What is ASTM D3588?
ASTM D3588 is the standard practice for calculating the heating value, compressibility factor and relative density of gaseous fuels from their composition. It is the method the natural gas industry uses together with GPA 2172 and the physical constants of GPA 2145: instead of burning the gas in a calorimeter, you take the gas chromatograph analysis and add up the contribution of each component.
It is used in custody transfer and energy-based gas billing, in flow calculations (AGA 3 and AGA 7 need the relative density), in pipeline gas quality control and to compare gases with the Wobbe index.
Gross and net heating value
The gross heating value (HHV, higher heating value) assumes that the water formed by combustion condenses and returns its latent heat; the net heating value (LHV, lower heating value) assumes it leaves as vapor. For natural gas the LHV is about 10 % lower. Gas contracts usually use the dry gross heating value.
How it is calculated
1. Ideal heating value: Hid = Σ xᵢ·Hᵢ, where xᵢ is the mole fraction and Hᵢ the ideal heating value of each component (methane 1010.0 Btu/ft³, ethane 1769.7 and propane 2516.1, at 60 °F and 14.696 psia).
2. Compressibility factor: Z = 1 − P·(Σ xᵢ·√bᵢ)², with P the base pressure in psia and √bᵢ the summation factor of each component (methane 0.0116).
3. Real heating value: H = Hid / Z. Since Z is below 1, the real value is slightly higher than the ideal one (about 0.2 % for pipeline gas).
4. Relative density: Gid = M / 28.9625 and G = Gid · Zair / Z, with Zair = 0.99959.
5. Wobbe index: W = H / √G. Two gases with the same Wobbe index deliver almost the same energy to a burner at the same supply pressure.
Worked example
Pipeline gas with 94.0 % methane, 3.2 % ethane, 0.6 % propane, 0.1 % i-butane, 0.1 % n-butane, 0.03 % i-pentane, 0.02 % n-pentane, 0.05 % hexane, 0.8 % nitrogen and 1.1 % CO₂, at 60 °F and 14.696 psia:
Ideal HHV: 1032.0 Btu/ft³ (methane contributes 949.4, ethane 56.6 and propane 15.1).
Σ xᵢ·√bᵢ = 0.01229, so Z = 1 − 14.696 × 0.01229² = 0.99778.
Real HHV: 1032.0 / 0.99778 = 1034.3 Btu/ft³ = 38.54 MJ/m³; real LHV: 932.7 Btu/ft³.
Molar mass 17.210 g/mol: ideal relative density 0.5942 and real 0.5953.
Wobbe index: 1034.3 / √0.5953 = 1340.6 Btu/ft³.
You can reproduce it with the calculator's "Example: pipeline gas" button; the step-by-step calculation shows every intermediate value.
Validation
The calculator was checked against the public heating value spreadsheet of the U.S. Bureau of Land Management (BLM, 43 CFR 3175), which uses GPA 2172 with GPA 2145 constants: for its test gas it gives the same ideal heating value (1359.3 Btu/ft³), the same Z (0.9955) and the same real relative density (0.8139), and 1368.7 Btu/ft³ at 14.73 psia.
Dry basis, wet basis and base pressure
The result is on a dry basis. If the gas is saturated with water at 60 °F, the volumetric heating value drops by about 1.7 %, because the water vapor takes up volume without adding energy. The base pressure also changes the volumetric value: 14.696 psia (101.325 kPa) is the standard atmosphere and 14.73 psia is the most common in U.S. contracts.
Frequently asked questions
What is a BTU per cubic foot? It is the energy, in British thermal units, released by burning one standard cubic foot of gas. Pipeline natural gas is usually between 1000 and 1100 Btu/ft³ (37 to 41 MJ/m³).
How do I convert Btu/ft³ to MJ/m³? Multiply by 0.0372589, at the same base pressure and temperature. For example, 1034.3 Btu/ft³ = 38.54 MJ/m³. If your standard uses 15 °C or 0 °C as the base, the value changes slightly with temperature.
What do I do with C6+? If the analysis lumps hexanes and heavier as C6+, enter it as hexane or split it as your lab indicates; putting it all as hexane gives a slightly lower value when heptanes and octanes are present.
Can it be used for gas billing? It is a support tool with the constants published in GPA 2145; for custody transfer use the edition of the standard and the base pressure set by the contract.
Source: ASTM D3588, Standard Practice for Calculating Heat Value, Compressibility Factor, and Relative Density of Gaseous Fuels; GPA 2172 and GPA 2145.