This 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 composition of the gas by mole fraction is known.
The methods provide a means of 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 of not less than 0.5 in mole fraction.
Table 1 to Table 5 of this Standard give the basic properties required for the calculations, and Appendix A gives the values of auxiliary constants used in the calculation.
Note:
1 If the composition of the gas is known by volume fractions these must be converted to mole fractions (see Appendix B). Note, however, that the derived mole fractions will have uncertainties greater than those of the original volume fractions.
2 For the purposes of this Standard, the sum of the mole fractions used must be unity to the nearest 0.000 1, and all components with mole fractions greater than 0.000 05 must be accounted for.
3 For the calorific value calculated on a volumetric basis, there are limitations on the amounts of components other than methane which may be present. Generally, N2 should not be present in amounts exceeding 0.3 mole fraction; C02 and C2H6 should each not exceed 0.15 mole fraction; no other component should exceed 0.05 mole fraction. Given these limits, the uncertainty of the calculation is within 0.1 %.
4 The effects of water vapor on the calorific value, either directly measured or calculated, are discussed in Appendix C.
5 For the methods of calculation described to be valid, the gas must be above its hydrocarbon dew-point at the prescribed reference conditions.
1 Scope 1
2 Definition 1
3 Method Principle 3
4 Behaviour of Ideal and Real Gases 3
4.1 Enthalpy of Combustion 3
4.2 Calculation of Compression Factor 3
5 Calculation of Calorific Value on a Molar Basis 4
6 Calculation of Calorific Value on a Mass Basis 4
7 Calculation of Calorific Value on a Volumetric Basis 5
8 Calculation of Relative Density, Density and Wobbe Index 6
9 Accuracy 8
10 Trueness 9
11 Expression of Results 10
Appendix A (Normative) Values of Auxiliary Constants 23
Appendix B (Suggestive) Conversion of Volume Fractions to Mole Fractions 24
Appendix C (Suggestive) Effects of Water Vapour on Calorific Value 25
Appendix D (Suggestive) Approximate Conversion Factors between Reference Conditions 29
Appendix E (Suggestive) Examples of Calculations 32
