Circulating Fluidized Bed (CFB) Boilers
Life-Time Extension of Tubes in CFB Biomass and Waste to Energy Boilers
Biomass and waste fueled CFB boilers have been developed based on the advantages of CFB design, offering high combustion efficiency, low emissions and high fuel mix flexibility. Typically designed for medium or large installations, CFB boilers produce steam by burning biomass, recycled biomass, or pre-treated industrial or urban waste. This design spectrum also includes the conversion of units initially design for coal or lignite. Fuel flexibility allows the reduction of related costs and opens up operational flexibility to both renewable energy sources and conventional fuels.
Erosion is the main challenge for CFBs, with hard particle circulation into the boiler, so most designs use refractory protection in the furnace area. Erosion resistant coating is then applied on the evaporator surfaces to prevent thickness losses on the tubes. With fuel containing plastics and other chemicals generating Chlorine, Sulphur, and other alkali and heavy metals, the corrosion can dramatically increase the thickness loss rate.
Several WtE plants designed with CFB met reliability challenges leading to many unexpected outages and high maintenance cost so despite the efficiency and emission benefits, this technology is less developed for corrosive fuel, but material improvements can lead to longer life of the CFB boilers.
Corrosion Mechanisms in WtE and Biomass CFB fired Boilers
CFB boilers are preferred for their high efficiency and flexibility in fuel usage, thus high pressures and temperatures are found within these units. The more corrosive agents introduced through the fuel, the higher corrosion will develop on the non-protected heat exchange steel surfaces of the super-heaters.
Most corrosion protection mechanisms consist of generating a corrosion barrier on the base metal by forming an oxide layer. Inside CFB boilers this layer is quickly eroded, forcing the formation of another layer and consequently leading to erosion-corrosion phenomenon. Erosion-corrosion thinning can be quick with soft or poor erosion resistance materials.
With such conditions it can be expected that ashes will not build up so less corrosion by salts occurs. This is true for the surfaces seeing high flow speed, but in some areas the ashes can build up and typical under ash salt corrosion can develop due to flow turbulence. Even without an ongoing erosion mechanism, salt corrosion will depend on ash composition and tube skin temperature. With materials softening with temperature, erosion resistance performance of most materials declines and is something that must be addressed in CFBs and WtE units.
High-temperature chlorine corrosion in WtE and Biomass CFBs
The presence of chlorine can drive high-temperature corrosion in waste-to-energy and biomass boilers. Chlorine dissolves in the flue gas during fuel combustion. The saturated chlorine salts condense at the relatively cold heat exchanger surfaces (cold trap) and can directly cause corrosion on metallic surfaces. The amount of the chlorides near the steel surface is a major contributor to corrosion. The partial pressure of the chlorine salts near the steel surface increases where there is high heat flux, which increases the reactivity of the chlorides. The reaction between the chlorides and the iron on the steel surface results in iron chloride. The partial pressure of iron chloride gas is high.Consequently, it diffuses into the fouling where it contacts with oxygen to form iron oxide (Fe2O3 and Fe3O4). Sometimes chlorine corrosion is called active oxidation. Chlorine released diffuses back to the steel to cause a further corrosion. The result is an iron chloride layer directly on the tube surface, followed by a thicker layer of iron oxides, often penetrated by chloride salts.
The content of chlorine species in the flue gas is not as important for the intensity of the high-temperature corrosion as the content of specific chlorides inside the fouling and the heat flux density at the tube wall.
Hot Molten Salt Corrosion of WTE Boilers
The hot molten salt corrosion process starts when the temperature of the tube surface is higher than the melting point of the precipitated salts so that salt melts directly on to the metal surface. The resulting reaction dissolves existing oxide layers, which let the corrosive chlorine gas dissipate to the metal surface. Hot salt melts also change the structure of the fouling from a porous structure to a dense cover, trapping the iron chloride and closing it off from the gaseous oxygen thus increasing chlorine corrosion. Pitting is an even more dangerous mechanism of the molten salts reacting as a liquid electrolyte directly with the steel and dissipating it. If the equipment is not protected, costs and lost time can rapidly accumulate when something goes wrong.
Dew-point corrosion in WtE and Biomass Boilers
When the temperature of the flue gas decreases, a specific gaseous species reach saturation, droplets of the liquid start to condense on solid surfaces. The saturation temperature depends on the load of the concerned species and, for sulfuric acid, on the humidity of the flue gas. The liquid electrolytes forms wet spots on the surfaces, causing general corrosion and material loss in the form of pitting. In waste-to-energy plants, dew point corrosion is less common than in coal-fired plants, because SO3 reacts with chlorine species, making the formation of sulfuric acid less likely.
WtE Boiler Corrosion and Erosion Protection Experience
Refractory protection was the first protection against corrosive flue gases but have limited heat exchange properties as the thermal efficiency is low, thus they do not always offer the best protection against erosion mechanisms. Refractory alloys can be used, but due to the excessive cost of materials and limited erosion resistance surface protection rises as the best cost-effective solution.
Ceramic coatings are fragile and create a thermal expansion mismatch making them unreliable. The coatings trend to crack and corrosion can develop underneath the protective layer, which can peel off leaving your equipment utterly unprotected.
Most of the hard-facing alloys which can be applied by welding have high propensity to cracking, which can even propagate into the base metal. Such solutions are avoided on pressure parts but may be used on the mechanical structure.
The complex mechanisms of corrosion-erosion sometimes result in premature failure of the factory-installed boiler protection measures. One of IGS´s strengths is the fact that the material selection of the protective metal coating is based on a thorough analysis your specific wastage mechanisms, and will perform in the conditions of that exact boiler. IGS applies the solution in a matter of days during planned turn-around periods, preventing further metal loss and saving costs.
Erosion-Corrosion Resistant HVTS Metal Spray Coatings
HVTS coating has been used for decades in erosion application inside CFB boilers fueled with coal or lignite.
High Velocity Thermal Spray is designed for applying a metal layer with very low porosity and sealability to better protect the base metal in high corrosion environments. This technology allows the use of solid or cored wire, enabling easy material modification compared to technologies depending on the market available solid wire. Since the process doesn’t generate any dilution with the base metal, the quality of the coating is not altered by chemistry mix.
IGS brings to bear decades of experience to bring you the best and most efficient technology for field application:
- We work to reduce critical path schedule as much as possible and comply with the schedules we commit to. We offer robust equipment and reliable technicians, with full compliance to all relevant EHS conditions and rules
- We are familiar with the most difficult conditions in the most extreme environments.
- Safety is a priority at IGS. Be it in the scaffolding, during clean up, or at any other time, we use a proactive, multi-level safety system to ensure a safe working environment for our employees and everyone around us.
Integrated Global Services Europe provides on-site thermal spray coating inside WtE and Biomass BFB boilers in all countries of Europe, including the UK, Germany, the Netherlands, Italy, Switzerland, and France, from our operations center in the Czech Republic. We spray coatings in the United States, Middle East, Japan, South-East Asia, as well as Africa.
Leave us a note or start a chat with our operator to learn more about what we offer in corrosion and erosion prevention in WtE and Biomass CFB boilers.