*3.2. Composition of Flue Gases during Oxy-Combustion*

The effect of the oxidizing atmosphere on the instantaneous concentrations of NO for all tested fuels is shown in Figure 2. The oxidizing atmosphere and type of fuel have a large influence on NO emissions. Concentrations of NO increased with increasing initial O2 content in the O2/CO2 mixture. The highest values of NO concentrations were measured in the 40%O2/60%CO2 atmosphere and the lowest in the 21%O2/79%CO2 mixture. This is associated with the temperature of fuel particles [21]. A higher temperature of the fuel sample causes higher emissions of NO [12]. Additionally, a higher concentration of O2 in the combustion chamber enhances the combustion of VM and leads to a rise in NO formation. This finding has been also confirmed by the results presented in previous papers [26,27].

The highest NO concentrations were observed in the combustion of VM, especially for biomass fuels. The highest values of the instantaneous NO emissions in the oxy-combustion of energy crops were detected for *Miscanthus giganteus* and the lowest for *Sida hermaphrodita*. During combustion of *Miscanthus giganteus* in the 40%O2/60%CO2 atmosphere, the highest value of the NO concentration was 80 ppm, and it was approximately 30 ppm higher than that in the 21%O2/79%CO2 atmosphere (Figure 2b). The maximum amount of NO for *Sida hermaphrodita* and wheat straw was approximately twice as high in the 40% O2 mixture than that in the 21% O2 mixture. The highest instantaneous concentrations of NO during air-combustion were similar to those in the 30%O2/70%CO2 atmosphere for *Salix viminalis* and wheat straw. In other cases, combustion in the 30% O2 mixture caused higher emissions of NO. The lowest emissions of NO during oxy-combustion were detected for the reference coal due to the lowest VM content in this fuel.

**Figure 2.** Effect of oxidizing atmosphere on instantaneous concentrations of NO for all tested fuels. (**a**) *Sida hemaphrodita.* (**b**) *Miscanthus giganteus.* (**c**) *Salix viminalis.* (**d**) Wheat straw [12]. (**e**) Scots pine [12]. (**f**) Bituminous coal [12].

Emissions of N2O in the air- and oxy-combustion for all tested fuels are shown in Figure 3. Nitrous oxide concentrations were much lower in comparison with NO emissions. N2O was formed at the same time as NO, which suggests that the formation of N2O was proceeded by the direct oxidation of fuel-N instead of the reduction of NO [12]. The shapes of N2O curves in air combustion are bimodal. The highest N2O emissions were detected in the combustion of VM in the 21%O2/79%CO2 atmosphere. The highest value of N2O concentration for energy crops, approximately 11 ppm, was observed for the combustion of *Miscanthus giganteus* (Figure 3b). As the oxygen concentration in the O2/CO2 atmosphere

increased, N2O concentrations decreased. The lowest N2O emissions (below 8 ppm) for energy crops were observed for the combustion of *Sida hermaphrodita* both for air and oxy-combustion.

**Figure 3.** Effect of oxidizing atmosphere on instantaneous concentrations of N2O for all tested fuels. (**a**) *Sida hemaphrodita.* (**b**) *Miscanthus giganteus.* (**c**) *Salix viminalis.* (**d**) Wheat straw [12]. (**e**) Scots pine [12]. (**f**) Bituminous coal [12].

HCN and NH3 are the most important precursors of NOx formation. Higher concentrations of HCN were generated in oxy-fuel combustion. The highest emissions of HCN

were observed in 21%O2/79%CO2, mainly in the combustion of char. However, the highest values of the instantaneous HCN concentrations did not exceed 6 ppm for both energy crops and reference fuels. Analogous findings were reported in our previous study [23] and in reference [28]. The maximum ammonia emissions were observed during combustion in the mixture of 30%O2 and 70%CO2, but the values of the instantaneous concentrations were very low, below 3 ppm, for all samples. Large amounts of CO2 inhibited the oxidation of NH3 and HCN to NOx [12].

Emissions of NO2 were noted during the combustion of char only in the 21%O2/79%CO2 mixture. The maximum values of NO2 concentration did not exceed 3 ppm for energy plants. This observation is consistent with the results obtained in references [25,28].

Figure 4 shows the influence of the oxidizing atmosphere on concentrations of SO2 for energy crops and reference fuels.

**Figure 4.** Effect of oxidizing atmosphere on instantaneous concentrations of SO2 for all tested fuels. (**a**) *Sida hemaphrodita.* (**b**) *Miscanthus giganteus.* (**c**) *Salix viminalis.* (**d**) Wheat straw [12]. (**e**) Scots pine [12]. (**f**) Bituminous coal [12].

The amount of SO2 formed during oxy-combustion of energy crops was much lower than that for the reference coal. For all tested samples, the maximum values of SO2 were detected for the VM combustion period. An increase in SO2 emissions was caused by an increase in the inlet concentration of O2. The maximum value of SO2 for energy crops was 16 ppm for *Miscanthus giganteus* in the oxy-40% atmosphere. However, the highest SO2 concentration in the oxy-40% atmosphere was 68 ppm for coal. Oxy-combustion of *Miscanthus giganteus* in the mixture of 40% O2 resulted in three times as high emissions of SO2 than that in the mixture of 21% O2. Comparable results for biomass and fossil fuels are reported in reference [28].

The effect of the oxidizing atmosphere on concentrations of CO during the combustion of energy crops and reference fuels is shown in Figure 5.

**Figure 5.** Effect of oxidizing atmosphere on instantaneous concentrations of CO for all tested fuels. (**a**) *Sida hemaphrodita.* (**b**) *Miscanthus giganteus.* (**c**) *Salix viminalis.* (**d**) Wheat straw [12]. (**e**) Scots pine [12]. (**f**) Bituminous coal [12].

The charts show the great influence of the oxidizing atmosphere on the amount of CO in exhaust gas. The oxy-combustion of energy crops in the mixture of 21% O2 and 79 CO2 caused a drastic increase in CO emissions compared to conventional combustion. The maximum values of the instantaneous CO concentrations in the oxy-21% atmosphere for energy crops were 650 ppm and 610 ppm, for *Miscanthus giganteus* and *Sida hermaphrodita*, respectively. This phenomenon could be described by the impact of the following reactions [12,28]:

$$\text{ClO}\_2 + \text{C} \leftrightarrow \text{2CO} \tag{1}$$

$$\text{CO}\_2 + \text{H} \leftrightarrow \text{CO} + \text{OH} \tag{2}$$

$$\text{CO}\_2 \leftrightarrow \text{CO} + \frac{1}{2}\text{O}\_2\tag{3}$$

The first reaction, called Boudouard's reaction, appears to be predominant in the conversion of carbon monoxide under characteristic conditions for fluidized combustion [12,28]. The amount of CO in the flue gases decreased with an increase in the initial concentration of O2. During combustion in the mixture of 40% O2 and 60% CO2, the maximum values of CO concentrations were thrice lower for *Sida hermaphrodita* and *Salix viminalis* than those in the oxy-21% mixture. Similar trends for biomass fuels were reported in references [25,28].
