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Bubbling Fluidized Bed (BFB) Boilers

Life Extension of Furnace Tube in Biomass and Waste to Energy Boilers

Biomass fired bubbling fluidized bed (BFB) boilers can suffer fireside corrosion of the furnace wall tubes, which restricts steam conditions and downgrades thermal efficiencies, also resulting in frequent maintenance of the water wall tubing. IGS metal coatings provide protection from the extremes of fireside corrosion wastage. IGS provides cost-effective HVTS in-situ cladding having proven corrosion and erosion resistant qualities.

Corrosion-Erosion Mechanisms in WtE and Biomass BFB Boilers

Fireside corrosion-erosion is the main cause of metal loss on furnace wall tubing in biomass and waste fired BFB boilers. The rate of metal wastage is influenced by metal temperature (boiler pressure), gas temperature (heat flux). and combustion environment. The latter being a function of fuel chemistry and whether combustion is complete. Boiler furnace and final steam design temperatures are limited by the combusted fuel, as manufacturers adopt particular firing methods and temperature limits for a given fuel chemistry. Designers have to decrease temperatures with increase of chlorine, alkali and heavy metal contents present in the fuel. Chlorine readily reacts with volatile alkali metals to produce alkali metal chlorides, (Na+K)Cl, in the gaseous vapor phase. Alkali chlorides readily condense as ash deposits on gas tube metal surfaces, where it interacts with the normally protective oxide scales. 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. Under the influence of a heat flux, this leads to rapid diffusion of corrodents and corrosion products.

In addition to the alkali metals found in clean wood fuels, waste fuels frequently contain heavy metals such as lead and zinc. These metals 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 that have much reduced mechanical properties. These scales are vulnerable to erosion damage and loss in a process termed corrosion-erosion, in which the corrosion scales are lost, leaving the underlying tube metal unprotected from further corrosion. Typically, higher gas velocity further increases the corrosion-erosion rate.

Corrosion Resistant Coatings in the Market

The most widespread solution to manage excessive fireside corrosion in the furnace section is to weld several millimeters of Inconel 625 cladding to the plain carbon steel tubing, either in the workshop prior to installation or on-site during a turnaround outage. Ordinarily Inconel 625 is expected to have a lower wastage rate than plain carbon or low alloy steels. However, because it has low erosion resistance, Inconel 625 experiences accelerated metal loss due to abrasion in many BFB waste-to-energy and biomass boilers.

The use of thermally sprayed coatings is gaining greater acceptance as an alternative to Inconel 625 weld cladding due to resistance to corrosion-erosion phenomena. Initial attempts to employ traditional twin wire arc spray (TWAS) with NiCrMo alloy wire feedstock were relatively unsuccessful. High levels of coating permeation by corrosive species in high-moisture content environments caused corrosion at the coating interface and delamination. Coating permeation may occur through chemical and structural weaknesses in an arc-sprayed coating. These are simple porosity, high oxide content, particle micro-cracking and/or through coating macro-cracking due to elevated stresses.

Erosion-Corrosion Resistant HVTS Metal Spray Coatings

The use of IGS High Velocity Thermal Spray process ensures finely structured coating, with and 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, thermal spray may 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. Moreover the IGS HVTS generates no Heat Affected Zone on the substrate neither residual stresses on the base metal since the temperature of the base metal remains low even without water in the boiler tubes.

The thermally sprayed IGS coatings exhibits little if any observable corrosion damage or surface recession in the Biomass BFB applications.  The IGS 5460 coating exhibited a very minor increase in the oxidation on the outermost surface only. There is no discernible corrosion scale or penetration of combustion derived elements of the IGS HVTS coating. In the field tests where unprotected tube sections suffer metal losses of hundreds of microns, IGS HVTS coatings suffer approximately 1 – 3µm of material deterioration, meaning close to zero wastage rates.

Integrated Global Services Europe provides on-site thermal spray coating inside WtE and Biomass BFB boilers in all countries of European Union, including the UK, Germany, the Netherlands, Italy, Switzerland, and France, from our operations centre in the Czech Republic. As an international company, we spray coatings in the United States, Middle East, Japan, South-East Asia, and Africa. We have a global infrastructure established so that we can bring solutions to our clients where and when they need them.

IGS HVTS systems demonstrate extremely good corrosion resistance with little or no indications of metal loss in extreme conditions. 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.

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