Study On The Oxidation Characteristics Of Lignite And Its Upgraded Coals At Low Temperature

With the continuous reduction of high quality coal resource reserves inChina, the utilization of the low rank coal especially lignite is being paid moreattention. The high moisture content in lignite restricts its direct utilization, sothe drying process is necessary. However, lignite and its upgraded coals are easyto be oxidized during transportation and storage, which brings a lot of damage.The mechanism of low temperature oxidation and prevention of coalspontaneous combustion are the key problems to be solved in the utilization oflignite. Based on the above, XM lignite was selected as a research object in thispaper. According to the influencing factors of physical and chemical structure ofthe upgraded lignite，the effects of drying temperature and drying time on lowtemperature oxidation characteristics of upgraded lignite were investigated. Theunderstanding of the oxidation behavior of the lignite and its upgraded coals cansupply some theoretical basises for the efficient and safe utilization of lignite.Based on the changes of mass, heat and active groups of upgraded lignite, thelow temperature oxidation characteristics of different upgraded coals were researched and analysed. The main results in this study are summarized asfollows:(1) The oxygen adsorption amounts of lignite and its upgraded coals areinfluenced by many factors, mainly including the content and forms of water，the degree of damage to pore structure and the contents of aliphatic C-Hcomponents etc. The drying time and drying temperature work together to thebehavior of upgraded lignite adsorbing oxygen.(2) Drying temperature is important influencing factors on the changes ofmass, heat and active groups in the low temperature oxidation process ofupgraded lignite. When the oxidation temperature is lower than150oC, ligniteupgraded at relatively high drying temperature is not easy to be oxidized, due tothe decrease of the active component. When the oxidation temperature is higherthan150oC, for the lignite upgraded at relatively high drying temperature, theoxygen-containing compounds with high stability were generated, and theincrease of mass and heat release were higher during the process of oxidation.(3) Effects of drying time on the oxidation characteristics of upgraded ligniteat low temperature are connected with the drying temperature. For the ligniteupgraded at80oC, the longer drying time of coal was, the easier oxidation was.In the process of low temperature oxidation the increase of mass, heat release,conversion rate of aliphatic C-H groups and generation rate of oxygen-containing compounds (aldehydes, carboxylic acids, quinones and ketones, etc.)were increased with the prolonging of drying time. For the lignite upgraded at 160oC,240oC and320oC, with the change of drying time, the mass, heat andactive groups did not change significantly during low temperature oxidation.(4) The distinctions of existing forms and contents of aliphatic C-Hcomponent and C=O containing species are the major reasons for the differentbehaviors of low-temperature oxidation among different upgraded coals. Self-oxidation of the coal can be initiated by the oxidation reaction between oxygenand activated aliphatic C-H component, and generate C=O containing species.The reaction can take different paths according to the different functional groupsattached to the activated aliphatic C-H component. These differences causediverse reaction pathways of low-temperature oxidation among differentupgraded coals.