Soot Deposits and Fires in Exhaust Gas Boiler - Martin's Marine ...

oC1501401301201101000Fig. 11: Sulphuric acid dew point of exhaust gas shown as a function ofthe sulphur content in the fuelC3500300250200Approximated sulphuric acid dew point0 1 2 3 4 5 wt %Sulphur (S) content in fuelExhaust gas temperature after T/C -/+ 15 CFig. 12: Influence of ambient air temperature on the exhaust gas temperature afterturbocharger for a 6S60MC-Cworse due to the low quality residual fuelson the market today.Low pinch point and soot depositsThe pinch point is therefore a parameterthat may influence the occurrenceof soot deposits when the pinch pointTropical (45 C air)ISO (25 C air)Winter (10 C air)30 40 50 60 70 80 90 100 110 % SMCRooooEngine shaft powerand thus the gas velocity is low. Conversely,a boiler designed with a highpinch point need not be a boiler with ahigh gas velocity. Such a boiler can, inprinciple, also be designed with a lowgas velocity, i.e. a low gas pressureloss across the boiler.Sulphuric acid corrosionA high degree of utilisation of the exhaustgas heat requires the lowes possibleexhaust gas boiler outlet temperaturewhich, if the required steam pressureand thereby the evaporation temperatureis sufficiently low, is limited mainlyby the risk of corrosion of the exhaustgas boiler heating surfaces due tosulphuric acid condensation.Corrosion starts when the temperatureof the boiler tube surfaces is equal to, orlower than, the dew point temperatureof the sulphuric acid. Furthermore, thetemperature of the boiler tube surfaces(gas side) is almost equal to the watertemperature in the boiler, due to the factthat the heat transfer coefficient on thegas side is extremely low compared tothat on the water side.The sulphuric acid dew point temperaturedepends especially on the contentof sulphur in the fuel oil and of oxygen inthe exhaust gas, but is rather difficult toestablish. The chemical reactions areas follows:a. at fuel combustion:S + O 2 SO 2b. at cooling of exhaust gas in the temperaturerange of 560° - 200°C:2SO 2+ O 2 2SO 3c. at reaction with water:SO 3+ H 2O H 2SO 4The chemical reaction b), in particular,is rather difficult to establish, the reasonbeing that the reaction takes placerather slowly and is catalysed by sootdeposits, etc., on the heating surfaces.Valid for the exhaust gas after turbochargerfrom MC/MC-C or ME/ME-Cmain engines, Fig. 11 shows, as aguide, the sulphuric acid dew point as afunction of the sulphur content in thefuel. With an average 2.9 % sulphurcontent in the fuel, the dew point of10This document, and more, is available for download at Martin's Marine Engineering Page - www.dieselduck.net