Effects of CO2 atmosphere on slow pyrolysis of high-ash lignite

Abstract

Slow pyrolysis of a typical high-ash lignite in China was carried out in atmospheres of N2, 20%CO2/N2, 40%CO2/N2, and 60%CO2/N2 in a fixed bed reactor. The evolution of char, tar, and gases in yield and characteristics, and the physico-chemical characteristics of chars in different atmospheres were compared. Results revealed that CO2 almost behaves as an inert gas in the evolution of char and CO before 600 °C. The decrease in char and CO2 yields and the increase in CO yield at temperatures higher than 600 °C are ascribed to the occurrence of char-CO2 gasification. The higher the CO2 level present in the atmosphere, the higher the severity of the gasification becomes. Introducing CO2 into the atmosphere raises tar and water yields but cuts down H2, CH4, and C2H6 yields. The promotion of tar yield results from the higher phenols and aliphatics yields in the presence of CO2. The reduction of H2 yield is associated with the increase in water yield, while the decline of light hydrocarbon gases is connected with the suppression effects of CO2 on methyl decomposition. The role of inherent minerals was also investigated by comparing product yields from raw coal and demineralized coal. The comparison indicated that the char-CO2 gasification rate is sharply reduced to nearly zero without the catalytic effects of calcium, iron, and magnesium minerals. The evolution of tar and light hydrocarbon gases in the CO2-containing atmosphere greatly depends on inherent minerals. Without minerals, tar yield is inhibited in the presence of CO2. In the absence of minerals, CO2 hardly has influences on CH4 and C2H6 yields. Keywords: CO2, Inherent minerals, Lignite, Tar