4.2.1. NOx Emissions

Figures 8 and 9 show the time-resolved emissions of NO and N2O during oxy-fuel combustion of all tested fuels. During oxy-combustion, the highest NO emissions were detected during the volatiles combustion after 14 s for Scots pine (Figure 8c), 21 s for wheat straw (Figure 8a) and willow (Figure 8b), and 28 s for coal (Figure 8d). The lowest NO concentrations in a flue gas were observed during combustion in 21% O2/79% CO2 environment for all fuels, due to the lower temperature of fuel particle [23,24]. During oxy-21 combustion of wheat straw (Figure 8a), the highest NO concentration was approximately 50 ppm and it was 25 ppm lower than that in air combustion (Figure 1a). Concentrations of NO increased with increasing O2 content in the O2/CO2 mixture. The highest NO concentrations were detected in 40% O2/60% CO2 atmosphere and were approximately

95 ppm and 75 ppm for wheat straw and *Salix viminalis*, respectively. The maximum value of NO concentration for wheat straw was about twice as high in oxy-40 than that in oxy-21.

**Figure 8.** The instantaneous concentrations of NO during oxy-combustion of tested fuels.

Emissions of N2O were significantly lower in comparison with NO emissions. Nitrous oxide was formed simultaneously with NO, which implies that formation of N2O proceeded by direct oxidation of fuel nitrogen rather than by the reduction of NO. The highest N2O concentrations were detected during volatile matter combustion in the oxy-21 atmosphere. The highest value of N2O concentration, approximately 18 ppm, was observed for the combustion of wheat straw (Figure 9a). As the oxygen concentration in O2/CO2 atmosphere increased, N2O concentrations decreased. The lowest N2O emissions (below 5 ppm) were observed for oxy-combustion of Scots pine, the fuel with the lowest nitrogen content.

Emissions of NO2 were only detected during char combustion in oxy-21 atmosphere. The highest instantaneous NO2 concentration did not exceed 6 ppm for coal and 4 ppm for wheat straw. Figure 10 shows total NO, N2O and NO2 emissions during combustion in the oxy-21 atmosphere. Total emissions of NO2 were approximately 20 ppm and 10 ppm for coal and wheat straw, respectively. This finding is consistent with the results obtained by Lasek et al. [31].

**Figure 9.** The instantaneous concentrations of N2O during oxy-combustion of tested fuels.

**Figure 10.** Total emissions of NO, N2O and NO2 during combustion in oxy-21 atmosphere. Vertical bars represent standard deviation.

Figure 11 shows the split of NO and N2O emissions between volatile-N and char-N oxidation during burning of biomass fuels and coal in all atmospheres. Oxidation of volatile matter accounts for approximately 75% of total NO formed and char oxidation contributed 25% for wheat straw in the oxy-21 atmosphere while for coal 40% and 60%. Combustion of wheat straw in the oxy-40 atmospheres caused oxidation of volatile-N for approximately 60% of total NO formed, and char oxidation contributed 40% for wheat straw (68% and 32% for *Salix viminalis*; 80% and 20% for Scots pine; 60% and 40% for coal).

**Figure 11.** Effect of oxidizing atmosphere on NO emissions during combustion. Vertical bars represent standard deviation.

In the case of N2O (Figure 12), during oxy-combustion, contributions of volatile-N and char-N are completely different than that in air-combustion. Oxidation of volatile matter accounts for approximately 85% of total N2O formed, and char oxidation contributed 15% for wheat straw and Scots pine in the oxy-21 atmosphere. (93% and 7% for *Salix viminalis*; 80% and 20% for Scots pine; 51% and 49% for coal).

Combustion of wheat straw in the oxy-40 atmospheres caused oxidation of volatile matter for approximately 77% of total N2O formed and, char oxidation contributed 23% for wheat straw (75% and 25% for Salix viminalis; 90% and 10% for Scots pine; 70% and 30% for coal).

The influence of oxygen concentration in the O2/CO2 mixture on the total emissions of NO is shown in Figure 13. As expected, emissions of NO increase with increasing O2 content in the mixture, particularly for biomass fuels. The emission of NO also increased with the particle temperature due to the decrease in CO emission. Higher particle temperatures helped the oxidation of NCO to NO, too [29]. Moreover, higher concentration of O2 in the riser enhance the combustion of volatile matter and char and lead to an increase in NO formation. This remark has been confirmed by the results obtained by Czakiert et al. [32] and Jankowska et al. [33].

**Figure 12.** Effect of oxidizing atmosphere on N2O emissions during combustion. Vertical bars represent standard deviation.

**Figure 13.** Influence of oxygen concentration on the total emission of NO during oxy-combustion. Vertical bars represent standard deviation.

The opposite trend can be observed for N2O emissions (Figure 14). As the oxygen concentration in O2/CO2 mixture increases, N2O concentrations in a flue gas decrease, mainly for coal.

**Figure 14.** Effect of the oxygen concentration on the total emissions of N2O during oxy-combustion. Vertical bars represent standard deviation.

Hydrogen cyanide is a main precursor of N2O formation. It should be mentioned that HCN is a very toxic gas (approximately 20 times more toxic than CO) that already affects human beings at ppm levels [31]. Higher concentrations of HCN were generated during oxy-fuel combustion. Emissions of HCN were the highest in oxy-21 atmosphere, particularly during char combustion. The highest instantaneous concentrations of HCN did not exceed 6 ppm both for biomass fuels and coal. Similar results were reported in our previous study [22] and by Lasek et al. [31].

The highest NH3 emissions were observed during combustion in oxy-30 and oxy-21 atmospheres. However, the instantaneous concentrations of NH3 were very low; they did not exceed 3 ppm for all fuels. High concentrations of CO2 inhibited oxidation of HCN and NH3 to NOx.
