At present, the main technical approaches for the deep processing of medium- and low-temperature coal tar are as follows; as for high-temperature coal tar, they are…

At present, the processing of medium- and low-temperature coal tar is receiving considerable attention in China, but hydrogenation is the main method used. It is understood that the primary processing techniques include: 1. Distilling medium- and low-temperature coal tar to remove asphaltenes – followed by hydrogenation； 2. Medium and low-temperature coal tar distillation – Hydrogenation of light fractions (heavier fractions are subjected to delayed coking, and the resulting oils are also hydrogenated) ； 3. Medium- and low-temperature coal tar distillation – light fractions are used to extract phenols, which are then processed according to methods 1 and 2 ； 4. Distillation and phenol extraction to produce various oils. Are there any other processing methods? If anyone knows, please share; thank you so much!

Coal tar contains large amounts of unsaturated hydrocarbons such as olefins and polycyclic aromatic hydrocarbons, as well as sulfur and nitrogen compounds; it has a high acidity and high gum content, resulting in poor stability of the product, making it unsuitable for release as a finished product. By using the hydroprocessing technique, desulfurization, saturation of unsaturated hydrocarbons, denitration, and saturation of aromatics can be carried out, thereby improving stability, reducing sulfur content, and lowering aromatic content to obtain naphtha and high-quality fuel oil. The product quality can meet the standards of gasoline and diesel blend oils. 　　This technological approach enables full utilization of the by-products of coking, namely tar and coke oven gas, to achieve a circular economy in the coking industry as well as value addition to its products. With rising crude oil prices, some experts consider this technology to represent another route for converting coal into oil, alongside direct liquefaction and indirect liquefaction.

Harbin gasification manufacturers use the low-temperature tar fractions produced by coal gasification, those with temperatures below 350°C, along with light oil, and feed them into the hydrogenation reactor; generally, the catalyst needs to be replaced after a operation cycle of 4–5 months. Shaanxi Shenmu Tianyuan Chemical started operations in April 2008; it ceased production due to equipment issues and is currently under renovation, using medium-temperature coal tar as raw material ； There is one facility in Qitaihe, Heilongjiang that has been under construction yet has not started operations ； Several are currently in the planning stage in Shaanxi and Inner Mongolia. Those in Shaanxi are all watching the operations of Shenmu Tianyuan Chemical ； Hydrogenation of high-temperature coal tar is difficult and uneconomical, mainly due to the extremely high pressures required and high hydrogen consumption; it is better to perform vacuum distillation of high-temperature tar and use the fractions obtained before 420°C for hydrogenation.

In the suspended-bed process, fine powdered catalyst is pre-mixed with the feedstock, and then both are fed into the reactor along with hydrogen; they flow from bottom to top, where hydrocracking takes place. The catalyst remains suspended in the liquid phase and exits the reactor from the top along with the reaction products. Suspension bed reactors are generally of cylindrical design without a catalyst bed ； In a fluidized bed reactor, the catalyst is located inside the reactor; a catalyst supply pipe is provided at the upper part of the reactor, while a catalyst discharge outlet is located at the lower part (to remove old catalyst from the bottom and add new catalyst from the top). Hydrogen and feed oil enter the reactor from its lower part; as they pass through the catalyst-packed bed via a grid distributor, the void space between the catalyst particles increases gradually with the rise in flow rate, causing the volume of the catalyst bed to expand. The height of the catalyst bed is controlled by the flow rate of the circulating liquid (a circulation line is provided within the reactor; the reaction products are drawn out from the top of the reactor, pumped to the bottom of the reactor where they mix with the feedstock before entering the reactor again, thereby controlling the feedstock flow rate).

In 2017, Xinjiang Xuanli Environmental Protection Energy Co., Ltd. began construction of a hydrogenation plant capable of processing 500,000 tons per year of full-range medium- and low-temperature coal tar, in the Naomao Lake Industrial Park in Hami, Xinjiang. The plant came online in January 2018 and has since operated stably over long periods of time. In 2019, the enterprise, technology, and design teams conducted a 72-hour self-calibration of this device, and all parameters met the expected levels, with a maximum load of 105%. The test results show that the oxygen content in low- and medium-temperature coal tar in the whole fraction is 6%–8%; pretreatment techniques can reduce the total metal content to below 20 μg/g, the ash removal rate is >80%, and the yield of coal tar after purification is >99% (on an anhydrous basis) ； During the hydrogenation process, the hydrogen consumption is 6%–6.5%, and the yield of C5+ liquids is greater than 94%. Among these, the yield of the hydrogenated oil product is 20%–24%, while the aromatic content is greater than 65%, making it an excellent raw material for aromatics production ； The yield of hydrogenated oil 1 is 70%–75%, with a cetane number of ≥46 and a cetane value of ≥51; it is possible to produce diesel fractions of 0#, -10#, -20#, and -35# that meet National VI standards (GB 19147-2016) in a flexible manner, depending on seasonal changes.

Delayed coking of coal tar: In reality, the temperature inside the furnace tubes has already reached the temperature required for coke formation. What we do is use methods such as injecting steam, water, or light paraffin oil to increase the flow rate of the oil, thereby allowing it to move quickly from the furnace tubes to the coke tower where coke is formed. This is presumably what delayed coking entails

Our country has made significant progress in the development of hydrogenation technologies for medium- and low-temperature coal tar (hereinafter, “coal tar” refers to “medium- and low-temperature coal tar”). A variety of hydrogenation technologies have been developed, which can be classified into the following 4 categories based on their characteristics: The first category is coal tar hydrogenation refining/hydroprocessing technologies; the second category is the combined delayed coking-hydrocracking process technology; the third category is fixed-bed hydrocracking technologies for coal tar; and the fourth category is suspension/slurry-bed hydrocracking technologies for coal tar. 1. Coal tar hydrogenation refining/hydroprocessing technology: This technology features the use of fixed-bed hydrogenation refining or hydroprocessing to remove impurities such as sulfur, nitrogen, oxygen, metals, as well as saturated olefins and aromatics from coal tar, thereby producing target products such as naphtha, diesel fuel, heavy fuel oils with low sulfur and nitrogen content, or raw materials for carbon materials. The advantages of coal tar hydrogenation refining/hydroprocessing technology are: a relatively simple process flow, as well as lower investment and operating costs. Its disadvantages are: lower yields of naphtha and diesel, which depend mainly on the content of light oils in the feedstock coal tar, as well as a low utilization rate of coal tar resources. 2 Delayed coking-hydrogenation combined process technology. The main principle of this technology is to convert the heavy oil fraction in coal tar into light fractions and coke through delayed coking; thereafter, both the light fractions from coal tar and those obtained from delayed coking are subjected to hydrorefining or hydrorefining/hydroisomerization in order to produce naphtha and diesel products. The basic process flow of the delayed coking–hydrorefining/hydrocracking combined process: First, the full-range coal tar is subjected to delayed coking, yielding gas, coke, light fractions (naphtha and diesel fractions), and heavy fractions (at 350–500 °C). Then, the light fractions are subjected to hydrorefining, while the heavy fractions are used as feedstock for hydrocracking; ultimately, naphtha and diesel products are obtained. The basic process of the delayed coking-hydrorefining combination process: Coal tar is first distilled into light oil (< 360 °C) and heavy oil (> 360 °C). The heavy oil serves as the feedstock for delayed coking, and the delayed coking unit operates using a full circulation system for fractions with temperatures above 360 °C. During this process, all light fractions (< 360 °C) are subjected to hydrorefining, resulting in naphtha and diesel products. The main operating conditions for such technologies are a delayed coking reaction temperature of 450–550 °C and a reaction pressure of 0.1–3.0 MPa; for the hydrogenation reaction, the temperature is 300–450 °C and the pressure is 6.0–20.0 MPa. Comparing the two process technologies mentioned above, the former requires higher investment but yields a higher liquid output. Shenmu Tianyuan Chemical Co., Ltd., part of Shaanxi Coal and Chemical Industry Group, uses a delayed coking–hydrorefining/hydrocracking process to process medium- and low-temperature coal tar; this represents a new method for coal tar processing. In this process, the oil yield from the delayed coking unit is approximately 80%, while the coke yield is about 16%. The advantage of the delayed coking-hydrogenation combined process is that it converts a portion of heavy coal tar into light oil products; the disadvantage is that the process is relatively complex, and it converts part of the coal tar into coke, resulting in incomplete utilization of the coal tar resources. . 3 Coal tar fixed-bed hydrocracking technology: The concept behind coal tar fixed-bed hydrocracking technology is to use the fixed-bed hydrocracking method to convert the heavy oils in coal tar (with a boiling point > 350 °C) into light oil products, thereby increasing the yield of such light oil products. Since coal tar contains a high amount of heteroatoms such as sulfur, nitrogen, and oxygen, as well as catalyst contaminants like resins, asphalts, and metals, on the one hand, these contaminants in the feed oil can easily deactivate the catalysts in the hydrorefining section and clog the catalyst beds. On the other hand, the ammonia generated in the hydrorefining section can affect the activity of the hydrocracking catalysts, while the water produced can cause permanent deactivation of those catalysts. Therefore, preserving the activity of the catalysts and ensuring their long-term operation is key to this type of technology. In terms of methods to protect the catalyst activity in the hydrorefining section, various techniques employ a similar approach – namely, the use of multiple catalyst grades, which can effectively prevent the impact of contaminants present in the feedstock. To safeguard the catalyst activity in the hydrocracking section, different hydrocracking process technologies have been developed, such as those with two stages in series [24–26] and two stages in parallel [27–29]; these approaches prevent ammonia and water generated in the hydrorefining section from entering the reactors in the hydrocracking section. If it is necessary to further increase the cetane number of diesel products, the diesel fractions produced by the above hydrocracking can be further subjected to hydrorefining. Furthermore, in order to reduce the hydrogen consumption during the hydrogenation process, it is also possible to remove phenols from the phenol-containing fractions in coal tar before carrying out hydrogenation on it [25]. This approach helps to reduce hydrogen usage during hydrogenation and also allows for the production of certain phenolic products. The main operating conditions for such technologies are: hydrogenation reaction temperature of 300 to 450 °C, reaction pressure of 5 to 19 MPa, space velocity of 0.5 to 3.0 h⁻¹, and hydrogen-to-oil volume ratio of 600 to 3,500. The advantage of fixed-bed hydrocracking technology is that it converts most of the heavy oils in coal tar into light oil fractions, thereby increasing the yield of light oil products and the utilization rate of coal tar resources. It also maximizes the cetane number of diesel products, which can reach over 40. Its disadvantages are that the process flow is relatively complex, and there are certain restrictions on the feedstock oil; to enable long-term operation, the dry point of the feedstock oil must be below 600 °C, and ideally below 580 °C. Such technologies are not yet used in industrial production. 4 Suspension bed/slurry bed hydrocracking technology for coal tar: Suspension bed, slurry bed, and fluidized bed reactors typically use catalysts that can be used once or replaced online, which allows the impact of contaminants in the feed oil on catalyst activity to be eliminated and keeps the catalyst activity within the reactor at an optimal level. Therefore, such reactors are ideal for processing feed oils with high levels of contaminants. The most prominent advantages of the suspended bed/slurry bed hydrocracking technology for coal tar are: wide adaptability to feed oils, high utilization rate of coal tar resources, high yield of light oil products, and good product quality. Such technologies have not yet been applied in industrial production, but they are expected to be used in some new coal chemical projects in the near future.