Effects of de-ashing on the micro-structural transformation and CO2 reactivity of two Chinese bituminous coal chars

Abstract

Two Chinese bituminous coals were de-ashed by hydrochloric and hydrofluoric acids; and their gasification reactivity with CO 2 were investigated using isothermal thermo-gravimetric method at constant temperatures of 900-1200°C, respectively. The effects of micro-structural transformation of coal chars before and after de-ashing were analyzed using X-ray diffraction (XRD) and laser Raman. Results indicate that the de-ashing treatment gives rise to the partial damage of carbon structure, and then forming more amorphous carbon during pyrolysis. Such amorphous carbon with high D band fractions of Raman spectrum demonstrate relatively less regular structure. The disordered structure allows more gasification agent (i. e. CO 2 ) penetrate into the inner of char, which probably promotes the gasification reactivity of it. The nano-rod structure forms during the raw coal char CO 2 gasification by the strong polymerization of pyrolysis gas. The de-ashing process is considered to diminish the support point for secondary polymerization, which causes more decomposition. The conversion ratio and reaction rate are increased. As a result, the reactivity of Shenhua coal char exhibits less change after de-ashing due to the competitive effect between vanishing of catalysis of ash and micro-structural transformation of char; in contrast, the reactivity of Yanzhou coal char after de-ashing presents apparent improving because of the less catalysis of ash and more micro-structural damage.