Figure 5. Shell Acid Dew Point Curves (1978)
In such cases the corrosive wear rate will depend on the alkalinity of the lube oil (BN), the lube oil feed rate and the specific rate of acid neutralisation by the alkaline additive components [8].
This apparent discrepancy between prediction and practice has prompted a further investigation into the sulphuric acid dew points (the author's are much indebted to W. J. Crama of the Shell Research and Technology Centre in Amsterdam, for his valuable contribution to this investigation). This has resulted in a revised set of these dew point curves.
4.1 Revised Acid Dew Point Curves
Upon combustion of fuel sulphur, acid gases SO2 and SO3 are formed and together with abundantly present water vapour from fuel combustion, condensation of acid (with water) is possible. Only sulphuric acid (from SO3) is usually taken into account and not sulphurous acid (from SO2), since the contribution to corrosion of the latter is generally regarded as insignificant: it is a much weaker acid with a much lower dew point.
Based on an experimental thermodynamic data set of sulphuric acid-water mixtures [9] an accurate model description was developed for the liquid phase. Assuming an ideal behaviour for the vapour phase, an overall thermodynamic model description for the two-phase system could thus be generated. This model description was subsequently used for the calculation of dew points of combustion gases from a heavy fuel of typical composition and variable sulphur content (C / H = 8.21 and S = 1 to 5%m) at various pressures assuming thermodynamic equilibrium. The degree of sulphur conversion to its highest oxidation state (%SO3), the excess of air applied in practice to achieve complete fuel carbon and hydrogen combustion (λ) and the extent of combustion at the particular cylinder pressure of the engine cycle (n) were also taken into account. It was possible to correlate the calculated dew points by a mathematical expression, allowing results to be reproduced readily in a computerised manner.
Fuel sulphur is always rapidly and entirely oxidised to SO2 and a small fraction of this SO2 is further oxidised to SO3. It is normally assumed that all SO3 is formed at the high combustion temperatures in the cylinder by homogeneous (atomic oxygen) and heterogeneous (metal catalysed) oxidation and no further SO3 is formed later in the colder exhaust [10,11]. Various own measurements in diesel engine exhausts have given values of 0.3 - 1.5 for this percentage of SO2 that is converted into SO3, while in the literature values up to about 7% are mentioned [12]
Another parameter is the excess of air in the combustion chamber. For all practical purposes this can be fixed at λ = 2, being equivalent to an air-to-fuel ratio of about 28: 1 (kg:kg). Finally, close to top-dead-centre not all fuel will have combusted, therewith resulting in less than the potential amount of acidic species at the highest pressures. In theory, this may be corrected by choosing a conversion factor n between 0 and 1. It is probably so, however, that all sulphur will very quickly be converted to SO2 and then n may be assumed to be equal to 1. A convenient set of the new dew point curves is given in Figure 6, where it is assumed that the fraction of S converted into SO3 = 3.0%, λ = 2 and n = 1.
Figure 6. Shell Acid Dew Point Curves (2000)
