1. What Is the Wobbe Index?
The Wobbe Index (WI) is the single most important parameter for assessing whether one fuel gas can replace another in combustion equipment without modification. Named after Italian engineer Goffredo Wobbe, it quantifies the thermal energy delivered through a fixed orifice at a given supply pressure, accounting for both heating value and gas density.
Definition
WI = HHV / √SG
Gross heating value divided by the square root of specific gravity (air = 1.0).
Physical meaning
Thermal input at constant pressure
Equal Wobbe Index means equal heat release through the same burner at the same gas pressure.
Net Wobbe Index
WInet = LHV / √SG
Uses lower heating value. Preferred in Europe (EN 437) and for gas turbine applications.
Modified Wobbe Index
MWI = HHV / √(SG × T/Tref)
Temperature-corrected. Critical for gas turbine fuel specifications where fuel gas temperature varies.
Why the Wobbe Index Matters
- Gas substitution: Two gases with different compositions but the same Wobbe Index will deliver the same heat output through the same burner orifice at the same supply pressure, eliminating the need for equipment changes.
- LNG trade: LNG cargoes from different sources (Qatar, Australia, US Gulf Coast) have different compositions. The Wobbe Index determines whether a cargo is compatible with a receiving terminal and its downstream consumers.
- Pipeline blending: When streams with different compositions merge in a pipeline network, the resulting Wobbe Index must remain within the acceptable interchangeability band.
- Gas quality tariffs: Pipeline operators specify allowable Wobbe Index ranges in their tariffs to protect downstream consumers and equipment.
- Hydrogen blending: As hydrogen is blended into natural gas networks for decarbonization, the Wobbe Index helps determine maximum allowable H2 concentration.
Critical concept: A gas with a higher heating value does not necessarily have a higher Wobbe Index. A rich gas with high HHV also has a high specific gravity, which partially offsets the heating value in the Wobbe calculation. This is why the Wobbe Index, not HHV alone, is the correct measure of interchangeability.
Typical Wobbe Index Values
| Gas Type | HHV (Btu/scf) | SG (air=1) | WI (Btu/scf) | Notes |
|---|---|---|---|---|
| Pure methane | 1,010 | 0.554 | 1,357 | Reference baseline |
| Lean pipeline gas (95% C1) | 1,020 | 0.58 | 1,340 | Typical US transmission |
| Rich pipeline gas (85% C1) | 1,100 | 0.65 | 1,364 | Gathering system gas |
| LNG – Qatar | 1,110 | 0.64 | 1,388 | High ethane content |
| LNG – US Gulf Coast | 1,030 | 0.59 | 1,341 | Lean, treated gas |
| LNG – Australia (NWS) | 1,060 | 0.61 | 1,357 | Moderate richness |
| 10% H2 blend | 940 | 0.51 | 1,316 | Hydrogen-natural gas blend |
| 20% H2 blend | 870 | 0.44 | 1,311 | Near lower interchangeability limit |
| Landfill gas (50% CH4) | 505 | 0.85 | 548 | Not interchangeable without upgrading |
| Pure propane | 2,516 | 1.522 | 2,039 | Different gas family entirely |
2. Derivation & Physics
The Wobbe Index derives from the fundamental relationship between orifice flow, gas density, and thermal energy release. Understanding the derivation explains why this single parameter captures gas interchangeability so effectively.
Orifice Flow and Heat Release
Volumetric flow through an orifice:
Q = Cd × A × √(2 × ΔP / ρ)
Where:
Q = Volumetric flow rate (scf/hr)
Cd = Discharge coefficient (dimensionless)
A = Orifice area (ft²)
ΔP = Pressure drop across orifice (lbf/ft²)
ρ = Gas density at supply conditions (lb/ft³)
Since ρ = SG × ρair:
Q = Cd × A × √(2 × ΔP / (SG × ρair))
Q = K × 1 / √SG
Where K = Cd × A × √(2 × ΔP / ρair) is constant for fixed geometry and pressure.
Thermal Input Derivation
Heat release rate at burner:
˙Qthermal = Q × HHV
Substituting the orifice equation:
˙Qthermal = K × (1 / √SG) × HHV
˙Qthermal = K × (HHV / √SG)
˙Qthermal = K × WI
Therefore:
For a fixed burner (K = constant), the heat release rate is directly
proportional to the Wobbe Index. Two gases with the same WI deliver
the same thermal input regardless of their individual HHV and SG values.
This is the fundamental principle of gas interchangeability.
Modified Wobbe Index
The standard Wobbe Index assumes gas arrives at the reference temperature (60°F or 15°C). When gas temperature differs from the reference, density changes and flow through the orifice is affected. The Modified Wobbe Index corrects for this.
Modified Wobbe Index (MWI):
MWI = HHV / √(SG × Tgas / Tref)
Where:
Tgas = Actual gas temperature (Rankine or Kelvin)
Tref = Reference temperature (519.67°R = 60°F, or 288.15 K = 15°C)
Physical reasoning:
Gas density at actual temperature: ρ = ρref × (Tref / Tgas)
Higher gas temperature → lower density → higher volume flow → more gas through orifice
But energy per unit volume remains the same (HHV is per standard cubic foot)
Implications:
- At Tgas = Tref: MWI = WI (no correction needed)
- At Tgas > Tref: MWI < WI (hot gas delivers less energy than WI alone suggests)
- At Tgas < Tref: MWI > WI (cold gas delivers more energy)
Gas turbine application:
Gas turbines are highly sensitive to fuel Wobbe Index. Fuel gas may
arrive at 50–400°F depending on fuel treatment and heating.
Typical GT MWI specification: 40–55 MJ/m³ (or equivalent Btu/scf range).
Unit Systems
| Parameter | US Customary | SI / ISO | Conversion |
|---|---|---|---|
| Heating Value (HHV) | Btu/scf | MJ/m³ | 1 Btu/scf = 0.03732 MJ/m³ |
| Wobbe Index | Btu/scf | MJ/m³ | 1 Btu/scf = 0.03732 MJ/m³ |
| Reference Temperature | 60°F (14.696 psia) | 15°C (101.325 kPa) | Volume correction factor applies |
| Specific Gravity | Dimensionless (air=1) | Dimensionless (air=1) | Same value in both systems |
Reference conditions matter: Heating values and Wobbe Index change with reference conditions. US standard (60°F, 14.696 psia) gives different numerical values than ISO standard (15°C, 101.325 kPa). Always verify which reference conditions are being used when comparing Wobbe values from different sources.
3. AGA Interchangeability Indices
While the Wobbe Index is the primary interchangeability criterion, it does not capture all combustion phenomena. The American Gas Association (AGA) developed additional indices to assess flame stability, flashback tendency, and incomplete combustion risk. These are defined in AGA Bulletin No. 36 and AGA Report No. 5.
The Three AGA Indices
IL — Lifting Index (Flame Stability)
IL (Lifting Index):
Measures the tendency of the flame to lift off the burner port.
Physical basis:
- Flame is anchored to the burner port by a balance between gas velocity and flame speed.
- If gas exits too fast relative to its flame speed, the flame lifts and may blow off.
- If gas exits too slowly, the flame may flash back into the burner.
IL relates to the heat release rate and flame speed of the substitute gas
compared to the reference gas.
Acceptable range: IL = 0.8 to 1.2
IL < 0.8 → Flame may lift or blow off (under-fired)
IL = 1.0 → Same flame characteristics as reference gas
IL > 1.2 → Flame may flash back or burn too close to port (over-fired)
Factors that increase IL:
- Higher Wobbe Index (more heat release per unit area)
- Higher flame speed (H2, C2H6 have faster flames than CH4)
Factors that decrease IL:
- Higher inert content (N2, CO2 dilute flame)
- Lower heating value
IF — Flashback Index
IF (Flashback Index):
Measures the tendency of the flame to propagate back into the burner
mixing chamber (flashback).
Physical basis:
- Flashback occurs when flame speed exceeds gas velocity at the burner port.
- Higher flame speed gases (H2, C2H6) are more prone to flashback.
- Lower density gases flow faster through the orifice, partially compensating.
IF is driven primarily by the flame speed ratio between substitute and
reference gas, with a density correction.
Acceptable limit: IF < 1.2
IF < 1.0 → Lower flashback risk than reference gas
IF = 1.0 → Same flashback characteristics
IF > 1.2 → Increased flashback risk; burner modification may be needed
Critical factor:
Hydrogen has a laminar flame speed of ~312 cm/s vs. 40 cm/s for methane.
Even small H2 additions significantly increase IF.
IY — Yellow Tip Index (Incomplete Combustion)
IY (Yellow Tip Index):
Measures the tendency toward incomplete combustion and carbon (soot)
formation, visible as yellow tipping of the flame.
Physical basis:
- Heavier hydrocarbons (C3+) require more air for complete combustion.
- If air supply is insufficient, carbon particles form and glow yellow.
- Residential burners are typically designed for lean gas (C1-dominant).
IY relates to the stoichiometric air requirement and density of the
substitute gas relative to the reference gas.
Acceptable limit: IY < 0.8
IY < 0.6 → Clean combustion, well within limits
IY = 0.8 → At threshold; yellow tips beginning to appear
IY > 0.8 → Incomplete combustion likely; CO emissions increase
Factors that increase IY:
- Higher C3+ content (more air needed per unit volume)
- Higher specific gravity
- Higher HHV (more fuel energy, more air needed)
Factors that decrease IY:
- Lean gas (high C1, low C3+)
- Higher inert content (reduces combustible fraction)
AGA Index Summary
| Index | What It Measures | Acceptable Range | Key Driver | Failure Consequence |
|---|---|---|---|---|
| IL | Flame stability / lifting | 0.8 – 1.2 | Heat release × flame speed | Flame lifts off or blows out |
| IF | Flashback tendency | < 1.2 | Flame speed / gas density | Flame enters mixing chamber |
| IY | Incomplete combustion | < 0.8 | Air requirement / fuel richness | Yellow tips, CO, soot |
Weaver Interchangeability Method
The Weaver method is an alternative approach that uses six indices to characterize gas interchangeability more completely. It was developed by E.R. Weaver at the National Bureau of Standards and is referenced in AGA literature alongside the primary IL/IF/IY indices.
- JL (Lifting): Similar to IL; based on primary air entrainment and flame speed balance
- JF (Flashback): Similar to IF; accounts for burner port geometry effects
- JY (Yellow Tip): Similar to IY; includes secondary air availability
- JH (Heating Value): Direct heat input comparison for appliance capacity
- JI (Incomplete Combustion): CO production tendency under air-lean conditions
- JS (Speed of Combustion): Flame propagation velocity impact on burner performance
Practical guidance: For most pipeline and distribution engineering purposes, the Wobbe Index combined with IL, IF, and IY provides sufficient assessment. The full Weaver method is primarily used in detailed appliance testing and certification programs.
4. Burner Design & Combustion
Understanding how burners interact with varying gas quality is essential for applying the Wobbe Index correctly. Different burner types have different sensitivities to gas composition changes.
Atmospheric Burners (Residential/Commercial)
- Operation: Gas exits an orifice, entrains primary air by momentum (Venturi effect), and burns at port
- Air supply: Typically 40–60% primary air, remainder from secondary air around flame
- Wobbe sensitivity: High — fixed orifice, no automatic compensation
- Interchangeability band: ±5% Wobbe Index recommended
- Typical equipment: Residential furnaces, water heaters, cooking appliances, commercial boilers
Forced-Draft Burners (Industrial)
- Operation: Combustion air supplied by fan; fuel injected through nozzle or lance
- Air supply: Air-fuel ratio controlled by linkage, valve, or electronic control
- Wobbe sensitivity: Moderate — air-fuel ratio adjustment provides some compensation
- Interchangeability band: ±10% with ratio adjustment, ±5% without
- Typical equipment: Process heaters, boilers, thermal oxidizers
Gas Turbines
- Operation: Premixed or diffusion flame in combustion chamber; fuel through multiple nozzles
- Fuel specification: Modified Wobbe Index is the primary control parameter
- Wobbe sensitivity: Very high — affects combustion dynamics, emissions, and turbine life
- Typical MWI spec: ±5% of design value (tight control required)
- Concerns: Flame holding, combustion instability (dynamics), NOx/CO emissions, hot gas path temperature
Combustion Effects of Gas Quality Variation
| Gas Quality Change | Effect on WI | Flame Behavior | Operational Impact |
|---|---|---|---|
| Higher C3+ content | WI increases slightly | Longer, more luminous flame; yellow tips | Increased CO, soot; flame impingement risk |
| Higher N2 content | WI decreases | Shorter flame; lifting tendency | Reduced capacity; flame instability |
| Higher CO2 content | WI decreases | Cooler flame; lower stability | Reduced capacity; flame-out risk |
| Higher H2 content | WI decreases (moderate) | Faster flame speed; shorter flame | Flashback risk; higher NOx at high H2 |
| Higher gas temperature | MWI decreases | Higher flow rate; richer mixture | Capacity increase; possible over-firing |
| Lower gas pressure | No WI change | Lower flow; flame closer to port | Reduced capacity; flashback at very low P |
Flame Speed Considerations
Laminar flame speed at standard conditions (1 atm, 25°C):
Component flame speeds (cm/s):
- Methane (CH4): 40 cm/s
- Ethane (C2H6): 43 cm/s
- Propane (C3H8): 46 cm/s
- n-Butane (C4H10): 45 cm/s
- Hydrogen (H2): 312 cm/s
- Carbon monoxide: 47 cm/s
Key observation:
Hydrogen flame speed is ~8x that of methane. This is why hydrogen
blending significantly affects flashback index (IF) even at low
concentrations.
For a gas mixture, the effective flame speed is approximately:
SL,mix = Σ(yi × SL,i) / Σ(yi) (combustible components only)
This linear mixing rule is approximate. Actual flame speeds depend
on mixture stoichiometry, temperature, pressure, and turbulence.
Design practice: Gas turbine OEMs (GE, Siemens, Solar, Rolls-Royce) specify fuel gas requirements in terms of Modified Wobbe Index, heating value range, and maximum H2 content. Always verify compliance with the specific OEM fuel specification before operating on non-standard gas compositions.
5. Gas Quality & Pipeline Tariffs
Pipeline operators and gas utilities specify allowable gas quality parameters in their tariffs to protect downstream equipment and consumers. The Wobbe Index is increasingly used alongside traditional compositional limits.
Typical US Pipeline Gas Quality Specifications
| Parameter | Typical US Pipeline | Gulf Coast | Northeast US | LNG Feed |
|---|---|---|---|---|
| HHV (Btu/scf) | 950 – 1,100 | 967 – 1,110 | 970 – 1,100 | 1,000 – 1,100 |
| Wobbe Index (Btu/scf) | 1,310 – 1,390 | 1,310 – 1,400 | 1,310 – 1,385 | 1,340 – 1,400 |
| CO2 (mol%) | ≤ 2.0 | ≤ 2.0 | ≤ 2.0 | ≤ 0.05 |
| N2 (mol%) | ≤ 3.0 | ≤ 3.0 | ≤ 3.0 | ≤ 1.0 |
| Total Inerts (mol%) | ≤ 4.0 | ≤ 4.0 | ≤ 4.0 | ≤ 1.5 |
| H2S (grains/100scf) | ≤ 0.25 | ≤ 0.25 | ≤ 0.25 | ≤ 0.25 |
| O2 (mol%) | ≤ 0.2 | ≤ 0.2 | ≤ 0.1 | ≤ 0.01 |
| Water Dewpoint (°F) | ≤ −30 | ≤ −30 | ≤ −40 | ≤ −150 |
European Gas Quality Standards
European gas quality is governed by EN 437 (gas appliance categories) and national standards. Europe uses the Net Wobbe Index (based on LHV) as the primary parameter.
| Gas Family | EN 437 Group | Net Wobbe (MJ/m³) | Gross Wobbe (MJ/m³) | Typical Application |
|---|---|---|---|---|
| Group H (High) | 2H | 45.7 – 54.7 | 48.2 – 56.5 | Natural gas (most of Europe) |
| Group L (Low) | 2L | 39.1 – 44.8 | 41.2 – 46.8 | Low-cal gas (Netherlands, Belgium) |
| Group E (Extended) | 2E | 40.9 – 54.7 | 43.0 – 56.5 | Harmonized European range (future) |
LNG Cargo Assessment
LNG terminals must evaluate each cargo for compatibility with the downstream gas network. The Wobbe Index is the primary screening parameter.
LNG Cargo Acceptance Criteria:
Step 1: Determine regasified LNG composition
- From ship's GC analysis or bill of lading
- Account for boil-off gas (BOG) composition change during voyage
Step 2: Calculate Wobbe Index
- WI = HHV / √SG at standard conditions (60°F, 14.696 psia)
Step 3: Compare to terminal specification
- Typical US LNG terminal: WI = 1,310 – 1,420 Btu/scf
- If outside range: nitrogen ballasting or blending required
Step 4: Mitigation options for off-spec LNG
- Nitrogen injection: Reduces HHV and WI (1 mol% N2 ≈ −10 Btu/scf HHV)
- Blending with pipeline gas: Dilutes rich LNG with leaner pipeline gas
- LPG extraction: Removes C3+ to reduce HHV (used at some terminals)
- Cargo rejection: Last resort if no mitigation feasible
Example:
Qatar LNG: HHV = 1,110 Btu/scf, SG = 0.64
WI = 1,110 / √0.64 = 1,388 Btu/scf
US pipeline spec WI max: 1,390 Btu/scf
Result: Marginally within spec. N2 ballasting may be needed for
safety margin.
Gas Blending for Wobbe Adjustment
Blending Calculation (Volume Basis):
For two gas streams A and B mixed:
HHVmix = xA × HHVA + xB × HHVB
SGmix = xA × SGA + xB × SGB
Where xA + xB = 1.0 (volume fractions)
WImix = HHVmix / √SGmix
Note: WImix is NOT a linear blend of WIA and WIB because
the square root function makes the relationship non-linear.
To find blend ratio for target WI, solve iteratively:
1. Assume xA
2. Calculate HHVmix and SGmix
3. Calculate WImix
4. Adjust xA until WImix = WItarget
Example:
Stream A: Rich LNG, WI = 1,410 Btu/scf (HHV=1,120, SG=0.63)
Stream B: Lean pipeline, WI = 1,330 Btu/scf (HHV=1,010, SG=0.58)
Target: WI = 1,360 Btu/scf
At xA = 0.40:
HHVmix = 0.40 × 1,120 + 0.60 × 1,010 = 1,054
SGmix = 0.40 × 0.63 + 0.60 × 0.58 = 0.600
WImix = 1,054 / √0.600 = 1,361 Btu/scf (acceptable)
Tariff compliance: Pipeline operators may reject gas that does not meet Wobbe Index specifications. Producers and shippers must ensure their gas meets tariff requirements before delivery. Non-compliant gas may be subject to penalties, processing charges, or outright rejection at receipt points.
6. Hydrogen Blending Effects
Blending hydrogen into natural gas pipelines is a key strategy for decarbonizing energy systems. However, hydrogen has fundamentally different combustion properties than methane, and the Wobbe Index is an essential tool for determining safe blending limits.
Hydrogen vs. Methane Properties
| Property | Methane (CH4) | Hydrogen (H2) | Ratio H2/CH4 | Impact |
|---|---|---|---|---|
| Molecular weight | 16.04 | 2.016 | 0.126 | Much lighter gas |
| HHV (Btu/scf) | 1,010 | 324 | 0.321 | 1/3 the energy per scf |
| HHV (Btu/lb) | 23,890 | 61,100 | 2.56 | 2.5x energy per unit mass |
| Specific gravity | 0.554 | 0.070 | 0.126 | Very low density |
| Wobbe Index (Btu/scf) | 1,357 | 1,225 | 0.903 | ~10% lower than CH4 |
| Flame speed (cm/s) | 40 | 312 | 7.8 | 8x faster flame |
| Adiabatic flame temp (°F) | 3,542 | 3,807 | 1.07 | Slightly hotter flame |
| Flammability range (vol%) | 5 – 15 | 4 – 75 | — | Much wider range |
Wobbe Index at Various H2 Blending Levels
H2 blending effect on natural gas (base: 95% CH4, 3% C2, 2% N2):
0% H2: HHV = 1,023, SG = 0.586, WI = 1,336 Btu/scf (reference)
5% H2: HHV = 989, SG = 0.541, WI = 1,344 Btu/scf (+0.6%)
10% H2: HHV = 953, SG = 0.495, WI = 1,354 Btu/scf (+1.3%)
15% H2: HHV = 918, SG = 0.450, WI = 1,369 Btu/scf (+2.5%)
20% H2: HHV = 883, SG = 0.404, WI = 1,389 Btu/scf (+4.0%)
25% H2: HHV = 848, SG = 0.358, WI = 1,417 Btu/scf (+6.1%)
30% H2: HHV = 814, SG = 0.313, WI = 1,454 Btu/scf (+8.8%)
Key observation:
The Wobbe Index initially INCREASES with hydrogen blending (up to ~25%)
because the density decrease (denominator) outweighs the heating value
decrease (numerator). This is counterintuitive but physically correct.
At very high H2 levels (>50%), WI starts to decrease:
50% H2: HHV = 667, SG = 0.177, WI = 1,585 Btu/scf
100% H2: HHV = 324, SG = 0.070, WI = 1,225 Btu/scf
The non-monotonic behavior means WI alone is insufficient for
assessing hydrogen blending safety. Flame speed indices (IF) and
methane number must also be checked.
H2 Blending Limits by Application
| Application / Equipment | Max H2 (vol%) | Limiting Factor | Standard / Reference |
|---|---|---|---|
| Residential appliances (generic) | 5 – 20% | Flashback, flame detection | ASME/CSA; varies by appliance age |
| Industrial burners (pre-mix) | 5 – 15% | Flashback at burner port | Manufacturer specification |
| Gas turbines (DLN combustors) | 5 – 15% | Combustion dynamics, flashback | OEM fuel spec (GE, Siemens) |
| Gas turbines (diffusion) | 30 – 100% | Materials, NOx emissions | OEM fuel spec |
| Gas engines (lean-burn) | 2 – 10% | Knock / pre-ignition | Methane number > 65 |
| CNG vehicles | 2 – 5% | Tank embrittlement, seals | ISO 15403, SAE J1616 |
| Pipeline steel (X52–X70) | 10 – 20% | Hydrogen embrittlement | ASME B31.12 |
| Gas meters (diaphragm) | 20 – 30% | Measurement accuracy | EN 1359, OIML R137 |
Methane Number and Gas Engines
Methane Number (MN):
Definition: Knock resistance rating for gas engine fuel, analogous to
octane number for gasoline. Scale from 0 (hydrogen) to 100 (pure methane).
MN = 100: Pure methane (most knock-resistant)
MN = 80: Typical pipeline gas (acceptable for most engines)
MN = 65: Minimum for lean-burn gas engines (Caterpillar, Wartsila, Jenbacher)
MN = 44: Ethane
MN = 20: Propane
MN = 0: Hydrogen (most knock-prone)
Effect of H2 blending on MN:
Each 1% H2 reduces MN by approximately 0.5–1.0 points.
At 10% H2 in pipeline gas: MN drops from ~80 to ~70–75
At 20% H2: MN drops to ~60–65 (marginal for lean-burn engines)
Gas engine derating:
Below MN 80, many engines require derating (reduced power output)
to prevent knocking. Typical derating: 1–2% power per MN point
below 80.
Hydrogen blending challenge: The Wobbe Index suggests H2 blending up to 20–25% is acceptable from a thermal input perspective. However, the dramatically higher flame speed of hydrogen creates flashback and safety risks well before that level. In practice, 5–20% H2 is the typical limit for existing infrastructure, constrained by the weakest link in the gas chain (usually residential appliances or gas engines).
7. Industry Applications
The Wobbe Index finds application across the entire natural gas value chain, from upstream production through processing, transmission, distribution, and end use.
Upstream & Gathering Systems
- Field gas characterization: Wobbe Index screening determines whether field gas needs processing before pipeline delivery. Gas with WI outside tariff limits requires treatment (NGL recovery, N2 rejection, CO2 removal).
- Commingled production: When gas from multiple wells or formations is combined at a gathering facility, the blended Wobbe Index must remain within pipeline tariff specifications.
- Fuel gas selection: On-site compressor and heater fuel gas must meet equipment specifications. Low-WI gas (high inerts) may not sustain stable combustion.
Gas Processing Plants
- NGL recovery optimization: Deeper NGL recovery (more C3+ removal) lowers HHV but also lowers SG. The net effect on Wobbe Index depends on which changes more. Plant operators monitor WI to ensure residue gas meets tariff requirements.
- Nitrogen rejection: N2 removal increases both HHV and SG (removing a light diluent), generally increasing WI. NRU product gas is typically richer than feed gas.
- CO2 removal: Amine treating removes CO2, increasing HHV while reducing SG (CO2 is denser than methane). WI typically increases after treating.
Transmission Pipelines
- Receipt point quality: Gas quality gates at pipeline receipt points verify incoming gas meets tariff WI specifications. Non-conforming gas is rejected or subject to penalties.
- Blending at interconnects: Where pipelines interconnect, gases from different sources blend. Operators track WI at key points to ensure the blended gas remains within spec throughout the network.
- Storage injection/withdrawal: Gas injected into storage may have different composition than gas withdrawn (due to mixing with cushion gas). WI monitoring ensures withdrawn gas meets pipeline specs.
LNG Value Chain
- Feed gas specification: LNG plant feed gas must meet tight composition specs for cryogenic processing. WI is used for initial screening.
- Cargo quality management: Each LNG cargo has a unique composition profile. WI determines whether the cargo is compatible with the destination terminal and downstream market.
- Regasification terminal: Terminals may need nitrogen ballasting to reduce WI for rich LNG cargoes, or LPG extraction for very rich cargoes.
- Market compatibility: Different markets (US, Europe, Asia) have different WI specifications. A cargo acceptable in one market may not be acceptable in another without treatment.
Distribution & End Use
- City gate stations: Gas distribution companies monitor WI at city gate stations to ensure gas delivered to residential and commercial customers is within appliance design specifications.
- Appliance certification: Gas appliances are tested and certified for specific gas families defined by Wobbe Index ranges (EN 437 gas groups). Operating outside these ranges voids certification.
- Gas blending stations: Some utilities operate blending stations where propane-air or LNG is blended with pipeline gas to manage WI during peak demand or supply disruptions.
Renewable and Synthetic Gas
- Biomethane (RNG): Upgraded biogas (95%+ CH4) typically has WI close to pipeline gas and is interchangeable without issues. Raw biogas (50–60% CH4, 40–50% CO2) has a very low WI and is not pipeline-compatible.
- Synthetic natural gas (SNG): Methanation of hydrogen and CO2 produces SNG with WI similar to pipeline gas. Quality depends on conversion efficiency and residual H2/CO2.
- Power-to-gas: Electrolytic hydrogen blended into gas networks. WI is used alongside flame speed indices to determine maximum injection rates per the discussion in Section 6.
Measurement and Monitoring
Real-time gas quality analysis:
Online instruments for WI monitoring:
- Gas chromatograph (GC): Full composition analysis, WI calculated
- Cycle time: 3–10 minutes
- Accuracy: ±0.1% on composition, ±1 Btu/scf on HHV
- Calorimeter (direct measurement):
- Cutler-Hammer or Therm-O-Flow type
- Measures HHV directly by combustion
- Requires separate SG measurement for WI
- Correlative sensors (speed of sound, infrared):
- Faster response (seconds vs. minutes)
- Lower accuracy (±5–10 Btu/scf on HHV)
- Useful for trending and alarm purposes
- Portable analyzers:
- FTIR or laser-based for spot checks
- Suitable for field verification
Monitoring frequency:
- Pipeline receipt points: Continuous GC analysis (every 5–10 min)
- City gate stations: Hourly or continuous
- Industrial fuel gas: Continuous for gas turbines, periodic for boilers
- LNG terminals: Per-cargo analysis + continuous blend monitoring
Best Practices Summary
- Always check WI, not just HHV: Two gases with the same HHV can have different Wobbe Indices if their specific gravities differ. WI is the correct interchangeability criterion.
- Use all three AGA indices: The Wobbe Index addresses heat input but not flame stability, flashback, or soot formation. Check IL, IF, and IY for complete assessment.
- Specify reference conditions: Always state whether WI is at 60°F/14.696 psia (US) or 15°C/101.325 kPa (ISO). Numerical values differ between systems.
- Account for temperature: Use Modified Wobbe Index for gas turbine applications and when gas temperature differs significantly from the reference.
- Design for the weakest link: The maximum allowable Wobbe Index variation is determined by the most sensitive equipment in the gas chain (typically residential appliances or lean-burn gas engines).
- Monitor continuously: Gas composition can change due to upstream processing changes, well workovers, storage cycling, or LNG cargo switches. Continuous WI monitoring prevents off-spec gas from reaching sensitive consumers.
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