A comprehensive guide to exhaust gas aftertreatment systems for marine diesel engines

09/28/2021

An exhaust aftertreatment system cleans the exhaust gases produced by marine diesel engines. This reduces nitrogen oxide emissions and soot particles in the ambient air surrounding the ship and on board.

Modern exhaust systems consist of different components which can be incorporated into an exhaust silencer.

Why install an exhaust gas aftertreatment system?
The harmful pollutants in exhaust gases are created by two factors.

• Soot particles form because the combustion process is always partially incomplete and

• Harmful nitrogen oxides form when the high temperatures to which air and fuel are exposed during combustion convert nitrogen to nitrogen oxides.

How does an exhaust gas aftertreatment system work?
Dirty exhaust gasses are fed from the exhaust outlet of the turbo engine into the exhaust gas aftertreatment system where they are cleaned.

The Emigreen diesel particulate filter (DPF) traps and regenerates soot and other particles and converts the soot particles into ash through active or passive regeneration. This reduces the amount of particulate matter.

If there is also a need or requirement to reduce nitrogen oxide (NOx) emissions, a selective catalytic reduction (SCR) unit can be installed next to the DPF system. These systems are often installed together but can also operate as stand-alone units.

Selective catalytic reduction reduces NOx emissions by injecting an ammonia solution (urea) into the hot exhaust gas flow.

This creates a homogeneous mixture of NOx and ammonia, which flows through the SCR reactor where it is converted into harmless nitrogen gas (N2) and water vapour (H2O), reducing harmful emissions to between 1.8 and 2.1 grams per kilowatt hour.

This process is illustrated below.

The combined power of two systems

Emigreen® specialises in the design and delivery of exhaust gas aftertreatment systems for marine diesel engines.

Unique benefits of our systems:

• Built to your specifications so they can also be installed in small spaces.
• Designed to be maintained by the crew.
• Compliant with increasingly stringent international maritime regulations and, if necessary, specific local regulations.

Our solution incorporates the combined power of two systems: a diesel particulate filter (DPF) and a selective catalytic NOx reduction (SCR) unit; complemented by an exhaust silencer.

Emigreen® SCR NOx reduction
Emigreen®’s modern exhaust gas aftertreatment systems typically consist of two or three components:

• A DPF combined with a silencer
• An SCR combined with a silencer
• A combination of a DPF and an SCR, combined with a silencer.

Our SCR’s, DPF’s and silencers can be a standard design, or a custom design.

An exhaust gas cleaning system uses a catalyst (ammonia) to cause a chemical reaction that converts nitrogen oxide (NOx) into diatomic nitrogen (N2) and water vapour (H2O).

The water vapour evaporates into the air.

How this process works step by step:

• A dosing pump sucks urea from the urea tank and injects it into the flow of exhaust gases.
• The amount of urea injected into the exhaust stream depends on the base levels of NOx emitted by the engine and the desired reduction.
• A series of static mixers and homogenisers convert the exhaust gas and urea mixture into ammonia.
• The gasses are fed past catalytic elements that convert the NOx into nitrogen (N2) and water vapour (H2O).
• The SCR unit releases the purified exhaust gases into the atmosphere.

Depending on performance requirements, this system can also be configured with an integrated ammonia slip catalytic converter to eliminate ammonia slip.

A SCR system is essential to ensure compliance with IMO Tier III emission standards.

Emigreen® DPF soot filter
A diesel particulate filter (DPF) is a ceramic soot filter that traps carbon particles produced by diesel engines. During the regeneration process the soot trapped in the filter is converted to ash.

We offer two types of DPF system.

A DPF system with passive regeneration (Emigreen Delta)

• Thermal energy is required to enable filter regeneration.
• In engines with sufficiently high exhaust temperatures, the exhaust stream contains enough thermal energy to initiate and sustain the regeneration process.

A DPF system with active regeneration (Emigreen Alfa Alfa)

• This system is used when it cannot be guaranteed that the exhaust stream will contain enough thermal energy to initiate and sustain the regeneration process.
• With an active DPF system, regeneration can be forced if the exhaust stream does not contain enough thermal energy (flow and temperature) to trigger passive regeneration.

Emigreen® exhaust silencer
The release of hot gas into the atmosphere increases the level of exhaust noise. This may mean that noise reduction is required.

The solution must address the full spectrum of exhaust noise, which contains

• Low
• Medium and
• High frequencies

Each frequency requires a different acoustic approach. By using a different technology to attenuate each frequency it is possible to achieve maximum noise reduction.

These carefully tailored adjustments help prevent excessive back pressure that would otherwise reduce engine efficiency.