We protect grate-fired WtE (EfW) boiler tubes from fireside corrosion on-site with high-alloy corrosion-resistant cladding.
Waste to Energy boilers with grate firing can suffer fireside corrosion of the furnace and boiler wall tubes. This can lead to reliability issues since the corrosion allowance from the initial design can quickly disappear and leaks would generate unexpected outages. OEMs typically protect most of the first pass with refractory limiting the heat exchange. Even these are often suffering from degradation leading to tube corrosion and high maintenance costs. Alloy 625 weld overlay is now typical corrosion protection above the refractory in the remaining first pass, often in the second pass, and sometimes even in the third pass. Superheaters, evaporators, and economizers can also be overlaid, shielded, and/or clad for protection against corrosion and often erosion.
Corrosion inside grate fired boilers has always existed, but newer designs are more focused on thermal efficiency driving the pressure and temperature of the water and steam higher and higher. When heat is recovered only for district heating at relatively low pressure, i.e. 20 bar, the use of corrosion protection is often not necessary. But new designs generate power at high pressures and temperatures, ie 60 bar to above 100 bar with superheated steam well above 400°C. The conversion of coal boiler to biomass and RDF fuel is also a source of high corrosion rates since those units operate at even higher pressures and temperatures. Efforts to reduce material corrosion limit performance.
Reliability of operation is another critical economical factor since a leak leads to an unexpected outage of several days with significant losses. The frequency of planned outages increases to have maximum operating hours so reliable operation during the longest period without outage is the main target. But there is a balance between this longest reliable operation and the investment in maintenance cost.
Without protection, carbon steel will rust in the presence of oxygen. This well-known process will not stop since the corrosion layer is not protective like with some other metals. The corrosion rate depends on a lot of parameters: oxygen content, temperature, moisture, and other corrosion agents like chlorine, sulphur and alkali.
Boiler tubes are cooled with pressurized water or steam and heated up by corrosive combustion flue gases from about 1000°C down to around 250°C. Corrosion occurs at the tube surface which is typically the water temperature plus a gradient through the thickness between 40°C and 20°C. The skin temperature can also change with the building of ashes on the tube. Changing conditions typically make the corrosion process even more complex thus even more difficult to treat.
The grate-fired combustion of waste with various quantity of plastics or recycled wood containing paint or glue generates a broad range of volatile molecules containing chlorine, sulphur, lots of alkali and heavy metals. The chloride-rich ash inhibits the growth of a dense, duplex oxide scale which would ordinarily act as a diffusion barrier to limit further corrosion. Frequently a chloride rich corrosion scale is observed between the oxide scale and the metal surface. Heat flux leads to the rapid diffusion of corrodents and corrosion products. The alkali metals, lead and zinc react readily with chlorine and further contribute to the transport of chloride rich ash to the tube metal surface, increasing the corrosivity of the ash deposit and combustion environment.
Under these conditions, the dense corrosion scales are frequently disrupted forming instead defective, multi-laminated scales with reduced mechanical properties. These scales are vulnerable to erosion damage, exposing the metal to the corrosive environment thus leading to a process termed corrosion-erosion.
Even further critical conditions are experienced with online cleaning systems. To keep the heat exchange surfaces as clean as possible for thermal exchange performance and avoiding fouling various solutions have been developed: soot blowers spraying steam on the surface, water spraying, ice spraying, shot cleaning, rapping systems, explosion cleaning, micro-explosion. These techniques shock to the surface mechanically and thermally to remove the ashes but also accelerate the corrosion and erosion mechanisms. Often the erosion factor becomes a locally preponderant thinning process.
Refractory was the first protection against corrosive flue gases but limited heat exchange properties cut thermal efficiency. Refractory alloys can be used but due to the excessive cost of such material the main material used for building boiler is carbon steel or low alloyed steel. The implementation of a surface protection layer is the most cost-effective solution. Even a thin layer can deliver significant improvement on corrosion resistance.
The use of ceramic coating seems an attractive approach, however, the thermal expansion mismatch and the fragility of such coating make it unreliable. The coating trend to crack and corrosion can develop underneath the protective layer which can peel off then no more protection remains.
In the 80’s the application of Alloy 625 using the weld overlay process created a new opportunity in corrosion protection. Despite significant costs, the weldoverlay has been applied inside many Waste to Energy boilers.
The thermal conductivity of Alloy 625 is lower than carbon steel so too thick of a layer leads to higher surface temperatures unsuitable for corrosion mechanisms and lower heat exchange unsuitable for boiler performance. The application of a thin layer is limited by the welding process generating dilution with the base metal. This dilution alters the metallurgy of the alloy thus making the cladding less corrosion resistant. The cost of this solution is mainly driven by the cost of the material, so the thickness and metallurgy optimization is the target.
A protective layer can also be applied using thermal spray technology. Thermal spray technologies have been developed for over 100 years but mainly for erosion protection. Indeed, most thermal spray processes leave some porosity in the applied materials which is crippling for a corrosion protection layer. The Spray & Fuse process helps, however the implement for onsite application is unreliable and generates a lot of heat input to the structure, leading to stresses and distortions.
The great benefit of Thermal Spray is the application of a thin layer of material without dilution and heat-affected zone (HAZ).
IGS High Velocity Thermal Spray (HVTS) processes ensure finely structured cladding, with an order of magnitude lower permeability and greater homogeneity. Changes in feedstock chemistry have further improved performance by significantly reducing stress, increasing bond strength, and mitigating oxide content. By addressing the root cause of these microstructural defects through the process and material chemistry, HVTS can be successfully employed in the field. We work during shutdowns, with high production rates and low costs. As with welding and other cladding technologies, surface preparation, material, process and procedure criteria are defined in specifications. HVTS generates no Heat Affected Zone on the pressure parts, and does not place residual stresses on the base metal since the temperature of the base metal remains low even without water in the boiler tubes.
Integrated Global Services provides on-site thermal spray cladding inside WtE and Biomass BFB boilers in the European Union, including UK, Germany, Netherlands, Sweden and France, from our operations center in the Czech Republic. Being an international company, we clad in the United States, Middle East, Japan, South-East Asia, and Africa.
The IGS HVTS systems demonstrate excellent corrosion resistance with little or no indications of metal loss in extreme conditions of waste incinerators and refuse-derived fuel (RDF) grate-fired boilers. Whereas 625 weld-overlaid components must be replaced at end of life, IGS HVTS can be reliably refurbished or even replaced in-situ without replacing the underlying component. The corrosion resistance coupled with the ease of application in-situ makes the selection of such IGS claddings favorable in areas where weld alloy 625 fails.