Numerical Simulation Study on Liquefied Natural Gas and Boil-off Gas Behaviour Changes, Boil-off Gas Recovery and Conversion to CNG fuel at LNG Receiving Terminal

Abstract

As the energy market continues to grow, the liquefied natural gas has attracted attention as a cheap and environmentally friendly option to carry natural gas for distant markets. As a result, the liquefied natural gas supply chain has also witnessed a steady increase. The boil-off gas (BOG) generated at LNG receiving terminals is significant and industry practice is to flare and vent it to the atmosphere, which causes environmental damage. This work proposes a technical strategy to minimize the flaring and venting of excess BOG by using it as an alternative fuel for CNG-powered vehicles at the LNG receiving terminal. Dynamic simulation studies were carried out to quantify and recover the dynamic generation of BOG during LNG regasification, ships unloading and holding mode, under hourly ambient temperature between 26th May and 13th June 2024 for the hypothetical Maputo LNG receiving terminal of a 200,000 m³ aboveground full containment LNG storage tank. The heat ingress and BOG recovery at the LNG receiving terminal were simulated with LNG and generated BOG property changes analysis. Whilst the BOG conversion into CNG was investigated with the use of multistage compressor. The model was simulated for BOG generated by three different LNGs to analyse the resulted CNG suitable for to be used as vehicle fuel. The simulation results show that BOG recovered from lean, medium and rich LNG respectively is 315913.51kg, 290373.34kg and 307147.70kg during LNG regasification, 186482.32kg, 156570.73kg, and 173210.30kg during ships unloading with 168628.30kg, 140249.76kg and 159199.1kg solely during LNG pumping and piping, and 22462.53kg, 22370.38kg, and 19279.79kg during holding mode. A 100% excess BOG produced were recovered. During LNG regasification, lean, medium, and rich LNG storage tank served 361 hours, 383 hours and 406 hours respectively with a constant 200,000 kg hourly LNG regasification. During ships unloading mode, lean, medium and rich LNG storage tank took 15.2497 hours, 15.2496 hours, and 15.2492 hours to get at 95% storage capacity. The ships unloading mode produced more CNG hourly, more energy consumption for a CNG kilogram produced during ship unloading mode except in medium LNG, and CNG from lean and medium LNG’s BOG are on spec to be used as a vehicle fuel.