Автор: Kupich V.A., Butuzova L.F., Boyko V.N.
Источник: Young scientists’ researches and achievements in science: сборник докладов научно-технической конференции для молодых учёных (Донецк, 16 апреля 2020 г.) / ответств. за вып. Е.Н. Кушниренко. – Донецк: ДонНТУ, 2020. – 160 с.
Kupich V.A., Butuzova L.F., Boyko V.N. The peculiarities of the low-temperature semi-coke process and methods influenced the yield and properties of its products. The article considers the process of low-temperature semi-coking, analyzes the methods of production of semi-coking, and studies the properties of the products obtained, as well as their application.
semi-coking, pyrolysis, tar, solid combustible fossil, volatile.
Semi-coking is the process of thermal processing of solid fuels (coal, brown coal, and shale) without air access to temperatures of 500-6000C. For semi-coking, mainly coals with a high yield of volatile substances are used, giving a large yield of the primary resin. The yield of the primary resin and semi-coke depends on the quality of the feedstock, the design and the mode of the furnaces. Most of the volatile substances released during low-temperature pyrolysis, with the exception of free moisture vapors, are formed in the hottest areas of the plastic layer. The products of semi-coking are semi-coke, primary resin, pyrogenetic water, and primary gas. Semi-coking products are called primary products, since they do not undergo far-reaching thermal decomposition processes.
The process is carried out in special semi-coking furnaces. Depending on the heating method, all existing semi-coking furnaces can be divided into two groups: with external heating and with internal heating.
Semi-coke is the main product of the low-temperature pyrolysis process, with a solid residue of up to 90% from coal weight. Semi-coke fineness depends on its size, strength, thermal stability (change in strength, the occurrence of deformation and disintegration into smaller pieces when heated), of the original bulk material coal and semi-coking technology (furnace device and mechanical loads into pieces of coal inside the furnace, speed of heat and temperature.) Usually coals with the size of 20-80 mm are used in furnaces for semi-coking.
Semi coke is widely used as an energy-industrial fuel for direct combustion in the furnaces or industrial units such as power, cement, glass and ceramic plants. Consumption of lumpy half-coke is the most common. Due to its special features, semi-coke during combustion makes it possible to provide higher temperatures in furnaces with less fuel consumption.
Semi-coke is also used as household fuel. In the countries where the use of semi-coke in household stove is common, a number of special requirements are made to semi-coke, primarily ensuring smokeless combustion, smooth lumpiness, etc. Therefore, only some varieties are used for household purposes.
Recently, semi-coke has been used as a semi processed unit in the production of molded metallurgical coke. Brown-coal coke can be used in the charge for coking in chamber furnaces, where it in some cases successfully replaces the thinning components of the charge. A significant effect is achieved when the crushed semi-coke is injected into the blast furnace, where it plays the role of fuel, as well as a chemical reagent, which allows you to save a significant amount of blast-furnace coke from expensive and scarce sintering coals.
The primary resin is also called tar
or semi-coking resin
. Liquid products that condense from the vapor-gas phase formed during the semi-coking of combustible minerals are called primary resin. Primary resin differ in composition and properties depending on the nature of the fossil fuels. The density of primary resin is close to the density of water and varies between 0.95-1.05 g / cm3, so the resin are poorly protected from water. The yield of the primary resin is an important characteristic of the semi-coking process. Modern high-speed processes are focused on obtaining the maximum possible number of primary resins. Their yield depends on both the genetic characteristics of the solid combustible fossil (SCF) and the technological parameters of the thermal processing. Resins are dark brown liquids; the density of the resins varies from 0.95 to 1.1 g / cm3 depending on the method of semi-coking. The composition of semi-coke resins can include up to 35% phenols, 3-5% olefins, up to 10% naphthenic, 15-25% aromatic hydrocarbons, 1-2% organic bases and 2-10% paraffin hydrocarbons. Semi-coke resins can be used as raw materials for motor fuel, phenol, paraffin, and aromatic hydrocarbons production. Phenol is used in the production of plastic, lacquer, synthetic fibers, and pharmaceuticals. Paraffin serves as a raw material for the surfactants and detergents production.
Primary gas is a mixture of gaseous products formed during semi-coking. After gas gasoline extracting, resin products consists mainly of methane, its homologues, other hydrocarbons and hydrogen. The composition is also determined by the type of SCF that is subjected to semi-coking. The composition of the primary gas is characterized by a high content of methane and its homologues, which contributes to a high combustion temperature. The main amount of semi-coke gas is spent on heating the fuel and other needs at the enterprise where the semi-coke is carried out. The excess of semi-coke gas can be used as household fuel, as well as for organic synthesis.
Factors affecting the yield and quality of semi-coking products.
The yield and quality of semi-coking products depend on the properties of the fuel being processed, the heating conditions, in particular, the rate of heat input, the final heating temperature, and the pressure. The type of furnaces used, the heating method, the residence time of volatile substances in high-temperature zones, and other factors that determine the uniformity of the temperature field and influence the formation of final products are also of great importance. The yield of resins during semi-coking of brown coals of various types can vary widely – from 4.5 to 15-17%, the yield of resin formed during semi-coking of coal also depends on the characteristics of their structure and varies from 1.5 to 20% and above. As metamorphism increases (from gas coals to lean coals), the yield of semi-coke resin decreases. The only exception is fatty coal, which often produces as many primary resins as gas coal when heated to 6000C.
The final heating temperature of the fuel significantly affects the yield and properties of the semi-coking products, since high-temperature transformations occur as the heat is supplied. Usually the final temperature of semi-coking is 6000C. At this temperature the resin formation processes are almost completed. Higher-temperature more than 6000C transformations are typical for the transition stage of semi-coke to coke (dehydrogenation, dealkylation, the interaction of hydrogen with nitrogen-containing heterocycles, their subsequent splitting and the formation of ammonia and molecular nitrogen). An additional amount of gases (hydrogen, methane, ammonia, nitrogen, etc.) is formed, the output of the solid residue decreases, and its quality changes. As the temperature increases, the reactivity of the product decreases and the structural strength increases. An increase of the final heating temperature in the real process affects the yield and composition of the resins. To increase the temperature in the load, it is necessary to increase the temperature of the heat transfer gas in furnaces with internal heat supply or the temperature of the heating walls in furnaces with external heating. It causes additional pyrolysis of volatile substances.
Thus, an increase in temperature above 6000C leads to a decrease in the yield of solid product and resins, and an increase in the amount of gases.
An important factor is the heating rate. With more intensive heat input, the output of the semi-coke decreases and the output of the resin increases. When the volatile components pass through the fuel layers, they undergo greater pyrolysis both in the fuel mass, mixing with the more heated coolant, and at the heating walls. As a result of secondary pyrolysis, the resin yield can significantly decrease and the amount of gaseous products can increase.
A noticeable effect on the yield of semi-coking products is provided by the size of the pieces of processed fuel. Usually, the larger the size of the pieces, the less resin but more semi-coke is formed.
A similar effect on the semi-coking process is exerted by an increase in pressure: the yield of the resin decreases, but an additional amount of semi-coking and gaseous products is formed. When the pressure increases, not only the yield of semi-coke increases, but its strength increases as well. It is explained by the difficulty of separating volatiles, increasing their impact with solid and non-volatile liquid-phase products.
There are two main methods of semi-coking, in accordance with the method of heating furnaces for semi-coking, when the obtained products are quantitatively and qualitatively different. Stoves heating can be: internal and external. The simplest type of semi-coking is semi-coking in a vertical (shaft) furnace, in which fuel is supplied from above, and from below the preheated heat carrier gas is supplied evenly throughout the entire section of the furnace. After passing through the thickness of the fuel load, the gas gives its physical heat, due to which the process of semi-coking occurs, and is removed at the top, taking from the furnace liquid and gaseous products of semi-coking. This relatively rapid removal of volatile parts of the distillation creates favorable conditions for obtaining primary products. The higher the gas velocity in the furnace is, the more favorable conditions for obtaining primary products are. It is not always possible to supply a large amount of heat transfer gas to the furnace, since the resistance of the fuel layer increases in proportion to the squared velocity as the gas speed increases, and this greatly complicates the process. In industrial conditions, where you have to deal with different in size pieces fuel, compliance with the same regime as for the large fuel or for the fine one is impossible for the same reasons. The most common coolant is gas, produced in the same semi-coke oven.
The internal heating of the furnaces makes it possible to semi-coke some varieties of sintering coals. Due to the fact that the gaseous heat transfer supplied into the furnace reduces the partial pressure of the coal liquid and gaseous products formed during the decomposition of coal, their removal in vapor form occurs before reaching the temperature of the plastic state of the coal and therefore the semi-coke is poorly sintered. If there is not enough coolant, i.e. at low speeds, the bitumen begins to evaporate at the temperature of the formation of a plastic layer, pieces of fuel stick together and coal freezes in the furnace.
In furnaces with external heating, the coal is poured into a chamber whose walls are heated by flue gases produced in a separate furnace. After giving heat through the furnace wall of the fuel load, the combustion products are removed to the atmosphere, and the liquid and gaseous products of semi-coking are removed to the condensation equipment through a special hole in the top of the furnace. The fuel is heated from the wall to the center of the furnace. Semi-coke is therefore unevenly coked throughout the entire thickness of the load and the least amount of volatiles is contained at the walls. The greater the thickness of the heated fuel layer is the less uniform of the semi-coke will be. Therefore, the design of furnaces with external heating is tended to choose the smallest possible thickness of the fuel layer. As a result, the fuel decomposes more evenly and completely.
Thus, such issues as the process of low-temperature coking, its features, methods of semi-coking, product characteristics, as well as their application, factors affecting the output and properties of products were considered in this work.
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