Study On Gasification Mechanism Of Lignite Under Carbon Dioxide Atmosphere

Lignite is a typical low rank coal which features with low heat value and combustion efficiency,high content of inherent moisture,volatiles and ash.It is suitable to be gasified and produce syngas.However,conventional coal water slurry?CWS?technology is not satisfied for lignite due to the low solid concentration in CWS and thermal efficiency of gasification system.Liquid CO₂/lignite coal slurry can improve the solid concentration in slurry.The thermal efficiency is also enhanced due to lower temperature and latent heat of liquid CO₂.Three kinds of lignite from Zhaotong?ZT?in Yunnan,Baiyinhua?BYH?and Wulagai?WLG?in Inner Mongolia,were adopted to study the gasification mechanism under CO₂ atmosphere in this work.Firstly,thermo-gravimetric analysis method?TGA?was applied to analyze gasification procedure of three lignite coal under different heating rate?30 ? /min and 50 ? /min?,atmosphere and concentration,and different coal pretreatment method.It can be observed that the lignite gasification at CO₂ atmosphere happened commonly in three stages?dehydration,pyrolysis and char gasification?.In the third stage?char gasification?,there was more weight loss at high heating rate.The second stage?pyrolysis?was not affected by gasification atmosphere.However,more carbon dioxide would improve the gasification reaction in the third stage.Pre-drying of lignite had diverse effect on three lignite coals.Lignite with low coalification degree was easily affected by pre-drying treatment during the gasification reaction with CO₂.Secondly,the kinetic analysis of lignite gasification under CO₂ atmosphere was performed.Differential method was applied.The “)(aay-” curve was adopted to determine the most probable mechanism function for different reaction stages.The apparent activation energy E and pre-exponential factor A were calculated.The kinetic parameters of gasification reaction in dehydration and char gasification stages could accurately describe the reaction in these two stages.However,the reaction in stage of pyrolysis was more complex.At last,TG-GC was applied to analyze the gasification reaction product?mainly including CH4,CO and H2?at heating rate of 30?/min.It turned out that the release rate and temperature of CH4,CO and H2 were different between the three coal samples.The content of CO was much higher than CH4 and H2.Their release temperatures were overlapped with each other for each lignite.Furthermore,same gas had different release temperature range for different lignite.