### 9.17.2. Advantages and Disadvantages of Multistep Reaction Mechanisms

Multi-step schemes enable predicting a larger number of species and intermediate species like CO. They also apply if the fuel is a mixture of several species (for example, when modeling natural gas as a mixture of CH4, CO, and H2).

For multi-step schemes, the products limiter in the Eddy Dissipation model should not be turned on, because global extinction of the flame would happen with the commonly used value B = 0.5. Because of this, when applying multi-step schemes to premixed or partially-premixed systems it is recommended that you use the combined FRC/EDM model.

It is important to keep in mind that Finite Rate Chemistry does not account for turbulent fluctuations of temperature when computing temperature dependent reaction rates, unless the Arrhenius with Temperature PDF is used for the reaction rates.

Another consideration is that even though several reactions steps can be modeled, this usually remains a strong simplification from reality where dozens of species may be involved in hundreds of reactions simultaneously.

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