May 16, 2023
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The shipping Industry is facing an increasingly fierce regulatory environment, especially in terms of limits imposed upon air emissions. International Maritime Organization (IMO) NOx Tier III limits have been major drivers for performance and development of marine engines during the latest years. Whilst focus in the past could be put on improving only the engine efficiency, more stringent legislations coming into force have led to a shift in focus towards reduced emissions altogether, focusing on all of nitrogen oxides (NOx), sulphur oxides (SOx), and carbon dioxides (CO2).
As per Annex VI of MARPOL, in the second stage, a reduction in NOx emissions by about 20% compared to the previous Tier I value is required. A further reduction in the order of 80% (Tier III) applies to certain keel laying dates and operational areas. In addition to the IMO global standards, there is also a wide range of local regulations existing for NOx emissions. The low emission initiatives are mainly focused on the European Union (EU) and United States (US), other parts of Asia and Australia will follow as well.
All these emission legislations have triggered a vast amount of research activities at marine engine OEM’s in order to find the best possible options for fulfilling the limits. An overview of the different engine technology options available for fulfilling future NOx regulations as well as a list of criteria used from an engine original equipment manufacturer (OEM) perspective regarding the choice of best technology has been given together with some case study investigations on selected ship applications.
This technology is based on the effect of the Miller cycle (early inlet valve closure before bottom dead centre) on the combustion temperature. By anticipating the inlet valve closure the effective compression ratio is reduced leading to a lower temperature in the combustion and expansion phase. Since NOX formation is thermal activated, Miller cycle has a remarkable potential in terms of NOX reduction. To keep the same air fuel ratio, higher boost level is needed to compensate the reduction of the effective displacement during the intake stroke.
Consequently the biggest limiting factor in applying the Miller cycle is the pressure ratio demand from the turbocharger. Based on the test result the boost pressure needed to maximize the gains in terms of NOX reduction is clearly beyond the single stage Turbo charging system potential, as a consequence the Two Stage Turbo charging system is the optimum solution for such technology.
Other benefits of the two stage system of the superior turbocharger efficiency resulting and improved engine fuel consumption and the possibility to increase the engine output. The main drawback of the two stage system is a part load behaviour where the combination of turbocharger system characteristics and the Miller timing leads to a generally poor performance. Variable inlet timing system enables the Miller timing tuning at different load.
NOX reduction potential up to 50% is possible.
The low NOX compression tuning is achieved by increasing the compression ratio as close as possible to Max firing pressure and, at the same time, by slowing the injection rate in order to minimise the pressure increase rate in the combustion chamber. The target is to get close to the constant pressure combustion resulting in a lower combustion peak temperatures and consequently lower NOX emissions. Piston top design and injector layout are optimised accordingly. Emission potential is in the range of 10 to 15% with Limited fuel consumption penalty.
By utilising the flexibility of the electronic fuel injection equipment it is possible to tune the combustion for low NOX emission along the whole engine operative field, for this reason the common rail technology is as well part of the concept.
NOx emissions can be reduced by internal engine technology without the use of after treatment devices. By recirculating cooled exhaust gases in the combustion chamber the effective lambada (air fuel ratio) is reduced without impacting on the engine thermal load. As a result of the lower air to fuel ratio and consequently of the lower oxygen concentration remarkable reduction on NOX emission can be achieved. Main drawback of this technology for the Marine application is the incompatibility with the high sulphur fuel.
Selective catalytic reduction is one of the most effective ways to reduce NOX emission. Injected urea in the exhaust pipe vaporizers to form Ammonia that reacts on the catalytic substrate reducing NOx to N2. Test results have shown NOx levels reduction up to 90%.Also the total hydrocarbon and particulate matter emissions are positively affected. The NOR Technology allows to meet immediately the IMO Tier III level. SCR is well known as “add on” exhaust treatment system and as such does not interfere with the basic design. The NOR has in fact a specific operating temperature range as a function of fuel quality (sulphur content) and this has to match with the engine performance (exhaust gas temperature).
An important factor to be noted that the Tier III requirements are for new ships, meaning that additional space has to be allocated at the ship design stage.
The use of LNG, LPG, methanol or ethylene in lean burn gas engines offers another means to reduce the NOx emissions. Lean burn gas engines are capable of reducing NOx emissions up to 90% and meeting the Tier III compliance. Differently from the Gas Diesel concept where the gas is directly injected in the combustion chamber in the dual fuel and SG engines, the gas is injected at low pressure in the intake port. The combustion mode is homogenously premixed and lean resulting in very cold and even combustion temperature leading to very low NOx emission level
The advantage here is that both SOx and NOx compliance is achievable. This is a low OPEX solution .LNG fuel tank requires additional space.
Water can be used as a mean to reduce the NOx emission. Water vapour acts as a temperature dumper and dilutes the oxygen concentration in the combustion air reducing the NOx formation rate. If water is directly injected in the combustion chamber it has also the affect to cool directly the combustion process (latent heat of evaporation).
In the Wetpac H (Humidification) concept the combustion air is saturated with water immediately after the compressor cooler is possible to reach water to fuel ratio up to 1.3 resulting in NOx reduction of 40%. In the Wetpac E (Emulsion) concept the fuel is mixed with water before the injection. Achievable water fuel ratio is related to injection system capacity and normally it is possible up to 0.3 with a resulting NOx reduction of 20%.
Main concerns in using Wet technologies are linked to the water consumption, its availabity on board and cost to treat it.
There are key points for the ship Owner consideration to select the technologies:
• Evaluation of total cost of Ownership towards selection of a technology
• One factor influencing the choice of technology is also the impact on storage space on-board the ship and how large a footprint all the needed extra technologies take. Often the cargo space size is what defines the rate a shipping company can charge for its services and thus it will have large implications on the choice of technology
• Compliance testing and emission measurements on board the ship-Non OEM SCR systems
Some of the other aspects to be considered are fuel flexibility due to vessel operation in NECA and outside NECA areas. Resale value of the ship plays an important role as tier III compliance vessel may fetch higher values at resale.
Installing NOx Tier III compliant technology could be beneficial beyond just achieving compliance with emission regulations, which includes scheme like NOx fund (Norway), Finnish investment aid, Hong Kong FWC, Maritime Singapore green initiative etc.
Case studies have been made to show benefits of certain technologies for specific customer Applications. The first case study is for a 5500 dead weight ton (DWT) PSV running on Marine Gas Oil (MGO) and with an electric propulsion having totally 7800 kW engine power installed. Operation time is estimated to 6000 h/year with the profile, and 100% of the time within an ECA.
PSV operating profile-Around 27% of the operating time is spent on steaming at 14.5 or 12 knots speed for transportation from Harbour to the Rigs and utilising either 3 or two of the four install engine at about 90% load. During dynamic positioning operations at the Rigs either 3 engines are needed at 77% load for heavy DP or only two engines at 40% load for light DP operation. Stand by and harbour operation demands 1 engine at 20 to 60% load.
Alternatives investigated including the following:
• Selective catalytic reduction
• Exhaust gas recirculation
• Two Stage turbocharging +SCR
• Two stage turbocharging + exhaust gas recirculation
• Dual fuel engine
With EGR an increase of 3.9% in fuel consumption and costs is foreseen due to worse combustion but by combining with a 2 stage Turbo charging solution the full cost are almost back at the reference level which is considered to be gensets with SCR. Despite this, the operating cost of the EGR solutions are always higher than the reference mainly due to an increase in estimated maintenance costs of almost 50% with the EGR system that is related to an expected increase in the corrosion level with the recirculated exhaust gases and thus more overhauls, cleaning, and component replacement needed on the air side. The other alternative showing a benefit, compared to the SCR reference is a solution with a 2 stage Turbo charging and SCR system.
On the investment side EGR is a strong showing overall CAPEX than the reference. But for the overall analysis, the relatively high increase in operating expenses needs to be taken into account. With a 2 stage turbocharging solution, the investment in SCR is smaller due to a slightly smaller catalyst element need with lower NOx emission. With this solution overall operating expense would also decrease and give a certain payback time for the investment.
Ro-Ro ships have the advantage of providing fast port turnaround without special cargo-handling facilities due to access ramps and open decks allowing fast manoeuvring of fork lift trucks, trailers, etc. Most of the Ropax vessel have operation in ECA. This means that the choice of engine emission abatement technologies is very important for this shipping sector with a large requirement for low emissions already now and a probable large increase in the future with more ECA zones implemented worldwide.
The second case study is made is for a 203 m long and 25 m wide RoPax ferry taking approx. 700 passengers, operating on a regular route three times per day and accumulating 8000 running hours in a year. The overall propulsion power installed is 38 MW, and the hotel load is 1.5-4 MW, all provided by the four engines installed. The operating profile is with most of the time operating time (35%) spent in cruising at a speed of 27 knots and with all the four engine operating at about 75% load.
Alternatives investigated for the Ro Pax ferry include:
The EGR alternative is not considered here since it will not be an economical alternative, demanding operation on ULSF which is needed to avoid corrosion, compared with the solutions operating on either HFO or LNG.
RoPax vessel operators demand solutions with the lowest possible OPEX to enable offering the lowest rate. The gas system Capex is relatively lower and makes the dual fuel engine solution more competitive. Consequently the 2 Stage turbocharging solution is a clear winner, together with Dual fuel engine solutions, because of the expected increase in LNG as fuel for short haul shipping and thus eventually lower LNG prices with a more developed infrastructure available. The chosen Ro-Pax vessel case shows benefit for LNG operation at LNG prices<1.35*HFO.
There are technologies available for the Tier 3 solution. Common rail and variable valve timing are ready for the flexibility Tier 3 requires.
Choice of technology for fulfilling future emission legislations is a compromise between CAPEX and OPEX but as important are factors like good serviceability and flexibility of the technologies. Fuel flexibility and access to multiple fuel markets remains a key aspect.
Overall, the studies have shown that even if the best option always depends on many different factors, LNG is a very strong candidate when emission legislations are enforced to a larger extent in combination with increasing availabity and decreasing LNG prices. The solution would have a payback time depending on the fuel price relationships. The drawbacks would be the need for larger fuel tanks hence less loading capacity and the dependency on having LNG bunkering station availability around the world. But as more capacity is built, the trend is clearly moving towards LNG as a valid option for ships operating within ECA areas.
Emission technologies versus fuel quality compatibility remains an ongoing development area.
About The Author:
Mr Pankaj Misra is a Marine Engineer from Marine Engineering College (DMET), Kolkata and possesses First Class Engineer (MOTOR) certificate of competency from Ministry of Transport, Government of India. Mr. Pankaj is a MBA (Finance) from SIES College Mumbai.
He has experience of 23 years in Maritime Industry with core capabilities in Technical Operations, technical management and business development. Mr. Pankaj has been working with Wartsila India in the commercial business of Marine Solutions, since last ten years.
He has earlier served as Technical Superintendent for three years and sailed as Chief engineer on board Merchant ships.
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