Current Status, New Technologies, and Development Trends in the Deep Processing of Coal Tar

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The coal tar industry is a relatively traditional sector; despite fierce competition from the petrochemical industry over the past 30 years, it still holds considerable potential for development. In recent years, especially with the growth of the new materials and steel industries, efficient utilization of coal tar resources has once again attracted attention. China is a major producer of coke, accounting for around 36% of the world’s total production. The production of coke generates a large amount of by-product – coal tar. In China, the annual output of coal tar is around 5–6 million tons, with a processing capacity of about 4.5 million tons; the capacity of projects that are under construction, being expanded, or planned is approximately 2 million tons. At present, there are roughly over 50 companies engaged in coal tar processing. The most advanced facility of this kind is the one owned by Baosteel Group, which was introduced from Japan in the last century; its processing capacity is 260,000 tons per year, and it can produce 26 different types of products. Following in rank are Ansteel, Wuhan Iron and Steel Group, and Benxi Iron and Steel Group. Most of the coal tar production outside of this is relatively scattered and relies mainly on home-based coking methods; this not only wastes a large amount of non-renewable resources but also pollutes the environment. 　　With the continuous development of China’s economy and the growing demands for environmental protection, the deep processing of coal tar has become an urgent issue to be addressed. Judging from the current development of the coal tar industry, China’s coal coking sector is going through an important period of integration and transformation. The coal tar industry in the future will move toward greater concentration, more precise separation processes, deeper processing, and the synthesis of new materials. 　　1 Production of coal tar in our country Coal tar is an organic mixture primarily composed of aromatic hydrocarbons, containing more than 10,000 different compounds; around 200 of these can be extracted. At present, about 50 of them are useful and can be extracted economically. The products obtained through further processing of coal tar, such as light oil, phenol, naphthalene, washed oil, anthracene, carbazole, indole, and asphalt, serve as basic raw materials for the production of synthetic plastics, synthetic fibers, pesticides, dyes, pharmaceuticals, coatings, additives, and other fine chemical products. They are also essential raw materials for industries such as metallurgy, synthesis, construction, textiles, papermaking, and transportation. Many of these products cannot be obtained from the petroleum chemical industry. Therefore, the processing of coal tar deep oil can promote the development of these industries. 　　In modern coking production processes, the main primary chemical products recovered from coal gas are coal tar, ammonia (mainly ammonium sulfate), and crude benzene. The yield of coal tar varies depending on the types of coal used in coking production. The higher the volatiles content in the coal blend, the greater the tar recovery rate and the higher the tar output. 　　2 Current Status of Coal Tar Processing at Home and Abroad 　　2.1 Production Scale 　　The capacity of each tar distillation unit in countries such as Japan, Germany, France, and Russia is between 100,000 and 500,000 tons per year. Theoretically, the greater the capability, the better the economies of scale. 　　With limited resources, choosing a processing unit with a capacity of 100,000 tons per year can maximize the processing value of the product. Only when the volume of tar collected is sufficient can a 500,000 t/a tar processing plant be built. Domestic single-unit tar distillation units are available in various capacities of 0.6, 1.2, 3, 5, 7.5, 10, and 150,000 t/a. Scale capacities of 30,000 t/a and above all use continuous distillation processes, while those below 30,000 t/a use batch distillation processes. The reasons for the varying scales of tar processing are as follows: ① Tar processing plants have different levels of tar production, so they are built to corresponding scales based on their own output; ② Tar processing is not regarded as an independent industry; rather, it is treated merely as a supplementary processing unit within coking plants; ③ The technological level required for large-scale tar processing remains insufficient; ④ Environmental protection and energy utilization do not receive the same level of attention as in developed countries. 　　2.2 Product schemes
2.2.1 There are three production models for the processing of coal tar abroad:
The first is to produce a wide range of products in various types by purifying and formulating them into different specifications and grades; the second involves producing further processed products based on coal tar derivatives, which are used in the fields of fine chemicals, dyes, and pharmaceuticals; the third focuses on processing asphaltic products. 　　The representative of the first model is the German company Lütgé. There are over 220 products separated and formulated from tar; naphthalene is available in 4 grades, resins in 5 grades, anthracene in 7 grades, and asphalt binders and impregnating materials in 20 grades. Based on market demands, the operating parameters can be changed on the same device to produce products of different grades, thereby achieving the versatility of the device. 　　A representative of the second model is Japan’s Sumikin Chemical, which focuses solely on purifying or further processing pure compounds derived from coal tar. It has developed and produced 180 different products; among them, there are 21 phenolic derivatives, 32 quinoline and its derivatives, and 60 naphthalene derivatives. 　　Representatives of the third model include Japan’s Mitsubishi Corporation, the U.S.-based RiUy Company, and Australia’s Koppem Company; all of them offer distinctive products in the processing of coal tar pitch. These companies do not process any other fractions from coal tar distillation; they sell them as blended oils, and only process the asphalt produced during distillation. Since the asphalt yield in the coal tar processing process is over 50%, proper processing of asphalt to increase its added value can ensure the overall profitability of the tar processing project. 　　2.2.2 Domestic coal tar processing The main products are phenols, naphthalene, washed oil, crude anthracene, asphalt, etc. The product quality and quantity produced by various plants are largely similar, resulting in mediocre efficiency in tar processing; there is still a significant gap compared to foreign countries. The main reasons are as follows: the scale of various tar processing facilities is generally small; there are few high-quality, high-value-added products; manufacturing enterprises have poor capacity to adapt to market changes; the market for deeply processed tar products needs to be developed, and it is particularly difficult to introduce new products onto the market. 　　2.3 Process Flow The processes for tar distillation both domestically and internationally are similar, involving dehydration and fractionation; however, the processes used abroad are more diverse than those in China. Compared with foreign processes, the tar distillation technology in China is not very different; it’s just suitable for different applications. With some improvements in equipment, instrument control, and energy utilization of the domestic process, it can be transformed into an advanced and practical process. 　　2.4 Environmental protection level
Environmental protection mainly refers to the treatment of wastewater, waste gas, and solid waste generated during the tar processing process. For wastewater generated from tar processing, the measures taken both at home and abroad are basically the same: it is collected centrally and then sent to the sewage treatment facilities of coking plants for treatment before discharge. The difference is that the quality indicators of the treated wastewater in China are slightly poorer. Waste gas treatment mainly refers to the treatment of off-gases and asphalt fumes generated during the tar processing process. Foreign tar processing plants collect this exhaust gas and channel it to scrubbers, where it is purified and cooled before being sent to tubular furnaces for incineration. Some tar processing plants also apply nitrogen sealing to the tops of their oil tanks, reducing the likelihood of vent gases being released. In China, only a few tar processing units collect and treat the vent gases centrally, while most units release them freely. Therefore, improvements are needed in exhaust gas treatment. The only waste residue generated from tar processing is tar residue. The treatment methods employed in all countries are the same: it is collected centrally and then mixed with coal. The facilities for coal distribution within the country are poor, and there is serious littering in some factories. However, with strict management and careful handling, it is entirely possible to reach foreign standards of treatment. 　　2.5 Energy conservation level　Energy conservation and consumption reduction are important indicators for the unit. Tar processing is a highly energy-intensive process. Abroad, better control is exercised over water, steam, and coal consumption through the adoption of technologies such as air cooling, heat exchange between hot and cold fluids, multi-stage circulating water systems, low-temperature vacuum distillation, and heat recovery via steam generation. However, electricity consumption abroad is actually higher than in China. With the adjustment of the domestic energy structure, it is an inevitable trend to make more use of electricity and reduce the consumption of water, steam, and gas. 　　2.6 Equipment Level The equipment level of a facility is closely related to the capabilities in mechanical manufacturing and automatic control. The level of equipment used for tar processing in China lags far behind that in foreign countries, mainly due to an overemphasis on the investment cost of such projects. It is difficult to find suitable domestic manufacturers for equipment designed for high-temperature operation, corrosion-resistant materials, and testing instruments for high-temperature, high-viscosity media. Even when equipment is imported from abroad, the standards of maintenance vary significantly. 　　3 Progress in Domestic Coal Tar Processing Technologies 　　3.1 Tar Distillation Technology 　　In China, distillation processes that operate at atmospheric pressure and use a single tower, with separation of two or three fractions, are commonly employed. The introduced coal tar distillation unit features the following characteristics: it employs continuous dehydration and light oil removal, with the distillation column operating under reduced pressure; phenol oil is obtained at the top of the column at a pressure of 13.3 kPa, while the product at the bottom is soft asphalt with a softening point of 65°C. A square-box tube furnace is used, with the temperature of the tar exiting the furnace being 330°C. Waste heat is made effective use of – soft asphalt exchanges heat with the tar, and low-pressure steam at 0.3 MPa is generated using a steam generator for each fraction. The oil vapor at the top of the distillation column is cooled using an air condenser, and the operation under reduced pressure allows for energy savings of around 15% to 50%. The exhaust gas extracted under reduced pressure, along with the separated phenol and water, are sent to the tube furnace for combustion. The material used for the distillation column is corrosion-resistant low-carbon alloy steel. 　　3.2 Distillation technology for industrial naphthalene　Currently, most domestic coking plants produce unacid-washed 95% industrial naphthalene; only those plants that recover quinoline produce dilute-acid-washed 95% industrial naphthalene. Additionally, the raw materials used to produce 95% industrial naphthalene also vary: narrow fractions (i.e., naphthalene oil fractions), four-component mixtures (light oil, phenol, naphthalene, wash oil), three-component mixtures (phenol, naphthalene, wash oil), two-component mixtures (naphthalene, wash oil), etc. The industrial naphthalene distillation processes can be divided into atmospheric batch distillation in batch reactors, vacuum batch distillation in batch reactors, atmospheric continuous distillation using two reactors and two towers, atmospheric continuous distillation using two furnaces and two towers, atmospheric continuous distillation using one furnace and two towers, atmospheric continuous distillation using one furnace and one tower, and high-pressure atmospheric continuous distillation using one furnace and two towers, among others. In terms of the actual number of trays in the distillation column, it started at 50 trays and later increased to 63, 64, and 70 trays. The types of distillation columns used include packed columns (with ceramic rings, Pall rings, corrugated plates, etc.), sieve tray columns, bar-type bubble cap columns, inclined hole plate columns, valve trays columns, etc. Currently, most large coking plants employ a 70-stage valve tray column in an atmospheric-pressure continuous distillation process featuring two or three mixed fractions as feedstock, along with two furnaces and two columns. The atmospheric-pressure single-furnace, double-tower continuous process is quite common; meanwhile, Baoshan Iron & Steel’s atmospheric- and pressurized single-furnace double-tower continuous process has the lowest energy consumption. With the application of computers, the single-column, single-tower continuous distillation process holds promise for development. 　　3.3 Washing techniques for the fractions obtained from tar distillation This refers to devices for alkali-eluting phenol or acid-eluting quinoline, which can respectively yield phenolate and quinoline sulfate. Generally, phenol is removed first, followed by quinoline; alternatively, only phenol can be removed while quinoline remains. The raw materials vary depending on the fractions obtained from tar distillation; there are narrow fractions and broad fractions. The washing process can operate intermittently or continuously. Washing equipment comes in various types, such as air agitation, mechanical agitation, pump mixing, static mixers, jet mixers, etc. The latter two types of scrubbers are more advanced, offer better cleaning performance, and are suitable for continuous operation and automatic control. The main controlling factors in alkali elution of phenol include alkali concentration, washing temperature, separation time, and the number of washing stages, etc. The washing requirement for each fraction is that its phenol content be less than 0.5%. Baosteel has adopted a fully continuous alkaline dephenolization process, with a low concentration of alkali solution, ranging from 8% to 10%. Both light oil and phenol oil undergo dephenolization in a single stage, with dephenolization efficiencies of approximately 38% and 88%, respectively. Its light oil defenolization serves to purify phenoxide salts. For naphthalene oil, a three-stage dephenolization process is employed, achieving a dephenolization efficiency of 79%; the dephenolization equipment used is a static mixer. Furthermore, quinoline removal was carried out via continuous acid washing only on dephenolated phenol oil and methylnaphthalene oil respectively, using an acid concentration of 30% to 39%, with efficiencies of 38.5% and 52.2% respectively. Static mixers are also used in the equipment. 　　3.4 Technology for producing crude anthracene　　Domestic manufacturers all adopt a batch operation process, with drum crystallizers as the equipment. To improve the yield of crude anthracene, a two-stage crystallization method was developed. The process introduced by Baoshan Iron & Steel employs fully continuous program-controlled operations, including: anthracene oil charging, cooling and crystallization, discharging, centrifugation, etc., taking a total of 44 hours. Later, it was improved to a combination of natural and forced cooling, reducing the time to 35 hours; this resulted in larger crystal particles. The equipment used is a vertical cooling crystallizer, which facilitates continuous operation. The crude anthracene obtained has an anthracene content as high as 38%, while its oil content is very low. 　　3.5 Decomposition technology for sodium phenolate　　In China, the sulfuric acid decomposition method is mostly used; its drawback is that it produces concentrated phenolic water, which is difficult to treat. The flue gas decomposition method was developed in the 1970s, but the problem of secondary pollution remains. Baosteel’s adopted process uses the blast furnace gas decomposition method, operating in two stages with a decomposition rate of 98%; it is also equipped with a causticization unit that enables the production of caustic alkali solution with a concentration of 8% to 10%, achieving a causticization rate of 77%, and there is no issue of secondary pollution. 　　3.6 Technology for producing refined naphthalene 　In China, the concentrated sulfuric acid purification method has been used traditionally; its drawback is that it generates large amounts of waste acid that are difficult to handle, and it also results in high energy consumption and low yields. In the 1980s, the batch-operated stepwise crystallization method was developed and widely applied. In recent years, the “Praobd” process technology has been adopted for box-type fractional crystallization, achieving a naphthalene purity of 90%; the entire process is under automatic control and operates continuously. 　　3.7 Refinement technology for crude phenol　　In China, the process of atmospheric dehydration followed by vacuum slag removal and distillation is widely used; however, the quality of the resulting phenolic products is poor. The introduced process utilizes a 5-tower continuous operation for dehydration, deslagging, and distillation, with the 6th tower operating in a batch mode. All towers operate under reduced pressure. The phenol recovery rate reaches as high as 42%, which is about 10% higher than that in China. The product quality is excellent; it includes special-grade phenol (with a crystallization point of 40°C or above), ortho-cresol (crystallization point above 29°C), meta- and para-cresols, dimethylphenols, etc. 　　3.8 Refining technologies for crude pyridine and crude quinoline　　In China, sodium hydroxide solution is used to neutralize and decompose quinoline sulfate; abroad, liquid ammonia is more commonly employed for this purpose. The refining of both crude pyridine and crude quinoline follows a process flow involving batch operation, azeotropic dehydration, and vacuum distillation. Unlike domestic systems, the introduced unit uses 6-tower batch dehydration and vacuum distillation operations, and incorporates air coolers to save cooling water. 　　3.9 Production technologies for purified anthracene, purified carbazole, and anthraquinone　　In China, crude anthracene is used as raw material; it is processed via a solvent distillation method to obtain purified anthracene, which is then catalytically oxidized to produce anthraquinone. Baosteel has adopted the Praobcl technology, which uses anthracene oil as raw material; a solvent is first added for distributed crystallization (i.e., the solvent crystallization method), followed by vacuum distillation to obtain pure anthracene (with an anthracene content of over 95%) and pure carbazole (with a purity of over 90%). The anthraquinone production process is a technology from the Swiss company Ciba Geigy; through multiple stages of fixed-bed catalytic oxidation and cooling, anthraquinone with a purity of over 99% is obtained. Compared to domestic levels, the standards in terms of technology and equipment are roughly the same. It is characterized by very little waste liquid generated throughout the entire production process, which can be sent to an activated sludge treatment system. Although a relatively large amount of exhaust gas is produced, it can be recovered and filtered, and then destroyed in an exhaust gas combustion device before being released; thus, it poses no harm to the environment. 　　4 Conclusion The level of further processing of coal tar depends on investment and market conditions. Generally speaking, as the degree of processing increases, the added value of the products rises, while investments also increase. In deep processing, it is necessary to consider the flexibility of the equipment in order to adjust product quality and varieties in a timely manner according to market demands. The centralized processing of coal tar is an inevitable trend in the development of modern industry; it is also a key technical policy. Moving forward, the focus will be on improving resource utilization rates, expanding the variety of products, and carrying out deeper processing. Efforts will also be made to extend the product structure, develop new products, reduce pollution, enhance information exchange both domestically and internationally, further open up to the outside world, increase efforts to attract investment, and thereby inject vitality into enterprise development. Additionally, an information and technical cooperation network for coal tar chemicals will be established. Strengthen collaboration among coal coking enterprises, scientific research and educational institutions, and information research organizations to ensure the rational utilization of resources.