# How Marine SCR Systems Work: Urea Dosing, Catalyst Chemistry and Consumption

A marine SCR (Selective Catalytic Reduction) system injects AUS 40 (40% urea solution) into engine exhaust at 300–450 °C; ammonia from urea decomposition reduces NOx to nitrogen and water over a catalyst, up to 95% conversion.

## System architecture (exhaust-flow order)

1. **Urea storage & supply**: stainless/HDPE tanks, pumps, filtered lines
2. **Dosing unit**: meters AUS 40 flow commanded by the controller
3. **Injection & mixing duct**: atomising lance + static mixers
4. **SCR reactor**: vanadium-pentoxide/titania or zeolite honeycomb catalyst
5. **Sensors & control**: NOx/temperature sensors, closed-loop controller, compliance logger

Two-strokes often mount the reactor pre-turbine (keeps catalyst hot); four-strokes after the turbocharger.

## Dosing control

The controller targets a molar NH₃:NOx ratio near 1:1, from engine load/speed, engine-out NOx, exhaust flow and temperature. Modern systems are closed-loop via downstream NOx sensors. Dosing maps assume **40% urea concentration per ISO 18611**. Weaker fluid (32.5% AdBlue) silently under-doses by ~19%.

## Consumption rule of thumb

**~15–25 g AUS 40 per kWh** for Tier III operation. Example: 10 MW engine, 85% load, 24 h inside an ECA ≈ 204,000 kWh → at 20 g/kWh ≈ **4.1 tonnes/day**. Confirm against the engine maker's Technical File.

## Failure modes

- **Ammonia slip**: over-dosing, poor mixing, low temperature, aged catalyst
- **Urea deposits**: incomplete decomposition at low temperature (urea, biuret, cyanuric acid) choking injectors/mixers
- **Catalyst poisoning**: trace metals/phosphate outside ISO 18611 / IS 17661 limits permanently deactivate the catalyst

All trace back to fluid quality or dosing discipline. Use certified AUS 40 with per-batch Certificate of Analysis.

Related: [What is AUS 40](https://aus40.in/what-is-aus-40-marine-urea.html) · [AUS 40 storage & handling](https://aus40.in/aus-40-storage-handling-shelf-life.html) · [AUS 32 vs AUS 40](https://aus40.in/difference-between-aus32-and-aus40.html)

PROTICKS: ISO 18611 & IS 17661 certified AUS 40, Jamnagar, India. info@proticks.com · +91-74909-17799