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

رقم المستند: NL ISO 6976 : 2015
القطاع: Petroleum and related Technologies
TC: NL TC 28
ICS:
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 composition of the gas by mole fraction is known. The methods provide a means of calculating the properties of the gas mixture at commonly used metric reference 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, together with their meanings, are given in annex A. 2 The qualifiers “higher”, “upper”, “total” and “gross” are, for the purposes of this International Standard, synonymous with “superior”; likewise, “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 fractions 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 unity to the nearest 0,000 I, 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: N, should not be present in amounts exceeding 0,3 mole fraction; CO, and C,H, should each not exceed 0,15 mole fraction; no other component should exceed 0,05 mole fraction. Given these limits, the expected trueness of the calculation is within 0,l %. 6 The effects of water vapour on the calorific value, either 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 prescribed reference conditions. 8 The values of basic physical property data are subject to revision as more accurate values become available from authoritative sources.