## 3 Results and Discussion
### Combustion Test
The study by [PERSON] et al. [49] included the combustion of biodiesel and petrodiesel in utility power plant boilers. They noticed that the contribution of the percent of thermal and prompt NO into total NO varied by a wide range. For example, the contribution of thermal NO changed in the range of 16-73% when combustion pressure changed in the range of 8-22 bar. [PERSON] et al. [50] noticed significantly higher NO\({}_{\rm x}\) emission (473 ppm, 3% O\({}_{2}\)) during the combustion of HFO using a 1.4 MW\({}_{\rm th}\) experimental rig. The experimental setup included a regenerator-burner system with highly preheated combustion air at 1200 \({}^{\circ}\)C and a furnace temperature at the level of 1300 \({}^{\circ}\)C. Thus, despite moderate N-content (i.e., N 0.21 w.%) and due to the thermal-NO\({}_{\rm x}\) mechanism, the NO\({}_{\rm x}\) emission was higher compared with the combustion of HFO at a similar or higher level. Figure 11 presents the comparison of the NO\({}_{\rm x}\) emission as a function of N-fuel content. The data were divided into two main groups: heavy fuel oils, HFO, and bio-liquid fuels. The relationship between the emission of NO\({}_{\rm x}\) and nitrogen content in combustion fuels is a complex issue. An overall conclusion from the data collected in Table 5 is that the concentration of NO during the combustion of liquid fuels is in the range of 31-432 ppm (ref. to 3 v.% O\({}_{2}\)). Increase in NO\({}_{\rm x}\) emission was also observed when diesel was substituted by biodiesel in diesel engines [51]. Figure 11: The emission of NOx in the function of N-fuel content, data from [9; 12; 16; 18; 43; 47; 50]. \begin{table}
\begin{tabular}{c c c c c c c} \hline  
The emission of polycyclic aromatic hydrocarbons (PAHs) during the combustion of HFO, FADR, and micro wax [22] is presented in Table 6. It was observed that in both cases of alternative fuels (i.e., FADR and micro wax), the total emission of PAHs (355.5 \(\upmu\)g/m\({}^{3}\)n and 198.5 \(\upmu\)g/m\({}^{3}\)n) was lower compared with the value in the case of HFO combustion (i.e., 810.4 \(\upmu\)g/m\({}^{3}\)n). [PERSON] et al. [22] concluded that during the combustion of HFO, the average value of PAH emission was 488 \(\upmu\)g/m\({}^{3}\)n; nevertheless, the highest emission was in the range of 3530 \(\upmu\)g/m\({}^{3}\)n. PAHs are created using the incomplete combustion of organic material, and the main factors influencing the value of emission are the type of fuel and additive, air pollution control devices, combustion control, and performance (air distribution, temperature, residence time, etc.). During the combustion of FADR, the highest values of emission (i.e., in the range of 50 \(\upmu\)g/m\({}^{3}\)n) were observed for acenaphthylene, acenaphthrene, fluorine, phenanthrene, and anthracene. In the case of fluoranthene, pyrene, and benzo (b + k) fluoranthene, the emission was in the range of 20-30 \(\upmu\)g/m\({}^{3}\)n. The emission of the rest of the compounds was less than 5 \(\upmu\)g/m\({}^{3}\)n. It is also worth underscoring the low emission of benzo(a)pyrene during the combustion of FADR (i.e., 0.73 \(\upmu\)g/m\({}^{3}\)n). The emission of benzo(a)pyrene during the combustion of HFO was significantly higher (i.e., 30.6 \(\upmu\)g/m\({}^{3}\)n). The reduction in PAH emissions from diesel engines was observed when diesel was substituted with biodiesel [51].