Gas 2025 Issue

Dynamic simulation of an LNG plant fuel gas system

Scenario modelling pinpoints pathways to prevent turbine trips seen with temperature, pressure, and instantaneous fuel gas compositional changes, ensuring reliability

Harry Z Ha, Cole Beattie and Javeed Mohammed Fluor Canada Ltd

T he global natural gas (NG) market has expanded sig- nificantly in recent years, with abundant supply from North America fuelling growing demand in Asia and Europe. Asian and European nations are looking to lique- fied natural gas (LNG) as a pathway to pivot away from coal-fired power generation and lower their greenhouse gas emissions. Energy security is also a growing concern for these same nations, and access to a responsible and reliable supply of energy has bolstered demand for North American LNG. With this growing demand, the reliability of LNG production and export facilities in North America is paramount, as any time spent offline could lead to lost rev - enue and giving up market share to other LNG producers in the Middle East and Asia. LNG is produced by liquefying NG with a refrigerant, which needs to be compressed as part of the refrigeration cycle. NG turbines are typically used to power the gear - boxes driving the refrigerant compressors in LNG facilities. If these turbines are interrupted, entire trains of LNG pro - duction can trip offline. Preventing turbine trips is a cru - cial step in ensuring LNG facility reliability. Given lengthy shutdown and start-up procedures for turbines and com - pressors within the LNG train, unscheduled downtime on an LNG train costs owners and operators millions of dollars per day. Gas turbines require a consistent supply of fuel gas (FG), and careful consideration must be taken in the design of the FG system to ensure the turbines can handle expected interruptions to both the gas supply and demand within LNG facilities. Any sudden changes to the FG pressure or the FG composition can lead to a trip in the gas turbines. Accounting for these interruptions ensures the overall reli - ability of the LNG facility and that it can meet its production targets and commercial obligations. LNG plant FG system The FG supply for the gas turbines in an LNG facility is nor - mally sourced from the overhead vapour space in the LNG storage tanks. As the LNG storage tanks are filled or the ambient air temperature rises, the vapour pressure in the tank rises, requiring venting of vapour within the tank. This vapour, known as ‘boil-off gas’ (BOG), is an economical fuel source for LNG facilities. In many LNG facilities, this BOG

is compressed using BOG compressors to fuel the gas tur - bines needed for LNG production. If the LNG storage tanks are being emptied, or there is any disruption or maintenance impacting the BOG com - pressors, the BOG supply to the gas turbines will be inter - rupted. To prevent gas turbine trips in these likely scenarios, a backup fuel supply is provided by NG, typically from the main LNG plant supply. Changing the fuel source from BOG to NG introduces a compositional change to the fuel supply, which the gas turbines cannot handle instantaneously. To accommodate this, FG mixing drums are installed in LNG facilities to suffi - ciently mix the FG supply fed to the gas turbines. The com - position of the FG is defined with a Modified Wobbe Index (MWI). Typically, the Wobbe Index is used to measure the ratio of a gas heating value (HV) and square root of its spe - cific gravity (SG), relative to air. However, to ensure better consistency between different fuels supplied at different conditions, it is ‘modified’ by controlling for temperature (T gas ) in degree Rankine and uses the lower heating value (LHV) in BTU/scf, as shown in Equation 1 :1

(1)

The MWI is used to compare the combustion energy output of different fuels at different temperatures. While gas turbine designs vary, typically, the FG composition fed to the gas turbine indicated by the MWI cannot vary by more than ±5% per minute. FG mixing drums are critical to ensure that when the fuel supply is changed, the transition is slowed to prevent the FG composition from changing so quickly that the gas turbines are tripped. Inlet pressure and temperature of the FG are also key parameters affecting gas turbine performance. Any reduc - tion in temperature or pressure to the gas turbine inlet will reduce the gas turbine efficiency and increase steam con - sumption in other auxiliary steam turbines. While gas tur - bine design can vary, the turbines considered in this article are designed to handle a maximum pressure swing of 10 psi per second. The overall FG system, including its compressors and turbines, is protected from major upsets with a valve that

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