## 3 Results and Discussion
### Combustion Test  
The results of combustion performance in terms of temperature inside the combustion chamber, air flow rate, and fuel flow rate, as well as the liquid fuel overpressure behind the  
Figure 4: Power loss due to demands on fuel transportation and heating, data from [16; 17; 22; 36], assumed nominal power 100 kW\({}_{\text{th}}\). Figure 5: Energy per fuel mass loss due to demands on fuel transportation (\(E_{pressure}\)) and heating (\(E_{heat}\)), data from [16; 17; 22; 36]. was compared with the values obtained by other research groups. It should be underscored that the emission of SO\({}_{2}\) was significantly decreased, and the emission of NO\({}_{x}\) was a little increased during the combustion of FADR. The lower emission of SO\({}_{2}\) was caused by the limitation of sulfur content in the fuel, which is a main parameter influencing SO\({}_{2}\) emission. A decrease in SO\({}_{2}\) (from 1180 ppm to 40 ppm) and NO\({}_{x}\) (from 201 ppm to 126 ppm) due to a decrease in S- and N-fuel content was observed by [PERSON] et al. [43], who presented the CFD modeling and experimental results of HFO and bioliquid, BL (a blend of palm oil, its residue, and animal fat), combustion in a 400-MW\({}_{e}\) power boiler. Fuel-N content in HFO and BL was 0.46 w.% and 0.19 w.%, respectively. Sulfur content in HFO and BL was 2.04 w.% and 0.04 w.%, respectively. A similar phenomenon was observed by [PERSON] et al. [44]. Apart from fuel NO, NO\({}_{x}\) primarily originates from thermal NO and prompt NO [45]. The emission of NO\({}_{x}\) during the combustion of liquid fuels is a complex process influenced by different factors. The difference in the NO\({}_{x}\) emissions between non-food fat-type  
Figure 8: The concentration of CO\({}_{2}\), O\({}_{2}\), and CO in flue gas during the combustion of natural gas and FADR. Figure 10: The emission of formaldehyde and carbon monoxide as a function of time during combustion of natural gas and FADR. Figure 9: The emission of butane and toluene as a function of time during combustion of natural gas and FADR. fuels, NF1 and NF2, in investigations by [PERSON] et al. [9] was caused (inter alia) by the significant difference in N-content. Namely, N-content in NF1 and NF2 was 0.16 w.% and 0.05 w.%, respectively; nevertheless, during the combustion of liquid fuels, N-fuel content is not the only factor that influences NO\({}_{\rm x}\) emission. Some comparisons of the experimental results of liquid fuel combustion confirm that the increase in N-fuel content did not increase NO\({}_{\rm x}\) emission in all cases. For example, [PERSON] and [PERSON] [16] noticed lower emission of NO\({}_{\rm x}\) during the combustion of animal fat (164 ppm referred to as 3 vol% O\({}_{2}\)) compared with HFO combustion (295 ppm referred to as 3 vol% O\({}_{2}\)), despite the higher N-content in animal fat (i.e., 0.35 w.%) compared with HFO (i.e., 0.3 w.%). [PERSON] and colleagues [46] explained that the total NO\({}_{\rm x}\) is a combination of thermal and fuel-bound nitrogen mechanisms. In some cases, lower flame temperature significantly reduces the emission of NO\({}_{\rm x}\). [PERSON] et al. [47] noticed that the emission of NOx during HFO combustion can be significantly influenced (in the range of 300-450 ppm) by the change in air distribution, swirl ratio of secondary air (0.5-1.4), mean droplet size (25-150 um), and fuel injection velocity (20-120 m/s). Among the other crucial effects, [PERSON] and [PERSON] [48] listed the atomizing air-to-fuel mass ratio (AFR), secondary air swirl number (Ss), air staging, burning using methane and propane as secondary fuels, and fuel atomization using mixtures of air and methane. [PERSON] et al. [49] concluded that an \"increase in the combustion pressure leads to more uniform combustion (better vaporization and propagation of fuel in the chamber) and the formation of high-quality sprays (smaller droplet size, deep penetration of sprays in chambers). This contributes to high local temperatures and disappearance of fuel local rich zones\".