Epsilon Archive for Student Projects

Abstract

The number of fuel stations for liquefied biogas or liquefied natural gas (LBG or LNG, both composed of liquid methane) in Sweden will increase in the coming few years, from 6 stations to over 30 in total. While there still are very few LNG heavy-duty vehicles (HDV's) on Swedish roads, technical problems may arise at fuel stations due to low consumption. As boil-off gas (BOG) accumulates and causes overpressure, there will be a need to vent or torch BOG to reduce this pressure, unless the BOG is handled some other way. There are several BOG management techniques. At an LBG fuel station with LCBG liquefied to compressed biogas) equipment, a compressor can be added to compress BOG to the CBG storage. If there are vehicles demanding CBG fuel, they may consume BOG and compensate for low LBG demand. There are over 50,000 CBG vehicles in Sweden, and their demand for CBG could help manage BOG problems at underutilised LBG fuel stations.

An LBG fuel station was modelled using Matlab Simulink's Simscape components. The modelled station was equipped with LCBG equipment and BOG compression. Low levels of demand for LBG and CBG at the station were simulated, for the purpose of finding its limits of operation. It was found that four or more LBG HDV's refuelling each day will keep the pressure under 8 bar until the storage is
empty, if starting at the pressure 2,3 bar. The pressure could also be kept under 8 bar with fewer than four LBG vehicles refuelling per day, if CBG vehicles consumed BOG compressed from the storage. Removing BOG lowered the pressure about 100 times more than removing liquid methane, mak-ing it possible to replace LBG vehicles with CBG vehicles, consuming smaller amounts of gas.

In conclusion, very low levels of demand for either LBG or CBG were found to be sufficient to manage BOG at an LBG fuel station. Such low levels of demand would not give enough income to the fuel station and would not justify the investment. Financially sound levels of fuel demand would also reli-ably prevent overpressure in the LBG storage and reduce the need for BOG management. On the
other hand, if LCBG equipment were included in the LBG station for the purpose of selling CBG to a present market, BOG compression might be a good choice of BOG management strategy