There are impurities associated with heavy fuel oil which give rise to boiler corrosion, fouling and acidic emissions.
Sodium and vanadium impurities present in fuel oil tend to form complex, low melting point deposits which foul the heat transfer surfaces in the boiler combustion chamber. This reduces heat transfer efficiency and increases corrosion. The introduction of MgO into the combustion chamber leads to the formation of magnesium vanadates with a high melting point which is soft and friable and easily removed.
Some of the sulphur present in fuel oil is oxidised to sulphur trioxide in the combustion process and this will react with water vapour as the flue gas cools to form sulphuric acid. Sulphuric acid causes corrosion problems and can also condense onto carbon deposits on the side of the chimney which are then re-entrained in the exit gas and emitted as acidic particles that can cause damage if the land on cars for example. Injection of magnesium hydroxide powder into the flue gas with neutralise the acidity.
Magnesium based additives have enabled power plants to operate with:
- Higher superheated steam temperatures, hence higher efficiency without serious deposit and corrosion problems.
- Lower flue gas exit temperatures without serious air pre-heater blockage and corrosion, and minimal acidic emissions.
The types of products that are used are:
- Magnesium oxide dispersed in light fuel oil which is injected into the combustion chamber to modify deposits and reduce the catalytic activity of vanadium compounds which promote the conversion of S02 to SO3
- Fine particle size magnesium hydroxide powder usually injected into the “cold end” of the boiler, for example into the air heater inlet. This minimises low temperature corrosion and acidic emissions