Emission indices are important flame properties that can be used to compare flame characteristics in different flow fields. They are usually given as the mass of pollutant generated per kilogram fuel consumed, g/kg-fuel. Ansys Chemkin flame models, the Premixed Laminar flame model and Opposed-flow flame model, use formulations derived by Takeno and Nishioka [108] to evaluate emission indices. Brief descriptions of emission index calculations are provided below. For a detailed derivation of the formulations, see the cited paper.

Since emission indices are reported per kilogram of fuel consumed, it is important to identify fuel components in the inlets. The flame codes assume fuel is introduced to the system only through the fuel boundary at x = 0. Any species containing a C or H element in the fuel inlet will be treated as a fuel species, except CO₂ and H₂ O. The total fuel-mass flow rate coming into the system can then be obtained from fuel inlet properties as

(14–11)

where subscript F and fuel denote fuel boundary and all fuel species of the system, respectively. The fraction of fuel consumed at a given location x can be computed by the ratio of accumulated fuel consumption and total fuel mass flow rate as

(14–12)

and the fraction of unburned fuel is .

The emission index of a pollutant, for example NO, can be calculated as

(14–13)

and the total NO emission index from the flame is

(14–14)

The NOx emission index is reported as grams of NO₂ -equivalent formed per kilogram of fuel consumed, that is,

(14–15)

Emission indices of CO ( EICO), NO (EINO), and NOx (EINOx) are available in the Ansys Chemkin Post-Processor for selected reactor models.