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.