Coal tar is a black or dark brown viscous liquid with irritating odor generated during coal carbonization.Coal tar can be divided intoLow temperature coal tar、Medium temperature coal tarandHigh temperature coal tarThe coal tar obtained in coke production belongs to high temperature coal tar.It is one of the purification products of coke oven gas condensed and separated during the cooling process of raw gas.[1]
Coal tar is generally used as the raw material for processing and refining to produce various chemical products, and can also be directly used, such as as the ingredient component of the binder used for industrial briquette, formed coke and coal based activated carbon, as well as fuel oil, blast furnace injection fuel, wood preservative oil and raw materials for burning carbon black.[1]
The volatile matter released from the charged coal during heating in the air insulated carbonization chamber enters the furnace top space through the gap between the coke cake and the wall of the carbonization chamber, the gap between the hot coke and the semi coke itself, and the gap between the coal materials,The secondary thermal decomposition reaction (cracking and aromatization of alkanes, dehydrogenation of cycloalkanes, dehydration and condensation of phenols, dealkylation of aromatic hydrocarbons and heterocyclic compounds, etc.) occurs at a high temperature of 700~850 ℃.These gaseous coal carbonization products at about 700 ℃ carry coal dust, coke dust and pyrolytic carbon, enter the gas collector through the riser, and are quenched to 70~80 ℃ by circulating ammonia water.In the gas collector, most of the coal dust, coke dust, pyrolytic carbon and some high boiling coal tar are condensed and washed down.The remaining coal tar in fog or vapor state is gradually condensed and separated in the coke oven gas primary cooler and the gas purification equipment after it.The condensed and separated coal tar is collected into the tar ammonia separator to preliminarily remove ammonia water and tar residue and separate the coal tar.When coal is carbonized at high temperature, the yield of coal tar is generally 3~4.5% of the quality of dry coal.[1]
Compositional nature
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Coal tar is a complex mixture of highly aromatic hydrocarbons, most of which are polycyclic and fused ring compounds with or without side chains and heterocyclic compounds containing oxygen, sulfur and nitrogen, and contain a small amount of aliphatic hydrocarbons, cycloalkanes and unsaturated hydrocarbons, as well as coal dust, coke dust and pyrolysis carbon.The newly recovered coal tar also contains about 5% water dissolved with various inorganic salts and other impurities.Due to the existence of extremely fine pyrolytic carbon, water often forms a stable emulsion with oil.Most components of coal tar have high melting points, but due to the mutual dissolution of a large number of monomer compounds, a low eutectic mixture is formed, making the coal tar still in a liquid state at room temperature.Many components of coal tar also form a large number of multicomponent azeotropic systems, which makes distillation and separation very difficult.High temperature coal tar contains more than 10000 compounds, which can be divided into neutral hydrocarbons, acidic phenols, and basic pyridine and quinoline compounds according to chemical properties.In 1819, the British Garden and Brand found naphthalene in coal tar, which was the first compound found in coal tar.Later, British and German scientists found anthracene, phenol, aniline, quinoline, pyridine, pyrene and benzene in succession.By 1972, 480 compounds had been identified, accounting for 55% of the coal tar, including 174 neutral compounds, 63 acidic compounds, 113 basic compounds, and the rest were fused ring and heterocyclic compounds containing oxygen and sulfur.[1]
Many countries have formulated quality standards for coal tar, and classified coal tar grades according to indicators.Density and toluene insoluble content are the two most important quality indicators of coal tar, with the general range of 1.13~1.22g/cm and 3.5~9% respectively.[1]
The composition and properties of medium and low temperature coal tar are quite different from those of high temperature coal tar. Medium and low temperature coal tar contains more oxygenated compounds and chain hydrocarbons, of which phenol and its derivatives account for 10%~30%, and alkane hydrocarbons account for about 20%. At the same time, the content of heavy oil (tar pitch) is relatively small, which is more suitable for the production of vehicle engine fuel oil and chemicals by hydrogenation technology.
The composition and properties of coal tar produced by different raw coals and pyrolysis processes are quite different.
The relative density of high temperature coal tar is more than 1.0 and contains a large amount of asphalt. It is almost a complex mixture composed of aromatic compounds. It is estimated that the total number of components is about 10000. About 500 single compounds have been separated and identified from it, accounting for about 55% of the total amount of tar.
There are only more than 10 compounds with mass fraction ≥ 1.0% in high-temperature tar, which are naphthalene (10.0%), phenanthrene (5.0%), fluoranthene (3.3%), pyrene (2.1%), acenaphthene (2.0%), fluorene (2.0%), anthracene (1.5%), 2-methylnaphthalene (1.5%), carbazole (1.5%), indene (1.0%) and oxyfluorene (1.0%).[2]
influence factor
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The properties of raw coal, coke oven structure and condition, as well as coking pressure and temperature system, have a great impact on the yield, properties and component content of coal tar.The composition and yield of volatile matter from the primary thermal analysis of coal are mainly affected by the nature of coal. The higher the volatile matter of raw coal is, the lower the softening temperature of coal is, and the larger the temperature range for forming colloid is, the higher the yield of coal tar is, and the larger the proportion of hydrogen in the element composition of coal tar is.However, secondary thermal decomposition can weaken the influence of coal quality difference, so that the final composition and properties of coal tar mainly depend on the conditions of secondary thermal decomposition.With the coking temperature andFurnace top space temperatureThe secondary thermal decomposition degree of volatile matter is deepened, the yield of coal tar is reduced, the density is increased, the content of toluene insoluble and quinoline insoluble is increased, the content of side chain compounds such as toluene, xylene and methyl naphthalene is reduced, and the side chain is shortened, the content of non side chain compounds such as benzene and naphthalene is increased, and the proportion of hydrogen in the element composition is reduced.Especially when the temperature of coke oven top space exceeds 750 ℃, the rate of coal tar density increase sharply.For example, the relative density of coal tar is 1.17~1.18 at 800 ℃, while it exceeds 1.22 at 850 ℃.When the coal in the carbonization chamber is not fully loaded, the top space will increase, the residence time of volatile matter in the top space will extend, and the degree of secondary thermal decomposition will deepen, leading to a series of changes such as the decrease of coal tar yield and the increase of density.The rise of the pressure in the carbonization chamber will increase the leakage of volatile matter, while the low pressure will cause the air leaked into the carbonization chamber to burn some volatile matter, both of which will lead to the loss of chemical products and reduce the yield of coal tar.Low moisture content of charging coal (such as dry coal orCoking with preheated coal）When the content of fine powder is high, or the high pressure ammonia water (or steam) injection is used for smokeless coal loading, the toluene insoluble and quinoline insoluble content of tar increases, the viscosity increases, and the processing performance becomes poor.[1]
Coal tar is generally used as the raw material for processing and refining to produce various chemical products, and can also be directly used, such as as the ingredient component of the binder used for industrial briquette, formed coke and coal based activated carbon, as well as fuel oil, blast furnace injection fuel, wood preservative oil and raw materials for burning carbon black.[1]
Processing technology
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Preparation of coal tar before distillation
1. The crude tar shall be sent to the tar depot for quality equalization, preliminary dehydration and deslagging.The tar depot shall be equipped with at least three storage tanks: one for receiving tar, one for standing dehydration, and one for sending oil to the tubular furnace. The three tanks shall be used in turn.
2. Tar dehydration
The moisture of tar must be removed before distillation. Dehydrated tar can reduce the heat consumption of distillation process, increase the production capacity of equipment, and reduce the system resistance of continuous distillation heating.
Tar dehydration can be divided into preliminary dehydration and final dehydration.The preliminary dehydration of tar generally adopts the method of heating and standing dehydration, that is, the tar is heated and insulated to about 80 ℃ in the storage tank with a coiled tube, standing for more than 36h, and the tar and water are separated due to different densities.The water content in tar can be preliminarily reduced to less than 4% after standing for dehydration.In addition, there are centrifugal dehydration method and pressurized dehydration method for preliminary dehydration of tar.
According to different production scales, the final dehydration methods of tar mainly include intermittent kettle dehydration, tubular furnace dehydration and steam heating dehydration.Among them, intermittent tar distillation process uses intermittent kettle for dehydration, and continuous tar distillation process uses tubular furnace for dehydration.The tubular furnace dehydration method is to send the preliminarily dehydrated tar into the tubular furnace for continuous heating to 120~130 ℃, and then send it to the primary evaporator (dehydration tower) to remove some light oil and water, which can reduce the water content of tar to 0.3%~0.5%.[2]
In the domestic continuous tubular furnace tar distillation process, the final dehydration of most manufacturers is carried out in the convection section of the tubular furnace.
3. Tar desalting is in the water (dilute ammonia water) contained in tar, and a small part of ammonia is NHfourOH exists in the form of ammonium salt.Tar desalting is mainly aimed at ammonium salt.Ammonium salt is divided into volatile ammonium salt and fixed ammonium salt.Volatile ammonium salt can be removed in the final dehydration stage of tar, but fixed ammonium salt cannot be removed.
These fixed ammonium salts can be decomposed to produce ammonia, hydrogen chloride, hydrogen sulfide, etc. in the tar distillation stage, which severely corrodes equipment and pipelines.At the same time, ammonium salt will make the distillate and water riseEmulsificationIt is very unfavorable for the dephenolization of naphthalene oil fraction.Therefore, tar must be desalted before distillation.
Before the tar enters the tubular furnace for final dehydration, it is necessary to continuously add 80~120g/L NatwoCOthreeSolution, neutralize with fixed ammonium salt to generate stable NaCl solution.
These sodium salts are completely residual in asphalt during tar distillation and become ash. If 0.1g fixed ammonium salt is removed from each kilogram of tar, the ash content of asphalt will increase by about 0.08%, so NatwoCOthreeThe amount of solution shall be appropriate.[2]
Tar distillation process
The purpose of tar distillation is to concentrate compounds with similar boiling points in tar into corresponding fractions for further processing and separation of monomer products.Tar distillation process can be divided into batch distillation and continuous distillation, and modern tar distillation adopts continuous distillation.
It has developed into continuous atmospheric and vacuum multi tower distillation.Continuous distillation has the characteristics of good separation effect, high yield of each fraction, and high concentration of phenol and naphthalene in a certain fraction.The main equipment of tar distillation includes tubular heating furnace, primary evaporator, secondary evaporator and distillation tower.The process of coal tar distillation at home and abroad is similar to that of dehydration and fractionation.[2]
Processing of coal tar fraction
1. Light oil fraction Light oil is the fraction before 170 ℃, and the yield is 0.4%~0.8% of anhydrous tar.In the conventional continuous tar distillation process, the light oil fraction comes from two sources: one is the light oil fraction obtained from the dehydration of the tar in the first stage evaporator, referred to as the first stage light oil for short;The second is the light oil fraction obtained from the top of the fractionator, referred to as the second stage light oil for short.The light oil fraction is generally incorporated into the washing oil after benzene washing or into the crude benzene for further processing, and benzene products, solvent oil and coumarone are separated.
2. Phenolic oil distillate Phenolic oil distillate is 170~210 ℃ distillate, and the yield is 1.0%~2.5%.Phenolic compounds are the products of thermal decomposition of coal, and their composition and output are related to the total oxygen content in coal, coal blending quality and coking temperature.Phenolic compounds mainly exist in phenol oil, naphthalene oil and wash oil fractions.
The process of extracting phenolic compounds from tar fraction includes alkali elution of phenol from the fraction, purification of crude sodium phenolate and decomposition of purified sodium phenolate.The purification of crude phenol is a process that uses the difference of boiling point of phenol compounds and adopts distillation method to obtain phenol products.The main products of crude phenol refining include phenol, industrial phenol, o-cresol, industrial o-cresol, m-p-mixed cresol, tri mixed cresol and xylenol.The process flow of crude phenol refining includes vacuum intermittent distillation and vacuum continuous distillation.
3. Naphthalene oil fraction Naphthalene oil fraction is 210~230 ℃ fraction.The content of naphthalene in coal tar is related to coking temperature, residence time and temperature of coal pyrolysis products in the top space of coke oven carbonization chamber. Generally, the content of naphthalene in high-temperature coking tar is about 10%.
The naphthalene containing fraction is mainly used to produce industrial naphthalene, which can be further processed into refined naphthalene.Distillation is commonly used to extract naphthalene from naphthalene oil fraction.The distillation processes for producing industrial naphthalene mainly include double furnace and double tower, single furnace and single tower, and single furnace and double tower pressurized continuous distillation.
4. Wash oil fraction Wash oil fraction is 230~300 ℃ fraction.The yield is generally 4.5%~6.5% of anhydrous tar.The washing oil fraction is mainly used to wash and absorb benzene group hydrocarbons in gas and extract quinoline compounds, phenolic compounds, methyl naphthalene, dimethyl naphthalene, naphthalene, indole, biphenyl, acenaphthene, oxyfluorene, fluorene and other products from it.[2]
Wash oil fractions with acid and alkali respectively, extract quinoline compounds and phenolic compounds, and cut narrow fractions in three floating valve towers with 60~70 trays.Among them, naphthalene oil fraction is the raw material for the production of industrial naphthalene, naphthalene residue and heavy wash oil are the raw materials for the extraction of oxyfluorene and fluorene, and methyl naphthalene fraction is the extractionα-Methylnaphthalene andβ-The raw material of methyl naphthalene, light residual oil and acenaphthene removal residual oil can be mixed with low naphthalene washing oil to extract industrial acenaphthene, and the medium quality washing oil can be used as an absorbent for recovering benzene hydrocarbon.
5. Anthracene oil fraction - Anthracene oil fraction is 280~360 ℃ fraction, and the yield is 16%~22%.The initial boiling point of the anthracene oil fraction is 310 ℃, and the yield is 4%~6% when the distillate is 50%, which is 400 ℃.Crude anthracene is a yellow green crystal obtained by cooling, crystallization and centrifugation of anthracene oil fraction or anthracene oil fraction.Production process of crude anthracene: the traditional process of anthracene oil is crystallization vacuum filtration centrifugation, with long process and poor production conditions.Now the crystallization centrifugation method has been adopted, and the horizontal scraper discharge centrifuge has been used to replace the intermittent centrifuge.Refined anthracene, phenanthrene, carbazole and other important chemical raw materials are obtained from crude anthracene prepared from anthracene oil.The refining methods of crude anthracene include solvent extraction, solvent extraction distillation, azeotropic distillation, adsorption, etc.
6. Coal tar pitch Coal tar pitch is the residue after distilling coal tar. According to the softening point, it can be divided into low temperature pitch, medium temperature pitch and high temperature pitch.Coal tar pitch is a bulk product separated from coal tar processing, accounting for 50%~60% of coal tar. Its processing and utilization level is crucial to the entire coal tar processing technology.
Coal tar pitch can only be further classified by extraction instead of distillation.Coal tar pitch is mainly used to produce pitch coke, binder for electrode and anode paste (modified asphalt), briquette binder, road asphalt, various asphalt anti-corrosion paints, etc.
Medium temperature asphalt can be modified to produce modified asphalt, and then to produce pre baked anode blocks for electrolytic aluminum industry and graphite electrodes for electric furnace steelmaking.The production technology of modified asphalt mainly includes oxidation thermal polymerization, heating polymerization, pressure thermal polymerization, Lutiger thermal polymerization and Cherry-T method (C-T method for short, taking coal tar as raw material).[2]
Coal tar hydro upgrading
Medium and low temperature coal tar is suitable for hydro upgrading to produce vehicle engine fuel oil and chemicals.With the soaring international oil price, the production of diesel, gasoline and other vehicle engine fuel oils from coal tar hydrogenation has attracted increasing attention. Some foreign countries that are short of oil resources have been preparing large-scale semicoke production projects to use by-product coal tar hydrogenation to produce diesel and gasoline.Hydroupgrading reaction is a reaction that takes place under the conditions of high pressure, hydrogen and catalyst.The main purpose is to remove sulfur, oxygen, nitrogen, and metal impurities from the oil, saturate the olefins, and change the stability, color, smell, and combustion performance of the oil, so as to change the nature of the oil and improve its use value.[2]
The original coal tar hydrogenation technology has certain defects, such as low utilization rate of coal tar, complex process flow, and certain restrictions on raw oil.In 2010, the General Research Institute of Coal Sciences proposed a heterogeneous catalyst coal tar suspension bed/slurry bed hydrogenation process and supporting catalyst technology based on the direct coal hydrogenation liquefaction process technology and petroleum residue hydrogenation process technology. This technology has the outstanding advantages of high utilization rate of coal tar and high yield of light oil products, and is the development direction of medium and low temperature coal tar processing technology.
High temperature coal tar hydrogenation to fuel oil is more difficult than medium low temperature coal tar hydrogenation, and it is still in the primary stage of industrialization in China.For the hydrogenation of high temperature coal tar, the combination of hydrofining and hydrocracking is necessary to obtain higher fuel oil yield.[2]
Development and utilization
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In 1882, Britain first established a coal tar processing and distillation plant to separate aromatic compounds, sleeper anti-corrosion oil, asphalt and other chemical products from coal tar, some of which can only be obtained from coal tar.The output of coal tar in the world is about 20 million t/a, of which 5 million t/a of various chemical products can be obtained.[3]
Germany is the first country to develop the added value of coal tar products. Germany Steele and France Corbis have first-class coal tar processing processes.The coal tar processing capacity of LUTEG Company is 1.5 million t/a. Its process flow has the advantages of low energy consumption, more micro components separated, high degree of automation, environmental protection, etc. The utilization rate of coal tar ranks first in the world.
The coal tar distillation process of Nippon Steel Pipe Company does not have a separate flash separation tower.Some developed countries in the world, such as the United States, Japan, Russia and Germany, have quite mature technologies in the field of coal tar.
There are 46 large and medium-sized coal tar processing enterprises in China, and 25 enterprises above 100000 tons, which are basically simple operations with low added value.These enterprises generally adopt atmospheric distillation process, which is not highly modernized and belongs to extensive operation.
Domestic coal tar products mainly include phenols, naphthalene, wash oil, crude anthracene, asphalt, etc.The product types and quality of each enterprise are roughly the same, and the deep processing products need to be developed.[3]
purpose
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The phenolic compounds in the middle and low temperature coal tar are mainly low-grade phenols, which can be extracted by chemical extraction as chemical raw materials. The fraction after extracting low-grade phenols will be an excellent raw material for hydrogenation to produce high cetane number diesel oil.
Medium and low temperature coal tar can be hydrotreated to produce diesel, gasoline and other automotive engine fuel oils and chemicals.
High temperature coal tar hydrogenation to fuel oil is more difficult than medium and low temperature coal tar hydrogenation. According to the traditional processing technology, each fraction of high temperature coal tar can be further processed to separate a variety of chemical products. The main products and uses of extraction and processing are as follows:[2]
fraction
purpose
Phenolic oil fraction
phenol
Phenolic resin, synthetic fiber, medicine
Cresol
Fungicides, plasticizers, mineral processing agents, herbicides, spices, flame retardants
Medicine, pesticide, dye, color developing agent.[2]
Anthracene oil fraction
anthracene
Dyes, paper pulp steaming aids, emulsifiers.
phenanthrene
pesticides.
carbazole
Dyes.
carbon black
Tire manufacturing, other rubber, plastic additives, dyes, printing ink and other industries, anti-corrosion oil.
Dianthracene oil fraction
Fluoranthene
Flaw detection agent.
pyrene
Dyes, engineering plastics.
Development prospect
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The properties and composition of medium low temperature coal tar and high temperature coal tar are obviously different and should be processed separately.Medium and low temperature coal tar can be hydrotreated to produce diesel, gasoline and other automotive engine fuel oils and chemicals.
High temperature coal tar hydrogenation to fuel oil is more difficult than medium and low temperature coal tar hydrogenation. According to the traditional processing technology, various fractions of high temperature coal tar can be further processed to separate a variety of chemical products.
Domestic tar processing units are generally scattered, small in scale, high in energy consumption and seriously polluting the environment, with few product varieties, especially high quality and high added value products, poor in quality, high in production cost and poor in economic benefits.In the future, coal tar should be processed in a centralized and large-scale way, increasing the variety of deep processed products of coal tar, and improving the quality of deep processed products.[2]
