Cutting NOx Emissions from Ships

A new ICCT study confirms feasibility of implementing new IMO standards for NOx emissions from ships on schedule in 2016. The study is a survey of technical literature and industry reports to assess equipment costs, environmental side effects, urea and catalyst availability and disposal, and overall system costs of SCR in the marine sector.

2014.03.28 - Cutting NOx Emissions from Ships Figure 1

Selective catalytic reduction is a technically feasible, cost-effective way to achieve the limits on nitrogen oxide (NOx) emissions from ships mandated by the International Maritime Organization (IMO) for 2016, according to a study released today by the International Council on Clean Transportation (ICCT). The study undercuts arguments in favor of a proposal to delay implementation of the so-called Tier III standard by five years, to 2021, which will be taken up at next week’s meeting of the IMO’s Marine Environmental Protection Committee (MEPC).

Selective catalytic reduction (SCR), using ammonia as the reducing agent, has proved effective in cutting diesel NOx emissions from a variety of mobile sources, including heavy-duty trucks and buses, passenger vehicles, and off-road equipment. It is the only technology currently available to achieve compliance with the IMO’s Tier III NOx standards for all applicable engines.

Annex VI of the International Convention for the Prevention of Pollution from Ships (MARPOL 73/78) introduced a stepwise approach to the reduction of emissions of NOx from new marine vessels. The third step, Tier III, applies to diesel engines installed on ships built on or after January 1, 2016, and to engines that undergo a major conversion on or after that date. It applies only to ships sailing in North American Emission Control Areas (ECA).

At the May 2013 meeting of the MEPC, the Russian Federation recommended that the IMO prepare an amendment to delay implementation of Tier III requirements to 2021, citing technical barriers to the adoption of control strategies such as SCR. That amendment will be considered at MEPC-66 in April 2014.

The ICCT study surveyed technical literature and industry reports to assess the equipment costs, environmental side effects, urea and catalyst availability and disposal, and overall system costs of SCR in the marine sector. Approximately 1,250 SCR systems have been installed on marine vessels in the past decade. Those vessels with the longest track records have accumulated more than 80,000 hours of operation over the past two decades. Currently there are at least 21 companies based in Europe, the US, and Asia developing engine, SCR, and catalyst technologies capable of meeting current and future NOx reduction requirements.

2014.03.28 - Cutting NOx Emissions from Ships Figure 2

Total number of vessels with SCR systems installed by year.

2014.03.28 - Cutting NOx Emissions from Ships Figure 3

Breakdown of SCR systems by vessel type.

The study estimated costs of technology application using a model produced by the International Association for Catalytic Control of Ship Emissions to Air (IACCSEA). That model showed a total (undiscounted) operation cost of between $104,000 and $224,000 per year, or approximately $900 to $2000 per tonne of NOx reduced. Based on historical trends in other sectors, the paper’s authors noted, the cost of operating marine SCR systems can be expected to fall.

The study also found that, because engines certified to current NOx emission standards must be tuned to operate at off-optimal combustion conditions, installation of SCR systems, by making it possible to run engines more optimally, could produce fuel efficiency gains on the order of 2% to 4%.

The study assessed other potential barriers to the adoption of SCR as well, concluding that none should significantly inhibit implementation of the 2016 NOx limits on schedule. Vanadium-based SCR systems, supplemented where necessary with strategies to boost exhaust temperature in low-load operations, will be capable of reducing NOx over a sufficient range of operational conditions, particularly when paired with the 0.1% sulfur fuel that will be made available in sulfur emission control areas. Production and distribution of urea to marine vessels should be manageable given the relatively small volumes to be delivered, the limited number of ports that need to be served, and the identification of best practices in Europe. The average consumption of urea for reaching the IMO Tier NOX level is approximately 15 l/MWh. If one assumes 40% urea solution price variation between 0.30 and 0.60 €/l, this will result in a typical operating cost of 4.5 to 9 €/MWh. Environmental byproducts, notably ammonia slip and excess CO2 emissions, are not expected to be generated in significant volumes.

More information can be found in the ICCT study “Feasibility of IMO Annex VI Tier III implementation using Selective Catalytic Reduction“.


Alyson Azzara, Researcher at the ICCT.  Alyson holds a Ph.D. in Marine Biology from Texas A&M University at Galveston and an MSc in Oceanography from Texas A&M University. She is a member of the marine program team of the ICCT and has experience on issues of environmental stewardship and ocean policy.

Dr. Dan Rutherford, Program Director for Marine and Aviation at the ICCT. He is the ICCT’s chief representative to the International Maritime Organization (IMO) on policies to reduce conventional pollutant and greenhouse gas emissions from oceangoing vessels. Dr. Rutherford holds a B.A. in Chemistry from the University of Minnesota at Morris and a M.S. and Ph.D. in Environmental Engineering and Science from Stanford University.

Haifeng Wang, Policy Analyst working on the Marine Team at the ICCT. Haifeng holds BA and MA in economics from the Ocean University of China and holds a Ph.D. in marine science and policy from the University of Delaware.


The International Council on Clean Transportation (ICCT) is an independent research organization working on behalf of transportation air quality and energy efficiency regulators worldwide. It was founded in 2001 to provide reliable, independent technical expertise on vehicles and fuels to the public sector. The ICCT maintains offices in Berlin, Brussels, China, and London, as well as in the U.S. It is funded principally by private foundations, such as the Hewlett Foundation and ClimateWorks in the United States and Stiftung Mercator in Europe.

Content from the study is reproduced in this article with the authors’ kind permission.

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