4.2.2. Emissions of SO2

The SO2 emissions during biomass fuels and coal combustion in various environments are shown in Figure 15. The instantaneous SO2 concentrations during oxy-combustion of biomass were much lower than those for the combustion of reference coal. The times of SO2 release were 40–90 s for biomass and 380–580 s for coal in all O2/CO2 mixtures. The highest emissions of SO2 were observed during the combustion of volatiles, for both biomass and coal. Compared with air-combustion, burning both biomass fuels and coal in oxy-21 atmosphere caused a minor increase in emissions of SO2. The highest SO2 emission in oxy-21 mixture was approximately 62 ppm for coal. An increase in the oxygen inlet concentration caused higher emissions of SO2. The highest SO2 emissions were detected in oxy-40 atmosphere and was 86 ppm for coal. The highest concentrations of SO2 in oxy-40 for biomass fuels were 21 ppm, 13 ppm and 10 ppm for wheat straw, *Salix viminalis* and Scots pine, respectively.

**Figure 15.** The instantaneous concentrations of SO2 during oxy-combustion of the tested fuels.

Figure 16 shows the influence of the oxygen concentration on the total emission of SO2. It can be seen that an increase in the oxygen inlet concentration resulted in higher SO2 emissions, particularly for coal. Combustion of wheat straw in the oxy-40 atmosphere caused twice as higher emission of SO2 than that in oxy-21. A similar trend for coal and biomass fuels during oxy-combustion was found in references [19,31,34,35].

**Figure 16.** Influence of the oxygen concentration on total emissions of SO2 during oxy-combustion. Vertical bars represent standard deviation.
