A Review and Economic Analysis of Different Emission Reduction Techniques for Marine Diesel Engines

The maritime industry is currently facing the challenges of adopting new technologies and operational practices with stricter international, national and local rules in order to reduce exhaust gas emissions from ships. The most objective of regulations introduced and presented by the Worldwide Sea Organization such as International Maritime Organization (IMO) and the US Environmental Protection Agency (EPA) is to lessen the commitment shipping makes to worldwide and local discharges. This paper analyzes emissions from marine engines and the process of waste exhaust gas formation and provides a summary of the emission reduction technologies to satisfy MARPOL NOx tier III and EPA tier IV rules. The results showed the possibility of achieving a valuable emission reduction percentage if future diesel engines are equipped with pre-treatment, internal-treatment and/or post-treatment techniques. Economics impact for medium and low speed for category 3 marine diesel engines is also presented.


Introduction
In recent years, marine diesel engine manufacturers have had to address the challenge of tightening controls on baneful exhaust gas emissions obligatory by regional, national and international authorities responding to concern over atmospheric pollution and its impact on human health and climate amendment. Open Journal of Marine Science International Maritime Organization (IMO) regulations concerning gas oxides had planned to accomplish emissions reduction emitted by diesel engines (DE) through three tiers [1].
Tier I for vessels launched after first of January 2000, tier II, for vessels launched after first of January 2011 with 15% exhaust gas emissions reduction of tier I, while tier III for vessels launched after first of January 2016 with 85% exhaust gas emissions reduction of tier I [2]. The current allowable NO x discharge level as per IMO control relies upon the speed class of the engine and ranges [3].
Besides, the regulations for the prevention of Air pollution from vessels (Annex VI) additionally forced a 1.5 percent Sulphur limit on marine power engines in Emission Control Areas (ECAs) viable since 2006, this limit decreased to 1.0% Sulphur compelling from first of July 2010 and was additionally diminished to 0.1% Sulphur starting on January 2015 [4]. Furthermore, in May 2004, the US Environmental Protection Agency (EPA) signed the final rule introducing Tier IV emission standards, which are phased-in over the period 2008-2015. The tier IV rules necessitate that outflows of particulate matter (PM) and NO x be additionally decreased by 99% [5]. Figure 1 shows the emissions regulations  Figure 2). Moreover, in order to achieve the level of emission control required to meet tier IV interim and tier IV final rules, some form of exhaust after-treatment will be required. The structure of the present paper is as follows. Section 2 presents the main pollutants from marine diesel engines, particularly nitrogen oxides (NO x ), Sulphur Oxides (SO x ) and smoke in general. In Section 3, NO x and SO x emissions control technologies such as internal-treatment and add-on technologies are presented. In Section 4 we evaluate the economic impact of the selected technology for category 3 marine DEs in order to determine the estimated cost of compliance with potential future emission regulations. Finally, Section 5 provides a conclusion of our study.   medium speed DE burning heavy fuel oil with a mean of 3 percent Sulphur content [8].

Exhaust Gas Emissions from Marine Diesel Propulsion Engines
According to Figure 4, the principle emissions are nitrogen (N 2 ), Oxygen (O 2 ), water (H 2 O) and carbon dioxide (CO 2 ). Among these gases, carbon dioxide is considered as potential source for greenhouse gas (GHG) development. Different studies have evaluated that 3 to 4 percent of universal CO 2 emissions are attributable to marine vessels [10]. A 10 percent reduction in cruise speed is an effective operational methodology that offers a significant reduction in CO 2 and fuel consumption up to 20 percent over the same distance [11]. Below are described the main pollutants emitted by a marine diesel engine which contribute to air pollution and human health risks.

Nitrogen (N2) and Oxygen (O2)
Nitrogen and Oxygen are not poisonous. N 2 and O 2 are the main components of both air intake and exhaust emissions from engines. Nitrogen which forms 78 percent by volume of intake air, mostly do not react during the combustion operation, despite a very small portion will react chemically creating diverse oxides Open Journal of Marine Science of nitrogen (NO x ). O 2 which forms 21 percent of the intake air, will be moderately transformed by the combustion operation, therefore, the free O 2 of the exhaust will depend on the excess air ratio along which engine is operated [12].

Carbon Dioxide (CO2) and Water Vapor (H2O)
The production of both carbon dioxide and water vapor depends on the quantity of fuel burned and the fundamental composition of the fuel burned. CO 2 and H 2 O vapor will be created in all combustion forms such as completed or nearly completed. Carbon dioxide is considered as a GHG that absorbs and transmits radiation inside the thermal infrared spectrum length [13].

Micro Pollutants
In the context of DE emissions, micro pollutants will enclose organic and heavy metals micro pollutants. Organic micro pollutants generally consist of dioxins, furans and polyaromatic hydrocarbons (PAH). However, PAH is well identified and known as a carcinogen for humans and is well documented. Furthermore, polychlorinated dibenzofurans (PCDF) has been reported as highly toxic and has been identified in the engine exhaust system. Heavy metals such as copper, mercury, cadmium, chromium, nickel and zinc are highly toxic and can cause serious damage by reducing the diversity of aquatic ecosystems, over fish kills to cancer in humans.

Sulphur Oxides (SOX)
Sulphur oxides are related precisely from the Sulphur content of the used fuels.
Oxidation of the Sulphur inside the combustion chamber allows the formation of the Sulphur dioxide (SO 2 ) and a smaller proportion of the Sulphur trioxide (SO 3 ). SO x have bad odor and are a bigger source of acid rain. SO x represent around 60% of universal transport SO x emissions and symbolize dangerous source to human health. New strict regulations to reduce SO x emissions are adopted recently such as the regulation 14 of the MARPOL Annex VI which imposes Sulphur limitations to the fuel used for marine applications.

Oxides of Nitrogen (NOX)
The development of nitrogen oxides is caused by the oxidation of molecular nitrogen in the combustion air or by the oxidation of organic nitrogen in the fuel.
Depending on the fuel used, important portion of the total nitrogen oxides emissions, are related for engines using HFO. It is known that the evolution of nitrogen oxides growths with the combustion temperature. As a result, slow speed marine DE produce high level of NO x compared to a medium or high-speed marine DE.  such as liquified natural gas, biofuels and light marine fuel oil (LMFO). Figure 5 illustrates the DE-NO X and SO x reduction emissions based on pre-treatment, internal treatment and post treatment technologies.

Marine Diesel Engine Emissions Reduction Solutions Using Pre-Treatment Technologies
Pre-treatment is the easiest and fastest way to comply with the emissions regulation recently adopted by the IMO. However, low-Sulphur substitute fuel such as methanol and liquified natural gas (LNG) face several challenges in terms of adaptability on board. In terms of engineering, they require dual-fuel engines and additional special fuel storage tanks. Figure 5. Methods of reducing SO x and NO x emissions from marine diesel engines using pre-treatment, internal-treatment and post treatment solutions [14]. Open Journal of Marine Science Methanol has the advantage to reduce SO x , NO x and PM emissions by an amount of about 99%, 60% and 95% compared to heavy fuel oil (HFO) for marine use, while LNG has the advantage to reduce SO x , NO x , CO 2 and PM by 98%, 86%, 11% and 96% respectively [15], Figure 6. However, methanol raises the possibility of corrosion and must be faced with adequate upgrading of fuel storage tanks. On the other hand, LNG retains about 85% of the energy stocked per unit volume compared to traditional oil fuel. One of the most vital challenges of the use of LNG as a marine fuel, is the higher size of his tanks (3 -4 times greater) than the marine diesel oil tanks [16]. This further increases the costs of installations for a retrofit. Recently, a study conducted and published by DNV-GL [17], confirms that LNG-fueled fleet will increase very-quickly over the next years, especially in zones with existing bunkering abilities. Moreover, a previous study assessed by Wärtsilä in order to evaluate the advantages of changing from HFO fueled engine equipped with a Sea scrubber to LNG fueled engine [18] has shown additional savings from the annual machinery cost (maintenance, oil lubricating, scrubber and SCR with annual capital) by an amount of 500 $/kW.

Furthermore, the expansion in navigation time in emission control areas (ECAs)
where high quality fuel is required will add more economic interests to LNG than HFO-powered engines where stricter emission regulations are approved and implemented. Figure   phenomenon [19]. To conclude this section, many DE manufacturers such as Wärtsila and MAN B&W have developed DEs that can be run on natural gas, marine diesel oil (MDO) or heavy fuel oil (HFO). This is known as fuel switching technology or dual-fuel technology [20]. The dual-fuel technology provides shipowners and operators with outstanding benefits. In gas mode, without any secondary exhaust gas purification systems, the engine is already compliant with IMO Tier III regulations [21]. Furthermore, dual-fuel technology offers reduced emissions of SO x and CO 2 as well as smokeless operation in gas mode. On the other hand, dual-fuel technology allows the operator to select the type of fuel to be used based on the market price variation. However, there are several disadvantages [22]. First, a spark ignited gas engine's power output is lower than that of a diesel engine of similar size. This translates during initial installation into a higher capital investment. There are also high maintenance costs for the spark ignition system. While many producers continue to provide in the development of longer-life spark plugs, there is still concern about their operating life. Furthermore, spark ignited engines run hotter than their diesel complements, resulting in significantly higher valve seat wear rates. Table 1 summarizes the benefits and limitations between alternative fuels for marine use considered as pre-treatment solutions.

Marine Diesel Engine Emissions Reduction Solutions Using Internal-Treatment Technologies
Internal treatment consists of a direct modification in the diesel engine. This is  DWI technology uses an injector composed of two parts, one to spray water and the other to inject fuel oil. During the fuel injection phase, the water-fuel density 0.4 -0.7 high pressure water is injected into the combustion chamber and the mixture of water and combustion gas is completed, allowing a reduction of combustion temperatures and NO x emissions by up to 60 percent [23]. Another advantage of using this technology appears in the fact that it does not require an extra space or additional cost and can be integrated for a medium speed marine diesel engine. However, this technology can bring to lightly more fuel consumption rate by 2% approximately. On the other hand, Miller cycle was initially proposed by Ralph Miller in 1947 and consists to use the Early Intake Valve Closing (EIVC) to achieve internal cooling before compression in order to reduce the compression cycle work [24]. The Miller cycle is considered as a cold cycle and allows a lower NO x emission up to 40% -60% and increase the efficiency of the engine [25]. Furthermore, Miller cycle can be used on four stroke marine diesel engine to complete low scavenge air temperature [26]. By reducing the scavenge air temperature, combustion temperatures and NO x are also reduced. According to Holtbecker, M. [27], for each 3˚C reduction, nitrogen oxide decreases approximatively by 1 percent. Moreover, internal engine technology such as Exhaust Gas Recirculation (EGR) results in combustion temperature reduction and small NO x composition. It is considered as the principal technology to reduce NO x from DE. Figure 8 illustrates the schematic diagram of EGR technology [28]. The resulting combination of exhaust gas with the fresh air has a low volume calorific value, which reduces the combustion chamber temperatures, and allows NO x less formation by 40 percent and more. However, the production of PM is increased due to the reduction of oxygen in the combustion chamber when applying EGR [29]. The deliberate reduction in the cylinder's oxygen available will reduce the engine's peak power. For this reason, when full power is required, the EGR is usually shut down, so in this situation, the EGR approach to controlling NO x fails. Table 2 summarizes the benefits and limitations of the selected Internal-treatment technologies.

Marine Diesel Engine Emissions Reduction Solutions Using
Post-Treatment Technologies  Nevertheless, closed loop fresh water scrubber systems have much smaller dis- • Weight and balance: weights depending on the scrubber rating and type will Open Journal of Marine Science vary significantly. The primarily weights of concern are the scrubbers themselves and even a 20 ton wet weight could be considerable concern for ships stability. 3 kPa of back pressure, exceeding the ratio will degrade performance by 1%.
• Electrical power: these systems' needs can reach 2% of nominal power significantly. Additional generator must be taken into consideration.
• Arrangements for machinery and stacking: for retrofits, it will be a significant challenge to fit the scrubber into existing spaces.
Otherwise, Selective Catalytic Reduction (SCR) technology offers the largest reduction of nitrogen oxide up to 90 percent on Des [38] [39]. The functioning principle is that the waste exhaust gas is combined with ammonia (NH 3 ) or urea before passing over a special catalyst layer at a high temperature between 300˚C -400˚C, reducing the NO x to N 2 and water (H 2 O) (2) and (3).
According to Wärtsilä manufacturer, SCR is the best technology to achieve tier III compliance. Table 3 summarizes the benefits and limitations of the selected Post-treatment technologies.

Methodology for Estimating Cost
We did not define a single model's costs to estimate the economic impact of all Open Journal of Marine Science  Fixed costs reflect the need for manufacturers to focus on adapting emission controls to specific applications for marine diesel engines, with significant engine calibration required to optimize these controls over a wide range of ship types and operating conditions. Fixed cost/engine was supplied by EPA.

Emulsified Fuel
Costs for emulsified fuel include water storage tank costs, heat exchanger, ultrasonic homogenizer, distilled water and various pumps and pipes. These are detailed in Table 5. It is supposed that the water tank is made of cold rolled steel

Direct Water Injection (DWI)
DWI contains charges for a low and high-pressure module, water storage tank, water injectors, flow fuses control unit and the related piping. Table 6 shows costs the detailed information.

Selective Catalytic Reduction (SCR)
Selective Catalytic Reduction charges include reactor, the urea tank, dosage pump, injectors, control system, a bypass valve, cleaning probe and the acoustic horn.
Retooling charges are for redesign of the exhaust system to entertain the SCR system. Table 7 shows costs the detailed information.

Exhaust Gas Recirculation (EGR)
EGR charges contain a sludge tank, supply pump, piping, waste pump, a recirculation pump, an EGR Valve, separator, a scrubber system and control system.  Based on an average EGR rate of 20 percent, sludge is supposed to grow at 0.005 g/kWh with a sludge density of 1300 kg/m 3 . The sludge tank is intended to be made of 1mm thick cold rolled steel. The tank will hold sludge from engine operation over 500 hours. Table 8 illustrates the cost of emission reduction technologies using the exhaust gas recirculation.

Seawater Scrubber
Scrubber charges contain the sludge tank, supply pump, a waste pump, a recirculation pump, an SO 2 monitor, oil and water separator, the scrubber system and control system. Sludge tank acquires a sludge buildup rate of 0.25 g/kWh and a sludge density of 1300 kg/m 3 .
It is envisioned to be made of cold rolled steel 1mm thick and will hold sludge generated from engine operation over 500 hours. Table 9 illustrates the cost of emission reduction technologies using the seawater scrubber. Open Journal of Marine Science  Table 10 shows the cost of emission reduction technologies using the fuel switching.
Economics impact for low and medium speed for category 3 marine diesel engines are shown in Figure 9 (for low speed marine engines) and in Figure 10 (for medium speed marine engines), where they are associated to kW/hour and kg fuel burned commonly for different engine grades and different emission reduction technology.
Based on the analysis and calculations, fuel switching technologies is the least Open Journal of Marine Science It can also be seen that the prices vary proportionally depending on the power rate of the engine but in the case of the DWI technique, the cost of adaptation on a 15,000 kW (for low speed) is slightly higher than that of 8500 kW due to the number of hours required for assembly and installation, while it is also higher than that of 48,000 kW because the cost of 48,000 kW is twice that of 15,000 kW but 3, 2 times bigger in terms of power allowing a cost reduction cost by an average of 33%. However, a combination of two or three technologies may prove attractive for shipowners to achieve significant emission reductions. In the future, the integration of multiple technologies will be the research direction of many vessels equipped with a marine diesel engine. Open Journal of Marine Science

Conclusions
Ship emissions are one of the major issues affecting those interested in the maritime domain, as they have a negative impact on the marine environment. In this paper, a review and economic impact analysis of different emission reduction techniques for marine diesel engines have been presented.
According to Figure 9 and Figure 10, using fuel switching technique as pre-treatment technology onboard ships appeared the best methods at long-term from the point of view of cost, while sea water scrubbers as post-treatment technology will be costly and add more ship operating cost. On the other hand, pre-treatment technologies such as the use of Methanol or LNG as marine fuel compared to HFO, reduce dependency on conventional fuels and have environmental benefits through an average reduction. However, due to the fact that both gases have significant differences in terms of characteristics and properties than Open Journal of Marine Science  conventional marine fuels, they are subject to storage challenges and require high capital cost in order to integrate them aboard ships in service. Appropriate integration for these gases as marine fuel is suitable for future ships to come.
It also was shown that internal-treatment technologies such as EGR and DWI in order to reduce oxides of nitrogen are mostly mature and are present in new marine DEs. The most advanced technology that can come in force with internal-treatment is the post-treatment, principally using SCR. However, due to the high price of reactor and the storage of urea in proportion to the fuel carried, SCR becomes the second most expensive technique after the scrubber.
Finally, the combination of two or three technologies is possible and can offer new solutions with high efficiency from the viewpoint of environmental and economic issues.