Liquefied natural gas

Liquefied natural gas (LNG) is natural gas (predominantly methane, CH₄, with some mixture of ethane, C₂H₆) that has been cooled down to liquid form for ease and safety of non-pressurized storage or transport. It takes up about 1/600th the volume of natural gas in the gaseous state at standard conditions for temperature and pressure.

"LNG" redirects here. For other uses, see LNG (disambiguation).

LNG is odorless, colorless, non-toxic and non-corrosive. Hazards include flammability after vaporization into a gaseous state, freezing and asphyxia. The liquefaction process involves removal of certain components, such as dust, acid gases, helium, water, and heavy hydrocarbons, which could cause difficulty downstream. The natural gas is then condensed into a liquid at close to atmospheric pressure by cooling it to approximately −162 °C (−260 °F); maximum transport pressure is set at around 25 kPa (4 psi) (gauge pressure), which is about 1.25 times atmospheric pressure at sea level.

The gas extracted from underground hydrocarbon deposits contains a varying mix of hydrocarbon components, which usually includes mostly methane (CH₄), along with ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀). Other gases also occur in natural gas, notably CO₂. These gases have wide-ranging boiling points and also different heating values, allowing different routes to commercialization and also different uses. The "acidic" elements such as hydrogen sulphide (H₂S) and carbon dioxide (CO₂), together with oil, mud, water, and mercury, are removed from the gas to deliver a clean sweetened stream of gas. Failure to remove much or all of such acidic molecules, mercury, and other impurities could result in damage to the equipment. Corrosion of steel pipes and amalgamization of mercury to aluminum within cryogenic heat exchangers could cause expensive damage.

The gas stream is typically separated into the liquefied petroleum fractions (butane and propane), which can be stored in liquid form at relatively low pressure, and the lighter ethane and methane fractions. These lighter fractions of methane and ethane are then liquefied to make up the bulk of LNG that is shipped.

Natural gas was considered during the 20th century to be economically unimportant wherever gas-producing oil or gas fields were distant from gas pipelines or located in offshore locations where pipelines were not viable. In the past this usually meant that natural gas produced was typically flared, especially since unlike oil, no viable method for natural gas storage or transport existed other than compressed gas pipelines to end users of the same gas. This meant that natural gas markets were historically entirely local, and any production had to be consumed within the local or regional network.

Developments of production processes, cryogenic storage, and transportation effectively created the tools required to commercialize natural gas into a global market which now competes with other fuels. Furthermore, the development of LNG storage also introduced a reliability in networks which was previously thought impossible. Given that storage of other fuels is relatively easily secured using simple tanks, a supply for several months could be kept in storage. With the advent of large-scale cryogenic storage, it became possible to create long term gas storage reserves. These reserves of liquefied gas could be deployed at a moment's notice through regasification processes, and today are the main means for networks to handle local peak shaving requirements.^[1]

Specific energy content and energy density[edit]

The heating value depends on the source of gas that is used and the process that is used to liquefy the gas. The range of heating value can span ±10 to 15 percent. A typical value of the higher heating value of LNG is approximately 50 MJ/kg or 21,500 BTU/lb.^[2] A typical value of the lower heating value of LNG is 45 MJ/kg or 19,350 BTU/lb.

For the purpose of comparison of different fuels, the heating value may be expressed in terms of energy per volume, which is known as the energy density expressed in MJ/litre. The density of LNG is roughly 0.41 kg/litre to 0.5 kg/litre, depending on temperature, pressure, and composition,^[3] compared to water at 1.0 kg/litre. Using the median value of 0.45 kg/litre, the typical energy density values are 22.5 MJ/litre (based on higher heating value) or 20.3 MJ/litre (based on lower heating value).

The volumetric energy density of LNG is approximately 2.4 times that of compressed natural gas (CNG), which makes it economical to transport natural gas by ship in the form of LNG. The energy density of LNG is comparable to propane and ethanol but is only 60 percent that of diesel and 70 percent that of gasoline.^[4]

Northwest Shelf Train 5: 4.4 MTPA

: 9.6 MTPA

Sakhalin-II

: 6.7 MTPA

Yemen LNG

Tangguh: 7.6 MTPA

: 15.6 MTPA

Qatargas

Qatar: 15.6 MTPA

Rasgas

Oil indexed contract, used primarily in Japan, Korea, Taiwan and China;

Oil, oil products and other energy carriers indexed contracts, used primarily in Continental Europe; and

[106]

Market indexed contracts, used in the US and the UK.

Asia (Japan, Korea, Taiwan), where gas distributed is rich, with a gross calorific value (GCV) higher than 43 MJ/m³(n), i.e. 1,090 Btu/scf,

the UK and the US, where distributed gas is lean, with a GCV usually lower than 42 MJ/m³(n), i.e. 1,065 Btu/scf,

Continental Europe, where the acceptable GCV range is quite wide: approx. 39 to 46 MJ/m³(n), i.e. 990 to 1,160 Btu/scf.

LNG quality is one of the most important issues in the LNG business. Any gas which does not conform to the agreed specifications in the sale and purchase agreement is regarded as "off-specification" (off-spec) or "off-quality" gas or LNG. Quality regulations serve three purposes:^[159]

In the case of off-spec gas or LNG the buyer can refuse to accept the gas or LNG and the seller has to pay liquidated damages for the respective off-spec gas volumes.

The quality of gas or LNG is measured at delivery point by using an instrument such as a gas chromatograph.

The most important gas quality concerns involve the sulphur and mercury content and the calorific value. Due to the sensitivity of liquefaction facilities to sulfur and mercury elements, the gas being sent to the liquefaction process shall be accurately refined and tested in order to assure the minimum possible concentration of these two elements before entering the liquefaction plant, hence there is not much concern about them.

However, the main concern is the heating value of gas. Usually natural gas markets can be divided in three markets in terms of heating value:^[159]

There are some methods to modify the heating value of produced LNG to the desired level. For the purpose of increasing the heating value, injecting propane and butane is a solution. For the purpose of decreasing heating value, nitrogen injecting and extracting butane and propane are proven solutions. Blending with gas or LNG can be a solution; however all of these solutions while theoretically viable can be costly and logistically difficult to manage in large scale. Lean LNG price in terms of energy value is lower than the rich LNG price.^[160]

Cleveland, Ohio, U.S. The East Ohio Natural Gas Co. experienced a failure of an LNG tank.^[184] 128 people perished in the explosion and fire. The tank did not have a dike retaining wall, and it was made during World War II, when metal rationing was very strict. The steel of the tank was made with an extremely low amount of nickel, which meant the tank was brittle when exposed to the cryogenic nature of LNG. The tank ruptured, spilling LNG into the city sewer system. The LNG vaporized and turned into gas, which exploded and burned.

October 20, 1944

Staten Island, New York, U.S. During a cleaning operation, 42 workers were inside one of the TETCo LNG tanks, which had supposedly been completely drained ten months earlier. However, ignition occurred, causing a plume of combusting gas to rise within the tank. Two workers near the top felt the heat and rushed to the safety of scaffolding outside, while the other 40 workers died as the concrete cap on the tank rose 20–30 feet in the air and then came crashing back down, crushing them to death.^[185]^[186]

February 10, 1973

October 6, 1979, , US. A pump seal failed at the Cove Point LNG import facility, releasing natural gas vapors (not LNG), which entered an electrical conduit.^[184] A worker switched off a circuit breaker, which ignited the gas vapors. The resulting explosion killed a worker, severely injured another and caused heavy damage to the building. A safety analysis was not required at the time, and none was performed during the planning, design or construction of the facility.^[187] National fire codes were changed as a result of the accident.

Lusby, Maryland

January 19, 2004, , Algeria. Explosion at Sonatrach LNG liquefaction facility.^[184] 27 killed, 56 injured, three LNG trains destroyed, a marine berth damaged. 2004 production was reduced by 76 percent. Total loss was US$900 million. A steam boiler that was part of an LNG liquefaction train exploded, triggering a massive hydrocarbon gas explosion. The explosion occurred where propane and ethane refrigeration storage were located. Site distribution of the units caused a domino effect of explosions.^[188]^[189] It remains unclear if LNG or LNG vapour, or other hydrocarbon gases forming part of the liquefaction process initiated the explosions. One report, of the US Government Team Site Inspection of the Sonatrach Skikda LNG Plant in Skikda, Algeria, March 12–16, 2004, has cited it was a leak of hydrocarbons from the refrigerant (liquefaction) process system.