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); English version of DIN EN ISO 6976
This International Standard specifies methods for the
calculation of the superior calorific value, inferior
calorific value, density, relative density and Wobbe
index of dry natural gases, natural gas substitutes and
other combustible gaseous fuels, when the compo-
sition of the gas by mole fraction is known. The
methods provide a means of calculating the proper-
ties of the gas mixture at commonly used metric ref-
erence conditions.
The methods of calculation require values for various
physical properties of the pure components; these
values are provided in tables and their sources are
identified.
Methods are given for estimating the precision of
calculated properties.
The methods of calculation of the values of properties
on either a molar or mass basis are applicable to any
dry natural gas, natural gas substitute or other
combustible fuel which is normally gaseous. For the
calculation of the values of properties on a volumetric
basis, the methods are restricted to gases consisting
preponderantly of methane (not less than 0,5 mole
fraction).
Examples of calculations are given in annex D for the
recommended methods of calculation.
NOTES
1 The symbols used in this International Standard. to-
gether with their meanings, are given in annex A.
2 The qualifiers "higher", "upper", "total" and "gross"
are, for the purposes of this International Standard, syn-
onymous with "superior"; fikewise, "lower" and "net" are
synonymous with "inferior". The term "heating value" is
synonymous with "calorific value"; "specific gravity" is
synonymous with "relative density"; "Wobbe number" is
synonymous with 'Wobbe index"; "compressibility factor"
is synonymous with "compression factor".
3 If the composition of the gas is known by volume frac-
tions these must be converted to mole fractions (see
annex C). Note, however, that the derived mole fractions
will have uncertainties greater than those of the original
volume fractions.
4 For the purposes of this International Standard, the sum
of the mole fractions used must be umty to the nearest
0,000 1, and all components with mole fractions greater
than 0,000 05 must be accounted for.
5 For the calorific value calculated on a volumetric basis,
there are limitations on the amounts of components other
than methane which may be present. It is impossible to be
definitive on this matter, but the following guidelines may
be useful:
N2 should not be present in amounts exceeding 0,3
mole fraction;
CO2 and C2H6 should each not exceed 0,15 mole frac-
tion;
no other component should exceed 0,05 mole fraction.
Given these limits, the expected trueness of the calculation
is within 0,1%.
6 The effects of water vapour on the calortfic value, e~ther
directly measured or calculated, are discussed in annex F
7 For the methods of calculation described to be valid, the
gas must be above its hydrocarbon dew-point at the pre-
scribed reference conditions.
8 The values of basic physical property data are subject to
revwsion as more accurate values become ava~labie from
authoritat,ve sources.