Case Study 05
Tilbury Power Station.
Recovery and use of stockpiled PFA.
Introduction.
Coal-fired power stations play a vital role in meeting the UK’s demand for energy, providing up to a third of our needs each year. As a result, fly ash – a secondary raw material produced by the combustion process used at coal-fired power stations – has become an important construction material, used in variety of products, from concrete to bricks, blocks and engineering fill.
Easily accessible and readily available, fly ash – also known as pulverised fuel ash or PFA – has a number of technical, environmental and practical benefits compared with alternatives – and it’s longterm usability means it can be taken either direct from the station or recovered from the UK’s ash fields. An example of this is RWE Generation UK’s Tilbury Power Station, which ceased operation in 2013 but continues to be a plentiful source of coal ash for the construction industry, and in particular, H+H UK Ltd – the UK’s largest manufacturer of aircrete blocks and systems.
Figure 1 – Tilbury Power Station.
Tilbury Power Station.
Tilbury B Power Station stopped generating electricity on 13 August 2013, after 46 years of successful operation. The station was scheduled to close under the EU’s Large Combustion Plant (LCPD) Directive, giving it 20,000 hours of operation from 1 January 2008. After over 40 years as a coal-fired plant, in 2011 Tilbury ceased to burn coal and commenced generation on 100% sustainable biomass – a world first – for the remainder of its LCPD hours.
During operation Tilbury generated up to 1,467MW, which equates to a total of 168.8TWh in total. As a coalfired powered station for the majority of operation, it also generated a minimum of 200kt of fly ash a year. As a result, Tilbury had a well established market for fly ash in the construction industry – but supply nearly always outstripped demand. The surplus ash produced during operation was moved into long-term storage on site and is now contained within Tilbury’s four existing ash fields.
The Project.
Following the closure of Tilbury Power Station, RWE Generation UK has worked to find a way to recover usable fly ash from the station’s ash fields to sustain a viable market from Tilbury – the only reserves of stockpiled fly ash in the south east of England.
Having used fresh Tilbury fly ash (i.e. not stockpiled) since 1996, H+H UK Ltd is by far its biggest ongoing customer, using almost 29kt of fly ash per annum during station operation. The closure of the power station had a huge impact on the fly ash supply chain from Tilbury and the businesses that depended on it as the producer of a reliable, cost-efficient alternative to primary raw materials.
Through a series of investigations and thorough testing, it’s been shown that as a robust aggregate, fly ash stored for long periods can still be suitable for use in a wide variety of construction materials. The stockpiled fly ash on Tilbury’s ash fields was traditionally used as engineering fill for projects such as the widening of the M25. However RWE Generation UK’s ash team, Generation Aggregates, has established a method for carefully excavating, screening and testing stockpiled ash at Tilbury which is safe for use and meets the BS EN13055-1 industry standard for lightweight aggregates in concrete.
Generation Aggregates extracts ash from the ash fields using a 360 excavator and puts it through a 4mm screen to remove inconsistencies. It can also be supplied without being screened, but given the age of some of the recovered ash it can be lumpy, which would restrict it to applications such as filler. It is stockpiled and loaded on to lorries meaning that it can go direct from Tilbury to user without any need for further refinement.
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Conclusions.
The recovery project has been a great success and as a result the station has increased supply to H+H UK fourfold to 400-500 tonnes a day, equivalent to approximately 20 million blocks a year. This has not only ensured fly ash continues to have a viable future but it has also supported a burgeoning recovered ash market and boosted availability in general.
This in turn is supporting the growth of manufacturers like H+H UK and the construction materials market as a whole. With house builders and builders merchants increasing their demand for high quality, sustainable materials, the Tilbury project shows how ash producers and manufacturers are rising to meet this demand and successfully helping to tackle the challenges associated with supply.
Robert Carroll, Technical Director, UK Quality Ash Association (UKQAA): “Supporting the use of recovered ash in the UK is important for maintaining the availability of fly ash. While a number of coal-fired power stations have or will shortly be coming to the end of their operating life and will cease to produce coal ashes there are stockpiles of almost 50m tonnes of usable fly ash available in ash fields across the UK. This is an opportunity for both producers and users to benefit by utilising this large source of environmentally acceptable by-product.
“Thanks to significant work being done by RWE Generation UK and H+H UK Ltd to extract and use fly ash recovered from the ash fields at Tilbury, it’s been shown that there’s a viable future for the material in the construction industry. Coal-fired power stations will therefore play an important role in providing raw material for the construction industry, well beyond decommissioning.”
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The use of fly ash in Aircrete.
H+H UK is the UK’s largest manufacturer of aircrete blocks and systems. A popular building material, the cellular structure of aircrete combines compressive strength with low density which gives a load-bearing but thermally insulating structural unit. A widespread application is the construction of walls and floors within residential and commercial buildings.
The materials used in the manufacture of aircrete blocks are fly ash, sand, cement, lime, aluminium and water. The manufacturing process begins by mixing fly ash, sand and water to form a slurry. The slurry is mixed with cement and lime, and finally a small quantity of aluminium powder is evenly dispersed through the mixture before it is poured into moulds.
The aluminium initiates a chemical reaction, generating the minute bubbles which form the characteristic aerated structure and appearance. When the mixture has partially set the resultant ‘cakes’ are wire-cut into blocks of predetermined size, and transferred to autoclaves for high pressure steam curing. During this process the ingredients combine forming the stable calcium silicate hydrates which generate the compressive strength of the finished product.
Originally manufactured from fresh stocks of fly ash, aircrete can also be manufactured using recovered ash which meet the industry standards BS EN 771-4 and BS EN 13055-1 and retains the same environmental benefits:
- Manufacture is resource-efficient as fly ash is a by-product recovered from a stockpile, rather than a primary raw material that must be quarried
- It has a low embodied carbon
- It has a low thermal conductivity meaning it gives a high level of thermal insulation
Acknowledgements.
The UKQAA would like to thank RWE Generation UK and H+H UK Ltd for their co-operation in producing this Case Study.
Information provided in this document is intended for those who will evaluate its significance and take responsibility for its use and application. UKQAA will accept no liability (including that for negligence) for any loss resulting from the advice or information contained in this document. It is up to the user to ensure they obtain the latest version of this document as the UKQAA revises and updates its publications. Advice should be taken from a competent person before taking or refraining from any action as a result of the comments in this guide which is only intended as a brief introduction to the subject.
In general usage the term ‘fly ash’ is used for pulverized coal ash but it can also cover ash from burning other materials. Such ‘fly ash’ may have significantly differing properties and may not offer the same advantages as ash from burning pulverized coal. UKQAA datasheets only refer to PFA / fly ash produced from the burning of coal in power stations.
V1 November 2014