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【Technology】Various approaches to coal tar hydrogenation have their own advantages and disadvantages | By Reporter Zhang Xinggang?Coal Chemical Industry Network?2015-12-03?In the past two years, projects utilizing coal tar hydrogenation technology have emerged in large numbers across China. On April 30, 2014, the process package for the 500,000 tons per year coal tar slurry bed hydrocracking unit at Inner Mongolia Qinghua Group Wusitai Fine Chemicals Co., Ltd. was approved. On July 25, 2015, a 150,000-ton/year fluidized bed coal tar hydrogenation unit was successfully commissioned at Hebei Xinqiyuan Energy Technology Development Co., Ltd.; it was the first such unit of its kind in China. At present, the first phase of the demonstration project for the hydrogenation of coal tar across its entire spectrum, carried out by Yan Yuan Chemical Co., Ltd. under Yanchang Petroleum, which uses full-range slurry bed hydrocracking technology, has entered the trial operation stage for its 500,000-ton per year coal tar hydrogenation facility. To date, six types of coal tar hydrogenation technologies – fixed-bed hydrorefining technology, fixed-bed hydrocracking technology, full-range coal tar fixed-bed hydrocracking technology, delayed coking-fixed-bed hydrocracking combined processing technology, and suspended-bed and fluidized-bed hydrocracking technology – have all been put into use in China. So how are these technologies compared? Which technology will be the direction for application and shape the future of the coal tar hydrogenation industry? Industry experts provided their comments respectively. Coal tar is a viscous liquid product that is dark brown or pure black in color. It is classified according to the temperature at which coal is carbonized, into low-temperature coal tar, medium-temperature coal tar, and high-temperature coal tar. Coal tar hydrogenation involves the use of hydrogenation refining or treatment to remove impurities such as sulfur, nitrogen, oxygen, metals, as well as saturated alkenes and aromatics from coal tar, thereby producing target products like naphtha, diesel, and heavy fuel oils with low sulfur and nitrogen content. Industry insiders told reporters that the six types of technologies for coal tar hydrogenation share the same principle; the only difference lies in the pretreatment process, with fixed-bed hydrogenation being used in all cases thereafter. In the preprocessing stage, they each have their own characteristics. The simple fixed-bed hydrorefining technique involves atmospheric and vacuum distillation, during which the entire coal tar fraction is separated into light fractions and heavy oils. The light fractions are used as feedstock for fixed-bed hydrorefining, where sulfur, nitrogen, oxygen, and other impurities are removed from them, while the olefins and some aromatics present in these fractions are hydrogenated and saturated. As a result, naphtha and low-sulfur diesel fractions are obtained. The advantages of this technology are its simple process and low investment, but the yield of light oil products is the lowest, and it can only handle fractions with temperatures below 350 degrees Celsius. The fixed-bed hydrocracking technology adds a hydrocracking stage to the process flow, where coal tar is first separated into distillate oils and asphalt products. In this process, the distillate oil enters the hydrorefining section and the hydrocracking section in sequence. In the hydrorefining section, elements such as sulfur, nitrogen, and oxygen are removed from the feedstock, and olefins as well as some aromatics are hydrogenated and saturated. During the hydrocracking stage, the vacuum distillate oil at temperatures of 350 to 500 degrees Celsius is cracked into lighter oils with temperatures below 350 degrees Celsius, thereby increasing the yield of oil products. Theoretically, if the dry point of the vacuum distillate oil is well controlled, the utilization rate of coal tar can be increased by about 30%. But in reality, the dry point of distillate oils is generally difficult to control well. The feedstock for the full-range oil fixed-bed hydrocracking technology includes a tar pitch component, further improving the utilization rate of coal tar. Zhu Yufei, a senior engineer at the Beijing Institute of Low-Carbon Clean Energy, believes that if the target product are fuel oils such as gasoline and diesel fractions, hydrogenation is better than coking. Because hydrogenation enables more efficient utilization of coal tar as a resource, allowing it to be converted into light oil products to the greatest extent possible. The delayed coking-hydrogenation combined technology can be used as a method for processing coal tar. However, its obvious drawback is the high coke yield, as it converts a considerable proportion of liquid coal tar into solid coke, which is unfavorable from the perspective of resource utilization. Especially for high-temperature coal tar, the difficulty of delayed coking processing is even greater. Yao Lei, an engineer at Shaanxi Shenmu Tianyuan Chemical Co., Ltd., believes that the delayed coking process is a semi-continuous operation method that allows for a large volume of processing per batch, with high efficiency as well. Unlike other technologies that rely on multiple sets of hydrogenation catalysts, this approach significantly reduces the investment and operating costs associated with coal tar processing, thereby lowering the costs of coal tar processing and improving its economic viability to some extent. However, Zhang Jianqiang, deputy general manager of Gansu Honghui Energy Chemical Co., Ltd., believes that considering the longest operating cycle, although the yield of the delayed coking-hydrogenation combined process is low at only 75%, overall factors such as catalyst wear and startup/shutdown losses make this technology relatively economical and suitable for use. Suspension bed and fluidized bed hydrocracking technologies are currently quite prominent technologies. In this technical reactor, the catalyst is not in a fixed state but in a flowing state; the reaction heat generated during hydrogenation can be carried away by the reaction medium, thereby avoiding the problem of catalyst bed clogging caused by the deposition and coking of pollutants brought in with the feedstock. Some industry experts believe that suspension bed and fluidized bed reactors allow for the online replacement of catalysts, which helps maintain stable catalyst activity within the reactor and thus enables long-term stable operation. This leads to an increased conversion rate of coal tar and a higher yield of light oil products, with the oil recovery rate reaching 90% or even higher. Zhu Yufei believes that the slurry bed hydrogenation technology is more suitable for processing coal tar, as it can handle heavy oils with high asphaltene content, and it has significant advantages when dealing with raw materials containing many pollutants. He believes that not all suspended-bed hydrogenation technologies can be used for coal tar processing, as the reactivity of coal tar differs from that of petroleum fractions; appropriate catalysts and process conditions are required to achieve good hydrogenation conversion results. Wan Xuebing, general manager of Shandong Zibo Taitong Catalysis Technology Co., Ltd., believes that in the hydrogenation process of coal tar using a suspended-bed system, the catalyst is suspended within the coal tar; its density is similar to that of the reactants, which allows for uniform reaction between the catalyst and the coal tar, resulting in good reaction efficiency. Zhang Xiaojing, director of the Liquefaction Research Institute at the China Coal Research Institute, believes that suspended-bed and fluidized-bed hydrocracking technologies involve different reactors, distinct reaction systems, and varying levels of complexity; therefore, it is necessary to compare the operating costs, operational difficulties, and investment requirements. Zuo Tie, a senior engineer at Sinopec Luoyang Engineering Co., Ltd., further explained that the catalyst addition and removal system in fluidized bed reactors is complex, resulting in investment costs that are 10–15% higher than those for suspended bed reactors. The suspended bed technology involves the continuous replenishment of catalyst in the reactor, ensuring that the freshest catalyst is used for the reaction with coal tar. Almost all of the asphalt present in the tar is converted, resulting in high yield rates; the yield rate for medium and low-temperature coal tar can reach 92%, while that for high-temperature coal tar is 83%. He Yongde, honorary president of the Shaanxi Chemical Industry Society, conducted various comparisons: the use of full-range coal tar fixed-bed hydrocracking technology yields 10% more oil, but it requires more hydrogen; 1 ton of oil produced by this method consumes 200 cubic meters more hydrogen compared to the combined technology of delayed coking and fixed-bed hydrocracking. From an investment perspective, the full-range coal tar fixed-bed hydrocracking technology requires 25–30% less investment than the combined delayed coking and fixed-bed hydrocracking technology; for the same type of coal tar, overall, the economic viability of the two technologies is similar. The oil yield of suspended-bed and fluidized-bed hydrogenation technologies is higher than that of fixed-bed technologies. Industry experts such as Li Zhiyue, deputy general manager of Shoushan Coking Company in Pingdingshan, Henan, and Zhang Xinqiao, general manager of Henan Baoshuo Tar Chemical Co., Ltd., are of the view that suspended-bed and fluidized-bed hydrogenation represent the technical approaches for the hydrogenation of coal tar; these methods offer high yields of oil products and are suitable for the hydrogenation of the entire fraction of high-temperature coal tar. It’s just that some of them favor fluidized bed reactors, while others prefer suspended bed reactors. Zhang Jianqiang pointed out that suspended bed and fluidized bed differ only in the method of agitation, and both represent future development directions. However, to determine whether the system can operate stably at full load over a long period of time, it is necessary to run it continuously for a year. Experts such as Cao Xianyong, a senior engineer at Xinjiang Guanghui Energy Company, told reporters that suspended-bed and fluidized-bed reactors represent the most advanced technologies for coal tar hydrogenation at present, but the investment required is also relatively high. The route should be determined based on the quality of coal tar and the scale of coal tar processing. If the quality of coal tar is good and the processing scale is small, fixed-bed and delayed coking are also suitable; in fact, high-temperature coal tar with a small processing scale can also be processed using fixed-bed hydrogenation. For high-temperature coal tar with a large scale of tar processing, high asphaltene and metal contents, suspended-bed and fluidized-bed hydrocracking technologies are more suitable. Fan Anlin, the chief engineer of Shandong Jinneng Coal Chemical Co., Ltd., told reporters that there is no absolute answer as to whether a suspended bed or a bubbling bed is the better option. The choice of technical approach should be determined by considering factors such as the quality of coal tar, the availability of coal tar resources, the company’s financial resources, the quality of the fuel oil obtained after hydrogenation of coal tar, and the prices of refined oils, in order to make an economic assessment for the company. In addition to producing oils using coal tar hydrogenation technology, other approaches are also being explored in practice. It is understood that most existing coal tar hydrogenation processes produce naphtha and light diesel as their final products. A prominent feature of these products is their high content of aromatics and cycloparaffins. The potential aromatic content in them is generally between 70% and 80%, which is far higher than that in petroleum, making them a rare source of aromatic compounds. Zhu Yufei said that by using selective hydrocracking technology to convert the polycyclic aromatics in it into monocyclic aromatics, which can then be used to produce aromatic products, it is possible to avoid the problem of low cetane number in light diesel products, while also making effective use of the aromatic resources contained in this fraction of oil. Coal tar is used to produce aromatic products through hydrogenation conversion technology, and it can also reduce the hydrogen consumption during the production process. Industry experts such as Li Zhiyue also say that if coal tar is used to produce aromatic products, it can not only significantly improve the efficiency of catalytic reforming units but also fully leverage the advantages of this raw material. Aromatics have a high added value and are exempt from consumption tax; this is a viable direction, and it makes economic sense. However, some industry experts believe that the decision to launch aromatics production should also be based on the aromatics production and sales market in the region where the company is located.