## 2 Aviation Fuel Emissions
### Aviation \(NO_{x}\) Emissions
Consequently, differences in NO\({}_{x}\) emissions between SAFs and traditional aviation fuels are often minimal [63; 64]. [PERSON] et al. [65] found no difference in NO\({}_{x}\) emissions between medium-chain fatty acids to biojet (MB)), produced via hydrogenation from coconut oil, and Jet A-1 fuel. This can be attributed to the nearly identical turbine-inlet temperatures (ITI) of different fuels burning in gas turbine engines at various power levels, suggesting similar combustion temperatures. [PERSON] et al. [66], using a chemiluminescence detector (CLD) to measure NO\({}_{x}\) concentrations, observed minimal differences in NO\({}_{x}\) emissions between 100% aviation kerosene, FT-SPK fuel, and HEFA fuel, with no impact from variations in the proportion of SAFs in SAFs/aviation kerosene blend fuels. [PERSON] et al. [67] reported NO emission levels between 2-2.5 ppm for HEFA-SPK and JP-8 fuels, as well as their mixtures, under colorless distributed combustion (CDC) conditions. This reduction of over 95% compared with traditional air combustion conditions was attributed to decreasedthermal-NOx generation due to lower peak temperatures. Differences in NOx emissions between HEFA-SPK, JP-8, and four different blend ratios of mixed fuels were minimal under CDC conditions. [PERSON] et al. [68] similarly found no significant trend in NOx emissions for HEFA and Jet A-1 fuels, as well as their mixtures. However, at ratios of 20:80 and 50:50, mixed fuel NOx emissions were slightly lower than those of pure fuels, supporting the 50:50 blend ratio as the drop-in proportion for HEFA fuels approved by ASTM D7566. These differences may be related to the adiabatic flame temperature of the mixed fuels. [PERSON] et al. [69] studied microalgae Spirulina biofuel, finding that, as the proportion of added Jet-A fuel increased, the variability in NOx emissions remained low. However, with an increase in engine speed, NOx emissions increased for all tested fuels. For instance, as the engine speed increased from 30,000 rpm to 80,000 rpm, NOx emissions for Jet-A fuel and a 20% biofuel blend increased from 0.2 and 0.18 g per kilogram of fuel to 1.2 and 1.15 g/kg of fuel, respectively. [PERSON] et al. [70] conducted tests on a small-scale (<250 kW) non-proprietary RQL combustion system, evaluating emissions from three conventional aviation fuels, three SAFs (two AT) and one CHJ fuel), and three blended fuels at pressures of 1.0, 1.5, 2.0, and 2.5 bar. Their results show minimal differences in NOx emissions among different fuels, followed by CO2 and CO, with the greatest disparity observed in UHC emissions. [PERSON] et al. [71] evaluated the performance and emissions of four different drop-in aviation biofuels, including UOP HEFA-SPK, Gevo ATJ, Amyris Farnesane, and SB-JP-8. NOx emissions for all four fuels exhibited a similar trend with equivalence ratio variation, with the lowest and highest NO emissions occurring at equivalence ratios of around 0.6 and 1.0, respectively. The reason for this is that, when the equivalence ratio is less than 0.6, prompt NO has a greater impact as the equivalence ratio decreases. Conversely, when the equivalence ratio exceeds 0.6, combustion-induced temperature rise becomes the predominant factor in thermal-NOx generation. Compared with the other three fuels, UOP HEFA-SPK demonstrated a significant increase in NO emissions for equivalence ratios greater than 0.6. If biofuels exhibit higher NOx emissions, potential reasons include a high nitrogen content in the fuel composition or a low oxygen content, the latter of which can be addressed by incorporating pentanol or ethanol to increase oxygen content [72]. However, in the actual use of aviation fuel, oxygen content in the fuel is not permitted.