Definition - What does Ash mean?
Ash is the grayish-white to black powdery residue left when something is burned. It is also the general term used to describe the inorganic matter in a fuel. Ash of all fossil fuels, with the possible exception of natural gas, contains constituents that promote corrosion on the fire side of boiler components. Ash can cause corrosion in boilers or other heat exchangers.
Corrosionpedia explains Ash
Ash is the solid residue of combustion. Ash that does not rise is called bottom ash and ash that rises is known as fly ash. In an industrial context, fly ash usually refers to ash produced during combustion of coal.
Coal ash may be used as an additive for the production of cement or concrete. However, many biomass ashes contain much higher concentrations of alkali than the standards which regulate the use of coal ashes allow.
The chemical composition of an ash depends on that of the substance burned. Wood ash contains metal carbonates and oxides formed from metals originally compounded in the wood. Coal ash usually has a high mineral content and is sometimes contaminated with rock; during combustion the mineral matter may become partially fused, forming cinders or clinker. Bone ash is largely made up of calcium phosphate. Seaweed ash contains sodium carbonate, potassium carbonate, and iodine that can be extracted.
In the case of furnace-wall corrosion, melting points between 635° and 770°F (335° and 410°C) have been reported for ash constituents on furnace walls under severe coal-ash corrosion. Reducing conditions exacerbates fuel-ash corrosion. The presence of carbon monoxide and/or unburned carbon and hydrogen sulfide promote the formation of metallic sulfides.
Tube spacers, used in some boilers, rapidly deteriorate when oil of greater than 0.02% ash content is fired. Because these spacers are not cooled and are near flue gas temperatures, they are in a liquid ash environment. Fuel oil additives can greatly prolong the life of these components. Most dry fuel additive preparations are used to treat fuels with high ash content, such as coal, bark or black liquor. Metal oxides are used for this purpose.