Engine makers have explored many avenues to find ways to meet the increasingly stringent NOx regulations. While there may be more yet to come, there are effectively five means that are attracting the majority of attention. These include:-
• Engine Tuning (Miller timing)
• Fuel water emulsions or direct water injection
• Air humidification
• Exhaust Gas Recirculation (EGR)
• Selective Catalytic Reduction (SCR) - up to 95% reduction – more difficult but achievable on slow speed diesels due to lower exhaust gas temperature – allows engine to be tuned for minimum fuel consumption.
The first four options are under the control of engine manufacturers and will doubtless be incorporated into future engine models. Several makers have already announced Tier III compliant engines but that does not mean that other methods will not also be made use of not least because with some of the options there are drawbacks such as increased fuel consumption or sub-optimal operation.
Engine Tuning (Miller timing)
Engine tuning in a camshaft controlled four-stroke marine diesel engine works by closing the inlet valve before the piston reaches bottom dead centre. This has the effect of lowering the cylinder pressure as the piston continued downwards, as well as dropping the temperature of the air.
Although the engine is still doing work as the piston is descending on the inlet stroke, there is a saving in work during the compression stroke, and the maximum air temperature and pressure is reduced on compression. In camshaftless engines with variable valve timing, Miller timing is easier to achieve.
However, the earlier closing of the inlet valve reduces the amount of air in the cylinder and this clearly affects engine efficiency. This can be overcome with the addition of two-stage turbocharging Which can achieve compression ratios twice that of single-stage turbocharging. in which the exhaust gas passes first through the turbine of a high-pressure turbocharger before being led to a low-pressure turbocharger.
The charge air enters the system through the low-pressure turbocharger, passes through a cooler and the is further compressed in the high-pressure turbocharger before being again cooled and taken into the combustion chamber. A variation of miller timing can also be used for two-stroke engines with variable valve timing and two-stage turbocharging.
In some engines it may be possible to meet the NOx Tier III requirements through the use of Miller timing alone but in some others an additional means may be needed.
Reducing the combustion temperature is also the concept behind adding water to the fuel. This can be done in any of three ways: as an emulsion, direction injection or by humid air. Emulsified fuels have a further benefit in that the injected fuel is present in smaller agglomerations than when pure fuel is injected. This means that there is more complete combustion and the expansion of the water present adds to the power generated. This can result in a fuel saving which some claim could be as high as 5%.
Emulsified fuels do sometimes generate a white plume in the exhaust which gives the appearance of smoke but which is in fact harmless and innocuous water vapour. The vapour also softens any soot in the exhaust and helps to reduce PM emissions. One of the companies pioneering emulsified fuel equipment says that the reduction in NOx satisfies Tier II requirements but is insufficient to meet the more challenging Tier III requirements that came into force in January 2016.
However, the company concerned does say that when combined with SCR, the reduction in NOx in the exhaust places less demand on the SCR equipment reducing its operating and maintenance costs. A similar effect is expected if a SOx scrubber is installed since the prior removal of PMs makes that equipment more effective also.
Exhaust gas recirculation
The in-engine technique of EGR has been common in smaller road engines for some time and is now being rapidly adopted into marine engines. By re-circulating exhaust gas into the charge air, the oxygen content in the cylinder is reduced and the specific heat capacity increased. Both cause lower combustion temperatures and therefore fewer NOx emissions. If high sulphur fuel is to be used EGR can also be combined with an exhaust gas scrubber after the main engine exhaust receiver to achieve full SOx compliance in an ECA.
In a typical EGR system, a proportion of around 40% of the exhaust gas from the main engine exhaust receiver instead of being directed to the turbocharger is passed through a dedicated closed loop scrubber which removes PM and SOx which could cause engine damage and also cools the exhaust gas to be re-circulated. The re-circulated gases cause oxygen as O2 in the scavenge air to be replaced with CO2 which has a higher heat capacity than O2 and so helps reduce peak temperatures in the cylinder. The reduced O2 content in the scavenge air also reduces the combustion speed which further reduces peak temperatures in the cylinder and the cooler temperature reduces the formation of NO and therefore NOx towards Tier III compliance.
Tier III only applies when vessels subject to the rules are operating in ECAs that limit NOx emissions. When outside of such areas, the engines need only meet Tier II standards and this makes SCR a possibly attractive option. Presently EGR is a slightly more expensive option than SCR especially for smaller and mid size engines. The contaminated water from the scrubber must also be cleaned and the sludge generated disposed of ashore which usually involves an additional extra cost.
Selective Catalytic Reduction
SCR systems are arguably the more conventional way of reducing NOx as systems have been in place for several years on some vessels operating in Northern Europe particularly Norway where the NOx levy is in place.
In an SCR system a reducing agent (usually gaseous ammonia, aqueous ammonia or aqueous urea solution) is added into the stream of exhaust gas. The exhaust gases and reducing agent at a temperature of 300°C to 400ºC are absorbed onto a catalyst, upon which the nitrogen oxides are transformed on the catalytic surface into nitrogen (N2) and water (H2O). When urea is used then CO2 is also formed during the process. This was not an issue when the main purpose was to reduce NOx but it does become problematical when attempting to meet the EEDI requirements aimed at reducing CO2.
SCR is capable of removing up to 99% of the NOx which is comfortably in excess of the 80% reduction from Tier I levels required under Tier III. SCR systems are not fool proof. If the exhaust gas temperature is too high, the ammonia burns rather than forming a compound with nitric oxide. If it is too low, it forms ammonium hydrogen sulphate and gradually blocks the catalytic converter. The same happens if the sulphur content of the exhaust gas is too high. The minimum temperature required depends on the fuel’s sulphur content.
The catalyst in an SCR system consists of a ceramic carrier with the active catalyst an oxide of a metal such as tungsten or vanadium. SCR systems are separate from the engine and although leading engine makers are involved in their development, there are also independent suppliers.
Some of the companies producing SCR systems have formed a trade body known as the International Association for Catalytic Control of Ship Emissions to Air (IACCSEA).
SCR systems do have a relatively high capital cost and annual running costs to take into account. The catalyst will need replacing at intervals of around four to five years but because the catalysts are arranged in a layered system which allows for only damaged catalysts to be identified, removed and exchanged it is not necessary to replace the entire catalyst at the same time.
A limiting factor in the take-up of SCR beyond the fact that the need for them is really only just beginning has been the size and weight of the systems and the need to carry sufficient supplies of ammonia (normally in the form of urea). Even on the smallest ship type the reagent storage tanks would likely need to be 5m³ and on a large tanker, bulker or container ship possibly ten times larger than that. As regards the requirements of the NOx Technical Code, a ship fitted with an SCR system will need to also be fitted with continuous monitoring equipment to prove compliance.
Continual development to improve technology
The NOx Tier III levels came into effect for ships built from the beginning of 2016 meaning some ships have already needed to be compliant. However, this does not mean that the technologies employed are now mature and indeed some would argue that is very far from being the case.
The methods being used are in their early production stages and the intention is to refine them to reduce size, cost implications and robustness and reliability. In addition, the impending global reduction in fuel sulphur levels due in 2020 or 2025 will mean that exhaust gas cleaning systems capable of dealing with both NOx and SOx will need to be production ready in the not too distant future.
And as has already been mentioned, NOx reduction almost always results in increased CO2 production. Even with an SCR system that does not need a lower engine operating temperature and so does not reduce efficiency in that connection, extra weight and pumping systems are needed and lead to an increased power demand.
Alternative fuels can cut NOx
All of the technologies discussed above are based on ships continuing to burn oil fuels. It is possible to use fuels that do not produce NOx or do so in much smaller quantities. LNG is often proposed as the ideal solution to reduce NOx emissions and while it is true that the level of NOx from a gas burning engine is very low, it is only a solution for ships equipped with pure gas or dual fuel engines and for steam turbine powered LNG carriers.
While LNG has been used as a marine fuel since the early days of LNG carriers it is mainly in connection with steam turbine systems where the boil off gas from the cargo provides the fuel for the steam turbine boilers. Several LNG carriers have now been built with dual-fuel diesel engines and the number of other ship types has expanded quite rapidly. The take up of LNG is still behind early predictions but the advent of the IMO’s IGF Code which lays down international rules for gas-fuelled vessels has meant that fuel storage and delivery systems are accepted universally rather than by specific flag states. The take up is being further accelerated as an LNG bunkering infrastructure takes shape.
Aside from LNG, a series of large ethane carriers have been built with dual fuel engines able to burn ethane also supplied as a boil off gas from the cargo. This is very much a niche fuel but there is also interest and pioneering developments in using methanol and ethanol, the liquid alcohol derivatives of methane and ethane as marine fuels. There are issues to be overcome with these fuels if they are intended for wider commercial use because of fuel density issues which make them best suited for short voyages.
It should be noted that although the term LNG is equated with methane, the actual composition of LNG cargoes and potentially commercially supplied bunkers is variable and it is possible for an LNG cargo to contain high quantities of nitrogen and other chemicals. Because nitrogen has a much lower boil off point than methane it is likely that most nitrogen in a cargo will boil off early in the voyage and so NOx emissions will reduce as the voyage progresses.
As experience with LNG engines grows, it may be that some problems come to light. Some believe that in order to achieve maximum efficiency from a gas–burning engine it will be necessary to constantly monitor the fuel composition and adjust engine parameters accordingly. Equipment that can do this is not yet commercially available but is under development.
It is possible for some existing diesel engines to be converted to run on LNG but this is a major conversion and one that would have to be evaluated weighing up many factors. Thus far only one vessel – the feeder container ship Wes Amelie – has been so converted but others are said to be in the pipeline.
In anticipation of operators wishing to convert engines to dual-fuel versions, leading engine makers have for some time been developing engines with a high degree of modularisation. Most dual fuel engines have been based upon an existing diesel engine and it is therefore easier to convert one of the base diesel engines to a gas burner because of this. Conversion of older engines is probably not economically viable. It needs to be borne in mind that a converted engine will also need new fuel storage and conditioning systems to be installed.