4.1.2. SO2 Emissions

In the air-combustion process, the sulfur contained in the fuel is oxidized mostly to SO2 with only a small fraction converted to SO3 [2]. Biomass fuels contain much less sulfur than coals, therefore, their combustion or co-combustion with coal produces lower emissions of SO2 [7,8,30]. Figures 5 and 6 show the time-resolved concentrations of SO2 and the total SO2 emissions for tested fuels. Emissions of SO2 from combustion of the tested biomass were much lower than those from the combustion of coal. The highest instantaneous concentrations of SO2 were observed during combustion of volatiles and they were approximately 58 ppm for coal and less than 9 ppm for biomass fuels.

**Figure 5.** The instantaneous concentrations of SO2 during air combustion.

Sulphur dioxide is formed through oxidation of sulfur-containing species in the fuel (fuel-S), which are released during devolatilization (volatile-S) and char oxidation (char-S) stages (Figure 6). Oxidation of volatile-S was responsible for 60–65% of the total SO2 formed for biomass fuels and for only 30% in the case of coal.

**Figure 6.** Effect of fuel type on the total SO2 emissions during air combustion. Vertical bars represent standard deviation.
