Mechanochemical activation of high-carbon fly ash for enhanced carbon reburning

Abstract

The focus of the present study is the reduction of the residual unburnt carbon contained in fly ash from PC-fired boilers by reburning and/or beneficiation. More specifically, the study addresses the potential of enhancing oxyreactivity of the residual carbon contained in fly ash by mechanochemical activation in order to improve the effectiveness of ash reburning. The concept is tested with reference to a high-carbon (nearly 50% by mass) fly ash sampled at the last field of the electrostatic precipitator of a utility boiler. The raw fly ash displayed moderate-to-low combustion reactivity and extensive development of anisotropic order possibly associated with extended molecular-ordered carbon domains. Mechanochemical activation was performed by ball milling in a lab-scale apparatus at 500 rpm and durations variable between 5 and 30 min. Both raw and mechanochemically activated samples have been characterized from the standpoint of particle granulometry and morphology, proximate and elemental analysis, ash constitution. Moreover, the influence of mechanochemical activation on the ability of ash samples to build up surface oxides has been characterized by temperature-programmed-desorption experiments. The assessment of the effectiveness of mechanochemical activation was based on the characterization of the intrinsic combustion kinetics of residual carbon by non-isothermal and isothermal thermogravimetric analysis. Experimental results indicate that a noticeable modification of DTG profiles, indicative of enhanced reactivity, is observed as a result of mechanochemical activation.