**3. Results and Discussion**

In this study, compositions of flue gases (NOx, SO2, and CO) during the air- and oxycombustion of energy crops and reference biomass fuels and bituminous coal are reported and discussed.

### *3.1. Composition of Flue Gases during Air-Combustion*

The effect of the fuel type on the instantaneous concentrations of NO, N2O, SO2, and CO during combustion in air is shown in Figure 1. The shapes of NO curves (Figure 1a) are similar for all energy crops, and their numerical values depend on the content of nitrogen and VM in the fuel. The highest emission of NO, approximately 60 ppm, was recorded for *Salix viminalis*. On the other hand, the lowest NO emission among all tested fuels was measured for *Sida hermaphrodita*, which is associated with the lowest nitrogen content in this fuel (see Table 2).

Emissions of N2O (Figure 1b) were much lower than emissions of NO. Two peaks of N2O concentration were observed during the combustion of biomass fuels. The first peak was seen during the combustion of VM and the second during the combustion of remaining char. Emissions of N2O for energy crops did not exceed 8 ppm. The highest emissions of N2O were recorded for wheat straw and the lowest for *Sida hermaphrodita*.

Nitrogen dioxide (NO2) concentrations were below the detection limit for all tested fuels.

The concentration of SO2 (Figure 1c) from the combustion of coal was much higher than that from the combustion of biomass. The highest value of the SO2 instantaneous concentration was observed during the combustion of VM, and it was approximately 58 ppm. The highest SO2 emissions for biomass fuels were detected for wheat straw. Concentrations of SO2 for energy crops were below 4 ppm. The lowest SO2 emission among all tested fuels was measured for *Sida hermaphrodita*, which is associated with the lowest sulphur content in this fuel (see Table 2).

Concentrations of CO in the flue gas during conventional combustion are shown in Figure 1d. The highest values of CO emissions were noticed during the char combustion, both for coal and biomass. The highest values of the instantaneous concentrations of CO were 27 ppm and 25 ppm for *Sida hermaphrodita* and *Miscanthus giganteus*, respectively. The release of CO lasted for 120–140 s for biomass, and 650 s for coal. Overall CO emissions were much higher for bituminous coal, which is associated with a higher content of carbon.

**Figure 1.** Effect of fuel type on instantaneous concentration of pollutants during air-combustion.
