Click here to view Image Gallery
The electric power industry produces millions of tonnes of coal ash each year. In a time of increasing environmental concerns and regulations, Tildy Bayar uncovers the best practices being used by coal-fired power plant operators

In a landmark document issued in December 2014, the US Environmental Protection Agency (EPA) laid out the first-ever federal rules for coal ash disposal.

Under the new guidelines, America’s coal-fired power plants are required to store coal ash – the waste product from coal combustion for power production, also known as CCPs (coal combustion products) – only in landfills and disposal sites that meet minimum structural standards.

These facilities must be monitored for leaks, and newly built storage lagoons for liquidized coal ash (called ‘impoundments’ in the US) as well as landfill sites must be lined to prevent leaks into the groundwater. (Landfill sites do not contain dry ash, which cannot be stored in the open without raising tremendous amounts of dust. The landfilling process involves wetting the ash with about 20 per cent water.)

The EPA’s document also classified coal ash as a “non-hazardous” material, leading to protests from environmental groups but a collective sigh of relief from utilities and the coal industry, which had expected to be hit with a stricter definition and more costly disposal rules.

Ari Lewis, a toxicologist and risk assessor with US environmental consultancy Gradient, told PEi that while the utility industry supports aspects of the non-hazardous waste decision, there are “many other parts of the rule that may trigger some costly measures”.

In particular, she said that if certain criteria are met, the EPA’s rule will require companies to close surface impoundments, which can potentially cause more impacts to groundwater than landfills, especially if they are unlined.

“There is a lot of public pressure” on utilities to close impoundments, Lewis explained. States such as North Carolina, where well-publicized storage facility accidents have occurred, are now “aggressively” looking into how to close their impoundments, she said. Specifically, under some circumstances, utilities may be required to move the ash to landfills (an idea that is currently under EPA review) – which, Lewis says, “is very expensive and the benefits are not clear”. Building more landfills creates land use issues, she explained, and moving the ash raises dust as well as potentially creating problems in communities due to increased use of transport vehicles.

To assist in this process, Gradient, with support from the Electric Power Research Institute, has developed a framework for use by utilities that examines the risks and benefits of such movement. It takes into consideration “the possible human health and environmental impacts under different closure scenarios (i.e., in-place closure vs excavate and re-dispose)”. It looks at worker safety and resource use “in addition to examining the more traditional media-specific exposure pathways such as groundwater, surface water and outdoor air”, and allows the utility to evaluate the relative impacts of each potential closure alternative.

Storage

Storage, whether in lagoons, silos or landfills, rather than re-use, is the default solution for coal ash management in most countries. The UK’s Department of Environment, Food and Rural Affairs (Defra) estimates that, of the eight million tonnes of coal ash produced in the country each year, half is re-used, while over 300 million tonnes have been stored in lagoons or silos since the 1950s. The UK Quality Ash Association (UKQAA) estimates that 50 million tonnes are currently stored in landfills. (The association says it collects, but no longer shares its figures on total coal ash production “due to the difficulties of interpreting the data”.)

The American Coal Ash Association (ACAA) reports on US coal ash production and use each year. In 2013, 53 million tonnes of coal ash were generated, of which 23 million tonnes were re-used. Of the unused portion, the EPA says 36 per cent was stored in landfills, and 21 per cent in wet storage facilities.

Some countries are doing better, though: for example, the Netherlands recycles 100 per cent of its coal ash because landfill is not allowed in the country. In Germany, where around 10 million tonnes of coal ash are produced per year, around 97 per cent is re-used, with the rest stored only on a temporary basis. According to the European Coal Combustion Products Association (Ecoba), of the 48 million tonnes of coal ash produced in 15 EU countries in 2010 (the latest available figures), 13.8 million tonnes were re-used.

Storage is the number one solution for coal ash disposal largely because it is generally the easiest and cheapest option. Cost is a prime consideration, and the cost is lowest when there is an available disposal site near a power plant. According to the ACAA, if the coal ash can be piped to the site, rather than trucked, and the ash is easy to handle, costs could be around $3–$5 per tonne. However, when the disposal site is further away and a more complex transport solution is needed due to either higher moisture content or larger volume, the cost could rise to $20–$40 per tonne. If a new disposal site is needed, involving an extensive permitting process, total costs will be even greater.

Storage silo technology is well-developed, but the accompanying ash handling systems involve more steps and thus are more expensive than the landfill option.

Recover, re-use, recycle

Coal ash is discharged from a power plant’s boiler and collected in different ways, with so-called fly ash, or pulverized fuel ash (PFA), carried out with the large air flow through the boiler and collected by electrostatic or mechanical precipitators, which act as a filter. Fly ash particles are generally around 40 microns in diameter, but can be between one and 150 microns.

Bottom ash is the slag that forms inside the boiler itself, then drops off to the bottom due to gravity. It is too large (typically between one and 10 mm in diameter) to be carried out by the air flow. Due to its large particle size, it is unsuitable for some types of re-use.

In the UK, most bottom ash is used in the manufacture of cement blocks for construction. In the US and other parts of the world, where cement blocks are not as popular, bottom ash is either landfilled or used as an aggregate material in concrete production. In many countries, including the US, if the fly ash is also not re-used, it can be combined with bottom ash before landfilling.

In some newer European member states where dry ash collection systems were not previously used, with the fly ash and bottom ash typically sluiced away together, an infusion of funds since joining the EU has led to upgrade projects which convert wet collection systems to dry so that the ash can be used beneficially.

According to Ecoba, around 40 per cent of coal ash ends up as fly ash, which is used primarily by cement companies in a product known as blended cement. This mixture is typically around 25–30 per cent fly ash and offers several advantages over Portland cement, a common material manufactured from chalk and clay.

A certain percentage of fly ash is sometimes specified in cement projects because the blended product is stronger, less permeable and more resistant to the effects of chemicals such as sulfates, some acids and soft water. In addition, the use of one tonne of fly ash replacement in cement production typically saves one tonne of CO2. Perhaps the most important advantage for cement firms, though, is that fly ash is about one-third the price of cement – which is why some cement companies and power plants have entered into joint venture projects to recover fly ash.

However, coal ash use in the construction industry varies according to country. According to European power and heat technical association VGB PowerTech, most of Germany’s coal ash is used for the refilling and reclamation of depleted lignite mines, with much smaller percentages used in underground mining, soil beneficiation and cement and concrete production.

New developments

The coal industry’s recent downward market trend has created a fly ash shortage in the UK and US, resulting in cement companies starting to take an interest in mining landfill storage sites to recover the ash. Lewis Baker, European Technical Manager at ash processing technology firm Separation Technologies, says: “The market needs fly ash, and they have to get it from somewhere.”

The motivations for mining landfill sites are somewhat different in the US and the UK, he adds. In the US, “utilities could reduce liabilities going forward, and it’s an attractive proposition to find beneficial uses for landfills. In the UK the issue is more that the market needs more fly ash than is available.”

Indeed, in November 2014 the UKQAA and the University of Dundee in Scotland announced a collaborative research project into the recovery and use of high-quality fly ash for the construction sector. The study aims to measure how much ash can be recovered across the UK, and then to test it for suitability for mixing with concrete.

Dr Robert Carroll, Technical Director at the UKQAA, said: “This is a really exciting opportunity. Fly ash demand continues to rise to keep pace with a growing construction sector, but availability is dependent on our use of coal power. Unlocking the potential of stockpiled ash in ashfields across the UK could increase supply, meet construction demand and exploit an otherwise underused material.”

A growing area of interest in coal ash use is the extraction of desirable rare earth metals. Gradient’s Lewis says rare earth metals can become enriched in certain types of ash, and can reach levels found in mined ores. “If that can be utilized,” she says, “it is attractive from a process standpoint as it is already enriched and pulverized, so it involves a less intensive workstream that going out and mining it.” Lewis said the process is still at the research stage, with programmes underway at several chemicals and engineering firms, and one at the University of Kentucky.

Best practices

Any form of re-use would seem to fall under the best-practice rubric according to ST’s Baker. “Coal power plants producing fly ash will, and should, always be looking at ways to increase use of their fly ash, and reducing the amount sent for disposal,” he says, adding that “conversion of any wet collection systems to dry collection certainly falls under the umbrella of best practice, and opens up new markets for ash usage.”

‘Beneficiation’ is a term that refers to the process by which carbon is removed from fly ash to produce a higher-value material. It is not required for all re-use applications; however, it does increase the value of the ash, offering a power plant an increased revenue stream.

“In many instances,” says Baker, “beneficiation of the ash is required to produce ash of a quality for use in concrete. This can often significantly increase the proportion of ash sent for beneficial use.”

And reducing the landfill requiment may be an economic necessity in some cases, especially where the landfill tax is high. In the UK, a proposed reclassification of fly ash from inert to hazardous waste – which did not happen in the end – would have led to a significant rise in landfill tax and, potentially, dramatic growth in the market for coal ash. Says Baker: “Economic pressures from enhanced landfill tax, or environmental constraints from landfill availability can often play a part in finding innovative solutions to increasing ash usage.”

However, in order to re-use the ash without incurring prohibitive transport costs, a power plant needs an available market – and its location will be key.

“Coal plants located near large cities or major construction areas are at an advantage in finding markets for ash products,” Baker says. But his use of “near” rather than “in” large cities is perhaps significant given that a very large power plant can generate up to one million tonnes of ash per year, and a city may have multiple power plants which together could produce enough ash to flood the market. A power plant located close to a city, but far from any other power plants, would be ideally placed to market its ash.

Safety issues

US activist group Southeast Coal Ash Waste says some uses of coal ash are “safer than others, and some are downright dangerous”. It cites mine-filling, where coal ash is stored in abandoned mines and quarries where it “typically is in direct contact with the water table”, as an environmentally hazardous practice, and notes that unlined landfill is potentially dangerous due to the possibility of leakage.

In addition, it cites the potential for collapse of the earthen dams surrounding landfill sites, such as the 2008 disaster at the Tennessee Valley Authority’s Kingston coal power plant. Re-use in concrete and brick manufacturing, the group says, is the safest practice in that it fully encapsulates the material, allowing no release of toxic contaminants, as well as replacing the use of high-carbon-footprint materials.

While studies have examined the use of coal ash in the production of wallboard, cement and underlayer for roads and embankments, findings indicated levels of potentially toxic metals only slightly higher than those found in common soil. According to Gradient’s Lewis, her firm is “not aware of any risk assessments showing any adverse risk” from the beneficial use of coal ash.

In the end, it would appear that any re-use of coal ash is beneficial, offering an enhanced revenue stream for power plants and a source of material for the construction sector, as well as a means of decreasing the carbon footprint of both. With the key focus for many companies now being sustainability,” says ST’s Baker, putting fly ash to beneficial use is “an environmental win/win”.

More Power Engineering International Issue Articles
Power Engineering International Archives
View Power Generation Articles on PennEnergy.com