Beneficiation of coal fly ashes by oxygen chemisorption

This paper addresses the issue of unburnt carbon in fly ashes from coal-fired power stations. It has been shown that carbon-in-ash has much lower combustion reactivity than the original coal, because of the extensive thermal annealing experienced in the boiler. Thermal annealing reduces the rate of the first reaction step of which carbon combustion is composed, namely oxygen chemisorption. In the present work experiments have been carried out in order to verify if a preconditioning stage consisting of mild pre-oxidation with air is able to promote oxygen chemisorption thus increasing the combustion reactivity of ashes. Fly ashes with high LOI have been exposed to air at temperature <400 °C for times up to 300 min. Results show that a satisfactory extent of oxygen chemisorption can be attained at 300–400 °C with 1–2 h holding time. After this conditioning, samples have been tested to check their combustion reactivity by means of non isothermal thermogravimetric analysis. Additionally, combustion experiments have been carried in a purposely designed suspension reactor at temperatures up to 1000 °C. Results confirm that ash pre-conditioning reduces the burn-out time of carbon in ash. The concept has been finalized into an international patent application.

► Unburnt carbon in fly ashes has low combustion reactivity because of thermal annealing.
► Thermal annealing reduces the rate of oxygen chemisorption on carbon ashes.
► Preconditioning with air at mild temperature can increase ash reactivity.
► A satisfactory uptake of oxygen can be attained at 300–400 °C with 1–2 h holding time.
► Combustion experiments up to 1000 °C show that conditioning reduces ash burn-out time.