Research on the technology for producing jet fuel from refined coal tar
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Coal-to-oil technology: Research on the technology for producing jet fuel from refined coal tar. Author: Hua Xiaobei. Source: Coal Chemical Industry Information Network. Views: 567. Update date: 2024-01-24. Hydrogenation modification experiments were carried out using hydrogenated and refined coal tar produced in industrial facilities as raw material, under conditions of a pressure of 15.0 MPa, a temperature of 360°C, a space velocity of 0.8 h^-1, and a hydrogen-to-oil volume ratio of 1000, in a fixed-bed pilot plant. The results show that after hydroprocessing of the refined coal tar feedstock, its boiling range shifts significantly to lower values; the hydrocarbon ratio of the product decreases slightly, the content of saturated hydrocarbons increases markedly, while the content of aromatics decreases significantly. The yield of the jet fuel fraction obtained at 130–280°C increased significantly; its smoke point improved, its freezing point decreased, and the volume fraction of aromatics dropped markedly. The product meets the standards for Jet Fuel No. 3 as specified in the national standard (GB6537-2018). This technology features high feasibility, high raw material utilization rate, high liquid yield, and high product quality, offering excellent economic and social benefits. With reform and opening up as well as rapid economic development, China’s demand for jet fuel is increasing day by day. In 2018, China’s consumption of jet fuel for civil aviation was 34.63 million tons; by 2019 this figure rose to 36.84 million tons, representing a year-on-year increase of 6.4%. This has led to an increasing reliance by China on oil, the main fossil fuel used for jet fuel, from year to year. Fossil fuels form an important foundation for the sustainable development of China’s economy. The country has abundant coal resources, but faces shortages of oil and insufficient natural gas resources. The share of coal production and consumption in the total primary energy mix remains above 70%. Due to China’s unique energy structure characterized by abundant coal, limited oil, and scarce natural gas, in order to meet the basic requirements of **energy strategic security, China’s coal-dominated energy structure is unlikely to change significantly for a long time to come. With the current international situation and the growing shortage of global oil resources, there is increasing interest in oil alternatives. To address the shortage of oil resources in our country, the only solution is to use coal-based alternative fuels; making full use of coal resources to produce oil substitutes has become one of the important areas of research. Coal tar is an important by-product in the coal coking industry. Under appropriate temperature and pressure conditions, catalytic reactions are used to remove heteroatom compounds such as oxygen, nitrogen, and sulfur contained in the oil; at the same time, unsaturated compounds like olefins and polycyclic aromatic hydrocarbons are saturated. Cracking reactions are also employed to produce smaller molecules. By properly controlling these reactions, it is possible to reduce the density of the oil, thereby improving its stability, color, odor, and combustion properties, with the aim of enhancing its quality. After hydroprocessing, coal tar can be transformed into product oil with a high flash point, high smoke point, and low freezing point, possessing the basic characteristics of jet fuel. In this paper, the fixed-bed hydrorefining process was employed in a laboratory pilot plant to investigate the hydrorefining effects of refined coal tar from northern Shaanxi, and jet fuel grade 3 meeting the national standard (GB6537-2018) was successfully produced. 1 Experimental Section 1.1 Experimental Materials The hydrorefined coal tar obtained from an industrial facility of a company in northern Shaanxi was used as the raw material, and its main properties are shown in Table 1.
As can be seen from Table 1, the density of refined coal tar oil is 0.8666 g·cm-3, with a initial boiling point of 79.8°C and a final boiling point of 457.8°C. The distillation range is relatively wide; the temperature at 50% of the simulated distillation range is 266.6°C, which is close to the final boiling point of conventional jet fuel fractions. The temperature at 70% is 313.2°C, exceeding the final boiling point specified in the national standards for Jet Fuel No. 3, indicating that the refined coal tar feedstock contains a high proportion of heavy components. The viscosities of refined coal tar at 50°C and 100°C are 2.067 mm2·s-1 and 1.401 mm2·s-1, respectively, with a freezing point of 4°C. The carbon mass fraction of the refined coal tar feedstock is 87.31%, the hydrogen mass fraction is 12.68%, the sulfur mass fraction is 7.4 μg·g-1, and the nitrogen mass fraction is 79.8 μg·g-1. By analyzing the mass spectral composition of the refined coal tar feedstock, it can be determined that the mass fraction of alkanes in this feedstock is 20.7% ； The total cycloalkane mass fraction is 28.6%, of which monocycloalkanes account for 3.7%, dicycloalkanes for 8.1%, tricycloalkanes for 13%, and tetracycloalkanes for 3.8%. It can be seen that the refined coal tar contains a high proportion of heavy-distillate cycloalkanes of the tricyclic and tetracyclic types; therefore, directly extracting the jet fuel fraction does not facilitate control of its freezing point ； The total aromatic content in refined coal tar is 50.7%, of which monocyclic aromatics account for 44.6%, bicyclic aromatics for 4.8%, tricyclic aromatics for 0.9%, and tetracyclic aromatics for 0.1%. It can be seen that despite the hydroprocessing used to refine the coal tar, its aromatic content remains high; this may result in jet fuel fractions obtained through direct distillation containing a significant amount of aromatic compounds. Due to the higher density of aromatic compounds, an excessive amount of these compounds can lead to a significant decrease in the smoke point of the jet fuel, making it difficult for the density and smoke point values to meet the standards for Grade 3 jet fuel. 1.2 Selection of technical route: The main properties of the refined coal tar jet fuel fraction at 130–280°C are shown in Table 2. The refined coal tar was fractionated to obtain a jet fuel distillate in the 130–280°C range, with a yield of 45.31%. The density of this jet fuel was 0.8674 g·cm-3; its carbon mass fraction was 87.15%, hydrogen mass fraction 12.49%, sulfur mass fraction 6.4 μg·g-1, nitrogen mass fraction 53.6 μg·g-1. Its flash point (closed cup) was 45°C, freezing point was -27.6°C, smoke point was 13.5 mm, and the volume fraction of aromatics was 20.1%.
As can be seen from Table 2, the density, freezing point, smoke point, and aromatic volume fraction of the refined jet fuel fraction do not meet the requirements of the national standard for Jet Fuel No. 3 (GB6537-2018), which is consistent with the previous analysis results using coal tar as the raw material. Therefore, it is necessary to carry out hydroprocessing on this raw material, hydrogenating and saturating the aromatic components in the coal tar feedstock, thereby saturating a large amount of monocyclic and bicyclic aromatics into naphthenes. This can reduce the density of the jet fuel fraction to a certain extent, while also significantly increasing its smoke point and greatly lowering its freezing point. Since refined coal tar feedstock contains a high proportion of heavy components, the naphthenes and alkanes in these heavy components can be subjected to hydrocracking to open their rings and break their chains, thereby shifting the boiling range of these heavy components into the jet fuel fraction. This approach can effectively increase the yield of the jet fuel fraction. The principle flow diagram of the medium-scale hydrogenation experimental unit is shown in Figure 1. Refined coal tar is mixed with hydrogen and then subjected to hydrogenation reactions in the R1–R3 hydrogenation reforming reactors, which are filled with hydrogenation catalysts. The hydrogenation catalyst used in the experiments was a highly nitrogen-resistant hydrogenation reforming catalyst produced by Sinopec (Dalian) Research Institute of Petroleum Chemistry Co., Ltd. After being separated by a high-pressure separator and a low-pressure separator, the hydroprocessing effluent yields hydroprocessed oil; the separated gas is purified and used as recycled hydrogen. The hydroprocessed oil then enters the D1–D3 distillation systems, where it is distilled to produce jet fuel product fractions.
As can be seen from Table 3, under the conditions of a reaction pressure of 15.0 MPa, a volume space velocity of 0.8 h-1, a reaction temperature of 360°C, and a hydrogen-to-oil volume ratio of 1000, the yield of the light naphtha fraction at temperatures below 80°C was 2.61%, the yield of the heavy naphtha fraction at temperatures between 80°C and 130°C was 20.05%, the yield of the jet fuel fraction at temperatures between 130°C and 280°C was 59.63%, the yield of the diesel fraction at temperatures between 280°C and 350°C was 3.75%, and the yield of the tail oil fraction at temperatures above 350°C was also 3.75%. A comparison with the data on refined coal tar that was not subjected to hydrorefining shows that the yield of the jet fuel fraction in the 130–280°C range from the oil obtained through modification is significantly higher than that of the refined coal tar before hydrorefining. Table 4 Main properties of the oil products obtained from hydrorefining. As can be seen from Table 4, the density of the modified product oil is 0.8045 g·cm-3, which is slightly lower than that of the refined coal tar before hydrogenation. By simulating the distillation range data, it can be found that the initial boiling point of the oil obtained through modification is 67.4°C, the 70% boiling point is 235.4°C, and the final boiling point is 409.2°C. It is evident that the distillation range of refined coal tar increases significantly after hydrogenation modification, with a marked reduction in the heavy components. The freezing point of the modified product oil is -25°C, a significant decrease compared to its freezing point before hydrogenation. The carbon mass fraction of the modified crude oil is 85.43%, the hydrogen mass fraction is 14.56%, the sulfur mass fraction is 3.4 μg·g-1, and the nitrogen mass fraction is 1.1 μg·g-1. It can be seen that through hydroprocessing, the hydrogen content in the product increases while the carbon content decreases relatively. Based on the data from mass spectrometry analysis, the alkane content in the oil produced by hydroprocessing is 40.3%, which is higher than before hydroprocessing; this indicates that significant ring-opening reactions occurred during the hydroprocessing process ； The total cycloalkane mass fraction is 49.5%, with little change compared to before hydroprocessing ； The total aromatic hydrocarbon mass fraction is 10.2%, which shows a significant reduction compared to before hydroprocessing, indicating that most of the aromatics underwent saturation reactions through hydroprocessing.
Table 5 shows the main properties of the jet fuel fraction obtained from hydroprocessing at 130–280°C. The density of this jet fuel fraction is 0.8198 g·cm-3, which is slightly lower than that of the jet fuel fraction derived from refined coal tar at the same temperature range.