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The synthetic ammonia plant of Zhongyuan Dahua Group Co., Ltd. utilizes the UHDE-AMV technology. The energy-saving features and measures of this technology include reducing the load on the first-stage furnace, adding excess air to the second-stage furnace, using gas turbines, employing an improved phenylformaldehyde decarboxylation process, installing units for ammonia and hydrogen recovery, and using turbine generators; the energy consumption per ton of ammonia produced is 28.8 kJ. The designed capacity of this unit is 28.9 km3/h, with an ammonia production rate of 1,000 t/day; the natural gas used as raw material is supplied by the Zhongyuan oil field. Due to the large fluctuations in the supply of natural gas from the Zhongyuan oil field, the plant can rarely operate at full capacity. Since winter 2005, the supply of natural gas has been even more severely inadequate, forcing the ammonia synthesis plants to operate at extremely low load levels for extended periods, with plant loads ranging only from 50% to 60% (14.0–17.5 km3/h), which increases the difficulty of operating these plants. The author provides a brief explanation of the problems that occur when the device operates at ultra-low load levels and the measures taken to address them, and also offers a concise summary of the key points for operating the device under such conditions. 1 Problems occurring under ultra-low load operation 1.1 Damage to the air distributor The unit has been operating at an ultra-low load level of 50%–65% since the winter of 2005. During a major maintenance session in February 2006, it was found that the air distributor in the second furnace was severely damaged, having burned through and broken into three pieces. Analysis shows that the reason is long-term operation at ultra-low load, which results in a small amount of air and low pressure inside the air coil, as well as uneven air flow rates. This causes vibration of the air distributor, shortening of the flame length in the nozzles, and an upward shift of the combustion temperature zone; the high-temperature gases at 1,200°C then burn through the distributor. 1.2 Tripping of the feed gas compressor: When the unit is operating at an ultra-low load of 15.0 km3/h, the inlet pressure of the feed gas compressor is controlled at 0.11 MPa. As the composition of the natural gas supplied from the oil field suddenly increased from 102% to 122%, the inlet pressure of the feed gas compressor dropped rapidly. The compressor shut down due to the low inlet pressure, which led to a shutdown of the entire system. 1.3 The decarburization balance is difficult to control. When the unit operates at an ultra-low load, due to low system heat and pressure, it is hard to maintain the decarburization balance; the liquid level in the regeneration tower rises, resulting in poor regeneration efficiency and an impact on the absorption rate. 1.4 Syngas compressor surge: When the unit operates at an ultra-low load, the system pressure is low, resulting in a low speed and flow rate of the syngas compressor. On March 21, 2006, the anti-surge valve of the air compressor opened suddenly on site; the process air in the system was discharged, the circulation rate of the syngas compressor dropped rapidly, and surge occurred in the unit. 1.5 Failure in the regeneration of the dryer in the hydrogen recovery unit: When the plant operates at a very low load, the hydrogen recovery load is also low; as a result, the amount of exhaust gas generated during dryer regeneration does not meet the required levels. This often leads to incomplete regeneration within the specified time, causing excessive levels of H2O and NH3 at the dryer outlet, which in turn results in icing and blockages in the cold box as well as an increase in pressure difference. Due to thawing during the cold box shutdown, 4 tons of ammonia are produced less per hour. 2 Technical measures for ultra-low load operation 2.1 Protection of the secondary furnace air distributor During operation, medium-pressure steam should be introduced into the air coil to ensure a uniform gas flow velocity within the coil, with the air flow velocity exceeding 30 m/s. When the system load is between 50% and 65%, the steam flow rate within the coil must be greater than 6 t/h. This not only ensures the safe operation of the air distributor, but also prevents the air coil from overheating; it is also beneficial for the hydrocarbon steam conversion reaction in the second-stage furnace. 2.2 Maintaining decarburization equilibrium: During operation at ultra-low load levels, if it is difficult to maintain the decarburization equilibrium, the hydraulic turbine can be shut down to save heat. Additionally, the following measures can be taken: ① Increase the system pressure to reduce the amount of water carried in by the process gas ； ②Reduce the flushing water volume for the absorption tower and regeneration tower to the minimum (1 t/h and 4 t/h) ； ③Increase the temperature of the solution in the upper section of the absorption tower, from 70°C to 85°C℃ ； ④The conversion system increases the water-carbon ratio ； ⑤Appropriately reduce the circulation volume to lower the regenerative heat load ； ⑥Open the heat exchanger bypass to increase the temperature of the decarburization process gas. 2.3 Protecting unit safety: During operation at low load, it is necessary not only to reduce the speed and activate the anti-surge valves of the large units, but also to ensure that the unit’s flow rate is greater than the surge threshold, with the turbine speed exceeding 8,500 r/min, in order to prevent fluctuations in the governor and unit surge. 2.4 Reducing the load on auxiliary boilers: Coal-fired boilers should supply more qualified steam, reduce the number of burners in the auxiliary boilers, and bring the load down to its lowest level, namely 0.8 km3/h (with a full load of 6.0 km3/h). The fuel saved can be used to increase the system load and thus boost ammonia production. 2.5 Ensuring stable operation of the generator set: To reduce ammonia consumption, a generator set driven by a high-pressure steam turbine should be started; 80 t/h of superheated high-pressure steam is used to generate 2,500 kW of power, which is then fed into the internal power grid for use. The work done by the high-pressure steam-driven turbine is reduced as medium-pressure steam enters the medium-pressure steam pipeline network. To ensure the safe and stable operation of the generator set, it is required that the turbine steam flow be >62 t/h and the power generation capacity be >1,000 kW, in order to prevent the sudden shutdown of the generator set. 3 Key points for operation at ultra-low load 3.1 Controlling the inlet pressure of the feed gas compressor: The inlet pressure at the first stage must always be >0.12 MPa (shutdown occurs at 0.06 MPa); the anti-surge flow rate should be >26.0 km3/h to prevent shutdown due to low inlet pressure of the compressor. In the past, there have been instances where the compressor inlet pressure was only 0.11 MPa; when the composition of the feed gas suddenly changed, the inlet pressure of the unit dropped rapidly, causing the feed gas compressor to shut down and resulting in a shutdown of the entire system. 3.2 Control of steam temperature and pressure: In the original design of this facility, the high-pressure steam at 11.2 MPa and 530°C was supplied by 3 waste heat boilers and 1 auxiliary boiler. High-pressure steam drives the ice machine turbine and the generator turbine to perform work, which is then fed into the medium-pressure steam network at 5.15 MPa and 420°C. Medium-pressure steam drives the raw material gas compressor, syngas compressor, and CO2 compressor turbines. With the construction of new projects, high-pressure and medium-pressure steam is now mostly supplied by coal-fired boilers; therefore, it is crucial to ensure the quality of the steam generated by these coal-fired boilers. The key points for operation are: ① Ensure that the temperature parameters of the small amount of steam produced locally are within the acceptable range ； ②All the coolers in the on-site steam system can be turned off; if necessary, less steam can be used to reduce pressure ； ③The superheated high-pressure steam should be above 460°C, and the superheated medium-pressure steam should be above 380°C; otherwise, it is detrimental to both the unit and the catalyst. 3.3 Control system pressure: The pressure of the control system is 3.2 MPa, which ensures an adequate absorption rate for decarburization and a high level of CO2 purification, reduces the amount of water carried from the process gas into the decarburization unit, and maintains the heat exchange efficiency of the reboiler. 3.4 Control of the temperature in the first and second furnace sections: Due to the low load on the facility, it is also necessary to keep the temperature in the first furnace at a relatively low level, around 745–755°C (normal range: 770–780°C). This helps to ensure a high methane conversion rate, saves fuel, and is also beneficial for the temperature of the walls of the conversion tubes. Providing an appropriate amount of additional air to the two-stage furnace is beneficial for system pressure, improves steam quality, and also ensures a high conversion rate of methane in the two-stage furnace. 3.5 Controlling the hydrogen-to-nitrogen ratio in the synthesis loop: The conversion system can be supplied with an additional amount of air, and the hydrogen recovery unit should increase the amount of hydrogen returned to ensure that the hydrogen-to-nitrogen ratio in the synthesis loop is ≥60%. This is beneficial for the ammonia synthesis reaction, reduces the power consumption of the syngas compressor, and increases the saturated steam generated in the waste heat boiler at the outlet of the ammonia synthesis tower. 3.6 Controlling the load of the hydrogen recovery unit: By adding an appropriate amount of air to the conversion system and increasing the amount of hydrogen returned to the cold box, it is ensured that the load on the hydrogen recovery unit remains above 18.0 km3/h (the normal value being 24.0 km3/h). This not only increases the amount of combustion exhaust gas flowing to the downstream furnace but also reduces the power consumption of the syngas compressor. 3.7 Protection of air coils: Due to low load, there is relatively little process air in the system; to prevent overheating of the air coils in the convection section, medium-pressure steam at a rate of 5–7 t/h can be supplied to these coils. Lowering the outlet temperature of a converter section reduces the temperature of the combustion off-gases in the convection section, which also helps to protect the air coil. 3.8 Adjusting the gas turbine load: Due to the low system pressure, the outlet pressure of the air compressor can be kept slightly lower, thereby reducing the load on the gas turbine. It is required that the load on the low-pressure shaft of the compressor be greater than 93.5%, and the flow rate be greater than 43.5 km3/h, in order to prevent surge in the unit. In this way, the gas turbine can save 0.2 km3/h of pure fuel, and the saved natural gas can be used to increase the load, thereby reducing the ammonia consumption per ton. 3.9 Compressor anti-surge protection: To ensure that the system pressure remains above 3.2 MPa, it is common to reduce the compressor speed; however, it is necessary to first open the anti-surge valve to keep the unit’s flow rate above the minimum level, thereby preventing surge in the unit. 4 Conclusion (1) When the ammonia synthesis plant is operating at ultra-low load, operators should regularly monitor operational trends and be adept at noticing slight changes in parameters. (2) On the premise of stable production, measures should be taken to reduce the cost of synthetic ammonia, anticipate accidents more often, and conduct regular accident drills. (3) Once an accident occurs, roles should be clearly defined, everyone should perform their duties, and focus should be placed on controlling the situation to prevent it from escalating.