Novel algorithm for calculating the methane number of liquefied natural gas with defined uncertainty

Abstract

Liquefied natural gas (LNG) is increasing in importance both as an energy carrier and as a transport fuel. While the developments for an improved infrastructure for LNG are significantly advanced, no commonly agreed method for the characterization of LNG mixtures in terms of the so-called methane number (MN) exist. In this work we present a novel, simple and robust algorithm for calculating the methane number from LNG composition. It combines the detailed experimental data used to develop the commonly used method by AVL (“Anstalt für Verbrennungsmotoren Prof. H. List”) with automated calculation and optimisation routines that guarantee for a high degree of repeatability and reproducibility. This is in accordance with other modern MN calculation tools. The algorithm shows good agreement with other popular methods for a set of exemplar LNG mixtures covering a broad MN range between 60 and 99. Our comparison indicates that the observed differences between the methods might stem from different approaches used for the higher hydrocarbon and inert gas corrections. For the first time to our knowledge the algorithm also determines the uncertainty associated with the calculated MN yielding expanded uncertainties that vary between 0.2 and 0.7MN depending on the composition of the mixture. We believe that incorporating the uncertainty associated with the calculation of the MN is important for developing a legislation for LNG quality as it would significantly enhance the confidence provided by the results of the calculation tools. In this context the definition of reasonable uncertainty limits in addition to a lower MN limit is recommended.