Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts First experimental comparison of calorific value measurements of real biogas with reference and field calorimeters subjected to different standard methods F.J. Perez-Sanz a,* , S.M. Sarge a , A. van der Veen b , L. Culleton c , O. Beaumont d , F. Haloua d a PTB - Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116, Braunschweig, Germany b VSL - Van Swinden Laboratory, Thijsseweg 11, 2629, JA Delft, The Netherlands c NPL - National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, United Kingdom d LNE - Laboratoire National de M_etrologie et d’Essais, 1 rue Gaston Boissier, 75724, Paris Cedex 15, France ABSTRACT This study presents the first comparison of experimental calorific value measurements of real biogas performed with three different calorimeters: one reference gas calorimeter developed at the French metrology institute (LNE) and two field calorimeters (Union Instruments CWD, 2005 CT and Cutler Hammer Calorimeter) at the German metrology institute (PTB). All measurement results obtained for calorific values agree within their measurement uncertainties. Uncertainties vary from 0.2% to 2.0% (coverage factor, k = 2) depending on the calorimeter and calibration procedure. Two different standards (DIN 51899 and ISO 6143) were used to calibrate the field calorimeters and these have been compared. This comparison focuses on the calibration procedure, calibration frequency, number and composition of calibration gases and evaluation algorithm. 1. Introduction The global concern about greenhouse gases, global warming and other environmental issues has been present since 1896 [1]. This con- cern is still of high relevance leading to new strategies like, most re- cently, the European Directive on Renewable Energies [2]. This direc- tive aims at reducing the emission of greenhouse gases 20% (from those in 1990), using 20% of energy from renewable sources and increasing the energy efficiency in 20%. This led to an increment in use of re- newable energies like solar, wind or biogas [3]. Concretely, biogas sector developed more than 6800 new biogas plants in Europe between 2010 and 2014 producing 4200 MW [4] from different sources as agricultural, forest, industrial or household feedstocks. Energy pro- duction by solar and wind technologies had a larger development than biogas production, but biogas is a direct energy that can be easily stored. The commercial value of energy gases is given by their energy content and it is, among others, quantified by the gross calorific value. Therefore, accurate, reliable measurements of gas calorific value are of vital importance for the gas trading. This is highly challenging because the composition of biogas fluctuates significantly as shown in Table 1. Differences in concentration lead to a wide range of calorific values from 5.5 kW h∙m −3 - to 8.5 kW h∙m −3 . Indirect methods are often used to measure the calorific value for natural gas. Composition measurements are performed usually by gas chromatography, and then the calorific value of the mixtures is inferred from the composition data and calorific value of the pure components issued from the standard ISO 6976 [6]. This standard covers only CO 2 concentrations up to 15% and other components, like water, lower than 0.0005%, therefore chromatography analysis requires some adaptation before to be suitable for biogas measurements. Furthermore, this method is prohibitively expensive for small producers. Not only the acquisition costs are high, but since it requires high quality gases for its normal use and the calibration process, the maintenance and running costs are significant as well. It also has the disadvantage that biogas might have minor impurities impossible to detect by chromatography. Other techniques, like NDIR for Non-Destructive Infra-Red analysis, are used to measure biogas and biomethane calorific value but the relia- bility of the measurements is weak as these techniques are not linear and mainly single-point calibrated. Because of the measuring principle, direct methods like calorimetry display the calorific value of any mixture regardless of the composition within a reasonable working range. This measuring technique is also much simpler in terms of calibration process. Polynomial regressions are performed and for that different calibration gases with different calorific values are required. This study continues a previous research presented by Haloua et al. [7] in a frame of an European metrology project on non-conventional https://doi.org/10.1016/j.ijthermalsci.2018.06.034 Received 22 August 2017; Received in revised form 10 January 2018; Accepted 28 June 2018 * Corresponding author. E-mail address: fernando.perez@ptb.de (F.J. Perez-Sanz). International Journal of Thermal Sciences 135 (2019) 72–82 1290-0729/ © 2018 Published by Elsevier Masson SAS. T