Liquefied natural gas (LNG) transport vessels, regasification terminals, and liquefaction plants all must deal with boil-off gas (BOG) — ordinarily methane and nitrogen — during operations. BOG is created due to the infiltration of heat, volume displacement, and pressure variation during the storage and movement of LNG. Improvements of revenue, energy efficiency, and the environment have driven operators to optimize their systems to include BOG recovery.
The idea of optimizing BOG recovery during LNG storage and processing has been around for a while. For example, the August 2008 issue of LNG Industry covered Kobelco and its LNG BOG reciprocating gas compressors (PDF) used to reliquefy recovered gas in a receiving terminal. In 2013, a Hydrocarbon Processing article discussed the optimization of LNG BOG systems in regasification terminals. And a recently published paper in Journal of Engineering for Gas Turbines and Power specifically focused attention on these systems as used in liquefaction plants.
But what are some of the strategies being suggested for more efficient BOG recovery and monitoring? Some designs optimize operating pressure ranges of BOG recovery systems to minimize electrical and fuel gas costs of the process. Additional considerations go into choosing a reciprocating or centrifugal compressor, with many developers arguing for reciprocating compressors for their wide control ranges and ability to discharge to high pressures. And several researchers and operators have considered the benefits of using dynamic simulation of various BOG systems. As loads and pressures can fluctuate, a simulator with various control algorithms can be implemented to improve system reliability and energy efficiency.
One other technology useful to LNG facilities is the flow monitoring system. LNG tankers, for example, have increasingly been made to comply with CO2 emissions set by the International Maritime Organisation (IMO). The IMO regulations require measurement of energy efficiency, and as LNG tankers have begun using BOG to power the vessel, monitoring systems must be put into place. Companies like Fluid Components International have been improving flow control and monitoring systems for BOG so they may handle the complex measurements of various process gases and liquids for regulatory purposes. These control and tracking systems can of course also be applied to terminals and plants.