What’s the alternative? - Surface engineering in alternative energy

Materials World magazine
1 Sep 2009

Dr Bryan Allcock, Managing Director of Monitor Coatings Ltd in North Shields, UK, describes the benefits of surface engineering in alternative energy power stations.

The International Energy Agency is forecasting an annual worldwide increase of over 55% in energy demand by 2030. This surge, coupled with declining production from many of the world’s major oil fields, will drive up the price of oil. By contrast, alternative energy technologies are becoming more competitive as scale of production grows and prices fall.

The use of biomass is steadily increasing in energy generation. In the domestic sector and small-scale industrial applications, the numbers of large industrial and combined heat and power generation systems are growing. Generally, biomass is considered to be a clean renewable energy source. Emissions are lower when firing biomass instead of fossil fuel, and the amount of SO2 released to the atmosphere is minimal due to the low sulphur content of the fuel. Lifecycle CO2 emissions are virtually zero and, because of the indigenous nature of biomass, its price is not subject to supply uncertainties or fluctuations in the same way as imported fuels.

Although biomass can be fired alone in a modified boiler, technical and economic issues mean that co-firing with coal has proved the most cost-effective solution to date. The challenges in biomass firing, such as slag forming by-products and a high moisture content that produces large volumes of corrosive gas during combustion, push the boundaries of surface engineering. Operators face a number of problems:

• High temperature oxidation and corrosion at 800-1,200ºC on high chrome containing steels.
• Chlorine and alkaline earth metal corrosion.
• Solid particle erosion, including steam erosion during the ‘blow down’ cleaning process.
• Ash settling and the build up of slag and sinters that reduce the heat transfer efficiency in super-heater tubes.

Exposure of super-heater tubes to such environments can result in the loss of the substrate material and eventual thinning of the pipe walls. Given the positive pressure of steam running through the pipes, this can result in catastrophic failure.

Corroding compounds

When wood products are burnt with coal, the sulphur and aluminium silicates from the coal react to form alkali silicates and alkali sulphates releasing hydrochloric acid (HCl) into the flue gases. The alkali metals are bound in high melting point compounds that have no corroding effect, however, significant corrosion can occur when sodium chloride (NaCl) and potassium chloride (KCl) are present. They react with sulphur, aluminium and silicate compounds again releasing HCl which, in turn, can corrode the super-heaters if the chlorine (Cl) condenses onto the super-heater tubes –

2KCl + SO2 + 1/2O2 + H2O → K2SO4 + 2HCl

Al2O3 . 2SiO2 + 2KCl + H2O → K2O . Al2O3 . 2SiO2 + 2HCl

Currently, biomass is limited to around four per cent of feedstock in co-fired power stations because of the severe corrosive nature of these gases and the insulating effect of the build up of slag, which causes increased shutdowns and higher maintenance costs.

Recent field trials in the UK conducted by Monitor Coatings Ltd in 13.5MW and 38.5MW alternative energy power stations that combust pure biomass, have shown that specialist coatings can protect 
components and internal boiler furniture such as super-heater tubes in highly corrosive, high temperature combustion environments.

Pore principles

The coatings produced with arc spray and high velocity oxy-fuel are complemented with a range of over-lay and sealant systems that are capable of functioning in these hostile environments.

Researchers in Japan at Tocalo Co Ltd in Kobe and Hokkaido Electric Power Co Inc in Hokkaido have linked the desired properties of thermally sprayed coatings in boiler applications and made several hypotheses regarding the performance of materials in such demanding environments. 

These include:

• It is likely that the abrasion rate, due to ash erosion, will exceed the growth rate of any protective oxide film forming on the surface of super-heater tubing.
• The erosion resistant layers should be as close to the surface as possible.
• The oxide layer formed upon exposure to the high temperature boiler environment should be low, so that excessive consumption of the coating does not occur.
There are a recognised range of materials, deposited using thermal spray techniques, that are typically applied to boiler components and internal furniture (listed in the box, top right). A common element of these coatings is the intrinsic level of residual porosity present, characteristic of the coating process.

The porosity can be used as an anchor for over-lay coatings and sealants. This also provides a chemically impermeable barrier to the corrosive species present in the flue gas and prevents the permeation of corrosive elements.

Moniplex is a ceramic material usually applied to intersperse in the pores of the base coating. The material is based on a complex chromia-silica matrix typically combined with hard alumina particles. The coating is applied in liquid form by dipping or spraying, and subsequent thermo-chemical treatments produce a totally dense, pore-free layer that is chemically and crystallographically bonded to the substrate. Thickness can be varied between 0.5µm and 25µm with an as-coated surface finish typically of around one micrometre Ra. The hardness of the resulting composite material can be in excess of 2,000 VPN10. Due to the fluid nature of the sealants, they are well suited for application in small internal bores.

On site

In the recent UK trials, sections of a super-heater tube arc-coated with a nickel chrome alloy, over-lay chromia-silica and sealed with a chromia-based sealant showed excellent resistance to flue gases at temperatures in excess of 950ºC generated by firing poultry litter. The chlorine content of the flue gas exceeded 1,000mg/Nm3. A comparison with weld over-lay INCONEL 625 was made, which typically has a life expectancy of less than 1,000 hours in service. The modified coating system was removed after 2,000 hours of exposure. The coated super-heater tubes showed no evidence of corrosion or erosion and, more significantly, there was no slag or sinter build-up on the pipes.

Surface engineering has at least doubled the lifetime of the tubes and will allow the plants to operate at higher temperatures with improved efficiency. Thermal spray coatings have been shown to reduce oxidation and chlorination of the substrate, extending the lifetimes of the tubes. The performance of the coatings correlated to coating composition, density and the presence of sealants. Sealants help to prevent ash build up and solid particle erosion and can be used in conjunction with the sprayed coatings. It is thought that sealants may also reduce the rate of diffusion of gaseous species, such as chlorine, through the coating, which may result in improved lifetimes.

The work has implications for biomass combustion, co-firing applications and waste 
incineration plants, and offers the potential for coatings to mitigate corrosion, ash build up and slag formation, potentially increasing plant lifetimes, operating temperatures and efficiency. Further long-term testing is required to establish the performance and lifetimes of coated components in various biomass environments.

Materials typically applied to boiler components
Cr3C2-NiCr cermets (ceramic-metallic)
NiCrMo (Nickel 625) alloys
FeCr alloys
Nano-crystalline and amorphous coatings, FeCrB
Nano-steel coatings
Inter-metallic aluminide coatings, Fe3Al

Further information: Monitor Coatings Ltd