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Desulfurization of fuels The sulfur content of coal and fuel oil is about 0.5 to 5% (weight). When burned, most of it is converted into sulfur dioxide (SO□) gas and discharged into the atmosphere. About 5% of the SO□ is oxidized into sulfur trioxide (SO□) in the atmosphere. Therefore, removing sulfur contained in fuel before combustion is one of the main technical measures to prevent sulfur oxides from polluting the atmosphere. 1. Coal Desulfurization ① Physical Method: Sulfur mainly exists in coal in the form of inorganic iron sulfide and organic sulfide. Among them, sulfur in the form of iron sulfide (pyrite) accounts for about 2/3. The specific gravity of pyrite is greater than that of coal. At the same time, pyrite is a paramagnetic substance with a specific magnetization coefficient of about 25×10-6 g/cm.□ ； Coal is a diamagnetic material, with a specific magnetization coefficient of about -0.5×10-6 g/cm□. Therefore, the coal can be crushed, and then the pyrite can be removed using gravity separation or high gradient magnetic separation. Physical methods can remove about 50% of the sulfur content in fuel. ②chemical method: After the coal is crushed, it is mixed with an aqueous solution of iron sulfate and heated to 100-130°C in a reactor. The iron sulfate reacts with pyrite and converts it into ferrous sulfate and monomeric sulfur. At the same time, oxygen is introduced to regenerate ferrous sulfate into ferric sulfate, which is recycled in the process system. The coal is separated through the filter and solution, with sulfur becoming a by-product. ③Gasification method: Coal gasification is a thermochemical process carried out in a gas generator. Coal passes through the gasification agent at high temperatures of 1000 to 1300°C, causing it to undergo incomplete oxidation and become coal gas. If the gasification agent is air, an appropriate amount of air passes through the coal seam from the bottom, is first preheated by the ash layer, and then enters the oxide layer. The oxygen in the air reacts with the high-temperature coal to generate carbon dioxide (CO□) and a small amount of carbon monoxide (CO). CO□ rises through the reduction layer and reacts with the hot coal to be reduced to CO. The gas with CO as the main component is heated and rises and discharged out of the furnace to produce air gas. If the gasification agent is steam, the reaction products are CO and H□, and the gas produced is water gas. Coal gasification also includes pressurization and hydrogenation gasification methods, which allow carbon monoxide or carbon elements to react with hydrogen to generate hydrocarbon gases such as methane, which increases the calorific value of coal gas. In order to save coal mining, transportation and other production links, the Soviet Union, the United States, etc. * * Underground gasification is also used. China is also carrying out experimental research on underground gasification. Most of the sulfur in coal becomes hydrogen sulfide during gasification, which is fed into the coal gas and then removed by liquid absorption or solid adsorption. ④liquefaction method: Coal liquefaction includes synthesis method, direct cracking hydrogenation method and thermal solution hydrogenation method. The synthesis method is to convert coal into gas, add hydrogen according to a certain proportion, and synthesize liquid fuel under high pressure, high temperature and the action of a catalyst, the main components of which are alkanes and methanol. The direct cracking hydrogenation method is to dissolve coal in a solvent to make a paste, and then react with the added hydrogen through the action of a catalyst at 400 to 500°C and a pressure of about 200 atmospheres to become a liquid fuel. The hot solution hydrogenation method is to heat and dissolve pulverized coal in oil with properties similar to coal, add hydrogen at a pressure of about 100 atmospheres, and separate insoluble matter at the same time to obtain liquid coal. During the liquefaction process, sulfur reacts with hydrogen to form hydrogen sulfide, which results in high calorific value, low sulfur, and low ash fuel. 2. Fuel oil desulfurization: Fuel oil generally refers to heavy oil, which is made from the atmospheric distillation residue of crude oil. Heavy oil contains a lot of sulfur, and most of the sulfur is cyclic sulfide, such as biphenylthiocene, naphthylthiocene, etc. These sulfides have high molecular weight. If the sulfur content is 3%, the sulfide content can reach 1/3 of heavy oil. Therefore, heavy oil is not suitable for desulfurization by directly removing sulfides. At present, heavy oil desulfurization uses a high-pressure hydrogenation reaction under the action of a catalyst to cut off the bond between carbon and sulfur, and replace the carbon with hydrogen. At the same time, hydrogen and sulfur react to form hydrogen sulfide, which is separated from the heavy oil and removed by absorption. Another method is to partially burn and gasify heavy oil with steam and oxygen, convert the sulfur into hydrogen sulfide and a small amount of sulfur dioxide, and then process it. 3. The catalyst used in the reaction is chromium, molybdenum, tungsten, iron, cobalt, nickel, platinum, etc. It can also be used in combination of cobalt-molybdenum, nickel-molybdenum or nickel-cobalt-molybdenum, usually with alumina as the carrier. During the production process, if carbon is deposited on the surface of the catalyst, or organic metal compounds of vanadium and nickel in heavy oil react with hydrogen and deposit on the catalyst, the catalyst will be poisoned and lose its catalytic effect. To solve this problem, two processes, direct desulfurization and indirect desulfurization, have been developed. Direct desulfurization is to select a catalyst with good anti-poisoning performance, directly hydrodesulfurize, and take appropriate protective measures. For example, some processes add a protective tower in front of the reaction tower and fill it with other cheap catalysts to remove impurities and metal components as much as possible. Indirect desulfurization is to distill heavy oil under reduced pressure and separate it into distillate oil and residual oil. The distillate oil is hydrodesulfurized separately and then blended with the residual oil. Or use liquefied propane (or butane) as a solvent to treat the residual oil, separate the asphalt, and then mix it with the distillate oil for hydrogenation. 4. Most of the sulfur contained in natural gas is hydrogen sulfide, with a small amount of organic sulfide. Therefore, the method of removing hydrogen sulfide can be used to desulfurize (see hydrogen sulfide control).