Life Extension of Furnace Tube in Biomass and Waste to Energy Boilers
Biomass and Waste 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 initial design can quickly disappear and leaks would immediately generate unexpected outages. Most of first pass is typically protected with diverse types of refractory or tiles limiting the heat exchange, often suffering from degradation breaking the corrosion protection thus leading to tube corrosion, and high maintenance costs. Inconel 625 weld overlay is now the typical corrosion protection above the refractory in the remaining first pass, often in the second pass, and sometimes even in the third pass. Superheaters, other evaporators, and economizers can also be overlaid, shielded, and/or coated for protection against corrosion and often erosion as well.
Corrosion inside 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, ie 20bar, the use of corrosion protection is often not necessary. But new designs generate power at high pressures and temperatures, ie 60bar or even above 100bar 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 limits 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.
Corrosion Mechanisms in WtE and Biomass grate fired Boilers
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 will depend of a lot of parameters: oxygen content, temperature, moisture, and if other corrosion agents like Chlorine or Sulphur and even salts are present. 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. The 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 combustion of waste with various quantity of plastics or recycled wood containing paint, glue, or plastics generates a broad range of volatile molecules containing Chlorine, Sulphur, and 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, that under the influence of a heat flux leads to rapid diffusion of corrodents and corrosion products.
In addition, the alkali metals contain heavy metals such as lead and zinc. These metals similarly 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 various techniques create mechanical or thermal shock to the surface to remove the ashes but also accelerate the corrosion and erosion mechanisms. Often the erosion factor becomes a locally preponderant thinning process.
Corrosion Resistant Coatings in the Market
Refractory protection was the first protection against corrosive flue gases but having limited heat exchange properties means thermal efficiency is low. Refractory alloys can be used but due to the excessive cost of such material the main material used for building boiler is carbon steel sometimes low alloyed steel. The implementation of a surface protection layer is the best cost-effective solution since 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 Inconel 625 using weld overlay process created a new opportunity in corrosion protection. Despite significant cost, the Inconel weldoverlay was applied onsite inside many Waste to Energy boilers with promising performances. The welding process has been improved to optimize the deposition rate and metallurgy. Nowadays the same alloy is still used with various thicknesses typically from 1 to 3mm layer. The thermal conductivity of Inconel 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. Application of thin layer is limited by the welding process generating dilution with base metal. This dilution alters the metallurgy of the alloy thus the coating become less corrosion resistant. The cost of this solution is mainly driven by the cost of the material, so optimization of the thickness and metallurgy is the target. This proven solution is now widely used including from initial design by OEM with weld overlay being applied in shop for better quality and cost effectiveness.
An Inconel 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 (except for systems working with cathodic protection). 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.
Corrosion Resistant HVTS Metal Spray Coatings
The great benefit of Thermal Spray is the possible application of a thin layer of material without dilution. Material can be from wire or powder, allowing flexibility in the material development. Different Thermal Spray processes generate varying amounts of heat input to the substrate, since they melt the metal at various temperature and with either an ionized gas (plasma), flame or an electrical arc. A flame solution will bring higher heat input whereas an arc solution would suffer from higher porosity but lower heat input.
IGS High Velocity Thermal Spray (HVTS) processes offers the best of both worlds, ensuring finely structured coating, 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 process and material chemistry, HVTS can be successfully employed in the field, during shutdowns, with high production rates and low costs. As with welding and other coating technologies, surface preparation, material, process and procedure criteria should be well defined in coating specifications. HVTS generates no Heat Affected Zone on the substrate, 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 Europe provides on-site thermal spray coating inside WtE and Biomass BFB boilers in all countries of European Union, including UK, Germany, Netherlands, Italy, Swiss and France, from our operations centre in Czech Republic. Being an international company, we spray coatings in the United States, Middle East, Japan, South-East Asia, as well as Africa.
The IGS HVTS systems demonstrate excellent corrosion resistance with little or no indications of metal loss in extreme conditions of biomass and refuse derived fuel (RDF) grate-fired boilers. Whereas Inconel 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 coatings favorable in areas which may traditionally have been protected using Inconel 625 weld overlays.