4.2.3. CO Emissions

Concentrations of CO during oxy-fuel combustion are shown in Figures 17 and 18. These results reveal a significant impact of the composition of oxidizing gas mixture on CO emissions. Combustion of biomass or coal in the oxy-21 mixture caused a strong increase in CO concentrations in comparison with air-combustion (see Figure 3). The highest CO emissions were approximately 950 ppm and 760 ppm for wheat straw and coal, respectively. This phenomenon can be explained by the influence of three chemical reactions [2,3,31,32]: Boudouard's reaction Equation (1), the reaction of CO2 with H radical Equation (2) and the dissociation of CO2 Equation (3):

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

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

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

**Figure 17.** Effect of oxygen concentration on the instantaneous CO concentrations during oxycombustion.

**Figure 18.** Influence of the oxygen concentration on total emissions of CO during combustion of biomass fuels. Open points represent combustion in air. Vertical bars represent standard deviation.

Reaction (1) seems to play a leading role in the conversion of CO under conditions typical for fluidized-bed combustion [31]. As oxygen concentration increased, the emission of CO was reduced. In oxy-40 environment, the highest CO emissions were three times lower for biomass fuels than that in the oxy-21 mixture. The highest instantaneous CO concentrations were 760 ppm, 700 ppm and 465 ppm in oxy-21, oxy-30 and oxy-40, respectively. The analogous trends were reported in references [31,36–38].

Moreover, the reduction of NO to N2 in the presence of an active char and higher concentrations of CO is enhanced through the heterogeneous NO reduction reaction [37,39]:

$$\text{CO} + \text{NO} \leftrightarrow \text{CO}\_2 + \frac{1}{2}\text{N}\_2\tag{4}$$

According to results presented by Lupianez et al. [14–16], a high level of CO in the flue gas is a result of incomplete combustion, which is a symptom of lower combustion efficiency. Thus, in this case, the reduction of NO to N2 is a consequence of such deficiency (Figure 19).

The results of this study show that the tested biomass fuels are ideal renewable energy resources both in conventional and oxy-fuel conditions with a minor potential for environmental pollution. According to the EU's Large Combustion Plant Directive, emission limits for NO (as NO2), SO2 and CO are 200 mg/m<sup>3</sup> <sup>n</sup> (dry gas at 6% O2), and 20 mg/m3 n for particulate matter (fly ash). In the case of coal-fired CFB boilers, these limits can only be met with De-NOx (selective non-catalytic reduction) and De-SOx (in situ SO2 capture) systems. Biomass fuels usually contain much less nitrogen, sulfur and mineral matter than coals. Since the magnitude of NOx and SO2 emissions is strongly related to the content of N and S in the fuel, a proper combination of combustion technique, operating conditions, and biomass fuel can deliver a combustion system with significantly reduced requirements for De-NOx and De- SOx systems or even eliminate them.

These experimental results can be used to optimize the combustion process of biomass fuels in the CFB oxy-combustion technology. The data can also be useful in mathematical modeling of biomass combustion both in conventional and oxy-combustion processes.

**Figure 19.** NO and CO (ppm) profiles during combustion of wheat straw under oxy-combustion.
