Thread Content
As stated in the preface, the synthetic ammonia plant of Zhongyuan Dahua Group employs the internationally advanced UHDE-AMV low-energy consumption process technology, using natural gas from the Zhongyuan oil field as raw material to produce 1,000 tons of synthetic ammonia per day. At the end of 2005, a new coal chemical project was launched, with an annual production capacity of 500 kt of methanol; it began operations successfully in May 2008. Currently, affected by the tight supply of natural gas as a raw material, the production capacity of ammonia synthesis plants remains between 45% and 80% most of the time, resulting in increased consumption of ammonia per ton and rising production costs. After the methanol unit in the coal chemical industry was brought online successfully, the methanol off-gas was fed into the ammonia synthesis unit to produce ammonia as a by-product, thereby reducing the ammonia consumption per ton and enabling circular economy production; on May 26, 2008, the methanol off-gas was successfully introduced into the ammonia synthesis unit. 1 Process flow: The compositions of the ammonia synthesis conversion gas and the methanol purge gas are similar (see Tables 1 and 2); it is sufficient to heat and depressurize the purge gas before introducing it into the ammonia synthesis plant. As per the design, the vent gas is directed to the front of the high-temperature shift reactor (the designed flow rate is 30,080 km3/h, while the actual flow rate ranges from 15,000 to 30,080 km3/h). A new heat exchanger has been added in front of the shift reactor; the converted gas at the outlet of the shift reactor flows through the tube side, while the vent gas after passing through the pressure reduction valve flows through the shell side. The heated vent gas then mixes with the converted gas in front of the shift reactor before entering the high-temperature shift reactor for reaction. The new heat exchanger is equipped with a bypass valve to regulate the temperature at the inlet of the shift reactor. The process flow is shown in Figure 1. As can be seen from Table 2, the hydrogen content in the actual off-gas is lower than the designed value, while the nitrogen content is higher than the designed value. This is because, during the initial operation of the coal chemical gasification unit, nitrogen is used to transport coal powder, whereas carbon dioxide is intended for this purpose in the design. Table 1 Composition of synthetic ammonia conversion gas % Note: Pressure 4.2 MPa, temperature 370°C. Table 2 Composition of methanol purge gas % Note: The designed pressure for the purge gas is 7.49 MPa at a temperature of 55°C; the actual pressure is 5.85 MPa at a temperature of 52°C. Figure 1: High-temperature shift process after grid connection. 2: Effects of introducing vent gas on the ammonia synthesis plant. 2.1 Positive effects: (1) To maintain a steam-to-gas ratio of 0.45 in the high-temperature shift reactor, it is necessary to add more water vapor to the first stage of the reactor, thereby increasing the water-to-carbon ratio; calculations show that 3–4 t/h of additional steam is required. (2) Due to the high nitrogen content in the off-gases at present, in order to maintain the hydrogen-to-nitrogen ratio in the synthesis loop, it is necessary to reduce the amount of air supplied to the secondary furnace. When the nitrogen content in the off-gases is between 40% and 55%, 2000–5000 m3/h less of air needs to be used in the secondary furnace for the same production load. After the air volume decreases, in order to maintain the air flow velocity within the air distributor of the two-stage furnace, it is necessary to add 2–3 tons of steam per hour to the air coil, ensuring that the air flow velocity remains above 30 m/s as a means of protecting the air distributor. At the same time, as the air volume decreases, the air compressor can reduce its load slightly to save power consumption. (3) After the vent gas is connected to the grid, the reduction in air supply to the second-stage furnace leads to a decrease in the outlet temperature of this furnace and an increase in the methane content at its outlet. It is necessary to raise the outlet temperature of the first-stage furnace in order to increase the methane reaction load there, thereby reducing the methane content at the outlet of the first-stage furnace and ensuring that the methane content at the outlet of the second-stage furnace remains within acceptable levels. The temperature of one furnace section was increased from 760°C to 770–780°C. (4) Adjust the circulation rate of the K2CO3 solution in the purification section based on the levels of CO and CO2 in the off-gases, to ensure that the CO2 concentration after absorption in the purification section is within acceptable limits and to prevent any excess levels. Generally, when the CO and CO2 contents in the vent gas exceed 10% and 5% respectively, the circulation rate of the K2CO3 solution in the purification unit must be increased by 50–100 m3/h. (5) Due to the high nitrogen content in the vent gas, after connection to the grid the load on the hydrogen recovery unit increases; as a result, more combustion exhaust gases are returned to the fuel gas system after separation in the cold box, which allows for an average savings of around 2 km3/h of natural gas as fuel. (6) After introducing methanol off-gas, the average ammonia production can increase by 3–6 t/h depending on changes in composition and quantity, while the natural gas consumption per ton of ammonia decreases by 100–150 m3, as shown in Table 3. Table 3 Comparison before and after the introduction of methanol off-gas. Note: The designed full-load capacity of the ammonia synthesis plant is 28.9 km3/h. 2.2 Negative impacts: (1) The continuous operation cycle of coal chemical gasification and methanol production units is not long; the continuous introduction period for off-gas is also short, with the maximum duration being 7 days and the minimum just a few hours. The ammonia synthesis plant needs to adjust its operating conditions promptly in response to any interruptions in the supply of off-gas. (2) The components of the off-gas are unstable; therefore, during operation it is necessary to adjust the production conditions promptly in response to changes in certain key parameters related to ammonia synthesis. Such as: the hydrogen-to-nitrogen ratio of the fresh gas and that of the synthesis loop, the methane content at the outlet of the two-stage furnace, and the gas parameters at the outlet of the purification absorption tower, etc. (3) Due to the high nitrogen content in the bleed gas, the circulation volume of the syngas compressor increases, resulting in the compressor consuming 2 t/h more steam per hour. (4) After normal introduction of purge gas, the air supply to the second-stage furnace decreased and the outlet temperature dropped, resulting in an increase in the methane content at the outlet. Although measures such as raising the temperature of the first-stage furnace were taken, the methane content at the outlet of the second-stage furnace still increased slightly, although it exceeded the design specifications. (5) Due to the continuous release of gas for the reasons mentioned in (1) above, it is detrimental to the equipment, pipelines, and valves, affecting their service life. 3 Conclusion: After the methanol off-gas was properly introduced, the ammonia synthesis output increased significantly (by 100–150 tons per day), while the ammonia consumption per ton decreased markedly. The economic benefits were considerable, thereby achieving energy conservation, emission reduction, and circular economy production. It is known that in the coal chemical plants of some companies, methanol off-gas is wasted without being utilized, resulting in energy loss. At present, our company’s coal chemical production facilities are in the initial stage of operation, and the technology and experience available are not yet fully developed. As production continues and more technical expertise is accumulated, the operation will become increasingly stable. The introduction of methanol off-gas will then be more consistent and reliable, resulting in greater economic benefits.