Thermodynamics

Heating Value from Gas Composition

The heating value of natural gas determines its energy content and economic value. Accurate BTU determination from gas chromatograph analysis is essential for custody transfer, gas quality monitoring, tariff compliance, and burner interchangeability throughout the midstream value chain.

Launch Calculator GPA 2145 Data

Key equation

HHV = Σ(y_i × HHV_i)

Mole-fraction-weighted sum of component heating values from GPA 2145.

Typical range

950 - 1150 Btu/scf

Pipeline-quality natural gas. Pure methane = 1010 Btu/scf.

Key standards

GPA 2145, 2172, ISO 6976

Physical constants, calculation methods, and international calorific value standards.

Contents

Use this guide when you need to:

  • Calculate BTU content from a gas chromatograph analysis.
  • Understand HHV vs LHV and when each applies.
  • Evaluate gas interchangeability using the Wobbe Index.
  • Determine if gas meets pipeline tariff specifications.

1. What is Heating Value?

The heating value (also called calorific value or heat of combustion) of a gas is the amount of thermal energy released when a unit quantity of the gas undergoes complete combustion with oxygen. For natural gas, this is the primary measure of energy content and directly determines the economic value of the gas in custody transfer and sales agreements.

Fundamental Definition

Heating Value Definition: The heating value is the enthalpy of combustion measured at constant pressure, with reactants and products at the reference temperature. For methane (the primary component of natural gas): CH4 + 2O2 --> CO2 + 2H2O + 1010.0 Btu/scf (HHV at 60 degF, 14.696 psia) The heating value of a gas mixture is the mole-fraction-weighted sum of individual component heating values: HHV_mix = SUM(y_i x HHV_i) Where: y_i = Mole fraction of component i HHV_i = Gross heating value of component i (from GPA 2145) Units: US: Btu per standard cubic foot (Btu/scf) at 60 degF, 14.696 psia ISO: MJ per normal cubic meter (MJ/Nm3) at 15 degC, 101.325 kPa

Why Heating Value Matters

Custody transfer

Revenue determination

Natural gas is sold on an energy basis (MMBtu). The BTU factor (HHV/1000) converts volume (Mscf) to energy (MMBtu). A 1% error in HHV directly causes 1% error in revenue.

Tariff compliance

Pipeline quality

Pipeline tariffs specify minimum and maximum HHV (typically 950-1150 Btu/scf) along with limits on inerts, H2S, and water content. Off-spec gas may be rejected or subject to penalties.

Burner performance

Gas interchangeability

The Wobbe Index (HHV/sqrt(SG)) determines whether a gas can be substituted in existing burners without adjustment. Critical for LNG import terminals and gas blending operations.

Process design

Equipment sizing

Fired equipment (heaters, boilers, furnaces) is sized based on gas heating value. Changes in gas composition affect burner heat release, flame temperature, and emissions.

Economic significance: For a pipeline transporting 1 Bcf/d of natural gas at $3/MMBtu, a 1% error in heating value determination represents approximately $30,000/day ($11 million/year) in measurement uncertainty. This is why GPA 2145 physical constants and GPA 2172 calculation methods are contractually mandated for custody transfer.

2. HHV vs LHV (Gross vs Net Heating Value)

The distinction between gross heating value (HHV, Higher Heating Value) and net heating value (LHV, Lower Heating Value) centers on the treatment of water produced during combustion.

HHV (Higher Heating Value / Gross Heating Value): Assumes all water produced by combustion CONDENSES to liquid at the reference temperature. This recovers the latent heat of vaporization of the water vapor. HHV = Heat of combustion + Latent heat of water condensation LHV (Lower Heating Value / Net Heating Value): Assumes all water produced by combustion REMAINS AS VAPOR at the reference temperature. The latent heat is NOT recovered. LHV = Heat of combustion (water stays as vapor) Relationship: HHV = LHV + n_H2O x h_fg Where: n_H2O = Moles of water produced per mole of fuel h_fg = Latent heat of vaporization of water = 1059.1 Btu/lb at 60 degF (standard) = 2441.7 kJ/kg at 15 degC (ISO) For methane: CH4 + 2O2 --> CO2 + 2H2O HHV = 1010.0 Btu/scf LHV = 909.4 Btu/scf Difference = 100.6 Btu/scf (10.0%)

When to Use HHV vs LHV

Application Convention Reason
US gas sales / custody transfer HHV Industry standard per GPA 2172; contractual requirement
European gas trading HHV (GCV) ISO 6976 uses Gross Calorific Value (equivalent to HHV)
Gas turbine performance LHV Exhaust leaves at high temperature; water remains as vapor
Boiler efficiency HHV (US), LHV (EU) Condensing boilers can recover some latent heat
Emissions reporting (EPA) HHV 40 CFR Part 98 uses HHV for GHG calculations
Fuel cell systems LHV Exhaust water exits as vapor at operating temperature
Common error: Comparing HHV-based and LHV-based efficiencies without conversion leads to incorrect conclusions. A gas turbine rated at 40% efficiency (LHV basis) is actually about 36% efficient on an HHV basis. Always confirm which basis is used before comparing thermal efficiencies across equipment or jurisdictions.

3. Combustion Chemistry

The heating value of each gas component is determined by its combustion stoichiometry. Hydrocarbon combustion produces carbon dioxide and water; the energy released depends on the number and type of chemical bonds broken and formed.

Combustion Reactions for Key Components

Hydrocarbon Combustion (General): CnH(2n+2) + (3n+1)/2 O2 --> n CO2 + (n+1) H2O Specific Components: Methane: CH4 + 2 O2 --> CO2 + 2 H2O HHV = 1010.0 Btu/scf Ethane: C2H6 + 3.5 O2 --> 2 CO2 + 3 H2O HHV = 1769.7 Btu/scf Propane: C3H8 + 5 O2 --> 3 CO2 + 4 H2O HHV = 2516.1 Btu/scf n-Butane: C4H10 + 6.5 O2 --> 4 CO2 + 5 H2O HHV = 3262.3 Btu/scf n-Pentane: C5H12 + 8 O2 --> 5 CO2 + 6 H2O HHV = 4008.9 Btu/scf Non-Hydrocarbon Combustibles: Hydrogen: H2 + 0.5 O2 --> H2O HHV = 325.0 Btu/scf Hydrogen Sulfide: H2S + 1.5 O2 --> SO2 + H2O HHV = 637.1 Btu/scf Inerts (zero heating value): N2, CO2, He, O2, H2O contribute zero BTU but dilute the mixture.

Why Heavier Hydrocarbons Have Higher Heating Values

On a per-volume basis (Btu/scf), heavier hydrocarbons have progressively higher heating values because each additional carbon atom contributes roughly 500-750 additional Btu/scf. However, on a per-mass basis (Btu/lb), the values converge because heavier molecules also have proportionally greater molecular weights.

Component HHV (Btu/scf) HHV (Btu/lb) Ratio to CH4 (vol)
Methane 1,010 23,875 1.00x
Ethane 1,770 22,323 1.75x
Propane 2,516 21,646 2.49x
n-Butane 3,262 21,293 3.23x
n-Pentane 4,009 21,072 3.97x
Practical implication: Adding 1 mol% ethane to a methane stream increases the mixture HHV by approximately 7.6 Btu/scf. Adding 1 mol% propane increases it by approximately 15.1 Btu/scf. Rich gas from associated gas production often has HHV above 1100 Btu/scf due to high C2+ content.

4. GPA 2145 Physical Constants

GPA Standard 2145 (Table of Physical Constants of Paraffin Hydrocarbons and Other Components of Natural Gas) is the authoritative source for component properties used in natural gas heating value calculations. It is published by the Gas Processors Association and revised periodically to reflect improved experimental measurements.

Key Properties at Standard Conditions (60 deg F, 14.696 psia)

Component MW HHV (Btu/scf) LHV (Btu/scf) Ideal SG
Methane (C1) 16.043 1,010.0 909.4 0.5539
Ethane (C2) 30.070 1,769.7 1,618.7 1.0382
Propane (C3) 44.097 2,516.1 2,314.9 1.5226
i-Butane (iC4) 58.123 3,251.9 3,000.4 2.0068
n-Butane (nC4) 58.123 3,262.3 3,010.8 2.0068
i-Pentane (iC5) 72.150 4,000.9 3,699.0 2.4912
n-Pentane (nC5) 72.150 4,008.9 3,706.9 2.4912
Hexanes+ (C6+) 86.177 4,755.9 4,403.8 2.9755
Nitrogen (N2) 28.014 0 0 0.9672
CO2 44.010 0 0 1.5196
H2S 34.082 637.1 586.8 1.1767
Helium (He) 4.003 0 0 0.1382
Hydrogen (H2) 2.016 325.0 274.6 0.0696

Reference Conditions

GPA 2145 reports properties at two primary sets of reference conditions:

  • US Standard: 60 deg F (15.56 deg C) and 14.696 psia (101.325 kPa). This is the basis for US custody transfer.
  • ISO Standard (6976): 15 deg C (59 deg F) and 101.325 kPa (14.696 psia). Used in international gas trade. Values differ slightly from US standard due to temperature difference.
C6+ characterization: The Hexanes+ fraction is a lumped pseudo-component representing all hydrocarbons with 6 or more carbon atoms. GPA 2145 uses n-hexane properties as the default representation. For rich gas streams with significant C6+ content, a detailed C6+ characterization (MW, SG) from extended gas chromatography improves accuracy.

5. Calculation Method (GPA 2172)

GPA Standard 2172 defines the standard calculation procedure for determining gross heating value, relative density, compressibility factor, and theoretical hydrocarbon liquid content from a gas composition analysis.

GPA 2172 Calculation Procedure: Given: Gas composition {y_i} in mole fractions from gas chromatograph Step 1: Validate composition SUM(y_i) = 1.0 (normalize if necessary) Step 2: Mixture molecular weight MW_mix = SUM(y_i x MW_i) Step 3: Ideal specific gravity SG_ideal = MW_mix / MW_air Where MW_air = 28.9625 lb/lbmol Step 4: Real specific gravity (with compressibility correction) SG_real = SG_ideal / Z_mix Where Z_mix = SUM(y_i x b_i) at standard conditions b_i = summation factors from GPA 2172 Step 5: Gross heating value on volume basis HHV = SUM(y_i x HHV_i) Btu/scf (ideal, dry) Step 6: Net heating value on volume basis LHV = SUM(y_i x LHV_i) Btu/scf Step 7: Heating value on mass basis HHV_mass = HHV x V_m / MW_mix (Btu/lb) Where V_m = 379.49 scf/lbmol (ideal gas molar volume at std) Step 8: Wobbe Index WI = HHV / sqrt(SG_ideal) Step 9: BTU Factor BTU_factor = HHV / 1000 (MMBtu per Mscf)

Worked Example

Example: Typical pipeline gas Composition: CH4 = 85.0 mol%, C2H6 = 6.0%, C3H8 = 3.0% iC4 = 0.5%, nC4 = 0.5%, iC5 = 0.2%, nC5 = 0.2% C6+ = 0.1%, N2 = 2.0%, CO2 = 1.5%, others = 1.0% Step 2: Molecular weight MW = 0.85 x 16.043 + 0.06 x 30.070 + 0.03 x 44.097 + ... MW = 13.637 + 1.804 + 1.323 + 0.291 + 0.291 + 0.144 + 0.144 + 0.086 + 0.560 + 0.660 + ... MW = 19.12 lb/lbmol (approximately) Step 3: Ideal SG SG = 19.12 / 28.9625 = 0.660 Step 5: HHV HHV = 0.85 x 1010.0 + 0.06 x 1769.7 + 0.03 x 2516.1 + 0.005 x 3251.9 + 0.005 x 3262.3 + 0.002 x 4000.9 + 0.002 x 4008.9 + 0.001 x 4755.9 + ... HHV = 858.5 + 106.2 + 75.5 + 16.3 + 16.3 + 8.0 + 8.0 + 4.8 + 0 + 0 + ... HHV = 1093.6 Btu/scf (approximately) Step 8: Wobbe Index WI = 1093.6 / sqrt(0.660) = 1093.6 / 0.8124 = 1346 Btu/scf
Normalization: Gas chromatograph results do not always sum to exactly 100.00 mol%. GPA 2172 requires normalization before calculating properties. The most common approach is proportional normalization: multiply each component by (100/total). Normalization errors greater than 2% should be investigated before proceeding with the calculation.

6. Wobbe Index and Gas Interchangeability

The Wobbe Index (also Wobbe Number) is the single most important parameter for gas interchangeability assessment. It determines whether a substitute gas can be used in existing burner equipment without modification.

Wobbe Index: WI = HHV / sqrt(SG) Where: WI = Wobbe Index (Btu/scf) HHV = Gross heating value (Btu/scf) SG = Specific gravity (air = 1.0) Physical meaning: The Wobbe Index is proportional to the thermal input to a burner at constant supply pressure through a fixed orifice. Two gases with the same Wobbe Index will deliver the same heat release rate when burned at the same pressure through the same burner, regardless of their individual compositions. Derivation: Heat input = HHV x Q_gas Q_gas proportional to 1/sqrt(SG) (from orifice flow equation) Therefore: Heat input proportional to HHV/sqrt(SG) = WI

Wobbe Index Ranges

Gas Type Wobbe Index (Btu/scf) Application
US residential (AGA) 1,310 - 1,390 Standard household appliances, furnaces, water heaters
Typical pipeline gas 1,320 - 1,380 Most US pipeline specifications
LNG regasified 1,350 - 1,420 Varies by LNG source; Qatar LNG higher than US shale gas
Rich associated gas 1,380 - 1,500+ May require NGL extraction before pipeline injection
Hydrogen blend (10%) 1,200 - 1,280 Reduced WI due to low SG of hydrogen

Interchangeability Criteria

The American Gas Association (AGA) and other standards bodies define interchangeability limits based on several indices beyond the Wobbe Index:

  • Wobbe Index: Primary parameter. Must be within a specified range (typically +/- 4% of the reference gas) for burner compatibility.
  • Lifting Index (Weaver): Predicts flame lifting (detachment from burner port). Affected by flame speed and port loading.
  • Flashback Index: Risk of flame propagating back into the burner manifold. Higher with hydrogen-rich gases.
  • Yellow-tip Index: Likelihood of luminous (sooty) flame tips due to incomplete combustion. Higher with rich gas.
LNG import terminal implication: Different LNG sources have different Wobbe Indices. Qatar LNG (WI approximately 1,400) vs US shale gas (WI approximately 1,340) requires blending with nitrogen or lighter gas to meet local pipeline specifications. The Wobbe Index is the governing parameter for LNG interchangeability assessments worldwide.

7. Gas Quality Specifications

Pipeline tariffs and gas purchase contracts specify acceptable ranges for heating value and other quality parameters. Gas that fails to meet these specifications is termed off-spec and may be rejected, blended, or subject to financial penalties.

Typical US Pipeline Gas Quality Specifications

Parameter Specification Rationale
Gross Heating Value (HHV) 950 - 1,150 Btu/scf Burner compatibility, Wobbe Index range
Total Inerts (N2 + CO2) < 3-4 mol% Dilution of BTU content, pipeline capacity efficiency
CO2 Content < 2-3 mol% Corrosion when combined with water
H2S Content < 4 ppm (0.25 grain/100 scf) Toxicity, corrosion, SO2 emissions
Total Sulfur < 20 ppm (5-20 grain/100 scf) Corrosion, odorization interference
Water Content < 7 lb/MMscf Hydrate formation, corrosion
Oxygen (O2) < 0.2 - 1.0 mol% Corrosion, explosive mixtures with H2S
Hydrocarbon Dew Point < 15 - 45 deg F at delivery pressure Liquid dropout in pipeline, slug flow

Impact of Off-Spec Gas

Low BTU gas

HHV < 950 Btu/scf

High inert content (N2, CO2) dilutes energy content. Common in coalbed methane, landfill gas, and some Permian Basin CO2-rich wells. Remedies: CO2 removal (amine treating), N2 rejection unit, or blending with rich gas.

High BTU gas

HHV > 1150 Btu/scf

Excessive C2+ content (rich associated gas). Exceeds Wobbe Index limits and can cause yellow-tipping in burners. Remedies: NGL extraction (cryogenic or lean oil), or blending with lean gas or nitrogen.

BTU billing adjustment: When gas is sold on an energy basis ($/MMBtu), the BTU factor converts measured volume to energy. For example, gas with HHV = 1,050 Btu/scf has a BTU factor of 1.050 MMBtu/Mscf. If the meter measures 10,000 Mscf, the energy delivered is 10,000 x 1.050 = 10,500 MMBtu. This BTU adjustment is performed automatically by electronic flow measurement (EFM) systems using real-time gas chromatograph data.

8. Liquid Content (GPM)

GPM (gallons per Mscf) quantifies the theoretical volume of liquid hydrocarbons that can be recovered from a gas stream per thousand standard cubic feet. It is a key economic parameter for gas processing plant design and NGL recovery optimization.

GPM Calculation: GPM_i = y_i x 1000 / V_Li Where: GPM_i = Gallons per Mscf for component i y_i = Mole fraction of component i V_Li = Ideal gas volume per liquid gallon (scf/gal) from GPA 2145 Component liquid volumes (scf per liquid gallon): Ethane (C2): 36.5 scf/gal Propane (C3): 36.4 scf/gal i-Butane (iC4): 30.6 scf/gal n-Butane (nC4): 31.4 scf/gal i-Pentane (iC5): 27.4 scf/gal n-Pentane (nC5): 27.7 scf/gal Hexanes+ (C6+): 24.4 scf/gal GPM (C2+) = Total recoverable liquid content including ethane GPM (C3+) = Total recoverable liquid content excluding ethane GPM (C3+) is more commonly quoted for economic analysis because ethane recovery varies with market conditions.

Typical GPM Values

Gas Type GPM (C2+) GPM (C3+) Typical Source
Lean gas 1.0 - 2.0 0.3 - 0.8 Dry gas reservoirs, coal seam gas
Moderate gas 2.0 - 4.0 0.8 - 2.0 Conventional gas wells
Rich gas 4.0 - 8.0 2.0 - 5.0 Associated gas, wet gas reservoirs
Very rich gas > 8.0 > 5.0 High-pressure retrograde condensate
Processing economics: A gas plant processing 200 MMscfd of gas with 3.0 GPM (C3+) can theoretically recover 600,000 gallons/day of NGL. At NGL prices averaging $0.80/gallon, that represents $480,000/day in gross NGL revenue. GPM is the first metric gas processors evaluate when determining whether to build or expand a processing facility.

9. BTU Adjustment and Energy Measurement

In the US natural gas market, gas is sold on an energy basis (dollars per MMBtu) rather than a volume basis. The BTU factor converts measured gas volume to delivered energy and is the critical link between flow measurement and revenue accounting.

BTU Factor and Energy Conversion: BTU Factor = HHV / 1000 (MMBtu per Mscf) Energy Delivered = Volume (Mscf) x BTU Factor (MMBtu/Mscf) Example: Meter reads 50,000 Mscf of gas with HHV = 1,045 Btu/scf BTU Factor = 1045 / 1000 = 1.045 MMBtu/Mscf Energy = 50,000 x 1.045 = 52,250 MMBtu At $3.00/MMBtu: Revenue = 52,250 x $3.00 = $156,750 If HHV were incorrectly measured as 1,055 Btu/scf: Energy = 50,000 x 1.055 = 52,750 MMBtu Revenue = $158,250 Error = $1,500 on one measurement period (0.96%)

Real-Time BTU Determination

Modern custody transfer stations determine BTU content continuously using process gas chromatographs (GCs) that analyze composition every 3-10 minutes. The GC output feeds directly into the electronic flow measurement (EFM) system, which calculates heating value per GPA 2172 and applies it to the measured volume in real time.

  • Gas chromatograph (GC): Measures C1 through C6+, N2, and CO2 mole percentages. Extended analysis GCs also measure individual C6-C9 components.
  • Flow computer / EFM: Calculates HHV, SG, Z-factor, and volume at standard conditions. Applies BTU factor to determine energy in MMBtu.
  • Reporting period: Daily or hourly averages are used for accounting. Monthly statements reconcile gas volumes and energy totals.
Audit requirement: Gas chromatographs used for custody transfer must be calibrated against GPA reference standard gas mixtures at regular intervals (typically weekly or monthly). The calibration gas composition and certified heating value provide traceability to GPA 2145 physical constants. Failure to maintain calibration can result in measurement disputes and financial adjustments.

10. Practical Applications

Gas processing

NGL Recovery Optimization

Heating value and GPM together determine the economic value split between residue gas and NGL products. When ethane prices are low, processors may reject ethane (leaving it in the gas stream), which raises the residue gas HHV.

Blending operations

Gas Quality Management

When multiple gas sources with different compositions are blended (e.g., LNG regas + pipeline gas + RNG), the resulting HHV and Wobbe Index must meet downstream specifications. Linear blending applies: HHV_blend = SUM(Q_i x HHV_i) / SUM(Q_i).

Emissions calculations

CO2 Reporting

EPA greenhouse gas reporting (40 CFR Part 98) requires fuel-specific emission factors based on HHV. Natural gas: 53.06 kg CO2/MMBtu (EPA default). Actual emissions depend on composition, particularly the C2+ content.

Hydrogen blending

Decarbonization

Blending hydrogen into natural gas reduces HHV and Wobbe Index. At 10 vol% hydrogen, HHV drops approximately 7% and WI drops approximately 3%. Material compatibility and flame speed must also be assessed.

Renewable natural gas (RNG): Biogas from landfills and anaerobic digesters typically contains 50-60% methane with balance CO2, producing raw HHV of only 500-600 Btu/scf. After CO2 removal (upgrading), pipeline-quality RNG has HHV of 970-1010 Btu/scf, slightly lower than conventional natural gas due to residual CO2. RNG heating value must be verified before pipeline injection.

11. Industry Standards & References

Primary Standards

Standard Title Application
GPA 2145 Table of Physical Constants of Paraffin Hydrocarbons and Other Components of Natural Gas Component properties (MW, HHV, LHV, SG) used in all heating value calculations
GPA 2172 Calculation of Gross Heating Value, Relative Density, Compressibility, and Theoretical Hydrocarbon Liquid Content Standard calculation procedure for BTU determination from gas composition
GPA 2261 Analysis for Natural Gas and Similar Gaseous Mixtures by Gas Chromatography Standard method for gas chromatograph analysis providing composition input
ISO 6976 Natural Gas - Calculation of Calorific Values, Density, Relative Density, and Wobbe Index International standard for heating value calculation at 15 deg C / 101.325 kPa
ASTM D3588 Standard Practice for Calculating Heat Value of Natural Gas ASTM equivalent to GPA 2172 for non-GPA member organizations
AGA Report No. 5 Natural Gas Energy Measurement Framework for energy measurement in custody transfer

Key Technical References

  • GPSA Engineering Data Book: Chapter 2 (Product Specifications) and Chapter 23 (Physical Properties) provide comprehensive reference data and calculation examples for heating value determination.
  • API MPMS Chapter 14.5: Manual of Petroleum Measurement Standards covering energy measurement in natural gas custody transfer.
  • AGA Bulletin No. 36: Interchangeability of Other Fuel Gases with Natural Gas. Defines Wobbe Index limits and supplementary interchangeability indices.
Contractual requirement: Gas purchase and transportation agreements in the United States universally reference GPA 2145 and GPA 2172 for heating value determination. Using different physical constants or calculation methods can result in measurement disputes. Always verify which edition of GPA 2145 is contractually specified, as minor revisions in physical constants between editions can affect calculated HHV at the hundredth of a Btu/scf level.

Ready to calculate Heating Value?

→ Launch Heating Value Calculator