# Fuel-Water Emulsions as an Alternative Fuel for Gas Turbines: A Project Summary
## 1 Introduction  
A fuel-water emulsion is a mixture of at least two liquids, one of which is in the form of drops that is dispersed in the other. The dispersed droplets have microscopic or ultramicroscopic dimensions [1]. In issues related to the combustion process in engines, two-phase emulsions are most often used, in which the continuous phase is fuel, and the dispersed phase is water. Although research is also being carried out on the use of three-phase emulsions, they differ from two-phase emulsions in that inside the drops of the discrete phase, there are even smaller drops of liquid constituting the internal phase [2; 3; 4]. Water is a polar liquid, and hydrocarbon fuels are non-polar liquids, which means that they do not mix with each other. The process of mixing this type of liquid destroys the surface tension between them, which leads to the breakdown of the dispersed phase into drops. The resulting drops tend to combine with each other and separate from the continuous phase. This effect can be eliminated by adding a surfactant to the mixture, which reduces both the thermodynamic instability of the emulsion and the surface tension between liquids. It also limits the increase in interfacial free energy [5; 6; 7]. During the combustion process of emulsion fuels, secondary atomization of droplets may occur. These phenomena are called micro-explosion and puffing. The phenomenon of micro-explosion involves the sudden bursting of a fuel drop, while the puffing phenomenon is characterized by the separation of part of the liquid from the original drop of the fuel mixture [8; 9]. These phenomena are commonly observed in studies conducted on single drops of emulsion fuels [10; 11; 12; 13]. The quality of the atomization of emulsion fuel dropsis influenced by factors such as temperature, the size of the water drops constituting the discrete phase and the position of the water drop in the emulsion fuel drop. Moreover, the influence of each of the two variables can be compensated for by modifying the third one [14]. Mathematical models of the course of micro-explosions are being developed [15]. One of their representatives is the membrane model, in which attempts have been made to develop relationships that enable the calculation of the force of micro-explosions in a unified way for various types of emulsions [16; 17]. The emulsifier can take an active part in the combustion process, as proven by tests carried out on single fuel drops; for example, in the tests conducted on a fast compression machine, in which the continuous fuel phase was n-decane and the discrete phase was water with a share from 10% to 30%. Moreover, the mixture was enriched with sorbitan monooleate (emulsifier) in an amount of 2% [12]. As a result of these studies, it was observed that in emulsions containing up to 20% of water in their composition, after heating the drops, classic combustion and micro-explosions occur. The combustion stage of fuel drops, in which the emulsifier is burned, was also recorded in studies in which fuel drops with the addition of aluminum nanoparticles and sorbitan oleate (emulsifier) were burned [11]. Moreover, it was noticed that increasing the amount of emulsifier extends its combustion time. Apart from the research conducted on single drops of emulsion fuel, there have been very few studies on the combustion of fuel-water emulsions in gas turbines [4]. The objects of the research were the J79-GE-10 (General Electric) [18], 9000B (Alstom) [19], V94.V3 GT and V84.V3 GT (Siemens) [20; 21; 22] and C60 (Capstone Green Energy) turbines [23]. In all cases, apart from the one in which the test object was the C60 turbine, a reduction in NOx emissions was recorded as a result of replacing the traditional fuel with a fuel-water emulsion. In the case of the C60 turbine, an increase in NOx emissions was recorded compared with the reference case following the use of a fuel mixture containing up to 37.5% water in its composition. After increasing the amount of water to 50%, the NOx emissions changed to a decreasing trend [23].