Carbon capture using wastes: A review

Potential key strategies for the management of anthropogenic CO2 emissions include mineral carbonation and storage in oil wells and in the oceans. In Europe, a large-scale demonstration of carbon capture and storage (CCS) has recently been given the go-ahead, and the application of mineral carbonation technology (MCT) to serpentine and olive-type minerals. Although less controversial in its approach, MCT involves intensive pre-treatment of the mineral feedstock, and a consequent high sequestration cost USD100-120/tonne CO2 treated. Mineralisation by carbonation is reliant upon the long-term storage of CO2 in thermodynamically stable and environmentally benign carbonate-based reaction products that are persistent over geological-timescales. The use of solid industrial process wastes for storing carbon (via waste carbonation technology, WCT) may provide a shorter-term gain, as the industrialisation of CO2 mitigation technologies takes place. With WCT, CO2 is reacted with alkaline waste residues, to both risk-manage a high pH, and utilise waste CO2 gas, can be used as a pre-treatment prior to landfilling, facilitate valorisation and production of new materials. The present work examines the current status of waste carbonation and investigates the utilisation of seven ‘common’ alkaline industrial residues showing that they have potential to sequestrate 1Gtonne of CO2 worldwide. The projected average cost of USD38-95/tonne of CO2, is competitive with landfill and projected carbon taxes. If WCT is more widely commercially developed an option for the management of significant amounts of carbon could become more quickly established

Enhancement of accelerated carbonation of alkaline waste residues by ultrasound

The continuous growth of anthropogenic CO2 emissions into the atmosphere and the disposal of hazardous wastes into landfills present serious economic and environmental issues. Reaction of CO2 with alkaline residues or cementitius materials, known as accelerated carbonation, occurs rapidly under ambient temperature and pressure and is a proven and effective process of sequestering the gas. Moreover, further improvement of the reaction efficiency would increase the amount of CO2 that could be permanently sequestered into solid products. This paper examines the potential of enhancing the accelerated carbonation of air pollution control residues, cement bypass dust and ladle slag by applying ultrasound at various water-to-solid (w/s) ratios. Experimental results showed that application of ultrasound increased the CO2 uptake by up to four times at high w/s ratios, whereas the reactivity at low water content showed little change compared with controls. Upon sonication, the particle size of the waste residues decreased and the amount of calcite precipitates increased. Finally, the sonicated particles exhibited a rounded morphology when observed by scanning electron microscopy

Accelerated carbonation of wastes and minerals

Accelerated carbonation technology (ACT) could be used for the stabilisation of hazardous wastes, remediation of contaminated soils and re-use/recycling of various waste streams. ACT has also potential for storing anthropogenic CO2 emissions into mineral silicates and alkaline waste residues via mineral or waste carbonation. Compared to ocean and geological storage, mineral and waste carbonation offer several advantages such as long-term storage and low monitoring requirements. Currently, the biggest challenge of mineral carbonation is the low conversion rate of calcium and magnesium-based minerals into thermodynamically stable carbonates under ambient temperature and pressure. Also, literature offers little information about physical techniques or chemical substances that could enhance the efficacy of accelerated carbonation of alkaline wastes. In this study, various carbonation techniques were applied for increasing the carbonation reactivity of magnesium hydroxide. The experiments were conducted under low temperature and pressure, while the maximum reaction time was 24 hours. Under these conditions the associated costs are kept to a minimum. The possibility of producing monolithic products with value-added was investigated by using blended mixtures of magnesium and calcium hydroxide. These mixtures were cured in carbon dioxide for 7 and 28 days and their physical properties were measured and compared with the properties for normal and lightweight concrete. Moreover, several alkaline residues were carbonated with the aid of ultrasound and four candidate catalysts (acetic acid, ethanol, sodium hypochlorite and sodium nitrite) and their CO2 uptake was measured. During sonication the variables: ultrasonic frequency, water content and treatment time were examined, while the applied chemicals were added at three different molarities (0.1 M, 0.5M and 2.5M). Throughout this work a number of analytical techniques were used for the characterisation of the raw and carbonated materials. These techniques included XRay fluorescence, X-ray diffraction, wet laser analysis, total organic carbon analysis and scanning electron microscopy. The results showed that the CO2-reactivity of Mg(OH)2 was low due to thermodynamic constraints that inhibited the rapid diffusion of CO2 into the system. The mixtures composed of pure Mg showed improved compressive strength and bulk density. In addition, sonication at low water content was weak, as there was lack of enough water to facilitate cavitation. On the other hand, at high water content the achieved CO2 uptake of the products increased by up to four times, as the wet conditions enhanced the cavitation of the solid particles. Finally, it was found that ethanol and acetic acid promoted the hydration rate of CO2 during accelerated carbonation, while minerals phase analysis did not reveal the formation of toxic by-products. In conclusion, the findings of this study proved that sonication depends highly on water content and is favoured at wet conditions. Furthermore, acetic acid and ethanol are two chemicals with potential to ameliorate the accelerated carbonation of various industrial wastes without the formation of un-desired or toxic compounds