||Republic of South Africa (RSA) announced its nationally determined contribution (NDC) as a commitment to Paris Agreement, in which the country pledged to reduce its GreenHouse Gas (GHG) emissions by 42% below business-as-usual (reference year as 2005). Its detailed implementation plan and measures, including carbon tax, are under development; however, the carbon-intensive sectors (metals, cement, lime, chemicals, mining, power) are expected to share a significant burden of meeting GHG emission reduction targets.
Recently, besides Carbon Capture Storage (CCS) technology, Carbon Capture and Utilization (CCU) has also attracted much attention. In the CCU technologies CO2 is converted to useful materials, such as chemicals, fuels, carbonates and others. Among these CCU technologies, the technique of carbon mineralization, which utilize spontaneous carbonation reactions between alkali wastes/rocks and CO2 is advantageous due to its potentially low energy requirement and larger market size for the produced carbonates. However, to develop feasible mineralization processes, appropriate choice for alkali waste, selection of suitable mineralization process conditions and utilisation of process byproducts must be explored.
Demolished concrete (concrete waste) is generated in significant quantities in the course of building, demolition or renovation in both RSA and in Japan. In addition, waste coal fly ash is mainly generated in RSA as a coal combustion byproduct during power generation in coal power plants.
The cement hydrate component of concrete sludge and demolished concrete, in powder form, is an ideal reagent for carbon capture. It is dispersed in water and then carbon dioxide-containing exhaust gas is bubbled into the solution under ambient temperature and pressure without any additional chemical substances, leading to the formation of stable solid mineral carbonates. While fly ash particles are fine and require no grinding, the energy implication would be more favourable.
The hydrates in concrete materials and fly ash, such as calcium hydroxide, calcium silicate hydrate and ettringite, are well described. Several other metals and rare earths are also contained in fly ash. The hydrates are effective in remediation and sequestration process. The main mechanism is the provision of alkalinity, resulting in pH rise, metal hydroxide and Ca carbonate precipitation. Ion exchange with the ettringite structure is also possible. It is expected that both the cement hydrate fine powder (from concrete) and fly ash can be used simultaneously for CO2 sequestration and Acid Mine Drainage (AMD) treatment such that mineral carbonation by-products and clear water can be produced, which can be reused as process water or further processed to potable water through further treatment using reverse osmosis.
This project is categorised as developing a technique to fix carbon dioxide in the form of stable carbonate minerals using alkaline waste, thus mitigating against global warming. Many of the previous studies on carbon capture processes were directed at basic calcium-containing minerals which show high potential for carbon capture, also in the South African context.
Overall, toward a decarbonised society centred on the cement industry, the SATREPS project aims at developing a carbon recycling system based on carbonate mineralisation of basic wastes by introducing the concept of a circular economy. In the system, CO2 is fixed in minerals, while by-products are recycled into cement production. For non-recycled by-products, a new circular loop will be formed as environmental remediation agents. Since it is considerably important that both of CO2 mitigation technology and application of environmental remediation agents is easy to procure, operate and maintain equipment at low cost in developing countries, the project on the carbon recycling will be conducted by using concrete wastes and fly ash generated in RSA.
The Overall Goal
Carbon/material recycling systems using the proposed technology (MCC&U) is applied widely in Republic of South Africa (RSA) on the basis of circular economy toward an establishment of a decarbonized society.
Carbon recycling systems including resource recycling loops is developed to reduce process-related CO2 emissions from the cement industry.
Development of direct carbonation of demolished concrete (MCC&U1)
Pilot plant installation and its operation for concrete sludge indirect carbonation technology (MCC&U2)
Development of mineral carbonation process by bipolar membrane electrodialysis (MCC&U3)
Strategy for social implementation of MCC&U is presented
Dispatch of experts and researchers for utilization of carbon dioxide, environmental chemical engineering, metal resource recycling system, and so forth
Provision of equipment
a) Various carbonation reactors including pilot plant
b) Analytical equipments including Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES)
Training in Japan
[South African side]
C/P personnel staffing
Project office space with equipments and furnitures
Local cost expenditure
a) Operation and maintenance cost for pilot plant (Running cost for Pilot Plant operation)
b) Running public cost for utilities
c) Domestic travel expenses