Why urea injection is the preferred method for NOx control


urea nox control

The IMO’s targets for reducing nitrogen oxide (NOx) emissions by 2016 posed challenges for ship owners, driving the exploration of emission reduction technologies like engine adjustments, alternative fuels, and exhaust cleaning methods. Among these, the Selective Catalytic Reduction (SCR) system, involving urea injection into hot exhaust gases, emerged as an efficient solution for NOx reduction.

Due to an increase in vessels around the world, the amount of exhaust gases emitted forced the International Maritime Organization to issue maritime legislation for reducing the environmental impact arising from the increasing amount of exhaust gases.” (Seddiek, 2014)

The MEPC planned to reduce NOx emissions to the lowest level by the year 2016. To reach these targets set by the IMO, ship owners face technical challenges.

Technologies for emission reduction can be divided into three general areas: engine adjustment, alternative fuel, and exhaust cleaning technologies.

For NOx reduction, currently, many vessels use the SCR system. This system is considered the most efficient method of NOx reduction compared to the other methods due to its durability and compliance with most commercial vessels.

What is SCR?

Selective Catalytic Reduction is an advanced emissions control strategy system that reduces exhaust gas emissions of nitrogen oxides. The SCR is an active emissions control system. Hot exhaust gases flow out of the engine and into the injection spool.

Here, a water-based urea solution is injected into the exhaust gases. From here, the exhaust gases flow through an array of catalytic elements where the NOx is converted to water vapor and nitrogen gas. The exhaust gases then flow out of the stack. This system is commonly known as an SCR-Urea system.

What does urea injection do?

As mentioned before, urea is used in the SCR system to reduce NOx pollutants in the exhaust gases from diesel engines. Urea is injected as a water-based solution. Inside the solution, ammonia (NH3), which is produced as a result of the hydrolysis of urea in the mixing trajectory, will react with the NOx in the catalytic elements and is converted into water vapor and nitrogen gas.

he conversion process can be presented in the following simplified formula:

CO(NH2)2 + H2O → 2NH3 + CO2
4NO + 4NH3 + O2 → 4N2 + 6H2O

Urea can be injected into the SCR system in several solutions. These solutions have an industry standard from ISO. The most commonly used urea solutions in the maritime industry are AUS32 (ISO22241) and AUS40 (ISO18611). Here, AUS40 consists of 40% urea in water.

The difference in the use of both urea solutions is in the usage amount for the engine. In general, inland navigation can bunker urea at a higher frequency than on board seagoing vessels. Therefore, AUS40 is commonly used for seagoing applications.

Emigreen® SCR applications

For vessels, Emigreen® offers different solutions to reduce NOx from the exhaust gases. Depending on the power output of the system and therefore the amount of urea injected into the system, either the Emigreen® MINOx S or Emigreen® MINOx M can be applied on board the vessel.

Emigreen® MINOx S

The Emigreen® MINOx S system is commonly used for systems with a lower power output from the engine. In steps of 7.5 l/hr, the system can be adjusted up to 22.5 l/hr. In general, the system consists of the following components:

  • Emigreen® Injection spool
  • Emigreen® SiGAS silencer
  • Emigreen® NOx spool
  • Emigreen® MINOx S1-3 Dosing unit
  • Emigreen® MINOx S control cabinet

Emigreen® MINOx M

The Emigreen® MINOx M system is used for the higher power output engines on board the vessel. There are different systems available that can inject urea up to 50 l/hr. The system consists of the following components:

  • Emigreen® Injection spool
  • Emigreen® SiGAS silencer
  • Emigreen® NOx spool
  • Emigreen® MINOx M Dosing unit
  • Emigreen® MINOx M Air unit
  • Emigreen® MINOx M control cabinet


About the author

Jelle Westerhof is sales manager at Emigreen® and applies our industry-leading solutions to the projects of our clients. Feel free to reach out to Jelle for feedback on this article, for questions and for a free consultation.