To supplement domestic natural gas supplies, the U.S. is expected to increase its dependence on offshore liquefied natural gas suppliers in the coming years. However, the composition and hydrocarbon content of imported LNG may significantly vary from those of North American sources. Variation in fuel composition may lead to plants using fuel that violates their combustion turbine fuel specifications and may cause operational problems.
Offshore liquefied natural gas (LNG) suppliers are satisfying the difference between rising demand and declining U.S. domestic production of natural gas and Canadian imports. For example, Gazprom and Shell announced in April that LNG from the Sakhalin-2 project, located off Russia’s eastern coast, will enter the U.S. market through the Energia Costa Azul import terminal in Baja California, Mexico, and eventually make its way into Southern California. The Sakhalin-2 project is expected to produce about 5% of the world’s LNG next year (Figure 1). Although access to new supplies is a good development, Siemens’ research has indicated that the composition and hydrocarbon content of imported LNG can significantly vary from current domestic sources and may cause operational problems.
1. East meets West. The first Russian LNG plant on Sakhalin Island consists of two process trains, each having an annual production capacity of 4.8 million tons. The plant is projected to reach its design capacity (9.6 million tons per year) in 2010. Courtesy: Gazprom
The process of importing LNG begins with liquefying gaseous natural gas at the source to cryogenic temperatures on the order of – 260F. The liquefied gas is then loaded into well-insulated ship-mounted storage tanks for transport to one of several U.S. receiving terminals (currently, there are reportedly eight in operation). When the LNG arrives, the liquefied gas is regasified before entering the gas distribution pipelines. The fact that the Federal Energy Regulatory Commission is tracking 28 new approved LNG terminal projects or expansion projects indicates the expected growth in LNG imports in the coming years.
The liquefaction and regasification processes also enable removal of some of the inert components — such as nitrogen gas (N2), carbon dioxide (CO2), and higher hydrocarbons (C6+) — while retaining all of the original amounts of ethane, propane, and butane. The result can be an LNG-derived gas that can have a wide range of Wobbe Index (defined as higher heating value divided by the square root of specific gravity) and variable energy content per cubic foot when compared with domestic natural gas.
The rate of variation of constituents (fuel composition) in LNG-based gaseous fuels can adversely impact the operation of power generation turbines. The nature of LNG is such that the composition of the fuel may significantly change even from shipment to shipment over a matter of days or even several times in a given day. The rate of change in fuel constituents also depends on the location of the unit on the gas supply pipeline network. For example, a unit that is close to a pipeline node and receives gas from two or more sources could be more susceptible to fuel composition variation.
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