Fixed carbon in coal

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Fixed carbon in coal: After removing moisture, ash, and volatile matter from coal, what remains is the fixed carbon. 　　Like volatile matter, the fixed carbon in coal is also an indicator of the degree of coal metamorphism, increasing as the degree of metamorphism increases. Therefore, fixed carbon is used as an indicator for classifying coal. 　　Fixed carbon is an important source of the calorific value of coal; therefore, some use fixed carbon as the main parameter for calculating the calorific value of coal. Fixed carbon is also an important indicator for coal used in ammonia synthesis. 　　Formula for calculating fixed carbon:
(FC)ad = 100 – (Mad + Aad + Vad)
When the carbonate CO2 content in the coal sample is 2–12%:
(FC)ad = 100 – (Mad + Aad + Vad) – CO2,ad (in coal)
When the carbonate CO2 content in the coal sample is greater than 12%:
(FC)ad = 100 – (Mad + Aad + Vad)
Where:
(FC)ad – Fixed carbon in the analyzed coal sample, % ； 　　Mad — Analysis of moisture content in coal samples, % ； 　　Aad — Analysis of ash content in coal samples, % ； 　　Vad —— Analysis of volatile matter in coal samples, % ； 　　CO2,ad (coal) — Analysis of carbonate CO2 content in coal samples, % ； 　　CO2,ad (cokemass) – The proportion of CO2 in the cokemass compared to that in coal, % ； 5. Sulfur content in coal 　　(1) Forms of sulfur present in coal 　　The sulfur content in coal can be divided into organic sulfur and inorganic sulfur, depending on the form in which it exists. Some coals also contain a small amount of elemental sulfur. 　　Organic sulfur in coal exists in the form of organic compounds; it is structurally complex, and not enough is known about it to date. The main functional groups include thiol compounds, R-SH (–SH represents a thiol group) ； 　　Thiophenes, such as thiophene and benzothiophene; thioquinones, such as p-thioquinone; thioethers, R-S-R’; thienanthrenes, etc. Inorganic sulfur in coal exists in the coal in the form of inorganic substances. Inorganic sulfur is further divided into sulfide sulfur and sulfate sulfur. The vast majority of sulfur in sulfides is pyrite sulfur, with a smaller portion being marcasite sulfur; the two are polymorphs of the same substance. There are also small amounts of ZnS, PbS, etc. Sulfate sulfur is mainly present in CaSO4. 　　Sulfur in coal is classified into combustible sulfur and non-combustible sulfur based on its ability to burn in air. Organic sulfur, pyrite sulfur, and elemental sulfur can all burn in air; they are all combustible forms of sulfur. Sulfate sulfur cannot burn in air; it is non-flammable sulfur. 　　Sulfur remaining in the ash after coal combustion (mainly as sulfate sulfur), or sulfur remaining in coke after coking (mainly as organic sulfur, calcium sulfide, ferrous sulfide, etc.), is referred to as solid sulfur. Sulfur released during coal combustion, or sulfur that emerges along with gas and tar during coal coking, is known as volatile sulfur (primarily in the form of hydrogen sulfide and carbon monoxide sulfide (COS)). The fixed sulfur and volatile sulfur in coal are not constant; rather, they vary depending on factors such as the combustion or coking temperature, the rate of temperature increase, and the nature and quantity of mineral components. 　　The total amount of sulfur in various forms in coal is referred to as the total sulfur content of coal (St). The total sulfur in coal typically includes the sulfate sulfur (Ss), pyrite sulfur (Sp), and organic sulfur (So) of the coal. St = Ss + Sp + So. If there is elemental sulfur present in the coal, it should also be included in the total sulfur content. 　　(2) The impact of sulfur in coal on industrial utilization Sulfur is one of the harmful substances in coal. When coal is burned as a fuel, it produces SO2 and SO3, which not only corrode equipment but also pollute the air and even cause acid rain, posing a serious threat to plant growth and human health. When coal is used to produce semi-water gas for ammonia synthesis, gases such as hydrogen sulfide are present in large amounts in the gas and are difficult to remove completely; this can poison the synthesis catalyst and affect production. Coal is used for coking, and the sulfur in coal ends up in the coke, making steel brittle. Steel becomes scrap when its sulfur content exceeds 0.07%. To reduce sulfur in steel, limestone is added during blast furnace ironmaking, which reduces the effective volume of the blast furnace and also increases the slag discharge volume. During storage and transportation, when coal contains high levels of substances such as iron sulfide, it can catch fire spontaneously due to oxidation and rising temperatures. 　　The sulfur content in coal fields in our country varies. The sulfur content in coal fields in the Northeast and North China is relatively low, while coal mines such as those in Zaozhuang, Shandong, Wuda in Inner Mongolia, Fenshi in Shanxi, and Tongchuan in Shanxi have a higher sulfur content. The sulfur content in coal mines in Guizhou and Sichuan is even higher. In some coal mines in Sichuan, the sulfur content is as high as 4–6%, and it is difficult to reduce it to even 2% even after washing. 　　Removing sulfur from coal is an important issue in coal utilization. In this regard, Western countries such as the United States have made great progress in research on clean coal. First, they develop coal washing and processing (the proportion of raw coal that is washed ranges from 0 to over 80%, while in China it is less than 20%); through washing, the ash content in coal is reduced, and inorganic sulfur in coal is removed (organic sulfur cannot be removed by washing) ； The second is desulfurization during coal combustion and desulfurization of flue gases. This undoubtedly increases the cost of using coal. We are also conducting research on clean coal. Given that the utilization rate of coal washing plants for thermal coal in our country is currently only slightly over 50%, it is necessary to formulate and implement environmental protection laws related to coal use as soon as possible, in order to promote the development of coal washing processes and the application of clean coal technologies. 　　(3) Key points of testing in coal The testing of sulfur in coal includes the determination of total sulfur, pyrite sulfur, and sulfate sulfur. See GB214-83.