# Fuel-Water Emulsions as an Alternative Fuel for Gas Turbines: A Project Summary
## 3 Research Results and Discussion
### NOx Emissions  
In the combustion process in turbojet engine combustors, among the nitrogen oxides produced (generally designated as NOx), nitrogen oxide NO is the most important. There are three basic mechanisms of nitrogen oxide formation: thermal (thermal NO), over-equilibrium (prompt) and fuel (fuel NO). Most NOx in the combustion process in gas turbines is produced according to the thermal mechanism. It involves the oxidation of molecular nitrogen by atomic oxygen. The condition enabling the formation of NO according to this mechanism is the dissociation of molecular oxygen at high temperature [54; 55; 56; 57].  
The use of a fuel-water emulsion leads to a reduction in the temperature behind the combustion chamber with a simultaneous reduction in NOx emissions in the exhaust gases (Figure 9). In the graph, the measuring points represent water contents of 0%, 3%, 6%, 9% and 12%. In the GTM120 miniature gas turbine, NOx concentration reduction with the smallest temperature reduction after the combustion chamber occurs at a shaft rotation speed of 100 kPrm. Increasing the turbine load to the maximum significantly increases the possibility of NOx reduction but at the same time leads to the intensification of the temperature reduction process. It was also observed that as the turbine load increases, a lower water content is required to achieve a given NOx reduction level. Moreover, the NOx concentration reduction occurs intensively in all the cases presented when the temperature drop does not exceed 4%. After exceeding this value, the nature of changes in the reduction process becomes more complex, which suggests that the drop in this temperature cannot be directly linked to the drop in temperature in the primary combustion zone characterized by the highest temperatures, where most nitrogen oxides are produced according to the thermal mechanism.  
Based on the above, the reasons for the intensive reduction of NOx emissions in exhaust gases should be sought in changes in the flame structure in the primary combustion zone. Computer numerical calculations prove that the addition of water to fuel, even in the smallest amounts, causes a more even temperature distribution in the primary combustion zone with a reduced volume of maximum temperatures. This translates into a reduction in the area where NOx is formed most intensively and an increase in the area where the intensity of NOx formation is lower (Figure 10). These changes in the flame structure contribute to a significant reduction in NOx with a relatively small reduction  
Figure 9: Percentage change in NOx emissions as a function of the percentage change in temperature behind the combustion chamber.  
in the temperature behind the combustion chamber. Details regarding the experimental studies and numerical calculations have been presented in full detail in [40; 41]. Also noteworthy is the fact that the effects shown in Figure 10 remain valid even when a simpler equilibrium model is used for the numerical calculations, which does not take into account the chemical imbalance resulting from aerodynamic flame stresses under turbulence [58].  
When analyzing the combustion process of emulsion fuels, attention should be paid to the combustion of emulsions whose continuous phase consists of fuels containing significant amounts of nitrogen in their composition. In these cases, the use of a fuel-water emulsion as a way to reduce NOx emissions may have the opposite effect. This is proven by the previously mentioned study, in which the continuous phase of the emulsion was Redwood 650 [24]; the study is described in [31; 59; 60]. In the second case, the fuel constituting the continuous phase was JF-8, but the additive was the surfactant Clindrol 100 CG, containing 4.37% nitrogen. Moreover, the fuel-water emulsions used contained up to 20% water and up to 20% emulsifier, which resulted in an increase in water content and increased NOx emissions for each simulated engine load.