Design and optimization of an emergency system for cryogenic fuels

The use of light hydrocarbons is desirable in order to mitigate the human impact on the environment. Besides, this strategy is a forerunner solution for the implementation of a hydrogen economy in the long term. Cryogenic conditions represent an attractive option to make the use of these fuels more sustainable. Since the bottleneck for their large-scale adoption is represented by safety aspects, an emergency auto-thermal burner was designed. This safety system has the purpose to dispose of the content of a tank truck, in order to avoid the loss of containment and the accidental release of LNG or LH2. The disposal of the fuel includes its vaporization, the heating of the vapor phase up to a temperature suitable for combustion, the mixing with ambient air, and the combustion. The most important technical requirements are the duration of the discharge process and the portability of this device. The temperature of flue gases was estimated through numerical methods. Then, inner and outer heat transfer coefficients were determined for each coiled-tube heat exchanger, and their surface was calculated, solving the equation of energy balance along the pipe. Finally, the nozzle was designed in order to slow down the gaseous fuel slightly upstream of the burner. Two variants are proposed for the emergency burner for LNG: one with separate reboiler and another with evaporation in tube. While the former option is suitable for stationary applications, the latter is easy to transport, assemble, and put into operation, since it is lightweight and does not require any heat transfer fluid. The emergency burner for LH2 was designed with evaporation in tube only. The para-to-ortho conversion was considered since it results from the increase in hydrogen temperature. The enthalpy of this endothermic reaction represents an additional energy request. If the decrease of para-hydrogen fraction along the pipe was neglected, the length of the superheater would be significantly underestimated.