On the use of low carbon fuels. A phenomenological, safety, environmental and economic analysis.

This work presents an exhaustive analysis of the use of liquefied natural gas (LNG) and hydrogen. At first, the possible consequences of the accidental release of these fuels were analyzed. More specifically, ad-hoc experimental campaigns and detailed computational models suitable for cryogenic conditions were employed to fill the literature gap. Regarding experimental tests, either reactive or non-reactive scenarios were experienced. For LNG, 4 pool fires and more than 50 evaporation trials were conducted. In addition, direct and indirect water mitigation systems were employed. On the hydrogen side, 17 accidental releases of hydrogen from a high-pressure facility, operating up to 450 bar, in the presence of an ignition source were investigated. Concerning the numerical approach, computational fluid dynamics (CFD) models were employed to recreate hardly experimentally replicable scenarios and to scale the obtained experimental results to different boundary conditions. Related to the former, the consequences of the rapid phase transition (RPT) of LNG and liquid hydrogen (LH2) were analyzed. In addition, phenomenological aspects such as LNG visibility and stratification were also considered. More particularly, the characterization of the visible cloud boundaries generated in the case of an unintentional release of LNG was evaluated and compared with the standard value for the flammability of clouds based on the lower flammability limit (LFL). Related to the stratification, the effects of thermal and mass stratifications on combustion efficiency were analyzed, too. As a result, the safety distances calculated by CFD for stratified mixtures were found almost double than the corresponding values estimated for homogeneous cloud and by integral models commonly adopted in risk analyses. Besides, the presence of sulphur-based substances in the biogas (sour) produced from the digestion plant was also considered. Regarding the latter point (i.e., to scale the obtained experimental results to different boundary conditions), CFD models and quantum mechanical (QM) calculations were coupled to numerically mimic obtained hydrogen jet fire experimental data. This model was also extended and adapted to hydrogen-methane mixtures in the framework of injection of compressed hydrogen into the existing natural gas pipelines as a way forward to reduce the quantity of carbon-based fuel. Secondly, results were used to define possible general guidelines to be applied during bunkering and loading/unloading operations or, more generally, when the product is transferred from one storage point to another. Indeed, a possible method to identify the boundaries of the safety areas was reported. This method was consequently compared to standard procedures, which are often considered unreliable. Parallelly to these activities, a comprehensive analysis of the authorization path followed by the Venice LNG S.p.a., the main sponsor of the present work, was presented. The project consists of one atmospheric pressure tank that will be supplied by small and medium-sized LNG tankers, while re-distribution of the product will be guaranteed by small trucks and LNG carriers. In addition, the developed methodologies were applied to the specific case of Venice LNG. Moreover, economic and environmental analyses were conducted to face the main event of recent years. At this aim, an optimization of the plant aimed at reducing the initial investment was conducted. Subsequently, the possible commercial diffusion of low carbon fuels and the relative potential environmental impact were investigated.