About Combustion Reaction
Combustion is a rapid exothermic chemical reaction between a fuel and an oxidant — typically oxygen — that releases heat and light. The simulation models the complete combustion of methane (CH₄ + 2O₂ → CO₂ + 2H₂O), proceeding through a radical chain mechanism with three stages: initiation (bond homolysis produces hydroxyl and methyl radicals), propagation (radicals abstract hydrogen atoms in a chain), and termination (two radicals combine and the chain ends). Real-world applications range from internal combustion engines and gas turbines to understanding wildfire dynamics and atmospheric chemistry.
You can control the air-fuel ratio, initial temperature, and pressure to observe how these variables affect flame speed, maximum temperature, and the relative yields of CO₂ and H₂O. The animation tracks radical concentrations over time, letting you see how quickly the chain reaction accelerates and extinguishes.
Frequently Asked Questions
What is the stoichiometric air-fuel ratio for methane?
The stoichiometric ratio for methane combustion is approximately 17.2:1 by mass (air to fuel), or a molar ratio of 2:1 oxygen to methane. At this ratio all fuel and all oxidant are consumed simultaneously, producing the highest theoretical flame temperature — around 2,230 K in air under adiabatic conditions.
What is a radical chain mechanism?
A radical chain mechanism involves species with unpaired electrons (radicals) that react to form new radicals, sustaining the chain. In methane combustion, OH• radicals abstract hydrogen to form H•, which reacts with O₂ to give HO₂• and then more OH•. This autocatalytic cycle continues until the radicals are quenched by recombination at the termination stage.
Why does incomplete combustion produce carbon monoxide?
When the oxygen supply is limited (rich mixture), there is insufficient O₂ to fully oxidise every carbon atom to CO₂, so some carbon exits as CO instead. Carbon monoxide is highly toxic because it binds haemoglobin with around 200 times the affinity of oxygen, blocking oxygen transport in the blood.
How does pressure affect combustion speed?
Higher pressure increases the collision frequency between fuel and oxidant molecules, accelerating the chain-branching reactions. Laminar flame speed roughly scales with pressure as S𝒓 ∝ pⁿ where n is slightly negative for many hydrocarbons, meaning flame speed decreases slightly with pressure, but turbulent combustion and autoignition tendency (knocking) increase markedly with pressure.
What is the adiabatic flame temperature?
The adiabatic flame temperature is the maximum theoretical temperature reached when all combustion heat is retained in the products with no heat loss to the surroundings. For methane burning in air it is approximately 2,230 K; for pure oxygen the value rises to around 3,054 K. Real flames are always cooler due to radiation, conduction, and dissociation of products.
What causes a flame to have different colours?
Flame colour is determined by the emitting species and temperature. The blue inner cone of a gas flame comes from excited CH• and C₂• radicals emitting in the 430–470 nm range. The orange-yellow outer zone arises from incandescent soot particles and sodium impurity emission at 589 nm. A pure hydrogen flame is nearly invisible in daylight.
How does the ignition temperature differ from autoignition?
The ignition temperature (or flash point) refers to the minimum temperature at which a fuel-air mixture will ignite when given an external spark or flame. Autoignition temperature is the temperature at which the mixture spontaneously combusts without any external ignition source. For methane, autoignition occurs at around 537 ℃, well above the ignition temperature.
What role does nitrogen play in combustion?
Nitrogen is mostly inert during combustion but at high temperatures (above ~1,800 K) it reacts with oxygen to form thermal NOx (nitric oxide and nitrogen dioxide) via the Zeldovich mechanism. NOx is a major air pollutant that contributes to acid rain and tropospheric ozone formation, so modern combustors use staged combustion and lean premixed designs to minimise peak temperatures.
Can combustion occur without a visible flame?
Yes — smouldering combustion occurs in porous solids like wood, charcoal, or cigarettes where oxidation proceeds at the solid surface without a gas-phase flame. It is particularly dangerous because it can persist inside materials where it is not easily detected, releasing CO and other toxic gases even without visible fire.
What is the difference between deflagration and detonation?
Deflagration is subsonic combustion where the flame front propagates at speeds of 0.1–10 m/s driven by thermal conduction and diffusion. Detonation is a supersonic wave (typically 1,500–3,000 m/s) where the shock front itself compresses and ignites the mixture. Detonation produces much higher pressures and is used in pulse detonation engines but is destructive in conventional engines (engine knock).
How does the simulation model flame speed?
The simulation approximates laminar burning velocity using an Arrhenius-type rate expression: k = A⋅e⁻𝐸ₐ/(RT), where Eₐ is the activation energy, R is the gas constant, and T is the local temperature. As temperature rises during initiation, the exponential factor grows rapidly, causing the characteristic sudden acceleration of the chain reaction that the animation displays as a spreading flame front.