By: Tony Fry, National Physical Laboratory, UK

Although new fossil fuel burning power stations have a high efficiency, this is at the expense of harsh operation conditions – high temperatures and pressures – which can increase the risk of corrosion and premature component failure. Current models cannot predict how the materials used in such components will react to these conditions. Now a joint academic and business project has come to the rescue.

A new breed of more efficient power stations could be the answer to reducing the environmental impact of burning fossil fuels. But they create an unfamiliar environment for existing materials, which could increase the risk of corrosion and premature component failure.

Current models cannot predict how materials will react to these conditions. This has prompted new research into modelling material corrosion in advanced power stations by the National Physical Laboratory (NPL) in partnership with an academic institution and three commercial companies.

It aims to provide tools for the power industry to make informed, confident decisions about the impact of new conditions on different materials.

Change in power generation

The publication of the 2007 Energy White Paper confirms that the way the UK generates power is set to change. The government reached a view that it would be in the pubic interest to allow energy companies to invest in nuclear power and reduced its emphasis on conventional fossil fuels. It also suggested it would do everything it could to encourage investment in renewable sources of energy and made particular reference to supporting emerging technologies. The changes are the result of an increasing need to meet aggressive targets for reducing carbon emissions and the decrease in available oil and gas resources.

A clean coal plant in midwestern USA on a cold day Source: A. Olsen
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As the country starts to turn its attention to nuclear the government has also recognized that building the required infrastructure for next generation nuclear power stations could take in excess of 20 years – possibly too long for it to hit the 2050 target of a 60 per cent reduction in carbon emissions on the levels in 1990. During that time, power must continue to be generated through alternative methods, which can continue to help reduce emissions. These include renewable sources of energy such as wind power and conventional fossil fuel burning.

Renewable energy can support existing methods of power production, but the implementation costs are vast, the planning processes are complex, and they require a backup if, for example, the wind stops blowing. This means they cannot become a viable option as the UK’s primary energy source.

Fossil fuels, and coal in particular, will therefore have an important part to play in securing future energy supplies. Coal fired power stations still generate a third of the electricity needed to power the country. They are attractive because, unlike renewable plants, they are flexible and can be turned on and off quickly to meet the changing demands of the population.

So the job today is to reduce the carbon emissions from existing power plants. Both the government and the power industry are committed to tackling this challenge. The search for a cleaner way to burn fossil fuels has become a priority.

Options for cleaner burning

There are already several possible approaches. Supercritical and ultrasupercritical power stations are a new breed of power plant running at higher temperatures to increase efficiency. Biomass co-fired power stations mix crops with coal because burning crops is carbon neutral as they have already drawn carbon dioxide from the atmosphere. Other methods include oxy-fuel firing and using scrubbers to capture and compress carbon dioxide (CO2) plants funnel emissions (known as flue gases) back through the boilers, concentrating the CO2 so it can be pumped away for storage as a liquid or solid rather than being released as a gas.

These processes could help reduce emissions, but they each present unfamiliar and potentially very aggressive environments for boiler components. They increase the risk of higher corrosion rates and premature component failure. Ultrasupercritical power stations can run at temperatures of more than 100oC, hotter than traditional sites, and biomass co-firing produces noxious substances.

Coal coming from a conveyor belt Source: O. Lantzendörffer
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These new approaches could increase efficiency and reduce CO2 emissions, but they are also changing the way that our power is produced. Central to their development is new technology and readily available materials that can operate at increasingly higher temperatures and pressures.

To understand the impact of these more aggressive conditions, the NPL the UK’s National Measurement Institute, in partnership with Cranfield University, RWE npower, E.ON and Doosan Babcock Energy Ltd, will develop new methods for predicting the corrosion behaviour of high temperature materials in coal fired utility boilers.

The three-year research project, which is supported by the Technology Programme of the Department for Business, Enterprise and Regulatory Reform – formerly the Department of Trade and Industry – is being led by E.ON Power Technology.

Project’s goals

Each partner brings considerable experience to the project. E.ON, RWE npower and Doosan Babcock will provide empirical data on the corrosion rates inside boilers. They will insert probes into existing power station boilers. Experiments will also be run on E.ON’s combustion test facility – a large-scale model boiler that can simulate any possible method of energy generation without the risk of affecting consumer energy supplies. The facility reproduces accurate scale versions of real plant environments, using validated data obtained from coal, gas, oil and renewables power plants.

The results enabled predications about the behaviour and characteristics of fuels in full size plant worldwide and the evaluation and testing of all types of approaches for power generation. It enables users to combine different fuels with burner types and firing geometries to test a huge range of conditions.

Its wide range of applications includes investigating the most intense combustion regions and examining flue gas treatment technologies. It is designed to reproduce flame conditions near to the burner and give realistic in-furnace residence times.

Cranfield University of the United Kingdom will validate the empirical data from the power companies and conduct its own laboratory simulations to provide further corrosion rate data for consideration.

NPL will collate all the new and historic corrosion rate data into a searchable database. It will then use its expertise in modelling and neural networks to identify those parameters with the strongest links to corrosion rates. It is the relationships between these that will define how the new models should be developed. Cranfield University and NPL will work together to consider these relationships, refine them and convert them into viable models, which can predict the outcomes of changes in the selected parameters.

Project’s deliverables

The project by NPL and Cranfield Universtity has five deliverables as listed below:

A review of existing models – the team will identify how robust they are, how compatible they are to the project outcomes and what range of methods for power generation they cover. The team needs to understand what existing models can tell us about new methods of power generation.

Plant database – a database of information about the plant operational and environmental parameters influencing fireside corrosion rates will help to define the experimental conditions under which corrosion rates need to be measured. This database will comprise very granular information about each approach to power generation.

Corrosion database – a database of measured corrosion rates under a variety of plant operating and environmental conditions relevant to the proposed new methods of power generation. New models for fireside corrosion – NPL and Cranfield University will lead the development of new integrated fireside corrosion models.

Together they will analyse the corrosion database and identify the most relevant variables to consider when assessing the suitability of a particular material for advanced methods of power generation. The new model will be available to industry. It will help determine corrosion rates and therefore life expectancy and possible breakdown rates, helping power companies select the most appropriate materials for the application in question.

Corrosion guidelines document – the final deliverable will be a guide to using the new models. This ensures they are applied correctly and deliver accurate information.

Over the last five years, companies in the power industry have worked closely with each other and the government to embrace the responsibility of looking at greener alternatives to burning coal. They have increased investment in the research and development of new technologies and ways to make them work for both the industry and the planet.

Future coal fired power stations will be configured to minimise carbon dioxide emissions and be ready for carbon capture. Developing new models mean the power industry will finally have a chance to understand the impact of these changes on the materials and components in their power stations, taking greater confidence in the choices they make.

The NPL and its partners are leading the drive to develop these models. The combined expertise of this research team is expected to deliver results that will move industry’s perception of advanced power stations from caution to confidence.