Balance between interchangeability and heating value

Gas interchangeability is a common issue for natural gas (NG) distribution networks. Interchangeability must be addressed in major gas markets in which gas is supplied from many different sources with different qualities. In the US and the UK, gas interchangeability is yet to be fully resolved regarding national quality specification guidelines with respect to liquefied natural gas (LNG) imports and nonconventional gas sources.

Many international gas markets have adopted interchange-ability parameters to ensure end-user protection in dealing with multiple supply sources. Practically all global gas markets (and even some US regions) have adopted applying interchangeability parameters in contractual terms and conditions. The most commonly used reference is the Wobbe Index. This index adjusts the heating value with relative gas density and does consider burner performance. It is internationally the most widely accepted measure of interchangeability.

LNG parameters. Particularly in the UK, the Wobbe Index is frequently used as a parameter to determine the upper limit (major constraint) on imported rich gases. However, most US gas specifications are based upon heating value—not the Wobbe Index. Historically, this practice has been adequate because most NG supplies were sourced from interstate transmission pipelines and had very homogeneous gas compositions.

As domestic NG supplies lag behind demand, the US gas supply mix will increasingly involve more imported LNG, and pipeline-gas compositions will become less homogenous. Many participants in the US gas market have called for pipeline standards clarification on issues surrounding gas interchangeability to ensure consistent safe and reliable supply sources.

International standards. Different gas specifications and standards for the global expanding supplies markets will have an effect on the NG markets. Quality issues on LNG deliveries will be more complex and critical as receiving terminals balance and cope with different supply chains.

The growing receipt of high-heating-value, rich LNG adds challenges to receiving terminals that are connected to low-calo-rific-value gas pipeline networks. This situation for both ends of the LNG supply chain must be addressed.

Shipping side. LNG producers must decide whether to produce more than one LNG specification to satisfy all potential customers in Asia, Europe or North America. Such actions would involve additional capital investment and operating costs to produce several LNG products with varying qualities. Shipping only one quality (LNG product) could potentially limit marketing opportunities in an expanding commodity market.

Receiving side. Many US gas pipelines can accept rich gas from LNG terminals with high heating value limits—1,050 Btu/ scf—1,070 Btu/scf. Imported LNG from many sources can have

heating values ranging from 1,080 Btu/scf to 1,160 Btu/scf. To import rich (high-Btu) LNG and still comply with gas send-out requirements, receiving terminals must reduce the gas Btu value.

Options. The most common reduction approach is injecting inert gas (e.g., nitrogen) up to the pipeline content limits—usually 2% to 3%. Air injection is also technically feasible, but if it is only used for minor heating value adjustments to avoid exceeding the minimum free-oxygen content specifications, which are 0.01—0.2%. The Btu reduction by adding inerts is limited to 20-30 Btu/scf. This may be sufficient to adjust quality for some LNG streams, but insufficient for others. Under the current US heat content-based pipeline tariff specifications, few imported LNG supplies meet restrictive gas quality provisions. In 2005, only LNG from Trinidad and Tobago could be directly delivered into most gas markets along the US East Coast.

Conversion for interchangeability. However, if US pipelines translated current heat content specification into a corresponding Wobbe Index (1,330—1,370 Btu/scf), most global LNG supplies, when blended within allowable concentrations of an inert gas, could meet the tariff specification for interchange-ability on the Wobbe Index basis.

Extraction. In cases where inerts cannot meet the US heating value constraints, C2 and heavier components must be removed at extra costs to the LNG receiving terminal. Experiences demonstrate that systems to reduce C2+ or C3+ gases at LNG receiving terminals are more cost-effective in terms of capital investment and operating costs than systems injecting inerts or boosting gas send-out pressure to achieve heating value constraints. Also, C2+ removal solutions yield a high-value natural gas liquids (NGLs) stream at the receiving terminal. The NGLs have an economical benefit and can further enhance the total profitability of the receiving terminal.

Most liquefaction plants are designed to serve clearly identified markets with long-term supply contracts. These contracts also specify the particular quality parameters that the plant must meet. Clarity of gas specifications throughout the US gas network and in supply contracts, incorporating Wobbe Index numbers and dew-point ranges, would assist LNG receiving terminal operators. Such changes could provide flexibility to terminal managers on how to optimize LNG inventories and provide reliable NG supplies to pipeline end users.