Effects of CO_(2)atmosphere on slow pyrolysis of high-ash lignite

Slow pyrolysis of a typical high-ash lignite in China was carried out in atmospheres of N_(2),20%CO_(2)/N_(2),40%CO_(2)/N_(2),and 60%CO_(2)/N_(2)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 CO_(2)almost behaves as an inert gas in the evolution of char and CO before 600℃.The decrease in char and CO_(2)yields and the increase in CO yield at temperatures higher than 600℃are ascribed to the occurrence of char-CO_(2)gasification.The higher the CO_(2)level present in the atmosphere,the higher the severity of the gasification becomes.Introducing CO_(2)into the atmosphere raises tar and water yields but cuts down H_(2),CH_(4),and C_(2)H_(6)yields.The promotion of tar yield results from the higher phenols and aliphatics yields in the presence of CO_(2).The reduction of H_(2)yield is associated with the increase in water yield,while the decline of light hydrocarbon gases is connected with the suppression effects of CO_(2)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-CO_(2)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 CO_(2)-containing atmosphere greatly depends on inherent minerals.Without minerals,tar yield is inhibited in the presence of CO_(2).In the absence of minerals,CO_(2)hardly has influences on CH_(4)and C_(2)H_(6)yields.