Research On Microwave Dielectric Properties Of Coal Chars

The amount of residual or unburned carbon in fly ash is a major concern forthe performance and economy of power plants, and microwave techniques foron-line determination of unburned carbon are effective in guiding the operationof pulverized coal fired boilers. However, it has also been found that operationconditions and fuel properties greatly influence the monitoring results, whichprohibits the widespread use of microwave measuring apparatus. Themeasurement is carried out by microwave absorption and attenuation, and thedielectric property of fly ash along with unburned carbon plays a key role onaccurate results. Accordingly, studying dielectric properties and structuralchanges of coal chars during heat treatment is significant for thoroughunderstanding of the effects of various factors on the dielectric properties. Todiscover microwave loss mechanism of coal chars is also vital to real-timemonitoring carbon content in fly ash and raising measuring accuracy by themicrowave techniques.Three different coals (an anthracite coal from Jincheng, a bituminous coalfrom Fugu, a brown coal from Yunnan) were collected as raw materials, and thechars were prepared at a fast heating rate and a slow heating rate over thetemperature range of850~1600°C in a horizontal tube furnace. Dielectricproperties of coal chars were determined in the2~18GHz frequency by a vectornetwork analyzer. With the help of X-ray diffraction, Raman spectroscopy,high-resolution transmission electron microscopy, infrared spectroscopy and field emission scanning electron microscopy, the interaction mechanism of coalchar with microwave was discussed from a micro perspective in detail. Theimportant results are as follows.(1) Heating temperature played a prominent role in dielectric properties.The dielectric parameters (r′,r″and tane) indicated higher values for coalchars generated at higher temperatures. When the temperatures increased, thefraction of carbon ordering in coal chars increased. More parallel-alignedaromatic layers were presented, and the π bonds formed between parallel carbonlayers, which could absorb and waste energy in microwave field in the form ofdipole. The effect of heating temperature on the dielectric properties dependedon the development of char structure, and the dipole polarization could be theprincipal form of loss mechanism for microwave energy.(2) Dielectric properties largely depended on coal rank. The results showedthat the dielectric parameters for the three coal chars could be ordered asJincheng chars, Yunnan chars and Fugu chars. The higher stacking height ofcarbon layers and the higher concentration of aromatic carbon structure wereresponsible for the highest values of dielectric parameters of Jincheng chars.Although the aromatic layers suffered from an obvious compression owing tohigh sensibility to heat treatment, a higher level of aliphatic structure in Yunnanchars hindered the aromatic carbon structure, resulting in weaker dielectricresponse than Jincheng chars. The wider interlayer spacing of carbon layers inFugu chars largely restricted the polarization induced by microwave irradiationso Fugu chars had the smallest dielectric parameters. Effects of coal rank ondielectric properties resulted from the structure differences among chars fromdifferent coals. The differences were presented with the carbon skeletonsattained from the aromatic carbon layer and the different carbon groups. By thisway the relation of the polarization and the dielectric properties was explainedfor the coal chars in the microwave field.(3) An acid treatment for demineralization was introduced in order to study the influence of mineral matter transformation behavior on char structure andsubsequent dielectric properties in the heating process. A new phase siliconcarbide (SiC) generated from carbon thermal reduction of silicon dioxide (SiO2)with carbon at higher temperatures (1300°C and over) accompanied byproducing char. The development of carbon crystallite was significantlycatalyzed with the aid of carbide formation-decomposition, and the charstructural ordering and dielectric properties were improved. Nevertheless, thestructure and dielectric properties at lower temperatures (1150°C and below)was accelerated due to the migration of the mineral matter within carbon matrix,which brought about the reduction of resistance from mineral matter as impurityand favored the arrangement of carbon layers. Furthermore, the catalysis effectof transformation from SiO2to SiC was also related to mineral matter content.The higher the content is, the stronger catalysis for char structure and dielectricproperties.(4) During the process of char generating at1300°C under a slow heatingrate, the carbon thermal reduction reaction of the mineral matters resulted in aconsiderable amount of SiC whiskers. According to vapor-liquid-solidmechanism (VLS), the carbide existed in solid solution Si(N)C wherein thenitrogen atoms replaced or substituted for the carbon atoms. SiC whiskers had agreat potential for the microwave absorption, whose loss properties could begreatly promoted by the solid solution of the nitrogen atoms. Consequently, thedielectric properties for the coal chars at1300°C markedly exhibited an increasecompared with the coal chars at1150°C. The forms of the carbides influencedthe dielectric properties to a great extent.(5) Based on the principle model of microwave attenuation, the resultsshowed that heating temperatures, coal ranks and mineral matter influencedsignificantly the attenuation coefficients, the phase coefficients, microwavepower attenuation. The effect of mineral matters on the microwave attenuationproperties was weaker than heating temperatures and coal ranks.