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Wang Lingyi, Yang Cong: The energy consumption per ton of ammonia in China’s fertilizer industry is approximately (9000–11000 Mcal), or 37620–45980 MJ, which is still far below the advanced international levels (≤7000 Mcal). The low-temperature activity of methanation catalysts is directly related to the energy consumption in the methanation process. Below, a brief quantitative analysis of the relationship between the low-temperature activity of the methanation catalyst and energy consumption is presented, using the operational conditions of the Φ1000 methanation tower at Feicheng Fertilizer Factory in August 2006 as an example. The methanation process at Feicheng Fertilizer Factory utilizes the \"fully autothermal, non-isobaric alcohol-alkane methanation\" process developed by Nanjing Guochang Chemical for purifying raw materials used in ammonia synthesis. In this high-pressure dual-alkane process, the high-pressure alkanation system is capable of achieving self-thermal equilibrium within the ammonia synthesis system; during normal operation, neither the high-pressure alcoholation system nor the alkanation system requires the use of circulators or electric heaters. However, due to the low content of alcoholated gas (CO + CO2), it is difficult to achieve self-thermal equilibrium in the methanation reaction, necessitating the use of an external heat source. The heat source for the methanation reaction comes from the relatively stable temperature and pressure of the ammonia synthesis off-gases, which ensures stability in the temperature of the high-pressure alkylation tower. Heat exchange is carried out by a temperature-raising heat exchanger. Status of methanation in Feicheng on August 22, 2006: Area A – 256262263262262259; Area B – 255260261260261260. Flow rate into the methanation system: 55,000 Nm3/h, of which 17,000 Nm3/h is used to adjust the furnace temperature via a shortcut route, while 38,000 Nm3/h is used for heat exchange with the output gas from Stage 2 of the synthesis process. The output gas volume from Stage 2 of synthesis is 30,000 Nm3/h. The temperature of the hot gas inlet is 277.7°C, with a gas flow rate of 38,000 Nm3/h; the temperature of the cold gas inlet is 200.3°C, and its gas flow rate is 17,000 Nm3/h. After mixing, the outlet temperature is 256°C, and the gas flow rate at the outlet is 55,000 Nm3/h. The temperature of the synthesis gas outlet is 311.9°C, with a gas flow rate of 30,000 Nm3/h. The temperature of the methanation gas inlet is 200.3°C, and its gas flow rate is 38,000 Nm3/h. After heat exchange, the temperature of the second outlet is 204°C, while the temperature of the methanation gas outlet is 277.7°C. The above data pertains to operation with a methanation inlet temperature of 256°C; the outlet level is within acceptable limits, reaching the design requirement of 10 ppm. In fact, maintaining an inlet temperature of 240°C in high-pressure methanation is sufficient to meet the process requirements (the Hebei Hejian Fertilizer Factory has always kept the temperature at 240°C). Many manufacturers control the methanation temperature based on their own process characteristics and operational practices. Since many manufacturers’ methanation heat exchange units are not equipped with flow meters, and some use electric heaters to control the inlet temperature, it can only be said qualitatively that controlling the methanation inlet temperature at a lower level is beneficial for energy savings; however, it is not possible to conduct a quantitative analysis of the extent to which such temperature control contributes to energy savings. Since the heating exchanger at Feicheng Fertilizer Plant is equipped with a flow meter, an analysis is provided for this plant to compare the energy consumption when the methanation inlet temperature is 256°C versus 240°C. For hot and cold fluids, the heat balance equation is: Q = mhCph(T1 – T2) = mcCpc(t1 – t2). Here, mh and mc represent the mass flow rates of the hot and cold fluids, in Kg/s; Cph and Cpc are the specific constant-pressure heat capacities of the hot and cold fluids, in KJ/(Kg·K). T1 and t1 denote the inlet temperatures of the hot and cold fluids, in K, while T2 and t2 represent the outlet temperatures, also in K. Since the main components of the gas entering and leaving the methanation process as well as those in the syngas are similar, Cph ≈ Cpc. Let QL be the heat loss. mhCph (T1–T2) = mcCpc (t1–t2) + QL. Performing a heat balance based on Figure A: When the inlet temperature is 256°C: 38000 × Cph × (277.7 – 256) = 17000 × Cpc × (256 – 200.3) + QL. When the inlet temperature is 240°C: 38000 × Cph × (T1 – 240) = 17000 × Cpc × (240 – 200.3) + QL. T1 = 254°C. When the methaneation process operates at an inlet temperature of 240°C, with the same gas flow rate of 38000 Nm3/h, the required temperature is 254°C. Energy savings: (Q256°C – Q240°C) / Q256°C ≈ 35%. Performing a heat balance based on Diagram B: When the inlet temperature is 256°C: 30000×Cph×(311.9–204) = 38000×Cpc×(277.7–200.3) + QL. When the inlet temperature is 240°C: mh×Cph×(311.9–204) = 38000×Cpc×(254–200.3) + QL. Cph = 20641 Nm3/h; this means that 9359 Nm3/h less of synthetic gas is used compared to when the inlet temperature is 256°C. The energy savings amount to: (Q256°C – Q240°C) / Q256°C ≈ 31%. In summary: reducing the inlet temperature of the methanation catalyst by 16°C can result in approximately 30% energy savings. The energy savings are significant; therefore, when using methanation catalysts, it is recommended that manufacturers maintain the inlet temperature at around 245±5°C, provided that the system can operate stably over the long term. This applies especially to those manufacturers who use temperature-raising heat exchangers, as long as the process requirements can be met. It can not only preserve the low-temperature activity of the catalyst and extend its service life, but also reduce energy consumption. The methanation catalyst in Phase 1 of Shandong Union Chemical is operated at 250 degrees, while that in Phase 2 operates at 245 degrees, a reduction of 5 degrees; the manufacturer is very satisfied. Anhui Haoyuan initially kept the operating temperature at 240 degrees, making it the manufacturer that uses the lowest temperature for methanation. Nowadays, as all manufacturers focus on energy conservation and emission reduction, higher requirements are placed on catalysts; therefore, the use of catalysts with high activity and high quality is essential in the fertilizer industry.