While natural gas occurs in nature as a mixture of methane and other gases, propane is actually a byproduct of both petroleum refining and natural gas processing. Natural gas must be cleaned before being used, and byproducts of this process include hydrocarbons like propane in addition to butane, ethane, and pentane. The difference between propane and natural gas in domestic use comes down to their energy efficiency, cost, compression, storage, and risk factors. There is very little difference, however, when it comes to how well they perform in appliances for heating, cooking, or drying.
Energy vs. Cost
Propane provides more energy per unit of volume than does natural gas. While propane is usually measured in gallons (or liters), natural gas is found in cubic feet (or cubic meters). Heat is measured in British Thermal Units (BTUs), which is the amount of heat needed to increase the temperature of 1 pound (0.5 kg) of water by 1°F (0.56°C). When the amount of energy each produces is compared, both are measured in their gaseous form.
Natural gas provides just over 1,000 BTUs per cubic foot (0.0283 cubic meters); the same volume of propane in gaseous form provides about 2,500 BTUs. This means that propane contains about 2.5 times more usable energy content. So, less propane is needed to produce the same amount of energy as natural gas.
With that said, natural gas tends to be less expensive, at up to 1/6 the cost of propane, depending on the region. Cost considerations depend on the prices of local utility companies and propane companies, however. To compare values, it is typically necessary for a user to convert gallons (or liters) to cubic feet (or cubic meters); 1 US liquid gallon (3.785 liters) is equal to about 0.133 cubic feet (0.0037 cubic meters). Utilities in colder climates might supply natural gas for less money, especially during winter months, though some rural areas may not have access to a utility company that provides gas service. In most areas, however, availability and local laws will be the determining factors.
Performance and Preference
When used for heating or cooking, most people cannot tell much difference between propane and natural gas. Some barbecue enthusiasts prefer natural gas because it is a cleaner burning fuel, meaning that it doesn't release as many pollutants into the air. Exhaust from propane might affect the taste of food, but this is also considered to be part of the charm of grilling.
Both propane and compressed natural gas (CNG) can also be used to power alternative fuel vehicles. Vehicles that run on either type require special tanks to hold the fuel; many cars are actually bi-fuel, which means they have additional tanks to hold gasoline. CNG must be kept at much higher pressure than propane, also called liquid petroleum gas (LPG), so the tanks are often larger and heavier, which can lower the vehicle's mileage. Vehicles that run on CNG tend to be more expensive than those that use LPG, as are vehicle conversion kits, but the natural gas used as fuel costs less. This makes CNG a better choice for vehicles like buses or taxis that are on the road a lot.
Storage
One difference in the physical properties of propane and natural gas is how easily they liquefy and are transported. Propane turns into a liquid at -46°F (-43°C), but when it's held under pressure, it will stay a liquid even at much higher temperatures, allowing it to be stored and carried in the portable steel tanks that can be purchased at most gas stations. Once the pressure is released, as through a valve on a barbeque grill, the propane immediately becomes a gas as long as the temperature is above -44°F (-42°C). Though most often used with portables stoves or grills, propane also can be used as a fuel for heating elements.
Natural gas can be stored in several forms, including as compressed natural gas (CNG), liquified natural gas (LNG), and in an uncompressed form. In most cases, natural gas is compressed before it can be stored or transported; it is much easier to move and extract in this form, as it can be pushed through pipes and out of valves. While it is most often supplied via a public utility company using pipes, CNG can be put into storage tanks for use in those locations where it cannot be piped in. Gas in this form is under extremely high pressure, so there is a small risk that the storage units could burst if not properly maintained.
To turn natural gas into a liquid (LNG), it must be cooled to -260°F (-162°C). At such low temperatures, it takes up even less space than CNG, so larger amounts can be put into insulated tanks and transported. Although special insulated containers must be used to keep LNG in liquid form, this type of storage is safer because any leaks will evaporate into the air.
Risk Factors
Propane is heavier than air, which is heavier than natural gas. Both propane and natural gas will dissipate into the air if they are released in an open environment, and both can pose an explosive risk if they concentrate enough and are ignited. Because propane is heavier, however, it tends to fall to the ground, collect, and pose a greater explosive risk than natural gas, which tends to rise and dissipate into the air. CNG, which is stored at very high pressure, is more likely to explode if storage tanks are damaged.