1. Definition
   Excess air coefficient: $\alpha = \dfrac{L}{L_0}$, where $L$ is actual air supply and $L_0$ is theoretical air (just enough for complete combustion).

2. Back-calc from flue gas oxygen (common in practice)
   Let dry flue gas oxygen volume fraction be $O_2$ (%). Ignoring air leaks and measurement error, an approximation is:

• \alpha \approx \dfrac{21}{21-O_2(%)}

Example: if flue gas $O_2 = 3\%$, $\alpha \approx \dfrac{21}{18} = 1.167$.

3. Back-calc from flue gas $CO_2$ (needs fuel characteristics)
   When the maximum $CO_2$ (theoretical dry flue gas limit) is $CO_{2,\max}$ and measured $CO_2$ is $CO_2$:

• $\alpha \approx \dfrac{CO_{2,\max}}{CO_2}$
  This method depends on fuel composition and sampling location, so it is more sensitive to error.