Assessment of a Carbonation-Based CO₂ Utilization Process for the Valorization of CFBC Ash

Abstract

This work assesses a combined accelerated carbonation and wet granulation treatment applied to circulating fluidized bed combustion fly ash with the aim of producing secondary aggregates for civil engineering applications and of achieving a net storage of CO2. The experiments were carried out at both a laboratory scale and larger scale, and the effects of the CO2 content of the gas flow (40 or 100%), temperature (from 25 to 60 °C), and the use of an alkaline activator solution as binder for the granulation process were investigated. Specifically, the particle size distribution, aggregate crushing value, leaching behavior, and CO2 uptake of the products after 28 days curing under ambient air were analyzed. In addition, the energy requirements of the process were estimated on the basis of the results of the larger scale tests and were used to calculate the CO2 emissions of the process to estimate the net CO2 avoided that could be achieved per kilogram of produced aggregate. The carbo-granulation process allowed us to achieve a relevant increase in particle size with respect to the starting material. The conditions that yielded the best performance in terms of product properties (both technical and environmental) and the maximum amount of CO2 avoided (above 75 g of CO2/kg aggregate) was the carbo-granulation treatment performed at 60 °C with water as binder and a gas phase containing 40% CO2. Although the products obtained employing the alkaline activator solution presented a lower mobility of trace elements of potential environmental concern and generally a higher CO2 uptake compared to the granules produced with water as granulation binder, the carbon footprint of the additives (sodium silicate in particular) would make the process carbon positive, even considering the CO2 avoided by replacing natural aggregates.