The pyrolysis of Xinjiang Runbei coal were carried out on a small bubbling fluidized bed and the products were characterized by different methods

such as gas chromatography and SEM

to investigate the effects of CH_4 and CO_2 atmosphere on the pyrolysis characteristics of coal with semi-coke as the heat carrier.The results show that

compared with the single atmosphere

the gas product yield in the CH_4/CO_2 atmosphere reduced

which was contributed to the promoted reforming reaction between CH_4 and CO_2 by coal semi-coke.The decreasing degree increased with the temperature and CO_2 concentration.When the pyrolysis temperature reached above 600 ℃

the yields of CO

H2 and pyrolysis water significantly increased with the CO_2 concentration and temperature.The tar yield increased with the temperature in the CH_4/CO_2 atmosphere

due to the reactions of methane cracking

methane/carbon dioxide reformation

and carbon dioxide/carbon gasification.The tar yield reached its maximum

and achieved a large increase at about 600 ℃.Owing to the influence of some reactions

such as methane cracking reaction and carbon dioxide and semi-coke gasification

the yield of semi-coke increased in CH_4 atmosphere but decreased in CO_2 atmosphere.In the CH_4/CO_2 atmosphere

the yield and characteristics of semi-coke also exhibited distinct changes

which was more obvious with the increase of pyrolysis temperature.At low CO_2 concentrations(6%)

CO_2 promoted the cracking of CH_4 more than the gasification of CO_2 and carbon.Therefore

the yield of semi-coke increased and the semi-coke had smaller porosity and rougher surface.As the CO_2 concentration was increased to 15%

the gasification reaction of CO_2 and carbon was enhanced

which caused obvious reduction in semi-coke's yield and increase in its porosity.