The use of coke oven gas to produce synthetic natural gas SNG, compressed natural gas CNG and liquefied natural gas LNG is a new field of coke oven gas utilization. The process flow of methanation of coke oven gas to produce liquefied natural gas LNG was discussed and economic evaluation was carried out. The results show that the process of producing liquefied natural gas after methanation of coke oven gas, combined with the gas analysis system of Tianyu Intelligent Control and gas solutions in the coke oven gas process industry, has higher utilization rate, better environmental protection effect and better economic benefits.

        Natural gas is a very clean primary energy, but the current consumption of natural gas accounts for only 3-4% of China's primary energy consumption, which is much lower than the world average (25%) and lower than the Asian average (8.8%) ). Increasing the proportion of natural gas once again in energy consumption will help China's environmental protection.

        At the same time, China's natural gas market is in a period of rapid development. The Oil and Gas Division of the Energy Development Bureau of the National Development and Reform Commission predicts that the natural gas demand growth rate will remain at an average annual rate of about 8% from 2016 to 2010, and that the natural gas demand is expected to exceed 300 billion Nm³ by 2020, and the market gap will reach 90 billion Nm³. Moreover, China's natural gas prices are also rising. Calculated by calorific value equivalent, the ratio of domestic natural gas wellhead price to crude oil price is 0.24 ~ 0.4: 1, while the ratio of natural gas to crude oil in developed countries is usually 0.6: 1, there is more than 50% room for upward adjustment.

        At the same time, coking enterprises in China produce coke oven gas by-products while producing coke. In addition to reheating for domestic use and civilian use (as city gas), production of synthetic ammonia or methanol, the rich coke oven gas is about 20 billion Nm³ per year The value exceeds the first phase of the "West-East Gas Transmission" project. Rich coke oven gas combustion emissions or direct emissions not only cause huge waste of resources, but also cause serious environmental pollution. Therefore, the use of coke oven gas to produce synthetic natural gas (SNG), and then the production of compressed natural gas (CNG) or liquefied natural gas (LNG), can open up new sources of clean energy, and at the same time will produce more obvious economic, environmental and social benefits, It is of great significance to promote the technological progress and industrial development of the coking and energy industries.

1. Principle and process

1.1 Principle of Coke Oven Gas to Synthetic Natural Gas

Since the total content of CO-loaded CO2 in coke oven gas is about 10% (v / v), the content of polycarbon hydrocarbons is 2 to 3%, and about 55% (v / v) of H2, so the methanation reaction can be used To generate methane, the main reactions are shown in equations (1) and (2):

       It can be seen from equations (1), (2) and (3) that these three reactions are very exothermic reactions. During the reaction, the heat of reaction can raise the temperature of the methanation furnace to about 650 ° C. This not only causes the catalyst to carbonize due to the cracking of multi-carbon hydrocarbons, but also may easily sinter and deactivate the methanation catalyst that is not resistant to high temperatures.

1.2 Process flow of methanation of coke oven gas to produce liquefied natural gas LNG

   Methanation to synthesize LNG mainly contains three major processes, purification and separation, methanation and cryogenic liquefaction, of which the methanation process is the key to the technology.

Process flow of coke oven gas to LNG

    1. Purification and separation

    Mainly adopt dry method and wet method, take out ammonium chloride chlorobenzene, tar, fine desulfurization, and obtain standardized raw materials for the next chemical reaction. Purification and separation include several processes such as feed gas compression, pretreatment, secondary pressure increase, organic sulfur hydrolysis, and fine desulfurization. The coke oven gas contains impurities such as tar, benzene, unsaturated olefins, and sulfur. The unsaturated olefins in the coke oven gas will analyze carbon in the subsequent coke oven gas methanation reaction and affect the catalyst activity, which is caused by inorganic sulfur and organic The mixed sulfide composed of sulfur is also a poison of the methanation catalyst, which will cause the permanent poisoning of the methanation catalyst to lose its activity. Therefore, it must be strictly controlled and thoroughly removed in the purification and separation stage. After fine desulfurization, the content of mixed sulfur in coke oven gas will be removed to 0.1ppm.

    2. Methanation

    CH4 is obtained by the chemical reaction of carbon monoxide and carbon dioxide with hydrogen. The process of methanation is a strong exothermic process, the reaction temperature is 250 ℃ -600 ℃, so the process of controlling the reaction temperature and recovering the reaction heat is the key to the entire process, otherwise it is easy to cause explosion and other safety accidents. At the same time, the catalyst in the methanation process is very important. On the one hand, the quality of the catalyst is related to the conversion rate, which affects the output of methane; on the other hand, the high temperature environment puts forward higher requirements for the heat resistance of methane. The current representative technologies of methanation technology abroad are Topsoe Methanation in Denmark and David Methanation in Britain. Domestically, there are Southwest Chemical Design Institute, Dalian Kately Catalyst Engineering Co., Ltd., Xindi Energy Engineering Technology Co., Ltd., Wuhan Kelin Fine Chemical Co., Ltd., and Shanghai Huaxi Chemical Technology Co., Ltd. etc.

    3. Cryogenic liquefaction

    The cryogenic liquefaction process needs to determine the type of cryogenic liquefaction process according to the amount of coke oven gas processed. For larger-scale coking plants, the hybrid refrigeration process MRC can be used. The obtained LNG is mainly CH4 and N2, and the CH4 content is greater than 96%. Cryogenically cool the gas to -162 degrees and compress it 625 times to obtain liquid natural gas (LNG). The technology is relatively simple and mature.

1.3 Economic evaluation

       Take a coking enterprise with an annual output of 1 million tons of coke as an example. In addition to its own use, the amount of coke oven gas is about 2.5X104Nm³ / h. If the methanation technology is used to produce synthetic natural gas, the synthetic natural gas SNG or compressed natural gas CNG can be obtained as 10733Nm³ / h, or liquefied natural gas 9000Nm³ / h. Based on the calculation of coke oven gas price of 0.3 yuan / Nm³ and electricity of 0.5 yuan / Kwh, the production cost of synthetic natural gas is about 1.23 yuan / Nm³, the production cost of Yuanbu natural gas is about 1.32 yuan / Nm³, and the production cost of liquefied natural gas is about It is 1.69 yuan / Nm³. If the price of natural gas is 1.8 yuan / Nm³, the price of compressed natural gas is 2.5 yuan / Nm³, and the price of liquefied natural gas is 3.0 yuan / Nm³, the annual gross profit will be 48.94 million yuan and 101.31 million yuan respectively. And 94.31 million yuan, good economic benefits.

1.4 Gas solution for coke oven gas methanation synthesis LNG process

       Based on coking oven gas methanation synthesis LNG process, combined with domestic advanced gas analysis technology, Tianyu Intelligent Control has developed a coke oven gas to natural gas synthesis process gas analysis system and gas analyzer with independent intellectual property rights to monitor the process in real time The content of gas in the gas is then optimized and controlled to achieve the optimal output of the natural gas production process. At present, due to the excellent quality and high stability of Tianyu Intelligent Control Gas Analyzer, it has verified the rationality of the design structure, and has been unanimously praised by the majority of users, and its comprehensive performance can meet industrial applications.

1.5 Conclusion

       After the coke oven gas is compressed, TAS removes naphthalenebenzene, etc., and then hydro-converts crude desulfurization, and after fine desulfurization, the total sulfur content is reduced to 0.1ppm. Then, using methanation technology and separation technology, synthetic natural gas SNG and liquid natural gas LNG can be obtained . The synthetic natural gas fully complies with the national standard GB17820-1999 for pipeline natural gas, and part of the data is far superior to this standard. Combining the research of Wuhan Tianyu intelligent control coke oven gas to synthetic natural gas process and the development of gas gas analysis system, it shows that the use of gas analysis system can maximize the conversion of coal gas to synthetic natural gas, not only the energy utilization rate of raw materials is relatively high, Moreover, the technological process is simple, the investment is small, and the economic benefit is good.