Profit from flyash and slag

As industry tightens its belt against the tough times ahead, we take a timely look at how some power utilities are turning waste into gold.

Despite the mounting interest surrounding the renewable sector, coal-fired power stations still account for a large percentage of the world’s electricity generating capacity. Each year, some 3bnt of coal is burnt, creating around 650Mt of coal fly ash in the process. In China alone, over 300Mt of fly ash was produced in 2006 and the country’s cumulative fly ash production is excess of 2.5bnt. In the US, 72Mt was produced in 2006, up 50 per cent from that seen in 1993. Like all waste products, its exact composition varies significantly, depending on the type of coal burnt and the specifics of the burning process. However, they tend to be rich in silicon, iron and aluminium oxides, as well as calcium oxide (CaO) and contain traces of heavy metals. The presence of CaO confers a pozzolanic quality, allowing some types of fly ash to be used as a substitute for Portland cement. As cement production requires a great deal of energy and generates substantial volumes of CO₂, the use of alternatives such as fly ash is attractive, both economically and environmentally. In addition, the rise of carbon trading may well give this approach an added boost in the future.

Such uses for fly ash have become important to power plant operators in recent years, due to economics and policy directives. In the case of the former, the cost of landfill space has been a key driver in promoting greater recycling of fly ash. In the US, fears that heavy metals can leak from fly ash piles and into the local water supply have also been a contributing factor. Indeed Constellation Energy and a landfill site owner were forced to pay a US$1m fine last year for contaminating groundwater in Maryland. The site held 4.5Mt of fly ash.

Increasing awareness of the opportunities offered by fly ash has resulted in companies such as Evonik Power Minerals, (European-based and featured in our March issue) or Separation Technologies LLC (a US-based subsidiary of Titan America), which manage the transportation and handling of the ash from power stations to its end user. Separation Technologies LLC has seen its annual sales of fly ash to the concrete industry grow by 20 per cent per annum and markets the ash under its ProAsh brand. According to company representatives, 15-30 per cent of the ProAsh can be used to make cement, while some of the collected ash with remaining calorific value is returned to the power plant to be reburnt.

Separation Technologies signed a deal in July with Colorado Springs Utilities to process the 120,000-130,000st of fly ash produced each year by the city’s two coal-fired power plants. As a result, Colorado Springs will save US$620,000 in landfill costs and could net the utility up to US$2m/year in ash sales. The city is also looking to sell bottom ash to brick and cinder block manufacturers, potentially leading to further savings of US$27,250 in avoided landfill and generate up to US$10,000 in annual revenue. The deal took Separation Technologies’ number of clients up to 10.

Another promising use for fly ash is in the manufacture of dry walls, the most commonly used indoor building material in the US. A company called Spertech announced on October 27 that it has found a way of producing wallboards using 98 per cent fly ash without the need for heating.

There are two classes of fly ash as defined by the American Society for Testing and Materials: Class F and Class C (see Table 1). Class F is produced from the burning of hard coal (anthracite and bituminous) and has less than 10 per cent lime, while Class C typically contains over 20 per cent lime and has a higher sulphur content. It comes from the burning of lignite or sub-bituminous coals. Both classes have pozzolanic properties, but Class C differs from Class F in this regard, in that it does not require a chemical activator such as sodium silicate to be self-cementing.

The growing market for fly ash appears to be creating a symbiotic relationship between other fuel-intensive industries and the cement sector. For example, Nalco, Asia’s largest alumina producer has recently invited expressions of interest from ‘competent’ parties to set up a cement plant based on fly ash, according to RC Pradhan, the company’s chairman. Last year, Vedanta Resources, a major mining and aluminium producer, invited bids from cement companies with a view to setting up a similar project, preferably in the form of a joint venture.

The problem of fly ash storage is also an obstacle to obtaining planning permission for new power plants. For example, Santee Cooper, a South Carolina utility, is looking to build a new coal-fired power station and environmentalists are seizing on the potential dangers of heavy metal leakage from the proposed landfill site. However, although the company has admitted that some mercury will leach into the Great Pee Dee River, it is hoping to use the site as a staging area with the majority of the ash to be sold to cement companies. However, there are some environmental concerns associated with the use of fly ash in cement manufacture. High mercury emissions from a Lafarge cement plant, situated in New York state, have prompted an investigation, which is expected to report its findings later this year. It is thought that the inclusion of fly ash in the cement kiln and the resulting exposure to extremely high temperatures (around 1750ºC), is transforming heavy metals within the ash into vapour. The situation is made more complicated by the fact that as utilities have come under pressure to clean up their emissions from coal-fired power stations, the concentration of mercury in fly ash has been increasing. A study conducted by the EPA in 2007 found that the mercury content in fly ash has risen by 850 per cent due in part to tighter federal standards for emissions from power plants. Another worry is that mercury may not be the whole story, given the presence of other heavy metals in fly ash.

Figure 1: Total US slag cement shipments.
Includes both slag cement shipped as a separate
product and as a component of blended cement.
Source: Slag Cement Association.

Slag: surprisingly sophisticated

Coal-fired power plants aren’t the only source of a waste product with great value to the construction sector. Steelmaking produces large quantities of blast furnace slag, which can be used to make slag cement (also known as ground granulated blast-furnace slag). Blending slag cement with Portland cement results in a product with advantages over standard cement. These include lower permeability, a lighter colour and improved resistance to sulphate attack and the alkali-silica reaction. The latter is also true of Class F fly ash but not Class C. The composition of slag cement is much closer to Portland cement, than coal fly ash and therefore can be used in much higher quantities (replacing up to 50 per cent in normal concrete, and up to 80 per cent in special applications, such as mass concrete).

In comparison, fly ash typically can only replace 20-30 per cent of Portland cement. Slag cement is typically more uniform than fly ash, varying less from source to source. As can be seen from Figure 1, sales of slag cement and blended cement containing slag cement in the US have grown significantly over the past decade. The recent reversal is due to the sub-prime crisis which has impacted on the American construction industry. The 3.4Mt of slag used in 2007, resulted in the avoidance of 2.9Mt of CO₂ emissions, conserved 14.5tnBTU of energy and 5Mt of virgin materials. In China, fly ash and slag cement are used in combination, with the routine mix used for concrete being 50 per cent pure Portland cement, 25 per cent fly ash and 25 per cent slag. This proportion results in the highest compressive strength (MPa) after 28 days (Lan & Yuansheng, 2007).

Uranium from fly ash

One of the more novel approaches seen to date is being pioneered by Sparton Resources. This company is looking to extract commercially viable quantities of uranium from the fly ash generated from coal-fired power plants. It already has agreements in place in eight countries including China and South Africa. In March, it secured a patent from the Chinese government for its uranium extraction process. In the same month, it reported that initial leaching tests on coal fly ash from the Lincang germanium area of Yunnan province, succeeded in recovering 70-80 per cent of the uranium content from the ash (281ppm U3O8). In addition to the high yield, the results were seen as encouraging given the low acid consumption and the low lime content of the ash (three per cent). The fly ash samples were taken from Tianhao, a local power producer which has given Sparton permission to eventually extract uranium from its ash stockpiles which have been estimated to amount to 450,000t.

In addition, Sparton Resources is looking to extract uranium from the Xiaolongtang, Dalontang and the Kaiyuan power stations, also in Yennan province. The plant at Xiaolongtang alone produces some 0.9Mta of fly ash and has stockpiles of around 5Mt. According to company estimates, a total of 145t of uranium could be extracted from the fly ash produced annually by these power stations. Sparton has been working closely with the China National Nuclear Corporation subsidiary (ARCN) and the two organisations have formed a joint venture to further develop this technique. However, this approach is possible only due to the comparatively high uranium content of some Chinese coals. The practice of reclaiming uranium from fly ash had been used previously in China with some success but was abandoned when the uranium market collapsed in the early 1980s. The technique promises to help reduce the environmental impact of fly ash, while at the same time providing China with additional domestic supplies of uranium. This will be especially welcome as the country is currently looking to expand its fleet of nuclear reactors. The EIA has predicted that it will add 15-30GW of new capacity by 2020.

The US Geological Survey does not consider the radioactive components of fly ash to be a health issue and that, as pointed out by BE Scheetz of Pennsylvania State University, the extraction of uranium from fly ash is not a viable means of producing fissile material for nuclear proliferation purposes. If this was the case, then fly ash uranium extraction would have occurred on a large scale during the Cold War.

There are number of other applications for fly ash, including soil stabilisation, mine backfill and agriculture. The IEA’s Clean Coal Centre published a report in May 2005 which investigated the potential for these different approaches. It reported that fly ash has been successfully used to boost crop yields in many countries, allowing the use of less fertiliser, gypsum and irrigation, due in part to its moisture retention and boosting properties. However, the report highlighted that such approaches are often limited to the local vicinity of the plant where the fly ash is produced due to transportation costs. This also limits its use as a soil stabiliser for construction work.

An impressive example of the scale at which fly ash can be used, is the mine backfilling project at Northwich, UK. A total of 1.1Mt of pulverised fly ash was used to combat the risk of subsidence from the town’s disused salt mines. The fly ash originated from Drax, the UK’s largest coal-fired power plant. A study concluded that all other alternatives for the mine infilling were less suitable due to a variety of reasons “including hazardous nature, handling difficulties, consistency, availability and cost” (P. Brennan, 2007). The project made it possible to develop over 20ha of land around the northern part of the city.

Of course, fly ash and slag are not the only by-products from coal-fired power generation with financial value. FGD gypsum is produced as a by-product of the desulphurisation of flue gases and can be used as a substitute for natural gypsum which is used in the production of varnishes, adhesives and plastics. It is also used as a setting controller in cement production. plasterboard manufacturing. In addition to its use in brickmaking, bottom ash can be used in greenroof construction and to enrich soil.

Waste not…

The waste from coal-fired power generation and steel making can be used to turn an unattractive expense into a lucrative revenue stream for plant owners, while at the same time, substantially reducing energy consumption and the need for raw materials in the construction industry. This translates into significant CO₂ emission reductions. Given the growing maturity of companies set up to offer utilities waste product marketing services, this approach should be seriously considered by all large-scale coal consumers, especially when viewed in the light of the growing move towards sustainability as a vital part of corporate responsibility and a “license to operate.”

For more information consider visiting the following websites:
www.stiash.com
www.slagcement.org