Urea has the chemical formula CO(NH₂)₂, and agricultural-grade urea normally contains at least 46% nitrogen.
In China, around 75%–80% of urea is produced from coal, while the rest is mainly produced from natural gas. Although the upstream processes are different, both production routes finally provide the same two key raw materials for urea production: liquid ammonia and carbon dioxide (CO₂).
The basic chemical reaction is:
2NH₃ + CO₂ → CO(NH₂)₂ + H₂O
Route 1: Coal-Based Urea Production
Coal-based urea is the main production route in China.
Step 1: Syngas Production – Coal Gasification
Raw materials: Raw coal, such as lignite or bituminous coal.
Pulverized coal is fed into a gasifier together with steam and oxygen. Under high temperature and high pressure, the coal is converted into crude synthesis gas (syngas), mainly containing carbon monoxide (CO), hydrogen (H₂), and carbon dioxide (CO₂), together with impurities such as sulfur compounds and dust.
The crude syngas is then cooled and cleaned. Coal ash, dust and other solid impurities are removed from the gas stream.
Water-gas shift reaction: CO is converted into additional hydrogen and carbon dioxide.
The hydrogen will later be used for ammonia production, while the carbon dioxide becomes one of the main raw materials for urea synthesis.
Step 2: Gas Purification
The syngas must be further purified before it can be used for ammonia and urea production.
1. Desulfurization
Sulfur compounds, especially hydrogen sulfide (H₂S), are removed to prevent catalyst poisoning and protect the downstream equipment.
2. CO₂ Removal and Recovery
A large amount of CO₂ is separated from the gas mixture and recovered at high purity. The recovered CO₂ is then compressed and stored or sent directly to the urea synthesis section.
At this stage, the plant has prepared the two important components needed for the next processes:
High-purity hydrogen for ammonia production
Recovered CO₂ for urea production
Step 3: Ammonia Synthesis – Production of Liquid Ammonia
Nitrogen is obtained from air, while hydrogen comes from the coal gasification process.
The two gases react under high temperature and high pressure in the presence of an iron-based catalyst:
N₂ + 3H₂ → 2NH₃
The reaction produces ammonia gas. After cooling and compression, the ammonia is converted into liquid ammonia and stored for use in the urea plant.
At this point, the two key raw materials for urea synthesis are ready:
Liquid ammonia (NH₃) + Carbon dioxide (CO₂)
Step 4: High-Pressure Urea Synthesis
The high-pressure synthesis section is the core part of a urea production plant.
Typical operating conditions are approximately:
Pressure: 13.5–14.5 MPa
Temperature: 180–185°C
Liquid ammonia and compressed CO₂ are fed into the urea synthesis reactor in the required ratio.
The reaction takes place in two main stages.
1. Formation of Ammonium Carbamate
The first reaction is very fast. Ammonia reacts with CO₂ to form ammonium carbamate:
2NH₃ + CO₂ → NH₂COONH₄
2. Conversion of Ammonium Carbamate into Urea
Ammonium carbamate then undergoes dehydration to form urea and water:
NH₂COONH₄ → CO(NH₂)₂ + H₂O
An Important Point: The Conversion Is Not 100%
The single-pass conversion of ammonia and CO₂ into urea is only around 60%–70%.
This means a significant portion of the feed does not become urea in one pass. It cannot simply be discharged because that would result in a large loss of raw materials.
Instead, the unreacted ammonia, CO₂ and ammonium carbamate must be separated, recovered and recycled back to the synthesis section.
This recycling system is one of the key factors affecting the energy consumption and operating cost of a urea plant.
The outlet from the synthesis reactor is therefore not pure urea. It is a mixture containing:
Urea solution
Unreacted ammonium carbamate
Free ammonia
CO₂
Water
Step 5: Decomposition, Recovery and Recycling
The synthesis outlet is gradually depressurized and heated in the recovery section.
1. Ammonium Carbamate Decomposition
When heated and depressurized, ammonium carbamate decomposes and releases:
NH₃ + CO₂
2. Absorption and Condensation
The released ammonia and CO₂ are recovered through absorption and condensation processes.
They are then returned to the synthesis section for further reaction.
The efficiency of the recovery and recycling system has a direct impact on plant economics.
A well-designed recovery system means:
Less ammonia loss
Less CO₂ loss
Lower raw-material consumption
Lower energy consumption
Lower urea production cost
After most of the ammonia and CO₂ have been recovered, the remaining product is a dilute urea solution, typically containing around 60%–70% urea.
Step 6: Evaporation and Concentration
The dilute urea solution is concentrated through two-stage vacuum evaporation.
Most of the water is removed under vacuum conditions, producing a molten urea solution with a concentration of more than 99.5%.
Temperature Control Is Critical
The evaporation temperature must be carefully controlled.
If the temperature becomes too high, especially above approximately 140°C, urea may undergo side reactions and form biuret.
Excessive biuret content can be harmful to crops, particularly when the fertilizer comes into direct contact with plant leaves.
Therefore, proper evaporation temperature control is important for maintaining the quality of the final urea product and controlling biuret content.
Step 7: Urea Granulation
The concentrated molten urea is finally converted into solid fertilizer granules.
There are two widely used granulation technologies.
1. Prilling Tower – Standard Agricultural Urea
The molten urea is pumped to the top of a prilling tower, which may be several tens of meters high.
A rotating spray head breaks the molten urea into very small droplets.
The droplets fall freely through the tower while being cooled by an upward flow of cooling air. As they cool and solidify, they form small, round white urea prills.
The finished product is collected at the bottom of the tower.
Advantages:
High production capacity
Relatively low production cost
Simple and mature technology
Disadvantage:
Granule strength is generally lower than that of large-granule urea
Prilling is still widely used for producing standard agricultural urea.
2. Fluidized-Bed Granulation – Large Granule Urea
For large-granule urea, a fluidized-bed granulation system can be used.
A concentrated urea solution is sprayed onto small urea seed particles. The particles continuously grow as additional urea layers build up on their surface.
At the same time, cooling air removes heat and solidifies the urea coating.
This process produces larger and stronger granules with less dust.
Large-granule urea is particularly suitable for:
Deep fertilizer application
Mechanical fertilizer spreading
Broadcast spreading with fertilizer spreaders
Applications where higher granule strength is required
Step 8: Final Screening and Packaging
After granulation, the urea granules are sent to a vibrating screen for classification.
The screening system separates:
Fine particles and powder
Oversized particles
Lumps or agglomerated material
The qualified granules are then sent to the automatic packaging machine.
Depending on the market requirements, the finished urea is commonly packed in 40 kg or 50 kg bags.
The final product is then ready for storage, transportation and agricultural use.
From Coal to Finished Urea
The complete coal-based production chain can be summarized as:
Coal-Based Route
This process integrates coal gasification, ammonia synthesis and urea production into one large-scale chemical production system.
The efficiency of each section, especially gas purification, ammonia synthesis, high-pressure recovery and energy integration, has a major influence on the final production cost, product quality and overall plant efficiency.
At Mingwei, we deliver more than equipment. We deliver a complete, workable system that keeps your fertilizer quality consistent, batch after batch.
That's what we mean by:
Beyond Equipment. Total Solutions.