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NOx reduction: Chemical or mechanical?

ERG, NOx reduction
MAN’s EGR solution, tested aboard the ‘Maerk Cardiff’, features Alfa Laval’s PureNOx system coupled with a 6S80ME-C9.2 engine


Over the last couple of years, capital investment in energy and fuel efficiency has dropped off. But NOx compliance and reduction of particulate matter remains a crucial topic, writes Paul Morgan.

Until the beginning of 2016, engine manufacturers were able to achieve IMO Tier I and Tier II NOx emissions limits through operational set-up and minor engine setup modifications. But for Tier III compliance, external changes need be considered. The main contenders are selective catalytic reduction (SCR), an exhaust after-treatment that employs a catalytic converter and an additive (usually urea) to reduce the NOx generated in the combustion process, and exhaust gas recirculation (EGR), which consists of an internal engine process to prevent the formation of NOx.

As current newbuilding order books are not strong, it is difficult to predict any particular trends for which technology will emerge as the preferred choice. Both systems are technically complex to install, and have opex costs which, along with trade and route patterns, make the decision on which system to install far from straight forward.

A major deciding factor is whether the installation is going to be on a new build, or as a retrofit. Overall, capex for SCR is lower than EGR, but SCR has an extra opex cost due to urea consumption. This makes EGR is more suited for newbuilds, whereas SCR can be more efficiently retrofitted.


Why do we need either? Greenhouse gasses are a huge worldwide concern, and NOx emissions are a significant contributor to this. NOx emissions comprise about 95% nitric oxide (NO) and 5% nitrogen dioxide (NO2), which is formed as NO oxidizes in the engine after combustion.

Formation of the oxides produced is largely dependent on the temperature of the exhaust gasses produced in the engine. In general terms, the lower the temperature, the less NOx will be produced. However, if combustion temperatures rise above 1,200°C the formation is accelerated, and above 1,500°C the formation becomes significant.

When comparing two very different NOx reduction technologies like EGR and SCR, there are several factors to consider, not least the global economics of which system to choose and the installation cost. This can be linked to the operation of the vessel, and the main vessel trading area, as NOx control areas are going to be initially linked to the existing emission control areas (ECAs) in North America and the Caribbean, where sulphur limits are already in place. The sulphur compliance options in ECAs will be either low-sulphur fuels or sailing with scrubbers and burning heavy fuel oil.

This is relevant to NOx compliance technologies because higher sulphur fuels require a significantly higher specification water treatment system as part of the SCR process, due to the more aggressive nature of higher sulphur fuels, therefore increasing the cost of production of the units.

EGR lends itself more to large two-stroke slow speed engines, where the exhaust temperature can be too low for SCR, unless the unit is fitted in a high-pressure location, before the turbocharger. SCR on the other hand seems to be most popular on four-stroke engines.


MAN Diesel & Turbo is one of the engine designers at the forefront of developing EGR technology, and shop tested the first fully engine-integrated EGR system on a 6S80ME-C9.2 engine at Hyundai Heavy Industries in October 2012. EGR test runs were completed in January 2013 during sea trials on the Maersk Cardiff, with a similar engine installed.

Using EGR to reduce the NOx produced after combustion can come at a cost. The specific fuel oil consumption (SFOC), measured in g/kWh, will increase as a result of the power consumption needed for the EGR system. The main consumers are the EGR blower, scrubber water pump and the water cleaning plant. The total electrical power consumption is expected to be about 2% of the total main engine power.

According to MAN, a 10% increase in exhaust gas recirculation results in a 20% NOx reduction, but with a SFOC penalty in the region of 0.5 g/kWh. There are ways to mitigate the increase in SFOC using various engine setups and operational modes, but this will inevitably result in compromising the reduction in NOx emissions.

The SCR process meanwhile, works by injecting a urea solution (CO(NH2)2) or ammonia (NH3) into the exhaust gas at temperatures from 290°C to 350°C, resulting in the conversion of NOx into harmless nitrogen and water. To allow this process to take place, a catalyst unit is situated in the exhaust gas flow, where the reducing agents react with the nitrogen oxides forming nitrogen and water.

It has been shown that it is possible to reduce NOx by up to 95% using SCR systems, but realistic targets are usually set to 85% to 90% in order to reduce the risk of ‘ammonia slip’. Ammonia is itself a significant contributor to the greenhouse effect, and ammonia slip can occur when the total amount of injected urea does not react fully with the NOx. Ammonia is also very corrosive, so excess amounts in the exhaust system can cause a lot of corrosion.


Opex costs of using SCR must be taken into consideration, and the rule of thumb is that in order to reach a 90% reduction in NOx, approximately 15g of urea is needed per kW/h of energy produced by the engine. With the cost of urea currently ranging from US$200 to US$350 per tonne, the expense can escalate quickly for large engines.

While the basic engine design does not need any changes for applying SCR, available space is one of the main considerations when retrofitting equipment in an engine room. In addition to the SCR catalyst unit, an SCR system consists of a reactor tank, a pump and control system for dosage of ammonia/urea, and the storage of ammonia or urea.

Winterthur Gas & Diesel (WinGD) has been working on high pressure SCR for large two-stroke engines. One of the main reasons that the use of SCR has mainly been the domain of four-stroke engines rather than two-strokes is that the exhaust temperatures of two-stroke engines is usually a lot lower. Successful use of SCR depends on a higher exhaust temperature. By situating the SCR catalyst housing upstream of the turbocharger on a two-stroke engine (commonly known as high-pressure SCR), higher temperatures and pressures are experienced than if the unit is placed in a low-pressure location downstream of the turbocharger.

High-pressure SCR can operate under more variable temperature and pressure conditions than the low pressure systems more commonly used on four-stroke marine engines. When in use with high sulphur fuels, the design temperatures and pressures can be critical for operational efficiency, and to avoid process problems with the equipment. Temperature management of the catalyst when using high-pressure SCR is controlled through a series of dampers and valves.


Placing the SCR upstream of the turbocharger of a low-speed engine has significant design influences compared to conventional post turbine SCR. Normally the pressure would be one bar and the exhaust temperature is more even over the engine load range. The most important limitation in the use of SCRs on two-stroke engines is therefore the required minimum operating temperature.

Careful temperature control of the engine exhaust gases is vital to allow successful operation of the engine together with the SCR when using a high sulphur fuel. The condensation point of ammonium sulphate salts is a major consideration, as these salts can reduce the effectiveness of the catalyst. Predicting the amount of salt condensation in the pores of the catalyst will allow the SCR to be used with lower exhaust temperatures for a period of time without any detrimental effect, but should be avoided to reduce the risk of damage to the SCR system.

Engine temperatures also need to be matched with the catalyst to optimise the usage of the urea consumption at high loads, and to avoid deposits. The minimum exhaust temperature for long-term operation is limited by the condensation of ammonium- and sulphate-containing salts, and WinGD advises that the SCR must not run for long periods at temperatures where significant amounts of salt can condense in the catalyst pores.

Decreased catalyst activity at lower temperatures results in the inability to reduce NOx, but also potentially increases ammonia slip, which may lead to deposits on surfaces of the exhaust gas system. This is particularly relevant when using high-sulphur residual fuels. Low exhaust gas temperatures may result in the condensation of ammonium and sulphur salts on the catalyst surface or in the pores of the catalyst and again result in reduced effectiveness of the SCR system.

When there is a need to operate at lower temperatures, there is the risk of reduced catalyst activity due to ammonium sulphate condensation in the pores of the catalyst. It is therefore important to ensure a sufficient catalyst reserve to maintain performance.

However, the ammonium sulphate salt condensation reaction is reversible, and catalyst activity can be regained. A test run by WinGD using a 6.5MW, 500mm bore, four-cylinder test engine with a bypass stream SCR showed that total recovery can be obtained when the catalyst is operated above the bulk condensation temperature.


At low temperatures, SCR performance can be marginally improved by using a catalyst with a high vanadium content. However, this type of catalyst has potentially negative design implications so must be closely matched to the operating profile of the vessel. When operated at a high temperature, a high-vanadium SCR has an excessive reducing agent oxidation, and the potential to oxidize gaseous sulphur to S+VI, a compound that may result in a visible bluish exhaust gasses and also increases the risk of salt deposits in the heat recovery equipment downstream of the catalyst.

MAN Diesel & Turbo provides both systems for its two-stroke customers, and is perhaps best placed to judge uptake among large engine users. But to date, the orders for Tier III NOx abatement technologies are simply too small to allow any trends to be drawn.

As promotion & customer support manager Jan Vinder told delegates at the recent ‘Future Engine Forum’ hosted by The Motorship and ExxonMobil in Hamburg: “It’s a discussion we need to have for each project. It depends for instance on the initial cost, or whether you have waste heat recovery – in latter case we think low-pressure SCR is the solution.”

The technology behind both systems – and the potential implications for each on engine operations – is very different. With both striving to achieve the same goal, capex and opex considerations will be the major factors in the decision on which way to jump.

The Original Posted by MotorShip