Sulfuric Acid Mist Formation
Fossil fuels such as coal and oil contain a significant amount of sulfur. When fossil fuels are burned, about 95 percent or more of the sulfur is generally converted to sulfur dioxide (SO2). Such conversion happens under normal conditions of temperature and of oxygen present in the flue gas. However, there are circumstances, under which such reaction may not occur.
When flue gas has too much oxygen, the SO2 further oxidizes into sulfur trioxide (SO3). Too much oxygen is only one of the ways that SO3 is formed. Gas temperature is also an important factor. At about 800 °C, formation of SO3 is favored. Another way that SO3 can be formed is through catalysis by metals in the fuel. Such reaction is particularly true for heavy fuel oil, where a significant amount of vanadium is present. In whatever way SO3 is formed, it does not behave like SO2 in that it forms a liquid aerosol known as sulfuric acid (H2SO4) mist that is very difficult to remove. Generally, about 1% of the sulfur dioxide will be converted to SO3. Sulfuric acid mist is often the cause of the blue haze that often appears as the flue gas plume dissipates. Increasingly, this problem is being addressed by the use of wet electrostatic precipitators.
Read more about this topic: Flue-gas Desulfurization
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