## 4 Conclusions  
As a result of experimental studies, the characteristics of thermal decomposition and combustion processes were determined for Chernogorskiy coal, cedar needles, and their mixtures (coal 50% + biomass 50%) with different fuel particle sizes. Based on the thermal analysis, the temperature at which ignition of the coke residue occurs (with particle sizes of 100-200 and 300-400 \(\upmu\)m for coal 368 and 390 \({}^{\circ}\)C, respectively, for biomass 260 and 267 \({}^{\circ}\)C), the temperature at which the combustion process is completed (with particle sizes of 100-200 and 300-400 \(\upmu\)m for coal 733 and 788 \({}^{\circ}\)C, respectively, for biomass 542 and 642 \({}^{\circ}\)C), and the combustion index (with particle sizes of 100-200 and 300-400 \(\upmu\)m for coal 1.14 \(\times\) 10\({}^{-8}\) and 0.78 \(\times\) 10\({}^{-8}\) min\({}^{-2\circ}\)C\({}^{-3}\), respectively, for biomass 5.02 \(\times\) 10\({}^{-8}\) and 4.01 \(\times\) 10\({}^{-8}\) min\({}^{-2\circ}\)C\({}^{-3}\)) are established.  
The addition of 50% biomass to coal is the most promising for practical use, along with widespread solid natural fuels in different variations of its grate firing. The addition of 50 wt% biomass to coal made it possible to reduce the temperature at which ignition of the coke residue occurs with a particle size of 100-200 \(\upmu\)m by 33%, with a particle size of 300-400 \(\upmu\)m by 14%, and to reduce the temperature at which the combustion process is completed by 23 and 27%, respectively; the combustion index increased by 2.7 and 2.5 times, respectively. The activation energy of the mixture decreased by 16% compared to coal. An analysis of the mixture components interaction against each other showed the absence of the additivity principle, and the interaction had a positive effect on the main combustion characteristics (there was an increase in the maximum mass loss during coke residue combustion and a decrease in the temperature at which the combustion process is completed). Co-combustion of coal and biomass particles of different sizes (100-200, 200-300, 300-400 \(\upmu\)m) in a heated airflow at 500-800 \({}^{\circ}\)C made it possible to reduce the ignition delay times by 21-55%.  
The practical use of fuel mixtures with the addition of biomass helps to improve the energy characteristics of fuels, which is confirmed by the results obtained. Co-combustion of coal and biomass is a promising solution to environmental and energy problems, as well as the problem of the depletion of non-renewable energy resources. Cedar needles can be considered an additional resource when solving the problem of finding affordable energy raw materials. The results of the study provide a theoretical basis for the development of measures to convert power plants operating on solid fuels to co-combustion of coal and biomass.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app131911060/s1](https://www.mdpi.com/article/10.3390/app131911060/s1)]([https://www.mdpi.com/article/10.3390/app131911060/s1](https://www.mdpi.com/article/10.3390/app131911060/s1)), Figure S1: Ignition delay times of coal, biomass particles and their mixtures vs. particle sizes at an air flow with temperature of 700 \({}^{\circ}\)C.  
Conceptualization, P.K. and N.Z.; methodology, O.F. and P.K.; formal analysis, O.F. and N.Z.; investigation, A.P. and A.Z.; resources, P.K.; data curation, D.G.; writing--original draft preparation, A.P. and A.Z.; writing--review and editing, D.G., A.P. and A.Z.; visualization, A.P. and A.Z.; supervision, D.G. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was supported by the Russian Science Foundation [grant number 23-23-00280. URL (accessed on 1 October 2023): [[https://rscf.ru/project/23-23-00280/](https://rscf.ru/project/23-23-00280/)]([https://rscf.ru/project/23-23-00280/](https://rscf.ru/project/23-23-00280/))].  
**Conflicts of Interest:** The authors declare that they have no conflict of interest.