Equilibrium Calculator


The calculation of the chemical equilibrium of a reactive system is in many cases used as a first approach to obtain information about the expected chemical composition and temperature of a real application. Although such a calculation does not provide information about the time scale to reach this state, the estimation is often used for preliminary work on the design of reactive systems.

In order to use the calculator, first select a species set that is to be considered, then provide information on the thermodynamic state (pressure and temperature) and the initial composition (fuel, oxidizer and "air factor" which is the inverse of equivalence ratio). Select the calculation mode (which quantities to fix at a constant value) and then press the "Calculate" button.  The oxidator is always a mixture of O2 and N2 (i.e. air) but can vary by its mole fraction of O2.


Species set:
  Symbol Unit  
Pressure p bar
Temperature T K
Air factor λ -
O2 mole fraction in oxydizer XO2,ox  




    Fuel Air Mixture react.Mix.
T K        
ρ kg/m3        
λ W/m/K        
μ Pa s        
cp J/kg/K        



Ok, doing numerical calculations in your browser seems somewhat weird, but as a proof-of-concept...  Yes, this means that the equilibrium code is running on the client side (i.e. your browser), using the JavaScript language.

 The calculation of the equilibrium concentration is done minimizing numerically the Gibbs' free energy of the mixture. In principle the procedure is the same as the one proposed by W. C. Reynolds and realized in his famous STANJAN code.

[Reynolds, W. C. "The element potential method for chemical equilibrium analysis: Implementation in the interactive program STANJAN, version 3."Technical Rept." (1986)].

 As only thermodynamic data for all species considered in the mixture are needed for such calculations, these data are available in the literature. Thus, the following calculator is able to use some common data sets for species considered to be relevant for the combustion reaction.The thermodynamic data (and transport data, which are not used for calculation, but only displayed) for calculating the equilibrium concentrations are taken from various resources of the literature.

Calculation modes comprise several conditions:
  • T:P - constant temperature and pressure,
  • T:V - constant temperature and volume,
  • P:V - constant pressure and volume,
  • P:H - constant pressure and enthalpy,
  • V:U - constant volume and internal energy,
  • V:H - constant volume and enthalpy.
To enable a better overview, all species with a fraction of less than 1E-9 by mass are not displayed.

The query syntax of URL's can be used to give parameters already from the internet address. Examples are:

  • "?pressure=1&temperature=400&o2inair=0.9&luftzahl=1.5&CH4=0&C2H6=100"
  • "?mechanism=gri30&pressure=1.5&temperature=400&o2inair=0.21&luftzahl=1.3&H2=0&CH4=100"
  • "?mechanism=fuel-oxid-prod&mainfuel=C10H22&pressure=1&temperature=400&o2inair=0.9&luftzahl=1.7"
  • mechanism set is one of alkanes, gri30, hexadecane_global, kee, many-thermo, ndecane, notsomany, s2, s8 or fuel-oxid-prod
  • fuel-oxid-prod requires to specify mainfuel, too