## 3 Results  
To exemplify the concentration of some compounds during the combustion of wood, O\({}_{2}\), NO, NO\({}_{2}\), NO\({}_{x}\), CO\({}_{2}\), CO, and H\({}_{2}\) were measured with the TESTO 350 XL analyser (Lenzkirch, Germany) (specialised analyser for measuring combustion gases according to sensors on chemical compounds; sensors were used for detection O\({}_{2}\), NO, NO\({}_{2}\), NO\({}_{x}\), CO\({}_{2}\), CO, and H\({}_{2}\); also, measure the ambient air temperature and gas temperatures) without using the medium-voltage source to supply PRESP (Figure 4). The measurements were performed at the exit of the chimney. At the same time, the gas temperature at the chimney output and the ambient temperature were measured (Figure 5). Dry wood (less than 10% humidity) was burned in the stove (about 2.3-2.8 kg per load, from time to time). In Figures 4 and 5, the error bars for the measured values have been passed. For O\({}_{2}\) and CO\({}_{2}\) (Figure 4), the errors are below 0.8%, and for the temperature (Figure 5), the errors are below 1% (for this reason, the error bars are not visible on the graphs). During the supply of wood to the boiler, NO and NO\({}_{x}\) increase to high values (at the beginning of the operation to 80 ppm, after a few hours to 40-60 ppm), as well as the  
Figure 4: Evolution of some measured quantities (O\({}_{2}\), NO, NO\({}_{2}\), NO\({}_{x}\), and CO\({}_{2}\) measured with TESTO 350 XL) during the combustion of wood in the 20 kW power plant. Figure 5: Evolution of the ambient temperature (t\({}_{\rm air}\)) and gases in the chimney output (t\({}_{\rm gas}\)), measured with TESTO 350 XL during the burning of wood in the 20 kW power plant. temperature of the gases at the output of the chimney. After a period of operation (2-3 h), the operating temperature decreases, as do the concentrations of NO and NOx. During the beginning of wood burning, the concentration of O2 decreases and CO2 increases, and after the wood has burned, the evolution is reversed for O2 and CO2. Other quantities were also measured: CO, which had values above 5000 ppm (the device can measure up to 5000 ppm; lower values after burning the wood 3500-4500 ppm; in this stage, evaluations were also made on the operation of the plasma reactor); and H2, which had values above 2000 ppm (the device can measure up to 2000 ppm; lower values after burning the wood 1000-1700 ppm). The metallic chimney, together with the central electrode, forms a cylindrical capacitor, whose electrical capacitance is calculated as [32]  
\[C=\frac{2.\pi\cdot\varepsilon_{0}\cdot\varepsilon_{r}\cdot h}{ln\frac{\Omega} {\phi_{\mathrm{c}}}} \tag{5}\]  
After performing the calculations using the PRESP data, C = 48.867 pF. The measured electrical capacitance (consisting of the stainless steel chimney and the central discharge electrode) with air (\(\varepsilon_{r}\) = 1), when the boiler is not working, is 50 pF. During boiler operation (with smoke at high temperatures and humidity), as the temperature increases, the electrical capacitance increases (\(\varepsilon_{r}\) > 1): 67 pF at 198 \({}^{\circ}\)C, 68 pF at 202 \({}^{\circ}\)C, and 71 pF at 244 \({}^{\circ}\)C.  
Some factors in computing and evaluating insulation quality are the dielectric absorption ratio (D\({}_{\mathrm{ar}}\)) and the polarisation index (P\({}_{\mathrm{i}}\)) [32]. \[D_{ar}=\frac{R_{after\ 1\ min}}{R_{after\ 30\ s}} \tag{6}\]  
\[P_{i}=\frac{R_{after\ x\ min}}{R_{after\ 1\ min}} \tag{7}\]  
The values of D\({}_{\mathrm{ar}}\) and P\({}_{\mathrm{i}}\) are used to estimate the quality of the electric insulation (in principle, values above unity indicate better quality of insulation). This is achieved by plotting the insulation resistance with respect to time and calculating the D\({}_{\mathrm{ar}}\) and P\({}_{\mathrm{i}}\) values. For a medium-quality insulation material, the resistance should increase when a measuring voltage is applied [32].