Effect of pyrolysis conditions on the char gasification with mixtures of CO2 and H2O

Two kinds of char were prepared from a lignite by fast pyrolysis using a drop tube furnace and by slow pyrolysis using a fixed-bed furnace at the temperature of 1273 K. Scanning electron microscopy, X-ray diffractometry and the BET method were employed to characterize char properties. The chars were gasified with CO2, H2O and their mixtures in a thermogravimetric analyzer (TGA) system to investigate gasification kinetics and derive the rate expression. To validate the gasification rate equation derived from TGA, a fluidized-bed gasification experiment was also carried out. The results showed that both fast-char and slow-char were mainly composed of dense char. The shrinking core model was applicable to predict both gasification of fast-char and slow-char. It was found that the char gasification rate in the mixtures of CO2 and H2O was obviously lower than the sum of the two rates of the char independently reacting with CO2 and H2O but higher than the rate of each independent reaction, for both the fast-char gasification and slow-char gasification. Both of the results from the TGA and the fluidized-bed reactor showed that char-H2O reaction was independent on char-CO2 reaction, while char-CO2 reaction was inhibited by char-H2O reaction.