Surface issues for power - surface engineering for power plant
Dr David Whittaker of DW Associates, Wolverhampton, UK, looks at the surface engineering challenges presented by future systems of energy generation.
During 2007, the UK’s Materials Knowledge Transfer Network (KTN), through PowdermatriX (now known as the Materials KTN’s particulate engineering sector), carried out technology roadmapping analysis to identify opportunities for particulate materials/products to make a significant, in some cases unique, contribution to satisfying the future performance needs of power generation plant. This analysis was based on published information and guidance from industry experts, particularly the results of a parallel exercise carried out by the Energy Materials Group of Materials UK, the Materials KTN policy and legislation sector, aimed at defining a strategic research agenda.
The PowdermatriX analysis culminated in a detailed energy materials roadmap, which led to the February 2008 summary report, The Future of Power Generation – The Role of Particulate Materials.
Many of the identified opportunities for particulate materials involve their use as feedstocks in surface coating technologies.
New generation capacity
Authoritative sources predict substantial investment in new power generation capacity, whether the global market is considered or that of the UK in isolation.
Globally, this investment will be driven by the prediction that demand for electrical power will virtually double by 2030, with major contributions to this trend from developing economies. The anticipated demand increase in the UK will be more modest at around 25% over the same period, but here the new capacity requirement will be driven by the enforced retirement of a significant number of the existing fossil-fuelled and nuclear power plants.
Globally and in the UK, expectations are that 75-80% of new capacity will be based on combustion power generation (see graph, top right), so this is where major opportunities will arise.
This predicted continuing dominance of combustion power generation must be considered alongside the requirement to reduce CO2 and other greenhouse gas emissions. For instance, the UK Government has committed to 60% cuts in emissions from all activities/sources from 1990 levels by 2050, but with a target reduction in the energy sector (electricity generation, and upstream oil and gas production) by over 80%, with intermediate targets of around 15% by 2020 and 42% by 2030.
Efficiency improvements in combustion technology plant could lower emissions by around 20%, and co-firing with biomass might contribute a further 10%. Such measures may therefore help achieve the required levels up to 2020, but for the more aggressive goals beyond that date, carbon capture and storage (CCS) will be necessary.
Substantial developments are needed in all items of plant and, in most cases, surface engineering is expected to play a key role in providing the solutions. The required developments can be summarised as:
• Protecting the surfaces of boiler components against increasingly aggressive operating environments, both in terms of fireside corrosion – arising from the increased use of biomass co-firing and oxy-fuel firing to assist CCS – and of steam side oxidation from the adoption of ultra super critical steam temperatures and pressures to ‘claw back’ the efficiency penalties associated with CCS. A UK Technology Strategy Board-funded project, Advanced Surface Protection to Enable Carbon abatement Technologies (ASPECT), which began in October 2008, aims to provide novel surface coating solutions for these Issues. Partners in the project are Doosan Babcock, E-ON, RWE npower, Sulzer Metco, Monitor Coatings, Cranfield University and the National Physical Laboratory. Details of its proposed technologies and materials cannot be revealed at present.
• The need to protect steam turbine components against more aggressive conditions as a result of the increasing adoption of ultra super critical steam conditions. The prime issues to be addressed relate to steam droplet and particle erosion problems (created by the carry-over of exfoliated steam side oxide particles from the boiler with the steam), steam oxidation and the requirement for enhanced thermal barrier protection.
• Buffering gas turbine components against more aggressive conditions, arising from the need to raise operating temperatures and the desire to increase the use of ‘dirty’ syngas fuels for security of fuel supply. Again, corrosion, oxidation, particle erosion resistance and thermal barrier protection will be key.
The main types of protective coatings for gas turbine components are diffusion coatings, which are formed by surface enrichment of an alloy with aluminium, chromium or silicon, and overlay coatings, which are predominant in gas turbines and are applied by a variety of methods, including thermal and slurry spraying, and physical vapour and weld deposition. Examples of the latter coating include corrosion-resistance coatings, such as MCrAlYs, and thermal barrier coatings, which comprise a ceramic top coat (usually partially stabilised zirconia) attached to the substrate by an oxidation-resistant bond coat, typically an MCrAlY or diffusion aluminide coating.
In the short to medium term, technical advances will cover the application of new coating materials to existing substrate materials, but in the longer term, the substrate and coating materials/technologies will have to provide an overall system solution.
• The need for enhancements in sealing in gas and steam turbines. Surface coatings such as MCrAlY and nickel-graphite are already being applied as abradable rotor tip and rim seals, and research is ongoing to produce improved rotating seal materials. In static sealing situations, wear and fretting problems can be experienced. The development of hard facing systems is seen as a means of alleviating these issues.
• The use of certain surface coating technologies (such as thermal spraying, and weld or laser deposition) to lay down thicker deposits in the repair and refurbishment of certain turbine components.
There are also surface engineering applications associated with a number of the power generation technologies based on renewable sources.
Most of the global capacity increase in generation from ‘renewables’ is predicted to arise from new hydro-power installations. This source of energy uses mature turbine technology. Improving the performance of hydro turbines has been, and will remain, an incremental process of materials and engineering developments. Metal deposition technologies for the repair and refurbishment of components can combat degradation mechanisms such as cavitation, pitting and particle erosion.
Wind power is another alternative, although, its global contribution to the total capacity increase from all sources in the period to 2020 is predicted to only be around 0.1%. In view of the UK’s substantial wind resources, the penetration of wind power in the country’s energy mix can be expected to be more substantial. However, even a 0.1% contribution to total capacity increase equates to around 40,000 new wind turbines, providing a substantial business opportunity.
A major trend in wind turbines is predicted to be the use of synchronous drives without gear boxes (to reduce maintenance requirements). Such turbine drive trains will use substantial amounts of high performance (Nd-Fe-B) permanent magnets, largely in marine operating environments (for example, a three megawatt wind generator would employ around 1.7t of Nd-Fe-B magnets). Surface coating developments will be necessary to enhance the corrosion protection of these magnets.
Wave and tidal technologies comprise another suite of possible systems. Those that involve the building of barrages or containments are close derivatives of hydro-power, with similar material challenges. Others are closer to wind turbines, or employ linear generators that incorporate permanent magnets.
Materials KTN Technology Translators are continuing to work with the power generation supply chain to widen the understanding of the sector’s needs within the surface engineering community. Ultimately, the organisation aims to help identify developments in new materials or technologies that will produce a real difference to performance.
Further information: Dr David Whittaker