**5. Formation of the air pollutants during combustion process in marine diesel engine**

The major pollutants in diesel exhaust emissions are a direct result of the diesel combustion process itself. Typical concentrations of exhaust gas emissions from marine diesel engine largely comprise nitrogen approximately 76 %, oxygen abt. 13 %, carbon dioxide (CO2) abt. 5 % and water vapour abt. 5 %, with smaller quantities of pollutants: nitrogen oxide (NOX) abt. 1200 ppm, sulphur oxide (SOX) abt. 640 ppm, carbon monoxide (CO) abt. 60 ppm, partially reacted and non-combusted hydrocarbons (HC) abt. 180 ppm and particulate matter (PM) abt. 120 mg/Nm3 [15]. The composition of this gas mixture, liquids and solids that are actually emitted into the air will vary depending on engine type, engine power, operating conditions as well as fuel and lubricating oil type and also depends on whether the emission control system is present. Pollutant formation in marine diesel engine is discussed below.

#### **5.1. Nitrogen oxides (NOx)**

Nitrogen oxides (NOx) generate thermally from atmospheric nitrogen oxygen in the intake or scavenging air. The oxidation of atmospheric nitrogen is influenced by local conditions in the combustion chamber, such as the maximum cylinder pressure, local peak temperatures and local air- fuel ratio. The primary reaction product is nitric oxide (NO) by approximately 90 % of the volume, but about 5 % of it is converted into nitrogen dioxide (NO2) later in the combustion cycle, during expansion and during the flow through the exhaust system. At the same time, a very limited proportion of nitrous oxide N2O is also formed. Further oxidation of NO to NO2 subsequently continues at ambient temperatures after the exhaust gases have passed out to the atmosphere. Nitrogen oxide is of particular concern because of its detrimental effects on respiration and plant life, as well as its significant contribution to acid rain. In addition, NOx, together with volatile organic compounds (VOC), is also involved in a series of photochemical reactions that lead to an increase in troposphere ozone which, in turn, adversely affects human health and natural vegetation. These problems are only pronounced on land and especially in urban areas.

Analysis of the combustion process in the cylinder and the reactions which are involved in formation of NO has identified three main sources of NO formation of which, as mentioned above, some is converted to NO2 to give the NOx mixture. These sources are thermal NO formation, prompt NO formation and fuel source. A majority of the NO emission is generated by internal combustion engines through the thermal process.

#### **a.** *Thermal nitric oxide (NO) formation*

During the combustion process in diesel engine, high temperatures are reached. Around 1700 K, and above up to 2500 K, sufficient thermal energy is available to dissociate oxygen, nitrogen and also other molecules formed during the combustion process itself. The recombination of the elements leads to the formation of NO. The reaction processes are quite slow so that most nitrogen oxides are formed during the mixing of the stoichiometric combustion gases with excess air in the cylinder. In low- and medium-speed diesel engines, by far the most important part of NOx is generated in the thermal NO process.

Formation of nitric oxide can be represented with three chemical reactions based on Zeldovich mechanism as in [16]:

$$N\_2 + O \to NO + N\tag{10}$$

$$\rm N + O\_2 \rightarrow \rm NO + O \tag{11}$$

$$N + OH \rightarrow NO + H \tag{12}$$

The first two reactions show the formation of nitric oxide for the lean mixture and the third for the rich mixture. The first reaction is the rate-limiting step due to its very high temperature activation. The high activation energy is required to break the triple bond in the nitrogen molecule (:N≡N:), which occurs at high combustion temperature; this is named thermal nitrogen monoxide (NO). The formation rate of thermal NO is practically insignificant if the temperature is below 1700 K. On the other hand, if the temperature rises, especially over 2000 K, the formation of thermal NO is strongly accelerated. The formation of thermal NO may be reduced by lowering and controlling the temperature peaks and minimising flue gas residence at high temperatures. As in [17], the equation for the total formation rate of thermal nitrogen oxides (NOx) is

$$\frac{d\left[\,^{\rm{NO}}\right]\_x}{dt} = \frac{6 \cdot 10^{16}}{T^{0.5}} \cdot e^{\left(\frac{-69090}{T}\right)} \cdot \left[\,^{\rm{N}\_2}\right] \cdot \left[\,^{\rm{O}\_2}\right]^{0.5} \,,\tag{13}$$

where T is absolute flame temperature (K), N2 nitrogen molecule concentration (molcm-3), O2 oxygen molecule concentration (molcm- ) and dNOx/dt nitrogen oxide speed formation (molcm-3).

#### **b.** *Prompt nitric oxide (NO) formation*

**5.1. Nitrogen oxides (NOx)**

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on land and especially in urban areas.

**a.** *Thermal nitric oxide (NO) formation*

mechanism as in [16]:

by internal combustion engines through the thermal process.

part of NOx is generated in the thermal NO process.

Nitrogen oxides (NOx) generate thermally from atmospheric nitrogen oxygen in the intake or scavenging air. The oxidation of atmospheric nitrogen is influenced by local conditions in the combustion chamber, such as the maximum cylinder pressure, local peak temperatures and local air- fuel ratio. The primary reaction product is nitric oxide (NO) by approximately 90 % of the volume, but about 5 % of it is converted into nitrogen dioxide (NO2) later in the combustion cycle, during expansion and during the flow through the exhaust system. At the same time, a very limited proportion of nitrous oxide N2O is also formed. Further oxidation of NO to NO2 subsequently continues at ambient temperatures after the exhaust gases have passed out to the atmosphere. Nitrogen oxide is of particular concern because of its detrimental effects on respiration and plant life, as well as its significant contribution to acid rain. In addition, NOx, together with volatile organic compounds (VOC), is also involved in a series of photochemical reactions that lead to an increase in troposphere ozone which, in turn, adversely affects human health and natural vegetation. These problems are only pronounced

Analysis of the combustion process in the cylinder and the reactions which are involved in formation of NO has identified three main sources of NO formation of which, as mentioned above, some is converted to NO2 to give the NOx mixture. These sources are thermal NO formation, prompt NO formation and fuel source. A majority of the NO emission is generated

During the combustion process in diesel engine, high temperatures are reached. Around 1700 K, and above up to 2500 K, sufficient thermal energy is available to dissociate oxygen, nitrogen and also other molecules formed during the combustion process itself. The recombination of the elements leads to the formation of NO. The reaction processes are quite slow so that most nitrogen oxides are formed during the mixing of the stoichiometric combustion gases with excess air in the cylinder. In low- and medium-speed diesel engines, by far the most important

Formation of nitric oxide can be represented with three chemical reactions based on Zeldovich

The first two reactions show the formation of nitric oxide for the lean mixture and the third for the rich mixture. The first reaction is the rate-limiting step due to its very high temperature

*N O NO N* <sup>2</sup> +® + (10)

*N O NO O* <sup>2</sup> +® + (11)

*N OH NO H* +®+ (12)

Prompt nitric oxide can be formed promptly at the flame front by the presence of hydrocarbon radicals produced only at the flame front at relatively low temperature. Nitric oxide generated via this route is named 'prompt nitric oxide (NO)'. Hydrocarbon (HC) radicals react with nitrogen molecules with the following sequence of reaction steps:

$$\rm{CH} + \rm{N}\_2 \rightarrow \rm{HCN} + \rm{N} \tag{14}$$

$$H\text{CN} + N \rightarrow \cdots \rightarrow NO \tag{15}$$

Nitrogen reacts with an HC radical to produce hydrogen cyanide HCN, and further, HCN reacts with nitrogen to produce nitric oxide via a series of intermediate steps. In contrast to thermal NO mechanisms that have activation temperature above 1700 K from (160), prompt NO can be formed starting at low temperature, around 1000 K as in [17].

#### **c.** *Fuel sources of NO formation*

NO formation from fuel becomes important when using heavy fuel oil because such fuels contain more organic nitrogen than marine diesel oil and other distillate fuels. Heavy fuel oil can contain up to 0.5 % nitrogen which increases the total NOx emission by as much as 10 %.

#### **5.2. Sulphur oxides (SOx)**

Formation of sulphur oxides (SOx) in the exhaust gases is caused by the oxidation of the elemental sulphur in the fuel into sulphur monoxide (SO), sulphur dioxide (SO2) and sulphur trioxide (SO3) during the combustion process. SOx emissions in diesel engine exhaust gas mostly comprise of sulphur dioxide and a small amount of sulphur trioxide. The stable products such as sulphur dioxide (SO2), hydrogen sulphide (H2S), carbon disulfide (CS2) and disulfide (S2) are created during the combustion of the rich mixtures. The radical sulphur monoxide (SO) reacts with oxygen (O2) to produce sulphur dioxide (SO2) at high temperatures. The amount of sulphur dioxide emissions depends on the sulphur content of the fuel used and cannot be controlled by the combustion process. Furthermore, sulphur trioxide (SO3) cannot be created in the combustion under fuel-rich conditions, even when the combustion is near the stoichiometric point. However, if there is even a 1 % air excess, sulphur trioxide rapidly increases in its quantity. Typically, the amount of SO3 is 5 % of the amount of sulphur oxides (SO2 and SO3). For example, if the fuel contains 3 % sulphur, the volume of SOx generated is around 64 kg per tonne of fuel burned; if fuel with 1 % sulphur content is used, SOx emission amount is about 21 kg per tonne of fuel burned as in [18]. SOx formed from diesel exhaust is corrosive and is partly neutralised by an engine's lubricating oil which is used as a typical base. Moreover, sulphur oxides (SOx) combine with moisture to form sulphuric acid (H2SO4), which is then excreted in the form of acid rain. It has a harmful effect on plants and human health and can damage many objects including buildings. Sulphur dioxide emissions also negatively impact human health; sulphate particles particularly can induce asthma, bronchitis and heart disease.

#### **5.3. Carbon monoxide (CO)**

The formation of carbon monoxide (CO) is a result of incomplete combustion of organic material, which is due to a lack of oxygen or low temperature at some points in the combustion chamber. Also the same reasons lead to the formation of hydrocarbons (HC). Hydrocarbons can also be formed from evaporation of the lubrication oil towards the end of the firing period. In diesel engines, the formation of carbon monoxide is determined by the air/fuel mixture in the combustion chamber, and since diesel fuel has a consistently high fuel-air ratio and the efficient combustion process, formation of this toxic gas is minimal. Nevertheless, insufficient combustion can occur if the fuel droplets in a diesel engine are too large or the level of turbulence is insufficient or swirl is created in the combustion chamber. When burning heavy fuel oil, the hydrocarbon emissions are lower than from the light fuel oil combustion due to lower evaporating level.

#### **5.4. Hydrocarbon (HC)**

Hydrocarbon (HC) emissions as fraction of the exhaust gases from diesel engines predomi‐ nantly consist of unburned or partially burned fuel and lubricating oils as a result of insufficient temperature. This often occurs near the cylinder wall where the temperature of the air/fuel mixture is significantly less than in the centre of the cylinder. In the atmosphere, the hydro‐ carbons are subjected to photochemical reactions with nitrogen oxides forming the groundlevel ozone and smog. Hydrocarbon (HC) emissions are represented as total hydrocarbons (THC) or as non-methane hydrocarbons (NMHC), as in [19].

#### **5.5. Particulate matter (PM)**

mostly comprise of sulphur dioxide and a small amount of sulphur trioxide. The stable products such as sulphur dioxide (SO2), hydrogen sulphide (H2S), carbon disulfide (CS2) and disulfide (S2) are created during the combustion of the rich mixtures. The radical sulphur monoxide (SO) reacts with oxygen (O2) to produce sulphur dioxide (SO2) at high temperatures. The amount of sulphur dioxide emissions depends on the sulphur content of the fuel used and cannot be controlled by the combustion process. Furthermore, sulphur trioxide (SO3) cannot be created in the combustion under fuel-rich conditions, even when the combustion is near the stoichiometric point. However, if there is even a 1 % air excess, sulphur trioxide rapidly increases in its quantity. Typically, the amount of SO3 is 5 % of the amount of sulphur oxides (SO2 and SO3). For example, if the fuel contains 3 % sulphur, the volume of SOx generated is around 64 kg per tonne of fuel burned; if fuel with 1 % sulphur content is used, SOx emission amount is about 21 kg per tonne of fuel burned as in [18]. SOx formed from diesel exhaust is corrosive and is partly neutralised by an engine's lubricating oil which is used as a typical base. Moreover, sulphur oxides (SOx) combine with moisture to form sulphuric acid (H2SO4), which is then excreted in the form of acid rain. It has a harmful effect on plants and human health and can damage many objects including buildings. Sulphur dioxide emissions also negatively impact human health; sulphate particles particularly can induce asthma, bronchitis and heart

The formation of carbon monoxide (CO) is a result of incomplete combustion of organic material, which is due to a lack of oxygen or low temperature at some points in the combustion chamber. Also the same reasons lead to the formation of hydrocarbons (HC). Hydrocarbons can also be formed from evaporation of the lubrication oil towards the end of the firing period. In diesel engines, the formation of carbon monoxide is determined by the air/fuel mixture in the combustion chamber, and since diesel fuel has a consistently high fuel-air ratio and the efficient combustion process, formation of this toxic gas is minimal. Nevertheless, insufficient combustion can occur if the fuel droplets in a diesel engine are too large or the level of turbulence is insufficient or swirl is created in the combustion chamber. When burning heavy fuel oil, the hydrocarbon emissions are lower than from the light fuel oil combustion due to

Hydrocarbon (HC) emissions as fraction of the exhaust gases from diesel engines predomi‐ nantly consist of unburned or partially burned fuel and lubricating oils as a result of insufficient temperature. This often occurs near the cylinder wall where the temperature of the air/fuel mixture is significantly less than in the centre of the cylinder. In the atmosphere, the hydro‐ carbons are subjected to photochemical reactions with nitrogen oxides forming the groundlevel ozone and smog. Hydrocarbon (HC) emissions are represented as total hydrocarbons

(THC) or as non-methane hydrocarbons (NMHC), as in [19].

disease.

184 Current Air Quality Issues

**5.3. Carbon monoxide (CO)**

lower evaporating level.

**5.4. Hydrocarbon (HC)**

Particulate matter is a mixture of organic and inorganic substances largely comprising elemental carbon, ash minerals, heavy metals, condensed sulphur oxides, water and a variety of unburned or partially burned hydrocarbon components of the fuel and lubricating oils. More than half of the total particulate mass is soot (inorganic carbonaceous particles), of which the visible evidence is smoke. Some of the fuel particles do not burn completely, and they are emitted as droplets of heavy liquid or carbonaceous material. The incomplete burning is a result of locally low quantities of excess air. A mistimed or otherwise poorly operating fuel injection and poor mixing of fuel within the cylinder also result in incomplete combustion and increased the particulate matter emissions. Soot particles (unburned – elemental carbon) are not themselves toxic, but they can cause the build-up of aqueous hydrocarbons (HC), and some of them are believed to be carcinogens. Particulates constitute no more than around 0.003 % of the engine exhaust gases. Almost the entire diesel particle mass is in the fine particle range of 10 microns or less in diameter (PM10). Approximately 94 % of the mass of these particles are less than 2.5 microns (PM2.5) in diameter. Diesel PM is of specific concern because it poses as a lung cancer hazard for humans as well as a hazard from noncancer respiratory effects such as pulmonary inflammation. Because of their small size, the particles are readily respirable and can effectively reach the lowest airways of the lung along with the adsorbed compounds, many of which are known or suspected mutagens and carcinogens. Secondary reactions of NOx and SOx can also produce PM.

The most effective method of reducing particulate emissions is to use lighter distillate fuels; however, this leads to added expense. Additional reductions in particulate emissions can be achieved by increasing the fuel injection pressure to ensure that optimum air/fuel mixing is achieved. The third method of reducing particulate emissions is to use cyclone separators, which are effective for particle sizes greater than 0.5 µm.

## **5.6. Carbon dioxide (CO2)**

Carbon dioxide is one of the basic products of combustion and is not toxic; however, it has been linked to the 'greenhouse effect' and global warming. Diesel engine exhaust gases containing CO2 as a result of carbon and oxygen O2 combustion. The maximum concentration of carbon dioxide will be generated during stoichiometric combustion, i.e. when complete amount of fuel reacts with oxygen from the air during combustion. The actual concentration of CO2 depends on the relative contents of carbon (C), hydrogen (H) and other combustible elements in the fuel. The maximum values of carbon dioxide for common types of marine fuel are shown in Table 3 [20], assuming that the exhaust is dry.

The maximum value of carbon dioxide (CO2 max) can be calculated according to the following expressions:

$$\text{CO}\_2\text{max} = \frac{\text{No. of CO}\_2\text{ molecules produced by complete combustion of fuel}}{\text{Total no. of molecules of combustion products}}\tag{16}$$

#### 186 Current Air Quality Issues


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

For 'wet' exhaust gases

$$\text{CO}\_2\text{max} = \frac{c}{c + \frac{h}{2} + \frac{79,1}{20,9} \cdot \left(c + \frac{h}{4}\right)} \text{ \%} \tag{17}$$

For 'dry' exhaust gases

$$\text{CO}\_2\text{max} = \frac{c}{c + \frac{79.1}{20.9} \cdot \left(c + \frac{h}{4}\right)}\text{ \%} \tag{18}$$

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

$$\mathbb{E}\left[\text{CO}\_2\right] = \frac{\text{CO}\_2\max\cdot\left(20, 9 - \left[\text{O}\_2\right]\right)}{20, 9}.\tag{19}$$


**Table 4.** Summary of pollutants

Reduction of carbon dioxide emissions can be achieved by reducing specific fuel oil consump‐ tion (SFOC) since the amount of CO2 produced is directly proportional to the volume of fuel used and therefore to the engine efficiency. An alternative is to use fuel with a low carbon ratio relative to hydrogen, which greatly increases the price of marine fuel oils. Table 4 provides a summary for the pollutants discussed above.
