Effect of K₂CO₃ Addition on CO₂ Gasification Characteristics and Ash Sintering Behaviour of a Chinese Lignite at Different Temperatures and Pressures as Examined Using a High-Pressure Thermogravimetric Analyser

Abstract

The gasification characteristics and the morphology of the residue ash from pressurised K₂CO₃-catalysed gasification of a Chinese lignite in CO₂ was investigated using a High-Pressure Thermogravimetric Analyser operating at pressure of 2.0 or 3.5 MPa and temperature between 750 and 900 °C for at least 2 h, after being heated from room temperature at 10 °C/min. The K₂CO₃ addition was varied from 0 to 10% w/w. Gasification characteristics of the K₂CO₃-doped lignite was determined by analysing the weight loss and conversion rate as a function of time whereas the ash morphology was analysed by using SEM–EDS. Results showed that at 3.5 MPa the in-situ weight loss of the lignite increased as K₂CO₃ addition ratio increased, suggesting that K₂CO₃ addition promoted lignite gasification. The conversion rate of the lignite correspondingly increased from 61 to 92% as the temperature elevated to 750 °C. An increase in the final temperature to 900 °C significantly promoted lignite gasification when K₂CO₃ was less than 5%, however this was not obvious for lignite with 10% K₂CO₃ addition. This is because the conversion rate of the lignite with 10% K₂CO₃ addition had exceeded 90% before the final temperature of 900 °C was reached. Furthermore, as pressure decreased from 3.5 to 2.0 MPa, the lignite gasification rate slowed down, with or without K₂CO₃ addition. Conversion rate of the lignite decreased from 61 to 42% while the temperature initially elevated to 750 °C. SEM–EDS analysis revealed that sintering of the lignite ash was not observed at 750 °C, but became apparent at 1% K₂CO₃ addition. The degree of ash sintering further aggravated at 5 and 10% K₂CO₃ addition. As the temperature increased from 750 to 900 °C, the ashes of the raw lignite and 1% K₂CO₃ doped-lignite remained largely similar, whereas the sizes of the 10% K₂CO₃-doped lignite ash was increased and the particle surfaces became smooth, suggesting an enhanced sintering of the ash. The formation of K-aluminosilicate and Ca-aluminosilicate of low-melting points in the ash was responsible for possible deactivation of the doped catalyst K₂CO₃ and the observed ash behaviour.