Ash is product of combustion, it is unburned matter. Ash will flow with flue gas through furnace, superheater, reheater, economizer, air heater, multi cyclone / baghouse / electrostatic precipitator and then be discharged out to atmosphere through stack.
Ash can gives bad impact to steam boiler if maintenance program is not performed. Erosion, corrosion, slagging and fouling are the influence of ash. Erosion is reducing of surface metal because of impingement particle in ash with surface metal tubes, so the abrasion occurs. Erosion in steam boiler can be reduced but can not be eliminated, because particle abrasion ash include other unburned particle always presence in flue gas content, can not be eliminated 100%. Some of ways to reduce possibility of erosion are choose highest fuel efficiency and doing routine maintenance such us clean surface tubes with soot blower.
Another impact of ash is corrosion which is caused by either low temperatur corrosion or high temperature corrosion. Low temperature corrosion generally occur in economizer and air heater area because at temperature under 200 C, the water vapor will react with SOx content in flue gas to form sulfuric acid which lead to corrosion. High temperature corrosion generally occur in superheater and reheater area but the causes can not be fully known because amount of reduction regarding to corrosion is very small approximately microns per year.
Another impact of ash are slagging and fouling. Slagging and fouling can be distinguished based on heat transfer. Slagging is the deposits which attached on the surface furnace tubes by radiant heat while fouling attached on superheater, reheater, economizer and air heater by convection heat. Slagging and fouling influence on heat transfer from flue gas to water inside tube can not be optimal.