Treatment of waste gas from coal tar processing

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Application of Exhaust Gas Treatment Technologies for Advanced Tar Processing Abstract: The exhaust gases generated in advanced tar processing have varying compositions, and the processing facilities are scattered, making centralized treatment difficult. Treating them separately is constrained by factors such as investment costs, operating expenses, and technical limitations. A comprehensive treatment approach was adopted for the exhaust gases from the tar processing system, achieving good results in terms of low cost, high efficiency, and wide coverage. Keywords: deep processing of tar; exhaust gases; comprehensive treatment. In the deep processing of tar, it is essential to research exhaust gas treatment technologies that offer high capture efficiency and low costs. 1. Analysis of exhaust gas treatment technologies 1.1. The venturi tube method is widely used in exhaust gas treatment technologies; it relies on the suction generated by the venturi tube to power the capture and cleaning of exhaust gases. The exhaust gas is sprayed with cleaning oil and enters the venturi for the first cleaning step; after being cleaned a second time in the cleaning tower, it is then discharged. The advantages of this technology are its good cleaning effect, low oil cleaning costs, low initial investment, and low power consumption during operation; as a result, its use has been widespread. 1.2、The filler cleaning device: In the early stages of plant construction, the exhaust gas cleaning devices installed on various storage tanks were the filler cleaning devices. An exhaust gas cleaning tower is installed in the area where the storage tanks are located. Each storage tank is connected to the exhaust gas cleaning tower through an overhead gas collection pipe; the exhaust gas escaping from the tops of the tanks enters the tower under its own pressure. After being washed by the oil-circulating wash medium in the packing section, it is discharged outside. The advantage of this exhaust gas treatment technology, which uses the pressure of the exhaust gas itself as a power source, is low investment, lower operating costs, and no need for significant energy consumption ； The drawback is that the exhaust gas capture rate is limited by the sealing performance of the storage tank and collection pipes, resulting in a low capture rate. 1.3 The blower draws in air; some tar plants use the inlet pipe of the gas blower as a negative pressure source for exhaust gas treatment. The various storage tanks are connected to the exhaust gas cleaning tower through collection pipes, and after the exhaust gas has been cleaned, it is fed into the gas system via the gas blower. This technology does not require an additional power system, but it can easily disrupt the gas system; therefore, strict control over the oxygen content in the gas is necessary. An analysis of several existing exhaust gas treatment technologies in China shows that these technologies can be roughly divided into two steps: the first is exhaust gas capture, which involves directing all of the exhaust gas into a cleaning device; this is crucial for ensuring an effective rate of exhaust gas capture. Second is the cleaning of exhaust gases, which involves taking appropriate measures to clean the exhaust gases before releasing them. The capture and purification of exhaust gases are both measures to address the surface symptoms; in addition, it is necessary to improve the production process in order to reduce the amount of exhaust gases generated. Only by addressing both the surface symptoms and the root causes can low costs and good results be achieved. 2. Control measures: Based on the above analysis, and with a focus on ensuring effectiveness, addressing the issue at its source, and implementing process control, various measures are taken to reduce the formation of exhaust gases, improve the efficiency of exhaust gas capture, ensure clean emissions, and control costs. Different technical approaches are employed depending on the amount of exhaust gases generated, their composition, and their physical and chemical properties in different regions. 2.1 Improvement of pipeline purging method: Steam purging is used for all the oil transfer pipelines from the oil receiving party to various production units. Due to the long length of these pipelines and their frequent use, the purging process takes a long time, resulting in a high amount of exhaust gas escaping from the storage tanks in these units. Tests have shown that when using compressed air to purge certain delivery pipelines (without affecting production), the amount of exhaust gas is lower, and there are also fewer organic impurities present. Therefore, switching to compressed air for purging can help reduce the moisture content in the raw material tar and various oils. 2.2 The exhaust gas capture and cleaning device for the washing tank contains many acidic substances, with a temperature of around 90°C. Based on the characteristics of the exhaust gases from the oil storage tanks for various oils (high water vapor content along with light, volatile components) and the actual layout on site, we installed a new set of Venturi tube exhaust gas capture and cleaning devices using dilute alkaline solution tanks. A venturi tube is used to capture exhaust gases, and a dilute alkaline solution serves as the cleaning medium; the only additional equipment is one venturi tube. The insulation of the washing tank is removed to allow the tank itself to dissipate heat freely; if the temperature of the dilute alkaline solution is too low and this affects transportation, the heater inside the tank can be turned on. Under normal conditions, the heating effect of the exhaust gas on the dilute alkaline solution alone is sufficient to meet the temperature requirements for its transportation and circulation. The cooling effect of dilute alkali on this portion of the exhaust gas can reduce the total amount of water vapor and its organic entrainment. Furthermore, dilute alkalis also have a certain absorption effect on organic entrainers. While ensuring effective exhaust gas cleaning, it neither increases the total amount of dilute alkali nor reduces its consumption. The dilute alkali used for cleaning needs to be replaced regularly to prevent excessive levels of naphthalene from accumulating and precipitating, as this can have an adverse effect on the transportation and circulation of the dilute alkali. This technology makes maximum use of existing equipment, reducing both equipment costs and operating expenses. The specific equipment and main parameters are shown in Table 1. Table 1: Equipment List for Exhaust Gas Cleaning Devices in Various Oil Storage Areas
Equipment Name | Quantity | Main Parameters
Oil washing pump | 1 | H=36.5m, n=2900rpm, N=4kW, Q=9.5m3/h
Venturi tube | 1
Oil washing tank | 1 | Φ2400mm, V=20m3
Exhaust gas cleaning tower | 1 | Φ1200mm, 4-layer baffle tower

3. Modification of exhaust gas capture and cleaning devices for various intermediate oil tanks and asphalt flue gases
The exhaust gas capture and cleaning device in the tar intermediate tank area is a self-pressured type of exhaust gas cleaning device; it is responsible for capturing and cleaning the exhaust gases from 3 tar storage tanks that are used during startup and shutdown operations, 3 mixed oil tanks, phenol-water tanks, light oil tanks, I-anthracene oil tanks, II-anthracene oil tanks, underground tar tanks, as well as light oil water separators. However, this device has the following drawback: its exhaust gas capture efficiency is not high ； The exhaust treatment capacity is insufficient; the diameter of the exhaust outlet is only 150 mm, which not only limits the exhaust flow rate but also affects the efficiency of exhaust gas capture ； The cleaning effect is poor; the packing section is DN200×800. The exhaust gas in this area consists mainly of oil and gas, accompanied by frequent short-duration pipeline purging (about 4 to 7 times per day). We carried out the following technical upgrades. ①The original exhaust gas cleaning tower was removed, with only the circulating oil washing tank and the exhaust gas collection pipes remaining. ②By utilizing the suction force of a venturi tube as a negative pressure source for exhaust gas capture and cleaning, it is possible to reduce power consumption compared to packing-based capture systems, as well as cut down on one-time costs for pumps, motors, packing, and other components. ③To increase the processing capacity of the cleaning section, an exhaust gas cleaning tower was constructed using the existing pipelines; it is similar in design to the tar storage tank area, with a diameter of 2400 mm, and its bottom is connected to the original oil washing circulation tank. The negative-pressure pipeline connected to the venturi is connected to the chimney of the exhaust gas cleaning tower; actual measurements show that a suction force of -50 Pa can be generated at the bottom of the exhaust gas cleaning tower, which is sufficient to meet the requirements for exhaust gas capture. ④For exhaust gas cleaning, the original oil washing method and oil circulation pump are still used. The main parameters are shown in Table 2. Utilizing the negative pressure provided by the venturi, the exhaust gases from each tar tank are drawn into the exhaust gas cleaning tower; after being cleaned by circulating cleaning oil, they are released into the atmosphere through the coke oven chimney. Furthermore, since the exhaust components in this area are mainly oil and gas, the amount of oil used for washing is low; it needs to be replaced every two months, at a rate of 15 tons per replacement. Table 2: Equipment list for the exhaust gas cleaning device in the tar intermediate tank area. Equipment name, quantity, main parameters: Oil washing circulation pump – 4 units; H=45 m, n=2950 rpm, N=7.5 kW, Q=24 m3/h. Oil washing circulation tank – 4 units, Φ3000 mm, V=30 m3. Exhaust gas cleaning tower – 4 units, Φ2000 mm, 4-layer baffle tower. 3. Implementation results: (1) The vapor emissions from each oil tank have been significantly reduced; the phenomenon of “smoking” at the top of the tanks has been eliminated, and the odor in the production area and its surrounding areas has been greatly reduced. ⑵The modified exhaust gas cleaning device is used effectively; since it makes full use of the existing equipment, the investment required is low. ⑶ The upgraded exhaust gas cleaning facility has low operating costs, making it highly valuable for promotion and as a model to be followed. 4. Conclusions ⑴ The production system operates stably, but reducing or stabilizing the amount of exhaust gas generated is an important prerequisite for ensuring the efficiency of exhaust gas treatment. ⑵The best way to achieve complete capture of exhaust gases is to use negative pressure to guide them for capture. Therefore, finding a negative pressure source that is cost-effective and effective is key to the widespread application of exhaust gas capture technology. ⑶Depending on the composition of the exhaust gases, different media can be used to clean them, which not only ensures an effective cleaning efficiency but also reduces production costs. The exhaust gas cleaning stage in tar processing equipment aims to find cleaning agents that are both cheaper and more effective ； In the capture phase, the focus is on finding a negative pressure source that is cost-effective and highly adaptable. Using the suction force of a venturi is just one method; if the distance to each oil tank is too great, it will affect the effectiveness, and it is also impractical for companies that are solely engaged in tar processing. Some domestic tar processing plants already use gas blowers as a negative pressure source for exhaust gas capture, pumping the exhaust gas from storage tanks to exhaust gas purification towers where it is purified before being released. This technology clearly possesses greater processing power and regional adaptability. As **environmental protection requirements continue to rise, it will be an inevitable trend for exhaust gas treatment technologies in tar processing units to be designed in tandem with production systems and for process upgrades to be carried out, ultimately leading to the adoption of centralized automatic control systems.