Europe is undergoing unprecedented changes in its energy production. There has been a steep decline in the use of fossil fuels and peat due to the green/sustainability transition goals and the Russia´s war in Ukraine with related sanctions. At the same time, waste-to-energy conversion (e.g., incineration of municipal solid waste and sewage sludge) is expected to further increase as landfilling becomes a more and more discouraged waste management option. In the Baltic context, Estonia has significant oil shale utilization but, even though it is defined as a strategic energy resource, most of it will be phased out by 2030. However, power plants are not producing only the energy but also various ash fractions as side streams. Coal fly ash has been traditionally an important supplementary cementitious material (i.e., material replacing Portland cement clinker) in concrete. In fact, the combined decreasing availability of coal fly ash and blast furnace slag from iron production (these two are the most used supplementary cementitious materials) in the future may even create a small crisis in the cement industry. This is because most of cements contain a significant amount of clinker substitution (e.g., CEM III class can have only 5% of clinker). It has been estimated that coal fly ash use in cement decreases from 3 million tonnes (or 2% of cement content) in 2020 to less than 1 million tonne (less than 1% of cement content) in 2050 according to the European Cement Association.
The European Union member states are generating 28 million tonnes of ashes from municipal solid waste, biomass, and sewage sludge incineration. Even though some of these ashes are utilized in certain low-value applications, such as municipal solid waste incineration bottom ash as in earthworks or sewage sludge ash as soil supplement in agriculture, a large fraction is still landfilled (for some ash fractions, more than half). These ashes are rich in silicon, calcium, and aluminum (the main elements required for cementitious materials), but they also contain significant amounts of recoverable base and precious metals, nutrients, and even rare earth elements. In fact, certain ashes nearly match or even exceed the currently exploited crude ores in the content of the valuable elements. For example, the phosphorus content of sewage sludge ash varies typically within 35–99 g/kg while the phosphorus content of ores can be 110–160 g/kg. Another example is zinc in municipal solid waste fly ash with a 9,000–70,000 mg/kg content in comparison zinc ores of 50,000–150,000 mg/kg.
To tackle these challenges, the European Commission has funded a project Integration of Underutilized Ashes into Material Cycles by Industry-Urban Symbiosis (AshCycle, www.ashcycle.eu) from the Horizon Europe programme. The project takes a holistic approach for the abovementioned ashes by addressing the resource recovery potential, required pretreatments, and their utilization for example as supplementary cementitious materials, raw material for completely cement-free binders (i.e., alkali-activated materials), sand and gravel replacement, CO2 absorbing materials, or adsorbent granules for water treatment. The project partners (total 27 from eight countries) represent universities and research institutions, powerplants, waste management and recycling companies, water treatment, construction material industry, and there is also a company developing an AI-based software supporting the ash utilization. The technical parts of the project focus on the recovery, pretreatment, and utilization approaches for ashes first in laboratory and then verifying them in pilots, which range from a few kilograms to tens of tons in their scale. Examples of the pilots include electrodialytic resource recovery, concrete products (e.g., paving blocks, street furniture, barrier elements, ), fired or unfired bricks, and ash granules for road subbase. The learnings from the pilots are used for sustainability assessment and developing models for the industry-urban symbiosis (i.e., evaluating the material flows and business possibilities between different actors).
Based on the already executed pilots, some observations can be made. One challenge is the lacking recognition of the addressed ashes by the existing concrete standards (this is also addressed in the project). Ash pretreatment (e.g., milling, washing, or chemical treatment to remove harmful elements or to improve ash properties) is a key operation enabling its further use. More industrial actors would be needed for this role, that is, between the ash producers and for example construction sector to ensure consistent and high-enough quality raw material. To make such businesses and the required investments more appealing, there should be more policy incentives, for example via banning the ash landfilling completely. However, the key overall message is that there are no insurmountable technical obstacles for utilizing the currently underutilized ashes.
Tero Luukkonen
Associate Professor & Coordinator of the AshCycle project, Fibre and Particle Engineering Research Unit, University of Oulu
Finland
