Optimizing LNG Production via Boil Off Gas System Design
John Ergina – Technical Professional – Process, KBR
Sumit Sharma – Senior Technical Professional – Process, KBR
In all liquefied natural gas (LNG) facilities, low pressure operations and equipment generate flash vapor collectively termed as Boil Off Gas (BOG). The BOG is either utilized as fuel gas or recycled to the inlet of the LNG train for liquefaction in order to eliminate flaring as an option for disposal. Overall plant design and operating flexibility set the limitations for the BOG system design. Within the identified limits, design optimization achieves the balance between maximizing LNG production and generating enough BOG to meet plant fuel gas demand.
The design choice between on-site power generation and external power import significantly affects BOG system capacity. Self-sufficient LNG facilities with on-site power generation will have a larger BOG unit to support higher fuel gas demand. However, LNG facilities with convenient access to a local electrical power grid for plant power, such as in the US Gulf Coast, will have lower fuel gas demand and smaller BOG unit. These two scenarios create an envelope for fuel gas consumption and BOG system design.
Optimization of BOG system design is achieved by operating flexibility that shifts the refrigeration load away from the main refrigerant compressor. Warmer LNG exiting the cryogenic exchanger leads to reduced load on the refrigeration compressors, but results in higher BOG flow and larger BOG compression power. With the load shift, additional refrigerant compressor capacity becomes available for increasing LNG throughput.
The existence of BOG invariably adds capital cost to the LNG plant economics. A robust design of BOG system manages the added cost by optimizing net LNG production. In this paper, an overview of the BOG system is discussed with a focus on factors that determine BOG system design. Optimization strategies for BOG system design are also presented that indicate increasing BOG system capacity reaches a point of diminishing returns on overall project economics.