Urea

Urea (carbamide, formula CO(NH₂)₂) is a synthetic organic nitrogen compound, obtained by reacting ammonia with carbon dioxide under high pressure^[1][2]. It is the most concentrated solid nitrogen fertiliser, with a ureic nitrogen content of 46 %^[2]. The product is supplied as prills — spheres formed by solidifying droplets in free fall^[3][4] — or as granules, produced by spraying concentrated melt onto recycled seeds^[3].

Manufacture

The industrial process proceeds via two successive liquid-phase reactions in a single reactor: first, ammonium carbamate formation (exothermic, rapid), then dehydration to urea (endothermic, slower)^[1]. Typical conditions range between 140 and 250 bar and 180 to 210 °C, with a molar NH₃/CO₂ ratio of 2.8 to 4 and a residence time of 20 to 30 minutes^[1]. Conversion, assessed on a CO₂ basis, is in the order of 50 to 80 %^[1].

Several total-recycle technologies have been developed: CO₂ stripping processes (Stamicarbon, Toyo ACES), NH₃ stripping processes (Snamprogetti), and the IDR (Isobaric Double Recycling) process combining both^[5]. In modern plants, the finishing section (prilling or granulation) is the main emissions source, with a pollutant load of 0.4 to 0.6 % of total unit feed, i.e. at least ten times higher than that of the wet section^[6]. Granulation requires less air than prilling; the dust it generates is coarser, facilitating capture^[6].

Urea is intrinsically tied to ammonia production: units are located in immediate proximity, as CO₂ from steam methane reforming or other synthesis processes serves as feedstock^[7][8]. This integration explains why 46 % of ammonia produced is absorbed by urea manufacture^[9].

Agronomic and environmental uses

Ureic nitrogen is hydrolysed in soil by the urease enzyme into ammonium ions, then nitrified to nitrate^[2]. This transformation can lead to losses through ammonia volatilisation, particularly high when urea is surface-applied on calcareous soils at elevated temperature, without incorporation^[10]. Urea and UAN solutions thus generate higher volatilisation losses than nitrate-based fertilisers^[10].

From a climate perspective, urea constitutes ammonia stabilised with carbon: approximately 0.7 tonne of CO₂ is released per tonne of urea applied^[7]. Its decarbonisation requires access to renewable carbon, a limited and geographically dispersed source^[7]. Conversely, nitrates and NPK fertilisers contain no carbon in their molecule and can be produced from low-carbon ammonia without feedstock constraints^[7].

Market and trade

Global urea output crossed the 195 Mt threshold in 2023, rising by more than 6 % from the previous year; over the past decade, expansion has run at an average annual pace of 1.3 %^[11]. Two Asian giants, China and India, sit at the top of the table both as manufacturers and consumers: the former secures self-sufficiency in nitrogen fertilisers, whilst the latter remains heavily dependent on foreign purchases^[11]. The bulk of nitrogen capacity recently built or planned worldwide takes the form of urea units, which explains why this product is growing faster than raw ammonia or nitrogen as a whole^[11].

International trade reached 55.1 Mt in 2023^[12]. Countries or regions endowed with abundant gas resources and limited domestic markets have traditionally dominated exportable supply^[12]. Beijing commands a colossal industrial park; although this primarily serves domestic supply, surpluses have been sold abroad in recent years. Since late 2021, however, overseas sales have collapsed owing to restrictive measures aimed at preserving domestic prices below world levels; by end-2024, they were virtually nil^[12]. North America, Latin America, South and East Asia form the core of import demand^[12].

In 2022, urea accounted for 50 % of world nitrogen fertiliser consumption, well ahead of NPKs (16 %), nitrates and CAN (8 %) and UAN (5 %)^[13][14]. Its share in nitrogen consumed is rising^[13]. International quotations notably track prilled urea prices on the Black Sea and granular urea prices in Egypt, both expressed on an FOB basis^[15].