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Natural Gas

Natural Gas Energy Resources

Natural gas is a highly abundant fuel. Exploration and the development of new extraction technologies have increased the potential supply. A well-developed pipeline system already exists to transport the fuel. Natural gas is essentially methane, a carbon compound, that releases CO2 and water vapor when combusted. Technological developments have enhanced the efficiency of generation with natural gas to further reduce the carbon emission of gas-fired generation.

Conversion Technologies

Combined Cycle Gas Turbine

Combined Cycle Gas Turbine (CCGT) is the dominant gas-based technology for intermediate and baseload power (Figure 1). A CCGT plant is a dual-cycle power plant that consists of one or more gas turbine electricity generators; the second part of the CCGT is equipped with heat recovery steam generators (HRSG)s to capture heat from the gas turbine exhaust (Figure 1). HRSGs take advantage of the energy in the exhaust gases to create steam with recovered waste heat. Steam produced in the HRSG powers a steam turbine generator to produce additional electric power. The combustion turbine will produce 60% to 70% of the electric output, with the balance created by the steam turbine.


Figure 1. Combined-Cycle Gas Turbine Power Plant Schematic (Source: Calpine 2012)
Figure 1. Combined-Cycle Gas Turbine Power Plant Schematic (Source: Calpine 2012)

Storage Technologies

Underground Storage

Natural gas can be stored underground or aboveground in a number of different ways. It is most commonly held in inventory underground and under pressure in three types of facilities: (1) depleted reservoirs in oil and/or gas fields, (2) aquifers, and (3) salt cavern formations (Figure 2). Each storage type has its own physical characteristics (porosity, permeability, and retention capability) and economics (site preparation and maintenance costs, deliverability rates, and cycling capability) which govern its suitability to particular applications. Two of the most important characteristics of an underground storage reservoir are its capacity to hold natural gas for future use and the deliverability rate at which the gas inventory can be withdrawn.

Figure 2. Types of Underground Natural Gas Storage Facilities (Source: EIA 2004)
Figure 2. Types of Underground Natural Gas Storage Facilities (Source: EIA 2004)

Above-ground Storage of Liquified Natural Gas (LNG)

LNG is natural gas that is cooled to -260°F until it becomes a liquid and then stored at essentially atmospheric pressure. Converting natural gas to LNG, a process that reduces its volume by about 600 times, allows it to be transported internationally by specialized tankers with insulated walls. It is kept in liquid form by auto refrigeration, a process in which the LNG is kept at its boiling point, so that any heat additions are countered by the energy lost from LNG vapor that is vented out of storage and used to power the vessel. Once delivered to its destination in the United States or abroad, the LNG is warmed back into its original gaseous state so that it can be used just like existing natural gas supplies, by sending it through pipelines for distribution to homes and businesses. Figure 3 shows an example of an LNG storage and peak shaving facility.


Figure 3. LNG Storage and Peak Shaving Facility (Source: DOE 2005)
Figure 3. LNG Storage and Peak Shaving Facility (Source: DOE 2005)

References

Calpine Corporation, 2012, San Joaquin Valley Energy Center, CA, United States of America, http://www.power-technology.com/projects/san_joaquin/san_joaquin3.html. Accessed March 2013.

DOE (U.S. Department of Energy), 2005, Liquefied Natural Gas: Understanding the Basic Facts, http://www.fe.doe.gov/programs/oilgas/publications/lng/LNG_primerupd.pdf. Accessed March 2013.

EIA, 2004, The Basics of Underground Natural Gas Storage, Aug., http://www.eia.gov/pub/oil_gas/natural_gas/analysis_publications/storagebasics/storagebasics.html. Accessed March 2013.