# SIST EN ISO 6976:2005

(Main)## Natural gas - Calculation of calorific values, density, relative density and Wobbe index from composition (ISO 6976:1995 including Corrigendum 1:1997, Corrigendum 2:1997 and Corrigendum 3:1999)

## Natural gas - Calculation of calorific values, density, relative density and Wobbe index from composition (ISO 6976:1995 including Corrigendum 1:1997, Corrigendum 2:1997 and Corrigendum 3:1999)

Specifies methods for the calculation of the superior calorific value and the inferior calorific value, density, relative density and Wobbe index of dry natural gas and other combustible gaseous fuels, when the composition of the gas by mole fraction is known. Replaces the first edition, which has been technically revised.

## Erdgas - Berechnung von Brenn- und Heizwert, Dichte, relativer Dichte und Wobbeindex aus der Zusammensetzung (ISO 6976:1995 + Corrigendum 1:1997 + Corrigendum 2:1997 + Corrigendum 3:1999)

Diese Internationale Norm legt Verfahren zur Berechnung von Brennwert, Heizwert, Dichte, relativer Dichte und Wobbeindex von trockenen Erdgasen, Erdgasaustauschgasen und sonstigen Brenngasen fest, wenn die Zusammensetzung des Gases in Stoffmengenanteilen bekannt ist. Anhand dieser Verfahren können die Eigenschaften des Gasgemisches bei allgemein üblichen metrischen Referenzbedingungen berechnet werden.

Die Rechenverfahren benötigen Werte für verschiedene physikalische Eigenschaften der reinen Komponenten; diese Werte werden in Form von Tabellen und unter Angabe der jeweiligen Quellen bereitgestellt.

Des weiteren werden Verfahren zur Schätzung der Präzision der berechneten Eigenschaften beschrieben.

Die Verfahren zur Berechnung der molaren und massenbezogenen Eigenschaftswerte gelten für alle trockenen Erdgase, Erdgasaustauschgase oder sonstige in der Regel gasförmige Brennstoffe. Die Berechnung der volumenbezogenen Eigenschaftswerte ist auf Gase beschränkt, die überwiegend aus Methan bestehen (Stoffmengenanteil 3 0,5).

Beispiele zu den empfohlenen Rechenverfahren sind in Anhang D aufgeführt.

ANMERKUNG 1 Die in dieser Internationalen Norm verwendeten Symbole und ihre Bedeutungen sind in Anhang A erläutert.

ANMERKUNG 2 Die Qualifikationsmerkmale "higher", "upper", "total" und "gross" sind für diese Norm gleichbedeutend mit "superior" (calorific value, Brennwert); ebenso sind "lower" und "net" gleichbedeutend mit "inferior" (calorific value, Heizwert). Der Begriff "heating value" ist gleichbedeutend mit "calorific value"; "specific gravity" ist gleichbedeutend mit "relative density" (relative Dichte); "Wobbe number" ist gleichbedeutend mit "Wobbe index" (Wobbeindex); "compressibility factor" ist gleichbedeutend mit "compression factor" (Realgasfaktor).

ANMERKUNG 3 Ist die Zusammensetzung des Gases in Volumenanteilen bekannt, müssen diese in Stoffmengenanteile umgerechnet werden (siehe Anhang C). Dabei ist jedoch zu berücksichtigen, dass die abgeleiteten Stoffmen

## Gaz naturel - Calcul du pouvoir calorifique, de la masse volumique, de la densité relative et de l'indice de Wobbe a partir de la composition (ISO 6976:1995, Corrigendum 1:1997, Corrigendum 2:1997 et Corrigendum 3:1999 inclus)

La présente Norme internationale prescrit des méthodes pour le calcul des pouvoirs calorifiques supérieur et inférieur, de la masse volumique, de la densité relative et de l'indice de Wobbe du gaz naturel sec et des substituts du gaz naturel lorsque la composition du gaz en fraction molaire est connue. Ces méthodes permettent de calculer les propriétés du mélange de gaz dans les conditions de référence métriques généralement utilisées. Les méthodes de calcul exigent des valeurs pour les différentes propriétés physiques des composants purs; ces valeurs sont consignées dans les tableaux et leurs sources sont identifiées. Des méthodes sont données pour estimer la précision des propriétés calculées. Les méthodes de calcul des valeurs des propriétés sur une base molaire ou sur une base massique sont applicables à tout gaz naturel sec, à tout substitut du gaz naturel ou à tout autre combustible normalement gazeux. Pour le calcul des valeurs des propriétés sur une base volumétrique, les méthodes sont limitées aux gaz essentiellement constitués (fraction molaire supérieure ou égale à 0,5) de méthane. Des exemples de calcul sont donnés dans l'annexe D pour les méthodes de calcul recommandées.

NOTES 1 Les symboles utilisés dans la présente Norme internationale sont explicitées dans l'annexe A. 2 Pour les besoins de la présente Norme internationale, les qualificatifs «plus élevé», «total» et «brut» sont synonymes de «supérieur»; de la même façon «plus bas» et «net» sont synony 1786mes d'«inférieur». Le terme «pouvoir chauffant» est synonyme de «pouvoir calorifique»; le «poids spécifique» est synonyme de «densité relative»; le «nombre de Wobbe» est synonyme d'«indice de Wobbe». Le terme «facteur de compressibilité» est synonyme de «facteur de compression». 3 Si la composition du gaz est connue en fractions volumiques, il convient de les convertir en fractions molaires (voir annexe C). À noter toutefois que les fractions molaires dérivées auront d

## Zemeljski plin – Izračun kaloričnih vrednosti, gostote, relativne gostote in Wobbejevega indeksa iz kompozicije (ISO 6976:1995 vključujoč Popravek 2:1997 in Popravek 3:1999)

### General Information

### Relations

### Standards Content (Sample)

SLOVENSKI STANDARD

SIST EN ISO 6976:2005

01-julij-2005

=HPHOMVNLSOLQ±,]UDþXQNDORULþQLKYUHGQRVWLJRVWRWHUHODWLYQHJRVWRWHLQ

:REEHMHYHJDLQGHNVDL]NRPSR]LFLMH,62YNOMXþXMRþ3RSUDYHNLQ

3RSUDYHN

Natural gas - Calculation of calorific values, density, relative density and Wobbe index

from composition (ISO 6976:1995 including Corrigendum 1:1997, Corrigendum 2:1997and Corrigendum 3:1999)

Erdgas - Berechnung von Brenn- und Heizwert, Dichte, relativer Dichte und Wobbeindex

aus der Zusammensetzung (ISO 6976:1995 + Corrigendum 1:1997 + Corrigendum2:1997 + Corrigendum 3:1999)

Gaz naturel - Calcul du pouvoir calorifique, de la masse volumique, de la densité relative

et de l'indice de Wobbe a partir de la composition (ISO 6976:1995, Corrigendum 1:1997,

Corrigendum 2:1997 et Corrigendum 3:1999 inclus)Ta slovenski standard je istoveten z: EN ISO 6976:2005

ICS:

75.060 Zemeljski plin Natural gas

SIST EN ISO 6976:2005 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------SIST EN ISO 6976:2005

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SIST EN ISO 6976:2005

EUROPEAN STANDARD

EN ISO 6976

NORME EUROPÉENNE

EUROPÄISCHE NORM

May 2005

ICS 75.060

English version

Natural gas - Calculation of calorific values, density, relative

density and Wobbe index from composition (ISO 6976:1995

including Corrigendum 1:1997, Corrigendum 2:1997 and

Corrigendum 3:1999)

Gaz naturel - Calcul du pouvoir calorifique, de la masse Erdgas - Berechnung von Brenn- und Heizwert, Dichte,

volumique, de la densité relative et de l'indice de Wobbe à relativer Dichte und Wobbeindex aus der

partir de la composition (ISO 6976:1995, Corrigendum Zusammensetzung (ISO 6976:1995 + Corigendum 1:1997

1:1997, Corrigendum 2:1997 et Corrigendum 3:1999 inclus) + Corigendum 2:1997 + Corigendum 3:1999)

This European Standard was approved by CEN on 17 April 2005.CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European

Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national

standards may be obtained on application to the Central Secretariat or to any CEN member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by translation

under the responsibility of a CEN member into its own language and notified to the Central Secretariat has the same status as the official

versions.CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,

Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,

Slovenia, Spain, Sweden, Switzerland and United Kingdom.EUROPEAN COMMITTEE FOR STANDARDIZATION

COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

Management Centre: rue de Stassart, 36 B-1050 Brussels

© 2005 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 6976:2005: E

worldwide for CEN national Members.---------------------- Page: 3 ----------------------

SIST EN ISO 6976:2005

EN ISO 6976:2005 (E)

Foreword

The text of ISO 6976:1995 has been prepared by Technical Committee ISO/TC 193 "Natural

gas” of the International Organization for Standardization (ISO) and has been taken over as EN

ISO 6976:2005 by CMC.This European Standard shall be given the status of a national standard, either by publication of

an identical text or by endorsement, at the latest by November 2005, and conflicting national

standards shall be withdrawn at the latest by November 2005.According to the CEN/CENELEC Internal Regulations, the national standards organizations of

the following countries are bound to implement this European Standard: Austria, Belgium,

Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary,

Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland,

Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.Endorsement notice

The text of ISO 6976:1995 has been approved by CEN as EN ISO 6976:2005 without any

modifications.---------------------- Page: 4 ----------------------

SIST EN ISO 6976:2005

INTERNATIONAL

IS0

STANDARD

6976

Second edition

1995-l Z-01

Corrected and reprinted

1996-02-01

Natural gas - Calculation of calorific

values, density, relative density and Wobbe

index from composition

Gaz na turel - Calcul du pouvoir calorifique, de la masse volumique, de la

densit relative et de I’indice de Wobbe P partir de la composition

Reference number

IS0 6976:1995(E)

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SIST EN ISO 6976:2005

IS0 6976:1995(E)

Contents

Page

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

............................................. ............ ......................

2 Definitions . ....................................... .................. ................. 3

3 Principle ............

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

4 Behaviour of ideal and real gases........................ 3

4.1 Enthalpy of combustion ..................................

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

4.2 Calculation of compression factor. . . . . . . . . . . . . . . . . . . . . . . . . . 4

5 Calculation of calorific value on a molar basis

5.1 Ideal gas ............................ .................................... .................

........................................... ...................... ................. 4

5.2 Real gas. . . . . . . . . . . . . . . . . . . . . . . . . . 4

6 Calculation of calorific value on a mass basis

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

6.1 Ideal gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6.2 Real gas ............................. .................................... ................. 5

. . . . . . . . . . . . . . . . . . 57 Calculation of calorific value on a volumetric basis

............................. 5

7.1 Ideal gas ............................... .....................

........................ ................. ......................................... 5

7.2 Real gas. . . . . . . . 6

8 Calculation of relative density, density and Wobbe index

8.1 Ideal gas - . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . . . . . . . . . . . . . . . . . . . . . . .

8.2 Real gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Accuracy .......................... ........................... 69 ...............................

9.1 Precision ............................... ............... ................................... 6

9.2 Trueness ......................... ................................ ........................ 8

................................. ............................. 99.3 Expression of results

IO Tables of recommended data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IO

0 IS0 1995All rights reserved. Unless otherwise specified, no part of this publication may be reproduced

or utilized in any form or by any means, electronic or mechanical, including photocopyrng and

microfilm, without permission in writing from the publisher.International Organization for Standardization

Case Postale 56 l CH-1211 Geneve 20 l Switzerland

Printed in Switzerland

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SIST EN ISO 6976:2005

0 IS0

IS0 6976: 1995(E)

Annexes

A Symbols and units . . . . . ..*........................................... . . . . . . . . . . . . . . 15

B Values of auxiliary constants, etc. ........................... .............. 17

B.l Molar gas constant ........ .................................... ..................

B.2 Critical constants and acentric factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

B.3 Properties of dry air ..................................... .......... ........ ...... 17

B.4 Enthalpy of vaporization of water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

C Conversion of volume fractions to mole fractions . . . . . . . . . . . . . . . . 20

D Examples of calculations ..................................... .................. 21

D.l Calorific value on a molar basis (clause 5) . . . . . . . . . . . . . . . . . . . . . . . . . . 21

D.2 Calorific value on a mass basis (clause 6) . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

D.3 Calorific value on a volumetric basis (clause 7) . . . . . . . . . . . . . . . . . . 21

D.4 Relative density, density and Wobbe index (clause 8)....... 22

D.5 Precision (clause 9) .......................... .................................... 23

E Behaviour of ideal and real gases ........................... ............... 25

E.l Variation of ideal-gas enthalpy of combustion withtemperature ................ ........................................ ........ .......... 25

E.2 Corrections for non-ideality: volumetric effects ...................E.3 Corrections for non-ideality: enthalpic effects

..................... 28

F Effects of water vapour on calorific value

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

F.l General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

F.2 Excluded volume effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

F.3 Latent heat (enthalpic) effect ............................... ................ 31

F.4 Compression factor effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

G Summary, discussion and selection of the calorific value ofmethane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

H Derivation of equations relating to precision .........................H.l Methane by difference ........................................................ 36

H.2 Methane by analysis ............................ ................................

J Approximate conversion factors between reference states. . 38

K Computer implementation of recommended methods . . . . . . . . . 40

L Calorific values on a molar basis for 60 “F reference

............................ ...........

temperature ....... .............................. 43

iii

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SIST EN ISO 6976:2005

IS0 6976:1995(E) 0 IS0

IV! Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..*............

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0 IS0

IS0 6976:1995(E)

Foreword

IS0 (the International Organization for Standardization) is a worldwide

federation of national standards bodies (IS0 member bodies). The work

of preparing International Standards is normally carried out through IS0

technical committees. Each member body interested in a subject for

which a technical committee has been established has the right to be

represented on that committee. International organizations, governmental

and non-governmental, in liaison with ISO, also take part in the work. IS0

collaborates closely with the International Electrotechnical Commission

(IEC) on all matters of electrotechnical standardization.

Draft International Standards adopted by the technical committees are

circulated to the member bodies for voting. Publication as an International

Standard requires approval by at least 75 % of the member bodies casting

a vote.

International Standard IS0 6976 was prepared by Technical Committee

ISO/TC 193, Natural gas, Subcommittee SC I, Analysis of natural gas.

This second edition cancels and replaces the first edition

(IS0 6976:1983), of which it constitutes a technical revision.

Annexes A and B form an integral part of this International Standard. An-

nexes C, D, E, F, G, H, J, K, L and M are for information only.

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SIST EN ISO 6976:2005

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SIST EN ISO 6976:2005

IS0 6976:1995(E)

INTERNATIONAL STANDARD 0 IS0

- Calculation of calorific values, density,

Natural gas

relative density and Wobbe index from composition

synonymous with “calorific value ”; “specific gravity” is

1 Scope

synonymous with “relative density ”; “Wobbe number” is

synonymous with “Wobbe index ”; “compressibility factor”

is synonymous with “compression factor ”.

This International Standard specifies methods for the

3 If the composition of the gas is known by volume frac-

calculation of the superior calorific value, inferior

tions these must be converted to mole fractions (see

calorific value, density, relative density and Wobbe

annex C). Note, however, that the derived mole fractions

index of dry natural gases, natural gas substitutes and

will have uncertainties greater than those of the original

other combustible gaseous fuels, when the compo-

volume fractions.

sition of the gas by mole fraction is known. The

4 For the purposes of this International Standard, the sum

methods provide a means of calculating the proper-

of the mole fractions used must be unity to the nearest

ties of the gas mixture at commonly used metric ref-

0,000 I, and all components with mole fractions greater

erence conditions.

than 0,000 05 must be accounted for.

The methods of calculation require values for various

5 For the calorific value calculated on a volumetric basis,

physical properties of the pure components; these

there are limitations on the amounts of components other

values are provided in tables and their sources are

than methane which may be present. It is impossible to be

definitive on this matter, but the following guidelines may

identified.

be useful:

Methods are given for estimating the precision of

N, should not be present in amounts exceeding 0,3

calculated properties.

mole fraction;

The methods of calculation of the values of properties

CO, and C,H, should each not exceed 0,15 mole frac-

on either a molar or mass basis are applicable to any

tion;

dry natural gas, natural gas substitute or other

combustible fuel which is normally gaseous. For the

no other component should exceed 0,05 mole fraction.

calculation of the values of properties on a volumetric

basis, the methods are restricted to gases consisting

Given these limits, the expected trueness of the calculation

preponderantly of methane (not less than 0,5 mole

is within 0,l %.

fraction).

6 The effects of water vapour on the calorific value, either

directly measured or calculated, are discussed in annex F.

Examples of calculations are given in annex D for the

recommended methods of calculation.

7 For the methods of calculation described to be valid, the

gas must be above its hydrocarbon dew-point at the pre-

NOTES

scribed reference conditions.

1 The symbols used in this International Standard, to-

8 The values of basic physical property data are subject to

gether with their meanings, are given in annex A.

revision as more accurate values become available from

authoritative sources.

2 The qualifiers “higher ”, “upper ”, “total” and “gross”

are, for the purposes of this International Standard, syn-

onymous with “superior ”; likewise, “lower” and “net” are

synonymous with “inferior ”. The term “heating value” is

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SIST EN ISO 6976:2005

0 IS0

IS0 6976:1995(E)

On molar, mass and volumetric bases, the inferior

2 Definitions

calorific value is designated respectively as q (tl ,I+),

W,~Pl) and 4Kb P1w(~*~P2)1~

For the purposes of this International Standard, the

following definitions apply.

2.3 density: The mass of a gas sample divided by

its volume at specified conditions of pressure and

2.1 superior calorific value: The amount of heat

temperature.

which would be released by the complete combustion

in air of a specified quantity of gas, in such a way that

2.4 relative density: The density of a gas divided

the pressure p1 at which the reaction takes place re-

by the density of dry air of standard composition (see

mains constant, and all the products of combustion

annex B) at the same specified conditions of pressure

are returned to the same specified temperature t, as

and temperature. The term ideal relative density ap--

that of the reactants, all of these products being in the

plies when both gas and air are considered as fluids

gaseous state except for water formed by com-

which obey the ideal gas law (see 2.7); the term real

bustion, which is condensed to the liquid state at t,.

relative density applies when both gas and air are

considered as real fluids.

Where the quantity of gas is specified on a molar ba-

sis, the calorific value is designated as &(t,,pJ; on a

2.5 Wobbe index: The superior calorific value on a

mass basis the calorific value is designated as

volumetric basis at specified reference conditions,

Hs (4 IPI > - divided by the square root of the relative density at

the same specified metering reference conditions.

Where the quantity of gas is specified on a volumetric

value is designated as

basis, the calorific

2.6 enthalpy of transformation: The enthalpy of

is [t,,p,, V(t2,p2)], where t2 and p2 are the gas volume

transformation of a substance from state A to state

(metering) reference conditions (see figure 1).

B is thermodynamic terminology for the amount of

heat release which accompanies the transformation

2.2 inferior calorific value: The amount of heat between states. A positive heat release is taken by

convention to be a numerically identical negativewhich would be released by the complete combustion

enthalpy increment. The quantities enthalpy of com-

in air of a specified quantity of gas, in such a way that

bustion and enthalpy of vaporization therefore have

the pressure p1 at which the reaction takes place re-

meanings which should be contextually self-evident;

mains constant, and all the products of combustion

the term enthalpic correction refers to the (molar)

are returned to the same specified temperature t, as

that of the reactants, all of these products being in the enthalpy of transformation between the ideal and real

states of a gas.gaseous state.

Water as vapour

lnferio value I?,

rc alorif ic

Air

Metering

/-p--t--

at h PI

Gas at

tz, P2

Water as liquid

Superi or calorific value &

Combustion

Heat release

= Calorific value ti

Metered volume of gas

- Metering and combustion reference conditions

Figure 1 - Calorific value on a volumetric basis

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SIST EN ISO 6976:2005

0 ISQ

IS0 6976:1995(E)

sponding mole fraction, all the terms then being

2.7 ideal gas and real gas: An ideal gas is one

added together to obtain the “mole fraction average”

which obeys the ideal gas law:

of the property for the ideal-gas mixture. Values on a

. . .

= RmT

(1)

P=v,

volumetric basis are then converted to values for the

real-gas state by applying a volumetric correction fac-

where

tor.

is the absolute pressure;

NOTE 10 An enthalpic correction factor which is also, in

principle, required in calorific value calculations is deemed

T is the thermodynamic temperature;

to be negligible in all relevant cases.

is the volume per mole of gas;

In clause 10, values are given for the physical

properties of the pure components of natural gas on

R is the molar gas constant, in coherent

molar, mass and volumetric bases for the con- lmonly

units.

used reference conditions . Examples of calcu lations

No real gas obeys this law. For real gases, equation

are given in annex D.

(1) must be rewritten as

= Z(T,p) -RgT . . .

4 Behaviour of idea I and rea I gases

(2)

PT-ll

where Z(T,p) is a variable, often close to unity, and is

4.1 Enthalpy of combustion

known as the compression factor (see 2.8 and E.2).

The most fundamental physical quantities required in

2.8 compression factor: The actual (real) volume

the calculation of calorific values from first principles

of a given mass of gas at a specified pressure and

are the ideal-gas (standard) molar enthalpies of com-

temperature divided by its volume, under the same

bustion for the component gases of the mixture.

conditions, as calculated from the ideal gas law.

These quantities are complex functions of tempera-

ture; thus, the values required depend upon the

2.9 combustion reference conditions: The speci-

combustion reference temperature t,. For practical

fied temperature t, and pressure pl. These are the

reasons, it is not intended that the user himself car-

conditions at which the fuel is notionally burned (see

ries out calculations which give the appropriate values

figure 1).

at any arbitrary combustion reference temperature.

Instead, tabulations are given for the temperatures

2.10 metering reference conditions: The specified

t, = 25 “C, 20 “C, 15 “C and 0 “C. In clause E.l the

temperature + and pressure pz. These are the con-

derivations of the values tabulated are discussed; the

ditions at which the amount of fuel to be burned is

important point is that all four values for any sub-

notionally determined; there is no a priori reason for

stance are mutually consistent in a thermodynamic

these to be the same as the combustion reference

sense.

conditions (see figure I).

For the calorific value (on any of the three possible

NOTE 9 A range of reference conditions is in use

bases), a so-called enthalpic correction is, in principle,

throughout the world; appropriate data for the principal sets

required in order to convert the ideal-gas enthalpy of

of metric reference conditions are given in tables in this

combustion for the gas mixture to a value appropriate

International Standard.

to the real gas. This, however, is generally small

enough to be negligible. A discussion justifying such

2.11 dry natural gas: Gas which does not contain

neglect is given in clause E.3.

water vapour at a mole fraction greater than 0,000 05.

42 . Calculation of compression factor

3 Principle

For the volumetric-basis calorific value, a second real-

Methods are provided for the calculation of the gas correction is required to account for the deviation

calorific values, density, relative density and Wobbe of the gas from volumetric ideality, and this is gener-

index of any dry natural gas, natural gas substitute or ally not negligible. This correction is also required in

the calculation of density, relative density and, by im-other combustible gaseous fuel from a known com-

plication, Wobbe index. Clause E.2 gives the back-

position. These methods use equations in which, for

ground to the way in which corrections for volumetric

all individual molecular species of the gas mixture, the

values of ideal-gas thermophysical properties (which non-ideality should be applied, discusses the princi-

are given) are weighted in accordance with the corre- ples involved, and justifies the simplifications em-

---------------------- Page: 13 ----------------------SIST EN ISO 6976:2005

0 IS0

IS0 6976:1995(E)

have been derived from the 25 “C values in accord-

ployed which enable tractable calculations to be made

ance with the methods described in clause E.I.

without necessitating machine computation.

NOTES

Such corrections for volumetric non-ideality are made

using the compression factor Zmix. The formulation to

II Values of ??y are independent of pressure; conse-

be used for Zmix at the metering reference conditions,

quently the combustion reference pressure JI, is irrelevant

as required for the calculations described in clauses

for the ideal-gas case and is omitted from the nomenclature

5 to 9, is (equation E.17):

adopted.

12 The ideal-gas calorific value on a molar basis of a gas

. . .

or gas mixture is defined in this International Standard as a

zr-nix (t*lp*) = I - xj’ hj (3)

positive number. The values given in table3 are numerically

j=l

i I

equal to the standard molar enthalpies of combustion,

which are, however, conventionally expressed as negative

where the summation is taken over all N components

quantities (see 2.6).

of the mixture. Values of the so-called summation

factor bj are given in table2 (clause 10) at the

II-

three metering reference conditions of common in- 5.2 Real gas

terest, for all of the components of natural gas and

For the purposes of this International Standard the

substitute natural gas considered in this International

real-gas calorific value on a molar basis is taken as

Standard. Values are also given for all pure component

numerically equal to the corresponding ideal-gas

compression factors (or hypothetical compression

value.

factors) 5, from which the &i ’s have generally been

Zjs Any user re-

derived using the relationship-h, = 1 -

NOTE 13 A rigorous approach to the calculation of the

quiring greater detail should consult clause E.2.

real-gas calorific value on a molar basis from the ideal-gas

value would require the calculation of an enthalpic cor-

rection (see 4.1) for the mixture. In practice, this correction

5 Calculation of calorific value on a

is very small for typical natural gases, and can usually be

neglected with resultant errors not exceeding 50 J*mol- ’

molar basis

(approximately 0,005 %) (see clause E.3).

5.1 Ideal gas

6 Calculation of calorific value on a mass

The ideal-gas calorific value on a molar basis, at a basis

of a mixture of known composition is

temperature t,,

calculated from the equation

6.1 Ideal gas

;E70(tj) = F,Xj’T(tj)

. . .

(4) The ideal-gas calorific value on a mass basis, at a

j=l

temperature t,, of a mixture of known composition is

calculated from the equation

Ho(tl)

. . .

(5)

PO(+) is the ideal molar calorific value of the fiO(t,)

mixture (either superior or inferior);

where

HjT(t,) is the ideal molar calorific value of com-

ponent j (either superior or inferior);

M is the molar mass of the mixture, and is

calculated from the equation

is the mole fraction of component j.

Numerical values of T for t, = 25 “C are given rn

M = XjmMj . . .

(6)

table3 (clause 10); the values for (i?& are taken

i=l

from the original literature sources cited in annex M,

and the values for (fl,‘), derived using the accepted

value of the standard enthalpy of vaporization of water being the mole fraction of

at 25 “C (see annex B). component j;Values for $’ for other temperatures (t, = 20 “C, being the molar mass of com-

ponent j;

15 “C and 0 “C) are also given in table3; these values

---------------------- Page: 14 ----------------------

SIST EN ISO 6976:2005

IS0 6976:1995(E)

0 IS0

h ”(t-,) is the ideal calorific value on a mass basis is the molar gas constant

of the mixture (either superior or inferior). (= 8,314 510 J=mol- ‘=K-I,

see clause B.l);

Table 1 (clause IO) lists values of the molar mass for

all components considered in this International Stan- T2( = t2 + 273,15) is the absolute temperature, in

kelvins.dard.

The use of equation 1 represents the definitive

Use of equations (5) an$ (6) represents the definitive (8)

method for calculating H. An alternative method uses

method for calculating H . An alternative method uses

the equation

the equation

fi”(tj) = t Xj x -$

. . .

'y('l) (9)

(7)

( 1 =

j=l

J ’

A0 where Hj [tl,V(t2,p2)] is the ideal calorific value on a

wh ere Hi (t,) is the ideal calorific value on a mass ba-

volumetric basis of component j (either superior or

sis of component j (either superior or inferior).

values of Hj for four values

For convenience,

For convenience, values of fiJy for a variety of com-

of tl (25 “C, 20 “C, 15 “C and 0 “C) are given in

bustion and metering reference conditions are given

table4 (clause 1 O), in order that the user may avoid

in table 5 (clause IO), in order that the user may avoid

the necessity of using values of HI? as the starting

the necessity of using values of flJy as the starting

point of a calculation.

point of a calculation.

Numerical values obtained from either method will be

Numerical values obtained from either method will be

concordant to within 0,Ol MJ-kg-l, which is within

concordant to within 0,Ol MJmmB3, which is within

the limits of significance for the current state-of-the-

the limits of significance for the current state-of-the-

art.

art.

7.2 Real gas

6.2 Real gas

The real-gas calorific value on a volumetric basis, for

For the purposes of this International Standard, the

combustion at temperature t, and pressure p1 of a gas

real-gas calorific value on a mass basis is taken as

mixture metered at a temperature t2 and pressure p2

numerically equal to the corresponding ideal-gas

**...**

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