Safety & Process Engineering

Combustion & Flammability

Understand flammability limits (LEL/UEL), stoichiometric combustion, Le Chatelier's mixing rule, and flame propagation for gas processing safety per NFPA 68/69 and API 521.

Launch Calculator Le Chatelier's Rule

Methane LEL

5.0 vol%

Lower explosive limit in air at standard conditions. Below this, too lean to ignite.

Methane UEL

15.0 vol%

Upper explosive limit. Above this concentration, mixture is too rich to sustain combustion.

Auto-ignition

1076°F

Methane auto-ignition temperature. No external spark needed above this temperature.

Contents

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Calculate LEL/UEL, stoichiometric air, and combustion products for any gas or mixture.

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1. Flammability Limits

Every combustible gas has a range of concentrations in air within which it can ignite and sustain flame propagation. Below the Lower Explosive Limit (LEL), the mixture is too lean; above the Upper Explosive Limit (UEL), it is too rich.

Flammability range diagram showing LEL, UEL, and stoichiometric concentration

Key Definitions

Term Definition
LEL (LFL) Minimum concentration of gas in air that can propagate flame (vol%)
UEL (UFL) Maximum concentration of gas in air that can propagate flame (vol%)
Flammable Range UEL minus LEL — wider range means greater hazard
Stoichiometric Exact fuel-air ratio for complete combustion (maximum energy release)
AIT Auto-Ignition Temperature — minimum temperature for self-ignition without spark

Common Hydrocarbon Flammability Data

Gas LEL (vol%) UEL (vol%) AIT (°F) HHV (BTU/SCF)
Methane5.015.010761,012
Ethane3.012.49591,773
Propane2.19.58712,524
n-Butane1.88.47613,271
n-Pentane1.47.85884,017
Hydrogen4.075.01062325
H₂S4.044.0500647
CO12.574.01128321
Ethylene2.736.09141,613
Key trend: Heavier hydrocarbons have lower LELs but narrower flammable ranges. Hydrogen and H₂S have exceptionally wide ranges, making them more hazardous.

2. Le Chatelier's Mixing Rule

For gas mixtures, the flammability limits are calculated using Le Chatelier's rule, which weights each component by its mole fraction on a combustible-only basis:

Le Chatelier's Rule (NFPA 68): 1 / LEL_mix = Σ (y_i / LEL_i) 1 / UEL_mix = Σ (y_i / UEL_i) Where: y_i = mole fraction of combustible component i (on combustible-only basis, excluding inerts) LEL_i = LEL of pure component i

Handling Inert Components

Inert gases (N₂, CO₂) do not appear in Le Chatelier's equation directly, but they narrow the flammable range by acting as diluents. Their effect is accounted for when converting from combustible-only basis back to total mixture basis.

Example: Typical Natural Gas

Composition: 85% CH₄, 7% C₂H₆, 3% C₃H₈, 1% C₄H₁₀, 2% CO₂, 2% N₂

Combustibles only (96%): CH₄=88.5%, C₂H₆=7.3%, C₃H₈=3.1%, C₄H₁₀=1.0%
1/LEL = 0.885/5.0 + 0.073/3.0 + 0.031/2.1 + 0.010/1.8 = 0.216
LEL_comb = 4.63% → LEL_mix = 4.63% × 0.96 = 4.44%
UEL calculated similarly → ≈14.2%

3. Combustion Chemistry

Complete combustion of hydrocarbons produces carbon dioxide and water. The stoichiometric equation defines the exact air-to-fuel ratio needed.

General Hydrocarbon Combustion: CₙHₘ + (n + m/4) O₂ → n CO₂ + (m/2) H₂O For Methane: CH₄ + 2 O₂ → CO₂ + 2 H₂O Air required (stoichiometric): Air = O₂ required / 0.2095 (Air is 20.95% O₂ by volume)

Stoichiometric Air-Fuel Ratios

Fuel O₂ (mol/mol) Air (mol/mol) Stoich. Conc. (vol%)
Methane2.09.559.48%
Ethane3.516.715.64%
Propane5.023.874.02%
n-Butane6.531.033.12%
Hydrogen0.52.3929.5%
H₂S1.57.1612.3%

Excess Air

In practice, combustion equipment operates with excess air to ensure complete burn:

  • Process heaters/boilers: 10–20% excess air
  • Thermal oxidizers: 20–50% excess air
  • Flares: 50–100%+ (wind-assisted mixing)
  • Gas turbines: 200–300% excess air
Incomplete combustion: Insufficient air produces CO (carbon monoxide) instead of CO₂. At very low air ratios, soot and unburned hydrocarbons also form. This is both a safety hazard and emissions concern.

Special Combustion Cases

Compound Reaction Note
H₂S H₂S + 1.5 O₂ → SO₂ + H₂O Produces SO₂ — emissions regulated
CO CO + 0.5 O₂ → CO₂ No water produced
Hydrogen H₂ + 0.5 O₂ → H₂O Invisible flame — special detection needed

4. Temperature & Pressure Corrections

Flammability limits measured at standard conditions (77°F, 14.7 psia) change with temperature and pressure. These corrections are critical for process safety analysis at operating conditions.

Temperature Effect

Modified Burgess-Wheeler Law: LEL_T = LEL_25 × [1 - 0.000721 × (T - 25)] (T in °C) UEL_T = UEL_25 × [1 + 0.000721 × (T - 25)] Key effect: - Higher temperature → lower LEL, higher UEL - Flammable range widens with temperature - At auto-ignition temperature, no external ignition source needed

Pressure Effect

  • LEL: Relatively insensitive to pressure below ~300 psia
  • UEL: Increases significantly with pressure
  • At very high pressures (>1000 psia), flammable range can extend dramatically
UEL Pressure Correction: UEL_P = UEL_atm + 20.6 × (log₁₀(P_abs/14.696) + 1) Where P_abs is absolute pressure in psia
Practical impact: A gas processing plant operating at 1000 psig will have a much wider flammable range than atmospheric-pressure equipment. This affects hazardous area classification per API RP 500/505.

5. Applications

Hazardous Area Classification

  • LEL defines the threshold for gas detection alarm setpoints (typically 10–20% of LEL)
  • Area classification per API RP 500 (Class/Division) or API RP 505 (Zone) based on gas release likelihood
  • Equipment selection (explosion-proof, intrinsically safe) depends on gas group

Ventilation Design

  • NFPA 68: Deflagration venting for enclosed spaces
  • NFPA 69: Explosion prevention systems (inerting, suppression)
  • Dilution ventilation to maintain concentration below 25% of LEL

Flare and Combustion Equipment

  • Flare tip design requires gas within flammable range at ignition point
  • Purge gas calculations based on preventing air ingress below UEL
  • Burner management systems use combustion stoichiometry for air/fuel control

Relief System Design

  • API 521 fire-case relief sizing uses heat of combustion
  • Flare radiation calculations require combustion products
  • Dispersion modeling uses LEL as the hazard threshold

References

  • NFPA 68 — Standard on Explosion Protection by Deflagration Venting
  • NFPA 69 — Standard on Explosion Prevention Systems
  • API 521 — Pressure-relieving and Depressuring Systems
  • API RP 500 — Hazardous Area Classification (Class/Division)
  • API RP 505 — Hazardous Area Classification (Zone)
  • GPSA, Chapter 1 (General Information)
  • Zabetakis, M.G. — Flammability Characteristics of Combustible Gases and Vapors (Bureau of Mines Bulletin 627)

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