Liquefied Natural Gas Resource Use
Liquefied Natural Gas Resource Use
Liquefied Natural Gas (LNG) utilization enables the most environmentally friendly fossil fuel available to contribute to meeting growing energy demands from a resource that would otherwise go unused, thus requiring other fuels to fill the need. The process of producing and using LNG has unique environmental considerations that require careful study and management, as projects are evaluated and implemented to meet growing energy demands.
Historical Background and Scientific Foundations
The release of greenhouse gases into the atmosphere has been a naturally occurring phenomenon since Earth formed and life began, with gases released due to biodegradation of plant and animal matter, the digestive and respiratory processes of living beings, and naturally occurring hydrocarbon seepages from within Earth.
Natural gas discharges also occur as the result of moving water, oil, and other resources from beneath Earth’s surface. Gas exists in solution within water and/or oil while in the reservoir and is released when produced to the surface. Liquids are comparatively easy to capture and transport using readily available technology. Solution gas was routinely released into the atmosphere until technology enabled its capture and delivery to the consumer, while supplanting less clean burning fuels when pipelines could be installed. The liquefaction of natural gas enables transport from areas where pipelines do not exist and reduces the volume of gases released to the atmosphere. Environmental concerns relative to LNG come from the process of converting natural gas to a readily transportable state, transportation, and introduction into a delivery system. The steps in the process include:
- Processing to remove unwanted elements and impurities,
- Transportation in a liquid phase,
- Conversion back into a gas phase and pipeline distribution to homes and industry as the end users.
Liquified natural gas is 90+% methane (CH4) and is odorless, colorless, non-corrosive, non-toxic, and non-explosive in an unconfined environment. Liquefaction is a thermodynamic process requiring energy consumption to chill the gas causing the phase change to a dense liquid at -160°C (–256°F) at atmospheric pressure by compression to high pressure and rapid expansion back to atmospheric pressure. The density of LNG is 600 times that of an equivalent volume of natural gas at atmospheric pressure.
LNG is shipped at a constant atmospheric pressure, creating a boiling cryogen in a liquid phase in specially insulated and designed tanks. The process requires venting all gas produced as the liquid boils.
LNG is then delivered to a terminal, vaporized back into a gas phase, and shipped to its end users in homes and industry.
Impacts and Issues
The production of natural gas at the wellhead and transport to a processing plant typically has modest environmental impact once facilities are in place. Potential environmental and safety impacts of transporting LNG by container ships are a matter of considerable study. The Sandia National Laboratory in Albuquerque, New
WORDS TO KNOW
AMBIENT: Existing condition.
BOILING CRYOGEN: Low temperature liquid at its bubble point, the temperature and pressure at which evaporation begins.
LIQUEFACTION: The process of changing the state of something to a liquid.
PHASE: In the study of waves, a repeating or periodic wave’s phase is the relationship of its pattern of peaks and valleys to a fixed reference (such as time).
RESERVOIR: The collection of naturally occurring fluids in the porosity of a subsurface rock formation.
VAPORIZATION: The conversion of a solid or liquid into a gas.
Mexico, issued a risk analysis of transporting LNG by ship in 2004 that suggested a wide range of potential environmental consequences that could occur, depending on the specific details of any accident or incident involving LNG. The International Finance Corporation—World Bank—released guidelines for environmental safety at LNG facilities in 2007 that call for
management of greenhouse-gas and wastewater emissions that occur during a conversion process, as well as a buffer zone to minimize environmental impacts during an accident.
Any leak would release LNG as a cryogenic fluid at –256°F (-160°C) into the environment. The fluid would immediately begin to vaporize. The specific conditions of the leak and weather conditions would govern the extent of thermal shock and potential fire hazard to the immediate area. Both the Sandia and the World Bank studies discuss the range of possibilities and probabilities in detail.
The two processes in current use in the LNG vaporization stage are Open Rack Vaporization (ORV) or Submerged Combustion Vaporization (SCV). The ORV process is an open-loop system, which circulates large volumes of ambient temperature seawater through a heat exchanger in order to warm the cold LNG, causing vaporization back to a gas phase. The ORV process has the benefits requiring relatively lower construction and operating costs, uses seawater as a renewable heat exchange fluid, consumes less fuel, and emits less greenhouse gases than other processes. The related impact of the ORV process includes possible negative effects on plankton in the seawater and other aquatic life in the immediate area due to the thermal shock of the heat exchange process chilling the seawater, entrapment in the water intake system, and chemical shock of chemical agents needed to prevent bio growth clogging the process. These impacts can be substantial for some aquatic ecosystems, and as a result, ORV systems are usually restricted to certain areas.
The SCV process is a closed-loop system requiring fuel and air to heat a water bath sealed within a tank to warm the cold LNG as it passes through a heat exchanger submerged within the tank. The benefits of the SCV process include lower water usage and wastewater discharge, substantially lower impact on aquatic life, and easier construction. Environmental impacts of the SCV process include primarily, greater energy consumption of the heating requirements and, to a lesser degree, greater air quality emission issues, larger initial and operating costs, and larger land usage as compared to an ORV system.
As of mid-2007, more than 25 new LNG offloading and re-gasification facilities were planned for construction either in ports or offshore facilities throughout the world.
Center for Liquefied Natural Gas. “About LNG.” http://www.lngfacts.org/ (accessed May 2, 2008).
International Finance Corporation - World Bank. “Environmental, Health & Safety Guidelines - LNG Liquefied Natural Gas Facilities.” April 30, 2007. http://www.ifc.org/ifcext/enviro.nsf/AttachmentsByTitle/ggu_EHSGuidelinees200LNG/$FILE/Final+-+LNG.pdf (accessed May 2, 2008).
Sandia National Laboratories. “Guidance on Risk Analysis and Safety of a Large Liquefied Natural Gas (LNG) Spill Over Water, SAND2004-6258.” December 2004. http://www.fossil.energy.gov/programs/oilgas/storage//ng/sandia__ng_1204.pdf (accessed May 2, 2008).
U.S. Government Accountability Office. “Natural Gas Flaring and Venting: Opportunities to Improve Data and Reduce Emmissions.” August 13, 2004. http://www.gao.gov/htext/d04809.html (accessed May 2, 2008).
William J. Engle