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Summary of Process Technology Upgrades in Small Nitrogen Fertilizer Enterprises Abstract: Based on practical examples, this paper introduces the basic approaches adopted by small nitrogen fertilizer enterprises to address process-related issues. It points out that through process technology upgrades and the replacement of equipment, it is possible to continuously improve the technological level of production processes in such enterprises; this serves as an effective way to improve the operating environment, reduce production costs, enhance product quality, and achieve comprehensive utilization of resources. Keywords: low-nitrogen fertilizer ； Process issues ； Technical transformation ； Preface: The ammonia synthesis industry provides ammonia, an important basic chemical raw material for the chemical industry. The small-scale nitrogen fertilizer sector holds a significant position within China’s ammonia synthesis industry. The high degree of continuity required in ammonia synthesis demands a seamless process flow. Over the course of several decades, significant progress has been made in terms of equipment and process technology within the small nitrogen fertilizer industry; yet overall, the process still consists of three main stages: gas production, purification, and synthesis. Any problem that arises in a particular manufacturing process will hinder the normal operation of the entire factory. This paper conducts a systematic analysis of the current operating conditions and existing problems in small nitrogen fertilizer enterprises, and summarizes the experiences gained during the technical transformation process as follows. 1. Ensure high quality of raw materials, determine appropriate process parameters, and address the issue of high production efficiency and low consumption in coal gas production. There are currently many types of automated control systems available for gas production, which contribute to improving the efficiency of this process; however, if the quality of the coal fed into the furnace is not ensured and the process parameters are not set appropriately, these systems will still not be able to fulfill their intended functions. Some plant-type coal gasifiers have adopted the so-called third-generation microcomputer-based automatic control system for gas production, but due to an unreasonable distribution of the cycle time, the operation of these gasifiers is unstable, and neither the gas volume nor the quality meets the desired standards. Reasonable process parameters should be determined as follows: during the blowing stage, use \"strong wind for short periods\" to improve blowing efficiency and minimize reduction reactions during the blowing process. The ratio of the air volume fed into the furnace to the gas production volume should be 0.95:1. The blowing time should be such that the carbon content in the products discharged is less than 4% of the total carbon amount fed into the furnace, with the CO content in the blowing air being less than 6%. During the gas generation stage, the upper part is strong and short while the lower part is long and weak, thereby improving gas generation efficiency. It is advisable to keep the time allocation between upper and lower blowing at 65%–70% in order to stabilize the gasification layer and maintain a high furnace temperature for gas production. The difference in steam consumption between upper and lower blowing should be around 6%–8% on average, with the temperature difference between the coal layers at the upper and lower levels being less than 200°C. The cycle time allocation is as follows: blowing + recovery = 20%, top blowing = 26%, bottom blowing = 45%, secondary top blowing = 5%, final blowing = 4%. Ensuring the quality of coal fed into the furnace is a prerequisite for efficient gas production from briquettes; the required specifications for such coal are: a volatile content of around 5%, an ash content of less than 22%, and a fixed carbon content of over 58%. Requirements for the shaping process: proper combination of ingredients, enhanced crushing, secondary composting, three-stage screening, addition of sodium humate, and feeding into the furnace at low temperature. 2. Emphasize the use of theoretical knowledge and empirical data to improve desulfurization efficiency. For plants with medium to low sulfur content, wet oxidation methods such as the MSQ method, PDS method, and rubber-coating method can be employed; however, for gases with high sulfur content or prior to propylene decarboxylation, the rubber-coating method is preferable for desulfurization. For the dual-methane process, pre-treatment desulfurization at room temperature is necessary. However, some plants fail to address the desulfurization issue, and the problems can be summarized as follows: First, the design of the desulfurization tower is unreasonable; for packed towers, an operating gas velocity of 0.7–1.0 m/s is considered appropriate. Second, the amount of packing used is insufficient – in one plant, the secondary desulfurization process did not yield good results over time; it was found that the surface area of the packing was reduced by 1/3, amounting to about 1,000 m2, with around 10 cubic meters of φ50×φ50 stepped rings missing. Third, the regeneration process is ineffective, mainly because the pressure of the desulfurization liquid at the inlet of the injector is too low, below 0.23 MPa, which results in insufficient air suction. Additionally, the pumping capacity of the desulfurization pump is inadequate, as in the case of spray regeneration, the amount of air suction is proportional to the pumping capacity of the pump. Generally, for desulfurization, it is appropriate to control the liquid-to-gas ratio at 10–15 L/Nm3. 3 Great emphasis is placed on the quality of catalysts and the operating conditions to ensure long-term operation in production. Cobalt-molybdenum low-temperature shift catalysts allow for a significant reduction in ammonia vapor consumption per ton, which also enables an increase in the production capacity of the equipment. Some factories use catalysts of lower quality due to cost considerations, which results in only a short production cycle. Analysis shows that the low level of active components and too short immersion time are the key factors. Since the standards for this catalyst do not specify requirements regarding active components, some catalyst manufacturers cut corners by using an immersion time of only about 8 hours; as a result, the COO content is well below 1.6%. This can be confirmed by examining the catalyst in use – if the core inside remains white after it is cut open. In addition, great attention must be paid to the quality of the soda water in order to prevent the deposition of salts on the surface of the catalyst. Equal emphasis should be placed on the quality of manufacturing the main heat exchangers and the water temperature control heaters, to avoid leaks that could cause clumping on the catalyst surface and lead to oxidation and deactivation. These are the main reasons why low-temperature shift catalysts cannot operate for extended periods of time; strict controls must be implemented during the equipment manufacturing and production control stages. 4. Adopt a two-pronged approach to address both contradictions simultaneously in order to solve the decarbonization problem. Currently, the NHD method, DAE method, and carbon-propane method are used for decarbonization; the appropriate method can be chosen based on specific factors such as each plant’s steam balance, energy supply, and cooling water temperature. In general, satisfactory results can be achieved, but some plants do not see ideal outcomes. The main reasons are absorption and regeneration. Many plants have replaced the packing with 250Y structured packing, which increases the specific surface area by more than twice; however, this type of packing has a weak ability to distribute liquids evenly, and large tower diameters tend to lead to uneven flow. Static tests can be conducted after the packing is installed to observe the liquid distribution and make necessary improvements. It is reasonable to use tray distributors and redistributors, with the horizontal deviation needing to be less than 2 mm/m. The initial number of spray points per square meter should generally be between 80 and 120, and the height of each layer of packing should not exceed 5 meters. The criterion for the regeneration and purification process is that the residual CO2 level in the lean solution should be less than 0.2 mL/mL. However, chemical methods often result in significant errors; it is better to use chromatography for analysis. In one factory, chemical testing showed that the residual CO2 level in the lean solution was generally below 0.2 mL/mL during PC decarburization, but the actual decarburization effect was not good. After modifications were made by raising the position of the secondary flash and reducing the pressure, the load on vacuum and atmospheric pressure purification processes was reduced, which improved the regeneration efficiency, as well as the decarburization capacity and efficiency. 5. Every effort should be made to increase the air delivery capacity of the compressors in order to reduce the power consumption per ton of ammonia compressed. Some factories replaced their compressors with those having a higher capacity, but the reduction in power consumption per ton of ammonia was not significant enough to reach the designed values. According to our measurements across 6 factories, the average values are as follows: 3.3–17/320 yields 1069 kwh per ton of ammonia, while 4M8(3) results in 912 kwh per ton of ammonia. The designed values are: MH20-150/320 at 788 kwh per ton of ammonia, 4M8(3) or H8 at 877 kwh per ton of ammonia, and L3.3-17/320 at 942 kwh per ton of ammonia. The reasons for the high power consumption of these compressors include operation below full load, as well as the short service life of vulnerable components, particularly the valves, which results in an operating efficiency of less than 94% ； Furthermore, inadequate gas purification and poor electrostatic coking removal will also have a serious impact ； Excessive internal and external leakage in compressors is also a common problem among many enterprises. Industry standards require that the internal leakage rate be less than 2%, but our measurements on eight small nitrogen fertilizer plants showed that this rate was usually above 4%, with some cases reaching as high as 18%. This results in a high gas return rate, which in turn affects the compressor’s performance. The main causes of this issue are: internal leakage in circuits such as 2-to-1, 6-to-1, 7-to-1, and other related connection valves ； b: Internal leakage in the oil drainage valve ； c: Internal leakage in the balance section. This can be confirmed through gas composition analysis. Therefore, strengthening the inspection of valves in a timely manner and reducing the air return rate are effective measures to increase the output per unit machine and lower the electricity consumption per ton of ammonia produced. 6 Identify the root cause behind the symptoms in order to resolve the issue with the copper washing solution; currently, some factories still have levels that are slightly too high, which poses a significant threat to production. There are two cases: one is truly trace amounts. Over time, the catalyst continues to oxidize, reducing its service life. Many factories do not enforce strict control over the parameters of the recycling process; in particular, the temperature at which the recycled copper is discharged is too low. We conducted experiments and found that at atmospheric pressure, the copper melt must be heated to above 72°C for carbon dioxide to begin to escape from it. Otherwise, the residual amount of carbon dioxide in the copper melt remains above the acceptable level, and thus a precise control over its concentration cannot be achieved. The reason for this is that the production capacity is high while the recycling equipment is insufficiently large, the heating area is inadequate, the reflux tower is too small, or there is a short circuit in the copper melt within the recycler. The second issue is false trace amounts, which are mainly caused by unsaturated hydrocarbons; gas pretreatment can be carried out using a mixture of silver sulfate, mercury sulfate, and 95% concentrated sulfuric acid. The root cause of the copper washing solution issue is cuprous sulfide, which possesses surfactant properties. When the carbon removal process generates high levels of exhaust gases, foam forms in the copper solution, and cuprous sulfide helps to stabilize this foam, leading to overflow. Therefore, it is essential to control the H2S level after decarburization to less than 0.001 g/m3. After a major overhaul at a factory, copper carryover in the liquid persisted for as long as half a year. Efforts were made to clean the packing and improve the distributor, but the root cause of this carryover was ignored. Subsequent investigations revealed that an error had been made in preparing the sodium thiosulfate standard solution used for desulfurization analysis after carbonization; 0.1 mol/L was used instead of 0.01 mol/L, which reduced the H2S concentration by 10 times and led to incorrect analysis and judgments. 7 Emphasize actual production measurements to address the ammonia balance issue; according to theoretical calculations, in ammonia plants that use coal as a raw material, there is an excess of ammonia of around 13%. Some plants fail to manage their ammonia balance effectively, and the main reasons for this include: the characteristics of semi-water gas, leaks of useful gases during the production process, low efficiency in the utilization of ammonia during the carbonization process, excessive amounts of ammonia used for internal purposes (>5%), high levels of ammonia released during the synthesis process, inadequate protection of catalysts leading to a decline in their activity, and a lack of attention to ammonia recovery – as a result, 10% of the total ammonia is wasted. After the plant was initially put into operation, there was an ammonia imbalance. The causes were sought solely in the synthesis and carbonization processes. Subsequent measurements revealed that the amount of ammonia used in carbonization (calculated based on the amount of ammonia solution used) was much lower than the output of synthetic ammonia, thus identifying the root of the problem. Employees were then asked to investigate further, and it was found that the vent valve for non-condensable gases at the top of the water cooler in the refrigeration process was not closed, allowing a large amount of ammonia to escape. There are also some factories that pay very little attention to ammonia recovery methods. One such factory uses an old carbonization tower as a recovery tower in its carbonization process, but the resulting dilute ammonia water is not concentrated for recovery and is instead discharged directly, which results in both resource waste and environmental pollution. Tests have shown that the amount of ammonia discharged accounts for as much as 5.6% of the total production volume. Practice has proven that the recovery method of \"one ton of ammonia per ton of water, adding water gradually, concentrating it step by step, and recovering it at constant pressure\" is widely applicable. Some factories pay very little attention to the reduction of the ammonia catalyst, which results in poor catalyst activity and severely hinders production. In addition to strictly controlling the water vapor concentration, another key factor in this reduction process is to raise the overall catalyst temperature as much as possible to reach the temperature required for complete reduction; this is especially true for the catalyst in the lower sections, as it plays a crucial role in the later stages of production. After the plant began operations normally following its initial startup, an ammonia imbalance occurred. It was found that due to insufficient ammonia levels, the plant switched to light load operation too quickly at the end of the reduction process; as a result, there was a difference of over 30 degrees between the bottom temperature and the temperature required for complete reduction. After carrying out additional reduction measures (increasing hydrogen supply, increasing space velocity, and raising the temperature at the lower layer), the ammonia production improved significantly after another period of operation at light load