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The review presents the results of experimental and theoretical studies obtained in recent years within the framework of the main areas of research of gel fuels: choice of component composition and substantiation of fuel preparation technologies and their rheological characteristics; fuel transportation and atomization processes; and the processes of their ignition and combustion. The main advantages of gel fuels in comparison with widely used liquid and solid fuels are considered. The advantages and disadvantages of known experimental approaches to the study of the combustion processes of gel fuels are analyzed. The well-known physical and mathematical models of gel fuels ignition are given, including those under conditions of melt droplets dispersion. The tendencies of further development of gel fuels within the framework of the combustion theory are formulated.

Composite fuel is a promising energy source that allows for solving the problems of waste disposal with energy generation. Such fuel is the most accessible fuel and is cheap in comparison with fossil fuels widely used in industrial thermal power engineering. This paper presents the results of experimental studies on the effect of the initial temperature and the addition of combustible liquids and solid components on the ignition characteristics of composite fuel single droplets. Composite liquid fuels were prepared using the main components: bituminous coal, coal processing waste (filter cake), rapeseed oil, turbine oil, and water. The research was carried out for fuel droplets with an initial temperature from −60 to +60 and an ambient temperature from 700 to 1000 °C. The differences in the ignition delay times at conditions close to the limiting ones were 2–3.5 times. A promising direction for intensifying the processes of the ignition and combustion of composite liquid fuels under relatively intense heating is self-grinding into a large number of small fragments up to complete disintegration due to the dispersion effect. It has been experimentally found that the addition of highly flammable liquids (gasoline, kerosene, diesel fuel, formic acid) to the fuel composition in an amount of 5% is characterized by an intensification of ignition and burnout of droplets by about two times. The ignition delay time is reduced by 20–40%, while the size of the dispersion area is increased by 20–70%. The addition of formic acid to the composite fuel has a positive effect on the main ignition characteristics from 5 to 50%, and the addition of a similar amount of diesel fuel by 20–64%.

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