*2.3. Emmisions*

UNINA and SmartUp presented in [14] a general approach for emissions estimation, based on the results, produced with a gas turbine engine performance calculation tool, the semi-empirical approach illustrated in [15] and average data included in [16]. This emission model requires a manageable amount of input data: the engine's overall pressure ratio, fuel flow rate, operating conditions and ambient conditions are the only information required. The output of this model consists of the emission indices (EIs) for the following species: nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC), carbon dioxide (CO2), water vapor (H2O) and sulphur dioxide (SO2). For the EIs of CO2, H2O and SO2, constant average values listed in Table 2 were assumed.


**Table 2.** Average EIs for CO2, H2O and SO2 for conventional (Jet A-1) aviation fuel, assumed according to [16].

It can be assumed that the approach adopted by UNINA and SmartUp can be considered reliable as long as the fuel is conventional jet fuel (i.e., Jet A-1). For this reason, in the case of HEFA-SPK blends, calibration factors to correct the EI of the above species were considered in [14]. Specifically:


These deviations were all applied to the reference EI values calculated with the original approach established for conventional aviation fuel. With the help of these methods and estimates, the emission impact in the different scenarios can be estimated [14].
