can store roughly equal numbers of gas molecules, the user can get more energy from a tank of propane than a tank of natural gas. Piping allows for a continuous supply of either gas, so the more efficient methane is preferred.
The burning of propane and methane may also raise health and environmental concerns. If enough oxygen is present, both chemicals burn completely to produce only carbon dioxide and water (steam). When insufficient oxygen is present, incomplete combustion can lead to other products. In the case of natural gas, only carbon monoxide* is added to the list. Propane has the potential to generate several different partially oxidized products (including carbon monoxide) which can be toxic and/or pollutants. Because of this distinction, natural gas is often referred to as “clean burning”.
*An asphyxiant, carbon monoxide prevents the body from taking in oxygen and can be fatal in large doses. Released into the environment, carbon monoxide doesn’t last long and soon becomes carbon dioxide.
Aside from fire hazards, both gases also pose a risk of asphyxiation. When released into a room, they can lower the concentration of oxygen in the room to the point that humans lose consciousness, and eventually die from lack of oxygen. For this reason, people who work with these gases in confined areas wear SCBA (self-contained breathing apparatus) so that they have a reliable source of oxygen.
Tanks of either gas are under extreme pressure. While tanks are designed to contain the pressure, damage to the tank can lead to disaster. If the valve on a tank is broken off, for instance, the gas exits through the resulting hole with great force, pushing the tank in the opposite direction. The net result is effectively a metal torpedo that has enough power to go through concrete walls. If the side of a tank is damaged, the tank may rupture, exploding (with or without flames, depending on whether a spark is present) and sending shrapnel (pieces of the tank) in different directions. Tanks are carefully constructed to avoid these situations, but abuse of the tank can be dangerous.