Microwave Pyrolysis Of Zhundong Coal And Its Transformation To Functional Carbon Materials

Microwave heating?MW-heating?technology has potential advantages in increasing the yield of pyrolysis gas and reducing environmental impact in some coal processing processes due to its unique selective heating property.Due to its unique selective heating,the pyrolysis process of coal and the physical and chemical properties of coal char formed are quite different from those of conventional heating?CV-heating?.Based on the MW-pyrolysis of coal,the activated carbon?AC?and other functional carbon materials can be used as catalysts in the process of methane catalytic decomposition?CMD?.Therefore,this thesis focuses on three key issues: "the correlation between the change of coal char structure and the ability of temperature rise in the process of MW-pyrolysis","the difference of physical and chemical properties between chars obtained from MW-pyrolysis and conventional electric furnace pyrolysis" and "constructing carbon based and Fe doped carbon catalyst suitable for CMD process".A modified K-type thermocouple was used to study the temperature rise characteristics of Zhun Dong raw coal and coal-absorber mixtures during MW-pyrolysis.The role of microwave absorber was discussed.The evolution of carbon crystallite structure in semi-char during pyrolysiswas studied by means of X-ray diffraction.The relationship between the aromatic structure and the microwave absorption capability of semi-char was studied.The results showed that the assistant heating effect of absorber is only dominated in the initial devolatilization stage.Increasing the weight percentage of absorber can dramatically reduce the temperature rise time in the stage.In the intense devolatilization stage,the temperature rise rate of semi-char first increases dramatically and then decreases.The microwave absorption capability of semi-char is found to be controlled by the combined effects of the aromaticity and aromatic layer diameter.In this thesis,the evolution of char structure during the non-isothermal low temperature pyrolysis of Zhun Dong coal under MW-heating was investigated and compared with that under conventional?CV?heating.FTIR analysis indicated that MW-pyrolysis promote the breaking of aliphatic side chains below 400?.However,the release of functional groups and the evolution of carbon microcrystals of coal under MWheating are slower than that under CV-heating from 400? to 700?,which should be related to the directional movement of oxygen free radicals under MW-field.But above 700?,the pyrolysis rate of coal under MW-heating accelerated.CV-pyrolysis was more conducive to pores development than MW-pyrolysis.The S_BET of CV-char at 600? was 7.0 m²/g,while that of MW-char is 3.5 m²/g.The combustion performance of MW-char was inferior to that of CV-char.The Reax FF MD method was used to simulate the pyrolysis process of coal in the MW-field.Three stages of coal pyrolysis with or without MW-field were obtained: activation stage,rapid thermal decomposition stage and thermal condensation stage.Through the analysis of the change of pyrolysis components at typical temperature,it was found that MW-heating can inhibit the escape of small molecular products,such as C₂H₄ and CO₂,during the pyrolysis activation stage.The decomposition rate of coal by MW-heating was faster than that by CV-heating,so the pyrolysis system first entered the polycondensation stage.At the same time,the yield of tar by MW-pyrolysis was lower than that by CV-pyrolysis.Through the observation of multi-carbon components,MW-field could inhibit the polycondensation in the stage of rapid thermal decomposition.Therefore,the final yield of pyrolysis gas obtained by MW-heating was higher than that by CV-heating,especially C₃H₆ and CH₄.Meanwhile,MW-heating promoted the migration of nitrogen to the pyrolysis gas,and the nitrogen component was HCN.Temperature rise characteristics of subbituminous coal impregnated with KOH in a wide range of KOH/coal ratio?w/w,0.25?2?under MW-heating were investigated and the catalytic performance in CMD over the resultant AC was measured.The results showed that with the increase of KOH content,the temperature rise ability of the coal-KOH adduct under MW-field increases gradually,and the maximum temperature that can be increased from 170? to 880?.The increase of aromaticity due to the structural rearrangement of the coal aromatic layers during KOH impregnation and the release of volatiles during MW-heating provides the main driving force for the continuous increase of temperature.When KOH/coal ratio is 1.5,it took 224 s for coal-KOH adduct to rise from room temperature to 800?,with the development of specific surface area(S_BET)and pore volume to the maximum of 661.8 m²/g and 0.512 cm³/g,respectively.However,the catalytic activity of AC with the largest S_BET for CMD is not the highest.At 800? and higher temperatures,the increase of active site number on the surface by the potassium intercalation mechanism promotes the catalytic activity of AC.Therefore,the research core of carbon-based catalysts from coal is to increase the number of active site,not the S_BET alone.Adding Fe?NO₃?₃ to MW-assisted KOH activation of coal to construct catalyst suitable for CMD process.The results showed that this method shortened the preparation time of metal based catalyst.Meanwhile,due to the carbothermal reduction reaction between coal char and iron oxide,the iron doped carbon catalyst did not need hydrogen reduction before the process of CMD.The methane conversion of the optimum catalyst was still 46% at 120 min.At the end,the method of MW-heating the mixture of ferrocene and graphite powder to form iron doped carbon was also explored.The results showed that most of the primary iron nanoparticles were agglomerated.However,in this condition,iron nanoparticles obtained could not meet the needs for CMD catalyst,because in a very short time of 10s?30s,iron particles were coated with carbon nanotubes?CNTs?and other structures.Based on the analysis of the morphology of iron particles,it was proposed that the coalescence between the particles in the iron agglomerates accelerates the formation of suitable CNT growth sites.