How does the tower granulation process cope with new environmental regulations!
Thread Content
I. Elements related to environmental protection1. Harmful components in exhaust gases: (1) Urea dust; (2) Ammonia; (3) Aerosols and “white smoke trails”; (4) Noise.
2. Causes and impacts: (1) Urea dust: The formation of this substance is an inevitable result of the tower granulation process. Its concentration and emission rate are generally related to the following factors: a. Defects in nozzles and transmission components; b. Cooling rate of particles in the forming area; c. Particle size; d. Hourly production volume; e. Local weather conditions at the time. (2) Ammonia generation: During urea synthesis, residues remaining dissolved in the urine are present to some extent. Its relative concentration is related to the pre-granulation treatment processes and the cooling rate of particles inside the tower. (3) Aerosols and “white smoke trails” (harmful and having a significant impact): The main reason for these is that after washing, ammonia and urea dust dissolved in water form hydrates, leading to aerosols and “white smoke trails,” which severely affect regional PM2.5 levels. (4) Noise: It is usually caused by mechanical noise (such as fans and high-pressure pumps), airflow noise (excessively high flow rates), and improper airflow pathways.
3. Completeness of environmental protection measures and delays in regulatory implementation: Urea production enterprises typically not only have high total production volumes but also high exhaust gas output per tower (usually 600,000 Nm3/h), resulting in high emission rates and significant environmental damage； From the perspective of environmental protection trends, ammonia nitrogen-containing dusts are far more harmful than those from steel production or fly ash; they not only cause severe air pollution but also lead to contamination of nearby water bodies and groundwater. Strict controls will surely be imposed once the conditions become more favorable ； (1) Effects of dust: All particulate matter is harmful, and ammonia nitrogen is particularly so. Under the current conditions, emissions should be reduced as much as possible. In areas with strict regulatory controls, we recommend keeping emissions below 10 mg/Nm3; in remote areas or those far from water bodies, this limit can be relaxed to 20 mg/Nm3. This is a prudent approach to avoid unnecessary re-investment in projects, as environmental regulations could otherwise restrict production capacity in severe cases ； (2) Effects of ammonia: a. We should be aware that the relative hazard associated with ammonia varies greatly depending on its form of existence and the height at which it is emitted. b. The load capacity of an ammonia environment in its \"dry\" state is very high. This is because, in this state, ammonia has a relatively low concentration, the exhaust temperature is high, and its density is much lower than that of the atmosphere; as a result, it disperses well in the upper atmosphere, rising to several kilometers altitude where it gets greatly diluted. In nature, ammonia is primarily produced through the decay and fermentation of plants. It is generally harmless to humans, animals, buildings, and plants. Although its hazard level is relatively low, it still needs to be controlled to prevent potential damage under weather conditions with high air humidity and low air pressure. A safe level should be kept at 50 mg/Nm3 ； c. Hydrates of ammonia pose a significant threat to the environment, especially those found in near-surface areas. Due to their slightly higher density, hydrated ammonia tends to accumulate in the areas surrounding towers, where it combines with dust in the air (including urea particles) to form aerosols with high refractive indices. With favorable meteorological conditions, this state remains relatively stable, causing long-term damage to humans, animals, and buildings. It is recommended to use the ammonia escape rates associated with the ammonia-based desulfurization process as a reference for accurate data ； (3) Aerosol and white smoke “trailing” – Aerosol and white smoke trailing cannot occur in a moisture-free state or when the relative humidity of the flue gas is low. When a water washing process is used, a large amount of water evaporates from the exhaust gas, resulting in a humidity level that is close to or at saturation. When these conditions coincide with the meteorological conditions (most of the time throughout the year), the water in the flue gas condenses, and together with ammonia and particulates, this leads to the formation of a “trailing” effect. Currently, there are only two methods for eliminating this phenomenon: heating or cooling. For the large volume of gas handled by granulation towers, these methods involve high costs and energy consumption, and they are also difficult to implement at heights of nearly 100 meters; nevertheless, they must still be considered ； (4) Noise: In residential areas and relatively open environments, the propagation distance is long due to the height of the tower, so noise needs to be controlled. However, the noise power is usually low and its intensity is not high, making control relatively easy and the implementation cost low. II. Adaptability of Production and Cost Factors 1. Overview: The tower granulation process is widely used in the production of small to medium-sized particles due to its simple structure, low investment costs, and high operational reliability. Setting aside the issue of granulation quality, the production capacity of a single unit is greatly influenced by the temperature of the particles. Below, we will briefly analyze the factors that affect particle temperature: a. Hourly output: Under the same particle size, there is a linear relationship between hourly output and particle temperature ； b. Particle size: There is a non-linear relationship between particle size and particle temperature, with an essentially cubic proportional relationship. When the average particle size exceeds a certain value, it becomes difficult to reduce the particle temperature to a low level even with increased ventilation volume. This is the fundamental reason why large particles cannot be processed using tower granulation methods, as time – another key factor affecting cooling efficiency – plays a crucial role in this process ； c. Particle distribution: Making full use of the cross-section of the tower can effectively improve heat exchange efficiency and reduce the temperature of the particles; the impact of this factor is positive. The diameter of the circular normal distribution has an inverse square relationship with the temperature of the particles exiting the tower ； d. Effect of cooling air: The ventilation volume calculated based on heat transfer principles is clearly insufficient under forced ventilation conditions. Practice has shown that in plain areas, the deviation can be as high as over 50% in summer. The main reason for this, as analyzed from the temperature gradient data, is the low flow velocity within the tower, which results in poor heat and mass transfer between the cold and hot air as well as the particles. There is a significant difference between the temperature at the center of the cross-section and the temperature on the tower walls; actual test results show that this difference can be as much as 30°C or more in some cases. As the ventilation volume increases, these values also rise, which tells us that within a certain range, there is a linear inverse relationship between ventilation volume and particle temperature; however, once a certain value is reached, there is a noticeable reduction in the rate of change of these values. III. Other factors: 1. Dynamic and static loads: Lower dynamic and static loads contribute to the structural stability of the tower (including the weight of water and accumulated ash). 2. Wind resistance and seismic resistance: Load calculations should be carried out based on the structural characteristics of each tower, with corresponding measures and reinforcements implemented. 3. Forced ventilation granulation towers: Energy consumption and the durability of internal components must be taken into full consideration, with attention paid to factors such as resistance, efficiency, and stability of output.
IV. Comparison of granulation tower processes is as follows:
| Parameter | Process 1.0: Natural ventilation | Process 2.0: Washing | Process 3.0: Dry type (1st–2nd generation) | Process 4.0: Dry type (3rd generation) |
|-----------|-----------------------------------|---------------------|--------------------------------------------|----------------------------------------|
| Dust emission | 150–350 mg/Nm³ | 40–60 mg/Nm³ | ≤10 mg/Nm³ | ≤5 mg/Nm³ |
| NH4 levels | Usually ≤120 | Usually ≤50 | Slightly reduced by 15% | Can be reduced by 50% without additional costs |
| Aerosols and white smoke | None | Significant amounts of diffuse white smoke | None | None |
| Ventilation control | Depends on chimney effect and thermodynamics; has some self-regulation capability based on temperature differences inside and outside the tower; largely determined by climate conditions | Uses natural ventilation | Uses both natural and forced ventilation | Relies on forced ventilation; makes maximum use of chimney effect and thermodynamic principles to reduce energy consumption; has automatic tracking capability |
| Cooling effect and improvement | Affected by tower structure, climate conditions, and weather conditions | Affected by tower structure, climate, weather conditions; forced ventilation is influenced by exhaust fans | Affected by tower structure, climate, weather conditions; forced ventilation is influenced by exhaust fans | Affected by climate conditions and exhaust fans | ； Meteorological conditions have little impact; there is a high degree of compatibility with the tower structure. Climate and meteorological conditions are transformed into favorable factors, influenced simultaneously by the fan’s capacity and air flow, resulting in excellent heat and mass transfer efficiency and relatively low energy consumption. Changes and requirements regarding the tower: no additional loads. Additional factors include the weight of structural components and water; however, the increase is usually modest. High requirements exist for waterproofing within the tower, and there is some corrosive effect. The weight increase is significant, but there are few dynamic loads; thus, the tower itself must meet certain requirements. Compared to the first and second generations of dry-type systems, the weight is reduced by about 40%, with no dynamic loads or noise. Resource consumption: no cost involved, except for water resources and a small amount of electricity. More electricity is required, along with a small amount of compressed air; the electricity consumption is approximately 6–7 kWh per ton. It is also around 4 kWh per ton. Value of recovery: no recovery possible. For a plant with an annual production capacity of 400,000 tons (with a dust concentration of 250 mg), the average emission rate is about 225 kg/h; over an operating time of 8,000 hours, this amounts to about 1,800 tons. For the same plant, the average emission rate is about 45 kg/h, resulting in around 360 tons over 8,000 hours of operation. It can also be as low as 9 kg/h, leading to approximately 72 tons over 8,000 hours. In some cases, the average emission rate is around 4.5 kg/h, resulting in about 36 tons over 8,000 hours. Characteristics of the recovered material: no recovery possible. The recovered material is an aqueous solution containing impurities (mainly affected by water and air quality). It can also be in the form of dry powder, again depending on air quality. Methods for handling the recovered material: no recovery possible. Equipment used includes solution tanks, pumps, pipes, measuring instruments, monitoring devices, valves, etc. The wet recovery process is similar to those described above ； The dry recovery process allows for transportation by manual or mechanical means; the wet recovery process is similar ； The dry recovery process can use manual or mechanical means for transportation