Effect of ash on dielectric properties and micro-structure of high alkali coal at different temperature pyrolysis. Issue 4 (August 2020) - Liu, Haiyu
Abstract: The effect of ash on dielectric properties and micro-structure of high alkali coal at different temperature pyrolysis was studied, so as to provide theoretical basis for coal deep processing by microwave. An acid-washed method was adopted to remove ash in Zhundong coal for preparing coal chars at 700 °C–1300 °C. X-ray diffraction analysis was used to characterize the microcrystal structure. The thermal stability was characterized by thermogravimetric analyzer, and the dielectric properties were measured by a vector network analyzer. The results showed that when the pyrolysis temperature was below 1100 °C, the presence of ash hindered the development of carbon structure in raw coal char. The main reason is that the alkali metal oxides (K2 O and Na2 O) in the ash promoted the solution loss reaction during pyrolysis. The structure of the original carbon layer was damaged, thereby the graphitization degree, thermal stability and dielectric properties of raw coal char were weaker than the ash-free coal char. When the pyrolysis temperature reached 1300 °C, the minerals were completely melted. The reaction of phase transition of SiO2 in ash played a catalytic role on raw coal char structure, resulting in tighter arrangement of adjacent carbon layers. The raw coal char showed stronger dielectric properties and thermal stability. Highlights: Below 1100℃, alkali metals promoted the solution loss reaction, the dielectric properties of raw coal char were weaker. Reaching 1300℃, SiO2 underwent a phase transition reaction, the dielectric properties of raw coal char were stronger. The dielectric properties enhance with the increase of order of carbon structure and thermal stability.