Container Ship Clouds calm sea

IACCSEA Comment: on low temperature operation of SCR

The removal of NOx from an engine exhaust, via SCR is a chemical process.  For the chemical reaction (the neutralisation of acidic NOx with basic ammonia NH3) to occur, the appropriate conditions must be provided.  For example, there must be sufficient ammonia (NH3) in both quantity and spatial distribution matched to inlet NOx in combination with sufficient catalyst to meet the required target emissions levels.  Of key importance is the temperature as the SCR process requires operation within a specific range.

This temperature window is dependent on several factors such as fuel type and sulphur content and is typically in the range of 300 – 500℃ (but can be lower e.g. 270℃ for lower sulphur fuels).  If the exhaust temperature falls below the specific range for its operation – the efficiency of the SCR reactions drops and fouling of catalyst (with ammonium sulphate salts) may occur.   Without appropriate management, in addition to the NOx emissions, unreacted ammonia (NH3) will also be released to the atmosphere.

SCR systems can tolerate limited periods of low temperature operation (with fouling). With higher temperatures these ammonium salts return to the gaseous phase, with the ammonia available to neutralise NOx.

Therefore, managing the temperature and performance conditions is critical for the efficient and effective use of deNOx SCR.  Minimum temperatures of operation and/or recovery temperature capability can be achieved through multiple, known technologies, each with its own advantages for a given application including energy cost, modification of target performance levels, catalyst volume, etc.  Examples include:

  • Heating low temperature exhaust gas (via external sources such as burners),
  • Engine management where exhaust gas temperatures are maintained at lower loads,
  • Engine/battery systems that continue to maintain minimum exhaust gas temperatures with excess power (not needed a low load demand) being stored electrically,
  • More advanced monitoring equipment (CEMs) that informs the operator of real-world emissions offering significant improvement on the test cycle approach.

In conclusion, when vessels operate at lower load (and lower temperatures) such as close to shore this presents challenges to deNOx technology. However, the industry has established tools to improve net performance and ensure lower NOx emissions close to shore.

North Sea Giant 1

North Sea Giant’s SCR Retrofit: A Catalyst for Cleaner Oceans with Support from the Norwegian NOx Fund

In the maritime industry, environmental responsibility is a major concern. The installation of a Selective Catalytic Reduction (SCR) system on board the North Sea Giant vessel represents a significant step in reducing harmful nitrogen oxide (NOx) emissions.


Why SCR Should Be Installed

The decision to retrofit the North Sea Giant with an SCR system is grounded in the environmental responsibility of the ship owner – North Sea Shipping. Nitrogen oxides, released in high quantities by maritime engines, contribute to air pollution and are major contributors to smog, acid rain, and severe health problems such as respiratory diseases.


Improvement in Air Quality

The primary goal of the SCR system is to reduce NOx emissions. This project achieved an 80% reduction in NOx from 9 g/kWh to 1.5 g/kWh. The Norwegian NOx Fund plays a crucial role in improving the air quality in Norway as well as supporting SCR technology and other measures in marine and land-based environments.


Reduction of Social Costs

This collective commitment to environmental protection also helps to reduce the social costs associated with air pollution. Poor air quality leads to increased healthcare expenses due to higher incidences of respiratory diseases.


Overall, the SCR retrofit on the North Sea Giant, coupled with support from the Norwegian NOx Fund, is an exemplary initiative that showcases how retrofit projects can be made easy. This project ensures compliance with regulations, significantly enhances air quality, and curtails the social costs associated with air pollution. It serves as a model for how collaboration with environmental funds can drive us toward a cleaner and more sustainable future for our oceans and planet.

Shipping’s future fuels require monitoring and managing local pollutants

Last week, the International Energy Agency (IEA) published a report estimating the future fuel mix for shipping to 2050 under a net-zero scenario. According to the IEA, ammonia is projected to become the primary marine fuel at 44%, with hydrogen and biofuels at 19%, while methanol is limited at 1% [1].

This forecasted growth of alternative fuels is an encouraging step as the industry aims to decarbonise in line with the IMO’s GHG emissions reduction targets by 2050 [2]. However, as we embrace these new fuels, it’s vital to ensure that the benefits are maximised while mitigating unintended consequences like local pollutant leakage, including NOx, N2O, ammonia slip, or methane slip [3].

The cost of poor air quality from international shipping is staggering, resulting in approximately 400,000 premature deaths each year and a societal cost of over €58 billion annually [4].

This is why the IACCSEA strongly encourages the industry to adopt emissions control technology such as Selective Catalytic Reduction (SCR) and Continuous Emissions Monitoring (CEMs) to manage and monitor harmful local pollutants.

This can be achieved through retrofitting existing vessels or installing such technology on new vessels. By doing so, we can improve confidence, safety, and the efficacy of the shipping industry’s future fuel mix, safeguarding human and environmental health on the journey to decarbonisation.

IACCSEA is committed to supporting the shipping industry in aligning decarbonisation objectives with the effective management of local pollutants. We are ready to collaborate, provide resources, and facilitate initiatives to achieve cleaner shipping practices.

If you are interested in learning more about how IACCSEA works with the shipping industry to control ship emissions to the air, please to get in touch with us at



[1] International Energy Agency (IEA), 2023. Net Zero Roadmap: A Global Pathway to Keep the 1.5 °C Goal in Reach — 2023 Update. Available here:

[2] International Maritime Organization (IMO), 2023. Revised GHG reduction strategy for global shipping adopted. Available here:

[3] Mærsk Mc-Kinney Møller Center for Zero Carbon Shipping, 2023. Managing Emissions from Ammonia-Fueled Vessels. Available at:

[4] European Federation for Transport and Environment, 2023. Air Pollution. Available here:

Future Fuels in Shipping

Comparing future fuels in shipping

Using alternative fuels in marine transport can play a critical role in decarbonising the shipping sector and contributing towards climate change goals. The market for alternative fuels continues to develop through ship builders, engine manufacturers and classification societies under the guidance of MARPOL regulations (International Convention for the Prevention of Pollution from Ships) adopted by the International Marine Organisation (IMO).

One key consideration for marine alternative fuels is how current maritime legislation, largely shaped towards conventional fuel types, addresses the adoption of alternative fuels. For example, to meet NOx emission limits, an Engine International Air Pollution Prevention (EIAPP) Certificate under MARPOL is required when building a maritime diesel engine according to the NOx Technical code.  In the future EIAPP certificates will need to address emissions from combusting alternative fuels to ensure that the market can provide the necessary solutions to aid in the decarbonisation of the maritime industry.  The table below provides an overview of the major fuel options available within the marine sector. It considers the current advantages, disadvantages, emissions profile as well as where catalytic solutions exist in controlling and reducing these emissions.


Future fuels in shipping

Hydrotreated Vegetable Oil (HVO) / Heavy Fuel Oil (HFO) / Marine Diesel Oil (MDO) Advantages
  • Abundance
  • Low Cost
  • Baseline emissions are controllable with emissions control
  • Local & global
  • High baseline emissions
  • SCR is an established technology – lower reactor size is possible with a cleaner exhaust gas
Emissions profile
Catalytic solutions
  • SCR for NOx Filters for PM (including Black Carbon) at an earlier TRL
Ultra-Low Sulfur Diesel (ULSD) Advantages
  • Can have a more compact aftertreatment system
  • Potential to have a longer life time due to less poisoning
  • Alters the lubricity of diesel
Emissions profile
  • NOx SOX PM
Catalytic solutions
  • Deployment of DPF is possible and lower SOx enables other aftertreatment technologies
Bio / e LNG Advantages
  • Established Fuel – Improved CO2 emissions Lower NOx Lower SOx.
  • Suitable for use in alternative power generation technology.
  • Energy Density relative to diesel fuels.
  • GHG emissions CO2 & CH4
  • CH4 a GHG ~90 x CO2 equil.  Bio LNG emissions risk of catalyst poisoning
Emissions profile
  • NOx PM CO CO2 CH4
Catalytic solutions
  • SCR for NOx OxiCat for Com CH2O and CH4
Bio / e Methanol Advantages
  • Drop in fuel also suitable for Alt technologies – Electrochemical Power
  • Energy density / Toxicity.
  • Limited experience at sea
Emissions profile
Catalytic solutions
  • SCR for NOx OxiCat for CO and CH2O
Ammonia Advantages
  • Zero carbon at point of use.
  • Low lifecycle GHG if Green / Blue
  • Energy density / lack of experience at sea
  • N2O GHG ~300 x CO2 equil.
Emissions profile
  • NOx NH3 N2O
Catalytic solutions
  • SCR/ASC for NOx /NH3  Options for N2O
Hydrogen Advantages
  • Efficient & Zero carbon at point of use
  • Low lifecycle GHG if Green / Blue
  • Suitable for alternative power generation
  • Energy density
  • Lack of  experience at sea
Emissions profile
  • NOx & H2
Catalytic solutions
  • SCR for NOx (SCR for H2) oxicat for H2




SCR Selective Catalytic Reduction
NOx Nitrogen oxides
SOx Sulphur oxides
PM Particulate Matter
NMHC Nonmethane Hydrocarbon
CO Carbon monoxide
CO2 Carbon dioxide
 DPF Diesel particulate filters
GHG Greenhouse gas emissions
CH4 Methane
LNG Liquefied Natural Gas
CH2O Formaldehyde
N2O Nitrous oxide
NH3 Ammonia
ASC Ammonia Slip Catalyst
H2 Hydrogen gas
TRL Technology Readiness Level



Since its formation in 2011, IACCSEA has had a primary focus of demonstrating the technological and economic viability of catalytic technology for reducing emissions from shipping.

We do this by:

  • Sharing objective and factual technical information

We leverage our collective industry expertise and networks to gather and disseminate objective and factual technical information on marine catalytic emission control technologies (including costs and benefits) and promote awareness of this technology, including latest developments.

  • Contributing to industry groups and forums, and regulatory discussions

We use our voice to primarily inform regulators and the shipping community that proposed regulations for reducing emissions from shipping can be met through commercially available catalytic after treatment technology.

  • Working with others in the shipping community

We work closely with other stakeholders in the continued development and implementation of strategies that lead to cleaner shipping and raises awareness of the benefits of this technology among stakeholder groups and in the regulatory arena.

Danfoss IXA

Danfoss IXA announcement

Allan Skouboe, Chief Technical Officer at Danfoss IXA

Allan Skouboe, Chief Technical Officer at Danfoss IXA

IACCSEA is pleased to welcome Danfoss IXA as a new member. With extensive experience in exhaust gas after treatment, Danfoss are positioned to play a key role in the ‘green wave’ of technological development.

On joining IACCSEA, Allan Skouboe, Chief Technical Officer at Danfoss IXA, stated “We are happy to join an organization which is focused on the use of SCR technology in the maritime business. Our state-of-the art NOx sensor is widely used across the industry in connection with SCR and the membership of IACCSEA help us to stay ahead on industry requirements and trends.”.

Speaking on behalf of IACCSEA Ilkka Saarinen said “We look forward to engaging with Allan and the team in the years to come. Monitoring emissions from ship engines will grow in importance as shipping embraces being cleaner and greener”.


Since its formation in 2011, the International Association for Catalytic Control of Ship Emissions to Air (IACCSEA) has focussed on demonstrating the technological and economic viability of on board Selective Catalytic Reduction (SCR) technology to reduce Ox emissions.  We work closely with other stakeholders in the continued development and implementation of strategies that lead to cleaner shipping and to improve awareness of the benefits of SCR technology.

World Enviroment Day


June 5th marked the World Environment Day, which in 2019 focused on creating awareness on air pollution and encouraging action aimed at minimising its effects on people’s health and the environment. It is our mission at IACCSEA to be a catalyst for clean shipping, and in the spirit of the day we would like to take the opportunity to remind everyone about the effects of nitrogen oxide (NOx). Formed in the heat of the marine engine, NOx is a dangerous, acidic pollutant that can be transported over many hundreds of miles and deposited as acid rain. It promotes the formation of ground level ozone, detrimental to human health and is known to exacerbate heart and lung complaints. NOx acidifies its environment and damages plant life in the sea and on land.

There are ways, however, to neutralise the NOx in the exhaust of the marine engine. Selective Catalytic Reduction technologies are a simple, cost-effective and proven NOx reduction solution capable of achieving the emission limits set by the International Maritime Organisation, and in many cases up to 99% reduction of NOx emissions.

If you would like to learn more, please do get in touch with us at

For more information on International Environment Day:

IACCSEA to host a pre MEPC 74 event on 12 May to discuss its insights on why the IMO Tier III NOx regulations are not working yet

IACCSEA, the leading authority on catalytic reduction of NOx emissions from shipping, will host IMO MEPC 74 delegates and other esteemed guests from the industry on the evening of 12th of May. The programme will include thought-provoking presentations and discussions on the insights gained from the implementation of IMO Tier III NOx regulation in the North American Emission Control Areas to date and what difficulties need to be overcome in the future. Some of the topics we will discuss include:

  • Since the introduction of IMO Tier III regulation in 2016, there were very few calls by Tier III certified vessels in the North American ECA. This will mean that expected reduction in NOx emissions will not be achieved, as revealed interviews with major US and Canadian ports.
  • What are some of the common difficulties encountered in certifying IMO Tier III vessels and how to overcome them?

This is an invitation-only event held under Chatham House rules, so please get in touch with if you are interested in attending.

IACCSEA – to facilitate the transition to low emission shipping

London. The International Association for Catalytic Control of Ship Emissions to Air, IACCSEA, has been formed with a primary focus of demonstrating the technological and economic viability of Selective Catalytic Reduction (SCR) technology capable of reducing NOx emissions from marine engines.READ MORE