Sailing towards IMO Tier 3
In order to meet future emission limits for marine engines with respect to nitrogen oxides (NOx) and sulphur oxides (SOx), engine manufacturers have been developing a number of solutions. This paper from MAN Diesel sets out the options for medium-speed diesel engines.
In order to meet future emission limits for marine engines with respect to nitrogen oxides (NOx) and sulphur oxides (SOx), engine manufacturers have been developing a number of solutions. This paper from MAN Diesel sets out the options for medium speed diesel engines.
The IMO requires that all vessels with keel-laying after 1 January 2011 must comply with Tier 2 NOx limits worldwide and the necessary development has been completed, with MAN Diesel & Turbo able to offer a full range of IMO Tier 2 compliant engines.
The subsequent Tier 3 stage, however, which will be introduced for vessels with keel-laying after 1 January 2016, will not have one globally valid NOx limit, but a differentiation based on the area in which a vessel is operating. While the Tier 2 NOx limit will remain valid on the open seas after 2016, an 80% NOx reduction compared to IMO Tier 1 (this relates to an approximate 75% reduction compared to IMO Tier 2) will become mandatory within designated emission control areas (ECAs).
Additionally, a so-called ‘not-to-exceed’ clause will be introduced with IMO Tier 3 which states that the cycle average NOx limit of an engine must not be exceeded by more than 50% at every individual load point of the cycle.
In addition, the exhaust emission of SOx is also limited by the IMO. While the limits are expressed in terms of a maximum allowable fuel sulphur content, legislation also permits alternative abatement techniques leading to an equivalent level of SOx concentration in the exhaust gas. In contrast to NOx, the level of SOx emissions cannot be influenced by the combustion process itself and all sulphur entering the combustion chamber with the fuel must exit it in the form of SOx again. It should be noted that in addition to the general IMO limits, additional regional limits do apply, e.g. a maximum fuel sulphur content of 0.1% within harbours of the European Union.
A number of technologies, both engine internal and external, are known to have a potentialtoreduce NOx and SOx emissions (the latter only by external measures) in order to fulfil the upcoming IMO Tier 3 limits. MAN Diesel says it has evaluated these technologies, both from a technical and from an economical viewpoint, and is now focusing on the development of Tier 3 compliant solutions which will offer maximum customer benefit and are suitable for series production as well as being operationally reliable.
Possible NOx measures
MAN Diesel has continued to focus on its core competences by developing and producing injection, turbocharger and electronic control (SaCoSone) systems in-house to achieve the closest connection and best interaction of all emission-relevant engine internal processes.
For IMO Tier 2, NOx reduction of 20% and more is achieved by combining Miller cycle with high pressure turbocharging (single-stage) and optimised high pressure injection systems. For better part load performance, a variable valve timing (VVT) is additionally needed and MAN Diesel is already in series production of these optimised systems.
By using an even more efficient two-stage turbocharging system, the NOx reduction can be increased up to 40% which has already been proven by MAN Diesel in-house tests. Even if IMO Tier 3 NOx reduction of 80% for ECA operation will not be achieved by these internal measures alone, an optimised and flexible combustion system will be the base for all ongoing developments.
For reducing NOx emissions with water, two technologies could be used, i.e. fuel water emulsion (FWE) and humid air motor (HAM). Both concepts are based on diluting oxygen concentration of the combustion air and using the water vapour for cooling down the combustion temperature peak which leads to a NOx reduction.
The FWE concept involves mixing fuel and water before injection. MAN Diesel in-house tests have shown the potential of more than 30% NOx reduction but, due to an increasing fuel penalty above 30% water/fuel ratio, the FEW concept alone is not an economic solution for IMO Tier 3 ECA regulations.
The HAM concept is based on saturating the charge air with water. Field experiences have shown positive results up to 65% NOx reduction by using the MAN patented engine heat exchanging system. This concept is based on heating the injected water for achieving a higher saturating rate. In contrast to FEW, the HAM system can operate with sea water. The main limiting factors for this technology are increasing charge air pressures and restricted saturation behaviour of the air in view of limited charge air temperatures.
Other critical aspects to be evaluated might be the capital costs and the space demand in the engine room for the HAM module. In the same way as for FWE concept, MAN Diesel is already offering the HAM method for retrofit solutions since 1999 as a possible alternative for achieving NOx reductions to Tier 2 level and below.
Although the technical potential of wet technologies will not be the final solution for achieving IMO Tier 3 limits, the technologies can be used for special applications or niche markets now and in the future.
Selective catalytic reduction
The selective catalytic reduction (SCR) concept is currently the best tested and approved system for achieving NOx reduction rates up to 90%. Several applications with MAN engines have been running for many years successfully in the field and this method is based on injecting aqueous urea solution into the exhaust gas for reducing NOx to N2 on the catalytic substrate.
The main challenge for engine developers is to adjust the whole system consisting of engine, urea injection unit and SCR module. Ammonia slip has to be avoided by an intelligent urea injection control which should be integrated into the engine control system. Best performance can only be reached by controlling engine operation parameters and urea injection in parallel.
For medium speed engines, SCR modules can be easily integrated after the turbocharger in the exhaust stack which avoids complex engine room arrangement while the urea tank can be integrated into the ship design. Another advantage is the possibility of combining SCR with all basic developments for inner engine combustion optimisation as well as the flexibility to switch off the system outside ECAs. MAN Diesel has already introduced a standardised SCR package for all medium speed engines.
Exhaust gas recirculation
The exhaust gas recirculation (EGR) concept uses exhaust gas for diluting the oxygen concentration in the charge air which leads to a lower combustion temperature. In combination with Miller cycle, high pressure two-stage turbocharging and common rail systems, NOx reduction rates up to 80% percent have been reached during MAN Diesel in-house tests. Advantage of the EGR concept is the independency of additional needed media like urea or fresh water. A possible disadvantage might be the necessity of low sulphur fuels or even distillates to protect the engine against corrosion.
The EGR concept can only become a relevant solution for IMO Tier 3 if the overall cost calculation for the shipowner turns out advantageous compared to SCR arrangements. MAN Diesel is continuing to investigate EGR for medium speed diesel engines.
Technological concept evaluation
The various technological concepts are evaluated by calculating the life-cycle costs of the most promising solutions from the operators’ perspective taking into account the capital costs as well as consumables and maintenance.
For the medium speed engine programme of MAN Diesel, the recently introduced Tier 2 6L32/44CR was chosen as a reference. The engine is equipped with common rail injection and single-stage turbocharging. Variant (0) is used as reference for the life-cycle costs under IMO Tier 2 conditions. Engine variant (1) is equipped with an SCR in order to comply with IMO Tier 3. These two configurations are compared with variant (2), a 6L32/44CR with two-stage turbocharging, which is also equipped with an SCR catalyst.
Beside these two IMO Tier 3 engines, MAN Diesel is evaluating the economy of advanced inner engine technology, comprising EGR, two-stage turbocharging and a common rail system with multi-injection ability, engine variant (3). The focus of the first evaluation step is on comparing the two SCR variants (1) and (2) with the reference IMO Tier 2 variant (0). In the second stage, the most economical SCR variant will be compared with the engine configuration variant (3).
The results of the life-cycle cost calculation show that the future IMO Tier 3 scenario will most probably result in a cost increase compared with the IMO Tier 2 reference scenario. The cost increase is expected to be in the range of 1.6% to 7.6%, depending on ECA share and engine technology. Comparing the two SCR variants (1) and (2) from the life-cycle cost point of view, the variant (2) turns out to have a cost advantage of approximately 4% over a 10 year period.
The operating costs of both engine variants (1) and (2) increases due to the additional urea consumption of the SCR catalyst when operating in ECAs. When operating outside the ECAs, engine variant (3) has similar operational advantages regarding SFOC as engine variant (2). In order to comply with the IMO Tier 3 limits, the EGR system needs to be switched on inside the ECA which results in a fuel oil consumption penalty in the range of 2% to 4%. This fuel oil penalty can directly be compared with the urea costs of engine variant (2).
In addition to this cost difference, additional investment costs for engine variant (3) can be expected due to the EGR system. Taking into account the differences in investment and operational expenses, the engine variant (3) seems to be an interesting alternative for an
ECA profile greater than 50%. It is clear that engine variants (2) and (3) are quite close regarding life-cycle costs, as the total difference after 10 years of operation is in the range of €100,000 at 100% ECA share.
From today’s technological viewpoint, the combination of IMO Tier 2 technology in combination with SCR seems to be the most straightforward solution. Additional cost savings can be generated by the application of two-stage turbocharging. For application with high ECA profiles, advanced engine technology combining EGR, CR and two-stage turbocharging may become an interesting alternative.
Two-stage turbocharging for medium speed engines in combination with Millervalve timing is increasingly interesting because of advantages in fuel oil consumption and in NOx emissions.
In framework 6 of the EU supported research project Hercules, an MAN Diesel type 6L 32/44 CR prototype engine was equipped with both a flexible, two-stage turbocharging system and variable Miller valve timing. Compared with the single-stage turbocharged Tier 1 engine it was possible to achieve a fuel consumption-neutral reduction in NOx emissions of more than 40% while simultaneously increasing cylinder output from 560kW to 640kW. It should be pointed out that these excellent results were achieved in combination with a very good smoke emissions behaviour and that the load acceptance characteristics of the engine are comparable with that of a standard engine with single-stage turbocharging.
The decisive factor for this excellent overall outcome is the chosen technology package of variable valve timing (VVT) for the inlet valves and a flexible charging system employing a variable turbine area (VTA) nozzle ring in the high pressure turbocharger stage.
It is hence possible to achieve very good values for fuel consumption and emissions of oxides of nitrogen and, by ‘switching off’ the Millerprocess and raising charge air pressure using the VTA system at part load, good smoke emission values and favourable load acceptance characteristics.
The combination of reduced emissions of both CO2 and NOx is also extremely interesting as a base for Tier 3 strategies. Investigations showed a two stage turbocharged 32/44 CR with Tier 2 emission level would have a SFOC advantage of some 3% to 5% compared with a single stage turbocharged Tier 2 engine.
Unfortunately the exhaust temperature control of a SCR catalyst is more challenging as extreme Millervalve timings, increased overall turbine pressure ratios and the temperature limits of HFO operated engines lead to low temperatures downstream from the LP turbine.
Countermeasures have already been investigated. Again the VTA technology is very helpful because the exhaust gas temperatures can be controlled and increased almost without drawbacks in SFOC.
Dealing with sulphur
The easiest way to meet the SOx limits is to use suitable low-sulphur bunker fuel which means using the more expensive MGO. Running an engine developed for residual fuels on MGO requires some care since the lower lubricity and viscosity need measures to protect inlet valve seating and injection pumps. These measures are already available and in operation.
To continue taking advantage of the lower cost of HFO requires an after-treatment measure in order to meet the legislation. Currently two major principles can be found in marine applications.
Scrubbers based on the use of sea water or fresh water with an alkaline reagent like caustic soda is well-known and already installed on several vessels. Another way of after treatment is the absorption of SOx in reactors using limestone granules. Both systems reach efficiencies above 90% and require significant space for installation and storage room for consumables.
Although wet scrubbers can be part of the EGR system to clean the recirculated exhaust gas before re-entering the engine, the temperature of the exhaust gas after the wet scrubber is far too low for a proper SCR operation and the high sulphur content in the exhaust gas before the scrubber challenges the temperature control. A way around this problem can be the use of dry systems using limestone granules which is neutral to the exhaust gas temperature and allows the operation of the SCR in the cleaned exhaust gas afterwards. The exhaust gas passes through the granules and the SOx reacts with the limestone to gypsum. First ongoing test trials onboard sea-going ships show promising results.
Gas engines - the panacea?
The use of natural gas is another option to meet the NOx and SOx limits in one step. The fuel is practically sulphur-free and meets Tier 3 NOx limits. The drawback of LNG is the significant larger amount of storage room in a range of roughly 2.5 to 3 times compared to fuel oils. A good compromise is the use of dual-fuel engines giving low emission in ECAs and full range outside.
MAN Diesel has evaluated multiple technologies to fulfil IMO Tier 3 emission limits with its medium speed engine programme. While several technologies do have the potential to reach Tier 3 emission levels, an economical life-cycle cost analysis shows that the greatest potential lies in SCR after-treatment and EGR.
For MAN Diesel’s medium speed engine program, the SCR system has been chosen as the primary method to meet Tier 3 compliance. Its major benefits are flexibility, robustness and a high level of maturity. The entire MAN Diesel medium speed engine program fulfilling Tier 3 NOx requirements with SCR technology will be available well before 2016.
In parallel, the development of an EGR system will be continued for medium speed engines. Once this system has proven its competitiveness with respect to capital cost, operating cost and robustness, it will be a viable option. Furthermore, MAN Diesel is looking into scrubber technologies to reduce SOx emissions from the exhaust gas as this technology represents the only chance to operate marine engines on heavy fuel oil within ECAs after 1 January 2015.
Lastly, optimisation of conventional engine technologies such as turbocharging, fuel injection, combustion strategy and electronic controls will be continued. This will be crucial in order to have the best base engine available to incorporate new abatement technologies and to tailor the best overall system.
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