Fouling and Corrosion
The waste-fired plant cannot be designed with steam parameters similar to those of traditional power plants fired with coal, gas, or oil. This is because waste differs from fossil fuel, particularly in terms of the content of chlorine, which - combined with sulphur - may lead to high-temperature corrosion, even at relatively low temperatures. The risk of corrosion and erosion can be reduced by observing a number of specific design criteria and by designing the boiler for moderate steam parameters (pressure and temperature).
The primary features to minimise fouling and corrosion are as follows:
• optimised combustion chamber dimensions and low gas velocity to reduce ash entrainment;
• long gas residence time in the radiation passes before entering the convective pass;
• horizontal flue gas flows in the convective pass, which have higher thermal efficiency;
• on-line cleaning systems in the convective pass are made of mechanical rapping devices rather than soot blowers; and
• wide tube spacing in the horizontal pass including free space for additional heating surface.
Some combustion processes may, furthermore, carry a risk of CO corrosion. The corrosive nature of the flue gas from waste incineration usually limits the steam parameters to a maximum temperature of approximately 400 °C and a pressure of approximately 40 bar. The temperature of the water returning to the boiler (feed water) is maintained at a minimum of 125 to 130 °C to limit the risk of low temperature corrosion in the coldest part of the boiler.
Hoppers are provided under the vertical and horizontal passes to collect boiler ash. Collected dust is discharged by mechanical conveyors into the ash silos. Boiler ash is considered to be part of the produced fly ash.