Accurate experimental ($p$, $\rho$, $T$) data of natural gas mixtures for the assessment of reference equations of state when dealing with hydrogen-enriched natural gas

TL;DR

This study provides high-precision experimental density data for natural gas mixtures, including hydrogen-enriched variants, to evaluate and improve the accuracy of existing equations of state like GERG-2008 and AGA8-DC92.

Contribution

The paper presents new experimental density data for natural gas mixtures with and without hydrogen enrichment, testing the performance of standard equations of state on these real samples.

Findings

GERG-2008 shows an average deviation of 0.027% for conventional natural gas.

AGA8-DC92 has an average deviation of 0.078% for conventional natural gas.

Deviations increase for hydrogen-enriched mixtures, indicating areas for model improvement.

Abstract

The GERG-2008 equation of state is the approved ISO standard (ISO 20765-2) for the calculation of thermophysical properties of natural gas mixtures. The composition of natural gas can vary considerably due to the diversity of origin. Further diversification was generated by adding hydrogen, biogas, or other non-conventional energy gases. In this work, high-precision experimental ($p$, $\rho$, $T$) data for two gravimetrically prepared synthetic natural gas mixtures are reported. One mixture resembled a conventional natural gas of 11 components (11 M) with a nominal mixture composition (amount-of-substance fraction) of 0.8845 for methane as the matrix compound. The other mixture was a 13-component hydrogen-enriched natural gas with a low calorific value featuring a nominal composition of 0.7885 for methane. Density measurements were performed in an isothermal operational mode at temperatures between 260 and 350 K and at pressures up to 20 MPa by using a single-sinker densimeter with magnetic suspension coupling. The data were compared with the corresponding densities calculated from both GERG-2008 and AGA8-DC92 equations of state to test their performance on real mixtures. The average absolute deviation from GERG-2008 (AGA8-DC92) is 0.027% (0.078%) for 11 M and 0.095% (0.062%) for the 13-component $H_{2}$-enriched mixture, respectively. The corresponding maximum relative deviation from GERG-2008 (AGA8-DC92) amounts to 0.095% (0.127%) for 11 M and 0.291% (0.193%) for the $H_{2}$-enriched mixture.