## 4 Results and Discussion
In a laboratory-scale microwave-assisted burner with an output power of 4.5 kW, the specific microwave consumption per unit of coal was determined to be 0.08 kWh/kg [6]. As can be observed, this ratio decreases as the system scale increases. The flame resulting from the interaction of coal with plasma shows a cylindrical form extending along the burner axis. Typically, at a distance of about 15 cm from the end of the coaxial line, there is a bright luminous formation--the plasma flame--with a diameter of approximately 20-25 cm, resulting from the microwave energy converting the air into plasma. Primary and secondary air flow rates control the plasma flame position. After the plasma flame interacts with the coal, a temperature value of 1200 to 1350 \({}^{\circ}\)C is measured using the pyrometer mounted on the burner, as shown in Figure 1. As the coal feed rate increases, the length of the flame gradually increases. When the coal feed rate reaches approximately 3000 kg/h, the flame length can extend up to 4 m. Figure 8 shows a photograph of the flame extending from the burner. Figure 8: Flame extending up to 4 m at a coal feed rate of approximately 3000 kg/h. The results obtained show that, compared to conventional burners, in a microwave burner, the coal powder and air mixture ignites early and reliably within the plasma flame even at quite low temperatures, around 30 \({}^{\circ}\)C, and a stable flame is formed. The process of igniting and burning pulverized coal within the plasma created by microwaves is similar to the process in the plasma formed by direct current and alternating current electric arcs. First, when coal particles are exposed to the high temperature of the plasma flame, they disintegrate into very small pieces due to thermal shock. This leads to a significant increase in the total surface area of the coal particles and results in the release of a greater amount of volatile matter. Secondly, the electrons and ions present in the plasma environment intensify the chemical reactions of coal oxidation. In the case of microwave plasma, these processes are enhanced due to the plasma being highly unstable and the electron temperature being as high as 11,600 K [24]. Experiments have shown that the coal-air mixture fed into the burner can easily ignite within the plasma formed by converting secondary air into plasma with microwave energy. In this case, a stable flame of plasma and burning coal particles emerges almost instantaneously. Assessing the economics of using plasma in a commercial process is challenging. However, the main costs include capital equipment (CAPEX), operational expenditure (OPEX), and utilities. In terms of cost, magnetron-based plasma equipment (generator + impedance matching) operating at frequencies of 2.45 GHz or 915 MHz can be obtained at comparable prices per watt, typically ranging from 1 to 3 Euros/Watt, depending on the type of power supply and the complexity of the impedance matching/tuning systems [25]. The total cost of the newly developed microwave-assisted auxiliary burner, including capital equipment (CAPEX), operational expenditure (OPEX), and utilities, for a 75 kW system is estimated to be around 180,000 Euros, which falls within the typical range of 1 to 3 Euros/Watt for magnetron-based plasma systems. On the other hand, it is stated that in a boiler producing 75 tons per hour of steam, approximately 6 tons of fuel oil is consumed during boiler start-up [10]. This amount corresponds to the minimum of about 15% of the total fuel oil consumption used when auxiliary burners are activated to maintain a stable amount of process steam production in case of a malfunction or a change in coal properties. The tests of the newly developed auxiliary burner were conducted in a pulverized coal-fired boiler with an electricity generation capacity of 22 MW\({}_{e}\), producing 96 tons of steam per hour. Each start-up of this boiler requires approximately 8 tons of fuel oil.