Maintenance down, lifetime up, using thermal coatings in waste plant

TWI Bulletin, January - February 2010

Thermal spray coatings reduce corrosion of superheater tubes in dedicated biomass plants

Melissa Riley joined TWI in February 2003 and is a senior project leader in surface engineering involved in managing projects and providing technical expertise in the areas of thermal spraying and coating technologies. Melissa is a Member of the IOM3 and a Chartered Engineer and has been responsible for leading TWI's work on the development of coating systems for biomass and waste to energy power plants. 

The world is increasingly looking to renewable energy sources to meet future energy needs, reduce CO₂ emissions and for security of supply.

For example the UK Government and EU aims to achieve 20% energy generation from renewable sources by 2020. Biomass is one source of renewable energy and includes fuels such as wood, straw, poultry litter, meat and bone meal and dedicated biomass crops. Melissa Riley discusses challenges associated with combustion of biomass and waste fuels and recent developments in coating technologies to mitigate corrosion, erosion and slagging which currently limit operating temperatures and efficiency and result in increased shutdowns and high maintenance costs in many plants. This work includes key results of the TSB funded HiCoat project and TWI's work on boiler degradation in biomass and waste-to-energy plants as well as details of a new group sponsored project.

Incentives for using biomass have led the drive towards co-firing these fuels in existing fossil fuelled power plant along with the development of new biomass fuelled plant. In addition to this, the continued rise in landfill tax is increasing interest in waste to energy (WtE) power generation. However severe, chlorine induced, corrosion and slagging cause significant operational challenges in both biomass and WtE plants. As a result, biomass is limited to ~5% of feedstock in co-fired power stations. These factors reduce plant durability resuting in increased shutdowns and higher maintenance costs for plant operators.

Biomass fuels

The characteristics of biomass are very different to those of coal and therefore use of solid biofuels and wastes sets new demands for boiler process control and boiler design as well as for combustion technologies, fuel blends control and fuel handling systems. The volatile matter in wood based biomass is generally close to 80% compared to 30% for coal. Furthermore as biomass has a high moisture content it has a relatively low net calorific value compared to coal. Biomass covers a wide range of fuels including dedicated crops, residues and by-products and wastes for example:

• Dedicated plantation eg short rotation forestry (willow), perennial crops (miscanthus) and arable crops (rapeseed, sugarcane, sugar beet).
• Residues, eg wood and wood felling residues from forestry, straw from cereals and food and industrial crop residues (sugarcane, tea, coffee, rubber trees, oil and coconut palms)
• By-products and wastes eg sawmill waste, manure such as poultry litter, sewage sludge, organic fraction of municipal waste, used vegetable oils and fats and meat and bone meal (MBM)

Co-firing biomass

Co-firing biomass eg straw, willow and miscanthus with coal utilises existing power plants and at the same time helps to reduce emissions of pollutants (SO₂ NO_x) and greenhouse gases from traditional power plants. However the proportion of biomass is limited to around 5-10% of the fuel due to the challenges associated with the properties of the fuels. The ash content, melting behaviours and chemical composition of the biomass fuels have significant effects on corrosion and fouling of the boiler. Alkali and alkaline metals and chlorine species present in the fuel react to form harmful alkali and alkaline metal and chlorine compounds, which settle onto heat transfer surfaces causing significant corrosion. In addition the melting behaviour of the ash residues can also result in slagging on tube surfaces. Therefore, the proportion of biomass is limited to avoid the need to reduce plant operating temperatures and efficiency.

Dedicated biomass plants

There are many dedicated biomass plants specialising in biomass combustion, including large scale plants (12-39MW) and smaller scale, combined heat and power, domestic biomass boilers. The UK leads the way in terms of biomass combustion with Energy Power Resources operating some of the largest (~38MW) biomass combustion plants in the world, fuelled by straw, poultry littler and meat and bone meal. However these fuels present their own challenges.

Straw fired boilers suffer from rapid deposit accumulation and corrosion rates due to the high chlorine and potassium content of the fuel, and therefore, operating temperatures are limited to prevent rapid corrosion of the components. Meat and bone meal also has extremely high chlorine and sulphur contents compared to other biomass fuels and presents significant problems for plant operators. CO, CO₂, SO₂ and Cl₂ are produced during MBM combustion. Alkali and alkaline metals (Na, Mg, K and Ca) from the fuel react with chlorine species to form metal salts, such as NaCl and KCl, which in turn can react with SO₂ and O₂ releasing HCl or Cl₂ gas. The ash consists of calcium phosphate, the mineral component of bone, and sodium and potassium compounds that also react with Cl₂ to form additional metal salts. Ash hinders the formation of protective oxide scales on tube surfaces. Corrosion, solid particle erosion and ash build up are worse on the side of the tubes exposed directly to the gas flows. A combination of these mechanisms results in excessive wall, thinning and failure of the tubes: in extreme cases, super-heater tubes can fail in a matter of weeks (Fig.1). Consequently, operating temperatures and efficiency are often limited with steam temperatures typically being around 400°C compared to >600°C in conventional coal fired plant.

Waste to energy

Waste to energy plants burning municipal solid waste (MSW) also suffer from serious chlorine induced corrosion which limits operating conditions and efficiency. However, the continued rise in landfill tax is increasing interest in WtE power generation. A number of plant operators have investigated the use of nickel alloy 625 weld overlay clad low alloy steel super-heater tubes to mitigate the effects of corrosion, erosion and slagging. Overlays are extremely costly but can extend the lifetime of the plants considerably. However, even where weld overlays have been used, corrosion rates of 1.3mm/year have been observed in particularly aggressive waste to energy (WtE) plants.

MSW combustion produces CO₂, H₂O, SO₂ and HCl, together with O₂, where HCl is considered to be the most corrosive. Heavy metals eg Pb, Zn, and alkali/alkaline metals are known to be present in the fuel and these react with chlorine to form various low melting point chlorides (Fig.2).

Under typical operating conditions experienced in WtE plants, most, if not all of the salts identified in Fig.2 can form and exist as molten salts on the surface of the superheater tubes resulting in severe attack. Analysis of nickel alloy 625 weld overlay superheater tubes retrieved from a WtE plant after four years service (Fig.3), confirms that Pb and Zn salts exist on the surface of the WtE superheater tubes and form part of the corrosion front, with molten lead chlorides, such as PbCl₂, and zinc chlorides being the most likely mechanism of attack.

Alkali metal salts are also present on tube surfaces and in the ash. Ash impingement results in erosion and roughening of the tube surface (solid particle erosion) and contributes to tube failures, especially on the side exposed to the gas flow (Fig.4). Roughening results in further ash build up and more corrosive metal salts being in direct contact with the metal surface thus increasing the rate of corrosion compared to gaseous corrosion and oxidation alone.

Thermal spray coatings

Although weld overlays can extend the lifetime of superheater tubes, many biomass and WtE plant operators are looking for more cost-effective corrosion mitigation technologies. TWI has recently completed a collaborative programme to develop thermal spray coatings for high temperature corrosion mitigation in biomass plants. The TSB-funded HiCoat project ('High Corrosion Resistant Coatings for Biomass Plant') evaluated the performance of thermal spray coatings combined with advanced sealant technologies in specific biomass environments with the aim of developing a low cost coating system which:

• provides high-temperature corrosion resistance to achieve protection on low carbon steel applicable to biomass plant operators
• provides protection to resist high temperature chloride and alkali metal salt corrosion
• is capable of withstanding solid particle erosion and steam cleaning
• is capable of resisting ash settling and build up which leads to sintering and reduced efficiency

Field trials

Field trials in a 13.5 MW renewable energy power station combusting pure meat and bone meal, indicated that the specialist coatings developed can provide protection of superheater tubes in highly corrosive, high temperature combustion environments. The use of coatings resulted in a doubling of the lifetime of the tubes compared to uncoated superheater tubes and allowed the plant to operate at higher temperatures with improved efficiency.

Coatings were prepared by arc and high velocity oxy-fuel (HVOF) thermal spraying. A number of nickel chromium based coatings were evaluated including 50:50NiCr, Cr3C₂-25NiCr, Ni alloy C-276 and Ni alloy 625. Some were sealed with advanced slurry coatings to reduce ash build up on, and slagging of, the tube surfaces by sealing the thermal sprayed coatings and providing a smooth finish. The coated superheater tubes were incorporated into a large-scale MBM biomass plant for 1760 hours where they were subjected to gas temperatures of 700-800°C (equivalent to steam temperatures of 400-420°C) and exposed to corrosive species including oxides (CO, CO₂, SO₂), Cl₂, Na and K.

On removal from the plant, the tubes were visually assessed in terms of ash build-up, solid particle erosion and general coating condition. Coatings with sealants exhibited less ash build up and less solid particle erosion (roughening) compared to coatings tested in the as-sprayed condition. Where ash was present on sealed tubes, it was less adherent than on unsealed tubes. Coating composition and coating process were also shown to have a significant effect on the performance of the coated tubes, with nickel alloys C-276 and 625 appearing to perform better than NiCr and Cr₃C₂-NiCr coatings. In terms of visual inspection of general coating condition, solid particle erosion and ash build-up, the HVOF coatings generally appeared to perform better than the arc sprayed equivalents.

Effects of coating process and coating material

When sectioned, less degradation of the substrate was observed under HVOF coatings compared to arc sprayed coatings. Denser HVOF coatings have fewer pathways for aggressive species to diffuse through the coating and therefore reduce the rate of attack at the substrate-coating interface.

C-276 coatings generally performed better than Ni625 coatings based on visual examination on removal from the plant. However, sectioning revealed that the Ni625 appeared to be better at resisting substrate-coating interface attack than C-276 coatings. Element mapping of C-276 coatings showed oxygen and chlorine present at the substrate-coating interface (Fig.5). Oxygen levels are similar for all C-276 coatings, whereas the amount of chlorine appears to be lower for HVOF coatings and sealed arc sprayed coatings compared to unsealed arc sprayed coatings. Although oxygen levels are greater at the interface, chlorine is expected to cause greater degradation at the substrate-coating interface due to its ability to react with iron and chromium to form unstable chlorides and presents the greatest concern.

Fig.5. Elemental mapping showing presence of oxygen (middle) and chlorine (right) species in as-deposited (a) arc, (b) HVOF sprayed coating and sealed (c) arc sprayed C-276 alloy coatings subjected to 1780 hours exposure in a dedicated biomass plant. Nominal coating thickness, 300m

Levels of chlorine detected for the arc sprayed Ni625 coatings (Fig.6a) were comparable with those observed for HVOF C-276 coatings (Fig.5b). Chlorine was not detected at the substrate-coating interface for Ni625 HVOF coatings (Fig.6b) suggesting coating composition and not just coating structure/density has an effect on the levels of chlorine at the interface.

Fig.6. Elemental mapping showing presence of oxygen (middle) and chlorine (right) species in as-sprayed, unsealed (a) arc and (b) HVOF sprayed Ni625 alloy coatings subjected to 1780 hours exposure in a dedicated biomass plant.

Sealed coatings

Sealants appear to improve the performance of thermal spray coatings in biomass plants by minimising ash build up and solid particle erosion.

For example, as-deposited, unsealed Cr₃C₂-NiCr coatings were shown to suffer from severe material loss, through erosion of the coating on the side of the tube exposed to the gas flow, in contrast to the sealed coating of the same composition (Figure 7).

Fig.7. As-deposited, unsealed (a) and sealed (b) Cr₃C₂-NiCr alloy coatings deposited by HVOF spraying subjected to 1780 hours exposure in a dedicated biomass plant

Sealants also help to reduce the amount of corrosive species on the surface of the coated tubes, therefore reducing the risk of attack from corrosive salts. They also seem to offer some benefits in terms of minimising gas diffusion through the coatings. Element mapping of arc sprayed alloy C-276 coatings with and without sealants (Fig.5a and 5c) show they do not prevent diffusion of gas through the coatings as both are present at the coating-substrate interface. However, the amount of chlorine detected is significantly lower than that detected for the unsealed coating. Further work is required to assess whether the sealants may help to reduce corrosion at the substrate-coating interface.

Summary

Thermal spray coatings have been shown to reduce corrosion (oxidation and chlorination of the substrate) of superheater tubes in dedicated biomass plants resulting in increased lifetimes. The performance of coated superheater tubes correlates to coating composition, coating density and the presence of sealants. HVOF sprayed coatings provide better protection of the substrate than arc sprayed coatings of the same material due to their higher density. Alloy C-276 coatings gave the best overall performance based on visual assessment, but alloy 625 gave best protection against attack at the substrate-coating interface which can ultimately lead to failure. Sealants help to prevent ash-build up and solid particle erosion when applied to sprayed coatings and it is thought they also reduce the rate of diffusion of gaseous species, eg chlorine species, through the coating, resulting in improved lifetimes.

Further work...the future of biomass and waste to energy combustion

The HiCoat work has implications for biomass combustion, co-firing applications and waste incineration plants and offers the potential for coatings to be used to mitigate corrosion, ash-build up and slagging and ultimately to increase plant lifetimes, operating temperatures and efficiency. Further testing is required to establish the performance and lifetimes of coated components in a wider range of biomass and waste environments. To assist with further coating development, TWI has developed a high temperature test facility to allow controlled assessment of coatings for biomass and WtE environments. A group sponsored project will be starting during 2010 to develop the technology further. The project will use the high temperature test facility in addition to field trials in biomass and WtE plants to produce industry qualified coating systems. For further information about this project contact: melissa.riley@twi.co.uk or dave.harvey@twi.co.uk

Acknowledgement

The author would like to thank the HiCoat project consortium (TWI, Energy Power Resources, Talbotts, Monitor Coatings, Metallisation, Ecka Granules and ADAS) and the State of Jersey waste incineration plant for their contributions to this work.