SCR can cut ship NOx emissions

Ocean-going vessels emit about 15 per cent of total anthropogenic NOx emissions. Photo: Paul Hart./ by

The technology to drastically cut ship NOx emissions is widely available, performs well, and may even slightly reduce ship fuel consumption. The costs of installation and operation are modest and expected to fall over time.

International shipping is a major source of emissions of nitrogen oxides (NOx). Globally, ocean-going vessels emitted about 25 million tons of NOx in 2007, representing about 15 per cent of total anthropogenic emissions. While NOx emissions from land-based sources in industrialised countries are gradually coming down, those from shipping show a continuous increase.

Ship NOx emissions can be mitigated by several means, including engine controls such as exhaust gas recirculation (EGR), exhaust gas after-treatment such as Selective Catalytic Reduction (SCR), or the use of alternative fuels such as gas or methanol.
In a new report, the International Council on Clean Transportation (ICCT) has investigated the viability of SCR technology to achieve compliance with the international Tier III NOx standards that will apply to new ships in designated NOx Emission Control Areas (NECAs) as from 2016. The Tier III standards require approximately 75 per cent lower emissions as compared to the Tier II standards that apply globally for new ship engines built after 1 January 2011.

ICCT notes that SCR technology is already used in millions of vehicles and power plants with a cumulative capacity of half a million megawatts worldwide, and that it is the only technology currently available to achieve compliance with the Tier III NOx standards for all applicable ship engines. (Other technologies can either achieve Tier II standard or achieve Tier III standard for only a subset of applicable ship engines.) State-of-the-art SCR systems can reduce NOx emissions by more than 90 per cent.

The maritime sector has had more than two decades of experience with SCR, and the report found that overall, approximately 1250 SCR systems have been installed on marine vessels in the past decade. Those vessels with the longest track records have accumulated up to 80,000 hours of operation over the past two decades.

SCR has been used on a variety of vessel and engine types using various fuels, including low-sulphur distillate fuel and high-sulphur residual fuel. Many current SCR applications are retrofits, where the after-treatment system has been retroactively applied to existing engines.

Engines certified to NOx emission standards are typically tuned to reduce emissions by operating at off-optimal combustion conditions, with negative impacts on fuel efficiency. When applying SCR, however, such engines can instead be tuned for maximum fuel efficiency. For example, SCR was estimated to provide a fuel economy benefit of 3–5 per cent under the EU’s Euro V standards for heavy-duty vehicles, with fuel cost savings partially offset by the additional cost of the urea used for SCR.

According to the ICCT, for ships, slightly lower fuel efficiency gains, on the order of 2–4 per cent, are expected under Tier III given that engine combustion conditions are currently less constrained under Tier II than equivalent standards for other modes. Thus, it may be possible to simultaneously reduce both CO2 and NOx emissions when moving from Tier II to Tier III compliance.

The International Association for Catalytic Control of Ship Emissions to Air (IACCSEA) has developed a cost estimation model for SCR installation and operation. Using this model, the ICCT calculated the total (undiscounted) operating cost of SCR for a 10 MW engine powering a vessel of 20,000 DWT using heavy fuel oil (HFO) and that spends 1500 hours annually in a NOx Emission Control Area to between US$104,000 and 224,000 per year, or approximately UD$900 to 2000 per tonne of NOx reduced.

ICCT concludes that SCR is a well-proven technology, and that a large number of companies based in Europe, the US, and Asia are currently delivering marine SCR systems to meet current and future NOx reduction requirements. The costs of installing and operating SCR are modest and are expected to fall over time as application of the Tier III standards generates greater innovation and competition among manufacturers and suppliers.

Based on this evaluation of technological capabilities and a history of successful application of SCR technology to maritime vessels, the ICCT sees no substantial equipment, supply chain, or cost barriers that would significantly inhibit the implementation of the NOx Tier III standards for applicable vessels in 2016 as established by the International Maritime Organization in 2008.

Christer Ågren

Feasibility of IMO Annex VI Tier III implementation using Selective Catalytic Reduction (March 2014). ICCT working paper 2014-4. By A. Azzara, D. Rutherford & H. Wang. 

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