Study On Microwave Drying Characteristics And Pyrolysis Kinetics Of Zhaotong Lignite

Coal becomes the important way of solving the energy shortage problems due to the mass consumption of oil and natural gas.As the low-rank coals,reserves abundant lignite has captured much attentions with increasing high-rank coals consumption,which has low market prices and low sulfur content,composing approximately 13%of the national coal reserves in China.Clean and efficient utilization of reserves abundant lignite resources are currently being used as coal pyrolysis,coal gasification,coal liquefaction,and coal combustion for power generation.However,the high water content,high ash,and low calorific value in lignite are major problems for its clean and efficient utilization.Therefore,the drying of lignite is the first and essential step in its clean and efficient utilization process.Lignite pyrolysis is the important way of realizing the clean conversion of lignite,but is the premises of lignite gasification,which has the important effects on the gasification,liquefaction and combustion of lignite,respectively.However,how to realize the green efficient drying and demineralization pyrolysis upgrading of high moisture content and low calorific value in lignite,which has been the problems of efficient utilization in lignite.Compared with the traditional heating methods,microwave heating provides several advantages,including selective heating,rapid heating and effective energy saving,which has been widely used in different fields.The high moisture value of ZhaoTong lignite was selected as the coal sample of study.For one,the thin-layer drying characteristics and condensate water quality during drying process were studied by means of microwave heating.For another,the effects of heating rate and ash content on the pyrolysis characteristics of lignite were studied by non-isothermal heating methods.The main conclusion is summarized as follows:?1?The instantaneous surface temperature rise characteristics of lignite and the temperature of final drying increased with increasing the microwave power during microwave drying process.Microwave power level,lignite sample mass and the inital moisture value of lignite were the basic factors of influencing the microwave drying characteristics of lignite.The FTIR curve fitting method was used to analyze the changes of the oxygen-containing groups of lignite,while the content of OH and the total content of C=O,COO^-,and COOH decreased with the increasing in microwave power level.The COD and pH of waste water produced from microwave drying process of lignite were increased then decreased with increasing the microwave power level.And the minimum COD of waste water produced from microwave drying process of lignite was 99.89 mg O₂ L^-1,which was greatly friendly to environment.Organic matter of waste water in this microwave power level was analysed by GC-MS.According to the classification of organic functional groups,the relative content of organic matter was that aliphatic hydrocarbon>aromatic hydrocarbon>phenols and alcohols>acids>esters>ketones>ethers,respectively.?2?As microwave absorbers,lignite char and NaNO₃ mixed with lignite evenly during microwave drying process,which can greatly enhance the microwave absorption capacity of lignite.The drying rate and the moisture diffusion coefficient of lignite enhanced with increasing the microwave power level,NaNO₃ content,and the weight ratio of lignite char to lignite,respectively,and the rate of inner moisture diffusion to the surface of lignite enhanced,which can shorten the drying time.The Page model best described the characteristics of the lignite char on the lignite microwave upgrading,while the Henderson and Pabis model was the most suitable one with addition of NaNO₃,which both have the larger R² and the smaller of RSS and?².While drying rate constant increased with increasing NaNO₃ content and the weight ratio of lignite char to lignite,respectively.The apparent activation energy of different microwave power with0.25?5 of the weight ratio of lignite char to lignite and 5.0%of weight content of NaNO₃ from the modified Arrhenius equation were 579.44W/g and 286.87W/g,respectively.The instantaneous surface temperature rising rate of lignite can be improved with the addition of lignite char and NaNO₃ to lignite,thus resulted in the increase of the surface temperature of lignite,which expressed in the two ways of temperature rising.The unit electric power consumption of moisture evaporating and effective energy of microwave drying system of lignite decreased with the increase in the NaNO₃ content and the weight ratio of lignite char to lignite,respectively.Therefore,as microwave absorbers,lignite char and NaNO₃ acted as the catalyst and reduced energy consumption during microwave drying process.?3?ZhaoTong lignite was treated by demineralization method,it indicated that COOH content of lignite increased with the increase in the acid washing time,and hydroxyl content and the absorption peaks of minerals of lignite decreased with increasing the demineralizing time,respectively.The frist order kinetics model had the higher linear fitting correlation on the non-isothermal pyrolysis model of lignite,which increased its theory activation energy with increasing the temperature rising rate,but decreased with increasing the ash content of lignite.Therefore,ash content of lignite acted as the catalyst and decreased the activation energy of lignite pyrolysis during the non-isothermal pyrolysis process.