Low Rank Coal Pyrolysis Kinetics And Pyrolysis Char Properties

Coal pyrolysis(or called dry distillation)is the basic step in the process of coal gasification and liquefaction.It is a key way by coal classification conversion to improve the utilization efficiency of low rank coal.The coal pyrolysis process comprises complex chemical reactions and physical changes.Coal pyrolysis includes a series of complex physical and chemical changes,operating conditions have an important influence on the distribution of pyrolysis products.It is difficult to be accurately described with qualitative and quantitative analysis of pyrolysis process.Rapid pyrolysis could improve the yield of tar,which is significant to the pyrolysis and upgrading.Wire mesh reactor is suitable to study coal pyrolysis reaction kinetics,because the sample has an independent layout and adjustable heating rate range.In the paper,the apparent pyrolysis kinetics of low rank coals were studied by thermogravimetric analysis firstly,and the overall n-th order reaction model and distributed activation energy model were discussed.The low rank coal were pyrolysed in wire mesh reactor at different operation conditions,With the help of Raman spectrum analysis,the change of coal structure in the pyrolysis process was investigated.Then,the kinetic process of fast pyrolysis was investigated and the kinetic parameters were obtained.The pyrolysis solid products were characterized by thermal balance(TG),gas chromatography-mass spectrometry(GC-MS),Raman spectrometer and scanning electron microscope(SEM)to explore how the heating rate influence coal pyrolysis process.The results and conclusions are as follows:1.Under fast pyrolysis conditions,temperature,heating rate and pyrolysis time would affect the the total pyrolysis volatile yield of coal.When the temperature were between 400 and 750 °C,total pyrolysis volatile yield would increase with the temperature.The devolatilization process occured mainly during the heating period,87% of total pyrolysis volatile were generated at the stage.When the heating rate is higher,more volatile could separate out.The tar of fast pyrolysis had more heavy components,including saturated alkanes(C18-31)and phenols.2.The paper compared physical and chemical properties of pyrolysis char at different heating rates.The results shows that with the heating rate is higher,the pyrolysis reaction become more intense.At the same time,more weak bonds break,the char will have more ordered structure,aromatic structure decreased while graphite structure increased.Because the large amount of volatile components separated out,the char would suffer more swelling stress and greater shrinkage at higher temperature,which brought more surface cracks.3.By the TG and DTG curves,the apparent activation energy of the pyrolysis of the two kinds of lignite was obtained with the overall n-th reaction model and the distributed activation energy model.Over all n-th reaction model divided the pyrolysis process into three stages,average activation energy was obtained at each stage,the apparent activation energy of Hulun Buir lignite were 39.62,29.41 and 61.23 kJ/mol;Shengli lignite was 34.15,33.28 and 76.16 kJ/mol.The apparent activation energy of two kinds of lignite increased with raising conversion rate by the distribution of activation energy,the activation energy of pyrolysis of Hulun Buir lignite is 183.01~490.83 kJ/mol,and Shengli lignite is 256.15~538.52 k J/mol.Because the base of two models are different,the kinetic parameters obtained are quite different,and the distributed activation energy model is more suitable to study the pyrolysis kinetics of low rank coal.4.In the wire mesh reactor,the relationship between total volatile yield and temperature were acquired at the heating rate of 1000°C /s.In the process of rapid pyrolysis of coal particles,the influence of mass transfer and heat transfer is overcome,pyrolysis process were controled by reaction rates,overall n-th reaction model was applied in the process.The intrinsic activation energy of three lignite by Coats-Redfern methed were Xiaolongtan lignite(78.12 kJ/mol)> Shengli lignite(58.87 kJ/mol)> Hulun Buir lignite(53.26 kJ/mol).