Combustion Chemistry: Radical Chains, Flame Temperature & Structure

Fuel oxidation: a thermodynamic summary, not a mechanism

At the most fundamental level, combustion is rapid oxidation: a fuel molecule donates electrons while oxygen accepts them. The global equation for methane is deceptively tidy:

CH₄ + 2 O₂ → CO₂ + 2 H₂O    ΔH° = −890 kJ/mol

Equivalence ratio Φ = (fuel/air)_actual / (fuel/air)_stoichiometric
  Φ = 1.0 → stoichiometric      Φ < 1.0 → lean (excess air)
  Φ > 1.0 → rich (excess fuel)

No methane molecule actually collides with two oxygen molecules and produces CO₂ and water in one step. That single equation is a thermodynamic bookkeeping summary; the real chemistry runs through thousands of intermediate species and hundreds of elementary reactions in a reaction zone often just 0.1–1 mm thick.

Radical chains: why flames self-sustain

A flame is a branching chain reaction carried by species with unpaired electrons — chiefly H•, O•, OH• and HO₂•. One reaction dominates everything:

Chain branching (multiplies radicals):
  H• + O₂ → OH• + O•
  O• + H₂ → OH• + H•

Competing termination (dominant below ~1,000 K):
  H• + O₂ + M → HO₂• + M

Branching converts one radical into two, exponentially accelerating the reaction; its competition with three-body recombination sets a crossover temperature around 1,000 K, below which a mixture cannot self-sustain a flame. Above it, branching wins and the mixture ignites — this is why cool oxidation proceeds slowly until suddenly it doesn't.

Adiabatic flame temperature: the theoretical ceiling

The adiabatic flame temperature (AFT) is what you'd get if every joule of reaction heat stayed in the products with none escaping — found from an enthalpy balance at constant pressure, ΔH_comb = Σnⱼ·Cp,j·(T_ad − T_ref):

CH₄ / air, Φ=1.0:  T_ad ≈ 2,230 K   (maximum, right at stoichiometry)
CH₄ / air, Φ=0.6:  T_ad ≈ 1,820 K   (excess air absorbs heat)
CH₄ / O₂ (no N₂):  T_ad ≈ 3,054 K   (nitrogen was the real heat sink)

Real flames run cooler than AFT because of radiation losses, heat transfer to surroundings, and thermal dissociation of CO₂ and H₂O above roughly 2,500 K. Removing nitrogen entirely — oxy-fuel combustion — raises AFT by 600–800 K, which is exactly how cutting torches and glass furnaces get hot enough.

Premixed vs diffusion flames — and why one is blue, one is yellow

In a premixed flame (Bunsen burner, petrol engine), fuel and air mix before reacting and the front propagates at the laminar burning velocity S_L — about 0.40 m/s for methane in air, but 2.65 m/s for hydrogen, which is one reason hydrogen leaks are so much more dangerous. A candle, by contrast, is a diffusion flame: fuel and air meet only where they diffuse together, at the mixture fraction equal to stoichiometric. Wax vapour pyrolyses in oxygen-poor pockets, growing soot particles that glow yellow-orange at 1,200–1,500 K, while a stoichiometric Bunsen flame burns hot and clean enough (~1,800 K) that its blue glow comes from excited CH• and C₂• radicals, not soot at all.

Frequently asked questions

Try it live

Everything above runs in your browser — open Combustion Reaction and watch methane and oxygen molecules collide, ignite, and settle into CO₂ and H₂O as you adjust temperature and the CH₄/O₂ ratio. Nothing is installed, nothing is uploaded.