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 (kcat′) 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. kcat′ 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 kcat′ 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 <2.0 nm) having relatively small kcat′, although it increased gradually. At X > 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 kcat′.