Liquefied natural gas (LNG) is increasingly important in meeting the US growing energy demand. The US Energy Information Administration (EIA) estimated domestic natural gas (NG) consumption at approximately 22 trillion cubic feet (Tcf) for 2004.1 The EIA also forecasts that NG consumption will increase to 26 Tcf by 2010 and further increase to approximately 31 Tcf by 2025. Imported LNG supplements domestic NG demand; LNG imports will increase from 0.65 Tcf in 2004 to approximately 6.4 Tcf by 2025.
The LNG supplies originating from different NG fields around the world are unique and different from US supplies. One common difference is higher hydrocarbon content. Also, the NG derived from LNG is very dry with minimal inert content. The total effect is a higher gas caloric value—also referred to as high heating value (HHV)—of the LNG delivered to receiving terminal and distribution pipelines. The changing supplies and quality characteristics associated with the higher hydrocarbon content do impact market (economic) and operational practices of interstate NG pipelines.
US pipeline gas quality. NG pipeline quality specifications in the US have evolved over the last 150 years as technical solutions were developed to resolve transportation problems, such as liquid dropout, hydrate formation, corrosion and improved mechanical design for higher pressure pipelines. Along with dew-point control requirements to avoid liquid dropout in the pipeline, the economic value for gasoline extraction (C5+ components) was recognized. Later, the economic value for liquefied petroleum gas (propane and butane or C3s and C4S) extraction and finally extraction of ethane or natural gas liquids (NGL) for petrochemical industry feedstocks gained importance in the US Gulf Coast regions.
The result was a gas specification for interstate pipelines, which was very lean with an HHV not exceeding 1,075-1,100 Btu/scf. This lean gas is delivered to industries and residential customers in various regions. According to the US Department of Energy (DOE), most of the NG flowing in interstate pipelines is preprocessed to strip out ethane and heavier components, thus leaving mostly methane in the NG.2 The existing infrastructure for gas processing and petrochemical industries in the Southern region supports these processing methods. But similar NGL pipeline and petrochemical infrastructures do not exist in the East Coast and Northeast region.
Gas quality—current trend. Due to tight NG supplies, pressure has mounted to allow NG with some ethane and heavier components to be imported. This "richer" gas with higher HHV is not desirable; when rich NG is combusted, the flames may be too large or too hot for certain applications and appliances.
Over the last three years, there has been a concerted effort to address this issue. The Federal Energy Regulatory Commission (FERC) hosted several public meetings. Current responsibility lies with the FERC to establish a formal set of gas-quality standards that address these issues. At the request of the FERC, the DOE has commissioned the National Energy Technology Laboratory (NETL) to conduct a detailed study regarding gas quality and interchangeability. The FERC is likely to resist issuing national standards due to geographic variation in gas compositions. They are more likely to issue a broad set of guidelines based on the SoCal and NGC white paper, the NETL study and findings based on cases such as AES v. FGT.3'4 It is likely that the final rule making will address some Wobbe Index (WI) characteristics, similar to the one utilized in Europe, to establish gas-pipeline quality standards for certain regions.
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