How to handle boil-off gases from LNG trucks

Abstract: This master thesis project aims to investigate the circumstances of boil-off for heavy vehicle transports, using LNG as fuel, and suggest possible ways of handling these gases otherwise released into the atmosphere. LNG, Liquefied Natural Gas, is when natural gas is cooled below its vaporisation point, turning it into liquid phase which is a much more dense way of storing and transporting the fuel. Trucks running on LNG store their fuel in super insulated tanks, but some heat are transferred to the fuel anyway, causing it to vaporise at a steady rate. During driving of these trucks, this vaporised gas is consumed and the pressure are kept at a certain level of 10 bar. Once the truck is turned off, consumption stops and the pressure starts to increase. After a period of standstill, normally several days, the pressure within the tank has increased to 16 bar where a valve opens to release gas from the tank. This is a safety feature, causing the pressure not to increase further creating hazards. While natural gas, mostly containing methane, is released, fuel is lost and an environmentally unfriendly substance is let out into the atmosphere. This should be avoided, to improve the environmental aspects of using LNG as a fuel for trucks, which most likely will be regulated by laws yet to come. Since the release of boil-off gases rarely should happen during the regular use of these trucks, but a system handling these gases should work at any time and place, a cheap and lightweight unit are to be fitted to these trucks. Equipment already on the truck should be used as much as possible, keeping additional costs and weight low. The only practical way of storing this gas, which is the most resource efficient way of handling these boil-off gases, is to re-liquefy it and transferring it back to the usual LNG tank on the vehicle. The second best option is to consume the gas, making it less environmentally unfriendly. While consuming the gas, as much as possible of its energy should be utilised as electricity and heat. Preferably, electricity should be produced as much as possible, charging the batteries on the truck, decreasing the fuel consumption while running and increasing the lifespan of the batteries. The most efficient way of managing the heat generated is to distribute it to the coolant system on the truck, providing it to the engine and several other components. Distributing the energy is also a matter of safety, as very hot areas otherwise might cause risks of fire. The most simple concept, that is easiest to implement in the near future, is to use a burner similar to the auxiliary diesel heaters fitted to some trucks today. This consumes fuel, generating heat to the coolant system. An additional cooler is needed, to cool of excess heat from the system keeping the temperatures to a certain level. Using this system for an extensive period of time needs an external power supply, since no electricity is generated from consuming the gas. Other technologies that could be used in the future, also generating electricity, is thermoelectric generators and solid oxide fuel cells. These are technologies now being further developed and adapted to the industry of vehicles. These technologies are especially interesting when they are implemented to these trucks for use within other systems as well, for instance utilisation of the heat within exhaust gases. A small scale re-liquefaction unit mounted to the truck is however seen as the most resource efficient solution, making it possible to keep using the gas for it intended purpose of propelling the trucks forward. This technology has to be made more compact, adapting it to the use on a truck.