Corrosion protection
Chemical combinations and corrosion
Household and industrial waste is a fuel which contains most of the elements in the periodic table. Depending on the character of the incineration, various chemical combinations will be formed.
Depending on local temperatures and oxygen supply, the Na, Ca, Cu, K, Cl, S, Cr, Pb, Zn, Fe, Sn and Al contents in particular will take part in various chemical reactions, the end-products of which can cause corrosion.
The use of waste as fuel therefore makes major demands on the plant’s resistance. This section exclusively considers the flue gas area.
Corrosion attack in a waste-fired boiler can in principle be divided into two main types.
- Low-temperature corrosion
- High-temperature corrosion
Low-temperature corrosion
Low-temperature corrosion appears in the boiler as well as on other surfaces where the temperature is under approx. 135°C. It is caused by condensation of the acidic sulphurous and chlorine-containing gases.
This type of corrosion is temperature-dependent. New plants are being designed differently in order to avoid low-temperature corrosion.
High-temperature corrosion
High-temperature corrosion (HT-corrosion) is more complex and only appears on the heating surfaces in steam boilers in particular on:
- Evaporator heating surfaces
- Superheater tubes
Tube wall temperatures (metal temperature) above 260°C can cause attacks of various HT-corrosion types depending on local conditions.
In general, increasing metal temperatures and increasing gas temperatures will increase the corrosion rate, which is measured in mm tube wall per year, where the metal temperature is the most important factor.
The corrosion speed is influenced by the thickness and composition of the ash layer, the HCl content of the gas, and the surplus/deficiency of oxygen in the gas at the tube wall.
The corrosion rate can be slowed by using high-alloy tubes and/or by protecting the tubes with refractory lining or welded high-alloy materials such as Inconel.
Corrosion protection: Refractory lining or Inconel?
The choice between refractory lining or Inconel depends on:
- The plant size in MW
- Regulatory requirements on the cooling process for the flue gases, including EU rules on retention time
- Which type of heating surface and design is chosen
If refractory lining is chosen as protection, the choice between the different lining types will be based on requirements regarding the resultant heat conduction, the residual porosity and the smoothness of the lining surface.
Reputable suppliers’ as well as our own experience will always enter into the considerations.
We have substantial experience with Inconel
If Inconel protection is chosen, we possess the necessary know-how with respect to material selection, welding (on site or workshop), maintenance inspection for guarantee purposes etc.
Up to year 2004, we have supplied approx. 5,000 m2 of Inconel-protected heating surface to approx. 30 plants.
