【Professional Analysis】Summary of the LNG production process using coke oven gas and the operation status of related facilities

Thread Content

This post was last edited by Ruling on 2016-4-12 21:12. Authors: Liu Xuefei1, Liu Pengxiang1, Zhou Long2, Wei Long1, Chang Junshi1. Keywords: coke oven gas； Methanation ； Liquefied natural gas ； Methane synthesis ； Liquefaction. Author’s institutions: 1. Beijing R&D Center of Xindi Energy Engineering Technology Co., Ltd.; 2. Henan Jingbao Xin’ao New Energy Co., Ltd. In May 2013, the technology for converting coke oven gas into liquefied natural gas, which was independently developed by Xindi Energy Engineering Technology Co., Ltd., was successfully applied for the first time in the Henan Jingbao coke oven gas to LNG project. Qualified LNG was produced, with a capacity of 300,000 cubic meters per day of liquefied natural gas – this represents a new application direction and example for the reuse of coke oven gas. Next, let the editor guide you through a summary of the operational experience related to the use of this technology: 1. Process description: This device is primarily used to convert the useful gases in coke oven gas into methane, which is then separated through cryogenic processes to produce LNG. It is divided into three sections: the purification section, the methane synthesis section, and the liquidation section. Its features are as follows. (1) Purification section: The task of the purification section is to pressurize and purify coke oven gas, ensuring that the impurity content in it meets the requirements of methane synthesis catalysts. (2) Methane synthesis section: The methane synthesis section is primarily used to convert the useful gases in coke oven gas into methane; these useful gases include CO, CO2, benzene, alkanes, etc. The methane synthesis reaction system employs a high-temperature multi-reactor configuration to ensure maximum utilization of the useful gases in coke oven gas, while meeting the gas requirements of the liquidation section. Additionally, a pinch-point technology is used to design a hierarchical heat exchange network for the efficient recovery of heat within the system. (3) Liquidification section: The task of the liquidification section is to perform cryogenic separation on the gas sent from the synthesis section to obtain LNG. The separated liquid-phase product is sent to an LNG storage tank for storage. The gas first undergoes pretreatment to remove impurities such as H2O, acidic gases (including CO2, H2S, COS, and organic sulfides), and mercury from the raw material gas. It is then liquefied through a mixed refrigeration cycle process, and nitrogen and hydrogen in methane are separated using a low-temperature distillation process. 2. Equipment operation status and experience summary (1) Fluctuations in the composition of coke oven gas: Due to issues at the coking plant, the composition of coke oven gas experiences fluctuations. The hydrogen-to-carbon ratio of the components ranges from as low as 3.2 to as high as 4.1. Fluctuations in the hydrogen-to-carbon ratio cause fluctuations in CO2 levels in the gas exiting the methane synthesis section. When the hydrogen-to-carbon ratio is above 3.5, the CO2 content in the outlet gas of the methane synthesis unit ranges from 6 to 10 ppm ； When the hydrogen-to-carbon ratio is below 3.5, the CO2 content in the gas exiting the methane synthesis section begins to increase significantly. During operation, the highest CO2 concentration reached nearly 300 ppm (v/v). The liquid processing section was designed with additional protection for the decarboxylation tower, so it had no impact on the subsequent liquid processing sections. At the beginning of operation, the oxygen content in coke oven gas is unstable and often fluctuates. This causes fluctuations in the bed temperature of the hydrogenation reactor in the purification section, making it difficult to stabilize the conditions in that section. It once caused an overheating in the hydrogenation reactor at high oxygen levels. After coordination, once the oxygen content in the coke oven gas was stabilized, the operations in the purification section became more stable. (2) Impurities in coke oven gas The impurities in coke oven gas mainly include benzene, naphthalene, tar, NH3, sulfur, etc. After passing through the purification section, NH3 and a small amount of benzene enter the methane synthesis section. After passing through the methane synthesis section, NH3 is still present and will enter the liquidification section. Benzene was not detected. It can be seen from this that the methane synthesis section in high-temperature reactions has a purifying effect on benzene, as it is possible to hydrogenate benzene to ultimately produce methane. In the liquid chemical processing section, the issue of cold box blockage caused by benzene can basically be disregarded. (3) During the operation of the ammonia cold box freezing blockage prevention device, problems of freezing blockage in the liquidification section cold box occurred twice. During on-site handling, an ammonia smell was detected. Upon checking, the melting point of ammonia is -77.7°C, and the operating temperature of the cryogenic tank is below -150°C; theoretically, these conditions allow ammonia to cause freezing blockages. Subsequent laboratory analysis confirmed that the substance causing the freezing blockage was primarily ammonia. Based on on-site sampling and analytical testing, the ammonia that enters the liquidation section originates from two sources: one is the ammonia contained within the coke oven gas itself ； The other is the side reactions in the methane synthesis section. According to on-site measurements, when the reaction temperature is around 600°C, the ammonia content in the gas exiting the methane synthesis section ranges from 130 to 160 ppm. When the reaction temperature is below 560°C, the ammonia content is below 80 ppm. After discussion and agreement, a deamination tower was added after the methane synthesis section. After passing through the deamination tower, the ammonia content entering the liquid treatment section drops below 10 ppm, resolving the issue of freezing and blockage in the cold box caused by ammonia. (4) Catalyst carbon deposition control: After the device operated at high temperatures for a period of time, an increase in pressure drop across a section of the reactor bed was observed. Analysis suggests that the increased pressure drop may be caused by carbon buildup. The main components of coke oven gas are hydrogen, carbon monoxide, carbon dioxide, methane, nitrogen, etc. Through experimental research, it has been shown that both carbon monoxide and methane can undergo carbon deposition reactions at high temperatures; in this reducing atmosphere of coke oven gas containing an excess of hydrogen, it is primarily carbon monoxide that undergoes such reactions. Further research has shown that when the temperature is reduced to a certain level, the carbon deposition reaction decreases sharply to a minimal extent, and increasing the amount of water vapor can also reduce the intensity of this reaction. Therefore, by changing the operating conditions of the device to keep the reactor temperature below the carbon deposition temperature and by adding an appropriate amount of steam, we were able to successfully suppress the occurrence of carbon deposition reactions. (5) Effect of operating pressure: The chemical equations for the methanation of carbon monoxide and carbon dioxide are as follows: CO + 3H2 = CH4 + H2O; CO2 + 3H2 = CH4 + 2H2O. It can be seen from these equations that in the methanation reactions of carbon monoxide and carbon dioxide, the total number of moles of substances decreases after the reaction compared to before, which means it is a volume-reducing reaction. Therefore, increasing pressure facilitates the forward progress of the methanation reactions of carbon monoxide and carbon dioxide. However, there is a significant excess of hydrogen in the coke oven gas, and it has been found through operation that carbon monoxide can be almost completely reacted under both low and high pressures, with no traces of it detected in the gases exiting the methane synthesis section. At low pressures, the concentration of carbon dioxide in the gas exiting the methane synthesis unit is between 500 and 800 ppm; as the pressure increases, the CO2 concentration decreases. When the pressure exceeds 1.5 MPa, the CO2 concentration drops rapidly to the level observed under normal operating conditions, and it no longer changes with further increases in pressure. It can be seen that carbon monoxide undergoes methanation prior to carbon dioxide, and in order to meet the CO2 content requirements in the liquidification stage, the plant should operate at a pressure higher than 1.5 MPa. Original address: http://www.gasheat.cn/Thesis/574.html#rd