What is Liquefied Natural Gas (LNG)?
NOTE: Information on this page is general knowledge on the LNG industry. Not all information may apply to any one specific proposed facility. The specifications of each proposal must be know to find which aspects of these descriptions apply.
Liquefied natural gas (LNG) is natural gas that has been cooled to a liquid state, at about -260° Fahrenheit, for shipping and storage. The volume of natural gas in its liquid state is about 600 times smaller than its volume in its gaseous state. This process makes it possible to transport natural gas to places pipelines do not reach.
Liquefying natural gas is a way to move natural gas long distances when pipeline transport is not feasible. Markets that are too far away from producing regions to be connected directly to pipelines have access to natural gas because of LNG. In its compact liquid form, natural gas can be shipped in special tankers to terminals around the world. At these terminals, the LNG is returned to its gaseous state and transported by pipeline to distribution companies, industrial consumers, and power plants.
What happens at LNG facilities?
LNG facilities are, in essence, refineries for methane gas. They receive relatively “raw” methane gas by pipeline, remove impurities, and liquefy the refined gas for storage or export. Some of the impurities, such as water vapor and sour gas, are removed to meet pipeline specifications or because they damage equipment. Other impurities, such as propane, butane, and ethane, are removed for sale—they often have higher sales values than the pipeline gas itself. Much of North America’s LNG exports are “unconventional” gas resources—meaning the gas is fracked.
After raw methane gas has been processed, the refined gas that will be distributed to consumers is 85 to 95 percent methane, depending on the source of the gas. It also contains small amounts of other gases such as ethane, propane, and nitrogen. Some of the refined gas powers equipment at the facility, while most of it is piped to equipment that cools the gas to -260° F (-162° C). At this temperature, it changes state from gas to liquid, taking up 1/600th of the volume it occupied as a gas. This volume reduction means companies can store or transport the natural gas in much larger quantities.
The equipment used to cool the gas makes up a “liquefaction train,” which has similar components to a household refrigerator. It includes a device where the gas is chilled (called a “heat exchanger” or “cold box”), a compressor that chills the refrigerants, and a power source that drives the compressor. The power source is typically a gas or steam turbine, similar to those found in oil refineries, power plants, and large chemical plants. Gas-driven turbines have been standard for over 30 years, largely replacing steam turbines. Some LNG facilities have just one liquefaction train, but most LNG facilities have more than one, producing greater energy demand and higher emissions.
After liquefaction, LNG is stored in a tank that keeps it cold enough to remain in liquid form. Maintaining the cryogenic temperature of LNG consumes a lot of energy, which is why large facilities usually have power generation stations on-site. Liquefaction and power equipment generate the most sizeable share of emissions at an LNG facility.
At a “peak shaving” facility, methane gas is refined and stored as LNG for peak demand during winter months, when it is re-gasified and distributed to natural gas customers.
At an export facility, methane gas is refined and stored as LNG until it is loaded onto export tankers.
At other facilities, such as marine bunkering facilities or import terminals, the LNG is stored until needed for regional sales or to fuel marine vessels.
LNG facilities produce emissions in at least five different ways:
- Fuel-fired equipment powers a facility’s gas refining and refrigeration processes by burning methane gas and produces the largest portion of a facility’s emissions.
- Venting removes surplus gases from processing equipment and disposes of waste gas. As a normal part of liquefaction and storage at an LNG facility, venting releases the greenhouse gases methane and CO2 directly into the atmosphere.
- Flaring burns waste gases, converting them into carbon dioxide and nitrogen oxide as well as other pollutants like carbon monoxide and particulate matter.
- “Fugitive emissions” of methane gas escape directly into the atmosphere from various pieces of equipment, including pipelines that feed gas to facility equipment.
- Marine emissions are released from the LNG tankers, many of which run on heavy fuel oil and emit related pollutants, including particulate matter, a significant contributor to lung and heart conditions in humans. An LNG tanker typically spends around 24 hours at port, and some facilities would likely have multiple vessel calls per week.
Health and Safety / Environmental Impact
Cooling Water and Cold Water Streams
The use of water for process cooling at LNG liquefaction facilities and for re-vaporization heating at LNG receiving terminals may result in significant water use and discharge streams. Recommendations to control once-through thermal process water use and discharge streams include the following:
Water conservation opportunities should be considered for LNG facility cooling systems (e.g.,air-cooled heat exchangers in place of water-cooled heat exchangers, and if applicable, opportunities for the integration of cold water discharges with other proximate industrial or power plant facilities). The selection of the preferred system should balance environmental benefits and safety implications of the proposed choice. Additional guidance on water conservation is provided in the General EHS Guidelines.
Cooling or cold water should be discharged to surface waters in a location that will allow maximum mixing of the thermal plume to ensure that the temperature is within 3 degrees Celsius of ambient temperature at the edge of the mixing zone, according to guidance provided in the General EHS Guideline and as noted in Table 1 of Section 2.1 of this Guideline.
If biocides/chemical use is necessary, carefully select chemical additives in terms of dose concentration, toxicity, biodegradability, bioavailability, and bioaccumulation potential. Consideration should be given to residual effects at discharge using techniques such as risk-based assessment.
» Environmental, Health and Safety Guidelines for Liquefied Natural Gas Facilities, IFC / World Bank Group, April 11, 2017