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Uses of PFA

 

A European Issue

Approximately 44 million tonnes of fly ash were produced in Europe (EU15) in 2003 of which 21.1 million tonnes (48%) were utilised, leaving about 23 million tonnes to dispose of.

Approximately 20 million tonnes of PFA were produced in the same year by the new 10 EU member states.

Production of PFA in the UK alone is in the region of 10 million tonnes per year

Construction

PFA has many uses in the construction industry: load bearing fill, concrete and cement manufacture, grouting, lightweight aggregate, cement-stabilised PFA for hard standings etc, building blocks as well as many other more specialised uses. Dry PFA combines with free lime released during the hydration of Portland cement by pozzolanic reaction. This allows it to replace part of the cement component in concrete, offering both technical and economic benefits. Technology has recently been developed to manufacture high quality bricks from PFA using a sintering process.

 

The following table lists some large-scale uses of PFA. In most cases the PFA-based products give superior performance to those based solely on conventional materials.

Application

Details

Bulk poured concrete Concrete containing fuel ash achieve lower hydration temperatures than conventional concrete making them more suitable for large solid structures requiring high integrity.
Roller Compacted Concrete (RCC)
450,000 cu.m. Predominantly constructed of 4.5:1 brown-coal fly ash to portland cement RCC.
Underwater concrete
Concrete basements constructed from special steel fibre underwater concrete containing up to 280 kg/cu.m hard coal fly ash, using about 100,000 tonnes of PFA in total
Weather resistant concrete
Constructed in the early 1980s.

Lightweight aggregate (LWA) and Lightweight aggregate concrete (LWAGfor bridge construction, floorings in high-rise buildings, etc.

Sintered fly ash pellets can be used to make artificial gravel with an average density of 850kg/cu.m which in turn makes lightweight high-strength concrete achieving weight reductions of up to 20% compared with gravel concrete of comparable strength.
Grouts for void filling, structural repairs, subsidence prevention, filling of old mine workings, etc.
£33M project utilising PFA from Drax Power Station (more details)
Highway construction

PFA and FBA can be used in the sub-base construction of highways to achieve superior load-bearing strength compared with conventional natural aggregates.

Fly ash bound mixtures (FABM) have been used for roadbase, sub-base and capping in road construction in the UK.

Fly ash can be used as a filler in ashphalt as an alternative to ground limestone.

The 1.4km Ramsgate Harbour approach road, constructed c 2000, used approximately 430 tonnes of PFA mainly sourced from Didcot.

Highway Safety Barriers

Concrete step barriers are currently are currently being installed to replace the familiar steel safety barriers along central reservations on motorways and dual carriageways as the old barriers come to the end of their lives. PFA both can and should be used in the manufacture of this concrete thus reducing the environmental impact of the construction.

Some 5,000 miles (at least) of barrier can be expected to be replaced over the next 20 years – and that’s not including barriers along road edges and along new build.

 

High Performance Concrete PFA is used in the manufacture of high performance concretes offering high acid resistance (SRB) and reststance against chemical attack, abrasion and frost.
Sulphate resistant cement compounds
 
Concrete blocks

PFA is a major component of Thermalite building blocks.

See also geopolymer concrete below.

Geopolymer concrete

E-crete, a new cheap and green construction material that is currently being manufactured and marketed in Australia, initially for small-scale low-performance-demand applications.
Land restoration Utilised 800,000 tonnes of PFA for ground remediation, lightweight engineering fill, lime/cement stabilised sub-base material and landscaping.
General Fill Utilised 100,000 tonnes of PFA from Ironbridge Power Station to raise the ground level of the site.
 

Industrial scale reprocessing of PFA into high quality building materials, such as high performance concretes, special cements, and blocks, is both technically feasible and well-proven and is being promoted in RWE’s own home country, Germany. The resultant product(s)can be used as cement and concrete substitutes, and as substitutes for, or supplements to, traditional building materials, such as clay bricks and concrete blocks. Because of its low porosity and chemical resistance, it can be used as a liner for waste pits and as a means of immobilising toxic waste, at the same time as helping to solve the the PFA “waste” disposal problem itself.

In Britain, the RockTron beneficiation process, another well established technology which has been available for about 15 years, is capable of recovering saleable products from any power station ash ash with no waste, ie all the ash is recycled. Recovered products include high-grade carbon products, pozzolanic cementaceous products with <2% LOI, and cenospheres. The process can be applied to dry or wet ash, including ash recovered from landfill. The cementaceous products can replace products that would otherwise have to be produced by energy-expensive kilning of limestone and thus contribute to a low-carbon economy.

In Australia, scientists have developed ways of making bricks and aggregates entirely from fly ash, using a technology that has been licensed to British and US markets.

A promising new technology for the manufacture of concrete, without using any portland cement at all, is e-crete, which is a form of concrete in which the silicates and aluminates in fly ash and slag waste are polymerised by the addition of alkali. The manufacture of e-crete does not involve the large amounts of energy consumption and CO2 production involved in the production of ordinary concrete. It is therefore both cheap and ‘green’, and is currently being manufactured in Australia for use (initially) in small-scale low-performance-demand applications, until its durability and usability are better proven. A considerable amount of interest has been generated in this product by the article which appeared in New Scientist in January 2008. E-crete may not be a completely proven technology yet, although there is evidence from analyses of alkaline slag-based concretes used in 40-year old constructions in the Soviet Union that it will prove to be as durable as ordinary concrete. It also yet again serves to illustrate the potential of fly ash as a raw material.

Concrete step barriers (CSBs) are currently being constructed along Britain’s motoways and trunk roads to replace the familiar steel safety barriers. CSBs offer improved performance, durability, and lifespan compared with steel barriers. They are maintenance free, do not generally need replacing or repairing following accidents, and are expected to last 50 years (compared with 20 years for steel barriers, which invariably need replacing following accidents.) They also offer improved safety performance due to greater impact strength and the absence of posts presents a reduced hazard to road users, especially motorcycles.

Concrete safety barriers are constructed by means of a continuous slip-forming process, which requires that the rheological properties of the concrete need to be tightly specified. Cement replacements, including fly ash, are deemed suitable for CSB applicationsThe specification also requires that the construction should be sustainable and that recyclable materials should be used. Many thousands of miles of CSBs must therefore be constructed over the next 15-20 years and we would expect that this would extensively exploit available supplies of PFA, and other cement substitutes, over that period.

The manufacture of bricks from PFA is now being promoted, in other countries, notably the USA (where building materials that contain coal ash are considered to be “green” products in a market that is receptive to green issues) as well as India and China, where PFA disposal is particularly problematic, as a means of reducing atmospheric pollution (from conventional brick manufacture) and water pollution (from disposal of PFA in landfill). It is being presented as a green and sustainable means of reducing the impact of coal burning on the environment.

This possibility is also important for the UK. The manufacture of bricks and light-weight aggregates from PFA is perhaps its single most important application. UK Government wishes to build around 300,000 new homes in the SE in the next 10 years, and bricks will undoubtedly be one of the main construction materials. Also, new infrastructure will be required to support the needs of the people that will occupy these homes. This will require concrete and aggregates.PFA bricks, for example, are superior, in virtually every every respect, to conventional clay bricks: they are stronger and lighter, adhere better to ordinary mortar and require less energy (and hence produce less CO2) for their manufacture and transportation. They possess low water absorbency, are highly resistent to frost and chemical attack. They can be manufactured in virtually any colour, shape and surface texture. Moreover the raw material, fly ash, comes at “zero cost”. The potential of this application for exploiting PFA produced by Britain’s power stations cannot be over emphasised. Unlike concrete manufacture, which cannot guarantee a steady a market for PFA and which requires major projects to take appreciable amounts of ash, brick manufacture can easily soak up all of the country’s PFA production, both in the future as well as any that is currently “stockpiled” in mounds or landfill.

The current practice of digging up the countryside to extract primary aggregates, and clay (for brick manufacture) only to have to fill the holes back up with PFA, is madness. Government needs to follow the example of other countries and introduce stronger incentives to ensure the take up of fly-ash-based materials by the construction industry, in line with existing policy.

PFA is not waste. It is in fact a valuable raw material that is being allowed to go to waste at the expense of causing unnecessary further damage to the environment by clay, sand and gravel extraction.

Other Applications

Other uses of PFA include

  • Mineral filler for plastics, paints, resins etc
  • Soil conditioning

Cenospheres

Cenospheres are a particularly valuable product used in pattern and mould making, foam-filled glass-fibre panels, fire-resistant coatings, urethanes, PVC frames, putty etc etc. Cenospheres are used in the manufacture of the Space Shuttle’s heat shield.

Why is PFA being Under-utilised in the UK?

RWE npower say that they are unable to dispose of most of their PFA on the market and are thus forced to throw it away. Four reasons for this are put forward: There are also two further reasons.

  1. Lack of demand/market capacity. While Didcot cannot force construction companies/concrete manufacturers to buy from them, it is clear that PFA is an under-utilised valuable resource. PFA offers an oft-superior alternative to sand, gravel and limestone, which are nevertheless being extensively mined throughout the country. The Government should apply some pressure to change this. However power station operators could be more proactive in seeking out and exploiting new opportunities.
  2. Didcot PFA, due to high LOI content, fails to meet the BS EN 206, BS 8500 standards for pozzolanic materials suitable for concrete manufacture, which considerably reduces its value. Clean PFA from continental power stations is preferred, and is even being imported for this reason. This situation could be remedied by more investment, eg in ash benefication or the deployment of catalytic removal of NOx from flue emissions. The Government needs to apply some incentives for the power companies to invest in these technologies, eg item 4 below.
  3. The very low rate of landfill tax paid by Power Stations. This is a mere £2 per tonne. If this were increased to say £20 per tonne, it would have little impact on the cost of electricity, but it would be a powerful incentive for the generating companies to invest in more sustainable methods of disposal.
  4. The classification of PFA in the UK as waste (See below). This means that anyone taking PFA for recycling has to comply with a raft of unnecessary waste handling legislation. This adds considerable cost and inconvenience and generally mitigates against reuse.
  5. However this obstacle may be a more of an excuse put forward by power station operators, rather than a valid reason why their PFA is under-utilised. Waste legislation does effectively prevent the unecessary transportation and potentially damaging dumping of unwanted PFA, which is wholly a good thing. The co-location of processing plant on power station sites generally obviates these problems, provided that the would-be operators of such plant can gain access to such a site and to the PFA. This is a major problem for companies wishing to engage in PFA reprocessing, as it requires the cooperation of the power station operators. Government could provide more incentives to ensure that this occurs.
  6. Finally, the power station operators generally have very little incentive to investigate or invest in PFA recycling technologies. If they already have a viable 24/7 disposal option, which, as we have found, they will go to any lengths to keep, they are simply not interested.

Is PFA Waste?

The classification of, even high-grade, PFA as waste is a bureaucratic absurdity. In consequence, anyone handling, transporting, storing or reprocessing PFA has to comply with 11 separate pieces of legislation. A system set up with the laudable aim of ensuring that waste is managed and disposed of in a safe and appropriate manner, is being interpreted in a manner that mitigates against the reuse or recycling of such materials, and means that, in this instance, a readily available valuable raw material is being discarded in favour of the mining of virgin sand, gravel and limestone (see above). The strange thing is that this problem has been known about and been allowed to persist for decades with nothing being done about it, while great piles of ash, and unnecessary mines and quarries, threaten to despoil the countryside.

A refrigerator is (hazardous) waste, but becomes so only after it is thrown away. Why does this “concession” not apply to Fuel ash, which apparently becomes waste as soon as it is produced?

More information can be found at the following links:

+ further links below.

[Update November 2008] 

To resolve this issue, the UK Environment Agency, in association with the Waste and Resources Action Programme (WRAP), as part of the Waste Protocols Project, which encompasses many other reusable ‘waste’ materials, is currently preparing a Quality Protocol for PFA and FBA, that will define the circumstances under which PFA destined for reuse may be designated as non-waste. A draft protocol has been published and a consultation is currently underway. Participants in this consultation include ash producers (power companies) and consumers (construction and manufacturing industry). Save Radley Lakes is also a participant. It is expected that the protocol will go live in late 2009. It will then be possible for PFA that is to be used in construction and manufacturing to no longer be designated as waste and thereby have to comply with the waste regulations. It is hoped that this will help reduce the amount of PFA, and other potentially useful materials, being sent uselessly to landfill.

 

[Update October 2010] 

The new Quality Protocol for the production and reuse of PFA was launched on 4th October 2010. See News Item.

ECOBA

It is reasonable to suppose that RWE npower fully concur with the aspiration that fuel ash should be recycled and utilised, since they are fully signed up members of ECOBA, the European Coal Combustion Products Association whose mission is “to encourage the development of the technology for the use of all coal combustion products, both on the industrial and the environmental level, with regard to relevant industrial and environmental demands“. ECOBA believes that coal combustion products (CCP) combustion residues, coal ashes and desulphurization products generated in coal-fired power plants, are valuable raw and construction materials, which can be utilised in various environmentally compatible ways; and that it is their task to propagate this message, especially amongst legislative and standardising institutions, and to communicate the economic and ecological benefits of CCP utilisation.

Some Options for Didcot

The following list some alternative options for RWE npower to consider. The list is not intended to be exhaustive.

With the Government set to expand house building in the Southeast to around 30,000 homes per year, demand for primary building materials can be expected to rise sharply. An estimated 2-3 million tonnes of “bricks and mortar” per year will be required to meet the demand for the construction of the houses alone, while road and other infrastructure construction could easily double this figure. Rather than throwing a valuable material away as waste, and damaging the environment in the process, Didcot should be looking to how it can best take advantage of its situation to meet much of this anticipated demand.

Investing in the RockTron separation process is one way to go. Scottish and Southern Electricity have already agreed to proceed with a project to construct and operate a RockTron plant at the Fiddler’s Ferry Power Station in Cheshire.

Another is to look into the possibility of manufacturing bricks and aggregates at Didcot. While this requires a larger investment (by around a factor of 3) than would be required for a Rocktron plant, the finished products (bricks) are of significantly higher value and demand is more assured, particularly in SE England.

Both of the above processes can utilise the ash in any condition, even after lengthy storage in lagoons. Brick and aggregate manufacture, for example, involve sintering and do not rely on pozzolanic reaction. This means that ash can be stockpiled in the environs of the power station (landfill at Sutton Courtenay for example) for as long as necessary while the reprocessing plant is commissioned. As any disposal at Sutton Courtenay would be temporary, costs would be anticipated to be much smaller than for permanent disposal. The power station would effectively be leasing, rather than consuming, the disposal resource. The end result is win-win for all: the power station saves some of its disposal costs (including, presumably, landfill tax) and could benefit from the profits from the product sales; the reprocessing plant owner would have a profitable business, people would benefit from extra jobs; the environment would benefit from building construction using greener more easliy transported materials, and the Radley Lakes would be saved from further damage.

After the power station has closed and all the ash has been used up, the reprocessing plant can be moved to another site. It is not necessary to write of its entire cost over the current projected life of the power station.

 
The usefulness of PFA products for containing and immobilising toxic waste suggests that a significant quality of Didcot’s PFA could be usefully deployed (if not disposed of) on the waste site adjacent to the power station itself. It may be that this is already being done.
 

Any use of PFA for landfill should be restricted to the beneficial infilling of unwanted voids, provided that any pollution risk from leachate is properly understood and managed. For example, PFA from Drax Power Station is being used in a £33M project to stabilise the Northwich Salt Mines in Cheshire, much to the relief of the citizens of Northwich whose homes were becoming increasingly at risk from the mines collapsing under them.

The current Oxfordshire Structure Plan (section 8.15) refers to an anticipated need to extract gravel from the Wallingford-Benson area of South Oxfordshire without creating gravel pits, even temporarily, which would attract birds that would present an unacceptable hazard to operations at RAF Benson. This would require fuel ash from Didcot.

Above ground disposal or stockpiling should be preferred to “below ground-level” disposal, since this results in significantly reduced pollution of groundwater, and makes the PFA more readily available for future recovery should this be required in the future. This is the preferred disposal method used by several other power stations, such as Drax where, making virtue out of necessity, the Power Station operators have used their ash to enhance the natural landscape rather than destroy it. (Click here to see aerial photo of the award-winning Barlow Mound at Drax. (Compare this with the situation at Radley.) Above-ground disposal on land close to the Didcot Power Station is another option that should be given more serious consideration. Experience of operating the large mound at Drax has shown that dust emissions, provided proper measures are put in place, are not a problem.

Finally, stricter measures need to be taken by power station operators, as well as by Government and its agencies, to limit damage done to the environment by PFA disposal, where this is necessary, by ensuring the adoption of appropriate waste minimisation policies and ensuring that the process is managed sensitively with proper monitoring and control of all its aspects.

 
While the Radley gravel pits may have been, for some, an unwanted intrusion on the landscape 25 years ago, when aerial photos show most of south Radley to be apparently underwater, this is not the case anymore. There is virtual universal public condemnation for the proposal to destroy the remaining lakes.

Other Links and Sources of Information

Permanent link to this article: http://www.radleyvillage.org.uk/ourvillage/didcot-fuel-ash-disposal/uses-of-pfa/