Microwave Dielectric Response And Microwave Pyrolysis Characteristics Of Low-rank Coal

Microwave heating has unique characteristics of selective heating,uniform heating,high thermal efficiency,low energy consumption and easy control.Microwave pyrolysis technology has good development potential and application prospect in coal classification and transformation.The microwave dielectric response parameters of the coal and the semi-coke are obtained by the test and theoretical calculation,and the efficiency of the microwave energy is analyzed.Through the in-depth study of the pyrolysis products,the main technological conditions that affect the microwave pyrolysis of coal are revealed.The mechanism and mechanism of microwave pyrolysis of coal are deeply analyzed.The strengthening and catalytic processes and methods are optimized,and the technological parameters are optimized,providing theoretical basis and technological basis for the clean and efficient utilization of coal based on microwave effect.Four low-rank coals,Huaxia long flame coal,Shenshan long flame coal,Shengli lignite and Ulanqab lignite,were selected as the research object.The influence of coal properties on microwave dielectric response was deeply analyzed,and the effect of char transformation and property transformation on dielectric loss mechanism transformation during microwave pyrolysis is further clarifying,and the microwave response mechanism of microwave pyrolysis process were also clarifying.On this basis,taking Huaxia long flame coal as the main research object,and comparing conventional pyrolysis,the effects of different pyrolysis conditions on the heating up characteristics,the distribution and composition of pyrolysis products were systematically studied.According to the research results,a microwave intensified pyrolysis process of coal under the action of absorbents was proposed.The dielectric response and microwave heating characteristics of different absorbents were systematically investigated.The optimum microwave absorbents and the amount of absorbents for the test coal were selected and determined by the microwave absorbents intensifying experiment.Combined with the advantages of microwave pyrolysis and the characteristics of catalytic pyrolysis,the catalytic microwave pyrolysis of coal under the promotion of catalyst was further proposed.The heating and pyrolysis characteristics of coal microwave heating were studied under the action of catalyst.The mechanism of microwave catalytic pyrolysis was explored in depth analysis of product distribution and product properties.According to the characteristics and advantages of catalytic pyrolysis of microwave pyrolysis,further puts forward the catalyst for catalytic microwave pyrolysis process of coal under the action of catalyst to promote coal by microwave heating temperature and pyrolysis characteristics,in-depth analysis of the product distribution and product quality,to explore the mechanism of microwave catalytic pyrolysis.The main conclusions of this study are as follows:1.Study on microwave dielectric response of low-rank coal?1?The quantitative relationship between microwave energy utilization efficiency and dielectric complex relative dielectric constant and two important parameters,microwave absorption coefficient and dielectric loss average power density,which describe the efficiency of microwave energy utilization are deduced,is derived.The complex relative permittivity of coal samples is tested by vector network analyzer.The microwave dielectric response capability of low-rank coal is evaluated by theoretical calculation and penetration depth,microwave absorption coefficient and average power density of dielectric loss.The results show that moderate moisture and kaolinite,feldspar,pyrite and other better dielectric minerals can help to promote the microwave dielectric response of low-rank coal,and increase the microwave absorption coefficient and dielectric loss average power density.The particle size should not be greater than the penetration depth,otherwise the heating inhomogeneous phenomenon may appear.Through the relationship between coal properties,microwave heating and temperature response and dielectric response parameters,we can reasonably predict the heating characteristics of coal samples at low temperature stage,and provide a theoretical basis for coal selection,pyrolysis process and equipment selection.?2?With the increase of microwave irradiation time,the phenomenon of"graphitization"in the semi-coke occurres,and the dielectric properties increases,and the electrical resistivity decreases.The mutation occurred in the 15²0 min stage.The theoretical calculation values of the imaginary part of the complex relative permittivity??_r??obtained by the semi focal resistivity are basically the same as that of the measured values.It is shown that the increase of the conductance loss of the semi coke in the process of microwave pyrolysis of coal compensates the decrease of polarization loss due to the loss of polar groups.2.Study on microwave pyrolysis characteristics of low-rank coal under different pyrolysis conconditions.?1?When the microwave power is 800 W,the final pyrolysis temperature is 600?,the limited-temperature is 30 min,the pyrolysis degree is relatively deep,the output of gas is relatively large,and the tar yield reaches the maximum.When the microwave power is 800 W and microwave continuous radiation,the gas output increases significantly again.After 30 min,the change of the yield of the pyrolysis products tends to be flat.Compared with the conventional pyrolysis,the output of microwave pyrolysis gas is generally increased,and the content of H₂,CH₄and CO is above 82%.Among them,the content of H₂ is the largest,usually around 50%.?2?FTIR analysis showes that,compared with conventional pyrolysis tar,the aliphatic rate of carbon decreased,the aromatic rate of carbon increased,the average condensation ring number and ring condensation index increased.The average condensation ring number of tar under the condition of limited-temperature radiation and continuous radiation is between 2.56².87.With the increase of continuous radiation time,the ring condensation index RCI of tar increased as a whole.Under different limited-temperature radiation time,the ring condensation index RCI basically did not change.The content sequence of tar is as follows:asphaltene>colloid>aromatic hydrocarbon>saturated hydrocarbon.The content of asphaltene is about 50%,which is far greater than that of conventional pyrolysis,and the content of colloid,aromatic hydrocarbon and saturated hydrocarbon is generally lower than that of conventional pyrolysis.When the microwave power was 800 W and the final pyrolysis temperature was 600?,the yield of each component was relatively high.The asphaltene content in the family composition was significantly increased,and the content of gum,aromatic hydrocarbon and saturated hydrocarbon decreases.The longer the time of microwave continuous radiation was,the precipitation of aromatic hydrocarbon components could be precipitated.GC-MS analysis showed that the content of fatty hydrocarbons in tar was significantly reduced under the conditions of microwave limited-temperature radiation30 min and continuous radiation of 30 min compared with conventional pyrolysis tar.The order of the content of benzene,its derivatives and polycyclic aromatic hydrocarbons was continuous radiation>limited-temperature radiation>conventional pyrolysis.The components of oxygen content are mainly phenol and its derivatives.The order of the content is:conventional pyrolysis>limited-temperature radiation>continuous radiation.The order of the content and size of light hydrocarbons and derivatives below C9 was:conventional pyrolysis>limited-temperature radiation>continuous radiation.?3?Compared with the raw coal,the high calorific value of the semi-coke under the limited-temperature radiation and the continuous radiation increased by 12.31%¹7.93%.Compared with the conventional pyrolysis char,the calorific value had little change.The calorific value of the semi-coke under continuous radiation is generally lower than the semicoke under the limited-temperature radiation.The limited-temperature radiation char and continuous radiation char contained abundant mesopores and macropores.The specific surface area was smaller than that of conventional pyrolysis char,and the total pore volume and average pore size were larger than those of conventional pyrolysis char.?4?A gasification reactivity parameter R_C,which is suitable for isothermal thermogravimetric analysis,is proposed.The gasification reactivity can be characterized accurately,and the coal char with low reactive activity can be accurately characterized.The R_C judgement showed that the CO₂reactivity of the microwave pyrolysis char was much lower than that of H₂O?g?,and the overall gasification reactivity of limited-temperature radiation char was stronger than that of the continuous radiation char.3.Study on the enhanced microwave pyrolysis of low-rank coal?1?The average heating rate of activated carbon,silicon carbide and recycle char as absorption agent increased by nearly 2 times,especially in the initial heating stage of 200?.Microwave absorbers could help the transfer and transfer of large molecules in the coal particles,and the yield of tar is obviously improved,and the yield of the semi coke is obviously reduced.?2?Activated carbon,silicon carbide and recycle char promoted the yield of H₂ significantly decreased,the yield of CH₄ and CO increased significantly,CO₂ yield increased slightly,C1^C2hydrocarbon content was significantly higher than that of no absorbent,and the calorific value of gas increased significantly.The yield of the constituent components of the middle group of tar is generally increased,but the promotion effect on each component is different.The content of hydrogen in tar is increased,the content of sulfur was reduced,the aliphatic rate of carbon was obviously increased,the aromatic rate of carbon was obviously reduced,and the average condensation ring number and the ring condensation index RCI decreased.The moisture of the semi-coke decreased,the ash content increased,the volatile matter increased,the fixed carbon decreased,the content of carbon and sulfur decreased,and the oxygen content increased.The absorbents had little effect on the calorific value of the semi coke.?3?Activated carbon,silicon carbide and cyclic semi-coke had good effect on microwave pyrolysis,and the effect was similar.From the aspects of product separation,recycling and economy,the circulating semi-coke produced by coal was more promising.With the increase of the addition of char,the yield of char decreased slowly,and the rate of gas,tar and pyrolysis continued to increase.At 30%,the output of gas increased obviously,and the semi-coke and the rate of pyrolysis water decreased.?4?As the amount of cyclic semi-coke increases,the H₂ content of gas was increasing,and the content of CO and C1^C2 hydrocarbons decreased.The yield of asphaltene and aromatics in the composition of tar was increasing,the H/C was increasing,and the change of O/C was not great.The aliphatic rate of carbon gradually increased,the aromatic rate of carbon gradually decreased,and the average condensation ring number and the ring condensation index RCI decreased.The content of aliphatic hydrocarbons in tar was significantly increased,and the content of benzene and its derivatives and polycyclic aromatic hydrocarbons decreased,and the phenols increased.The semi-coke produced a rich pore structure and increased the reaction activity,and the greater the amount of circulating semi-coke,the stronger the activity.4.Study on catalytic microwave pyrolysis of low-rank coal?1?Three metal compound catalysts?K₂CO₃,CaCl₂ and FeSO₄?were selected,and the results showed that they had the function of microwave absorbing agent,especially at the initial heating stage within 200?.The heating effect of ion conduction dielectric loss could increase the heating rate of coal samples and avoid the emergence of the rising temperature and gentle period,but the final temperature of heating was reduced.?2?K₂CO₃ and FeSO₄ promoted the yield of char significantly decreased,tar and pyrolysis rate decreased,the output of gas increased significantly,the product structure could be effectively reformed,and CaCl₂ had no obvious effect on product structure reforming.?3?Compared with no catalyst,the output of CO and CO₂ increased significantly under the catalysis of K₂CO₃ and FeSO₄,the content of C1^C2 hydrocarbon gas decreased and the calorific value of gas decreased significantly.Under the catalysis of CaCl₂,the yield of H₂ decreased,the content of C1^C2 hydrocarbon gas increased and the calorific value of gas increased.Under the catalysis of K₂CO₃,the content of asphaltene and aromatics in tar group composition decreased,and the gum and saturated hydrocarbons increased.The composition of the group changed little under the catalysis of CaCl₂.The content of colloid and aromatic hydrocarbons increased obviously under the catalysis of FeSO₄.The tar H/C increased under the action of three catalysts.GC-MS analysis showed that three kinds of catalyst for aliphatic hydrocarbons under the action of tar increases significantly,reduce the content of benzene and its derivatives,reduce the content of polycyclic aromatic hydrocarbons K₂CO₃ and FeSO₄,increase the content of polycyclic aromatic hydrocarbons CaCl₂;K₂CO₃ and FeSO₄ phenols decreased,CaCl₂ phenol increased.Under the action of three catalysts,the calorific value of the char was reduced,and CaCl₂ was the most significant.The cumulative pore volume of the semi-coke increased significantly,and the specific surface area of the semi-coke increased significantly under the action of CaCl₂ and K₂CO₃,and the FeSO₄ decreased significantly.Before 850?,the original pore structure of char promoted the activity of CO₂ greatly.After 850?,the pore structure of char had little effect on CO₂ reactivity,and the nature of catalyst itself had great influence.