Change in Catalytic Activity of Potassium during CO2 Gasification of Char

Potassium (K)-catalyzed CO2 gasification of lignite char was studied with a particular focus on the change in catalyst activity with the char conversion (X) at 800–900 °C. Char samples were prepared from an Indonesian lignite by a sequence of complete removal of inherent metallic species and mineral matter, K-loading by ion-exchange, and pyrolysis. The catalytic activity of K (k cat ′) was defined as the rate of catalytic gasification (after elimination of the rate of non-catalytic gasification and that of K volatilization from total mass release rate from char) per amount of K retained by the gasifying char. k cat ′ increased by a factor of 5–20 with X over its range up to 0.98–0.99, depending on the initial K concentration in the char (m cat,0), ranging 0.16–1.4 wt %-daf. Such significant increase in k cat ′ was due to the change in not the intrinsic reactivity of char but its porous nature, that is, the size and volume of pores that retained the K catalyst. At X < 0.4, the entire portion of the K catalyst was confined in micropores (width 0.4, the gasification created greater mesopores (width >2.0 nm), providing spaces for growth in the size of the K catalyst and allowing promotion of its activity. However, for low m cat,0, its major portion continued to stay in micropores with a limited increase in k cat ′.