1 standard cubic meters of natural gas after the full combustion heat release is called the calorific value of natural gas.
High calorific value refers to the 1 standard cubic meters of gas completely burned, the exhaust gas is cooled to the original temperature, while the water vapor in the condensed water discharged by the heat.
Low calorific value refers to the 1 standard cubic meters of gas completely burned, the exhaust gas is cooled to the original temperature, while the water vapor is still, the heat released by the steam state.
We usually say that the calorific value of natural gas are of low calorific value, about 37 trillion and 620 billion coke (9000 kcal) / standard cubic meters.

 Combustible gas and air mixture in case of fire can concentration range of the explosion occurred,  which called the flammable gas explosion limit. In the mixture, when the content of combustible gas reduced to the content cannot explode, referred to as the explosion limits of flammable gases. And when the content of combustible gas is increased to the content of the explosion can not occur, known as the upper limit of the explosion.
The mixture of gas and air must be within the explosion limits to fire, combustion and explosion. Natural gas - air mixture in the volume of natural gas content of 5% is the lower limit of the explosion, 15% is the upper limit of the explosion.

Natural gas is hidden in the formation of combustible gas, the composition of methane, containing a small amount of carbon dioxide, hydrogen sulfide, nitrogen and trace inert gas. Methane volume fraction is generally not less than 80%, calorific value is generally 90kcal/ square; density 0.7174Kg/Nm3, is lighter than air, explosion limit for 5% - 15% (natural gas concentration). Methane boiling point -161.49℃, at 0.1MPa, - 162 ℃can be converted into liquefied natural gas (LNG) by gaseous natural gas. Gas and liquid ratio of 625:1. Natural gas combustion generated after carbon dioxide and water,  and the flue gas does not pollute the atmosphere, is regarded as a clean energy. Natural gas is not only the quality of civil and industrial fuel,and is the ideal city gas source, but also the raw materials of chemical products.
In recent years, the global temperature rise and unstable weather patterns, the environmental problems caused by air pollution caused global concern. Therefore, many countries decided to invest more money in the cause of environmental protection. Natural gas is often hailed as one of the cleanest available mineral energy. With other energy sources compared, such as coal and oil, natural gas by the release of pollutants is minimal. Therefore, natural gas is regarded as a future energy utilization of choice for energy.