Evaluation Methods

Before examining all the statistics regarding the emission in maritime transport is meaningful to be aware how these figures are obtained. There are fundamentally two main methods for computing the CO2 emissions that are produced by a specific transport means (Psaraftis and Kontovas, 2009):

 Bottom-up approach: the emissions are calculated using simulation models calibrated on the ships activity;

 Top-down approach: this method basically computes the total emissions through the fuel sales data;

Estimates vary in respect to which of these two approaches is employed, besides the results are also affected by how data are elaborated and which assumption are made. In figure 3.3 are reported the result of the third IMO study as example of the relevant differences in the results between the bottom-up approach and the top-down approach.

Figure 3.3: CO2 emissions for the Top-Down approach and the Bottom-Up approach This graph regards the emissions of the international shipping.

Adapted from: (IMO, 2014), Table 2 and Table 3

625,5 624

596,4

647,5 648,9

884,9 920,9

855,1

771,4

849,5

0 100 200 300 400 500 600 700 800 900 1000

2007 2008 2009 2010 2011

[Mtonne] Emissions of CO2

Top-Down Bottom-Up

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3.1.1 T

-

A

The top-down approach is based on the fuel sales data, indeed it is also called “fuel-based”. Fundamentally, this method consists in computing the emissions multiplying the amount of fuel sold by the CO2 emission factor. Usually, in the maritime field different type of fuels are used for the main engine and the auxiliary engine. Ships principally use oil-based fuels such as HFO (heavy fuel oil) and MDO (maritime diesel oil). Therefore, if different fuels are taken into account, the total CO2 emissions, 𝐸𝑚𝑖𝑠𝑠𝑖𝑜𝑛𝑠_𝐶𝑂2 [tonne], can be calculated by the following equation:

𝐸𝑚𝑖𝑠𝑠𝑖𝑜𝑛𝑠_𝐶𝑂2 = ∑ 𝐹𝑆_𝑖 𝐸𝐹_{𝐶𝑂2,𝑖}

𝑖

(3.1) Where FSi is the amount of fuel sold ith [tonne] and 𝐸𝐹_{𝐶𝑂2,𝑖} is the emissions factor of such fuel. The value of emissions factor for the typical maritime fuel are reported in section 3.1.1. The data regarding the fuel sales are collected from database provided by the Energy Information Administration (EIA), the International Energy Agency (IEA) and the United Nations Framework Convention on Climate Change. For example, the IEA is the data source used in the inventory of CO2 emissions elaborated by (IMO, 2014). This approach would be the most reliable however the data about fuel sales are sometimes considered not dependable². Indeed, the results obtained from the top-down approach considerably differs from those furnished by the bottom-up approach.

3.1.2 B

-

A

The bottom-up method computes emissions by modelling the fleet activity, indeed this method is also called “activity-based”. Namely, this means that some activity data are required, such as travelled kilometres per year or day at sea per year. These activity data are then multiplied by some emission factors such as fuel consumption per km in tonnes or daily fuel consumption in tonnes respectively. Obviously, it is difficult to calculate a proper value of these emission factors hence many uncertainties are present in such studies. For instance, the fuel consumption per day of a vessel is a function of the sailing speed as well as of the payload and other factor therefore in order to compute the daily fuel consumption it is necessary to be aware about the vessel’s speed, the payload and other activity features. Moreover, once the daily consumption for the single ship is estimated, by this value it has to be calculated the global fleet’s total emissions and this is not a trivial challenge. Indeed, the sailing speed as well as the other activity

2The reasons that lead not to rely on fuel sales’ data are reported in (Psaraftis and Kontovas, 2009)

Speed optimization and environmental effect in container liner shipping Page 39 characteristics are different for each vessel, moreover these sort of data is not available, especially on a global scale. As a consequence, many assumptions a simplification are required. As an example, (Psaraftis and Kontovas, 2009) provide a study which estimate the CO2 emissionsof world commercial fleet, using the bottom-up approach. In this study, assuming the operative days per year, the time at sea hence the time in port and finally the daily fuel consumption at sea and the daily fuel consumption in port, the yearly emissions are computed for several size brackets and for different types of vessels such as container ships, tanker ships and bulk carriers. Some results of this study are reported and elaborated in the next section. Another example can be (Gkonis and Psaraftis, 2012) into which the emissions of the global fleet of a specific tanker segment are estimated.

Such study takes into account that the speed depends on both the bunker price and the freight rate, thus allowing to evaluate how these two factors influence the amount of emissions produced. According to (IMO, 2014), the best estimate for years’ emissions for GHG is provided by the bottom-up approach hence the results obtained from such analysis must be considered as benchmarks. Therefore, all the data provided in this thesis refers to the bottom-up method.

3.1.3

E

F

The emission factors EF are fundamentally coefficients that allow evaluating the emission of a certain gas. Multiplying the EF by the fuel consumption FC, for example in [tonne/day], permits to compute the amount of emissions E produced by burning the fuel:

𝐸 = 𝐹𝐶 𝐸𝐹 (3.2)

In fact, the EF is the number of gas tonnes produced per tonnes of burned fuel. The common values of EF are reported in table 3.1 for three different types of fuel, regularly used in maritime transport. However, in some articles a unique emission factor is used for each type of fuel. For instance, this was made in the first IMO GHG study of 2000 (Psaraftis and Kontovas, 2009) wherein the EF is equal to 3.17.

Similarly, the emission factors are furnished for each GHG and more in general for each pollutant agent whose environmental impact must be evaluated.

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CO2 Emissions Factors

Fuel Emissions Factor

HFO 3,021

MDO 3,082

LNG 2,7

Table 3.1: Emissions factor for HFO, MDO and LNG

The EF are in tonne of CO2 produced per tonne of fuel burned.

Adapted from: (Psaraftis and Kontovas, 2009)

Instead, in (IMO, 2014) a different value for the CO2 emission factor is provided, which is higher than the previous, as shown in table 3.2, such values are employed in the thesis to evaluate the emissions of the fleet. LNG contains less carbon than the other fuels hence the emissions of CO2 are lower. Nevertheless, using LNG increases the CH4 emissions (methane slip is the proper name for methane that is not used as a fuel and basically escapes into the atmosphere) hence the net effect of employing this type of fuel is a reduction by 15% of CO2eq.

CO2 Emissions Factors (IMO, 2014)

Fuel Emissions Factor

HFO 3,114

MDO 3,206

LNG 2,750

Table 3.2: Emission factor provided by the third IMO GHG study The EF are in tonne of CO2 produced per tonne of fuel burned.

Adapted from: (IMO, 2014), Page 248

Besides, in order to evaluate the effectiveness of using a specific fuel, it is also necessary to take into account the SFOC’s value (Specific Fuel Oil Consumption) for each type of bunker. Indeed, this parameter allows assessing the grams of fuel required to maintain a given power for one hour. This value depends on the vessel’s speed, however some values are reported in table 3.3 as indicative values.

SFOC [g/kWh]

Fuel Specific Fuel Oil Consumption

HFO 215

MDO 205

LNG 166

Table 3.3: SFOC for different fuel type Data source: (IMO, 2014), Table 24

Speed optimization and environmental effect in container liner shipping Page 41

3.1.4

C

D

E

As said in section 3, the main GHG is the carbon dioxide however also the methane CH4

and the nitrous oxide N2Oare greenhouse gases. These two gases are produced when the fuel is burned as well as the CO2. Therefore, their influence on pollution must be taken into account when emissions are computed. In order to assess the environmental effect of CH4 and N2Ois introduced a new concept: the carbon dioxide equivalency CO2e. As claimed in (IMO, 2014) the carbon dioxide equivalency is “a quantity that describes, for a given amount of GHG, the amount of CO2 that would have the same global warming potential (GWP) as another long-lived emitted substance, when measured over a specified timescale (generally, 100 years)”. The GWP expresses the contribution of a gas on the greenhouse effect relatively to effect of CO2. The GWP is equal to 25 and 298³ for methane and nitrous oxide respectively, considering a time scale of 100 years. This means that one tonne of N2O has the same consequence upon the greenhouse effect of 298 tonnes of CO2.

Table 3.3 reports the CO2e for each GHG and points out as the carbon dioxide is by far the most influential greenhouse gas, being responsible of the pollution about by 98%. As consequences, this thesis does not consider the pollution derived by N2Oand CH4, as it remarked in section 4.2.

CO2e Emissions [Mtonne]

2007 2008 2009 2010 2011 2012

CO2 884,900 920,900 855,100 771,400 849,500 795,700

CH4 5,929 6,568 6,323 7,969 9740 9,742

N2O 12,152 12,689 11,860 10,615 11,473 10,931

Table 3.4: CO2e emissions for GHGs in million tonnes produced This graph regards the emissions of the international shipping.

Adapted from: (IMO, 2014), Table 19

3 IPPC Fourth Assessment Report, Climate Change 2007-The physical science basis, Table TS.2

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