# Review of Coal-Fired Electrification and Magnetic Separation Desulfurization Technology
## 3 Comparison of Coal-Fired Desulfurization Methods  
At present, there are many methods for desulfurization of coal. According to the different desulfurization stages, the coal-fired desulfurization method can be divided into pre-combustion desulfurization, in-combustion desulfurization and post-combustion desulfurization. According to the different media environment, coal-fired desulfurization can be divided into dry desulfurization and wet desulfurization. The characteristics of pre-combustion desulfurization, in-combustion desulfurization and post-combustion desulfurization are as follows:  
1. Pre-combustion desulfurization is mainly achieved by gravity separation, flotation, magnetic separation, electric separation, microwave and jigging technologies to remove sulfur and ash from coal. Pre-combustion desulfurization can not only increase the utilization rate of coal, but also reduce the discharge of ash. It can also greatly reduce the corrosion of sulfur in coal to the equipment in the follow-up process to prolong the service life of the equipment, thus reducing the production cost and reducing the consumption of fuel [5]. 2. In-combustion desulfurization mainly refers to briquetite sulfur fixation technology and circulating vulcanization bed desulphurization technology, which can remove 50-60% sulfur. It can be seen that the efficiency of this method is not high, and there are problems such as easy slagging, abrasion and blockage in desulfurization process [9]. 3. Post-combustion desulfurization, also known as flue gas desulfurization technology, belongs to the end treatment, although the desulfurization rate can be as high as 90%, but its equipment and operation costs are extremely expensive, and secondary treatment of waste liquid problem is prominent, mostly used in America, Japan, Germany and other developed countries [10]. Pre-combustion desulfurization is the source technology of clean coal. It can not only desulphurize and remove ash, but also improve the efficiency of heat energy utilization, and its cost is much lower than that in-combustion desulfurization and post-combustion desulfurization. Therefore, from the perspective  
Figure 1: Distribution of sulfur in coal. of energy saving, green emission and high efficiency, pre-combustion desulfurization is the best choice. According to the principle of desulphurization, pre-combustion desulfurization method can be divided into physical desulfurization, chemical desulphurization and biological desulphurization. The principle of physical desulphurization is as follows: according to the difference of density, magnetic property, electrical conductivity and floatability of coal and sulfur-containing substances, the inorganic sulfur in coal is removed [11], and this method has been used on a large scale in the current industrial field; the chemical desulfurization method utilizes a chemical reagent such as a strong alkali, a strong acid or a strong oxidant to convert the sulfur in the coal into a liquid or gaseous sulfide by a chemical reaction such as oxidation, reduction or pyrolysis, thus achieving the purpose of desulfurization [12]; the biological desulphurization method is to suspend the coal powder in the suspension containing bacteria, under the action of the desulphurizing bacteria, the sulfur in the coal can be converted into sulphate, thus the purpose of desulfurization can be achieved [13]. Although chemical desulfurization and biological desulphurization can simultaneously remove inorganic and organic sulfur from coal, it is difficult to realize industrialization in the short term due to the complex production process, long production cycle and easy to produce secondary pollution. Therefore, the only real large-scale application in the industrial field is the physical desulphurization method [14]. Table 1 is a summary of some desulphurization methods and desulphurization rates.