3. Why Inject Sulfur Oxides?
- It's a common question. The answer lies in the difference between sulfur dioxide and sulfur trioxide. When high sulfur coals are burned, they yield large quantities of SO2 but very small amounts of SO3
SO3 however, is very important, because it naturally combines with moisture in flue gas to create sulfuric acid. Sulfuric acid (H2 SO4) is deposited on fly ash particles to form a thin conductive film which lowers the electrical resistance of the ash, allowing it to be readily collected by electrostatic precipitation.
- When low sulfur coals are burned, natural SO3 levels are reduced to the point that they no longer can create enough H2 SO4 to properly "condition" the fly ash and reduce its resistivity. High resistivity ash passes through precipitators sized for high sulfur coal, creating unacceptable emissions.
- Sulfur trioxide FGC systems, as developed and patented by Wahlco, injects small quantities of SO3 into the gas stream. This newly introduced SO3 reacts in the same way as naturally-occurring SO3, lowering ash resistivity, and allowing easy collection.
