## 2 Experimental Investigation  
### Materials  
The following solid fuels were selected for the study (Figure 1): Chernogorskiy coal (Minusinsk coal basin, Russia) and woody biomass (wood processing factory, Russia) in the form of cedar needles, which is a waste product of the wood industry. Chernogorskiy coal is used as an energy fuel for heat generation. Cedar is a common type of softwood from which building materials, sports equipment, and other goods are produced. In the process of wood processing at industrial enterprises, a large amount of waste is generated that must be disposed of, and one of the ways is to use it as an energy additive to solid fossil fuels.  
In this paper, the following solid fuels are considered: Chernogorskiy coal with a particle size of 100-200 \(\upmu\)m (hereinafter C-1); Chernogorskiy coal with a particle size of 300-400 \(\upmu\)m (hereinafter C-2); cedar needles with a particle size of 100-200 \(\upmu\)m (hereinafter B-1), and cedar needles with a particle size of 300-400 \(\upmu\)m (hereinafter B-2). Mixtures with the same particle sizes (100-200 \(\upmu\)m and 300-400 \(\upmu\)m), consisting of 50% Chernogorskiy coal and 50% cedar needles (hereinafter CB-1 and CB-2, respectively).  
Grinding of solid fuels was carried out in a mill, Retsch DM200 (Retsch GmbH, Haan, Germany). Using a screening machine from Retsch AS200 (Retsch GmbH, Haan, Germany), fuels with a particle size of 100-200 and 300-400 \(\upmu\)m were obtained using a set of sieves according to ISO 3310-1:2000. Fuel mixtures CB-1 and CB-2 were obtained by mixing coal and cedar needles.  
Proximate and ultimate analysis of fuels was carried out in accordance with the following standards: ISO 11722:1999; ISO 1171:2010; ISO 562:2010; ISO 1928:2009; and ASTM D5373-14e1. The combustion heat was determined using a calorimeter C6000 (IKA, Staufen, Germany); carbon, hydrogen, and nitrogen content were determined using a Vario  
Figure 1: Appearance of the solid fuels: (**a**) Chernogorskiy coal; (**b**) chopped cedar needles.  
MACRO cube (Elementar Analysensysteme GmbH, Langenselbold, Germany); moisture content was determined using an analyzer MA-150 (Sartorius, Guxhagen, Germany); and the volatiles and ash content of fuel were determined using a muffle furnace Snol 7.2/1300 (AB Umega, Utena, Lithuania). Table 1 shows the results of the proximate and ultimate analyses.  
When adding biomass to coal, various tasks are pursued, such as the possibility of utilizing biomass, reducing solid natural fuel consumption, and improving the environmental performance of power plants. In this work, the main task of adding biomass to coal is to improve the combustion characteristics of bituminous coal, since this coal is low-reactive. As a rule, the main influence on the fuel ignition and combustion processes is provided by the content of volatiles. There are 1.8 times fewer volatiles in coal than in biomass (Table 1).  
The biomass content in the mixture can reach 50% when burning solid fuel mixtures in boiler plants with stoker furnaces. A further increase in the mass fraction of biomass in the mixture affects a significant reduction in the fuel calorific value. The coal calorific value was 31.00 MJ kg\({}^{-1}\); that of biomass was 23.42 MJ kg\({}^{-1}\) (Table 1). Adding 50% biomass to coal reduces the calorific value of the mixture by 14%; a further increase will lead to a significant reduction in the mixture's calorific value. Therefore, in this work, the maximum mass concentration of biomass in a mixture of 50% was chosen.