⛽ Superior and inferior calorific value, density, relative density and Wobbe indices of natural gas and other gaseous fuels — calculation from the composition in mole fractions; PN-EN ISO 6976:2016-11

Petrochemistry PN-EN ISO 6976

Overview

WHAT THE STANDARD COVERS. Scope from the PKN card of PN-EN ISO 6976:2016-11 (Polish version), the main part in full (our translation): "This document gives methods for calculating the inferior calorific value, the superior calorific value, the density, the relative density, the superior Wobbe index and the inferior Wobbe index of natural gases, natural gas substitutes and other gaseous fuels, when the composition of the gas expressed in mole fractions is known. These methods give ways of calculating the properties of gas mixtures at commonly used reference conditions. The mole fractions by definition sum to unity. Guidance on meeting this requirement by chromatographic analysis is available in ISO 6974-1 and ISO 6974-2. The calculation methods require values of various physical properties of the pure components; these values, together with the associated uncertainties, are given in tables and their sources are identified. Methods are given for estimating the standard uncertainties of the calculated properties. The methods of calculating the values of properties on the basis of the composition expressed in mole, mass or volume fractions apply to any natural gas, natural gas substitute or other combustible fuel which is normally in the gaseous phase, except that for properties on the basis of the composition expressed in volume fractions the method is restricted to mixtures for which the compression factor at reference conditions is greater than 0.9. Example calculations for the recommended methods of calculation are given in Annex D." Further on, the card quotes notes 1–5 of the scope of the standard: synonyms of terms (e.g. "heating value" for "calorific value", "Wobbe number" for "Wobbe index"), no explicit limits of composition (an indirect limit is set by the condition of a compression factor greater than 0.9), calculation usually for dry gas with separate allowance for water vapour (ISO/TR 29922), assigning isomers of hydrocarbons with 7 or more carbon atoms to the "n-" isomer, and pseudo-components (e.g. the groups C6+ or C7+, as well as "spectator water" and non-combustible hydrogen sulfide, to which an enthalpy of combustion of zero is assigned). We read the content of the method in the sample extract of ISO 6976:2016 from the cover to clause 8.1, with the table of contents.

ACCORDING TO THE TEXT OF ISO 6976:2016 (foreword, introduction, clauses 1–8.1). The standard was prepared by ISO/TC 193 "Natural gas", subcommittee SC 1 "Analysis of natural gas"; the third edition (2016-08-15) replaced the second (ISO 6976:1995). Introduction: when gas is transferred between parties, both the quantity and the quality of the gas are needed; values calculated according to the 2016 edition differ only very slightly from those calculated according to the 1995 edition, but implementation has a cost (the software of instruments must be updated), changes the recorded and invoiced energy slightly, and, if implemented uncritically — e.g. at the entry to a transmission system but not at the exit — may cause a costly imbalance of settlements; commercial, contractual, regulatory and legal obligations must also be taken into account, so it may be appropriate to carry out an impact assessment and agree on an implementation date. References (2): ISO 6974-1 and ISO 6974-2 (composition and its uncertainty from gas chromatography) and ISO 14912:2003 (conversion of gas mixture composition data). Definitions (3): superior calorific value — the heat released by the complete combustion of a specified quantity of gas in oxygen at constant pressure p1, with all the products returned to the temperature t1 of the reactants, with the water condensed; inferior calorific value — the same, but with all the products in the gaseous state; relative density — the density of the gas divided by the density of dry air of reference composition under the same conditions (as ideal or real); superior and inferior Wobbe index — the superior or inferior calorific value on a volumetric basis divided by the square root of the relative density, whereby "Wobbe index" alone customarily means the superior one; compression factor — the actual volume divided by the volume calculated from the ideal gas equation; the reference conditions of combustion (t1, p1) and of volume metering (t2, p2) need not be the same, and many sets of them are used worldwide; 15.55 °C means 60 °F, and 101.325 kPa is taken as equal to 14.696 psi. Principles (5): the superior calorific value of the mixture on a molar basis is the sum of the tabulated ideal-gas values for the individual components weighted by their mole fractions; from it the inferior calorific value and the values on a mass and volumetric basis are calculated; the density and the relative density — from the mean molar mass weighted by mole fractions; for the real gas a volumetric correction (compression factor) is applied, and the enthalpic correction was found to be negligibly small (ISO/TR 29922), so the real-gas values on a molar and mass basis are taken as equal to the ideal-gas values; uncertainties are calculated according to GUM, taking into account correlations between mole fractions and between the molar masses of components (formulae in Annex B); the data tables are in clause 12, auxiliary data in Annex A, examples in Annex D. Enthalpies of combustion (6.1) are given for temperatures t1 = 25 °C, 20 °C, 15.55 °C (60 °F), 15 °C (59 °F) and 0 °C. The compression factor (6.2), needed for all values on a volumetric basis and for density, relative density and the Wobbe index, is calculated with formula (1) using summation factors from Table 2, valid for 90 < p2/kPa < 110 with p0 = 101.325 kPa; the same formula does not give acceptable values for pure hydrogen, helium and neon (Z > 1) or for components that are not gases under the metering conditions. Formulae (7.1–8.1): superior calorific value on a molar basis (Hc)G = Σ xj·[(Hc)G°]j; inferior calorific value (Hc)N = (Hc)G − Σ xj·(bj/2)·L°(t1), where bj is the number of hydrogen atoms in the molecule of the component, and L°(t1) — the standard enthalpy of vaporization of water; superior calorific value on a mass basis — the molar value divided by the molar mass of the mixture M = Σ xj·Mj.

STATUS (as of 08.10.2026). PKN cards of PN-EN ISO 6976:2016-11 — Polish version (publication 03-06-2019, 64 pages) and English version (publication 08-11-2016, 70 pages), both without a withdrawal label: title "Natural gas -- Calculation of calorific values, density, relative density and Wobbe index from composition" (our translation), price group V, Machinery and Engineering Sector, KT 277/PK 1 Measurement and Quality Assessment of Gaseous Fuels, "Introduces: EN ISO 6976:2016 [IDT], ISO 6976:2016 [IDT]", "Replaces: PN-EN ISO 6976:2008 - Polish version", ICS 75.060. PN-EN ISO 6976:2008 (Polish version, publication 28-01-2008), which introduced EN ISO 6976:2005, i.e. ISO 6976:1995 with corrigenda Cor 1:1997, Cor 2:1997 and Cor 3:1999, has the label "Withdrawn and replaced by PN-EN ISO 6976:2016-11 - English version" and the withdrawal date 08-11-2016. PKN search, term "ISO 6976" (08.10.2026): 6 items, all with this number; only the two versions of the 2016 edition are current, withdrawn are PN-ISO 6976:2003, PN-EN ISO 6976:2008 and both versions of PN-EN ISO 6976:2005. evs.ee catalogue: ISO 6976:2016 in force since 04.08.2016, in the history ISO 6976:1995 with corrigenda.

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database and current PCA documents, read on 08.10.2026). In the database copy the number appears in 27 records at 9 laboratories; in the current PCA documents (issues from 02.12.2024 to 01.10.2026) it occurs 24 times at the same laboratories; none of these entries is suspended (the suspension of part of the scope of AB 484 from 30.07.2026 does not cover them). Notations: "PN-EN ISO 6976:2016-11" — 8 laboratories (AB 081, AB 278, AB 300, AB 484, AB 511, AB 1220, AB 1228, AB 1780); AB 041 — "PN-EN ISO 6976:2008", i.e. the withdrawn edition based on ISO 6976:1995, in one entry and "PN-EN ISO 6976" without a year in two. The "Laboratories" tab (term "PN-EN ISO 6976") shows 8 laboratories and 22 records; it does not show AB 1220, whose notation in the database begins with the number of its own procedure "PB/02 wyd. 17 z 01.07.2024" (in the PCA document this procedure concerns the determination of the composition, and the standard — the superior and inferior calorific value). The term does not pick up standards with other numbers.

WHAT THE LABORATORIES TEST (PCA documents). All quantities are "calculated", on the basis of the composition: inferior calorific value — all laboratories; superior calorific value — all except AB 484; density — AB 041, AB 081, AB 278, AB 300, AB 484, AB 511, AB 1228; relative density — AB 041, AB 278, AB 300, AB 511, AB 1228, AB 1780; Wobbe index — AB 041, AB 278 and AB 1228 (superior and inferior), AB 300, AB 511, AB 1780. Subject: natural gas (AB 041, AB 278, AB 484, AB 511, AB 1220, AB 1228, AB 1780), gas mixtures (AB 041, AB 278, AB 1228), refinery gas and biogas (AB 041), coke oven gas (AB 081, AB 300, AB 1220), surplus gas, power gas and mine gas as well as biogas (AB 300), gases from technological processes — post-oxidation and hydrocarbon gases — and fuel gas (AB 484), converter gas, blast furnace gas and mixed gas (AB 1220). The laboratories determine the composition of the gas by gas chromatography (TCD and FID detectors) according to other documents — e.g. PN-EN ISO 6974-1 (AB 511), PN-EN 27941 and ASTM D 1946 (AB 484) or their own procedures.

WHERE A SEEMINGLY CORRECT RESULT IS EASY TO GET. First — reference conditions: a value without the stated combustion temperature t1 and the volume metering temperature and pressure t2, p2 is ambiguous; the standard gives tables for five combustion temperatures, and the metering conditions need not be the same as the combustion conditions. Second — volumetric basis: values per cubic metre, density, relative density and Wobbe indices require a correction for the compression factor; the volumetric basis is restricted to mixtures with a compression factor greater than 0.9 at reference conditions, and formula (1) is valid for pressures of 90–110 kPa. At this point the description of the scope on the PKN card speaks of "composition expressed in volume fractions", whereas the text of ISO 6976:2016 has "properties on the volume basis"; the input composition is always in mole fractions. Third — dry gas: the properties of dry gas are usually calculated, and the effect of water vapour is taken into account separately (note 3, ISO/TR 29922). Fourth — composition: mole fractions by definition sum to unity (guidance in ISO 6974-1 and ISO 6974-2), pseudo-components such as C6+ or C7+ are defined by the user to suit the application, and the uncertainty of the composition passes into the uncertainty of the result (clause 11). Fifth — the edition: AB 041 cites in one entry PN-EN ISO 6976:2008, i.e. ISO 6976:1995; according to the introduction to the 2016 edition the results differ little, but the change affects energy settlements and requires an agreed implementation. Sixth — industrial gases: the scope covers "other gaseous fuels" of known composition and does not set explicit limits of composition (note 2), so coke oven gas, blast furnace gas or converter gas fall within this wording; whether the tables of the standard cover all their components does not follow from the extract read.

WHAT WE DO NOT GIVE. We do not have clauses 8.2–12 or the annexes: the formulae for the inferior calorific value on a mass basis, for the volumetric basis, density, relative density and Wobbe indices (clauses 9–10), the methods of estimating uncertainty — analytical, general and fall-back (clause 11), the data tables for the pure components (clause 12), auxiliary constants, uncertainty formulae and example calculations (Annexes A–D). WE DO NOT GIVE numerical values from the tables.

Method principle

From the composition of the gas in mole fractions (usually from chromatographic analysis) the superior calorific value of the mixture is calculated as the sum of the tabulated values of the pure components weighted by their mole fractions, and from it the inferior calorific value, the values on a mass and volumetric basis, the density, the relative density and the superior and inferior Wobbe indices, at selected reference conditions; for the volumetric basis the compression factor of the mixture is applied (ISO 6976:2016, clauses 5–8). The standard also gives methods for estimating the uncertainty of the results according to GUM. The standard itself measures nothing — it calculates from the result of the composition analysis.

Applications

• Natural gas — superior and inferior calorific value, density and Wobbe indices by calculation; in the current accreditation scopes 9 laboratories
• Biogas, refinery gas and gas mixtures (AB 041)
• Industrial gases — coke oven, blast furnace, converter, mixed, surplus and mine gas and process gases (AB 081, AB 300, AB 484, AB 1220)
• Energy settlement of transferred gas — values together with the uncertainty, at agreed reference conditions

Key parameters

Standard

Standard number

PN-EN ISO 6976:2016-11 (wersja polska i wersja angielska; wprowadza EN ISO 6976:2016 [IDT] i ISO 6976:2016 [IDT])

Title (PL)

Gaz ziemny — Obliczanie wartości kalorycznych, gęstości, gęstości względnej i liczby Wobbego na podstawie składu

Title (EN)

Natural gas — Calculation of calorific values, density, relative density and Wobbe indices from composition

Step-by-step procedure

1. Gas composition

   Establish the composition in mole fractions summing to unity (usually by gas chromatography; guidance in ISO 6974-1 and ISO 6974-2); define pseudo-components, e.g. C6+, to suit the application (scope, notes 4–5).

2. Reference conditions

   Establish the combustion temperature t1 (25, 20, 15.55, 15 or 0 °C) and the volume metering temperature and pressure t2, p2 (3.11–3.12, 6.1).

3. Values on a molar basis

   Superior calorific value of the mixture = Σ xj·[(Hc)G°]j; inferior calorific value = superior calorific value − Σ xj·(bj/2)·L°(t1) (7.1–7.2).

4. Mass and volumetric basis

   Mass basis: divide by the molar mass of the mixture M = Σ xj·Mj (8.1); volumetric basis, density, relative density and Wobbe indices — with the correction for the compression factor according to formula (1), only when Z > 0.9 (6.2); WE DO NOT GIVE the formulae of clauses 9–10.

5. Uncertainty

   Estimate the uncertainty according to clause 11 and Annex B, taking into account the correlations between mole fractions (clause 5); WE DO NOT GIVE the formulae.

6. Result

   Give the values together with the reference conditions and the edition of the standard; WE DO NOT GIVE the rules for expressing the results (11.5).

Required equipment and apparatus

Health and safety (OHS)

• The method itself is a calculation; the hazards concern the sampling and analysis of combustible gases — according to the documents and instructions for those activities
• TECHNICAL NOTE: a result without reference conditions is ambiguous; the volumetric basis only for mixtures with Z > 0.9; the change of edition from 1995 to 2016 changes the results little, but affects energy settlements

Frequently asked questions