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

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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
sulphuric acid in the exhaust gas fromthe main engine can be expected to beabout 135°C, which means that in thiscase the temperature of boiler circulatingwater or feed water at the boiler inletshould be kept higher than 135°C.Steam production – Influenceof ambient temperaturesDuring normal operation of the ship, theambient air and seawater temperatureswill change, and this will have an influenceon the exhaust gas temperature.Thus, the exhaust temperature after turbochargerswill decrease about 1.6°C foreach 1.0°C reduction of the turbochargerintake air temperature, and vice versa.Steam productionkg/hISO ambient conditions (25 C)3,0002,0001,0001,00002,000040Extra steam needed40Surplus steamSteam consumptionSteam consumptionAs an example, valid for a 6S60MC-Cengine, Fig. 12 shows the influence ofthe turbocharger air intake temperatureon the exhaust gas temperature, validfor ISO reference conditions (25°C air/25°C c.w.), tropical air temperature of45°C and a winter air temperature of10°C, respectively.The similar example (see Fig. 13) validfor an Aframax tanker having a 6S60MC-Cmain engine installed, shows the correspondingsteam production of an exhaustgas boiler with an evaporator section only,and based on the steam pressure of 8bar abs., and 20°C pinch point, togetherwith the needed steam consumption forheating services. The upper graph for theISO (25°C air) based boiler design showsthat too much steam will be produced,50 60 70 80 90 100 %SMCRSteam productionkg/hWinter ambient conditions (10 C)50 60 70 80 90 100 %SMCREngine shaft powerFig. 13: Influence of ambient air temperarure on the steam production of an exhaust gasboiler installed on an Aframax tanker with main engine 6S60MC-CooTotal steamproductionTotal steamproductionand the surplus steam has to be dumpedby means of the atmospheric surplus condenser.However, in winter time (10°C air)with a lower exhaust gas temperature,the steam production will be lower, whereasthe steam consumption will increase,meaning that the oil fired boiler may occasionallyhave to start up to supplement thesteam production.Particulate emissions fromdiesel enginesLow speed diesels have been leadingthe way with regard to the acceptanceof low-grade fuels, low fuel consumptionand high reliability. In this process, thepresence of particulates in the exhaustgas, from an operational point of view,always has been, and no doubt alwayswill be, unavoidable.The typical exhaust gas emission valuesfor the most commonly discussed pollutants,NO x, SO x, CO, HC, and particulates,are shown in Fig. 14. In thecontext of this paper, only the particulate/sootemissions, and to some degreethe hydrocarbons (HC), are of interestand will be described in the following.Sources of particulate emissionsParticulates in the exhaust gas mayoriginate from a number of sources:• Agglomeration of very small particlesof partly burnt fuel• Ash content of fuel oil and cylinderlube oil• Partly burnt lube oil• Peeling-off of combustion chamber/exhaust system deposits.Typical form and rate of particulateemissionsOnce fuel is atomised in the combustionchamber of a diesel engine, theThis document, and more, is available for download at Martin's Marine Engineering Page - www.dieselduck.net11
Page 1 and 2: Soot Deposits and Fires in Exhaust
Page 3 and 4: Soot Deposits and Fires in Exhaust
Page 5: Chapter IBasic Information andBoile
Page 8 and 9: OC Temperature TExhaust gas boilerP
Page 12 and 13: 21% O 279% N 297% HC3% S97% HC2.5%
Page 14 and 15: Chapter IIBoiler Experience andDesi
Page 16 and 17: Number of cases15Troubledesign mean
Page 18 and 19: Analogy with snow (soot deposits)An
Page 20 and 21: )Preheated feed water during start-

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