A typical fly ash stockpile

Credit: UKQAA

As fly ash becomes an increasingly important construction material in the UK market, Dr Robert Carroll explains how both operating and decommissioned coal-fired plants are continuing to create opportunities for growth

Dr Robert Carroll

Given the prevalence of coal-fired power generation worldwide, fly ash is a readily available secondary material which has long been successfully used in the manufacture of construction products.

It therefore plays a vital role in helping to supply demand for minerals and aggregates and supporting established materials markets. In addition to supporting a resurgent global construction industry, it provides commercial opportunities for power companies.

Fly ash has a number of technical, practical and environmental benefits which make it an increasingly appealing alternative to finite primary raw materials in the manufacture of construction products.

Firstly, it’s a fine, lightweight raw material which is used as a constituent to make high-performance and durable construction products. For example, when used in concrete it gives a high strength, low permeability matrix, resilient to chemical attack, which protects steel reinforcement. Fly ash is also used in bricks, blocks, as an engineering fill and in diverse projects from housebuilding and roads to flood defences.

Secondly, thanks to the spread of coal-fired power stations, fly ash is produced at a number of locations across the UK on a regular basis. Stored within silos or relatively shallow ash fields (when compared with quarries), fly ash is easily accessible, less resource-intensive to extract and can be supplied direct from power stations to market on a local or regional basis. This makes it resource-efficient to supply, and significantly reduces the overall carbon footprint of a material supplier and the embodied carbon of the concrete products they produce.

Fly ash has very low embodied carbon dioxide associated with its production. It uses relatively small amounts of energy and water for production and processing. This further reduces its environmental impact if compared with the manufacture of cementious mixes with higher percentages of primary raw materials, and improves the sustainability of the construction product as a whole.

Performance, accessibility and sustainability mean, therefore, that fly ash is a valuable additional resource for the minerals and aggregates market, whether direct from silos or stockpiles.

Environmental implications

Fly ash is a valuable construction material, but it’s also important to bear in mind that the management and storage of fly ash stocks shouldn’t have a detrimental impact on the environment.

In Europe – and the UK specifically – the production, storage and processing of fly ash is regulated carefully to ensure that both producers and users protect the environment, while exploiting the benefits that this valuable by-product offers.

This means that dry fly ash collected in silos for short-term storage, and ash stored in lagoons or ash fields for long-term storage, are carefully managed in accordance with the requirements of environmental regulators. This involves regular monitoring and assessments to ensure that there are no hazards to the environment and to facilitate the recovery of the ash, if required.

Concrete blocks made with fly ash

Credit: H&H

Fly ash is supplied to relevant industrial product standards. For example, fly ash intended as a partial replacement for Portland cement, either within a composite cement or used directly in a concrete mix, complies with the harmonized European standard BS EN 450. Strict specifications and rigourous quality control procedures ensure that a high-quality and consistent raw material is delivered to the customer.

Tilbury Power Station’s stored ash is now used in concrete manufacture

Credit: UKQAA

Is the market sustainable?

In the UK – as with other European countries – due to a combination of emissions reduction targets and competition from alternative energy sources, some coal-fired power stations have closed or will shortly be coming to the end of their operating life. This shift in the energy market will have a significant impact on the industries which rely on traditional forms of energy generation and the supply chains sustained by them. This could potentially affect the future availability of raw materials such as fly ash, and could adversely affect the construction industry.

This change, however, is gradual. One third of our energy need is still met by coal-fired power, and that’s not likely change any time soon. Indeed, the UK government recently held a Capacity Market Auction to allow utilities to bid to supply future energy needs. This resulted in coal and biomass power stations securing contracts under the scheme until 2021. With investment in renewable energy uncertain, new nuclear unavailable for several years and gas susceptible to political and economic unpredictability, coal-fired power is therefore set to continue to be a main provider of energy in the UK for some years to come, securing the production of fresh fly ash for the foreseeable future.

The energy debate aside, at the UKQAA we’re conscious that safeguarding the supply of fly ash is essential to ensuring that it has a sustainable future. Ensuring plentiful supplies of fresh fly ash stored in silos is one thing but successfully assessing, extracting and using stockpile fly ash in ash fields is quite another – and it’s an area we’re pursuing.

Are ash fields the answer?

With a strong heritage in coal power in the UK, over the past half-century we’ve produced far more fly ash than the construction industry could ever use. Indeed, in recent years, on average, of the approximately six million tonnes of fly ash produced annually, typically two million tonnes has been surplus to demand. Over the years, this has amounted to some 50 million tonnes of potentially usable fly ash being stored in ash fields across the UK. These stockpiles of untapped raw material therefore represent huge opportunities for the ash and construction industries, from the point of production and manufacture right through to design and specification.

For example, Tilbury power station in southeast England ceased operation in 2013, after almost half a century of generation. Rather than decommission and landscape the ash fields, station owner RWE Generation took the opportunity to make use of the surplus fly ash stocks lying dormant in the ash fields by developing a process by which stockpiled fly ash, some of it five to 10 years old or more, could be used to support well-established supply chains. By carefully excavating, screening and testing ash recovered at Tilbury, the team has successfully ensured that it meets BS EN 13055-1, the industry standard for lightweight aggregates in concrete. As a result, the ash recovery programme has supplied construction firm H&H – one of the primary users of Tilbury ash – with up to 500 tonnes of quality ash per day, creating 20 million aerated concrete blocks a year.

What this project shows is that fly ash recovery from stockpiles offers immediate benefits for both users and producers of fly ash – but also highlights the potential for novel extraction and processing schemes, including the Innovative Processing Stockpile Ash Project at the University of Dundee.

This pioneering research project into the recovery and use of stockpile fly ash has a primary objective to develop a process route to transform stockpile ash into fly ash which is suitable for structural concrete.

Working in partnership with the Concrete Technology Unit (CTU) at the University of Dundee, the UKQAA is working to, first, establish an accurate picture of how much stockpiled ash is available for recovery in the UK, and then to analyze what proportion of this could be reclaimed. This involves collecting samples from across UK ash fields and testing them in laboratories to assess their performance as pozzolanas, when used as a partial cement replacement in concrete. The CTU will work with the UKQAA to develop a process route to transform usable stockpile ash into fly ash which meets the specification of BS EN 450, the recognized standard for fly ash to be used as a constituent in cement and concrete.

The research project will take three and a half years to complete, and will provide significant insight into the availability of usable fly ash in the UK and its possible applications. It will potentially identify significant opportunities for power station owners and create new supply chains, while boosting availability in the construction materials market.

This work underlines the importance that innovation and ingenuity can play in maintaining fly ash as a valuable asset. Not only does it help to future-proof supply and sustain confidence in the market, but it also suggests that there is opportunity beyond the decommissioning of a coal-fired power station.

Good practice currently used in the UK highlights the safe management and effective storage of fly ash, enabling exploitation of available material. This redefines the role that power stations can play, from energy generator to raw materials supplier.

Dr Robert Carroll is Technical Director of the UK Quality Ash Association (UKQAA). For more information on the UKQAA or how you can make use of fly, please visit www.ukqaa.org.uk.