## 2 Materials and Methods
### Graviometric Method for Measuring PM Emissions  
The basic quantity measured during all measurements was the value of PM emissions produced, which we determined using the gravimetric method. The measurement consists of taking a representative sample with a cascade cylindrical impactor from the flue gas in the flow rate measurement section, which lies directly behind the electrostatic precipitator. The impactor consists of a suction nozzle and a three-chamber space, where the flue gas flow rate is directed by conical inserts to a set of three sampling filters composed of glass fibers. The three-filter probe allows for simultaneous measurement of all three particle sizes: PM > 10, PM 10-2.5, and PM < 2.5. Along with the particle separation, the basic parameters such as chimney draft and temperature required for further calculations are also measured using the ANHOLM measuring control panel during the combustion process. It is also necessary to measure the velocity to ensure an isokinetic condition, which is determined by the same sampling rate as the flue gas flow rate in the chimney. The measuring probes that were used are shown in Figure 4.  
Measurements were performed using a TCR Tecora Isostack Basic device, which consists of probes for measuring chimney draught and temperature, and a sampling probe with filter columns for capturing PM from flue gases. The flue gases were then cooled  
Figure 4: Measuring probes: (**a**) probe for measuring PM production; (**b**) probe for measuring chimney temperature and flow rate; (**c**) layout of cascade sampling probe parts.  
Figure 3: Experimental HV power supply: (**a**) model CX-120A with power 100 W and voltage 20 kV; (**b**) model CX-600A with power and 300 W and voltage 60 kV.  
in a cooling and condensing unit and dried in a drying tower containing silica gel. The measurement itself consists of several basic steps. After setting the basic measurement parameters on the TCR Tecora Isostack Basic analyzer, it is necessary to prepare a set of collection filters. The filters must be dried and weighed dry. A Radwag WPS 50X drying balance was used for drying. Prior to each measurement, the impact must be thoroughly cleaned of settled impurities and heated to operating temperature to prevent condensation in the impactor cylinder. After a 30 min sampling of the particles, the measuring device notified us of the end of the measurement by writing out the evaluation tape with the basic measurement parameters, which are necessary for further calculation of the PM concentration in the flue gas. The filters must be dried again and weighed after the measurement. From the difference in the weight of the filters before and after the measurement, we determined the mass of collected particles. To calculate the final concentration of PM in the gas, it is necessary to insert the mass of particles together with the data from the output tape into the following relation [19]:  
\[C=\frac{(m_{2}-m_{1})}{V_{gn}}.1000 \tag{1}\]  
where \(C\) is the concentration converted to normal conditions in dry gas (mg.m\({}^{-3}\)), \(V_{gn}\) is flue gas sample volume \(\left(\text{m}^{3}\right)\), \(m_{2}\) is weight of filter after measurement (g), and \(m_{1}\) is weight of filter before measurement (g).  
From the value of the concentration of PM particles in the flue gas under standardized conditions \(C\), it is also possible to calculate the value of the concentration under reference conditions according to the legal regulation \(C^{\prime}\), which is defined by the relation:  
\[C^{\prime}=\frac{20.95-O_{2}^{ref}}{20.95-O_{2}^{prism}} \tag{2}\]  
where \(C^{\prime}\) is concentration for reference conditions according to the legislative regulation (mg.m\({}^{-3}\)), \(O_{2}^{prism}\) is measured average oxygen content in the gas volume (%), and \(O_{2}^{ref}\) is reference oxygen content (%).