Re-examination of Thermogravimetric Kinetic Analysis of Lignite Char Gasification

Abstract

Kinetic analysis of CO2 gasification by thermogravimetry was examined employing chars from a Victorian lignite. The gasification of char from the rigorously acid-washed lignite was extremely slow. More than 600 min was required for the complete gasification as a result of a near absence of inherent alkali and alkaline earth metallic (AAEM) species. It was nonetheless necessary to strictly choose the following conditions for eliminating the mass transfer effect: an initial mass of char of <1.3 mg, a particle size of char of <125 μm, and a total gas flow rate of ≥1000 mL standard temperature and pressure min–1. Such requirement arose from the inhibition of non-catalytic gasification by CO within the char particle. On the other hand, the char from the original lignite was gasified much faster than the non-catalytic gasification and completed within 50 min by catalysis of the inherent AAEM species. The measured rate of catalytic gasification was, unexpectedly, independent of the initial char mass and particle size and gas flow rate. This was reasonably explained by no or little importance of CO inhibition in the catalytic gasification. This study has also drawn another important conclusion that the non-catalytic gasification obeyed first-order kinetics with respect to the char mass. The rate constant was steady over the entire range of char conversion, while both the specific surface area and carbon-type distribution of the char varied along with the conversion.