Effect of Initial Potassium Catalyst Concentration on Its Activity and Fate in CO₂ Gasification of Lignite Char

K-catalyzed gasification of char and carbon has been studied for more than 40 years while many different types of relationships between specific rate of gasification (rsp = dXdt(1−X), X; char conversion) and K concentration in gasifying char/carbon (CK) have been reported. This work explored the mechanism causing such diversity in the rsp–CK profile. An ash-free lignite was impregnated with K2CO3, devolatilized, and then gasified with CO2 at 800 °C and CK at the start of gasification (CK,0) of 0.15–2.66 mol-K/kg-daf-char. CK was accurately determined by measuring the K volatilization rate as a function of X. For CK,0 = 0.15, rsp increased linearly with CK while the K catalyst activity (kc′ = rsp/CK) remained unchanged at 0.013 min–1 mol-K/kg-daf-char–1. The K species underwent transformation to much more active catalyst (kc′ ≈ 0.130 min–1 mol-K/kg-daf-char–1) for CK,0 = 0.64–2.66, where the rsp–CK profile was independent of CK,0, and rsp reached an upper limit due to saturation of gasifying char matrix with the catalyst. The catalyst transformation also occurred for CK,0 = 0.28–0.39 while kc′ of the transformed catalyst was steady at 0.03–0.08 depending on CK,0. Thus, the variety of the rsp–CK profile arose from that in CK,0. Higher CK,0 led to greater dX/dt and then dCK/dt under a steady supply of oxygen (as CO2) that anchored K on the char surface, allowing atomic-level dispersed C–O–K and/or K2CO3 to transform into clusters consisting of KxOy(CO2)z (y/x > 0.5, z/x ≈ 0.16). The most active cluster was formed even during the period of devolatilization for CK,0 > 1.69. Increasing CK,0 from 0.15 to 2.66 (by a factor of 18) resulted in shortening of the time required for X = 0.50 and 0.99 by factors as large as 1/200 and 1/100, respectively, due to the above-mentioned CK,0 effect on the rsp–CK relationship.