Development and industrialization of key technologies for the deep processing of coal tar, a by-product of coke ovens

1 Overview of coal tar: Coal tar contains tens of thousands of organic compounds; however, only 500 of them can be identified. In addition to various neutral, acidic, and basic components, it also includes other compounds containing oxygen and sulfur, some of which are components that cannot be obtained or are not economical to obtain from petrochemical raw materials. Coal tar is a liquid substance produced during the pyrolysis of coal. It is commonly classified into three forms based on the conditions of its pyrolysis: low-temperature, medium-temperature, and high-temperature. In-depth processing research and technological development are carried out for each of these forms. High-temperature coal tar is a by-product of coal coking, containing many heteroatoms, aromatic compounds, and heavy metals, as well as certain amounts of ash and other particulates such as semi-coke. The deep processing of high-temperature coal tar involves separating the chemical compounds contained in high-temperature coal tar through distillation to obtain valuable fractions, which are then further processed to produce a variety of aromatic hydrocarbon-based chemical raw materials and intermediates. With the rapid development of low-rank coal pyrolysis technologies, the production of medium- and low-temperature coal tar has increased significantly. The contents of alkanes, naphthenes, and polycyclic aromatic hydrocarbons in medium- and low-temperature coal tar are lower than those in the high-temperature tar used in conventional coking industries, making it suitable for use as clean fuels and chemical products with high added value. Research shows that through processes such as pretreatment of medium- and low-temperature coal tar, hydrorefining, and hydrocracking, it can be directly converted into clean energy. This not only helps to address the shortage of oil and gas resources but also promotes the efficient utilization of coal resources while reducing the likelihood of environmental pollution.

2 Current Status of Advanced Processing of Coal Tar To date, research on the advanced processing of coal tar is no longer limited to diverse areas such as the research and technological development related to coal and asphalt, which are by-products of coal tar; the advanced processing and separation technologies for the fractions obtained from coal tar washing; as well as studies aimed at analyzing and optimizing the processes and characteristics of tar recovery. Most studies focus on two aspects: the development of fine chemical routes from coal tar and the optimization of coal tar hydrogenation processes. 2.1 Fine chemical processing routes and development of coal tar: Due to the complex composition of coal tar, which has a significant impact on its properties as well as on its further processing, studying the components of coal tar and developing appropriate fine chemical processing routes for it remain hot topics in research in recent years. Research on the development of fine chemical industries based on coal tar components is as follows. 2.1.1 Detection of coal tar component contents and quality assessment: Gai Hengjun et al. comprehensively analyzed the properties and composition of tar, explained the reasons for the difficulty in oil-water separation, and developed separation methods using tar fractions as extractants. Through experiments, the optimal conditions are identified to meet the requirements of subsequent treatment devices, thereby reducing the biological toxicity of wastewater. The modification plan for the distillation unit and the key operating parameters were optimized through simulation. This method is simple, requires low investment, and is industrially feasible; it holds promise in solving the oil-water separation problem in coal pyrolysis. The contents of biphenyl, acenaphthene, and fluorene in coal tar play a crucial role and hold great significance in controlling the deep processing of coal tar and ensuring product quality. Jiang Xin and others proposed the key steps for detection through the analysis of tar properties, and designed tests for the dissolution and filtration residue rate as well as for naphthalene content. Experiments have shown that gas chromatography can be used to detect the contents of biphenyl, acenaphthene, and fluorene in coal tar, offering advantages such as fast speed and high accuracy ； A programmed temperature rise is used for the column temperature, which not only ensures good separation of the samples to be analyzed but also reduces the analysis time. At the same time, through experiments, the chromatographic separation conditions were determined to ensure complete separation of the components to be analyzed in the samples. Cai Weiyuan et al. pyrolyzed coal tar samples at different temperatures, and selected the stable value of oxygen content as the pyrolysis temperature. Different masses of benzoic acid were accurately weighed from small to large, cracked under experimental conditions, and a standard curve for oxygen content was established based on the relationship between the absolute mass of oxygen and the peak area. The results of the blank test and spiked recovery test show that this method can determine the oxygen content in coal tar quickly and accurately, fully meeting the requirements for quality testing of coal tar. Due to the strong penetrating ability of chloride ions, which can damage processing equipment and affect work efficiency, Liu Ying and her colleagues conducted in-depth research on the problems existing in the methods for determining the chlorine content in coal tar at Ansteel Chemical Technology Co., Ltd. They proposed targeted improvement measures; as a result, the precision and accuracy of chlorine content measurement in tar were significantly improved, work efficiency was enhanced, and the service life of the equipment was extended. These improvements met the requirements of operational sites and can serve as a reference for those involved in the analysis of chlorine content in coal tar. A high moisture content in coal tar can cause corrosion to pipelines and equipment, and it has a significant impact on the energy consumption associated with coal tar processing as well as on the safe and stable operation of production processes. Through measures such as controlling the quality of raw materials and enhancing the static dehydration process in storage tanks, Lei Hongqi and others can effectively reduce or eliminate the impact of moisture on the processing of coal tar, thereby ensuring its safe, continuous, and stable production. Meanwhile, the losses caused by coking processes, equipment, and devices were analyzed from multiple perspectives, and methods for controlling the water content in coal tar and chloride ions in coal tar at the source were proposed.

2.2 Optimization of the coal tar hydrogenation process In China, there is an overcapacity in coal tar production, while some jet fuels are in short supply and unable to meet market demand; therefore, converting coal tar into jet fuel through catalytic hydrogenation has become key to solving this problem, and it is also one of the main areas of research in the advanced processing of coal tar. Building on previous research regarding the routes and methods of coal tar hydrogenation technology, in recent years more studies have focused on exploring and researching the coal tar hydrogenation process from multiple perspectives, including pretreatment, catalysts, and reactors. 2.2.1 Refinement and development of pretreatment processes. Through analyzing common problems that occur during the hydrogenation pretreatment of coal tar—such as corrosion of atmospheric and vacuum distillation towers and pipelines—Chen Shuqun and Ma Jun optimized the equipment and pipelines based on field conditions and multiple engineering practices. This effectively addressed the issues related to corrosion and wear in the hydrogenation pipelines and equipment, ensuring the long-term stable operation of the facility ； By improving the heater, the problem of coking in the heater tubes was effectively resolved ； The adoption of one twin-screw pump and one mechanically sealed wash oil station system has reduced the energy consumption and costs of the bottom pump in the tower. Hu Honghui et al. treated high-concentration, low-quality wastewater from a coal tar slurry bed hydrogenation unit by employing a combined process of filtration-stripping-oil removal pretreatment, A/O-SBR biological treatment, and ozone oxidation-multiple aeration advanced treatment, and described in detail the processes for pretreatment, biological treatment, and advanced treatment respectively ； Experiments have confirmed that the combined treatment process of pretreatment + biochemical treatment + advanced treatment can effectively address the individual issues encountered in traditional wastewater treatment. It also allows for the full utilization of the advantages of various advanced wastewater treatment technologies, thereby achieving the degradation and removal of toxic and harmful pollutants, and ensuring that high-concentration coal wastewater can be discharged up to standard after treatment. Qi Jun and colleagues used the actual production data from a 500,000 t/a medium- and low-temperature coal tar hydrogenation plant in China as a reference, and employed Aspen Plus software to simulate the raw material pretreatment process in the coal tar hydrogenation process. Through simulation, the temperature distribution curve of the vacuum tower trays and the process parameters were obtained, which are generally consistent with the actual production parameters in the plant. This indicates that the process simulation of this unit can accurately reflect the operating conditions of the facility, laying a foundation for subsequent energy-saving optimizations. On this basis, the heat exchange network of the pretreatment unit was analyzed using the pinch analysis technique, and it was found that a large amount of heat from the bottom oil of the vacuum distillation tower was not being utilized properly. After optimization and modification, the heat from process streams is utilized effectively, reducing the load on the vacuum heating furnace, decreasing energy consumption, and increasing the annual cumulative benefits. To investigate the treatment efficiency of the iron-carbon microelectrolysis method for high-temperature coal tar hydrogenation wastewater, Song Zhiwei and colleagues examined the effects of four factors—iron-carbon dosage, reaction time, reaction pH value, and aeration volume—on the COD removal efficiency of the wastewater, and optimized the optimal process conditions through orthogonal experiments. The experimental results show that pretreating high-temperature coal tar hydrogenation wastewater using the iron-carbon microelectrolysis method is feasible, and it affects COD. The order of the main factors affecting the removal rate is initial pH value, iron-carbon dosage, reaction time, and aeration volume ； Meanwhile, through orthogonal experiment research, the optimal process for treating high-temperature coal tar hydrogenation wastewater using iron-carbon micro-electrolysis was determined. Using low-temperature coal tar as raw material, Tang Yingbiao combined techniques such as centrifugal separation, electrodesalination and dehydration, and chemical dechlorination to carry out a centralized pretreatment of coal tar via \"centrifugal separation + dechlorination reaction + electrodesalination\", and determined the optimal operating conditions. These conditions can significantly reduce the impurity content in coal tar, meeting the requirements of subsequent catalytic hydrogenation processes and providing technical support for the efficient utilization of coal tar. Furthermore, dynamic experiments have shown that this method can effectively reduce the impurities in coal tar, thereby solving the problem of coal tar pretreatment. It also helps to eliminate the corrosion caused by coal tar on subsequent hydrogenation equipment, ensuring their smooth operation and meeting the requirements of the subsequent hydrogenation processes.

2.2.2 Study and screening of catalyst properties: Lei Xiong et al. used γ-Al2O3 as a carrier to prepare 4 types of Ni-W/γ-Al2O3 catalysts with different nickel-tungsten mass ratios. Various methods were employed to characterize the physicochemical properties of these catalysts. Hydrogenation experiments were carried out to identify those catalysts that exhibited superior hydrogenation performance, and the effects of hydrogenation process conditions on the distribution of hydrogenation products as well as the removal of heteroatoms were analyzed. Studies have found that: 1) high temperatures are unfavorable for the lightening of oils and reduce the removal rate of thiophene-based organic sulfur compounds, but are beneficial for the removal of difficult-to-remove polycyclic nitrogen compounds ； 2) High pressure can suppress cracking and dehydrogenation reactions, increasing the yield of gasoline and diesel; it also facilitates the hydrogenation saturation of polycyclic nitrogenous compounds, thereby improving the denitration rate ； 3) Performing the hydrogenation reaction at low speeds is beneficial for the removal of heteroatoms and the lightening of the oil product. Li Zhifeng and others used micro-fixed-bed reactors for the hydrogenation of high-temperature coal tar, examining the deactivation of the hydrogenation catalyst over time. They also studied the regeneration effects of two methods: high-temperature calcination and solvent washing. Through preliminary analysis, high-temperature calcination and high-pressure hydrogen reduction were identified as effective ways to regenerate deactivated catalysts. This regeneration process enables the removal of organic substances from the surface or pores of the catalyst through high-temperature calcination, and it also helps to improve the dispersion and reduction of the active components involved in the hydrogenation reaction, thereby enhancing the performance of the regenerated catalyst. In contrast, when the regenerated catalyst is cleaned with a solvent, its catalytic activity cannot be fully restored to that of a fresh catalyst in terms of hydrogenation activity, because the impurities in the catalyst’s pores are not completely removed. Lü Mulan and others conducted hydrogenation experiments on coal tar mimics using the NiWP/A12O3 catalyst in a fixed-bed hydrogenation microreactor, in order to analyze the reasons for the rapid deactivation of the catalyst at the initial stage. The experiments showed that as the reaction progressed, the catalyst’s hydrodesulfurization and hydrodeoxygenation activities remained relatively stable, whereas its hydrodenitrogenation activity declined rapidly ； The main cause of this phenomenon is the formation of carbon deposits on the surface, which cover the reactive centers there and reduce the reactivity of the hydrogenation reaction ； The main components of this surface carbonization are aromatic hydrocarbons, with a small amount of aliphatic hydrocarbons as well. To address issues such as blockages between catalyst particles caused by Fe and Ca deposition, as well as the coverage of active centers and the rapid decline in bed pressure, Wu Yan studied the types and distribution patterns of metals deposited on catalysts used for the hydroprocessing of medium- and low-temperature coal tar (protectants and demetallizing agents). She found that Fe deposits along the axis of the catalyst bed show a downward trend from top to bottom in the reactor, while Ca does not exhibit any distinct distribution pattern. These findings provide foundational information for the development of coal tar preprocessing technologies.

2.2.3 Reactor performance and influencing factors Huang Ye et al. conducted experiments on the hydrorefining-hydrocracking of coal tar using a double-tube series fixed-bed reactor. Process optimization experiments were carried out using 4 different catalysts, and the optimal reaction temperature and optimal reaction space velocity for the hydrogenation of coal tar to produce fuel oils were ultimately determined. The hydrocarbon composition of the light fractions of coal tar hydrogenated gasoline and diesel was analyzed using gas chromatography-mass spectrometry. The results show that the octane number of the gasoline produced using the described process, as well as the cetane number of the diesel products, are low; however, the other parameters meet the **standard requirements. To study the effect of the conversion of oxygen-containing compounds during the hydrogenation of coal tar on the properties of the products, Song Zhaoyang and colleagues conducted hydrogenation deoxygenation (HDO) experiments on low-temperature coal tar fractions (<240°C) using a fixed-bed reactor with Ni-Mo/γ-Al2O3 as catalyst, thereby investigating the influence of different temperatures, pressures, and space velocities on the composition and properties of the hydrogenation deoxygenation products. The results showed that under conditions of high temperature, high pressure, and low flow rate, the oxygen, sulfur, and nitrogen contents of the hydrogenated products met the standards for automotive gasoline. The contents of alkylbenzenes and naphthenes increased, but the octane number (RON) and density of the hydrogenated products were low, so they could not be used directly as automotive gasoline. A kinetic model was developed to describe the hydrodeoxidation of coal tar, and the oxygen content in the hydrogenated product was in good agreement with the theoretical calculations. Li Guofeng analyzed the respective characteristics of coal tar hydrogenation technology on a fixed bed and that on a suspended bed, and subsequently conducted a comparative analysis of the hydrogenation processes using these two types of reactors. The report suggests that catalysts in suspended-bed reactors are less prone to coking and carbon deposition, exhibit better hydrogenation activity, and are thus more advantageous for treating coal tar with a high content of heteroatoms. Wang Huiqu used two types of series-connected fixed-bed reactors to carry out catalytic hydrogenation of coal tar to produce clean liquid fuels. Through preliminary screening experiments on catalysts, the optimal catalyst combination was ultimately determined, and the effect of pressure during the hydrogenation process on its efficiency was investigated. A relatively low hydrogen pressure of 6 MPa was successfully employed to obtain high-quality coal tar cracking products. Through catalyst lifetime tests, the catalyst demonstrated good activity stability, and catalytic hydrogenation in a series fixed-bed reactor can effectively improve the quality of raw coal tar. In response to the problem of excessive pressure rise in the refining hydrogenation reactor that often occurs in full-distillate coal tar hydrogenation units, He Yuling, taking into account the process configuration of such units and drawing on practical operational experience, proposed control measures for managing the pressure in the refining reactor from an operational perspective. This measure has proven effective in controlling the pressure difference in refinery reactors, thereby ensuring the safe, stable, long-term, full-load, and high-quality operation of the hydrogenation units. In the pilot-scale hydrocracking reaction using high-temperature coal tar as a feedstock in a autoclave suspension bed, Dai Xin investigated the effects of catalyst mass fraction, reaction temperature, reaction time, and initial hydrogen pressure on the yield of reaction products. Finally, it was tested under the optimal process conditions, and the experimental data obtained can lay a foundation for further industrial application.

3 Thoughts on the Issues in the Coal Tar Industry: China’s coal tar industry lags far behind foreign countries in terms of both processing technology and scale efficiency. Currently, the global annual production of coal tar is about 200 million tons, from which 5 million tons of various chemical products can be refined. In developed countries abroad, the distillation capacity of a single unit for coal tar is at least 100,000 tons per year; its processing and distillation capacity increases in proportion to scale benefits. Due to technical constraints, coal tar processing plants with an annual production capacity of over 500,000 tons can only be built when a large total amount of coal tar is available for collection, in order to achieve optimal economies of scale. However, most enterprises in our country operate in a simple manner and have not carried out in-depth development or industrialization of coal tar; their production processes have not been fully modernized. In light of this, and based on the above research, reflections and prospects are offered on the reasons behind the insufficient capacity for the deep processing of coal tar. 3.1 Cause analysis 1) The utilization of coal resources mainly relies on combustion and single conversion methods; limited by the scale of existing equipment and total production volumes, while there is a pursuit of larger scales, which results in low resource utilization efficiency ； 2) The integration of industry, academia, and research remains relatively weak; companies make limited use of new technologies and innovative R&D methods, resulting in a low level of processing technology ； 3) Low deep-processing capabilities, poor product quality and standard, resulting in weak competitiveness in the international market ； 4) In the research and development of coal chemical industry, the coupling of matter and energy between modern coal chemical technologies and traditional ones has been neglected, resulting in their isolated development ； 5) The industrial layout is unreasonable; environmental protection and energy utilization efficiency lag behind those of developed ** countries, thereby affecting the deep processing of related products.

3.2 Recommendations and Outlooks 3.2.1 Increasing investment in technology research and development Introducing advanced technologies from coal tar processing leaders such as Germany, the United States, and Japan is a quick way to overcome technological backwardness and improve economic efficiency. Enterprises can take advantage of the development opportunities presented by “Made in China 2025”. By considering their own actual circumstances and adopting measures such as Sino-foreign joint ventures, cooperation with foreign partners, and purchasing advanced foreign equipment, they can learn from overseas technologies. This will help them continuously enhance their capacity for independent innovation, actively promote the independent development of key equipment and technologies, foster cross-regional and cross-industry technological collaboration, establish a coal tar processing system suited to national conditions, and develop a high-level modern coal chemical industry, ultimately striving to achieve self-sufficiency in high-end products. 3.2.2 The coal tar processing industry is moving towards large-scale centralization. Conducting research on the deep processing of coal tar in a centralized manner not only aligns with the advocated scientific development concept, but also enables the integration and utilization of existing resources, thereby reducing energy consumption ； It is necessary to enhance the coupling between various coal chemical technologies and strengthen research and development on the optimization and integration of these technologies and systems. This will also make processing facilities larger in scale, more technologically advanced, and more modern, thereby improving the overall utilization rate of resources and the market competitiveness of products ； At the same time, it promotes technical exchanges among enterprises, facilitates coordination between them, and encourages partnerships between strong players, thereby creating an \"upstream-downstream integration\" of industrial chains and interest distribution mechanisms. 3.2.3 Integrating industry, academia, and research closely: The deep processing and industrialization of coal tar should not be carried out by companies acting on their own; nor should there be blind construction and investment. It is necessary to conduct thorough technical and market research. In our country, there are many higher education institutions and research organizations dedicated to the research, development, and design in the coal coking industry, such as the Shanxi Institute of Coal Chemistry under the Chinese Academy of Sciences, the Coal Science Research Institute, and Taiyuan University of Technology. Enterprises should strengthen cooperation with these research institutions, introduce new technologies and talent, establish their own research centers, and actively utilize new technologies to improve the utilization rate of coal tar while developing new products. For projects in this area, it is also possible to better anticipate risks and assess the scale of operations in advance. 3.2.4 Accelerate the development of new products that meet market demands. Coal tar products have a wide range of applications, such as in pharmaceuticals, chemicals, metallurgy, and many other fields. Different fields have diverse requirements regarding the quality and variety of products. Therefore, when designing products, it is necessary to conduct market research, adjust the product structure, and move towards more advanced and refined product designs, in order to produce products that meet the needs of different market segments. This will help improve the economic efficiency of processing enterprises and accelerate the development of coal tar processing in China. 3.2.5 Improving the clean and efficient conversion of coal. At present, the environmental pollution caused by coal tar processing enterprises in China cannot be ignored. Enterprises should enhance their environmental awareness, pursue development in the directions of differentiated quality, diversity, scale, and high-end standards, strengthen the development and application of energy-saving technologies, actively foster a circular economy, increase investment in research and development of new environmental protection technologies, and conduct research and development on the core technologies for zero-emission treatment of waste and wastewater commonly generated in coal chemical projects. They should also establish stable, efficient, and cost-effective processes and equipment for the recycling of materials in coal chemistry. Through continuous simulation and optimization, the concept of energy conservation and emission reduction is incorporated into the design, thereby maximizing the value of resources, improving production efficiency and environmental quality, ensuring a clean and efficient conversion process, and pursuing sustainable development. 3.2.6 **Provinces and regions can provide practical support in this regard. Coal is an important raw material for producing oil, natural gas, and high-value chemicals. The middle and upper reaches of the Yellow River are important coal-producing areas in China; most of the country’s large-scale coal bases are located here, and it is also a major source of coal tar. The development of technologies for the deep processing of coal tar and their industrialization is one of the key research and development projects in the modern coal chemical industry. It also serves as a crucial pathway toward the clean, efficient, low-carbon, and circular development of coal. This endeavor aligns with the guidelines regarding the development direction of fine chemicals outlined in documents such as the “Action Plan for Innovation in Energy Technology 2016–2030,” and it represents a concrete means of achieving the planned objectives. The developed key common technologies for the deep processing of coal tar can effectively promote research on modern coal chemical technologies in the middle and upper reaches of the Yellow River. They also meet the urgent need to transform hazardous waste—coal tar—into useful resources and enable its resourceful and high-value utilization. Furthermore, these technologies help improve the production environment in the coke industry and enhance the quality of economic development in the region. Therefore, the author hopes that **the relevant regional authorities can organize large-scale online or offline academic conferences to provide opportunities and platforms for exchanges between enterprises and research institutions, establish large-scale coal tar processing facilities to facilitate learning and cooperation among enterprises, and offer certain support in terms of policies and funding.

4 Conclusions and Outlook 1) In recent years, most research on the deep processing of coal tar has focused on the development of fine chemical processes for coal tar, with an emphasis on component analysis and the optimization of separation techniques; furthermore, efforts have been made to optimize the hydroprocessing of coal tar from the perspectives of pretreatment, catalysts, and reactors. 2) The main reason why the output of deeply processed coal tar products is much lower than that of other ** is that such processes are constrained by the scale of the equipment and total production volume, yet there is a desire to achieve larger scales ； The integration of industry, academia, and research remains weak; companies make limited use of new technologies and R&D processes, resulting in a low level of processing technology ； Lack of advanced processing capabilities results in poor competitiveness in the international market ； The various aspects of modern coal chemical technology and traditional coal chemical technology are not closely integrated, with each developing in isolation ； Due to the industrial layout, environmental protection and energy utilization efficiency have not reached developed **levels, which affects the further processing of related products. In the future, as efforts to advance the deep processing of coal tar continue, those involved should pay attention to the application of the latest industrial technologies and integrate them in light of the characteristics of the industry’s development. A network system centered on product information and technical cooperation should be established to foster enterprise collaboration and scientific research exchanges, thereby enabling the efficient and rational use of coal tar resources and ensuring the long-term stable operation of production facilities, as well as achieving energy savings, consumption reduction, and environmental protection.