## 2 Experimental Investigation
### TG/DTG Apparatus and Methods  
Determination of the main combustion characteristics and kinetic parameters of fuels was carried out using a thermal analyzer STA 449 F1 Jupiter (NETZSCH, Selb, Germany). The experiments were carried out in an oxidizing medium with an airflow rate of 50 mL/min, a heating rate of 10 \({}^{\circ}\)C/min, and a sample mass of 17 mg. Processing of the results of thermogravimetry (TG) and differential thermogravimetry (DTG) was performed using the software \"NETZSCH. Proteus Thermal Analysis 5.1.0\" supplied with the equipment. All experiments were duplicated to assess the repeatability and consistency of the results. The gross combustion index \(S\) (min\({}^{-2\circ}\)C\({}^{-3}\)) is usually used to compare the main combustion characteristics of different solid fuels. It is a quantitative indicator reflecting the temperature at which the coke residue ignition occurs, the temperature at which the combustion process is completed, and the average and maximum rate of mass loss [22; 23; 24; 25]. An increase in the combustion index indicates better fuel combustion at lower temperatures. The values \(T_{i}\) and \(T_{b}\) were determined by the crossing technique [26; 27; 28]. The combustion index was determined by the following Equation (1) [24; 25]:  
\[S=\frac{DTG_{\text{max}}\cdot DTG_{\text{mean}}}{T_{i}^{2}\cdot T_{b}}, \tag{1}\]  
where \(DTG_{\text{max}}\) is the maximum mass loss rate, %/min; \(DTG_{\text{mean}}\) is the average mass loss rate in the temperature range from ignition to complete burnout of its carbon residue, %/min; \(T_{i}\) and \(T_{b}\) are the temperatures corresponding to the conditions under which the coke residue ignition occurs and the combustion process ends, \({}^{\circ}\)C; \(S\) is the combustion index, min\({}^{-2\circ}\)C\({}^{-3}\). The analysis of the mixture component interaction during combustion was carried out by comparing the profiles of the DTG curves obtained experimentally (\(DTG_{\text{exp}}\)) and by calculation (\(DTG_{\text{est}}\)). Value \(DTG_{\text{est}}\) was calculated using the following Equation (2) [29; 30]:  
\[DTG_{\text{est}}=0.5 DTG_{1}+0.5 DTG_{2}, \tag{2}\]where \(DTG_{1}\) and \(DTG_{2}\) are the mass loss rates for each mixture component (\(DTG_{1}\)--coal; \(DTG_{2}\)--needles), %/min. The coincidence of the curve profiles of \(DTG_{\text{exp}}\) and \(DTG_{\text{est}}\) indicates the additivity of combustion. If the curve profiles do not coincide, then this indicates the interaction between the mixture components, which is reflected in the change in combustion characteristics. The combustion of coal, biomass, and their mixtures is usually considered a heterogeneous process to which the Arrhenius equation applies and can be described by the following Equation (3):  
\[\frac{d\alpha}{dt}=k(T)f(\alpha), \tag{3}\]  
where \(\alpha=(m_{i}-m)/(m_{i}-m_{f})\) is the degree of substance conversion obtained from the TG curve; \(m_{i}\) and \(m_{f}\) are the initial and final sample masses, respectively, kg; \(m\) is the sample mass for each temperature \(T\), kg. From the Arrhenius equation, the reaction rate constant can be expressed as Equation (4):  
\[k(T)=Aexp\left(\frac{-E}{RT}\right), \tag{4}\]  
where \(A\) is the pre-exponential factor, s\({}^{-1}\); \(R\) is the universal gas constant, J/(mol K); and \(E\) is the activation energy, J/mol. For non-isothermal processes with a constant heating rate \(\beta\) (\(\beta=d\Gamma/dt\)), the reaction rate equation can be written as Equation (5):  
\[\frac{d\alpha}{dT}=\frac{A}{\beta}exp\left(-\frac{E}{RT}\right)f(\alpha). \tag{5}\]  
In this work, the well-known Coats-Redfern method was used to determine the kinetic parameters of combustion [31]. The hypothetical combustion model is based on the \(n\)-th order reaction [32; 33], therefore, \(f\left(\alpha\right)=(1-\alpha)^{n}\).