Treatment of cooking fume emissions

Thread Content

Application and Discussion on Treatment Technologies for Tar Processing Exhaust Gases Zhang Jimin, Li Xianhong, Liu Lijun (Coking Plant, Anyang Iron and Steel Co., Ltd., Anyang 455004) Tar processing is a traditional industry that causes severe pollution. In recent years, with social progress and economic development, this industry has seen rapid growth; however, the conflict between pollution and development has become increasingly apparent, and pollution constitutes an important factor restricting the sustainable development of the tar processing industry. The exhaust sources are distributed across various production units, with different compositions; given the current layout of the processes, it is very difficult to handle them collectively, while treating them separately is constrained by investment costs, operating expenses, and technical limitations. Therefore, it is imperative to research exhaust gas treatment technologies with high capture efficiency and low costs. By integrating various technologies, our factory has treated the exhaust gases from the tar processing system, achieving excellent results in terms of low cost, high efficiency, and wide coverage. 1 Analysis of exhaust gas treatment technologies (1) Venturi tube method. The technology widely used in exhaust gas treatment relies on the suction generated by a venturi to power the capture and cleaning of exhaust gases. The exhaust gas is sprayed with washing oil and enters the venturi for the first cleaning, and after being cleaned a second time in the cleaning tower, it is discharged. The advantage of this technology is its good cleaning effect, while the disadvantages are a high one-time investment, the need for power to operate it, and high costs associated with oil cleaning. Thus, it has restricted the widespread application of this technology, which is only used in key areas such as asphalt sumps. (2) Filler washing device. Some tar processing plants use packing scrubbers as exhaust gas cleaning devices on various storage tanks. Tail gas cleaning towers are installed in areas where the storage tanks are concentrated. Each storage tank is connected to the tail gas cleaning tower through an overhead gas collection pipe; the tail gas escaping from the tops of the tanks enters the tail gas cleaning tower under its own pressure, and after being washed by oil in the packing section, it is discharged outside. The advantage of this exhaust gas treatment technology, which is powered by the pressure of the exhaust gas itself, is low investment, low 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. (3) Blower suction. Some coking 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 being cleaned, the exhaust gas 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. We analyzed several existing exhaust gas treatment technologies in the country and believe 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 measures to address the surface symptoms; in addition, it is necessary to improve production processes 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 Governance Measures Based on the above analysis, we take into account factors such as ensuring effectiveness, addressing the issue at its source, and controlling the process. By focusing on various aspects including reducing the generation of exhaust gases, improving the efficiency of exhaust gas capture, ensuring clean emissions, and controlling costs, we employ different technical approaches tailored to the amount of exhaust gases produced, their composition, and their physical and chemical properties in different regions. 2.1 Improvement of pipeline purging method: The tar transport pipelines from the recovery workshop to the tar distillation unit were originally purged using steam. Due to their long length and high frequency of use, the purging process took a long time, resulting in significant amounts of exhaust gas escaping from the three tar storage tanks. Tests have shown that when nitrogen is used to purge the tar delivery pipeline, there is less exhaust gas and fewer organic impurities,; therefore, using nitrogen for purging also helps to reduce the moisture content in the raw tar. 2.2 Tar storage tank exhaust gas capture and cleaning device: The raw tar contains 2%–4% moisture, at a temperature of around 90°C. Based on the characteristics of the exhaust gas from the tar storage tank (high water vapor content along with light components of tar) and the actual layout on site, we installed a new venturi-type exhaust gas capture and cleaning system using the tar dehydration tank. Negative pressure is used to capture exhaust gases, and the phenol-water mixture obtained through settling in a tar tank is used as the cleaning medium. The only new equipment added is a venturi tube; the cleaning tank is a phenol water tank, and the cleaning pump is a phenol water transfer pump. The insulation in the washing tank is removed to allow the tank itself to dissipate heat freely; if the temperature of the phenol 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 alone is sufficient to meet the temperature requirements for the transportation and circulation of the phenol solution. The cooling effect of phenol water on this portion of the exhaust gas can reduce the total amount of water vapor and its organic entrainers. In addition, phenol water also has a certain absorption effect on organic entrainers. While ensuring an effective exhaust gas cleaning effect, it neither increases the total amount of phenol water nor reduces the consumption of cleaning oil. The phenol solution used for cleaning must be replaced regularly while dehydrating the tar tank, in order to prevent excessive levels of naphthalene from accumulating and precipitating, as this could have an adverse effect on the transportation and circulation of the phenol solution. 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: List of Equipment for Exhaust Gas Cleaning Device in the Tar Storage Area
| Serial No. | Equipment Name | Quantity | Main Parameters |
|-------------|----------------|----------|-----------------|
| 1 | Phenol-water transfer pump | 2 units | H=45m, n=2950rpm, N=7.5kW, Q=24m3/h |
| 2 | Venturi tube | 1 unit | |
| 3 | Phenol-water tank | 1 unit | Φ2400mm, V=30m3 |
| 4 | Exhaust gas cleaning tower | 1 unit | Φ1200mm, 6-layer sieve tray tower |

2.3 Modification of the exhaust gas capture and cleaning device for the intermediate tar tank: The exhaust gas capture and cleaning device in the intermediate tar 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 tri-mixed naphthalene oil tanks, the phenol-water tank, the light oil tank, the I-anthracene oil tank, the II-anthracene oil tank, the underground tar tank, as well as the light oil water separator. However, this device has the following drawback: its exhaust gas capture efficiency is not high ； The exhaust gas 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 DN400×800. 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. (1) Remove the original exhaust gas cleaning tower, retaining only the circulating oil washing tank and the exhaust gas collection pipes. (2) Since the tar intermediate tank area is located close to the 130-m-high coke oven chimney, it was decided to utilize the suction force of the coke oven chimney as the negative pressure source for exhaust gas capture and purification. Compared with a venturi capture system, this approach not only saves energy consumption but also reduces the one-time investment in pumps, motors, venturis, and other equipment. (3) To increase the processing capacity of the cleaning section, an exhaust gas cleaning tower essentially identical to the tar storage tank area was constructed using old coal gas pipelines; it has a diameter of 1200 mm, with its bottom connected to the original oil washing circulation tank. The negative-pressure pipeline connected to the coke oven chimney was connected to the scrubber tower chimney, and it was actually measured that a suction force of 50 Pa could be generated at the bottom of the exhaust gas scrubber tower, which is sufficient to meet the requirements for exhaust gas capture. (4) The exhaust gas cleaning still uses the original oil washing method and oil washing circulation pump. The main parameters are shown in Table 2. Utilizing the negative pressure generated by the chimney suction, the exhaust gases from each tar tank are drawn into the exhaust gas cleaning tower; after being cleaned through circulating oil washing, they are released into the atmosphere via 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 once a month, at a rate of 1.5 tons per replacement. Table 2: Equipment list for the exhaust gas cleaning device in the tar intermediate tank area
Serial No. | Equipment Name | Quantity | Main Parameters
1 | Oil washing circulation pump | 1 unit | H=36.5m, n=2900rpm, N=4kW, Q=9.5m3/h
2 | Oil washing circulation tank | 1 unit | Φ1200mm, V=2m3
3 | Exhaust gas cleaning tower | 1 unit | Φ1200mm, 6-stage tray tower

3. Implementation results: By applying various techniques to modify the exhaust gas system, we achieved good results. (1) The vapor emissions from each oil tank have been significantly reduced, eliminating the phenomenon of \"smoking\" at the top of the tanks; the odor in the production area and its surrounding areas has also been markedly diminished. (2) The modified exhaust gas cleaning device is used effectively; since the existing equipment is fully utilized, the investment is low. (3) 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 (1) Ensuring the stable operation of the production system and reducing or stabilizing the amount of exhaust gas generated are important prerequisites for ensuring the efficiency of exhaust gas treatment. (2) The 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 efficient is key to the widespread application of exhaust gas capture technology. (3) Depending 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. 5 Discussion At present, the exhaust gas capture technology for most tar processing units in China is still in the research and development stage; the focus in the exhaust gas purification process is on finding purification 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. Utilizing the suction force of coke oven chimneys is just one method; if the distance from the chimneys is too great, its effectiveness is reduced, and it is also not practical for companies that deal solely with tar processing. Domestically, some tar processing enterprises 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 conjunction with the production systems and for process upgrades to be carried out, ultimately leading to the adoption of centralized automatic control systems.