Natural gas (NG) is the cleanest fossil fuel and the fastest growing energy resource for more than two decades. Liquefied Natural Gas (LNG) is the most common and economical option to transport natural gas over long distances. LNG is a cryogenic liquid at –169 at atmospheric pressure. In spite of heavy insulation, heat ingress into LNG during its storage and transport is inevitable. This causes a portion of the LNG to vaporize at to form Boil-Off Gas (BOG). Regasification terminals which store the LNG, and regasify it continuously to supply NG to gas distribution grids or customers are concentrated sources of the BOG. Unless reliquefied and/or reused, the BOG is a loss of valuable product, alters LNG composition and causes pollution if flared. Also, the power/energy required to handle the BOG forms most of the expenditure for a regasification terminal, and minimizing this undesirable energy consumption is important. This study focuses on BOG management at a typical regasification terminal. It aims to synthesize a thermodynamic process for BOG management to establish a lower bound on the terminal’s total power consumption. This thermodynamic lower bound can then be used to evaluate the actual performance of a regasification terminal.
The present literature has studied a spectrum of BOG management topics including BOG estimation from various sources, BOG minimization, exergy and/or exergoeconomic analyses of BOG management systems, etc. Reliquefaction, direct compression, linkage with a power generation cycle, and use as a fuel in another process plant are the prominent methods of managing the BOG at a terminal. However, most terminals are inconveniently located to form feasible/profitable links with process or power plants. Hence, a study of retrofit or grassroots design of a stand-alone terminal is a worthy undertaking. Liu et al.  reported multi-stage recondensation schemes to reduce the energy needed for reliquefying BOG in a regasification terminal. However, as our work shows they did not reach the minimum energy consumption. Park et al.  examined the retrofit design of a BOG reliquefaction system. While they achieved lower consumption levels than those of Liu et al. , but they did not consider some process options. Therefore, they also could not establish minimum energy benchmarks for a regasification terminal.
In this study, we develop a deeper insight into the regasification process and approach the thermodynamic minimum power consumption targets for an LNG regasification terminal. Using simple design/operation guidelines, we present a series of regasification schemes that eventually lead to the one with much lower power consumption than those reported in the literature. This scheme maximizes the use of LNG cold energy to minimize the total power consumption significantly. We have developed a nonlinear programming formulation to obtain the minimum power consumption for each regasification scheme. For the terminal presented by Park et al. , we obtain minimum energy needs for a range of BOG amounts, and show that our better schemes offer about 10-15% reduction in power consumption.
- Chaowei Liu JZ, Qiang Xu,and John L. Gossage. Thermodynamic-Analysis-Based Design and Operation for Boil-Off Gas Flare Minimization at LNG Receiving Terminals. Industrial & Engineering Chemistry Research. 2010; 49:7412–20.
- Park C, Song K, Lee S, Lim Y, Han C. Retrofit design of a boil-off gas handling process in liquefied natural gas receiving terminals. Energy. 2012; 44:69-78.
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