**6. Methods for reducing harmful pollutant emissions from marine diesel engines**

Different methods of reducing the pollutant emission from ship's diesel engines are briefly described in the following text. The focus is more on NOx and SOx than on other emissions, but some attention will be given to them as well.

### **6.1. NOx emission reduction technologies**

For 'wet' exhaust gases

186 Current Air Quality Issues

**Fuel CO2 max (%)**

**Table 3.** CO2 max values for marine fuel, assuming the gases are dry

Natural gas 11.9 Light fuel oil 15.5 Heavy fuel oil 15.8

For 'dry' exhaust gases

**Emission Source**

in rpm)

CO2 Function of combustion

**Table 4.** Summary of pollutants

SOx Function of fuel oil sulphur content

<sup>2</sup> max %, 79,1

<sup>2</sup> max % 79,1 20,9 4

*c c* <sup>=</sup> æ ö + ×+ ç ÷ è ø

Carbon dioxide (CO2) concentration can be calculated in the exhaust gas emissions according to equation (19), provided that oxygen concentration (O2), maximum concentration of carbon

2 2 ( )

NOx Function of peak combustion temperature, oxygen content and residence time (function of engine speed

max 20,9 . 20,9 *CO O CO* × - é ù

ë û é ù <sup>=</sup> ë û (19)

*<sup>c</sup> CO*

*<sup>c</sup> CO*

dioxide (CO2) max and fuel type are known, as in [20]:

2

CO Function of the air excess ratio, combustion temperature and air/fuel mixture

PM Originates from unburned fuel as well as ash content in fuel and lubricating oil

HC Function of the amount of fuel and lubricating oil left unburned during combustion process

2 20,9 4

è ø

*h*

(17)

(18)

*h h c c* <sup>=</sup> æ ö ++ × + ç ÷

> Nitrogen oxides (NOx) are an important air pollutant created as a by-product of combustion. Air contains primarily nitrogen (N2) and oxygen (O2). The heat generated during combustion causes nitrogen (N2) and oxygen (O2) to react to form NOX which is in direct proportion to peak combustion temperature and pressure. Therefore, NOX emissions can be mitigated by engine controls that decrease combustion temperature and/or aftertreatment of the exhaust gas. NOx reduction methods are generally categorised as primary methods or internal measure and secondary methods or aftertreatment. The primary methods include changes to the combus‐ tion process within the engine and can be divided into three main categories: combustion optimisation, water-based controls and exhaust gas recirculation. Many of the mentioned methods aim to reduce NOx emissions by reducing peak temperatures and pressures of the combustion process in the engine cylinder. Secondary methods or aftertreatment implies postcombustion abatement in which the exhaust gas is treated in order to remove NOX, either passing it through a catalyst or plasma system. Each of these methods is discussed below.

#### *6.1.1. Primary methods or internal measure*

The primary methods include changes to the combustion process within the engine and can be divided into three main categories. The first category is combustion optimisation; the second one is water-based control which consists of water injection, water/fuel emulsion and humid‐ ification; and the third category is exhaust gas recirculation. These methods have generally low impact on fuel consumption. Another drawback is to retrofit the system of existing engines. Tier II limits under MARPOL Annex VI, Regulation 13, can be achieved using primary controls.

**a.** *Combustion optimisation*

There are a number of ways to modify the combustion process, each aimed to reduce NOx emissions. Optimisation of the engine combustion process includes modifying the spray pattern by modification of the fuel valve design, injection timing, intensity of injection and injection rate profile (injection rate shaping), compression ratio, scavenge air pressure and scavenge air cooling. Delayed injection timing is very effective in reducing NOx but increases fuel consumption and smoke. It is usually combined with increased compression pressure and decreased injection duration to minimise or avoid increase in fuel consumption. Other operational modifications that could be made to reduce emissions are combustion chamber optimisation, variable valve timing, increasing the turbo efficiency, the use of a fuel injection system that can be easily adjusted (e.g. electronically controlled injection system) and decrease in the engine air intake temperature using Miller supercharging.4 Modification of the fuel valve design means replacing the conventional injectors with fuel efficient valves (e.g. slide valves) that optimise the fuel injected into the cylinder. These valves differ from conventional valves in their spray patterns, and they are designed to reduce the dripping of fuel from the injector into the postinjection combustion zone. This fuel entering late the combustion zone is subjected to lower temperatures and therefore results in the emission of unburnt fuel (PM) and VOCs. Changing the conventional fuel valves with slide valves has a significant impact on NOx reduction and PM emissions since PM is a product of incomplete combustion and unburnt fuel. Currently, the slide fuel valves are only applicable for slow-speed two-stroke diesel engines. However, all new engines of this type are supposed to have these valves fitted as the standard. The fuel nozzle was optimised for NOx simultaneously with the development of the slide valve. Tests on a 12K90MC engine (55MW at 94 rpm) at 90 % load showed a 23 % reduction in NOx emissions for a slide-type valve compared with a standard valve and nozzle and with a 1 % fuel consumption increase. Furthermore, increasing the number of injectors per cylinder enables the combustion process to be better controlled and therefore more efficient combus‐ tion. However, additional injectors, piping and associated equipment are associated with a cost penalty. Nowadays, modern slow-speed engines use three fuel injectors located near the outer edge of the combustion chamber. With sequential injection, each of the three nozzles in a cylinder is actuated with different timings. Pulsed injection gives about 20 % NOx reduction with about 7 % increase in fuel consumption. Sequential and pre-injection gave less NOx reduction and less fuel consumption increase. The effects are the result of changes in the overall pressure development and interaction between fuel sprays. Pre-injection can be used to shorten the delay period in medium-speed engines and thus decreases temperature and pressure during the early stages of combustion, resulting in reduced NOx. Pre-injection can reduce particulates which are increased by other NOx control measures, thus allowing greater flexibility in NOx control. Delayed injection combined with increased compression ratio have effect on reducing the maximum combustion pressure and hence temperature. By using this simple technique, a reduction of up to 30 % can be achieved. However, delayed injection increases fuel consumption up to 5 % in specific fuel oil consumption due to later burning, as less of the combustion energy release is subjected to the full expansion process and gas temperatures remain high later into the expansion stroke, resulting in more heat losses on the

<sup>4</sup> Reduced scavenge air temperature reduces combustion temperatures and thus NOx. For every 3 OC reduction, NOx may decrease by about 1 %. On four-stroke engines, the Miller concept can be applied to achieve low scavenge air temperature. Using a higher-than-normal pressure turbocharger, the inlet valve is closed before the piston reaches bottom dead centre on the intake stroke. The charge air then expands inside the engine cylinder as the piston moves towards bottom dead centre, resulting in a reduced temperature. Miller supercharging can reduce NOx by 20 % without increasing fuel consumption.

walls. In some engines, the timing adjustment can be made while in service. Smoke and emission of PM also increase due to reduced combustion temperatures and thus less oxidation of the soot produced earlier in the combustion, as in [21]. Furthermore, increasing injection pressure leads to better atomisation of the fuel and therefore to reduction in particulates and CO. Since combustion is cleaner, this technique tends to increase NOx reduction. There is also the new generation of the electronically controlled camshaftless engines that allow great flexibility for optimisation of the combustion process over the full range of operating condi‐ tions. These computer-controlled engines have allowed greater operational flexibility. As far as NOx is concerned, the main features are computer control of variable injection timing (VIT), injection rate shaping, variable injection pressure and variable exhaust valve closing (VEC). Variable exhaust closing gives the ability to change the effective compression ratio. With variable injection timing and variable exhaust valve closing, it is possible to optimise the injection timing delay and increased compression ratio over the whole load range to maintain peak pressures at low load while avoiding excessive peak pressures at high load. Computercontrolled camshaftless engines are equipped with common rail injection techniques which give high injection pressures and thus good spray characteristics even at low loads granting of NOx reduction emissions.

#### **b.** Water-based control

injection rate profile (injection rate shaping), compression ratio, scavenge air pressure and scavenge air cooling. Delayed injection timing is very effective in reducing NOx but increases fuel consumption and smoke. It is usually combined with increased compression pressure and decreased injection duration to minimise or avoid increase in fuel consumption. Other operational modifications that could be made to reduce emissions are combustion chamber optimisation, variable valve timing, increasing the turbo efficiency, the use of a fuel injection system that can be easily adjusted (e.g. electronically controlled injection system) and decrease

design means replacing the conventional injectors with fuel efficient valves (e.g. slide valves) that optimise the fuel injected into the cylinder. These valves differ from conventional valves in their spray patterns, and they are designed to reduce the dripping of fuel from the injector into the postinjection combustion zone. This fuel entering late the combustion zone is subjected to lower temperatures and therefore results in the emission of unburnt fuel (PM) and VOCs. Changing the conventional fuel valves with slide valves has a significant impact on NOx reduction and PM emissions since PM is a product of incomplete combustion and unburnt fuel. Currently, the slide fuel valves are only applicable for slow-speed two-stroke diesel engines. However, all new engines of this type are supposed to have these valves fitted as the standard. The fuel nozzle was optimised for NOx simultaneously with the development of the slide valve. Tests on a 12K90MC engine (55MW at 94 rpm) at 90 % load showed a 23 % reduction in NOx emissions for a slide-type valve compared with a standard valve and nozzle and with a 1 % fuel consumption increase. Furthermore, increasing the number of injectors per cylinder enables the combustion process to be better controlled and therefore more efficient combus‐ tion. However, additional injectors, piping and associated equipment are associated with a cost penalty. Nowadays, modern slow-speed engines use three fuel injectors located near the outer edge of the combustion chamber. With sequential injection, each of the three nozzles in a cylinder is actuated with different timings. Pulsed injection gives about 20 % NOx reduction with about 7 % increase in fuel consumption. Sequential and pre-injection gave less NOx reduction and less fuel consumption increase. The effects are the result of changes in the overall pressure development and interaction between fuel sprays. Pre-injection can be used to shorten the delay period in medium-speed engines and thus decreases temperature and pressure during the early stages of combustion, resulting in reduced NOx. Pre-injection can reduce particulates which are increased by other NOx control measures, thus allowing greater flexibility in NOx control. Delayed injection combined with increased compression ratio have effect on reducing the maximum combustion pressure and hence temperature. By using this simple technique, a reduction of up to 30 % can be achieved. However, delayed injection increases fuel consumption up to 5 % in specific fuel oil consumption due to later burning, as less of the combustion energy release is subjected to the full expansion process and gas temperatures remain high later into the expansion stroke, resulting in more heat losses on the

4 Reduced scavenge air temperature reduces combustion temperatures and thus NOx. For every 3 OC reduction, NOx may decrease by about 1 %. On four-stroke engines, the Miller concept can be applied to achieve low scavenge air temperature. Using a higher-than-normal pressure turbocharger, the inlet valve is closed before the piston reaches bottom dead centre on the intake stroke. The charge air then expands inside the engine cylinder as the piston moves towards bottom dead centre, resulting in a reduced temperature. Miller supercharging can reduce NOx by 20 % without increasing

fuel consumption.

188 Current Air Quality Issues

Modification of the fuel valve

in the engine air intake temperature using Miller supercharging.4

The second category is water-based controls consisting of water injection, water/fuel emulsion and humidification. Water-based controls reduce emissions of the NOx from diesel engines by introducing freshwater at different stages of the combustion process. Introduction of water into the combustion chamber reduces maximum combustion temperature due to the absorp‐ tion of energy for evaporation and the increase in the specific heat capacity of the cylinder gases and thus reduce emissions of the NOx [23]. The in-cylinder evaporation of the water also improves the atomisation of the fuel and causes it to burn more completely. Freshwater can be introduced in the charge air (humidification), through direct injection into the cylinder or through water/fuel emulsion. Water/fuel emulsion is the process of introducing water into the fuel prior to injection into the combustion cylinder and can reduce smoke, while humidification can increase smoke. Direct water injection is the process of introducing water directly into the combustion cylinder at pressures of 200–400 bar. The water is injected into the cylinder by a combined injection valve and nozzle that allow injection of water and fuel oil. The process is electronically controlled. Direct injection of water and water/fuel emulsions place the water more directly in the combustion region, where it has maximum effect on NOx production. Generally, direct water injection or water/fuel emulsions will yield about 1 % reduction in NOx for every 1 % of water-to-fuel ratio. This one-to-one ratio is consistent up to about 30 % water content, at which point the combustion temperature decreases too much, resulting in an increase in PM emissions. Alternative to water injection and water/fuel emulsion is the scavenge air humidification as a second category of the primary methods for NOx emission reduction that implies injection of very fine water mist in scavenge airstream after the turbocharger using special nozzles (Scavenge Air Saturation System). The fine water droplets evaporate fast, and further heat is introduced in the scavenge air. Humidification requires about twice as much water for the same NOx reduction compared with direct injection of water and water/fuel emulsions. Humidification can reduce NOx levels down to 2 to 3 g/kWh without fuel consumption penalty. Similar technique that is used for humidification of the engine scavenge air is the so-called humid air motor (HAM). In this system, hot compressed air from the turbocharger is led to a humidification tower and exposed to a large surface area and flushed with hot water. The water can be heated by a heat exchanger connected to the jacket cooling system or using an exhaust gas boiler. One manufacturer claims considerable success in service in reducing NOx emissions with the added claim of increasing the indicated power of the engine at certain loads, therefore reducing fuel consumption hence proportionally reducing CO2 emissions. The actual degree of NOx reduction varies from 10 % to over 60 %, depending on the engine type and which of the above reduction methods are adopted. For example, the experiment carried out on the Viking Line's MS Mariella has shown a NOx emission reduction from 17 to between 2.2 and 2.6 g/kWh and a decrease in fuel consumption of 2–3% using the HAM system [24].

#### **c.** *Exhaust gas recirculation (EGR)*

The exhaust gas recirculation (EGR) system is based on lowering of the combustion temperature and oxygen concentration thus lowering NOx. EGR reduces combustion temperatures by increasing the specific heat capacity of the cylinder gases and by reduc‐ ing the overall oxygen concentration, taking away a part of the exhaust gases and mixing it into the engine intake air. Some of the exhaust gas is cooled and cleaned before recircu‐ lation to the scavenge air side. The usage of the exhaust gas as intake air reduces the oxygen content in intake air from 21 % to 13 % which limits the NOx that can be formed and reduces the amount of combustion products that can take place. In engines operating on poor-quality fuel, external EGR can lead to fouling and corrosion problems. The residue from cooling and cleaning the exhaust gas on ships using heavy fuel oil contains sulphur in a form which is difficult to dispose of. The relative changes in measured emission parameters as a function of the recirculation amount at 75 % engine load show that at increased recirculation amounts, the HC and PM emissions are reduced corresponding to the reduction of the exhaust gas flow from the engine. Increased recirculation amount leads to increase in CO emissions due to lower cylinder excess air ratios and thus lack of oxygen in the combustion chamber. Furthermore, EGR tends to increase smoke by reducing the O2 concentration, increasing the combustion duration and decreasing the combustion temper‐ ature. All of that may be controlled using additional techniques such as water in fuel to achieve an optimum balance between NOX, CO and PM. Test engine work by MAN Diesel & Turbo has shown that, with 40 % recirculation, EGR has the potential to reduce NOX down to Tier III levels on a two-stroke low-speed marine engine and that increased fuel consumption, carbon monoxide emissions and PM emissions resulting from reduced combustion efficiency are manageable with engine adjustments. It is also reported that specific fuel consumption is greatly improved when using EGR to reduce NOX down to Tier II limits, when compared with using engine adjustments to achieve the same level of emissions, particularly at part load as in [25]. There are many different components to an EGR system such as high pressure exhaust gas scrubber fitted before the engine turbocharg‐ er, cooler to further reduce the temperature of the recirculated gas, water mist catcher to

remove entrained water droplets, high-pressure blower to increase recirculated gas pressure before reintroduction to the engine scavenge air and automated valves for isolation of the system. The scrubber in the EGR system is used to remove sulphur oxides and particu‐ late matter from the recirculated exhaust, to prevent corrosion and reduce fouling of the EGR system and engine components. The system requires the use of an electrostatic precipitator and catalysts to remove the particulates from the exhaust gas before injecting it as intake air. There is also a need for wet-scrubbing technology to remove the sulphur components of the exhaust stream prior to reintroduction into the engine. A cooling unit is also needed to reduce the temperature of the exhaust gas before it returns to the engine.

#### *6.1.2. Secondary methods or aftertreatment*

and water/fuel emulsions. Humidification can reduce NOx levels down to 2 to 3 g/kWh without fuel consumption penalty. Similar technique that is used for humidification of the engine scavenge air is the so-called humid air motor (HAM). In this system, hot compressed air from the turbocharger is led to a humidification tower and exposed to a large surface area and flushed with hot water. The water can be heated by a heat exchanger connected to the jacket cooling system or using an exhaust gas boiler. One manufacturer claims considerable success in service in reducing NOx emissions with the added claim of increasing the indicated power of the engine at certain loads, therefore reducing fuel consumption hence proportionally reducing CO2 emissions. The actual degree of NOx reduction varies from 10 % to over 60 %, depending on the engine type and which of the above reduction methods are adopted. For example, the experiment carried out on the Viking Line's MS Mariella has shown a NOx emission reduction from 17 to between 2.2 and 2.6 g/kWh and a decrease in fuel consumption

The exhaust gas recirculation (EGR) system is based on lowering of the combustion temperature and oxygen concentration thus lowering NOx. EGR reduces combustion temperatures by increasing the specific heat capacity of the cylinder gases and by reduc‐ ing the overall oxygen concentration, taking away a part of the exhaust gases and mixing it into the engine intake air. Some of the exhaust gas is cooled and cleaned before recircu‐ lation to the scavenge air side. The usage of the exhaust gas as intake air reduces the oxygen content in intake air from 21 % to 13 % which limits the NOx that can be formed and reduces the amount of combustion products that can take place. In engines operating on poor-quality fuel, external EGR can lead to fouling and corrosion problems. The residue from cooling and cleaning the exhaust gas on ships using heavy fuel oil contains sulphur in a form which is difficult to dispose of. The relative changes in measured emission parameters as a function of the recirculation amount at 75 % engine load show that at increased recirculation amounts, the HC and PM emissions are reduced corresponding to the reduction of the exhaust gas flow from the engine. Increased recirculation amount leads to increase in CO emissions due to lower cylinder excess air ratios and thus lack of oxygen in the combustion chamber. Furthermore, EGR tends to increase smoke by reducing the O2 concentration, increasing the combustion duration and decreasing the combustion temper‐ ature. All of that may be controlled using additional techniques such as water in fuel to achieve an optimum balance between NOX, CO and PM. Test engine work by MAN Diesel & Turbo has shown that, with 40 % recirculation, EGR has the potential to reduce NOX down to Tier III levels on a two-stroke low-speed marine engine and that increased fuel consumption, carbon monoxide emissions and PM emissions resulting from reduced combustion efficiency are manageable with engine adjustments. It is also reported that specific fuel consumption is greatly improved when using EGR to reduce NOX down to Tier II limits, when compared with using engine adjustments to achieve the same level of emissions, particularly at part load as in [25]. There are many different components to an EGR system such as high pressure exhaust gas scrubber fitted before the engine turbocharg‐ er, cooler to further reduce the temperature of the recirculated gas, water mist catcher to

of 2–3% using the HAM system [24].

**c.** *Exhaust gas recirculation (EGR)*

190 Current Air Quality Issues

Secondary methods, or aftertreatment, are based on treating the engine exhaust gas itself by passing it through a catalyst or plasma system. There has been much development in selective catalytic reduction (SCR) and nonthermal plasma (NTP) reduction systems over the last few years. Using these methods, NOx emission reductions of over 95 % can be achieved [26].

*Selective catalytic reduction* (SCR) is an exhaust gas treatment method by which the NOx generated in a marine diesel engine exhaust gas can be reduced to a level in compliance with the NOx Tier III requirements. The method involves mixing of ammonia as a reducing agent with the exhaust gas which is passed over a catalyst where more than 90 % of the NOx can be removed to below 2 g/kWh. The SCR system converts nitrogen oxides into harmless nitrogen and water, by means of a reducing agent injected into the engine exhaust stream before a catalyst. Hydrocarbons are also reduced. Exhaust emission abatement systems using SCR technologies usually use an ammonia (NH3) reductant introduced as a urea/water solution ((NH2)2CO) into the exhaust stream, prior to the catalyst blocks. For marine systems, a 40 % solution of urea in de-ionised water is typically used for safe handling and toxic risk reasons [27, 28]. The use of urea in the system breaks down the NOx emissions to N2 and H2O. The degree of NOx removal depends on the amount of ammonia added. A NOx reduction efficiency of 90 % can be achieved using a urea injection rate of 15 g/ kWh. NOx is reduced according to the following overall reaction scheme [26,27,29]:

Urea decomposition before entering the reactor:

$$\text{H}\_{3}\text{(NH}\_{2}\text{)}\_{2}\text{CO (urea)} \rightarrow \text{NH}\_{3}\text{(ammonium)} + \text{HONO (iosyncic acid)}\tag{20}$$

$$\text{HNCO} + \text{H}\_2\text{O} \rightarrow \text{NH}\_3 + \text{CO}\_2\tag{21}$$

The resulting quantity of CO2 is minor when compared with that resulting from fuel combus‐ tion.

NOX reduction at the catalyst:

$$4\text{NO} + 4\text{NH}\_3 + \text{O}\_2 \to 4\text{N}\_2 + 6\text{H}\_2\text{O}\tag{22}$$

$$\text{AlNO} + 2\text{NO}\_2 + 4\text{NH}\_3 \rightarrow 4\text{N}\_2 + 6\text{H}\_2\text{O} \tag{23}$$

$$\text{6NO}\_2 + \text{8NH}\_3 + \text{O}\_2 \rightarrow \text{3N}\_2 + \text{6H}\_2\text{O} \tag{24}$$

Equation 22 shows the main SCR reaction as nitric oxide dominates in the exhaust. The reaction shown at equation 23 occurs at the fastest rate up to an NO2:NO ratio of 1:1. However, at higher ratios, the excess NO2 reacts slowly as per equation 24. The rate of urea injection must be sufficient to reduce NOX emissions to the required level but not so great to avoid ammonia slip. Control is based on the load and speed of the engine with active feedback provided on some systems by NOX and ammonia emission monitoring. At engine start-up, urea injection is initiated once the catalyst reaches operating temperature, which is key for effective NOX reduction performance, deposit prevention and avoidance of ammonia slip. Catalysts have considerable heat capacity so the time taken to reach the injection trigger temperature is dependent on a number of factors including the minimum catalyst operating temperature recommended for the fuel type, the period of cooldown since the engine was last operated, the size of the catalyst and the engine load pattern at start-up. Injection can begin up to 30 min after a fully cold start, whereas it may begin within 10–15 min if the catalyst is still warm from running in the previous 6–10 h. SCR units are typically installed in the exhaust system of a diesel engine, if applicable, before the exhaust gas economiser and as close as possible to the engine because of the relatively high exhaust gas temperatures required by the catalysts for effective NOx reduction reactions. The SCR catalysts may also be integrated with the engine by close coupling to the engine, typically applicable to small high-speed diesel engines. For slow-speed diesel engines with inherently low relative exhaust gas temperatures, this may necessitate the integration of the SCR reaction chamber and catalysts before the turbocharger exhaust turbine. Depending on the engine load, the exhaust gas temperature on this side is 50–175 °C higher than on the low pressure side. Even though the reactor is placed before the turbine, the exhaust gas temperature will normally still be too low at low loads. To increase the temperature, a cylinder bypass from the scavenge air receiver to the turbine inlet is installed. The bypass is controlled by the cylinder bypass valve.

When opening the bypass, the mass of air through the cylinders will be reduced without losing the scavenge air pressure, and, accordingly, the exhaust gas temperature will increase. This system makes it possible to keep the temperatures above the required level. However, the cylinder bypass will increase the SFOC depending on the required temperature increase. Selective catalytic reduction is the only technology currently available to achieve compliance with the Tier III NOx standards for all applicable engines. Another option is selective noncatalytic reduction (SNCR), which works in a similar way with selective catalytic reduction but without the use of a catalyst. A reducing agent (ammonia or urea) injected during the combustion process converts the nitrogen oxides into nitrogen and water, reducing NOx emissions by 50 % [30]. The drawback of this system is that it is less efficient than the SCR method, because only 10–12 % of ammonia reacts with NOx. Since the cost of ammonia is relatively high and since the system requires extensive modification to the engine, the SNCR option does not appear to be competitive. Plasma reduction systems are based on the use of plasma. This is a partially ionised gas composed of a charge of a neutral mixture of atoms, molecules, free radicals, ions and electrons. Electrical power is converted into electron energy, and the electrons create free radicals, which destroy pollutants in exhaust emissions. Experi‐ ments have shown that plasma reduction systems can reduce NOx by up to 97 % [30]. It seems to be flexible in terms of size and shape and should be at relatively low cost. However, for marine use, it is still in the development phase.

#### **6.2. SOx reduction technologies**

32 2 2 4NO + 4NH + O 4N + 6H O ® (22)

2 3 22 2NO + 2NO + 4 NH 4N + 6H O ® (23)

2 32 2 2 6NO + 8NH + O 3N + 6H O ® (24)

Equation 22 shows the main SCR reaction as nitric oxide dominates in the exhaust. The reaction shown at equation 23 occurs at the fastest rate up to an NO2:NO ratio of 1:1. However, at higher ratios, the excess NO2 reacts slowly as per equation 24. The rate of urea injection must be sufficient to reduce NOX emissions to the required level but not so great to avoid ammonia slip. Control is based on the load and speed of the engine with active feedback provided on some systems by NOX and ammonia emission monitoring. At engine start-up, urea injection is initiated once the catalyst reaches operating temperature, which is key for effective NOX reduction performance, deposit prevention and avoidance of ammonia slip. Catalysts have considerable heat capacity so the time taken to reach the injection trigger temperature is dependent on a number of factors including the minimum catalyst operating temperature recommended for the fuel type, the period of cooldown since the engine was last operated, the size of the catalyst and the engine load pattern at start-up. Injection can begin up to 30 min after a fully cold start, whereas it may begin within 10–15 min if the catalyst is still warm from running in the previous 6–10 h. SCR units are typically installed in the exhaust system of a diesel engine, if applicable, before the exhaust gas economiser and as close as possible to the engine because of the relatively high exhaust gas temperatures required by the catalysts for effective NOx reduction reactions. The SCR catalysts may also be integrated with the engine by close coupling to the engine, typically applicable to small high-speed diesel engines. For slow-speed diesel engines with inherently low relative exhaust gas temperatures, this may necessitate the integration of the SCR reaction chamber and catalysts before the turbocharger exhaust turbine. Depending on the engine load, the exhaust gas temperature on this side is 50–175 °C higher than on the low pressure side. Even though the reactor is placed before the turbine, the exhaust gas temperature will normally still be too low at low loads. To increase the temperature, a cylinder bypass from the scavenge air receiver to the turbine inlet is

192 Current Air Quality Issues

installed. The bypass is controlled by the cylinder bypass valve.

When opening the bypass, the mass of air through the cylinders will be reduced without losing the scavenge air pressure, and, accordingly, the exhaust gas temperature will increase. This system makes it possible to keep the temperatures above the required level. However, the cylinder bypass will increase the SFOC depending on the required temperature increase. Selective catalytic reduction is the only technology currently available to achieve compliance with the Tier III NOx standards for all applicable engines. Another option is selective noncatalytic reduction (SNCR), which works in a similar way with selective catalytic reduction but without the use of a catalyst. A reducing agent (ammonia or urea) injected during the combustion process converts the nitrogen oxides into nitrogen and water, reducing NOx

The emission of sulphur dioxide is directly proportional to the content of sulphur in fuel. To meet the restrictions on emissions of sulphur oxides (SOx) and PM (particulate matter) that are determined by the MARPOL convention Annex VI which specifies a global and a local (ECA) limit on the sulphur content in marine fuel, there are only two possibilities for reducing SOx emissions: either use fuels with low sulphur content (there will be no restrictions in the use of heavy fuel oil) or apply an exhaust gas cleaning system to reduce the total emission of SOx. Although the SOx requirements can be met by using a low-sulphur fuel, the regulation allows alternative methods to reduce the emissions of SOx to an equivalent level. The process of exhaust gas cleaning is performed in a scrubber unit. There are two main types of SOX scrubber: wet scrubbers that use water (seawater or fresh) as the scrubbing medium and dry scrubbers that use a dry chemical. The wet scrubbing technology is based on the fact that sulphur oxides dissolve in water. This means that when the exhaust gas is sprayed with the alkaline water in the scrubber, the SOx will dissolve in the scrubbing water and be cleaned from the exhaust gas. The water is injected into the exhaust gas stream and is discharged from the bottom of the scrubber. The alkalinity in the scrubbing water will neutralise the SOx emissions. The scrubbing water must be cleaned of particulate matter and other contaminants before being discharged out into sea. Wet scrubbing systems which are normally fitted on marine engines may be categorised as open-loop system, closed-loop system or hybrid system.

*Open-loop systems* – In an open-loop technology, the water comes from the sea and goes directly to the scrubbers. After the scrubbing process, the water goes through water treatment and to the sea again. This system takes advantage of the natural alkalinity of seawater to buffer the acidity of SOX gases. The seawater flow rate in open-loop systems is approximately 45 m3 / MWh. Sulphur oxide removal rate is close to 98 % with full alkalinity of the seawater, meaning emissions from a 3.5 % sulphur fuel will be the equivalent of those from a 0.10 % sulphur fuel after scrubbing [29]. The sulphur oxides generated in the combustion process are dissolved and removed by the scrubber water. Sulphur dioxide (SO2) is dissolved and ionised to bisulphite and sulphite, which is then readily oxidised to sulphate in seawater containing oxygen. As in reference [29], similarly sulphuric acid, formed from SO3, and hydrogen sulphate dissociate completely to sulphate according to chemical reactions:

For SO2:

$$\text{SO}\_2 + \text{H}\_2\text{O} \rightarrow \text{H}\_2\text{SO}\_3 \text{(solphurous acid)} \rightarrow \text{H}^\* + \text{HSO}\_3 \text{(bisulphite)}\tag{25}$$

$$\mathrm{HSO}\_{3}^{-}\left(\mathrm{bisulphite}\right)\rightarrow\mathrm{H}^{\*}+\mathrm{SO}\_{3}^{2-}\left(\mathrm{sulphite}\right)\tag{26}$$

$$\text{LSO}\_3^{2-}\left(\text{sulphite}\right) + 1/2\,\text{O}\_2 \rightarrow \text{SO}\_4^{2-}\left(\text{sulphate}\right) \tag{27}$$

For SO3:

$$\text{LSO}\_3 + \text{H}\_2\text{O} \rightarrow \text{H}\_2\text{SO}\_4 \text{ (sulfur acid)}\tag{28}$$

$$H\_2SO\_4 + H\_2O \rightarrow HSO\_4^- \text{(hydrogen sulphate)} + H\_3O^+ \tag{29}$$

$$\mathrm{HSO}\_{4}^{-}\left(\mathrm{hidrogen}\,\mathrm{sulphate}\right) + \mathrm{H}\_{2}\mathrm{O} \rightarrow \mathrm{SO}\_{4}^{2-}\left(\mathrm{sulphate}\right) + \mathrm{H}\_{3}\mathrm{O}^{\*}\tag{30}$$

*Closed-loop systems* use freshwater treated with sodium hydroxide (NaOH) as the scrubbing media for the neutralisation of SOX. This results in the removal of SOX from the exhaust gas stream as sodium sulphate according to the following chemical reactions as in [29]:

For SO2:

$$\mathrm{Na}^+ + \mathrm{OH}^- + \mathrm{SO}\_2 \rightarrow \mathrm{NaHSO}\_3 \left( \mathrm{aq\ solidium} \,\mathrm{bisulphite} \right) \tag{31}$$

$$2\text{Na}^+ + 2\text{OH}^- + \text{SO}\_2 \rightarrow \text{Na}\_2\text{SO}\_3 \left( \text{aq sodium} \,\text{sulfite} \right) + \text{H}\_2\text{O} \tag{32}$$

$$2\text{Na}^+ + 2\text{OH}^- + \text{SO}\_2 + \text{1/2}\text{O}\_2 \rightarrow \text{Na}\_2\text{SO}\_4 \text{(aq sodium sulfate)} + \text{H}\_2\text{O} \tag{33}$$

For SO3:

$$\text{SO}\_3 + \text{H}\_2\text{O} \rightarrow \text{H}\_2\text{SO}\_4 \text{ (sulfur acid)}\tag{34}$$

$$2NaOH + H\_2SO\_4 \rightarrow Na\_2SO\_4 \left(aq\, sodium\,\,salt\right) + 2H\_2O\tag{35}$$

In a closed-loop technology, absolutely no water comes from the sea. The freshwater comes from a buffer tank and is cooled by the seawater. The freshwater is composed of NaOH and leaves the buffer tank to go to the scrubber. After the scrubbing process, the water comes back to the buffer tank, cleaned by a filter. The black water goes to a sludge tank and the clean water goes back to the scrubbing cycle. A big storage tank fills up the buffer tank. Closed-loop systems can also be operated when the ship is operating in enclosed waters where the alkalinity would be too low for open-loop operation. Closed-loop systems typically consume sodium hydroxide in a 50 % aqueous solution. The dosage rate is approximately 15 l/MWh of scrubbed engine power of a 2.70 % sulphur fuel is scrubbed to equivalent to 0.10 %. Using a closed-loop technology can have some advantages. First, there is a possibility to increase the pH level in order to reduce more SOx. Also there is no corrosion of the parts and less discharge water to clean. The running costs of the closed-loop technology are relatively high because it uses NaOH which is 0.2 €/kg and its required monitoring units. Also the sludge tanks have to be discharged at the harbour which costs a lot of money. The closed-loop reduces about 98 % SOx in the exhaust gas.

*Hybrid system* – A hybrid system is a mixture of both open loop and closed loop. In harbours and ECA, the system can operate with freshwater without generating any significant amount of sludge to be handed at port calls. At open sea, the system switches to the seawater open loop. Using a hybrid technology can have some advantages. First, if the ship is running at open sea, after switching to open loop, the accumulated water of the buffer tank can slowly be removed back to the sea. Also, the tank is slowly filled up again to prepare for the arrival at sensitive areas.

*Dry SOXscrubber* known as an 'absorber' brings the exhaust gas from diesel engine in the multistage absorber where contact with calcium hydroxide (Ca(OH)2) granules reacts with sulphur dioxide (SO2), forming calcium sulphite as in reference [29]:

$$\text{CaO}\_2 + \text{Ca(OH)}\_2 \rightarrow \text{CaSO}\_3\text{(calcium sulphate)} + \text{H}\_2\text{O} \tag{36}$$

The sulphite is then oxidised and hydrated in the exhaust stream to form calcium sulphate dihydrate, or gypsum:

$$2CaSO\_3 + O\_2 \rightarrow 2CaSO\_4 \left( calcium \, sulphate\right) \tag{37}$$

$$\text{CaSO}\_4 + 2\text{H}\_2\text{O} \rightarrow \text{CaSO}\_4 \cdot 2\text{H}\_2\text{O} \text{(calcium sulphate dihydrogen)}\tag{38}$$

Similarly, chemical reactions take place for SO3:

*SO H O H SO sulphurous acid H HSO bisulphite* 2 2 23 ( ) <sup>3</sup> ( ) <sup>+</sup> + ® ® + (25)


<sup>3</sup> 2 4 *SO sulphite O SO sulphate* 1/2 - - + ® (27)

*SO H O H SO sulphuric acid* 3 2 24 + ® ( ) (28)



*NA OH SO NaHSO aq sodium bisulphite* 2 3 ( ) + - + +® (31)

2 23 ( ) <sup>2</sup> 2 2 *NA OH SO Na SO aq sodium sulphite H O* + - + +® + (32)

*SO H O H SO sulphuric acid* 3 2 24 + ® ( ) (34)

24 24 ( ) <sup>2</sup> 2 2 *NaOH H SO Na SO aq sodium sulphate H O* + ® + (35)

2 2 24 ( ) <sup>2</sup> 2 2 *NA OH SO O Na SO aq sodium sulphate H O* 1/2 + - + ++ ® + (33)

( ) ( ) <sup>2</sup> *HSO bisulphite H SO sulphite* 3 3

*H SO H O HSO hidrogen sulphate H O* 24 2 <sup>4</sup> ( ) <sup>3</sup>

( ) ( ) <sup>2</sup> *HSO hidrogen sulphate H O SO sulphate H O* <sup>4</sup> 2 4 <sup>3</sup>

stream as sodium sulphate according to the following chemical reactions as in [29]:

*Closed-loop systems* use freshwater treated with sodium hydroxide (NaOH) as the scrubbing media for the neutralisation of SOX. This results in the removal of SOX from the exhaust gas

For SO3:

194 Current Air Quality Issues

For SO2:

For SO3:

( ) ( ) 2 2

$$\text{CaSO}\_3 + \text{Ca(OH)}\_2 + \text{H}\_2\text{O} \rightarrow \text{CaSO}\_4 \cdot 2\text{H}\_2\text{O} \text{(calcium sulphate dihydrogen)}\tag{39}$$

Using a dry scrubber can have some advantages. First, the good point of this technology is that the desulphurisation unit requires, aside from electrical energy, only Ca(OH)2 in the shape of spherical granulates. Also the dry scrubber further operates as a silencer. Dry scrubbers typically operate at exhaust temperatures between 240 °C and 450 °C. Calcium hydroxide granules are between 2 and 8 mm in diameter with a very high surface area to maximise contact with the exhaust gas. Within the absorber, the calcium hydroxide granules (Ca(OH)2) react with sulphur oxides to form gypsum (CaSO4 2H2O). To reduce SOX emissions to those equivalent to fuel with a 0.1 % sulphur content, a typical marine engine using residual fuel with a 2.70 % sulphur content would consume calcium hydroxide granules at a rate of 40 kg/ MWh (i.e. a 20 MW engine would require approximately 19 tonnes of granulate per day) [29, 30]. The dry scrubber reduces up to 99 % SOx in the exhaust gas. It will be absolutely no problem to fulfil all the IMO 3 requirements for 2016. The dry scrubber compared to the wet scrubber has lower investment costs and higher running costs and requires a lot of space which reduces the benefits. The efficiency of the SOx scrubber systems depends on the sulphur content in the fuel and generally ranges up to 97 %. Anyhow, the efficiency system must be sufficient to achieve a SOx emission level that is equal to or lower than the required limit.

On the other hand, it should be noted that the reduction of both NOx and SOx emissions from marine diesel engines be achieved by replacing conventional fuels with alternative fuels, e.g. liquefied natural gas (LNG). In case of using LNG, NOx emission can be reduced by 60 % and SOx emission by 90–100 %.

There is also another option to use onshore power supply at ports which is especially beneficial for local air quality. In this case, NOx and SOx emissions can be reduced by 90 %, while CO2 reduction depends on the source of electricity. The total CO2 emission reduction depends on how the used electricity is produced. In the European Union, the use of shore-side electricity rather than electricity generated by a ship using low-sulphur fuel will cut CO2 emissions by an average of 50 %.

One of the main benefits of shore connection systems stems from the fact that electricity generated on land by power plants has a smaller adverse impact on the ecosystem than that produced by ship engines. Namely, the main cause of air pollution from ships in ports is the use of auxiliary diesel engines to generate electricity on ships. Furthermore, experiments with wind and solar power, biofuels and fuel cells are ongoing and could be useful in the future to reduce air pollution from ships.

In Table 5, overview of different technologies and their potential for reduction of emissions from marine diesel engines are summarised as in [31].



**Table 5.** Different technologies and their reduction potential

## **7. Conclusion**

Using a dry scrubber can have some advantages. First, the good point of this technology is that the desulphurisation unit requires, aside from electrical energy, only Ca(OH)2 in the shape of spherical granulates. Also the dry scrubber further operates as a silencer. Dry scrubbers typically operate at exhaust temperatures between 240 °C and 450 °C. Calcium hydroxide granules are between 2 and 8 mm in diameter with a very high surface area to maximise contact with the exhaust gas. Within the absorber, the calcium hydroxide granules (Ca(OH)2) react with sulphur oxides to form gypsum (CaSO4 2H2O). To reduce SOX emissions to those equivalent to fuel with a 0.1 % sulphur content, a typical marine engine using residual fuel with a 2.70 % sulphur content would consume calcium hydroxide granules at a rate of 40 kg/ MWh (i.e. a 20 MW engine would require approximately 19 tonnes of granulate per day) [29, 30]. The dry scrubber reduces up to 99 % SOx in the exhaust gas. It will be absolutely no problem to fulfil all the IMO 3 requirements for 2016. The dry scrubber compared to the wet scrubber has lower investment costs and higher running costs and requires a lot of space which reduces the benefits. The efficiency of the SOx scrubber systems depends on the sulphur content in the fuel and generally ranges up to 97 %. Anyhow, the efficiency system must be sufficient to achieve a SOx emission level that is equal to or lower than the required limit.

On the other hand, it should be noted that the reduction of both NOx and SOx emissions from marine diesel engines be achieved by replacing conventional fuels with alternative fuels, e.g. liquefied natural gas (LNG). In case of using LNG, NOx emission can be reduced by 60 % and

There is also another option to use onshore power supply at ports which is especially beneficial for local air quality. In this case, NOx and SOx emissions can be reduced by 90 %, while CO2 reduction depends on the source of electricity. The total CO2 emission reduction depends on how the used electricity is produced. In the European Union, the use of shore-side electricity rather than electricity generated by a ship using low-sulphur fuel will cut CO2 emissions by

One of the main benefits of shore connection systems stems from the fact that electricity generated on land by power plants has a smaller adverse impact on the ecosystem than that produced by ship engines. Namely, the main cause of air pollution from ships in ports is the use of auxiliary diesel engines to generate electricity on ships. Furthermore, experiments with wind and solar power, biofuels and fuel cells are ongoing and could be useful in the future to

In Table 5, overview of different technologies and their potential for reduction of emissions

Optimise combustion 20–40% 0% 25–50% 0%

Water-based controlDirect water injection 50% 0% 0% 0%

**NOx SOx PM CO2**

SOx emission by 90–100 %.

196 Current Air Quality Issues

an average of 50 %.

reduce air pollution from ships.

**Category Technology aimed to reduce**

NOx

Engine modification

from marine diesel engines are summarised as in [31].

Owing to rapidly developed shipping industry and maritime traffic in recent decades, air pollution emissions from ocean-going ships are continuously growing. Exhaust gases from marine diesel engines are the primary source of emission harmful pollutants such as nitrogen oxides (NOx), sulphur dioxide (SO2), carbon monoxide (CO) and particulate matter (PM) which contribute significantly to environmental pollution, especially in port areas that are often located in or near urban areas, and a significant number of people are exposed to these emissions. The increased air pollutant concentrations and deposition have several negative effects. Nitrogen oxide and particulate matter can contribute to many serious health problems and increased morbidity and mortality (especially from cardiovascular and cardiopulmonary diseases). Nitrogen oxides also contribute to the formation of ground-level ozone, which has a harmful effect on plants and human health. Furthermore, sulphur dioxide and nitrogen oxide emissions increase acidification of sensitive forest ecosystems along the coastal areas and have a harmful effect on plants, aquatic animals and infrastructure by accelerating the deterioration process of various materials. Finally, ships are also a source of greenhouse gas, a pollutant which contributes to global warming. Recent studies indicate that the emission of CO2 by ship corresponds to about 3 % of the global anthropogenic emissions. If things remain the same, by 2020, shipping will have been the biggest single emitter of air pollution especially in areas of the dense maritime traffic such as Europe, North America and East Asia which surprisingly surpasses the emissions from all land-based sources together. Since harmful pollutant emissions from ships have great impact on the human health and the environment, it is required to tighten uniform regulations at the global level, bearing in mind that shipping is inherently international. The International Maritime Organization (IMO) responsible for the safety of life at sea and the protection of the marine environment reacts on NOx, SOx, PM and CO2 emissions from a ship by adoption of Annex VI of the MARPOL 73/78 Convention, titled 'Regulations for the Prevention of Air Pollution from Ships'. MARPOL Annex VI sets limits on NOx and SOx emissions from ship exhausts and prohibits deliberate emissions of ozonedepleting substances. Furthermore, the IMO marks out Emission Control Areas (ECAs) in cooperation with national governments with more stringent controls on sulphur emissions. These areas currently comprise the Baltic Sea, the North Sea, the English Channel, the US Caribbean Sea and the area outside North America (200 nautical miles). Ships are currently being permitted to burn fuel oils with sulphur content of less than 3.5 % while operating outside an ECA but must ensure that they burn fuel with a sulphur content of less than 1 % while within the sulphur Emission Control Areas. In accordance with EU's marine fuel sulphur directive, the sulphur content in marine gas oil within the territorial waters of an EU member state may not exceed 0.1 % by weight. This applies to all ships regardless of flag. Regarding reductions in nitrogen oxide emissions from marine engines, Annex VI introduced Tier I, II and III NOx emission standards for new engines. NOx emission limits are set for diesel engines depending on the engine's maximum operating speed. Tier I and II limits are global, while the Tier III standards apply only in NOx Emission Control Areas. Tier II NOx standards are currently being in force. In order to control CO2 emission from shipping, the first formal CO2 control regulations were adopted by the IMO, introducing a new chapter, Chapter 4, to Annex VI. Chapter 4 introduces two mandatory mechanisms intended to ensure an energy efficiency standard for ships: the first is the Energy Efficiency Design Index (EEDI), for new ships, and the second the Ship Energy Efficiency Management Plan (SEEMP) for all ships. The regulations apply to all ships of 400 gross tonnage and above and are entered into force on 1 January 2013. Energy Efficiency Design Index (EEDI) is the first globally binding climate change standard. It is anticipated that global CO2 reductions of 10 to 20 % could be obtained by implementation of EEDI and SEEMP. Detailed descriptions of the emission restrictions prescribed by Annex VI of the MARPOL 73/78 Convention are listed in section 2 entitled 'International regulation concerning air pollution from merchant shipping'. To meet these restrictions on emissions of harmful pollutants from marine diesel engines, different methods and technical solutions can be implemented. Nitrogen oxide reduction methods are generally categorised as primary methods or internal measure and secondary methods or aftertreatment. Primary methods include changes to the combustion process within the engine and can be divided into three main categories: combustion optimisation, water-based control and exhaust gas recirculation, while secondary methods, or aftertreatment, is based on treating the engine exhaust gas itself by passing it through a catalyst system. MARPOL Annex VI will reduce global ship sourced NOx emissions at a small rate because it only applies to new installations or major conversions. For NOx levels below MARPOL Annex VI, or for retrofitting, the main measures available now are water/fuel emulsions, direct water injection, inlet air humidification and catalyst system. Technical measures to reduce the sulphur oxide emission from ship's diesel engines include the adoption of low-sulphur fuels, the easiest way of reducing sulphur oxides emission. Usage of an exhaust gas cleaning system, i.e. scrubbers, is a possible alternative to low-sulphur fuels to reduce the total emission of SOx and considerably reduces emissions of other polluting particles. A detailed description of these methods is given in section 5 entitled 'Methods of reducing harmful pollutant emissions from marine diesel engines' As the air pollution emissions from ships are continuously growing, it is necessary to constantly improve and implement the efficient technologies and methods in order to reduce pollutant emissions from marine diesel engines and maintain them within the limits prescribed by MARPOL Annex VI as well as by other national and regional regulations. Some of these technologies and methods include the use of shore connection systems of which the main benefits stem from the fact that electricity generated on land by power plants has a smaller adverse impact on the ecosystem than that produced by ship engines, owing to that of particular concern is the pollution generated by ships at berth. Ships equipped with a green technology receive a higher grade. Furthermore, other technical measures for reducing air pollution from ships include the adoption of liquefied natural gas (LNG) as alternative fuel for marine engines. Wind, solar power, biofuels and fuel cells, the world of alternative energy is ongoing and could be useful for reducing air pollution from ships in the future. Finally, harmful pollutant emissions from ships require a stringent international standard due to their impact on the human health and the environment. It includes extending the SOx Emission Control Areas in the EU (e.g. in the Mediterranean, in the Black Sea, in the Irish Sea and in the North East Atlantic) and designating NOx Emission Control Areas as soon as possible.
