By Dr Paula Carey, Co-founder and Chief Technical Officer
2.01 billion tonnes of municipal solid waste are generated globally and 33% of that is not managed in an environmentally sound manner. Because at least 37% is dumped in either uncontrolled or controlled landfill, municipal solid waste is responsible for 5% of global CO2 emissions. Less than 20% of waste is recycled globally and the majority of this is in high-income countries.
High-income countries, particularly those with limited land space, generally consider Energy from Waste (EfW) a more sustainable management technique, when combined with good recycling schemes. It reduces the mass of waste by 95% and provides the ability to manage the gaseous emissions more effectively than in landfill while providing electricity and district heating.
Currently, EfW emissions are not included within measures such as the European CO2 trading Scheme (ETS) but there is increasing momentum to capture the CO2 from EfW plants. Indeed, the AVR plant in Duiven in the Netherlands has been capturing CO2 for use in greenhouses for several years. In the UK, there have also been proposals to make CCS a requirement for EfWs by the mid-2020s.
In terms of the environmental impact of EfW, the main concerns are the CO2 emissions and the remaining 5% of waste mass; residues from the incineration process, including Incineration Bottom Ash (IBA) and Air Pollution Control Residues (APCr). In the UK valuable metals are removed from the bottom ash (IBA) before it is mostly re-used as an aggregate. However, there are some concerns about the contamination within the IBA and in the UK there is a quality protocol to facilitate its reuse. This is where Accelerated Carbonation Technology (ACT) has major potential.
The Role of Carbonation
ACT is based on controlling, managing and accelerating the natural process of carbonation. In its simplest form, this refers to the reaction of metal ions, silicates, hydroxides and oxides with CO2.
Carbonation, particularly of the finer fraction of the IBA can reduce the availability (leaching) of the heavy metals that are typically problematic (lead, nickel, zinc) by lowering the pH of the material and forming stable carbonates. The majority of IBA is left to hydrate and carbonate naturally in the atmosphere before it is reused.
The APCr is produced by the injection of lime into the flue gas to absorb the chlorides and sulfates within the flue gas. The flue gas is then passed through filter bags to remove the excess lime, reacted lime and other particulates.
Because of the levels of lime with these residues they are a prime target for carbonation, particularly as their hazardous nature is the result of their high alkalinity and the content of heavy metals readily stabilized and solidified in the carbonation process.
Accelerated Carbonation Technology
Through the pressures to decarbonise and the availability of residues at EfW sites, ACT represents a means to address both. APCr can be used as a carbon sink; reducing emissions and the 5% of the waste remaining after EfW processes. The outcome of this is carbonated aggregate that has various applications within the construction industry, including concrete blocks.
The reactivity of the APCr to CO2 depends on the lime content and the content of calcium silicates in the APCr. This CO2 uptake varies between 3 and 15% by weight depending on how the lime is recirculated within the flue gas treatment system of a particular site. Using the average APCr produced by an EfW plant of 10,000 tonnes annually, 1,000 tonnes of CO2 can be permanently and safely stored using ACT.
Hence, EfW can provide electricity and district heating, while minimising emissions and waste by using ACT to manufacture a sustainable aggregate for the construction industry.
Interested in what ACT can do for your business or want to speak to one of our experts? Contact us now.