Accelerated carbonation for the treatment of MSWIr : optimisation and reaction modelling
Moist calcium silicate minerals are known to readily react with carbon dioxide (CO2). The Accelerated Carbonation of hazardous wastes is a controlled accelerated version of the naturally occurring process. The solid mixture is carbonated under a gaseous, CO2 rich environment at slightly positive pressures (3 bar), which promotes rapid stiffening of the non-hydrated product into a structural medium within minutes. In addition, an increased binding of toxic metals occurs as the solid carbonates. Today, Accelerated Carbonation is a developing technology, which may have potential for the treatment of wastes and contaminated soils and for the sequestration of CO2, an important greenhouse gas. The consequent significant improvement in the properties of certain treated materials can facilitate reuse in a variety of construction applications. Accelerated Carbonation represents a potential solution to sustainable waste management, the problem of decreasing landfill space in the UK, rising CO2 emission levels and the depletion of natural aggregate resources. This thesis reports on the application of Accelerated Carbonation for the treatment of Municipal Solid Waste Incinerator residues (MSWIr). The treatment imparts chemical and mineralogical changes to the residues, which reduce their environmental impact through encapsulation of hazardous components and cementation by carbonate precipitation. Given the viability of carbonation as a process to treat MSWIr, this investigation focused on optimising the fundamental parameters determining the extent and quality of carbonation of these residues. Major attention was also given to the modelling of the kinetics and mechanism of the carbonation reaction of Air Pollution Control Residues (APCr). The kinetics were studied in a batch carbonation rig designed and built at University College London. In addition, the major physical and chemical changes in APCr and Bottom Ashes (BA) after carbonation were evaluated using various analytical techniques. In addition, a commercial feasibility study has been carried out which confirmed the considerable and immediate potential for the commercialisation of Accelerated Carbonation technology for the treatment of municipal MSWIr. This conclusion was reached by analysing the market and industry for waste management, competing innovations and the capacity of Accelerated Carbonation to enable the recycling and reuse of CO2 and solid wastes. This work provides a fundamental understanding of the Accelerated Carbonation reaction of APCr essential to further ascertain the scale-up parameters required for the design of a large scale continuous process.