with the sulphur regulation of MARPOL Annex VI. Flag States must approve such systems and port States can (as part of their port state control) ensure that such systems are functioning correctly. If a scrubber system is not functioning probably (and the IMO procedures for such malfunctions are not adhered to), port States can sanction the ship. The United Nations Convention on the Law Of the Sea also bestows port States with a right to regulate (and even ban) the use of open loop scrubber systems within ports and internal waters. Sulfuric acid mist formation Fossil fuels such as coal and oil can 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 (). 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. can further oxidize into sulfur trioxide () when excess oxygen is present and gas temperatures are sufficiently high. At about 800 °C, formation of is favored. Another way that 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 is formed, it does not behave like in that it forms a liquid aerosol known as sulfuric acid () mist that is very difficult to remove. Generally, about 1% of the sulfur dioxide will be converted to . 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. FGD chemistry Basic principles Most FGD systems employ two stages: one for fly ash removal and the other for removal. Attempts have been made to remove both the fly ash and in one scrubbing vessel. However, these systems experienced severe maintenance problems and low removal efficiency. In wet scrubbing systems, the flue gas normally passes first through a fly ash removal device, either an electrostatic precipitator or a baghouse, and then into the -absorber. However, in dry injection or spray drying operations, the is first reacted with the lime, and then the flue gas passes through a particulate control device. Another important design consideration associated with wet FGD systems is that the flue