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Waste incineration slag and ash
Material description and quantity estimation
Waste incineration slag and ash is produced at co-incineration plants and waste-to-energy plants intended for the energy recovery of municipal waste. During the process, boiler slag and bottom ash is generated in the boiler, fly ash is produced in flue gas separation, and APC (air pollution control) waste is produced in flue gas cleaning. The quantity and quality of generated slag and ash are affected by the consistency of the incinerated waste, boiler’s operating principle, and flue gas treatment. (Kaartinen, Laine-Ylijoki & Wahlström 2007.)
The amount of produced boiler slag and bottom ash is approximately 20–30% of the mass of the waste utilised at the plant (Kaartinen, Laine-Ylijoki & Wahlström 2007). Boiler slag is granular sand or gravel-type substance, of which 15–45% consists of glass, metal, ceramics, rock material, and organic matter. The rest 55–85% of the mass of the slag consists of a variety of melting products. (Suomen Erityisjäte Oy 2017; Kaartinen, Laine-Ylijoki & Wahlström 2007.) In Finland, the waste incineration bottom ash contains significant amounts of iron (10–12%) and more copper when compared with the ash material concentrations at waste incineration plants in Denmark and the Netherlands (Rantsi 2015, 20–21). Flue gases are cleaned with the aid of cyclones, electricity, or fabric filters, which enables the separation of fly ash. Due to their consistency, fly ash and APC waste are classified as hazardous waste in Europe. They often contain arsenic, mercury, lead, cadmium, chrome, and soluble salts, such as chloride. (Kaartinen, Laine-Yljoki & Wahlström 2007.)
Before recovery, the waste incineration slag is first treated by aging and screening the metals. Since the beginning of 2018, treated waste incineration slag has been able to be utilised without an environmental permit in specified earth construction applications, which include route and field structures and the base structures of industrial and storage buildings. For the part of harmful substances, the treated slag must also meet the other quality requirements. (Mara-asetus 843/2017.) Boiler slag has a good thermal insulation capacity, and it is a frost resistant material (Saarenpää 2015). The most typical utilisation applications include substructures and base work related to soil structures and superstructures, such as parking area
s, sports fields, and pedestrian and bicycle ways. While taking the technical requirements into account, boiler slag can also be utilised in, e.g., landscaping, noise barriers, and pipe trench filling. (Kaartinen, Laine-Ylijoki & Wahlström 2010).
Boiler slag is primarily utilised in the field structures of waste centres, even though there are many other possible applications, as well (Rantsi 2015, 21). With the aid of a special dry separation method, valuable metals can be separated from the ash. After the separation, the ash is converted into artificial rock material. This material can be utilised instead of virginal sand and gravel material in, e.g., road construction and concrete industry products. (Suomen Erityisjäte Oy 2017.)
The utilisation of fly ash and APC waste is hindered by the concentrations of harmful substances in them (Koivunen 2007). At the moment, landfill acceptability assessments are conducted on fly ash and APC waste that are intended to be disposed of. When the requirements for landfill acceptability are met, the waste in question will be disposed of at a landfill site intended for non-hazardous waste. If the requirements are not met, the waste is considered hazardous waste and will (usually) be delivered to Fortum’s Riihimäki facility for disposal. (Kiertokaari Oy 2018.)
In 2016, a total of 1,515,000 tonnes of municipal waste was used in energy recovery (Suomen virallinen tilasto (SVT): Jätetilasto 2018). Approximately 378,750 tonnes of boiler slag and bottom ash is produced as the result of the incineration of this waste amount. Of this amount, the annual share of slag is approximately 300,000 tonnes (Sormunen 2017). The waste incineration slag and ash were identified as important material flows in the regions of those six largest cities in Finland with a currently active waste-to-energy plant. Such regions were the capital region (waste-to-energy plant of Vantaan Energia), Tampere region (waste-to-energy plant of Tammervoima Oy), and Oulu region (eco power plant of Oulun Energia).
In 2016, these plants produced the following amounts of slag and ash: approximately 71,000 tonnes in the capital region (HSY 2017), approximately 35,000 tonnes in the Tampere region (Tammervoima 2016), and approximately 24,000 tonnes in the Oulu region (Oulun Energia 2016).
Innovation needs
The following challenges are related to the utilisation of slag and ash:
- It is economical to separate metals from the particulates of slag, but, e.g., the gold separation techniques are expensive and disadvantageous due to the related dusting
- Such criteria and qualitative factors must be determined for the slag treatment that it is possible to utilise slag in operations outside the landfills, for instance, in earth construction, the asphalt industry, and the concrete and rock material industry
- Ensuring the uniformity of the slag consistency over a long period of time, as the quality of incinerated waste varies and the consistency may change over time
- Ensuring the uniformity of the slag’s grain size with the aid of, e.g., a proper screening if the slag is to be used in recycled products (asphalt, concrete, and rock products)
- Analysing the long-term effects of slag use to determine whether, e.g., harmful substances can be released from slag in asphalt use due to wear
- Challenges related to the economic profitability of ash and slag recovery: how the treatment can be, in reality, made profitable when compared to the use of virginal raw materials
- Possible negative attitudes to recycled materials must be changed. Since there are not that many pilot sites for slag use in Finland, the threshold for slag and ash recovery may be high due to the risks related to their use
- The value chain must be analysed in more detail. Finding operators that are interested in the use of slag and end customers that are interested in recycled materials at as early a stage as possible is essential for the recovery. Public operators could be pioneers in this kind of procurement.
Business-related challenges and opportunities
The possibilities of using boiler slag as recycled ore are good. Valuable metals can be separated in a cost-effective way. In the future, bioleaching and foaming will enable the separation of copper, aluminium, zinc, and other metals. (Kaartinen, Laine-Ylijoki…&Wahlström 2010.) In addition, high-quality concrete rock material can be refined from the raw slag mineral matter produced by incineration plants (Lehtonen 2017). In Finland, pilot studies have also been conducted on landscaping stones, in the manufacture of which the mineral matter separated from waste incineration slag has been utilised (Hautala 2017). According to the research results, the utilisation of mineral matter separated from waste incineration slag in the manufacture of landscaping stones does not cause heavy metals to be released into the soil at the utilisation site.
Ash can also be used in the manufacture of ceramic products (Silva ym., 2017). The high-temperature treatment eliminates harmful substances from the ash. The ash must, however, be very fine-grained, so the fine-grinding increases the treatment costs.
The production possibilities of the hydrogen gas contained by waste incineration bottom ash has also been studied in Japan (Saffarzadeh ym., 2016). The method provides one potential option for converting bottom ash into a resource with a higher value.
References (mainly in Finnish)
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Silva ym 2017. Silva. R.V., .de Brito. J., Lynn. C.J. & Dhir. R.K. 2017: Use of municipal solid waste incineration bottom ashes in alkali-activated materials, ceramics and granular applications: A review. Waste Management. Volume 68, October 2017, Pages 207-220
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