Understanding What is UREA of SCR System on Vessel

On a marine vessel, the UREA onboard SCR system is an exhaust-gas treatment system used to reduce NOx (nitrogen oxides) from the main engine or auxiliary engines. SCR stands for Selective Catalytic Reduction.

Basic principle

The system injects a urea solution (usually 40% urea / 60% water for marine applications, depending on the manufacturer) into the engine exhaust. The urea decomposes into ammonia (NH₃), which reacts with NOx over a catalyst.

NOx + NH₃ → N₂ + H₂O

So, the harmful NOx is converted mainly into nitrogen and water.

Typical onboard arrangement

A marine SCR system can be thought of as:

Engine → Exhaust pipe → Urea injection → Mixing section → SCR catalyst → Clean exhaust → Funnel

The major equipment normally includes:

• Urea storage tank – stores the urea solution.
• Urea transfer/pumping system – supplies urea to the SCR unit.
• Dosing/injection unit – accurately injects the required amount of urea.
• Mixing unit – ensures good mixing of urea-derived ammonia with the exhaust gas.
• SCR reactor/catalyst – where the NOx reduction reaction occurs.
• NOx sensors – measure NOx before/after treatment.
• Exhaust temperature sensors – SCR requires an appropriate exhaust temperature.
• Control system/PLC – controls dosing and monitors the system.
• Bypass valve/damper – depending on the installation, allows exhaust to bypass the SCR under certain conditions.

Why is it installed on ships?

The main purpose is to comply with IMO MARPOL Annex VI NOx emission requirements, particularly IMO Tier III requirements in applicable NOx Emission Control Areas (NECAs).

For example, when a vessel enters a NOx-controlled area, the SCR may be activated to achieve the required NOx reduction. The exact operating arrangement depends on the engine and SCR manufacturer.

What happens if urea runs out?

The consequences depend on the vessel’s system and control philosophy. Typically, the SCR system will generate alarms such as:

• Low urea level
• Urea pump failure
• Low urea pressure
• High/low exhaust temperature
• NOx sensor fault
• SCR catalyst differential-pressure/high pressure alarm
• Poor NOx conversion

If the vessel is required to meet Tier III limits in the applicable operating area, loss of SCR operation can become a compliance issue, so the crew normally needs to follow the vessel’s approved operating/emergency procedure.

Important distinction: Urea vs. fuel

The urea system is separate from the fuel system.

Fuel → engine combustion → exhaust → SCR + urea → reduced NOx

The urea is not burned in the engine. It is injected into the exhaust after combustion.