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.KeywordsAsh morphologyCatalytic gasificationGasification rateHP-TGAK2CO3Pressure