Biomass & Waste Energy
Life-Time Extension of Tube in CFB Biomass and Waste to Energy Boilers
Many biomass and waste fuelled CFB boilers were 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, these boilers produce steam by burning biomass, recycled biomass, or pre-treated industrial or urban waste, including conversion of units initially design for coal or lignite. Fuel flexibility offers a reduction of related costs and operational flexibility to cope with the pressure of renewable energy sources on the conventional fuels.
However, mixed fuel environments create extreme environments that can predispose equipment to damage. Erosion is the main challenge for CFB, with hard particles, such as bed, fuel and ash material, circulating in the boiler. Most designs use refractory protection in the furnace area to mitigate damage. Erosion resistant coating is then applied on the evaporator surfaces to prevent thickness losses of the tubes. When fuel containing plastics and other chemicals is used, Chlorine, Sulphur, and other alkali and heavy metals can generate, dramatically increasing the thickness loss rate through corrosion mechanisms.
Without proper preventative care, these conditions can lead to unexpected outages and high maintenance costs, reducing the efficiency and emission benefits of these boilers. However, the material improvement solutions offered by IGS can lead to a longer, better life for CFB boilers.
Corrosion Mechanisms in WtE and Biomass CFB Fired Boilers
The high efficiency of CFB boilers means higher pressures and temperatures inside the boiler tubes. Given the combination of new fuel types containing higher levels of corrosive agents with these higher temperatures and pressures which can lead to material softening, increased corrosion is likely to occur on the unprotected steel heat exchange surfaces of the super-heaters and erosion resistance is likely to decline.
Most corrosion protection mechanisms consist of generating a corrosion barrier on the base metal by forming an oxide layer. The challenge inside CFB boilers is that this layer is quickly eroded forcing the formation of another layer and consequently leading to erosion-corrosion phenomenon. Erosion-corrosion thinning can happen quickly when harsh conditions are combined with soft or poor erosion resistance materials.
It is sometimes assumed that in these conditions ashes do not build up thus there is less salt caused corrosion. While this holds true for surfaces seeing high flow speed, flow turbulence can create low flow areas, where ash builds up and ash salt corrosion develops. Even without additional erosion mechanisms, this type of corrosion is critical and highly influenced by ash composition and tube skin temperature.
High-Temperature Chlorine Corrosion in WtE and Biomass CFBs
Chlorine species are highly responsible for high-temperature corrosion in waste-to-energy and biomass boilers. Chlorine species dissolve 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 partial pressure of the chlorine salts near the steel surface increases at positions with high heat flux, which increase the reactivity of the chlorides driving corrosion.
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 corrosive attack. 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.
That being said, 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 similarly to the high-temperature corrosion described above under the condition that the temperature of the tube surface is higher than the melting point of the precipitated salts so that salt melts directly on the metal surface. The resulting salt melts dissolve existing oxide layers, which lets the corrosive chlorine gas dissipate to the metal surface. Hot salt melts also change the structure of the fouling from a porous to a dense cover, trapping the iron chloride and closing it off from the gaseous oxygen thus supporting 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.
Dew-point corrosion in WtE and Biomass Boilers
When the temperature of the flue gas decreases, gasses present in the environment reach saturation and droplets of liquid start to condense on solid surfaces. The saturation temperature depends on the load of the concerned gaseous species and (for sulfuric acid) on the humidity of the flue gas. The liquid electrolytes form wet spots on the surfaces, causing general corrosion and material loss in the form of pitting or almost uniform. In waste-to-energy plants the 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 is the first defense against corrosive flue gases and can also have excellent erosion properties, but has limited heat exchange properties as the thermal efficiency is low. Refractory alloys can be used, but due to the excessive material costs and limited erosion resistance the implementation of surface protection layer is often a more cost-effective solution.
The use of a thin ceramic coatings seems like an attractive approach, however, the thermal expansion mismatch and the fragility of such coatings make this solution unreliable. The coatings tend to crack and corrosion can develop underneath the protective layer which can then peel off.
Most of the hard-facing alloys which can be applied by welding have high propensity to cracking which can propagate into the base metal. Such solutions are best avoided on pressure parts but may be used on the mechanical structure.
One of IGS’s strengths is that we select our protective materials based on an in depth analysis of the wastage mechanisms present in your equipment’s unique environment. Our solutions preform in the conditions of that exact boiler to mitigate corrosion and erosion. We apply our protective materials in a matter days, during your planned turn around periods, with a highly mobile global team of trained and experienced personnel to provide the most convenient and effective solutions for your equipment.
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.