While oil and gas prices now are falling after the latest experience with fuel-price volatility, the Global Energy Decision fuels team is focused on modeling an integrated world-wide system of...
LNG Mitigation Costs: Who Will pick up the tab?
FERC issues a surprising order regarding responsibility for LNG-related retrofit costs.
normally used by an end-use customer in a combustion application. The measure most often used to calculate the interchangeability of natural gas is known as the Wobbe Index, which is determined by dividing the gross heating value (or higher heating value [HHV]) in Btu per standard cubic feet (scf) of a gas stream by the square root of the specific gravity of that stream. FERC has described the importance of the Wobbe Index as follows:
The formula for determining the Wobbe Index takes into account the fact that the heat release rates for a gas stream vary directly with its BTU content, but inversely with the gas’s specific gravity. That is because gas with a higher specific gravity has a lower volumetric flow rate. Therefore, if a gas stream with a higher gas gravity is substituted in a given burner with a fixed fuel-supply pressure, fewer cubic feet of gas will flow across the metering orifice. As a result, to ensure delivery of the same heat release rate to the burner, the substitute gas stream must have a higher heating value per cubic foot to offset the reduced volumetric flow rate. Conversely, if the substitute gas stream has a lower gravity, more gas volume will flow across the orifice during a given interval, and, hence, the heating value of the substitute stream must be lower to maintain the same Wobbe Index. 4
Simply put, the introduction into the United States natural-gas pipeline grid of large quantities of LNG containing a materially different composition than that contained in historic domestic natural-gas supplies, could have serious consequences for end-use combustion applications. As reported by the NGC+ Interchangeability Work Group—comprising 76 participating members from various stakeholder groups across the country, including LNG suppliers, natural-gas pipelines, utilities, power generators, feedstock suppliers, appliance manufacturers, research organizations, gas processors, and state officials—“varying natural-gas composition beyond acceptable limits can have the following effects in combustion equipment:
a. In appliances, it can result in soot formation, elevated levels of carbon monoxide and pollutant emissions, and yellow tipping. It can also shorten heat-exchanger life, and cause nuisance shutdowns from extinguished pilots or tripping of safety switches.
b. In reciprocating engines, it can result in engine knock, negatively affect engine performance and decrease parts life.
c. In combustion turbines, it can result in an increase in emissions, reduced reliability/availability, and decreased parts life.
d. In appliances, flame-stability issues including lifting are also a concern.
e. In industrial boilers, furnaces and heaters, it can result in degraded performance, damage to heat transfer equipment and noncompliance with emission requirements.” 5
After months of collaboration, the NGC+ Interchangeability Work Group published a set of guidelines regarding the quality of LNG received into interstate natural-gas pipelines to ensure that LNG can be received into the U.S. pipeline grid with minimal impacts on current end-user equipment. Specifically, the group recommended the following guidelines: (1) use of the local historical Wobbe Index average with an allowable range of variation of plus or minus four percent, with a maximum Wobbe Index of 1,400; (2) a maximum heating value limit