(159c) Flare Minimization of Boil-Off Gas during LNG Production

Yogesh Kurle and Qiang Xu

Dan F. Smith Department of Chemical Engineering

Lamar University, Beaumont, TX 77710, USA

Abstract

            With the continuously increasing of clean energy demands, the world-wide LNG (liquefied natural gas) production capacity is being expanded very fast and LNG is actually becoming the world’s fastest growing energy sector. United States Energy Information Administration (EIA) states that world natural gas trade, both by pipeline and by shipment in the form of LNG, is poised to increase in the future.  New LNG terminals, which are currently under construction, will increase the LNG production by 100 million tons per year (MTPA). Over 250 MTPA of liquefaction capacity has been proposed in North America alone. In 2013, 240 million tons of LNG was delivered worldwide.

LNG industries are also facing problems of LNG evaporation occurring at different places in its supply chain. The evaporated material is called Boil-off Gas (BOG), which is generated during LNG production, storage, loading, transportation, and unloading. Normal BOG generation at exporting terminals alone ranges from 2 to 6% of LNG produced. If this is not recovered and reused, the amount of material lost would be equivalent to at least one large-scale LNG plant capacity. Also due to stringent environmental regulations, flaring of BOG is not a viable option. Since LNG production is increasing, BOG generation problem would become more severe. If this issue is not addresses properly and in time, losses of valuable materials and energy plus air pollutions would be significantly greater than ever.

In this paper, the steady-state simulation tool Aspen Plus v8.2 was used to explore various options for BOG recovery.  LNG plant with 1100 m³/hr production rate and 10,000 m³/hr LNG loading rate was considered.  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 of LNG. BOG simulation results are validated using industrial data. The studied recovery options showed possible BOG recovery with energy usage less than 10% of the recovered energy from BOG. It is equivalent to more than 90% savings in energy when compared to complete flaring of BOG.

Since LNG loading is an intermittent and unsteady process, dynamic simulations are necessary to know the rate of change of BOG generation during loading, study the effect of BOG recycle on plant performance and controllability, and to study system behaviors with respect to changes in different parameters. In this paper, various factors were taken into consideration through dynamic simulations, viz., LNG composition and temperature, ambient temperature, jetty length, heat transfer coefficient of LNG tanks and pipelines.  Further control strategy was developed to make system capable of tolerating disturbances and to improve the system performance.  This study will help LNG plants choose a better option for BOG recovery based on individual plant environment and needs. Each LNG plant environment can be different in various ways like jetty length, LNG storage capacity, loading rate and frequency, available space for additional equipment. Similarly plants can have different demands of fuel gas, desired product specifications, controllability issues, which affects BOG recovery strategies.