Appendix II – Proof of the SO₂/CO₂ Ratio Method

  1 The SO₂/CO₂ ratio method enables direct monitoring of exhaust gas emissions to verify compliance with emissions limits set out in Table 1 in section 1.3 of these Guidelines. In the case of EGC systems that absorb CO₂ during the exhaust gas cleaning process it is necessary to measure the CO₂ prior to the cleaning process and use the CO₂ concentration before cleaning with the SO₂ concentration after cleaning. For conventional low alkali cleaning systems virtually no CO₂ is absorbed during exhaust gas cleaning and therefore monitoring of both gases can be undertaken after the cleaning process.

  2 Correspondence between the SO₂/CO₂ ratio can be determined by simple inspection of the respective carbon contents per unit mass of distillate and residual fuel. For this group of hydrocarbon fuels the carbon content as a percentage of mass remains closely similar, whereas the hydrogen content differs. Thus it can be concluded that for a given carbon consumption by combustion there will be a consumption of sulphur in proportion to the sulphur content of the fuel, or in other words a constant ratio between carbon and sulphur adjusted for the molecular weight of oxygen from combustion.

  3 The first development of the SO₂/CO₂ ratio considered its use to verify compliance with emissions from 1.5% S fuel. The limit of 65 (1ppm/%) SO₂/CO₂ for 1.5% sulphur in fuel can be demonstrated by first calculating the mass ratio of fuel sulphur to fuel carbon, which is tabulated in Table 1 in this appendix for various fuels and fuel sulphur contents; including 1.5% sulphur for both distillate and residual fuels. These ratios were used to solve for the corresponding SO₂ and CO₂ concentrations in exhaust, which are tabulated in Table 2 of this Appendix. Molecular weights (MW) were taken into account to convert mass fractions to mole fractions. For the 1.5% sulphur fuels in Table 2, the amount of CO₂ is set first at 8% and then changed to 0.5% to show that there is no effect due to changes in excess air. As expected, the absolute SO₂ concentration changes, but the SO₂/CO₂ ratio does not. This indicates that the SO₂/CO₂ ratio is independent of fuel-to-air ratios. Therefore, SO₂/CO₂ratio can be used robustly at any point of operation, including operation where no brake power is produced.

 Note that the SO₂/CO₂ ratio varies slightly from distillate to residual fuel. This occurs because of the very different atomic hydrogen-to-carbon ratios (H:C) of the two fuels. Figure 1 illustrates the extent of the SO₂/CO₂ ratios’ sensitivity to H:C over a broad range of H:C and fuel sulphur concentrations. From Figure 1, it can be concluded that for fuel sulphur levels less than 3.00% S, the difference in S/C ratios for distillate and residual fuel is less than 5.0%.

 In the case of using non-petroleum fuel oils, the appropriate SO₂/CO₂ ratio applicable to the values given in regulations 14.1 and/or 14.4 will be subject to approval by the Administration.

  4 Correspondence between 65 (¹ppm/%) SO₂/CO₂ and 6.0 g/kWh is demonstrated by showing that their S/C ratios are similar. This requires the additional assumption of a brake-specified fuel consumption value of 200 g/kWh. This is an appropriate average for marine diesel engines. The calculation is as follows:

  Note 1: The S/C mass ratios calculated above, based on 6.0 g/kWh and 200 g/kWh BSFC, are both within 0.10% of the S/C mass ratios in the emissions table (Table 2). Therefore, 65¹ (ppm/%) SO₂/CO₂ corresponds well to 6.0 g/kWh.

  Note 2: The value of 6.0 g/kWh, hence the 200g/kWh brake-specified fuel consumption is taken from MARPOL Annex VI as adopted by the 1997 MARPOL Conference.

  5 Thus, the working formulas are as follows:

 * Note: gas concentrations must be sampled or converted to the same residual water content (e.g., fully wet, fully dry).

  6 The following is the basis of using the (^footnoteppm/%) SO₂/CO₂ as the limit for determining compliance with regulation 14.1 or 14.4:

• (a) This limit can be used to determine compliance from fuel oil burners that do not produce mechanical power.

• (b) This limit can be used to determine compliance at any power output, including idle.

• (c) This limit only requires two gas concentration measurements at one sampling location.

• (d) There is no need to measure any engine parameters such as engine speed, engine torque, engine exhaust flow, or engine fuel flow.

• (e) If both gas concentration measurements are made at the same residual water content in the sample (e.g., fully wet, fully dry), no dry-to-wet conversion factors are required in the calculation.

• (f) This limit completely decouples the thermal efficiency of the fuel oil combustion unit from the EGC unit.

• (g) No fuel properties need to be known.

• (h) Because only two measurements are made at a single location, transient engine or EGCS unit effects can be minimized by aligning signals from just these two analysers. (Note that the most appropriate points to align are the points where each analyser responds to a step change in emissions at the sample probe by 50% of the steady-state value.)

• (i) This limit is independent of the amount of exhaust gas dilution. Dilution may occur due to evaporation of water in an EGC unit, and as part of an exhaust sampler’s preconditioning system.