White Rose: hoping to minimise CCS risks with oxycombustion21 May 2014
The oxycombustion technology proposed for the White Rose project, profiled here, may offer a possible way forward for pulverised coal power generation in the UK and elsewhere.
The White Rose oxycombustion carbon capture and storage project is one of two remaining contenders in the UK government's CCS Commercialisation Programme (the other being the Peterhead project, which envisages adding post combustion carbon capture to a gas fuelled combined cycle plant). White Rose is also the only CCS project still remaining in the running for support under phase 2 of the EU NER300 scheme, with evaluation expected to be complete by mid 2014.
The White Rose proposal is to build an advanced ultrasupercritical 426 MWe (gross) coal fired oxycombustion demonstration power plant, with full carbon capture and storage from the outset (resulting in a total parasitic load for the power plant of about 120 MW).
White Rose would be one of the world's first large scale oxycombustion plants. The use of 700°C steam has also been mentioned, which would be a world first in itself, necessitating advanced alloys.
The plant would also have the potential to co-fire biomass, up to about 10%, alongside coal.
Why White Rose?
Among the basic objectives of the White Rose plant are the following:
- To demonstrate that oxycombustion CCS technology can be used to generate reliable, flexible and affordable low carbon electricity.
- To help improve the UK's security of supply by providing a low carbon generation option using coal, which is relatively cheap.
- To act as an anchor project for the development of a carbon dioxide transport and storage network in the UK's most energy intensive region. The project proponents insist White Rose should not be seen as a one-off isolated project, but part of a cluster, with a cluster scaled CO2 pipeline.
Located on the 2000 acre Drax power station site near Selby in North Yorkshire, the CCS technology at White Rose would be capable of capturing two million tonnes of CO2 per year, some 90% of all CO2 emissions produced by the plant. Should biomass be co-fired with coal, White Rose could emit zero or even negative net CO2 emissions to the atmosphere.
To be built alongside the larger (6 x 660 MWe) existing coal power station, which is in the process of being converted to a predominantly biomass fuelled facility (with one unit fully converted and work underway on two more), White Rose would have a total footprint of approximately 27 hectares, providing enough space for the infrastructure that will include a boiler house, turbine hall, chimney stack, hybrid air and water cooling system and the CCS equipment.
Capture Power, the consortium proposing the White Rose project, has been formed by three companies, Alstom, Drax and BOC (member of the Linde Group).
Alstom would be responsible for construction and Drax for the operation and maintenance of the power plant including the CO2 capture facilities.
BOC would oversee construction, operation and maintenance of the cryogenic air separation unit (the largest ever built in the UK) that would provide oxygen for plant operations.
The project can trace its origins to some work Drax and National Grid did about six years ago to explore the potential for disposing of the CO2 off the eastern coast of the UK. This found that saline aquifers off the coast near the Humber estuary looked promising, and would be a feasible possibility for the Drax power plant.
Drax also looked into capture technologies and concluded that oxycombustion seemed to offer a relatively low risk profile, and Alstom was found to share this view. BOC subsequently joined the consortium (about three years ago), to provide the ASU technology.
The project is structured so that National Grid acts as a subcontractor to Capture Power, providing carbon dioxide transport and storage services.
The oxycombustion process proposed has a number of distinct advantages, proponents of the project believe:
- oxycombustion is very similar to conventional air-fired coal plant boiler practice and is developed from well-known systems and processes;
- the air and gas separation units have already been developed as part of other industrial processes;
- compared with post-combustion technologies, the oxycombustion process does not require large quantities of new chemicals on site;
- NOx emissions are inherently extremely low;
- oxycombustion technology has the potential to be retrofitted to existing plant if sufficient land is available; and
- the component technology has been proven through various smaller pilot projects around the world, notably the 30 MWt Schwarze Pumpe test boiler.
Overall, the key driver for selecting oxycombustion is that CCS technology risk is minimised compared with other capture options, the White Rose consortium thinks (although aiming for 700°C steam would appear to introduce technology risks of another kind).
The oxycombustion process entails using oxygen mixed with recycled CO2 instead of air for the combustion process. This essentially eliminates NOx from the system, resulting in a flue gas consisting of CO2, water and some inert gases (although the original planning application showed an SCR de NOx system in the power plant scheme, with presumably the possibility of residual NOx arising from nitrogen in the coal and air inleakage).
The CO2 rich flue gas is then further treated and compressed before being transported for storage.
Oxycombustion CCS technology requires additional infrastructure to be added to that typically used at conventional coal-fired power stations. The air separation unit produces nearly pure oxygen from air and a gas processing unit (GPU) is also required to clean and compress the captured carbon dioxide to meet the transport pipeline specifications.
The gas processing unit perhaps constitutes one of the highest technology risks associated with the oxycombustion CCS scheme as low levels of contaminants in the CO2 will cause corrosion. The gas processing unit technology is being studied in depth, taking on board lessons from Schwarze Pumpe experience in particular.
A few deviations from conventional pulverised coal power plant technology are also required. Some of the CO2 rich flue gas is partially recycled to maintain the required temperature and heat absorption rates in the boiler, while water needs to be removed from the flue gas, in a flue gas condenser.
Also, air leakage into the boiler and flue gas system has to be minimised so that the process remains efficient and NOx is not produced.
Most boilers operate at negative pressure so maintaining airtightness is crucial for the oxycombustion process (and would be an issue when considering oxycombustion retrofit for older boilers).
Yorkshire and Humber CCS trunkline project
In a separate but associated project, the Yorkshire and Humber CCS trunkline project, led by National Grid, the CO2 captured at White Rose will be transported by pipeline to a permanent geological storage site beneath the North Sea.
The onshore pipeline would be 75 km long and would use the same sort of technology as the UK national high-pressure gas pipeline network, owned and operated by National Grid. It would be up to 24 in (about 600 mm) in diameter and buried at least 1.2 metres below the ground. The CO2 would be transported in liquid form at a pressure of 150 barg.
The sub-sea pipeline would be the same size and on the seabed. Offshore, the CO2 would be further transported at a pressure of up to 200 barg to the storage site.
It is anticipated that the pipeline would have the capacity to transport up to 17 million tonnes of CO2 every year. The Yorkshire and Humber region is considered an ideal location for CCS project proposals due to the number of power stations and large industrial plants in relatively close proximity.
A CCS pipeline network connecting such facilities could help to develop CCS technology for other projects. White Rose would act as the anchor project and potential catalyst for development of a regional CCS pipeline network.
A report published by the research organisation CO2Sense suggests that a Yorkshire and Humber CCS cluster has the potential to cut the UK's CO2 emissions by up to 19% by transforming one of the UK's highest emitting regions into one of the lowest.
National Grid has completed an assessment of potential storage sites in the southern North Sea (starting with 257 wells in the target area) and has shortlisted two key sites for further appraisal, which is ongoing. Drilling of a first appraisal well was completed in summer 2013.
Public consultation is underway on the onshore pipeline route.
UK government policy
The White Rose and Yorkshire and Humber CCS trunkline projects are driven by evolving UK government policy that recognises there is an urgent requirement to develop new sources of low carbon electricity generation as energy demand continues to rise in the future.
With older coal-fired and nuclear power stations due for closure in the next few years, the UK government believes that up to 13 GW of new CCS plants could be deployed by 2030, rising to 40 GW by 2050. That could represent a significant element of all UK electricity demand, with 2013 demand peaking at approximately 50 GW.
The UK government's wider strategy to develop a world-leading CCS industry is explained in the UK CCS Roadmap published by the Department of Energy & Climate Change in 2012, which suggested that carbon capture and storage "has the potential to be one of the most cost effective technologies for the decarbonisation of the UK's power and industrial sectors, as well as those of economies worldwide." The CCS Roadmap contained a number of commitments, including:
- creating an electricity market that will enable CCS to compete with other low carbon technologies;
- launching the CCS Commercialisation Programme, with £1bn of capital support;
- working closely with industry to reduce costs, including through the establishment of a CCS cost reduction task force;
- removing barriers and obstacles to CCS deployment; and
- promoting the sharing of knowledge to accelerate deployment.
In December 2013, the government announced that the White Rose CCS project had been awarded multi-million pound front end engineering and design (FEED) study funding under the CCS Commercialisation Programme that will enable de-risking of the project before any final investment decision is taken. The project will need private funding and must be shown to be "bankable".
White Rose was the first project to be allocated funds under the aforementioned Commercialisation Programme, with the Peterhead announcement following a couple of months later. The FEED contract scope also includes development of the CO2 transportation and storage concept - the Yorkshire Humber CCS trunkline - to be undertaken by National Grid Carbon Limited.
Permitting for White Rose
Because the proposed White Rose plant exceeds 50 MWe output, it is classified as a nationally significant infrastructure project (NSIP). As an NSIP, a planning application will be made by means of a development consent order (DCO) via the Planning Inspectorate rather than being subject to local planning arrangements.
It is expected that a DCO application will be submitted in late 2014 to the Planning Inspectorate, which will consider the merits of the proposal before making a recommendation to the Secretary of State at the Department of Energy & Climate Change (DECC) who will make the final decision about whether White Rose gets the green light or not.
Between now and the submission of the DCO application, Capture Power will be facilitating an extensive consultation exercise with the local community to explain the purpose and details of the project and to consider any suggestions that might be put forward by interested individuals and organisations.
Technical consultation addressing issues such as visual impact, ecology, noise, traffic and air quality will also take place with statutory consultees including the neighbouring local authorities and other prescribed organisations.
Should White Rose gain planning consent, and subject to project funding being agreed, it is anticipated that a four-year construction programme could begin in 2015 enabling the plant to start generating power in 2019. It is expected that White Rose would have an operational life of at least 25 years.
Sources for this article included Capture Power Ltd and a presentation by Peter Emery of Drax, "White Rose carbon capture and storage project" at the New Age for Coal with Carbon Capture and Storage conference organised by the Society of Chemical Industry, London, UK, 7 November 2013.