Observation of water-gas shift equilibrium in diffusion flames

The kinetic behavior of the water-gas shift (WGS) sub-system has been studied computationally for hydrocarbon diffusion flames in the counterflow geometry. Of specific interest was the spatial domain in which the reactions in the WGS-complex exhibit characteristics that lead to species concentrations approaching those expected from equilibrium. Two scenarios were observed for WGS equilibrium: (1)full equilibrium in which every step of the WGS-mechanism is nearly in equilibrium;(2)“apparent” equilibrium, which is achieved via opposing but coherent shifts away from equilibrium for the two key elementary reactions that constitute the WGS reaction. This interesting phenomenon is caused by the supply of the H radical, which participates in the forward reaction of one step and the reverse reaction of the other. This new form of apparent equilibrium differs from well-known “quasi” equilibrium, which is typically caused by the combination of a fast reversible step and slow irreversible step. Parametric domains were found in which these two scenarios are observed. For a given fuel type it was shown that the two scenarios can be separated by the location of stoichiometry. On the oxidizer side, the full equilibrium of the overall WGS reaction is achieved, while on the fuel side, apparent equilibrium can occur.