Detailed Kinetic Analysis and Modeling of Steam Gasification of Char from Ca-Loaded Lignite

Abstract

A kinetic model of steam gasification of Ca-loaded lignite char has been proposed through the analysis of 41 sets of kinetic data obtained by thermogravimetry with different combinations of Ca concentration, temperature, and partial pressures of hydrogen and steam. The model quantitatively describes the change with time of the char conversion over its entire range by assuming progress of non-catalytic gasification and two different types of Ca-catalyzed gasification (Type-1 and Type-2) in parallel, all of which obey Langmuir–Hinshelwood mechanisms. The model attributes the catalysts for Type-1 and Type-2 to nanosized Ca-based particles and Ca species dispersed in an atomic scale, respectively. The initial concentration of Type-2 catalyst is saturated at a total Ca concentration over 1.0 wt %, while that of Type-1 increased in a linear manner with the total Ca concentration. The catalysis of Type-1 catalyst is more significant but diminished more quickly than that of the Type-2 one. Consequently, there was an optimum initial Ca concentration in the char around 2.0 wt % in terms of the time required for 99% char conversion. This trend is clearly different from that predicted from previous models that simply assumed loading saturation levels (LSLs). The kinetic model predicts characteristics of the steam gasification of the Ca-loaded char in an atmospheric bubbling fluidized bed at 800 °C. The characteristic time for the char gasification, which is given by the amount of in-bed char at steady state per feeding rate of the char (mc/Fc), is reduced by virtue of Ca by a factor of 7–8.