## 3 Evaluation Criteria  
Several criteria for evaluation were selected to determine the fuel mixture efficiency using MCDA methods. The criteria were divided into five groups (Figure 1): (i) economic, (ii) environmental, (iii) energy efficiency, (iv) social, and (v) production process safety. The criteria we selected are presented in Figure 1.  
The economic criteria reflect the expenses associated with using fuel, as well as a characterization of the fuel availability. They involve:  
* Fuel cost: A comparative cost of 1 kg of fuel. The cost of composite fuel is calculated as the total cost of its components, considering their fractions in the fuel mixture. * Fuel transportation costs (CTR) reflect the cost of transporting the fuel to an energy provider. * Fuel availability (\(A\)) characterizes the opportunity of using fuel or its separate components in the region under study. The environmental criteria were related to the environmental safety of using the fuel. Sulfur and nitrogen oxide concentrations in flue gases were chosen as environmental criteria. Undoubtedly, the combustion products of fossil fuels contain a lot of harmful substances, such as polycyclic aromatic hydrocarbon, particulate matter (PM), hydrogen chloride, metals, polychlorinated dibenzodioxins, sulfur oxides, nitrogen oxides, etc. Still, many studies (e.g., References [24; 25]) choose sulfur  
Figure 1: Fuel efficiency criteria regarding key indicators, which were divided into several groups. and nitrogen oxide emissions as the target parameters since they are the main pollutants in the combustion products of fossil fuels, they cause adverse air and climate change, and negatively affect human health [26]. According to known research findings [27; 28], the concentrations of SO\({}_{x}\), NOx, and PM are some of the main indicators of the environmental safety of boiler furnaces. Unfortunately, there are currently no conditions for a reliable measurement of PM; this is because it requires a flow of flue gases with a certain consumption and large volumes of the fuel for combustion. Therefore, we only considered the concentrations of SO\({}_{x}\) and NOx. These parameters were determined experimentally for several fuels at different temperatures. The basis of the experiments was the burning of fuel samples in a model combustion chamber (rotary muffle furnace R 50/250/13 by Nabertherm GmbH, Lilienthal/Bremen, Germany), while simultaneously analyzing the composition of flue gases using a gas analyzer (Testo-340 by Testo SE & Co. KGaA, Titisee-Neustadt, Germany). Using a hole in the combustion chamber, the gas analyzer probe was located in the fuel combustion zone and the flue gas was supplied to the sensors. The supply of flue gases and their analysis was carried out until the fuel was completely combusted. The methods and tools of measurement are described in detail in [PERSON] et al. [12]. The category of energy performance criteria contained parameters that characterized the fuel combustion and determined the indicators of the boiler plant operation:  
1. Ignition delay time of the gas-phase ignition (\(\tau_{d1}\)): duration of thermal impact, which is required for the gas-phase ignition of the fuel. 2. Ignition delay time of the heterogeneous ignition (\(\tau_{d2}\)): duration of thermal impact, which is required for the heterogeneous ignition of the fuel. 3. Complete combustion duration (\(\tau_{b}\)): interval from the fuel ignition until its complete burnout. 4. Specific heat of combustion (\(Q^{\prime}_{s}\)): amount of heat, released from the combustion of 1 kg of the fuel. 5. Minimum ignition temperature (\(T_{g}^{\mathit{min}}\)): the minimum heating temperature that ensures the gas-phase and heterogeneous ignition of the fuel. It enabled us to predict the heating temperature of the boiler for the stable ignition of the fuel supplied. 6. Ash content (\(A^{d}\)): the content of mineral non-combustible additives in the fuel. This parameter estimated the amount of ash that was left after the combustion of the organic part of the fuel.