Speeds of sound for a biogas mixture CH4 + N2 + CO2 + CO from p = (1–12) MPa at T = (273, 300 and 325) K measured with a spherical resonator

• Experimental speeds of sound for a biogas-like mixture of four components are reported.
• A spherical resonator was used for the measurements.
• Experimental data were fitted to a virial-type equation.
• Heat capacities and acoustic virial coefficients were obtained from the speeds of sound.
• All the results were compared with other equations such as GERG 2008 and AGA8.

The present work aims to measure speeds of sound c in a biogas mixture of CH4 + N2 + CO2 + CO, at p = (1–12) MPa and T = (273, 300 and 325) K, using a spherical acoustical resonator. The results are fitted to the virial acoustic equation of state, and the virial acoustic coefficients are obtained, βa and γa and extrapolated to zero pressure, determining the adiabatic coefficient as perfect gas, γpg  , and the isobaric and isochoric heat capacities as perfect gas, Cppg and CVpg, respectively. The speeds of sound are acquired with a mean expanded relative uncertainty of 165 parts in 106 (k = 2) and are compared with the results predicted by the reference equation of state for this kind of mixture (natural gas-like mixtures), EoS GERG-2008. Relative deviations between experimental data and values estimated by this model were less than 700 parts in 106 at T = 325 K, and below 400 parts in 106, and within measurement uncertainty of at T = 300 K, although appreciably higher at isotherm T = 273 K at the highest pressure data for this work, and even reaching values above 3,400 parts in 106.