Diesel Exhaust Fluid — sold under the AdBlue trademark internationally — is a 32.5% high-purity urea solution used in Selective Catalytic Reduction (SCR) exhaust aftertreatment on every BS-VI heavy-duty diesel vehicle in India. The chemistry is simple: urea and deionised water. The challenge is purity: ISO 22241-1 limits contaminants to single-digit ppm levels and a single mistake on raw material grade or deionised water quality can destroy the SCR catalyst on a Rs 60 lakh truck. This guide covers the chemistry, the manufacturing process, the ISO 22241 pass criteria and the practical realities of running a DEF blending plant in India.
In the SCR catalyst, DEF is injected into the exhaust gas stream upstream of a vanadium-pentoxide or zeolite catalyst. The urea solution decomposes thermally in two steps: thermolysis releases ammonia and isocyanic acid at >180 °C, then hydrolysis of the isocyanic acid produces a second mole of ammonia. The ammonia then reacts with NOx (NO and NO&sub2;) over the catalyst surface to produce harmless nitrogen and water:
(NH&sub2;)&sub2;CO → NH&sub3; + HNCO (thermolysis)
HNCO + H&sub2;O → NH&sub3; + CO&sub2; (hydrolysis)
4 NH&sub3; + 4 NO + O&sub2; → 4 N&sub2; + 6 H&sub2;O (SCR)
2 NH&sub3; + NO + NO&sub2; → 2 N&sub2; + 3 H&sub2;O (fast SCR)
The SCR catalyst is poisoned by metallic contaminants — calcium and sodium deposit as silicates or sulphates that block active sites; copper and iron interact destructively with the catalyst chemistry. This is why ISO 22241 is so strict on metal contamination: the catalyst cannot be cleaned in service, and once poisoned, NOx emissions rise above the BS-VI / Euro 6 limit, triggering the OBD warning and eventually a vehicle de-rate. DEF sits alongside the diesel-vehicle fluids in our wider automotive lubricant formulation practice.
| Component | Function | Typical % (m/m) | Specification |
|---|---|---|---|
| Automotive-grade urea (prilled or molten) | Active NOx-reducing component | 32.5% ± 0.7% | ISO 22241-1 raw material annex; biuret <0.5%, aldehyde <5 mg/kg, no anti-caking agents, metallic impurities below SCR poisoning thresholds |
| Deionised water | Solvent and decomposition reagent | 67.5% ± 0.7% | Conductivity <0.5 µS/cm at 25 °C; TDS <0.5 mg/kg; produced by RO + mixed-bed polisher; no microbial contamination |
No additives, no preservatives, no colourants, no surfactants. DEF is the simplest finished product in the entire automotive fluids portfolio — precisely because the SCR catalyst is so sensitive to anything else, and it demands none of the additive package development work that conventional lubricants require. The ISO 22241 raw material annex disqualifies any urea with formaldehyde-based coatings, anti-caking agents (which contain magnesium or calcium), or biuret >0.5%. Urea sold for fertiliser application meets none of these constraints — only specifically-produced automotive urea is acceptable.
| Property | Test Method | ISO 22241-1 Limit | Lubechem Design Target |
|---|---|---|---|
| Urea content | ISO 22241-2 B.1 | 31.8–33.2% | 32.4–32.6% |
| Density @ 20 °C | ISO 22241-2 B.2 | 1.0870–1.0930 g/ml | 1.0890–1.0905 g/ml |
| Refractive index @ 20 °C | ISO 22241-2 B.3 | 1.3814–1.3843 | 1.3823–1.3832 |
| Alkalinity as NH&sub3; | ISO 22241-2 B.4 | 0.2% max | <0.1% |
| Biuret | ISO 22241-2 B.5 | 0.3% max | <0.15% |
| Aldehyde | ISO 22241-2 B.6 | 5 mg/kg max | <2 mg/kg |
| Insoluble matter | ISO 22241-2 B.7 | 20 mg/kg max | <5 mg/kg |
| Phosphate as PO&sub4; | ISO 22241-2 B.8 | 0.5 mg/kg max | <0.2 mg/kg |
| Calcium | ICP-MS | 0.5 mg/kg max | <0.2 mg/kg |
| Magnesium | ICP-MS | 0.5 mg/kg max | <0.2 mg/kg |
| Sodium | ICP-MS | 0.5 mg/kg max | <0.2 mg/kg |
| Potassium | ICP-MS | 0.5 mg/kg max | <0.2 mg/kg |
| Iron | ICP-MS | 0.5 mg/kg max | <0.1 mg/kg |
| Copper | ICP-MS | 0.2 mg/kg max | <0.05 mg/kg |
| Zinc | ICP-MS | 0.2 mg/kg max | <0.1 mg/kg |
| Aluminium | ICP-MS | 0.5 mg/kg max | <0.2 mg/kg |
| Nickel | ICP-MS | 0.2 mg/kg max | <0.05 mg/kg |
| Chromium | ICP-MS | 0.2 mg/kg max | <0.05 mg/kg |
| Appearance | Visual | Clear, colourless | Clear, colourless |
The metal contamination limits are the hardest part to consistently meet — they come from incoming raw materials, from any non-stainless contact surface in the plant, and from packaging. Stainless 316L throughout (tanks, pipework, drum-filling line, even valve internals) is non-negotiable. Producers who run a DEF line frequently co-locate grease production — commonly lithium grease manufacturing, lithium complex grease manufacturing, sodium soap grease manufacturing, calcium soap grease manufacturing and polyurea grease manufacturing — to spread the capital cost of a shared site.
Diesel Exhaust Fluid (DEF) — sold under the AdBlue brand in many markets — is a 32.5% solution of high-purity urea in deionised water. It is injected into the exhaust stream of a Selective Catalytic Reduction (SCR) aftertreatment system on modern BS-VI diesel vehicles, where it decomposes to ammonia and converts NOx emissions to nitrogen and water.
BS-VI emission norms in India (April 2020) made DEF essential for all new heavy-duty diesel and many passenger diesel vehicles.
ISO 22241 is the international quality standard for DEF / AdBlue. ISO 22241-1 specifies the quality requirements (urea concentration 31.8–33.2%, density 1.0870–1.0930 at 20 °C, refractive index 1.3814–1.3843 at 20 °C, alkalinity ≤0.2%, biuret ≤0.3%, aldehyde ≤5 mg/kg, plus strict limits on metal contaminants). ISO 22241-2 covers test methods and ISO 22241-3 covers handling, transportation and storage. We support clients through ISO 22241 certification as part of our compliance service.
32.5% urea in water is the eutectic — the lowest-freezing composition. Pure water freezes at 0 °C; pure urea melts at 132 °C. A 32.5% solution has a eutectic freezing point of −11.5 °C, which is the minimum point on the binary phase diagram. Below 32.5%, water freezes first on cooling, leaving a concentrated urea phase; above 32.5%, urea crystallises first, leaving a dilute aqueous phase.
The 32.5% eutectic gives the widest operating temperature range and the lowest risk of phase separation in the vehicle tank.
Metallic contaminants — calcium, magnesium, sodium, potassium, iron, copper, zinc, aluminium — poison the SCR catalyst. ISO 22241-1 limits are 0.5 mg/kg each for Ca, Mg, Na, K and 0.2 mg/kg for Fe, Cu, Zn, Al; chromium and nickel limits are 0.2 mg/kg. Total phosphate is limited to 0.5 mg/kg.
These limits drive the requirement for deionised water (conductivity <0.5 µS/cm) as the dilution medium and for high-purity automotive-grade urea, not fertiliser-grade.
No. Fertiliser-grade urea contains additives (biuret, formaldehyde-treated coatings, anti-caking agents) and impurities that exceed ISO 22241 limits by orders of magnitude. Only ISO 22241-compliant 'technical grade' or 'automotive grade' urea — produced specifically for the DEF market — can be used.
Major Indian suppliers include Chambal Fertilisers' automotive urea plant, and imports from Yara, BASF and SABIC.
A modest DEF blending plant (2,000–5,000 L/hr) requires a water deionisation unit (reverse osmosis + mixed-bed polisher), stainless steel 316L blending tank with low-shear agitator, urea dissolution skid with weight dosing, in-line refractive index sensor, 1 µm filtration and stainless or PE-lined storage. Capital outlay typically Rs 1.2 to 2.5 crore depending on capacity. We scope DEF plants alongside our lubricant oil formulation consultant service so a single facility can also blend conventional oils.
Stainless 316L throughout is non-negotiable — DEF is mildly corrosive to mild steel and aluminium, and any contamination from non-stainless surfaces will fail the metals test.
Urea slowly hydrolyses in water to ammonia and carbon dioxide, and at higher temperatures to biuret (an intramolecular condensation product). At 25 °C, the urea content drops below the 31.8% lower limit in approximately 18 months; the biuret limit (0.3%) is breached in approximately 14–18 months. To guarantee field quality, the labelled shelf life is 12 months at 25 °C maximum storage. Higher storage temperature (30–35 °C) shortens shelf life to 6–9 months.
DEF is essentially non-toxic, non-flammable and non-hazardous — classified as a non-hazardous fluid for transport. It is mildly corrosive to copper, brass and aluminium on long contact and should not be stored in mild steel. Skin and eye contact require simple flushing with water. Major handling concern is contamination — spills must be cleaned up to prevent metal pickup if recovered, and dispensing equipment must be dedicated to DEF service only.
Share your target capacity, location and end-market (retail, OEM service fill, fuel station). We respond within one business day with a plant configuration, capex estimate and ISO 22241 quality roadmap.