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.

The work determines the characteristics of the processes of thermal decomposition and combustion when heating coal, cedar needles, and their mixtures with different fuel particle sizes. Based on the results of thermal analysis, the following characteristics were determined: the temperature at which the coke residue ignition occurs, the temperature at which the combustion process is completed, and the combustion index. An analysis was carried out of the interaction between the fuel mixture components on the characteristics of their combustion for compositions (50% coal and 50% biomass) with a particle size of 100–200 μm and 300–400 μm. The combustion kinetic parameters of individual solid fuels and their mixtures containing 50% coal and 50% biomass are compared. The activation energy for coal combustion was 60.3 kJ mol⁻¹, for biomass 24.6 kJ mol⁻¹, and for mixture 42.5 kJ mol⁻¹. The co-combustion of coal and biomass has a positive effect on the main combustion characteristics of solid fuels. Fuels with particle sizes of 100–200, 200–300, and 300–400 μm were studied at temperatures of 500–800 °C under heating conditions in a heated airflow. Using a hardware-software complex for high-speed video recording of fast processes, the ignition delay times were determined, the values of which for the considered fuels vary in the range from 0.01 to 0.20 s. Adding 50 wt% biomass with particle sizes of 100–200, 200–300, and 300–400 μm to coal reduces the ignition delay times of mixtures by 55, 41, and 27%, respectively. The results obtained can become the basis for the conversion and design of modern power plants operating on solid fuel mixtures to co-combust coal with biomass.