How Marine SCR Systems Work
& Urea Dosing Explained

A marine SCR installation has five functional blocks, in exhaust-flow order:

• Urea storage & supply. Dedicated AUS 40 tanks (stainless steel or compatible polymer), pumps and filtered supply lines. Correct materials matter. See our guides on AUS 40 storage and handling and unloading, pumping and discharge.
• Dosing unit. Meters the precise AUS 40 flow commanded by the control system, with compressed air or pressurised delivery to the injector.
• Injection & mixing duct. An injection lance atomises the fluid into the exhaust stream; static mixers homogenise the ammonia distribution before the catalyst.
• SCR reactor. Honeycomb catalyst blocks, typically vanadium-pentoxide/titanium-dioxide or zeolite formulations, where NOx reduction takes place. Soot blowers keep channels clear on residual-fuel installations.
• Sensors & control. NOx and temperature sensors upstream and downstream feed the closed-loop dosing controller and the compliance data logger.

On two-stroke engines the reactor is often installed pre-turbine (high-pressure side) to keep the catalyst hot enough; four-strokes usually mount it after the turbocharger. Either way, the chemistry, covered in our AUS 40 pillar guide, is identical.

The dosing controller solves one equation continuously: how much ammonia is needed to neutralise the NOx the engine is producing right now? Inputs are engine load and speed, engine-out NOx (mapped or measured), exhaust mass flow and temperature. The output is an AUS 40 injection rate.

Modern systems run closed-loop: a downstream NOx sensor verifies conversion and trims the dose in real time. The stoichiometric target is a molar NH₃:NOx ratio close to 1:1, enough ammonia to convert the NOx, but not so much that unreacted ammonia escapes the stack.

Estimating consumption

For voyage planning, a practical rule of thumb for Tier III operation is ~15–25 g of AUS 40 per kWh of engine output, depending on engine-out NOx and target reduction. Worked example: a 10 MW main engine at 85% load inside an ECA for 24 hours produces ~204,000 kWh. At 20 g/kWh that is roughly 4.1 tonnes of AUS 40 per day. Use the engine maker's Technical File figures for firm planning, and our consumption calculator for a quick estimate.

Three operational problems account for most SCR trouble at sea:

• Ammonia slip. Unreacted NH₃ passing the catalyst, from over-dosing, poor mixing, low temperature or catalyst ageing. Beyond smell and corrosion risk, slip wastes reductant and can trigger alarms. Mitigation: healthy closed-loop control, correct fluid, and (in some designs) a slip catalyst.
• Urea deposits. Incomplete decomposition at low exhaust temperature forms crystalline deposits (urea, biuret, cyanuric acid) on injectors and mixers, choking flow. High-purity, correctly concentrated AUS 40 and respecting the system's minimum-temperature dosing cutoff prevent most cases.
• Catalyst poisoning. Trace metals, phosphate and alkali contaminants permanently deactivate catalyst sites. This is precisely what the ISO 18611 / IS 17661 impurity limits exist to prevent, and why a per-batch Certificate of Analysis is worth insisting on.

Every one of these failure modes traces back to either fluid quality or dosing discipline. Sourcing certified AUS 40 from a manufacturer with traceable batches at the ports you actually call removes the fluid variable entirely. That network is mapped on our Indian port supply page.