Investigating Corrosion in Supercritical Fluids

This project proposes to study how turbine materials may fail in a new specialist energy production environment. The materials to be studied are superalloys, which are nickel- and cobalt-based alloys that can resist and work at high temperatures. These superalloys can be coated with thin ceramic layers, known as a thermal barrier coating (TBC), or metallic layers, to help protect them when they are in a high temperature environment.

It is important to know how the materials respond because they may be used in a new type of power plant which will expose them to an environment which is very unusual. This power cycle burns a fuel to drive a turbine and generate electricity when there is demand. The fuel can be natural gas or a synthetic gas made from coal, biomass or waste meaning that fuel supply is secure and power can be dispatched when needed. Unlike current power cycles using combustion, only oxygen, rather than air, is present with the fuel meaning carbon dioxide (CO2) and steam are the main products. The steam can be condensed out, and the CO2 kept. The CO2 then flows past the turbine at such high temperatures and pressures that it enters a special condition where it is neither liquid nor gas and is called supercritical-CO2.

This new type of power cycle has many potential advantages. As the superciritcal-CO2 is very dense, it's very good at pushing the turbine, so the energy from burning the fuel has a high efficiency of conversion into electricity, meaning electricity may be cheaper. It also means the turbine can be very small compared to other power cycles, so the new power plant can fit into small parcels of land, and can be put next to existing industrial structures for localised power generation. Finally, because the CO2 from burning the fuel is captured to drive the turbine, it doesn't have to be released into the atmosphere where it may contribute to climate change. Instead this CO2 can be captured, transported and used or stored. CO2 can be captured at 99% purity; this is better than specialist plant trying to remove CO2 from other power cycles, which aim to have 90% CO2 purity. This means the cycle can help make low-CO2 power while other renewable energy sources and storage options are developed.

However, in the supercritical-CO2 going through the turbine, there can be small amounts of chemical contaminants that can degrade the materials it is made from. As this power cycle recycles CO2 before transportation and use (it is a 'semi-closed' system), these chemicals can build up in concentration. To make sure that the plant built lasts for a long time and that there are no unexpected interruptions to power generation, it is important to know whether the turbine materials can survive these conditions as the supercritical-CO2 is at very high temperatures and pressures. By investigating the reliability of these materials, we can contribute to the confidence in these new, cleaner energy production systems, driving investment in and the spread of these options, rather than other cycles which may give off more CO2.

To meet this project's aim of understanding materials' degradation in this contaminated supercritical-CO2 operating environment experimental research much be carried out. Superalloy and ceramic coated samples will be provided by industry (see letters of support). These will be exposed at high temperatures (metal samples at 800-1000 C; TBC samples at 1100 C to simulate the cooling gradient anticipated through the component under operational conditions), high pressures (300 bar) and with chemical contaminants (such as H2O, SOX and NOX). Different superalloys will be used to see how differences in their chemistry, manufacturing and internal microstructure alters their reaction with supercritical-CO2. After a thousand hours exposure the samples will be looked at using specialist microscopy techniques to see how much metal has been lost and if any changes have taken place with the internal structure.

This project's impact is in providing enhanced understanding of long-term degradation mechanisms for turbine materials to inform the construction and operation of advanced, high efficiency, low plant-footprint advanced semi-closed supercritical-CO2 cycles. Despite the power cycle's potential, little is known about long-term effects on materials exposed to such operating conditions. Developing this understanding is especially important for the turbine's first stage, where blades experience combinations of high temperatures, pressures and corrosive contaminants. This research will provide confidence in the long-term use of specific superalloys and coatings, and oxidation/carburisation data obtained will directly impact industry, allowing the benefits of semi-closed supercritical CO2 power generation cycles to be realised, to the benefit of a wide range of stakeholders. Impact includes:

*Business/industry. The Materials and Manufacturing KTN vision places the UK at the forefront of new industries and associated supply chains by building on materials research. Examples of supply chain beneficiaries to this project include original equipment manufacturers (e.g. Siemens Corporation, Siemens Industrial Turbomachinery, Doosan Babcock Technology and Engineering), power generators, and companies supplying specialist components or maintenance services. Industry will benefit from long-term degradation data under simulated supercritical power generation cycle conditions and improved understanding of fundamental degradation mechanisms. This will aid future plant design, engagement in plant supply chains and inform safe operating guidance. Globally, research into this cycle is taking place in the USA. UK companies would benefit from access to materials performance know-how at higher temperatures than currently exists and the quantification of coatings to compete for work, thus this research would help keep UK research and industry at the forefront of new developments and contribute to UK economic success. Further, as this cycle has a small plant footprint, it can be located close to existing plant, or to novel fuel sources, allowing industries to integrate this technology, when developed, onto existing sites where space is often at a premium, prohibiting the use of other generation cycles.

*The Public Sector. Knowledge generated will provide governmental energy policy makers with more options when choosing which power generation methods to encourage. When setting policy, they must consider the cost, supply security and environmental impact. Uptake of this cycle can help lower CO2 emissions (in line with UK legislation e.g. the Climate Change Act and ratification of the Paris Treaty to cut greenhouse gases linked to limiting climate change). Additionally, the cycle's fuel flexibility (natural gas, coal, biomass and wastes) improves the UK's energy portfolio security and does not require the development of new energy storage technologies unlike other proposed low-carbon power generation techniques. Thus, this cycle can contribute to the UK's Industrial Strategy "Building a Britain Fit for the Future" by producing clean energy to drive growth.

*The general public. Semi-closed supercritical-CO2 based power cycles can be coupled to carbon capture and utilisation technologies for a clean energy supply. Benefits can be maximised by using alternative fuel sources to further reduce CO2 emissions and thus climate change. Fuels include biomass; conventionally considered a low carbon fuel as CO2 given off is assumed to be reabsorbed when the next crop grows. Using alternative fuel sources increases energy security by reducing dependence on a single source, reducing volatility in availability or cost of energy. Another advantage for the general public is related to the high efficiency of the cycle (generation efficiencies of 52-59% compared to 33-40% in coal plant), which has the potential to reduce power costs and thus lower electricity bills.