imo fuel conversion

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E

MARINE ENVIRONMENT PROTECTION COMMITTEE 60th session Agenda item 4

MEPC 60/WP.622 March 2010

Original: ENGLISH

DISCLAIMER

As at its date of issue, this document, in whole or in part, is subject to consideration by the IMO organ to which it has been submitted. Accordingly, its contents are subject to approval and amendment

of a substantive and drafting nature, which may be agreed after that date.

PREVENTION OF AIR POLLUTION FROM SHIPS

Communication with IPCC on CO2 Conversion Factors

Note by the Secretariat Introduction 1 MEPC 58 agreed that the carbon to CO2 conversion factors (CF) used by IMO should correspond to the factors used by IPCC (2006 IPCC guidelines). MEPC 58 also agreed that the same carbon to CO2 conversion factors should be used for the purpose of the EEDI and the EEOI, as well as in other IMO instruments. 2 INTERTANKO, in document GHG-WG 1/3/1, proposed modified carbon to CO2 conversion factors for Marine Diesel/Gas Oil, High Sulphur Fuel Oil and Low Sulphur Fuel Oil, according to the actual hydrocarbon content in each fuel type after deduction of impurities and other non-carbon components. 3 SIGTTO, in document MEPC 59/4/10, proposed new carbon to CO2 conversion factors for Liquid Petroleum Gas (LPG) and Liquid Natural Gas (LNG). 4 MEPC 59 agreed to use the carbon to CO2 conversion factors proposed by INTERTANKO and SIGTTO in the EEDI and the EEOI, and amended the two guidelines accordingly. 5 In accordance with the agreement at MEPC 58, carbon to CO2 conversion factors proposed by INTERTANKO and SIGTTO were conveyed by the Secretariat to IPCC for consideration and validation. The data forwarded by the Secretariat to IPCC is set out in the annex to this document. 6 The Secretariat received a response from IPCC stating the following:

The carbon to CO2 conversion factors proposed by IMO were considered by the EFDB Editorial Board (EB). The EB concluded that more information on the fuel composition would be necessary for the assessment, specifically:

MEPC 60/WP.6

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.1 further substantiation (references or documentation) for adopting a single hydrocarbon as representative of each fuel; and

.2 the associated uncertainties of this data would also be highly desirable.

In addition, the EB noted that it would be more useful for greenhouse gas inventory to have EFs expressed in terms of mass per energy unit.

Action requested of the Committee 7 The Committee is invited to note the information provided above and take action as appropriate.

***

MEPC 60/WP.6 Annex, page 1

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ANNEX

DATA FORWARDED BY THE SECRETARIAT TO IPCC 1 Conversion Factors currently used in MEPC/Circ.471 (CO2 operational Index),

based on 1996 IPCC Guidelines

Type of fuel ISO Specification Carbon content [m/m]

CCarbon [t CO2 / t Fuel]

Diesel/Gas Oil ISO 8217 Grades DMX through DMC 0.875 3.206000

Light Fuel Oil (LFO)

ISO 8217 Grades RMA through RMD 0.86 3.151040

Heavy Fuel Oil (HFO)

ISO 8217 Grades RME through RMK 0.85 3.114400

Liquid Petroleum Gas (LPG) 0.81 2.967840

Liquid Natural Gas (LNG) 0.80 2.931200

2 Proposed new Conversion Factors

Type of fuel CCarbon [t CO2 / t Fuel]

Marine diesel and marine gas oils (MDO/MGO) 3.082 Low Sulphur Fuel Oils (LSFO) 3.075 High Sulphur Fuel Oils (HSFO) 3.021 Liquefied Natural Gas (LNG) 2.750 Liquid Petroleum Gas (LPG) � Propane 3.000 Liquid Petroleum Gas (LPG) � Butane 3.030

3 Background of the proposed new Conversion Factors 3.1 For MDO/MGO, LSFO and HSFO

The amount of CO2 emissions is calculated as a function of the amount of fuel used by ships and as a function of the pure Carbon content in each fuel type. The bunker fuel oils used by ships are Marine Diesel/Gas Oil (MDO/MGO), High Sulphur Fuel Oil (HSFO) and Low Sulphur Fuel Oil (LSFO). The step-wise methodology to calculate the Carbon to CO2 conversion factor is as per MEPC/Circ.471 and is as follows: Step 1 � Establish the mean molecular structure for the 3 fuel oils used by ship � The Carbon content on each fuel type is determined to establish the mean molecular structure for each fuel oil. These are presented in Table 1 and are based upon reasonable and expert assessed mean molecular size of the average hydrocarbon content appropriate to each fuel type. The calculations assume that fuels contain only paraffins and contain no cyclic molecules whose hydrogen/carbon ratios differ due to their unsaturated status.


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Table 1

Fuel type Average Molecular structure MDO/MGO C18

HSFO/LSFO C30 Within the respective fuel oils, and CO2 as appropriate, the individual atoms and their molecular weights are shown in Table 2, which are used throughout the calculations.

Table 2

Hydrocarbon Atoms Molecular Weight Carbon 12.011

Hydrogen 1.0 Oxygen 15.9994

Step 2 � Establish the Hydrocarbon content in each of the fuels � This is obtained indirectly by assessing the non-hydrocarbon elements in these fuels (not containing atoms of carbon). The data for each fuel type are shown in Table 3.

Table 3

Percentage in fuel of non-Hydrocarbon elements

MDO/MGO HSFO LSFO Component type Sulphur Content 0.80% 2.70% 1.00% Ash and Metals 0.01% 0.15% 0.15% Water 0.20% 0.50% 0.50% Total non-Hydrocarbon elements 1.01% 3.35% 1.65% Hydrocarbon % (content in fuel) 98.99% 96.65% 98.35% Step 3 � Calculate the Carbon concentration (Carbon %) in the fuel � To determine the Carbon concentration in the fuel one needs the three elements as given by the following formula:

%nHydrocarbo x fueltheinnHydrocarbotheofweightmolecularaverage

fueltheincontentCarbonofweightmolecular%Carbon =

The Hydrocarbon content in each fuel (Hydrocarbon %) is given in Table 3. The molecular weight of the Carbon in the fuel is calculated with the formula:

Cn = n x 12.011 where "n" is the number of Carbon atoms in the molecule (i.e. n = 18 for the MDO/MGO and n = 30 for HSFO and LSFO � see Table 1). As an example for MDO/MGO:

C18 = 18 x 12.011 = 216.2


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Based upon the standard chemical equation for paraffins of nCH(n + 2), the molecular weight of the Hydrocarbon atoms in the fuel is calculated with the formula:

Cn = (n x 12.011) + (n x 2 + 2) As an example, the Carbon concentration in MDO/MGO is:

84.20%98.99%x254.2216.2%Carbon ==

Using the data for each fuel type, one can determine the Carbon concentration (Carbon %) in each of the fuel type, as given in Table 4:

Table 4 MDO/MGO

(C18) HSFO (C30)

LSFO (C30)

Carbon molecular weight 216.20 360.33 360.33 Hydrocarbon average molecular weight 254.20 422.33 422.33 Hydrocarbon content in fuel 98.99% 96.65% 98.35% Carbon concentration in the fuel 84.20% 82.46% 83.91% carbon concentration in MEPC/Circ.471 87.50% 85.00% N/A Step 4 � Calculate the Carbon to CO2 factor (similar for all the individual fuel calculations) � The Carbon to CO2 factor gives the amount of CO2 generated by one atom of carbon. It is calculated by the following formula:

CarbonofweightmolecularCOtheofweightmolecularfactorCOtoCarbon 2

2 =

The molecular weight of the CO2 is = molecular weight of the atom of Carbon + 2 x molecular weight of the atom of oxygen = 12.011 + 2 x 15.9994 = 44.01

Therefore, the Carbon to CO2 factor is 3.6612.01144.01

==

Step 5 � Calculate the Carbon to CO2 conversion factor for each fuel type � The conversion factor is derived by correcting the pure Carbon to CO2 factor for the concentration of Carbon in each fuel type or:

%Carbon x COforfactorCOtoCarbonfactorconversionCOtoCarbon 222 = For MDO/MGO the Carbon to CO2 Conversion factor = 3.66 x 0.842 = 3.082. This means that there would be 3.082 tonnes of CO2 emitted for each tonne of MDO/MGO fuel combusted. The calculations run for each fuel type and the results are presented in Table 5.


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Table 5 MDO/MGO HSFO LSFO Carbon to CO2 conversion factor for CO2 3.66 Carbon concentration in the fuel 84.20% 82.46% 83.91% Carbon to CO2 conversion factor for a fuel 3.082 3.021 3.075 carbon concentration in MEPC/Circ.471 3.206 3.114 N/A Conclusion The following Carbon to CO2 conversion factors may be used instead of those now referred to in MEPC/Circ.471.

FUEL TYPE Carbon to CO2 conversion factor Marine diesel and marine gas oils (MDO/MGO)

3.082

Low Sulphur Fuel Oils (LSFO) 3.075 High Sulphur Fuel Oils (HSFO) 3.021

3.2 For LNG and LPG

1 With reference to the nomenclature in the table used in MEPC/Circ.471, the following should be noted:

1.1 "Liquid Petroleum Gas (LPG)". The normal descriptor for this

product is "Liquefied Petroleum Gas (LPG)". There are commonly two grades of LPG, � one being predominantly Propane and one being predominantly Butane. These are generally carried as separate grades, but may occasionally be transported as a mixture.

1.2 "Natural Gas". Natural Gas is a very broad description of gas

whose main component is methane. Carriage of natural gas at sea, either as cargo, or, more rarely, as a propulsion fuel, is in the form of "Liquefied Natural Gas (LNG)". The natural gas is converted to LNG by means of cooling to about � 160°C. The demands of this process result in a product with a very limited range of compositions. In particular, CO2, which may be present in natural gas, has to be removed since it would form a solid in the liquefaction process. The following explanation arises, in part, from this limited range of compositions.

2 To simplify the explanation, the table has been re-composed showing the key figures that of CO2/tonne of fuel burnt expressed in tonnes/tonne. The explanation of the issues is clearer if consistent units are used.

Table 6 � CO2 Conversion Factors t/t of fuel

Diesel 3.206 Gasoil 3.206 LFO 3.151 HFO 3.114 LPG 2.968 LNG 2.931


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3 The main difference between the conventional liquid fuels and the gas fuels (LPG and LNG) is that the gas fuels are characterized in each case by having a single dominant component with a simple chemical formula whereas the liquid fuels are complex mixtures of hydrocarbon compounds. The section below titled "Derivation of Conversion Factors" expands on this point. 4 The following points should be noted.

4.1 The conversion factor for LNG, or more specifically, boil-off gas (BOG) used as fuel, the figure should be 2.75, not 2.931. This is derived from the chemical formula and laws of combustion. (Note: this assumes BOG is 100% methane, typically it contains a few percent nitrogen which would have the effect of reducing the value.) For LNG, as opposed to BOG, (this would arise if LNG is force-vaporized to provide supplementary fuel) the value will rise slightly, this is further discussed in paragraph 14.

4.2 LPG: for propane the value is 3.0 and butane 3.03. Commercial

grade LPGs may have traces of heavier hydrocarbons which would slightly increase the figure. Non-hyrdrocarbons/inerts would slightly decrease the figure.

5 From the perspective of the operators of LNG vessels, it is important to rectify the figure as per paragraph 4.1 above. The use of boil-off gas as fuel significantly decreases the CO2 emitted for a given power production compared to that if heavy fuel oil were used. It is important that the full and correct credit be given for these fuels if the desire to reduce CO2 emissions to the environment is to be realised. For LPG, the issue may be theoretical since, at the time of writing, there are no LPG-fuelled seagoing vessels and current codes do not permit the use of LPG as fuel in ships' engine rooms. Nevertheless, the figure should be corrected since the future is not foreseeable and the use of LPG as propulsion fuel could occur. Derivation of Conversion Factors1 6 As noted above, the boil-off gas from an LNG cargo is primarily methane. The scientific explanation of this can be found in paragraphs below. The start point for the derivation of the CO2 conversion factor for LNG is therefore to consider methane. 7 Fundamentals:

Atomic weight of Carbon (C) 12 Hydrogen (H) 1 Oxygen (O) 16

8 Methane � CH4

Combustion equation:

CH4 + 2O2 → CO2 + 2H2O (1) On both sides of the equation there are:

Carbon 1 atom Hydrogen 4 atoms Oxygen 4 atoms

Therefore the equation balances.

1 "Combustion Engineering and Gas Utilization", 3rd Edition.


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Putting in the atomic weights: (12 + 4) + (64) → (12 + 32) + (4 + 32) (2) 9 From the point of view of determining the CO2 conversion factor, the only interest is the fuel (CH4) and the CO2, i.e. the ratio of (12 + 4) to (12 + 32) or 16:44 or 2.75. (Note that the effects of excess air typically present in industrial/marine burning systems have no impact on the mass ratio of CO2 emitted to mass of fuel burnt.) 10 An identical process gives the following CO2 conversion factors for:

Table 7 � Conversion Factors

Methane CH4 2.75 Ethane* C2H6 2.93 Propane C3H8 3.0 Butane C4H10 3.03

* Note: Ethane is not generally carried as a bulk liquid cargo.

Sensitivities 11 Effects of Nitrogen in LNG As noted above, LNG vessels typically utilize boil-off gas from the cargo as fuel. An examination of Raoult's law:

nPXY 11

1 = (3)

∑ X P 1

Where: Y1 is the molar fraction of component "1" in the vapour phase X1 is the molar fraction of component "1" in the liquid phase P1 is the vapour pressure of component "1" at the prevailing

liquid temperature, quickly leads to the position that the impact of the ethane, propane, butane and C5+ in mixture that is LNG is negligible since their vapour pressure at a typical LNG temperature of -160°C is less than 5 mb in a container where the vapour pressure is of the order of 1100 mb absolute. 12 In assessing the composition of the boil-off gas, this only leaves the methane and nitrogen content of the LNG as contributors. Typical nitrogen contents of LNG are of the order of 0.1 to 0.35% at the point of export, see annex 1. Applying Raoult's Law, this gives a nitrogen content in the vapour space (mass base) in the range of 6.5% to 20%. This will decline exponentially as the voyage progresses and the nitrogen boils off preferentially. Nevertheless, the effect of nitrogen in LNG will be to reduce the theoretical CO2 conversion factor suggested above from 2.75, if the calculation is based on total cargo mass loss over the voyage.


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13 Effects of "Forced Boil-off" on LNG ships The contribution to the total fuel demand by boil-off gas varies depending on type of ship (e.g., membrane or Moss) and on whether the vessel is laden or in ballast. The range is wide and can vary from 40% to 95% of the total. Traditionally, the fuel used to supplement the shortfall of that available from boil-off gas has been heavy fuel oil, but there has been a trend towards use of LNG from the cargo to supplement the fuel demand. The implication is that, up to 60% of the fuel gas consumed may be of LNG composition rather than boil-off gas. Annex 1 contains compositional details of a range of commercial LNG as transported by sea. The average CO2 conversion factor for LNG form the compositions listed is 2.759 in a range of 2.730 to 2.774 and calculating out the CO2 conversion factor assuming a mixture of 40% boil-off gas and 60% LNG delivers a factor of 2.755. Sources of Inaccuracy and Uncertainty for LNG 14 Mass Measurement of Boil-off Gas The most accurate method of determining the mass of boil-off gas consumed over a voyage is by reference to the Custody Transfer System records. Accurate readings are taken before and at completion of cargo transfer in any port. From these it is possible to determine the mass loss from the cargo over the voyage. The process is described in reference2, which also determines that the overall accuracy of the measurement of the total mass of LNG (plus vapour) on board is of the order of 0.49%. 15 Ideal Gas The application of Raoult's law assumes an ideal gas. Real mixtures do not behave in this manner and there is some very small deviation from the ideal due to compressibility factors "Z". This only tends to become significant in calculations for gases over 20 bar and hence, at the pressures considered here, is a second order effect. Given the other uncertainties, it has been ignored for simplicity. 16 Atomic Weights The atomic weight figures used in equation (2) have been rounded and are widely used. Reference3 gives more accurate figures, but there are disputes in scientific circles over the exact figures. Reference (3) gives:

Hydrogen 1.00794 Carbon 12.0107 Oxygen 15.9994

17 Using these in the equation (2) above yields a CO2 conversion figure of 2.743. 18 Nitrogen in LNG The effect of the nitrogen component of LNG is described above. In determining a default figure for CO2 conversion factor, this probably represents the biggest area of uncertainty and, in extreme cases could lead to a 20% overestimation of environmental emission of CO2. The effect of nitrogen is specific to each cargo and voyage length and it is therefore not possible to arrive at a practical mean figure for use in a default calculation.

2 "LNG Custody Transfer Handbook" 2nd Edition. GIIGNL. 3 Periodic Table published by International Union of Pure and Applied Chemistry 2006.


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19 For LNG cargoes with nitrogen contents in the liquid phase of greater than 0.1%, the mass of boil-off gas determined from CTS readings should be reduced by the change in mass of the nitrogen content, derived from the compositional analysis performed as per reference (2). Sources of inaccuracy and uncertainty for LPG 20 Composition as transported The figures given in Table 7 are for pure components. In practice, "commercial" grades of propane and butane contain some other components. From the data sheets attached, this leads to a range of CO2 conversion factors shown in Table 8 below. From this, it is concluded that the use of the conversion factor for the pure product as a default value is acceptable.

Table 8 � CO2 Conversion Factors for LPG

Propane Pure 3.0 Representative 2.9995 Extreme 3.017

Butane Pure 3.03 Representative 3.0291 Extreme 3.0297

21 Use of LPG a fuel As noted above, LPG is not currently used as a fuel for propulsion of ships. Assuming that industrial practice is followed, the LPG will be supplied as liquid to a vaporizing system which vaporizes all components before being fed to the combustion process, then in consideration of environmental emissions of CO2, the composition as loaded should be used, rather than the composition in the cargo tank vapour space as for LNG ships. 22 Determination of mass There are uncertainties around the measurement of mass, the accuracy is not expected to be better than that for LNG above. Conclusions and recommendations 23 The table below gives recommended default values for the CO2 conversion factors. Note that, for LNG, the nitrogen content may need special consideration.

Table 9 � Recommended Conversion Factors

C Carbon [t CO2/t Fuel] C Carbon[gCO2 / t Fuel] LNG 2.75 2,750,000 LPG �Propane 3.0 3,000,000 LPG � Butane 3.03 3,030,000

* * *


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ANNEX 1

LNG COMPOSITION

The composition of LNG at the export point from 19 major exporters has been examined. Plants excluded from the analysis are Skikda � currently undergoing a major refurbishment and Marsa el Brega, whose LNG production composition is atypical. Marsa el Brega production represents about 2% of world capacity. The last column shows the calculated figure for Nitrogen content of the boil-off gas for the compositions of LNG shown. It has been included here to show the large scatter in the result.

Methane Ethane Propane Butane Nitrogen Pentane + Nitrogen mol % mol % mol % mol % mol % mol % in BOG mol % 87.4 8.6 2.4 0.05 0.35 0.02 21.788

91.23 4.3 2.95 1.4 0.12 0 8.383 90.4 5.2 2.8 1.5 0.07 0.02 5.111 97.7 1.8 0.22 0.2 0.08 0 5.389

84.83 13.39 1.34 0.28 0.17 0 12.235 93.4 6.5 0.1 0 0 0 0.000 97.2 2.3 0.3 0.2 0 0 0.000

91.09 5.51 2.48 0.88 0.03 0 2.240 89.33 7.14 2.22 1.17 0.08 0.01 5.865 99.8 0.1 0 0.1 0.1 0 6.516 89.4 6.3 2.8 1.3 0.05 0.05 3.745 91.8 5.7 1.3 0.4 0.8 0 37.743 87.9 7.3 2.9 1.6 0.4 0 24.045 90.1 6.2 2.3 1 0.4 0 23.596 96.2 3.26 0.42 0.07 0.008 0.01 0.575 90.1 6.47 2.27 0.6 0.25 0.03 16.179

89.02 7.33 2.56 1.03 0.06 0 4.479

* * *


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ANNEX 2

LPG COMPOSITION

The compositions of commercial grades of Propane and Butane as transported by sea are variable since differing compositional standards exist around the world The table below shows three compositions for each of Propane and Butane. One is the pure product, one is called representative and is derived from a review of actual cargo certificates held by a major international LPG ship operator and the third, labelled "extreme", is derived from limit conditions from various MSDS to give a high value for the CO2 conversion factor.

Ethane Propane Propylene Butane Pentane mol % mol % mol % mol % mol % Propane, pure 100 Representative 2 95 3 Extreme 5 85 5 5 Butane, pure 100 Representative 4 95 1 Extreme 3 95 2

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