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

Two Chinese bituminous coals were de-ashed by hydrochloric and hydrofluoric acids; and their gasification reactivity with CO2 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. CO2) penetrate into the inner of char, which probably promotes the gasification reactivity of it. The nano-rod structure forms during the raw coal char CO2 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.