Question:
Is LNG the same thing as natural gas, and how does it differ from LPG?

Answer:
Liquefied natural gas (LNG) is natural gas that has been temporarily converted to liquid form for ease of storage or transport. A reduction in volume during liquification makes it much more cost-efficient to transport over long distances where gas pipelines do not exist.

When natural gas is cooled to -259°F (-161°C), it becomes a clear, colorless, odorless liquid that is neither corrosive nor toxic. Natural gas is primarily methane (CH4) with low concentrations of other hydrocarbons, water, carbon dioxide, nitrogen, oxygen and some sulfur compounds. During the liquification process, natural gas is cooled below its boiling point, removing most of these compounds. The remaining natural gas is almost 100% methane with only very small amounts of other hydrocarbons. LNG weighs less than half the weight of water, so it will float if spilled on water. LNG is not odorized because the odorant would freeze out as a solid.

By contrast, as delivered to most heat-treatment shops, pipeline natural gas is not 100% methane but rather 85-90% methane in most cases (see reference 2).

Liquefied petroleum gas (also commonly referred to as LPG, GPL, LP Gas or even autogas) is a by-product of petroleum refining and consists of a mixture of hydrocarbon gases. LPG is primarily propane (C3H8). However, the most common composition is 60% propane and 40% butane (C4H10). Depending on the season, the mixture changes: in winter, more propane; in summer, more butane. Other hydrocarbons such as propylene (C3H6) and butylenes (C4H8) are commonly present in small concentration.

When propane/butane mixtures are lightly compressed (approximately 800 kPa or 120 psi), they change from a gaseous state to a liquid. LPG has a higher calorific value (94 MJ/m3 equivalent to 26.1 kWh/m3) than natural gas (38 MJ/m3 equivalent to 10.6 kWh/m3), which means that LPG cannot simply be substituted for natural gas. In order to allow the use of the same burner controls so as to achieve similar combustion characteristics, LPG is typically mixed with air to produce a synthetic natural gas (SNG). LPG is colorless, odorless and heavier than air. A chemical is added to give it a smell like rotten cabbage so that even a very small leak can be detected easily.