Managed pathways for CO2 mineralisation: analogy with nature and potential contribution to CCUS-led reduction targets.

The managed mineralisation of CO2 on mineral substrates has significant potential to mitigate CO2 emissions to atmosphere, using processes that are analogous to the formation of limestone in nature. High-temperatures and pressures or ambient conditions can be applied in processes that compare with the natural chemical, hydrothermal or biological formation of limestone. In the UK, recent policy developments recognise the potential of carbon utilisation and a reduction target of 40 Mt by 2030 has been set. In the present work, the analogies between natural and managed carbonate-production are briefly reviewed and the potential gains for mineralisation technology employing flue-gas as a direct source of CO2 are presented. With reference to selected UK solid waste arisings, our high-level analysis indicates mineralisation is capable of permanently sequestrating 1.2 Mt per year of CO2 in carbonated construction products. At a European level, nearly 7.8 Mt of CO2 can be managed in the same way. If indicative indirect CO2 savings are also considered, maximum total CO2 reductions of up to 3 and 30 Mt per year are possible in the UK and Europe, respectively. In respect of the UK's CCUS-led CO2 reductions for the 8 years to 2030, our high-level assessment suggests that up to 24 Mt, representing 60% of the 'target', may be met by the mineralisation of selected industrial process residues.

Valorisation of agricultural biomass-ash with CO2.

This work is part of a study of different types of plant-based biomass to elucidate their capacity for valorisation via a managed carbonation step involving gaseous carbon dioxide (CO2). The perspectives for broader biomass waste valorisation was reviewed, followed by a proposed closed-loop process for the valorisation of wood in earlier works. The present work newly focusses on combining agricultural biomass with mineralised CO2. Here, the reactivity of selected agricultural biomass ashes with CO2 and their ability to be bound by mineralised carbonate in a hardened product is examined. Three categories of agricultural biomass residues, including shell, fibre and soft peel, were incinerated at 900 ± 25 °C. The biomass ashes were moistened (10% w/w) and moulded into cylindrical samples and exposed to 100% CO2 gas at 50% RH for 24 h, during which they cemented into hardened monolithic products. The calcia in ashes formed a negative relationship with ash yield and the microstructure of the carbonate-cementing phase was distinct and related to the particular biomass feedstock. This work shows that in common with woody biomass residues, carbonated agricultural biomass ash-based monoliths have potential as novel low-carbon construction products.

Long-term performance of aged waste forms treated by stabilization/solidification.

Current regulatory testing of stabilized/solidified (S/S) soils is based on short-term performance tests and is insufficient to determine their long-term stability or expected service life. In view of this, and the significant lack of data on long-term field performance in the literature, S/S material has been extracted from full-scale remedial operations and examined using a variety of analytical techniques to evaluate field performance. The results, including those from X-ray analytical techniques, optical and electron microscopy and leaching tests are presented and discussed. The microstructure of retrieved samples was found to be analogous to other cement-based materials, but varied according to the soil type, the contaminants present, the treatment applied and the field exposure conditions. Summary of the key microstructural features in the USA and UK is presented in this work. The work has shown that during 16 years of service the S/S wastes investigated performed satisfactorily.

Accelerated carbonation treatment of industrial wastes.

The disposal of industrial waste presents major logistical, financial and environmental issues. Technologies that can reduce the hazardous properties of wastes are urgently required. In the present work, a number of industrial wastes arising from the cement, metallurgical, paper, waste disposal and energy industries were treated with accelerated carbonation. In this process carbonation was effected by exposing the waste to pure carbon dioxide gas. The paper and cement wastes chemically combined with up to 25% by weight of gas. The reactivity of the wastes to carbon dioxide was controlled by their constituent minerals, and not by their elemental composition, as previously postulated. Similarly, microstructural alteration upon carbonation was primarily influenced by mineralogy. Many of the thermal wastes tested were classified as hazardous, based upon regulated metal content and pH. Treatment by accelerated carbonation reduced the leaching of certain metals, aiding the disposal of many as stable non-reactive wastes. Significant volumes of carbon dioxide were sequestrated into the accelerated carbonated treated wastes.

Production of lightweight aggregate from industrial waste and carbon dioxide.

The concomitant recycling of waste and carbon dioxide emissions is the subject of developing technology designed to close the industrial process loop and facilitate the bulk-re-use of waste in, for example, construction. The present work discusses a treatment step that employs accelerated carbonation to convert gaseous carbon dioxide into solid calcium carbonate through a reaction with industrial thermal residues. Treatment by accelerated carbonation enabled a synthetic aggregate to be made from thermal residues and waste quarry fines. The aggregates produced had a bulk density below 1000 kg/m(3) and a high water absorption capacity. Aggregate crushing strengths were between 30% and 90% stronger than the proprietary lightweight expanded clay aggregate available in the UK. Cast concrete blocks containing the carbonated aggregate achieve compressive strengths of 24 MPa, making them suitable for use with concrete exposed to non-aggressive service environments. The energy intensive firing and sintering processes traditionally required to produce lightweight aggregates can now be augmented by a cold-bonding, low energy method that contributes to the reduction of green house gases to the atmosphere.

Current status and perspectives of accelerated carbonation processes on municipal waste combustion residues.

The increasing volumes of municipal solid waste produced worldwide are encouraging the development of processes to reduce the environmental impact of this waste stream. Combustion technology can facilitate volume reduction of up to 90%, with the inorganic contaminants being captured in furnace bottom ash, and fly ash/APC residues. The disposal or reuse of these residues is however governed by the potential release of constituent contaminants into the environment. Accelerated carbonation has been shown to have a potential for improving the chemical stability and leaching behaviour of both bottom ash and fly ash/APC residues. However, the efficacy of carbonation depends on whether the method of gas application is direct or indirect. Also important are the mineralogy, chemistry and physical properties of the fresh ash, the carbonation reaction conditions such as temperature, contact time, CO(2) partial pressure and relative humidity. This paper reviews the main issues pertaining to the application of accelerated carbonation to municipal waste combustion residues to elucidate the potential benefits on the stabilization of such residues and for reducing CO(2) emissions. In particular, the modification of ash properties that occur upon carbonation and the CO(2) sequestration potential possible under different conditions are discussed. Although accelerated carbonation is a developing technology, it could be introduced in new incinerator facilities as a "finishing step" for both ash treatment and reduction of CO(2) emissions.

Accelerated carbonation of municipal solid waste incineration fly ashes.

As a result of the EU Landfill Directive, the disposal of municipal solid waste incineration (MSWI) fly ash is restricted to only a few landfill sites in the UK. Alternative options for the management of fly ash, such as sintering, vitrification or stabilization/solidification, are either costly or not fully developed. In this paper an accelerated carbonation step is investigated for use with fly ash. The carbonation reaction involving fly ash was found to be optimum at a water/solid ratio of 0.3 under ambient temperature conditions. The study of ash mineralogy showed the disappearance of lime/portlandite/calcium chloride hydroxide and the formation of calcite as carbonation proceeded. The leaching properties of carbonated ash were examined. Release of soluble salts, such as SO4, Cl, was reduced after carbonation, but is still higher than the landfill acceptance limits for hazardous waste. It was also found that carbonation had a significant influence on lead leachability. The lead release from carbonated ash, with the exception of one of the fly ashes studied, was reduced by 2-3 orders of magnitude.