Effect Of Minerals On The Dielectric Properties And Micro-structures Of Coke Chars

In recent years,microwave radiation pyrolysis has been widely used in all walks of life especially in the coal processing and application,which has been favored by many scholars.However,there are many problems in the process of microwave pyrolysis,such as the pyrolysis products are always unsatisfactory,the pyrolysis time is too long,and the energy consumption is so much.In addition,during the process of pyrolysis,the phenomenon of burning out of material is often caused by thermal runaway.Therefore,it is necessary to put forward the solutions for different pyrolysis conditions.The microwave energy absorption ability depends on the dielectric properties of materials,at the same time,the dielectric properties are related to the components and micro-structures of materials.Accordingly,it is certainly need to investigate the variation of dielectric properties of coal chars prepared by microwave pyrolysis,so as to provide a practical significance for the rational control of microwave pyrolysis.This paper took XinJiang bituminous coal and YunNan lignitous coal as the research objects.These two kinds of coal were treated by acid-washing methods to reach the purpose of ash removal,and then the coke chars were prepared from the raw coal and demineralized coal,what's more,the range of the pyrolysis temperature was from 700? to 1300?.Micro-structures and surface functional groups of coke chars were detected by the X-ray diffraction and the infrared spectroscopy analysis technology;and the thermal stability of coke chars were tested and analyzed by thermogravimetric analysis;besides the dielectric properties of coke chars was determined by the vector network analyzer.9 kinds of minerals,KOH?NaOH?CaO?MgO?Fe₂O₃?Al₂O₃?SiO₂?B₂O₃?TiO₂,are added to the demineralized coal,the aim was to study the effect of different minerals on the dielectric properties and micro-structures of coal chars.In the experiment,the author specially made the transverse and vertical comparisons for the data of dielectric properties,crystal structures and heating characteristics of coal chars.The following conclusions are obtained through detailed experimental research and analysis:?1?The pyrolysis temperature has a great influence on the dielectric properties of coal chars.With the increase of the coke making temperature,the dielectric properties of coal chars are stronger and their microwave absorption abilities are stronger too;When the pyrolysis temperatures?900?;1100??are lower than the ash melting point,the ash content of the raw coal hinders the development of the dielectric properties of the coal chars,and the original carbon layer structures are destroyed by the ash of the raw coal,therefore,their thermal stabilities are poor and the graphitization degrees are low;When the pyrolysis temperature?1300??is greater than the ash melting point,the ash melting can promote the development of the dielectric properties of coal chars,because the melting of ash reduces the ability of coal mass transfer,it is difficult to form the developed pore structures,which making the coal chars have good graphitization order and strong dielectric properties.?2?When the coke making temperatures are 700?,900? and 1100??ash melting point below?,compared with the raw coal chars,ash free coal chars possess much stronger dielectric properties;and the results of XRD scanning show that the microcrystalline structures of the raw coal chars are poorer and the graphitization degrees are lower;in addition,it is found that there are more hydrogen and oxygen functional groups in raw coal chars,and their aromaticity and graphitization degrees are lower.?3?When the pyrolysis temperatures are 900? and 1100?,and the proportion of additives is 4% and 6% of the furnace coal,different minerals have different effects on the dielectric properties and structures of coal chars,The conclusions as follows:1.Alkaline metal oxides?K₂O,Na₂O,CaO,MgO?can inhibit the development of dielectric properties of coal chars,and the inhibition effect array from strong to weak is K₂O>Na₂O>CaO>MgO;at the same time,the additions of alkaline metal oxides destroy the ordered micro-structures of the coal chars,the damaging effect in an order from strong to weak is K₂O>Na₂O>CaO>MgO,which is consistent with the law of dielectric properties.2.The transition element oxides?Fe₂O₃?hinder the development of the dielectricproperties of the coal chars,which make the micro-structures of coal chars become worse;however,their destructive effects are weaker than that of alkaline metal oxides.3.The transition element oxides?TiO₂?,the carbon group element oxides?SiO₂?,as well as the boron group element oxides?B₂O₃?have promoting effects on the development of the dielectric properties of the coal chars,and the more oxides content,the stronger the effects,and the promotion effect of the three kind of oxides from strong to weak is B₂O₃ > TiO₂ > SiO₂;what's more,the above three additives can promote the development of the ordered graphitization structures of coal chars,whose regularity of the promotion effect follows the same law as dielectric properties;moreover,these three kinds of mineral oxides can improve the heating rate of coal chars during microwave pyrolysis,and the B₂O₃ is the strongest,TiO₂ times,SiO₂ is the weakest.4.The effect of boron group element oxides?Al₂O₃?on the dielectric properties and micro-structures of coal chars is too small,which can be neglected.