Appendix 2 - Emission Ratio
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Statutory Documents - IMO Publications and Documents - Resolutions - Marine Environment Protection Committee - Resolution MEPC.340(77) - 2021 Guidelines for Exhaust Gas Cleaning Systems - (adopted on 26 November 2021) - Appendix 2 - Emission Ratio

Appendix 2 - Emission Ratio

 1 This appendix is included to explain the background to the use of the Emission Ratio, defined in 2.3 of these Guidelines, as the criterion for the demonstration of equivalency with the fuel oil sulphur limits given in regulation 14 of MARPOL Annex VI. In addition, the basis of the Emission Ratio limit values as given in 1.3 of these Guidelines is also explained.

2 The carbon content of any fuel oil used for power generation by combustion exits that system essentially in the form of carbon dioxide (CO2). While certain amounts of the inflow carbon may form deposits within that system, be incorporated into any direct contact lubricant or exit in the exhaust gas as carbon monoxide or gaseous or particulate hydrocarbons, overall these quantities are not significant in comparison to the flow of CO2. This applies equally to all combustion systems: internal combustion engines, boilers and gas turbines.

3 Similarly, the sulphur content of a fuel oil used for combustion will exit that system essentially as sulphur dioxide (SO2) in the hot exhaust gas stream. Again, although a certain amount may be retained as sulphur compounds within the system or as other sulphur compounds in the exhaust gas stream, these are not significant in comparison to the flow of SO2.

4 Hence, although the CO2 concentration in the exhaust gas will vary in accordance with the excess air ratio applied, the ratio of CO2 to SO2 concentrations will be fixed by the carbon/sulphur ratio of the fuel oil used. In those instances where an exhaust gas cleaning system (EGCS) covered by these Guidelines is fitted, the effect will be to reduce the SO2, but not the CO2 content of the exhaust gas. Consequently, the SO2/CO2 ratio after the system will reflect the effectiveness of that system in removing SO2 from the exhaust gas.1 The post-EGCS SO2/CO2 ratio, the Emission Ratio, will largely correspond to that which would otherwise have been obtained if a lower sulphur fuel oil had been used but without the EGCS.

5 The principal elements present in petroleum-derived liquid fuel oils are carbon, hydrogen and sulphur and in some instances also nitrogen and oxygen. The actual proportions differ in each case. In order to derive the Emission Ratios corresponding to different fuel oil sulphur limit values, the fuel oil compositions given in 6.4.11.1.2 (table 9) of the NOX Technical Code 2008 are taken as the starting points in table 1 below. The given compositions for both distillate and residual fuel oils omit sulphur content, but these are simply the difference between the summation of the given values and 100% and hence are 0.20% for the distillate example and 2.60% for the residual. In order to estimate the carbon and hydrogen proportions of fuel oils with other sulphur content values the carbon/hydrogen ratio and the "nitrogen+oxygen" content are assumed to be unchanged for the respective fuel oils. In table 1 the carbon contents are calculated for fuel oil having a sulphur content for both the distillate and the residual fuel oil of 1.50% as has been used in earlier versions of these Guidelines.

6 From the derived carbon contents and selected sulphur content value the molar ratio of fuel sulphur to fuel carbon is obtained in table 2 and from those the corresponding ratios of SO2 and CO2. One of the particular features of petroleum-derived liquid fuel oils is that despite the wide range of physical properties, such as viscosity and density, between distillates and residuals there is only a very limited range in terms of carbon composition. Hence it is a reasonable proposition to use a single SO2/CO2 ratio in order to represent all such fuel oils; in this instance 65 has been taken to correspond to the Emission Ratio which would be obtained if using a fuel oil of 1.50% sulphur content.2 The value of 1.50% sulphur content was used as the basis of these calculations as that was the original limit value for Emission Control Areas as given by the MARPOL Annex VI text as adopted in 1997, and which has been subsequently amended.

7 From the Emission Ratio corresponding to 1.50% sulphur the Emission Ratios corresponding to the various sulphur limits now given in regulation 14 of MARPOL Annex VI are obtained (see table 3).

Table 1: Fuel oil carbon content values

Distillate fuel oil – petroleum-derived

Carbon Given % m/m 86.2  
  Calculated % m/m   85.08
Hydrogen Given % m/m 13.6  
  Calculated % m/m   13.42
Sulphur % m/m 0.2 1.50
Nitrogen + Oxygen % m/m 0 0
Carbon / Hydrogen ratio   6.338 6.338

Residual fuel oil – petroleum-derived

Carbon Given % m/m 86.1  
  Calculated % m/m   87.08
Hydrogen Given % m/m 10.9  
  Calculated % m/m   11.02
Sulphur % m/m 2.60 1.50
Nitrogen + Oxygen % m/m 0.40 0.40
Carbon / Hydrogen ratio   7.899 7.899

Table 2: Emission Ratio values for 1.50% sulphur fuel oil

             
        Distillate Residual  
  Fuel Carbon % m/m 85.08 87.08  
    Sulphur % m/m 1.50 1.50  
    Carbon mol/kg 70.90 72.57  
    Sulphur mol/kg 0.469 0.469  
    S/C ratio mol/mol 0.00661 0.00646  
  Exhaust gas Emission Ratio SO2 ppm / CO2 % 66.12 64.60  
  65  
     

Table 3: Emission Ratios corresponding to fuel oil sulphur content2

Fuel oil sulphur content % m/m Emission Ratio
1.50 65
0.50 21.7
0.10 4.3

Note 1. Should treatment systems be developed that also reduce the CO2 content, the core principle still applies except that in order to assess effectiveness in terms of SO2 reduction the CO2 value used would be that prior to that reduction i.e. CO2 being measured at a point upstream of that treatment device.

Note 2. The given Emission Ratios only apply where a petroleum-derived liquid fuel oil is being used. For other fuel oils specific Emission Ratio values would need to be determined, and approved by the Administration, based on the particular composition of the fuel oil in question.


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