## 2 Experimental Setup and Method  
### Materials  
Fuel pellets were produced from dry FC (Table 1). This waste was preliminarily dried at 105 \({}^{\circ}\)C (according to ISO 5068-1). The first fuel group (composition No. 1) consisted of coal dust (dry FC) with a particle size no more than 140 \(\upmu\)m without any additional components. The pellets of the second group of fuel compositions (compositions No. 2-No. 4) contained a 0.5 mm particle of porous polymer material in the central part of the experimental sample, which was impregnated with a combustible liquid and was surrounded by coal dust from all sides. We formulated a hypothesis, requiring experimental verification, that an additive (porous polymer material impregnated with a combustible liquid) in the fuel pellet composition must facilitate combustion. The ratio of components in the fuel compositions of the second group was 90:10%. The latter almost completely corresponds to the fraction of oil in the fuel, since the mass of a porous polymer material particle is negligible (less than 1%). The third group of compositions (compositions No. 5-No. 8) was a 70:30% mixture of coal dust with wood sawdust with a particle size less than 45 \(\upmu\)m, 45-100 \(\upmu\)m, 100-200 \(\upmu\)m, and 200-500 \(\upmu\)m. Wood sawdust is typical biomass, widely used as an additive to enhance the characteristics of solid fossil fuel combustion [26]. Thus, it is interesting to evaluate the effect of the dimensions of sawdust particles on the mechanism and characteristics of fuel pellet combustion. It is possible that particles of a larger size, when exposed to rapid heating, may release moisture and volatiles with a higher concentration. Hence, they may have a more significant effect on combustion. The analytical calculation of the heat of combustion was made using the methods from [27], and values of heat of combustion for fuel components (Tables 2 and 3). The obtained results are presented in Table 1.  
Dry processing waste of bituminous coal with a particle size less than 140 \(\upmu\)m was used as the main solid combustible component for preparing fuel compositions. FC was sieved through a sifter with a corresponding mesh size (in accordance with ISO 3310-1:2000) before producing pellets. The main characteristics of FC are listed in Table 2.  
An additional component of the second group of fuel compositions No. 2-No. 4 is a fine polymer material (construction sealant), impregnated with one of three combustible liquids: rapeseed oil (composition No. 2), water-oil emulsion based on rapeseed oil (composition No. 3) or water-oil emulsion based on used turbine oil (composition No. 4). The main characteristics of fuel components are presented in Table 3.  
The third group of compositions No. 5-No. 8 contained pine sawdust of different particle size as an additional combustible component. Air-dried wood was ground by an SW-2 cutting mill (HT Machinery, Japan-Taipei, Taiwan) and sieved using a laboratory plasnifter with a set of sieves (according to ISO 3310-1:2000) to obtain sawdust of different particle size: less than 45 \(\upmu\)m (composition No. 5), 45-100 \(\upmu\)m (composition No. 6), 100-200 \(\upmu\)m (composition No. 7), and 200-500 \(\upmu\)m (composition No. 8). See Table 2 for the characteristics of sawdust. Cylinder-shaped fuel pellets with a mass of 20 mg, 3 mm in diameter and 3 mm in height were produced by pressing coal dust with some other components or without them in a pellet die at a pressure of 5 MPa. A pellet die consists of three parts: a hollow steel cylinder with a through opening 3 mm in the internal diameter, which is located along the symmetry axis of the cylinder; a steel cap for air-tight sealing of the steel cylinder from one side; a steel piston to compress fuel pellets through the second opening in the steel cylinder. The said pressure was built up using a Heavy Duty Shop (Cluj-Napoca, Romania) Press 20 ton hydraulic press. This is a common method of briqueette and pellet production by pressing [25; 31; 32].