## 1 Introduction  
Coal dominates the primary energy consumption structure and will not change its dominant position in the energy mix in the next few years. However, due to the large amount of harmful substances emitted by coal burning, the deterioration of urban air quality and large-scale acid rain deposition have been caused. Therefore, actively analyzing the causes of coal-fired pollution and effectively removing sulfur from coal are of great significance for developing the economy, strengthening environmental protection, and managing severe acid rain. At present, there are many methods for desulfurization and deashing of burning coal. According to the different desulfurization stages, the coal-fired desulfurization method can be divided into desulfurization and ash reduction before combustion, desulfurization and ash reduction in combustion, and finally desulfurization and ash reduction after combustion. According to the different media environments, it can be divided into dry desulfurization and wet desulfurization. Desulfurization before combustion is the source technology of clean coal. It can not only desulfurize, but also remove ash. It can also improve the efficiency of heat energy utilization, and its cost is much lower than that during and after combustion. In addition, considering that most coal-fired power plants use pulverized coal stoves, the boiler must be fired with dry powder if the wet desulfurization cannot be used to do the online sorting. Therefore, from the perspective of energy conservation and emission reduction, green and efficient, dry desulfurization and ash reduction before combustion is the best choice [1]. [PERSON] [2] in the United States invented a frictional electric separator to remove pyrite and ash minerals from coal powder. The University of Western Ontario applied a dilute phase cyclone sorting machine to conduct coal sorting tests. The removal rate of pyrite was 47% [3]. [PERSON] [4] developeda novel rotary turboelectric separator (RTS) with the advantage of a suitable range of particle sizes. [PERSON] [5] conducted a triboelectric selection test of pulverized coal, and carried out single factor tests and orthogonal tests on air volume, voltage and particle size. The study found that there are optimal conditions for the effect of air volume and voltage on sorting. [PERSON] [6] used the surface modification method to pretreat the fine coal and used the rotary friction electric separator to separate it. The results show that the surface modification can significantly enhance the desluurization and ash reduction effect of electrostatic separation with rotary friction. [PERSON] [7] applied the innovative rotary friction electroseparator to study the effects of the key operating parameters such as charger speed, separation voltage and airflow velocity on the ash reduction effect of rotary friction electroseparation for micropulverized coal. The above several desluurization technologies belong to the frictional electrification technology. The main disadvantage of this method is the poor adaptability to different coal types. The sorting process is susceptible to air humidity and the upper limit of sorting is low. The Eriez company in America developed Reium100 permanent magnet HGMS, which has a field strength of 0.6 T. The magnetic material is vertically placed, and the recovery rate of magnetic material is 96-99% [8]. In order to improve the efficiency of sorting, Ganzhou Nonferrous Metallurgy Research Institute developed a new generation of Slon HGMS [9] and Slon-1000 dry vibration HGMS [10], which has significantly improved the level of high-gradient magnetic separation technology. [PERSON] [11] developed a new equipment based on the high-gradient magnetic separator, which combines the high-gradient magnetic separation and sedimentation centrifugation, effectively overcoming the shortcomings of the traditional high-gradient magnetic separator that cannot be continuously separated.