Boil-Off Gas Minimization and Recovery Options at LNG Loading Terminals

            In LNG plants, boil-off gas (BOG) is generated during production, storage and loading of LNG. Particularly during loading, huge amount of BOG is generated, which might require flaring. This results in wastage of material and energy. Also due to stringent environmental regulations, flaring of BOG is not a viable option. Proper handling of BOG is necessary to avoid BOG flaring. At LNG plant and exporting terminal, BOG is generated due to five main factors: (1) depressurization of LNG, (2) heat leaks through containers and pipelines, (3) tank breathing, (4) heat added by equipment like pumps, and (5) LNG carrying vessels being hot before loading LNG.

            Aspen Dynamic v8.0 was used to simulate the liquefaction and loading process. Heat transfer coefficients and heat capacities of tanks and pipelines were calculated based on their typical design and materials of construction used in reality. The simulation models heat transfer between LNG equipment and environment. Also the vapor displacement factor is considered in the dynamic simulation. Pipeline model is used to simulate hydraulics. The BOG generation amounts at different locations (viz. flash, storage, and jetty) were validated using reported industrial data. Various options for BOG recovery were considered: Use of BOG as fuel gas, feed gas, make-up gas for storage tanks; liquefy BOG on shore, liquefy BOG on jetty/ship (with and without NRU); store and reuse BOG etc. Based on plant environment and needs, different BOG recovery options might be feasible for different plants; and some might be more economic than the others.

            Most of the LNG plants have multiple trains and can have simultaneous loading of LNG vessels. This makes BOG handling more complex. These factors were considered to study their effects on BOG recovery in terms of equipment capacity needs, controllability and cost effectiveness; when a common facility is to be used to recover BOG. Capital cost and operating cost were calculated for each BOG recovery option based on requirement of additional equipment and energy; thereby providing comparison amongst those options. This study will help LNG plants choose a better option for BOG recovery based on individual plant environment and needs.