• MARPOL Legislation – this section gives a brief breakdown of the new MARPOL Legislation 73/78 ANNEX VI as it applies to the Marine Industry and diesel engines.
• Diesel Emissions – this section is written to provide the reader with a basic understanding of diesel emissions and their impact on the environment.
• Emission Reduction – there are a number of techniques which have been proven to reduce diesel emissions, however, their success highly depends on the role and purpose, age, speed, etc. of the diesel engines fitted onboard individual ships. This section will hopefully be aid in understanding these techniques and how they are suited for individual applications.
• Gas Monitoring – Along with ANNEX VI of MARPOL 73/78 is the technique code, which provides the procedures for measuring diesel emissions. This section provides a brief outline of the technical code and the tools available for measuring diesel emissions.

• References – Further reading is essential to totally understand why legislation is necessary to reduce diesel emissions, how it will be enforced and what is the way ahead for ship builders and owners. These references should provide the reader with sufficient understanding of the diesel emissions dilemma and where used extensively in the writing of this paper.

• What does the MARPOL legislation mean?
• What restrictions does the new legislation impose?
• What is the technical code?
• What are diesel emissions?
• Why are diesel emissions a concern?
• What are the different techniques available for emissions reduction?

• How are the different techniques for emission reduction suitable for each application?
• How are emissions measured?
• What emissions measuring equipment is most suitable for?
• What are engine groups and families?
• What is future technology bringing about to further reduce engine emissions?
• What other changes to legislation can be expected in the future?

1. Introduction
1. Background

The International Maritime Organisation (IMO) recently adopted Annex VI to the International Convention for the Prevention of Pollution from Ships (MARPOL 73/78). In part, Annex VI sets the limits for NOx emissions, which will be applicable to ship's propulsion and auxiliary engines greater than 130kW. This regulation, yet to be ratified, will affect all new vessels constructed after 1^st January 2000. Furthermore, this regulation will also affect engines over 130kW, which undergo major conversion after 1^st January 2000.

The adoption of this new legislation has far reaching affects for all ship builders and ship operators. Diesel manufacturers and researchers have been investigating a variety of techniques in the aim of reducing diesel emissions as far as reasonably practicable. These techniques have been divided into three areas of study: pre-treatment, primary (internal) methods and secondary (after-treatment) methods. All these techniques have to some extent been successful in reducing engine emissions however, their effectiveness and the effect they have on the engines themselves may not be completely known or understood. Furthermore, the intended role and purpose of different ships many mean that some techniques may not be adaptable for a given ship design.

Although emissions legislation has been predominately aimed at reducing NOx, it is widely anticipated that in the 21^st century, all forms of emissions will have regulations imposed. It is therefore important to understand what these emissions are, and how they are formed to ensure that by adopting one form of technology to combat NOx will not have a detrimental affect on other emissions, which will incur additional costs further down stream.

2. Aim

The aim of this paper is to discuss the diesel engine emissions and the technologies and methodologies available to help reduce these emissions and the MARPOL legislation. This paper first discusses the various emissions from diesel engines and then addresses the methods available to combat them. This paper is written as an informative guide and attempts to address those questions typical of a ship builder or operator who may wish to understand offered proposals to meet MARPOL legislation. Engine manufacturers will make these proposals in the first place, as only new engine builds and serious engine modifications would be considered. Most engine manufacturers are now able to offer engines, which conform to the IMO limits, however it remains useful to understand the mechanisms, employed.

3. Scope

• MARPOL Annex VI Legislation - section two of this paper discusses in some length what the new legislation entails and discusses ratification of the new legislation.
• Certificates of Compliance - this is also briefly discussed in section 2 of this report. Details of obtaining certificates of compliance are also discussed later in this paper.
• Emissions - the types of emissions from diesel engines, and their effect on the environment, are discussed in section 3 of this report.
• Emission Reduction Techniques - understanding what emissions are and what is being legislated will go a long way in understanding the techniques available to diesel manufacturers to combat emissions. These are discussed at length in section 4 of this paper along with the merits and demerits of each technique.
• Emission Testing - at some point, diesel manufacturers and ship owners will have to demonstrate that their engines are emission compliant. The emission testing procedure is described briefly in section 5.
• Emission Testing Equipment - continued on in section 5 is a brief list of equipment, which is acceptable for determining diesel emissions.

1. MARPOL Legislation

1. 17.0 g/kWh when the maximum engine speed is less than 130 rpm;
2. 45.0*n^(-0.2) g/kWh when the maximum engine speed (n) is more than 130 but less than 2000 rpm; and
3. 9.8 g/kWh when the maximum engine speed is greater than 2000 rpm.

1. Ratification

Although IMO have adopted the new Annex VI to the International Convention for the Prevention of Pollution from Ships, the Annex will not go into force until it is ratified by the participating nations.

Annex VI specifies that the Annex will not come into force until twelve months after the date on which not less than fifteen member states have signed up to the act. The total combined gross tonnage of those members who have signed up to the act must represent no less than 50 percent of the gross tonnage of the world’s merchant shipping. There is no time limit for this to occur so it could happen relatively quickly or take many years to be ratified, however, the best estimate is that ratification will occur by 2003.

2. Engine International Air Pollution Prevention Certificates

Once Annex VI of MARPOL 73/78 has been ratified, engine manufacturers will have to demonstrate that their engines meet the new guidelines. This will be accomplished by obtaining an Engine International Air Pollution Prevention (EIAPP) Certificate. These certificates will be issued either for individual engines or for an engine family or engine group after compliance with the NOx Technical Code is demonstrated. It is likely that the testing for diesel engine emissions will be delegated to classification societies. Full engine testing procedures are provided later on in this article.

3. Interim procedures

As it many take several years for Annex VI to be ratified, the Marine Environmental Protection Committee (MEPC) has issued Interim Draft Guidelines for the Application of the NOx Technical Code. This interim procedure is completely voluntary but will allow engine manufacturers to demonstrate that their engines installed on ships after 1^st January 2000 are NOx compliant prior to ratification of Annex VI. The interim programme will allow engine manufacturers to obtain a Statement of Compliance for their engines, which, it is envisaged, will be taken into account when EIAPP certificates are issued on ratification of the Annex.

4. Effect on Ship Builders and Owners

1. Emissions
1. Background

The major pollutants in diesel exhaust emissions are a direct result of the diesel combustion process itself. In general, the major components are as shown in Figure 3.1. While the type of fuel used plays a major part in determining the composition of the emissions, an important factor that determines the amount of NOx, is engine speed (the real factor is residence time). In way of comparison, Table 3.1 demonstrates typical emissions values for low and medium speed engines.

	Pollutant       NOx CO HC CO2 SO2	Medium Speed Engines (g/kWh) 12.0 1.6 0.5 600 3.6x%S where S = content	Low Speed Engines (g/kWh) 17 1.6 0.5 600 3.6x%S sulphur (%m/m)
Figure 3.1: - Typical emissions from low-speed diesel engine	Table 3.1:- Indicative emission comparison between low and medium speed diesels engines.

 

Each of these emissions will be discussed in turn in the following paragraphs and are summarised in Table 3.2.

2. Sulphur Oxides

The formation of Sulphur Oxides (SOx) in exhaust gases is caused by the oxidation of the sulphur in the fuel into SO₂ and SO₃ during the combustion process. As indicated in Table 3.1, the amount of SOx formed is a function of the sulphur content of the fuel used and therefore the only effective method of reducing SOx is by reducing the sulphur content of the fuel. Unfortunately, low-sulphur fuels are more expensive to purchase (10 to 20% greater cost, when switching from 3.5% to 1% sulphur) and there is a practical lower sulphur limit desired as desulphurisation of fuel lowers the lubricity of the fuel which can lead to increase wear on fuel pumps and injectors.

The regulation of SOx is predominately a regional issue. However, international pressure is growing for the oil producers to reduce the sulphur content of all fuels in order to control this problem at the source. The current EU Directive, which applies to all gas oil sold on land in the EU, is that the % sulphur content of fuels must remain below 0.2% with the aim of reducing this limit to 0.1% by the year 2008. Presently, most military navies use 1% low-sulphur fuels or lower. Special Areas have been set up, such as the Baltic, where the use of low sulphur fuels will be mandatory when Annex VI is ratified and will be limited to 1.5%.

If required, desulphurisation of diesel exhaust gases can be achieved by wet scrubbing. The flue gas is first passed through a quencher where it is cooled down to saturation temperature. The SOx is subsequently washed out with a neutralising agent (calcium bound in lime-milk or seawater) in a scrubber, however additional costs are incurred in disposing of the scrubbing products.

SOx formed from diesel exhaust is corrosive and in part is neutralised by an engines lubricating oil which is typically base. In the atmosphere however, SOx combines with moisture to form H₂SO₄, which then falls as acid rain, and has been linked to environmental damage.

3. Carbon Dioxide

CO₂ is one of the basic products of combustion and although diesels are one of the most efficient engines for the combustion of fossil fuels, the only way to reduce CO₂ is to either reduce the amount of fuel burned or by increasing thermal efficiency.

Diesel engines currently meet the CO₂ guidelines, however meeting stricter regulations on the permissible production of CO₂ is theoretically possible, but practically achieving these standards would be difficult. CO₂ is not toxic however, has been linked to the 'greenhouse effect' and global warming.

Alternative low carbon to hydrogen ratio fuels are already or will soon be used onboard ships, however, it is only a viable solution, if the ship sails with cargoes containing these fuels (i.e. LNG carriers) or cargoes where parts of the cargo evaporates in the form of volatile organic compounds (VOC) as, for instance, on shuttle tankers and crude oil carriers. The VOC gasses can be gathered and used as fuel in VOC diesel engines.

Finally, although CO2 is a concern with respect to the 'greenhouse effect', CO₂ emissions from Marine Transport, in terms of power produced, is the most favourable when compared with the Transport of goods world-wide by land, rail or air.

4. Carbon Monoxide

CO is formed due to the incomplete combustion of organic material where the oxidation process does not have enough time or reactant concentration to occur completely.

In diesel engines, the formation of CO is determined by the air/fuel mixture in the combustion chamber and as diesels have a consistently high air to fuel ratio, formation of this toxic gas is minimal. Nevertheless, insufficient combustion can occur if the fuel droplets in a diesel engine are too large or if insufficient turbulence or swirl is created in the combustion chamber. This will also be accompanied by a large increase in particulates (i.e. black smoke).

5. Hydrocarbons

The emission of unburned hydrocarbons (HC) generally results from fuel, which is unburned as a result of insufficient temperature or air supply. This often occurs near the cylinder wall (wall quenching) where the temperature of the air/fuel mixture is significantly less than in the centre of the cylinder. Bulk quenching can also occur as a result of insufficient pressure or temperature within the cylinder itself. Still further, HC production may also be a result of poorly designed fuel injection systems, injector needle bounce, excessive nozzle cavity volumes or fuel jets reaching a quench layer. Lubricating oil vapours also contribute to hydrocarbon emissions.

While HC emissions from diesel engines are generally within acceptable limits and are not included in the MARPOL legislation. HC reduction, for the most part, can be achieved by good engine design; however further reduction would most likely only be possible using secondary oxidation catalysts.

6. Smoke/Particulates

1. agglomeration of very small particles of partly burned fuel (HC);
2. partly burned lub oil (HC);
3. ash content of fuel oil and cylinder lub oil; or
4. sulphates and water.

1. Nitrogen Oxides

While SOx is predominately a regional issue, NOx is a global issue and the new MARPOL regulations will have a significant impact for ship owners and ship builders.

NOx is formed during the combustion process within the burning fuel sprays and is deemed one of the most harmful to the environment and contributes to acidification, formation of ozone, nutrient enrichment and to smog formation, and has become a considerable problem in most major cities world-wide.

The amount of NOx (ppm) produced is a function the maximum temperature in the cylinder, oxygen concentrations, and residence time. At cylinder temperatures, nitrogen from the intake air and fuel becomes active with the oxygen in the air forming oxides of nitrogen. Increasing the temperature of combustion increases the amount of NOx by as much as 3 fold for every 100^oC increase. NO is formed first in the cylinder followed by the formation of NO₂ and N₂O, typically at concentrations of 5% and 1%; respectively. NO is completely converted to NO₂ in the atmosphere within a few hours and being soluble is washed out by rain, which increases the acidity level of the soil.

The best way to reduce NOx generation is to reduce peak cylinder temperatures and there are a number of ways that this can be done. Without additional design changes, most of the methods cause a certain loss in engine efficiency which increases the engines sfc.

2. Summary

Emission	Legislated by IMO	Source
SOx	Ö	Function of fuel oil sulphur content
CO2	X	Function of combustion
CO	X	Function of the air excess ratio, combustion temperature and air/fuel mixture.
HC	X	Very engine dependant but a function of the amount of fuel and lub oil left unburned during combustion.
Smoke/Particulates	X	Originates from unburned fuel, ash content in fuel and lub oil.
NOx	Ö	Function of peak combustion temperatures, oxygen content and residence time.

1. NOx Emission Reduction Techniques
1. Background

There has been considerable research on methods for reducing emissions from diesel engines. The majority of this research has been centred on reducing NOx emissions, and will form the basis of further discussion in the following paragraphs. Nevertheless, the methodology in reducing other forms of emissions will also be discussed.

Competition by diesel manufacturers has meant that there are several viable solutions in achieving a NOx compliant engine, however, there must always be a monetary penalty for reducing NOx. The ship builder or owner must be aware that the final solution in achieving NOx reduction may be totally unique to the ship application and may be based on such things as operating profiles, water stowage facilities, electrical generation capabilities, etc. to name a few. This section of the report will clarify the issues surrounding the various NOx reduction techniques.

2. Classification of NOx Reduction Technologies

NOx reduction technologies can be divided into three basic categories namely: pre-treatment, internal measures and after-treatment. Pre-treatment methods are concentrated on lowering the adiabatic flame and/or combustion temperature by treatment or use of alternative fuels. Internal measure or primary methods alter the engine configuration to, in some form or another, alter the combustion process. After-treatment or secondary methods are fitted externally to the engine and are applied directly to the combustion gases. These methods are demonstrated in Figure 4.1. The various methods available to reduce NOx concentrations in exhaust gases is summarised in Table 4.2 at the end of this chapter.

3. Pre-treatment

• Denitration of fuel
• Using alternative fuels
• Water addition to fuel (water emulsification)

1. Denitration of fuel

Most of the fuel bound nitrogen becomes NOx. For each 0.1% nitrogen in the fuel, 0.6 g/kWh of NOx is produced. Diesel fuels has normally 0.1% N, while residual fuel oil has a nominal value of 0.3 - 0.4%. It can be seen therefore that by removing some of the nitrogen from the fuel, some NOx will be eliminated from the combustion gases. Unfortunately, there is no practical method of removing nitrogen from the fuel available within reach of industry.

2. Alternative Fuels

There are at this stage two alternative fuels under investigation for use in automotive or stationary diesel engines namely, methanol and liquid petroleum gas (LPG). Methanol has been the subject of much research over the last few years. Methanol does not contain any sulphur and therefore, SOx from emissions is completely eliminated. By combining methanol and EGR, NOx can be reduced as much as 50%. Nevertheless, all this does not come without a penalty and the reduction of this penalty has been the focus of the research, which is currently underway.

Methanol has bad ignition qualities and is corrosive in nature. The absence of sulphur means that the lubricity of this fuel is very low. Additionally the use of methanol would require modification to engine injection system. Methanol is a more expensive fuel than distillate and would also incur additional costs in modifications to fuel storage tanks and for the need for leak detection systems. There will also be large-scale logistic problems associated with the stowage and the obtaining of this type of fuel in some international ports.

The use of LNG and LPG, on the other hand, is well advanced and in place at many generating power plants and LNG and LPG tankers. The LNG is also low sulphur and combined with the use of pilot injection can reduce engine NOx emissions by 60%. Nevertheless, the major problem of storage on other than LNG ships would constitute a significant problem, which has yet to be overcome. As a conservative industry, ship builders and owners would not want to be compromise or be perceived as compromising safety on board their vessels.

3. Emulsified Fuel (Pre-treatment)

1. Primary Methods

• Modification of combustion
• Modification of scavenge/charging air
• Water injection
• Exhaust gas re-circulation
• Humid Air Motor

1. Modification of Combustion

• Injection timing retardation
• Increase in injection pressure
• Modification of Compression ratio
• Optimisation of induction swirl
• Modification of injector specification
• Change in number of injectors
• Pre-chamber type of combustion
• Modification of shape of combustion chamber.

1. Injection timing retardation/ rate modulation

Retarding the injection timing is one of the simplest techniques for reducing NOx. The effect of injection retardation is to reduce the maximum combustion pressure and hence temperature. By using this simple technique, a reduction of up to 30% can be achieved however, a penalty of up to 5% in sfc results, as the engine efficiency is reduced (Figure 4.4).

Figure 4.4: - Effect of fuel injection retardation on NOx generation

Alternatively, rate modulated injection can be used to smooth the cylinder pressure rise by adjusting the rate the fuel is injected into the combustion chamber. Fuel consumption is not affected as significantly with rate modulation but is typically, more effective in medium/high speed diesels at part load than at high loads.

With either of these methods, considerable component redesign (such as camshafts, electronic injection, etc) may be required and are therefore, more suitable to new engine designs.

2. Increase of Injection Pressure

Increasing the injection pressure on its own does not have any effect on reducing NOx. It is generally combined with other NOx reduction techniques such as injection retardation to reverse the fuel consumption penalty. Increasing Injection Pressure leads to better atomisation of the fuel and therefore a reduction in Particulates and CO. Since combustion is cleaner, the kernel temperature will be hotter, which will in fact increase NOx. Therefore, this method must be combined with other modifications to reduce NOx generation.

Increasing Injection Pressure requires stronger injection equipment at a potentially increased cost.

3. Modification of Compression Ratio

Increasing the compression ratio increases the maximum cylinder pressure and thus cylinder temperature and NOx. Therefore, in order to reduce NOx, this method must be combined with other NOx reduction techniques such as injection retardation. With injection retardation, a NOx reduction of 10 to 30% can be achieved with little penalty to sfc, if the maximum pressure is kept constant through the combination of the reduction measures.

The purpose of increasing the compression ratio would be to overcome some of the efficiency loss due to injection retardation and therefore decrease the sfc. Increasing the compression ratio but keeping p_max constant, by retarding the injection timing or reducing charge air, gives a very good trade-off between NOx and sfc. The idea behind this is to obtain constant pressure combustion, thereby reducing after combustion, which in practise leads to high local temperatures in the cylinder and therefore NOx.

Increasing the compression ratio has no real effects on engine price provided this is done during the engine design phase.

4. Optimisation of Induction Swirl

The optimisation of Induction Swirl improves the combustion process by assisting air/fuel mixing and again will not in itself reduce NOx. Thus this method must be combined with other techniques to achieve NOx reduction. The benefit is that there is no real additional cost associated with this technique. There is presently great debate as to the benefits of swirl with respect to NOx reduction. Nevertheless, the latest accepted opinion is that the piston crown must be designed for maximum local turbulence while ensuring that the turbulence does not impinge on the cylinder walls thereby increasing localised cooling.

5. Modification of Injector Specification

Changing the fuel nozzle design has proven to have significant impact on the formation of NOx, which can be reduced as much as 30% by optimising the spray pattern of the fuel within the combustion space. Recently much attention has been given to mini-sac type nozzles and slide valve which can reduce NOx emissions by 30%, but also have a considerable impact on the reduction of HC, CO, smoke and particulate emission. Table 4.1 demonstrates the effect on nozzle and fuel valve design on emissions generation. It should be noted that the fuel spray pattern for the slide fuel valve has been optimised for lower NOx emission than the standard and 6 hole nozzles. Care has also been taken to maintain low combustion chamber wall temperatures. Various fuel injectors are shown in figure 4.5.

Table 4.1	NOx	CO	Smoke	DSFOC
	ppm/15%/O2	ppm/15%/O2	BSN	g/bhph
Standard valve nozzle	1594	109	0.35	0.0
6-hole nozzle	1494	108	0.23	+0.4
Slide type fuel valve	1232	87	0.18	+1.8



Figure 4.5: - Modified fuel injectors (courtesy of MAN B&W)

6. Change in Number of Injectors

There has been some research into increasing the number of injectors per cylinder. Increasing the number of injectors enables the combustion process to be better controlled and therefore more efficient combustion. It has been reported that a decrease in NOx from 30% is achievable, however there is a cost penalty associated with the need to have additional injectors, piping and associated equipment. As well as additional equipment, a significant increase in maintenance costs can be expected.

7. Pre-chamber Type of Combustion Chamber

1. Modification of Air Intake System

Modification of the Air Intake System can take the form of either the modification of scavenge/charge-air cooling or the modification of the scavenge/charge air pressure.

1. Scavenge/Charge Air Cooling

Theoretically, providing cooler inlet air can lower the amount of NOx generated during combustion. Tests have shown that a 14% reduction is possible by lowering the scavenge temperature from 40 to 25^oC. However, the success of this method is greatly dependent on atmospheric and seawater conditions. Furthermore, the scavenge air temperature for low and medium speed engines is already fairly low and limited by the available cooling water temperature and therefore, this method is only suitable for high-speed diesels.

Improved cooling of the engine scavenge/charge air would increase the cost of an engine due to the additional water supply and cooler requirement. Also, cooling the air inlet temperature too much could lend itself to combustion stability problems.

2. Increasing the Scavenge/Charge Air Pressure

• If injection timing is retarded and compression ratio is kept constant the peak pressure will drop and sfc will increase substantially.
• If injection timing is retarded and compression ratio is increased to reach original peak pressure in the cylinder while charge pressure is kept constant, the sfc will slightly decrease.
• If injection timing is retarded and increase of charge pressure is used to reach original peak pressure while keeping compression ratio constant the sfc will increase.
• If injection timing and compression ratio both are kept the same an increase in charge pressure will cause a decrease in sfc.

1. Water Injection

Water injection (Figure 4.6) involves adding water directly into the cylinders during combustion through a special injector. As with emulsion fuel, NOx reduction of up to 40% is achieved by reducing the bulk temperature of combustion but has an advantage in that problems caused by ignition delay at part load may be avoided. Separate pumps for the fuel and water are needed along with modifications to the fuel delivery lines and injectors to accommodate a water/fuel ration of around 0.8. Potential drawbacks are higher engine costs and the potential for corrosion problems.

Figure 4.6: - Direct Water Injection

Stratification of water in fuel has also been investigated. This technique involves injecting water using the same injector as the fuel. Although this eliminates the need for an additional water injector, control of the altering the fuel/water injection process requires more complex electronic fuel injection equipment.

2. Exhaust Gas Re-circulation

Exhaust Gas Re-circulation (Figure 4.7) has proven to be successful at reducing NOx levels due to the higher specific heat capacities of the principal exhaust components (CO₂ and H₂O) in comparison with air. The overall effect is to reduce peak cylinder temperatures. The drawbacks with EGR include increased smoke and particulate levels and increased engine costs and increased engine wear when operating with high sulphur fuels. Furthermore, the exhaust gas must be cleaned from particles and sulphur, before entering the compression and scavenge air cooler. Reliable methods for exhaust gas cleaning are still under investigation. However, inserting a particle trap in the exhaust gas re-circulation path can reduce smoke and particulates.

On two stroke engines, scavenge efficiency is less than 100% and can be lowered to retain exhaust gases within the combustion chamber. This has the effect of achieving internal engine EGR without the need for external devices.

Figure 4.7: - Exhaust Gas Re-circulation

3. Humid Air Motor

1. Secondary Methods

Secondary, or after-treatment, is centred on treating the engine exhaust gas itself either by re-burning the exhaust gas or passing it through a catalyst or plasma system. There has been much development in selective catalytic reduction (SCR) and non-thermal plasma (NTP) systems over the last few years. Secondary methods, however, require a step change in capital cost, maintenance and through-life costs over primary methods.

Three way catalysts, such as those used on Petrol engines, can not be used, as diesel engines are non-stoichiometric engines

1. Re-burning

This technology consists of using fuel as a de-oxidiser into the exhaust system. Fuel is re-introduced into the exhaust gas, which is then re-heated in a boiler but at significant less temperature than the combustion within the diesel itself. Using this method significantly reduces NOx, however, the thermal efficiency of the cycle is significantly less than the diesel itself. Furthermore, there will be a significant increase in cost and space requirements.

2. Selective Catalytic Reduction

The Selective Catalytic Reduction (SCR) method makes use of the that, using a catalyst, NOx can be converted into nitrogen and water by reaction reducing agents such as ammonia (NH₃) or Urea (CO(NH₂)₂).

NOx + NH_{3 ®} N2 + H₂O

NOx + CO(NH₂)₂ ® N2 + H₂O + CO₂

The SCR technology is today installed and in operation on both low and medium speed diesel engines in the marine environment. Currently the most critical problems inherent with the SCR system for implementation in the marine environment are the investment and operation costs, the space problems for the catalyst elements and storage of either ammonia or Urea.

Besides this, minor disadvantages such as toxicity of ammonia, impact on engine performance during engine load changes, and ammonia slip during transient responses can be eliminated by proper design aof the combined engine and SCR system. The reward of the SCR system is up to a 95% reduction in NOx emission and it is yet the only solution that has been implemented to meet strict limits on the allowable NOx emission.

The reducing agent and sulphur from the fuel may react and create Ammonia-Sulphate at low exhaust gas temperatures. Ammonia-Sulphate can clog the SCR, if the engine runs at low loads over an extended period. Therefore, the injection of reducing agent, and thereby the NOx reduction, may need to be stopped for safety purposes at low engine loads. Alternatively, fuels with a low sulphur content may be used on engines fitted with SCR.

Superimposing the numerous primary methods available with SCR, does not have a significant effect on the total amount of NOx emission. However, it can reduce the consumption of reducing agent and thereby the operating costs, although some studies indicate a positive impact on cost. This depends on the specific engine configuration and requirement.

 

Figure 4.8: - A Selective Catalytic Reduction System (After turbocharger arrangement, and only feasible for fuels with low sulphur content)

3. Plasma Reduction Systems,

Plasma is a partially ionised gas comprised of a charge of neutral mixture of atoms, molecules, free radicals, ions and electrons. Electrical power is converted into electron energy and the electrons create free radicals, which destruct pollutants in exhaust emissions. The plasma is reactively hot but thermally cool which means that after treatment, little heating, if any, of the exhaust gas results.

Figure 4.9: - A schematic of a Non-Thermal Plasma System.

The production of viable technology for the creation of non-thermal plasma (NTP) at atmospheric pressure has been underway for a number of years, and to date, systems have been developed for incinerator flue gas clean-up, waste solvent treatment, air filtration, UV waste water treatment. Currently, this technology is undergoing further development for diesel engine exhausts. A schematic of a NTP System is shown in figure 4.9.

The prototype solution for diesel exhaust after-treatment is based on a surface discharge and can be assumed to be an electrically augmented catalyst. The plasma is generated using an alternating high voltage to breakdown the gas between two electrodes. The region between the two electrodes is packed with a material resulting in voltage breakdowns in the voids between the material. The duration of the voltage breakdowns is only of the few nanoseconds. A non-thermal plasma is produced which, augmented by the catalyst, breaks down the exhaust emissions.

Figure 4.10: - NOx Reduction Performance of Non-Thermal Plasma

Although the NTP System is still in is prototype phase for marine use, production costs should be relatively low cost. The system is compact and extremely flexible in terms of size and shape. Experiments to data have shown that a NOx reduction of up to 97% is achievable. When compared against the ISO 8178 C1 cycle, the NOx reduction performance of Figure 4.10 was achieved.

2. Summary

1. Exhaust Gas Monitoring Techniques
1. Introduction

To obtain an Interim Certificate of Compliance (called "Statement of Compliance"), - before the coming into force of Annex VI, an Engine International Air Pollution Prevention Certificate, the engine manufacturer must combine his engines into an engine group or an entire family. An engine from this group or family, representing the worst case of the group, i.e. adjustments/settings within the possible range leading to the highest NOx emission, is then selected for emissions testing. Alternatively, individual engines are to be tested. The tests to be conducted on this engine must meet the MARPOL Annex VI NOx technical code.

The easiest way to check the correct adjustment of the engine to meet the IMO requirement is the parameter check method.

This section discusses the types of emissions test equipment being used today and then goes on to discuss the ISO 8178 test cycles and emission measurement procedures.

2. Evaluation Systems

Emissions evaluation equipment can be divided into two categories, either extractive or non-extractive.

1. Extractive Systems

Extractive systems are permanently installed and require additional equipment to process the exhaust gas sample. The cost of extractive systems can very significantly however, they have the advantage of being able to be remotely located in a controlled environment, easier to operate, calibrate and maintain. Furthermore, extractive systems can be set up to monitor exhaust gas emissions from more than one engine.

2. Non-extractive Systems

Non-extractive systems predominately use infrared or ultra-violet techniques. The advantages to these types of systems are that they tend to be more portable and provide more rapid responses. However, these systems tend to be difficult to calibrate. Non-extractive systems measure the exhaust gas emissions without extracting the exhaust gas from the uptake system.

3. Chemiluminescence

Chemiluminescence or HCD (Heated Chemiluminescence Detector) is considered the accepted standard for laboratory and test cell measurement of NOx and was, in fact, the NOx Technical Code allows only the use of CLD/HCLD for NOx measurement.

Chemiluminescence analysers need to have a continuous supply of clean dry air, otherwise, damage to the analyser components will result. If installed onboard, instrument air, if available, can be used for this purpose.

If in the future should alternative methods should be adopted, the NOx Technical Code must first to be modified by IMO.

4. Infrared Analysers

Non-dispersive infrared (NDIR) analysers have been used and many will satisfy the NOx technical code requirements. The NDIR are considered non-extractive devices and are capable of measuring CO and CO₂. Although these devices allow for rapid response and therefore quick emissions measurement, they are difficult to calibrate.

5. Ultra-violet

The UV analysers are particularly useful for measuring SO₂ in the exhaust gas and come in extractive and non-extractive devices. UV analysers are not suitable for the measurement of NOx.

6. Test Procedures

An international test standard (ISO 8178) has been developed for all non-road diesel engines, which incorporates gaseous, and particulate emissions measurement. IMO has adopted these standards, if applicable to marine engines. The relevant parts are included in the NOx Technical Code. Air, cooling water temperature, air pressure and humidity all influence emissions and therefore the test standard allows correction to be incorporated to allow for this. The correction formulae are however, by no means fixed or certain.

The ISO Standard consists of ten parts as shown in Table 5.1 as is centred on providing engine manufacturers with standard procedures to enable them to achieve a Certificate of Conformity for their engines.

The ISO Standard standardises the test cycles to be used and the procedures for determining the emissions themselves.

Document	Title
Part 1	Test bed measurement of gaseous and particulate exhaust emissions
Part 2	Measurement of gaseous and particulate exhaust emissions at site
Part 3	Measurement of exhaust gas smoke under steady state conditions
Part 4	Test cycles for different engine applications
Part 5	Test rules
Part 6	Test report
Part 7	Engine family determination
Part 8	Engine group determination
Part 9	Test bed procedure for the measurement of smoke from off-road diesel engines
Part 10	In-use procedure for the measurement of smoke from off-road diesel engines

Table 5.1: - ISO 8178 Documentation

7. Test Cycles

Before commencing the emission tests, it is important to determine the test cycle as this determines the test cycle to be used. There are 15 difference test cycles of which only four can be readily applied for marine diesels. These test cycles are broken down as follows:

Test Cycle E2 is used for all constant speed marine engines for ship main propulsion including variable pitch propeller sets and electric propulsion.

 

 

 

	Speed	100%	100%	100%	100%
Test cycle type E2	Power	100%	75%	50%	25%
	Weighting Factor	0.2	0.5	0.15	0.15

Table 5.2: - The E2 Test Cycle

Test cycle type E3 is used for propeller law operated main and auxiliary engines.

	Speed	100%	91%	80%	63%
Test cycle type E3	Power	100%	75%	50%	25%
	Weighting Factor	0.2	0.5	0.15	0.15

Table 5.3: - The E3 Test Cycle

Test cycle type D2 is used for constant speed auxiliary diesel engines.

	Speed	100%	100%	100%	100%	100%
Test cycle type D2	Power	100%	75%	50%	25%	10%
	Weighting Factor	0.05	0.25	0.3	0.3	0.1

Table 5.4: - The D2 Test Cycle

Test cycle type C1 is used for variable speed, variable load auxiliary engines. The intermediate speed for this test cycle is determined by the manufacturer but generally falls within 60 to 75% of the rated speed.

	Speed	Rated				Intermediate		Idle
Test cycle type C1	Torque	100%	75%	50%	10%	100%	75%	50%	0%
	Weighting Factor	0.15	0.15	0.15	0.1	0.1	0.1	0.1	0.15

Table 5.5: - The C1 Test Cycle

8. Engine Families and Groups

As engine manufacturers have a variety of engines ranging in size and application, the NOx Technical Code allows the organising of engines into families or groups. This will reduce the amount of individual engine testing necessary to obtain Certification. The subdividing concept allows engines that are mass-produced to be divided into engine families and those which are built in small numbers to be divided into groups. By definition, an engine family is a manufacturer’s grouping which through their design, are expected to have similar exhaust emissions characteristics i.e., their basic design parameters are common. When testing an engine family, the engine, which can be expected to develop the worst emissions, is selected for testing.

The engine group concept has been derived for large marine diesels, which are not mass-produced. By definition, an engine group is a manufacturer’s grouping of engines that require adjustment or modification to ensure that they comply with the emissions standards and perform to these standards on site.

2. Acknowledgements

The authors would like to thank the members of the CIMAC Committee for their assistants in writing this paper. In particular, the authors would like to thank MAN B&W, Wärtsilä NSD Corporation and AEA Technologies for their contributions, which include the various figures and illustrations included in this paper and Kittiwake Developments Ltd for providing assistants in proofing this paper.



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1. About the Authors