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

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OC Temperature TExhaust gas boilerPreheaterEvaporatorSuperheaterExhaust gasSaturatedsteam forheatingservicesOil-firedboiler withsteam drumHeatexc.250SuperheatedsteamSaturatedsteam300200150100Exhaust gasSteam/water 7 bar absPinchpointTurbogenerator(steam turbine)CondenserCondensate pumpsFeedwaterpumpAtmosphericsurpluscondenserHot well5000% 20% 40% 60% 80% 100%FeedwaterHeat transmissionQSuperheater Evaporator PreheaterFig. 8: Special exhaust gas boiler system with turbo generator forelectricity productionSingle pressure steam system with preheater, evaporatorand superheater sectionsFig. 9: T/Q diagram for an exhaust gas boilercommon steam drum for the oil-firedboiler and the exhaust gas boiler.Separate steam drums may also beused, so that one boiler can be run ifthe other should malfunction.Because of its simplicity and low capitalcost, the above system is widely usedand is often entirely adequate when thesteam production is viewed as a meansof meeting the steam demand for heatingservices on the ship.The loss of water from the exhaust gasboiler is in the magnitude of about 1%of the steam production.Special exhaust gas boiler systemwith turbogeneratorWhen a turbogenerator, i.e. a steamturbine driven electrical generator, is installed(utilising the steam available afterdeduction of steam for heating services),the exhaust gas boiler systemwill be more advanced.An example of such a system is shownin Fig. 8. The boiler is, apart from theevaporator, also fitted with a preheaterand superheater. In this system too,the steam drum of the oil-fired boiler isnormally used as a common steam drum.The influence of a boiler’spinch pointA boiler’s pinch point is a parameter thatcan tell us a lot about the boiler’s designand potential behaviour in operation. Itwill therefore be defined below, and itsinfluence on some important boiler parameterswill be discussed in this section.A boiler’s T/Q diagram and definitionof pinch pointA temperature/heat transfer diagram, aso-called T/Q diagram, illustrates thecharacteristic temperature coursethrough the exhaust gas boiler. As anexample valid for the special exhaustgas boiler system shown in Fig. 8, a T/Qdiagram is shown in Fig. 9.The utilisation efficiency of an exhaustgas boiler is characterised by its pinchpoint. The pinch point is the lowesttemperature difference between the exhaustgas and the saturated steam, i.e.the temperature difference between theexhaust gas leaving the evaporatorsection and the saturated steam, seethe T/Q diagram in Fig. 9.Normally, the steam pressure will be above7 bar abs. (6 barg) and often equal to 88This document, and more, is available for download at Martin's Marine Engineering Page - www.dieselduck.net
Relativesteamproduction%125110105100755025TemperatureCo230220210200190180170160Steam productionOutlet gas temperatureSteam temperatureHeat transfer surfaceRelative heattransfer surfaceExample:132% larger exhaust gas boilerrequired to produce 10% moresteam at 5 C pinch pointcompared with 15 Coo232225200175150141125100755025080 60 50 40 30 20 15 10 5 3 Pinch point CoFig. 10: Influence of a boiler´s pinch point, relative to 15 o C. The graph shows the relative influence of the pinchpoint on an exhaust gas boiler´s heat transfer surface (size and investment) and steam production [3]bar abs. (7 barg), corresponding to aminimum evaporation temperature of165°C. According to the T/Q diagramthe gas outlet temperature, even for aboiler with feed water preheater section,will therefore not be lower than about165°C, when 20°C or above is used asthe pinch point.A boiler’s steam production andheat transfer surfaceThe influence of the pinch point on theexhaust gas boiler design will be evidentfrom the following example.The graphs in Fig. 10 show the influenceof the pinch point on the boiler’s heattransfer surface and steam production [3].By way of example, the graphs in Fig.10 indicate that an exhaust gas boilerwith a pinch point of 5°C, comparedwith one with a pinch point of 15°C, willproduce 10% more steam, but at theexpense of having a heat transfer surfaceabout 2.3 times that of the originalboiler surface, and the gas velocitythrough the boiler may be correspondinglyreduced, as otherwise the pressureloss across the boiler might be too high.A boiler’s pressure loss and gasvelocityIn principle, the pinch point may beconsidered a measure of how extensiveand how efficient the heat utilisation ofthe exhaust gas boiler is.The lower the pinch point, the larger theheat transfer surfaces and the more efficientis the exhaust gas boiler, and thehigher is the gas pressure loss acrossthe boiler. As the maximum permissiblegas pressure loss has a certain limitation,the boiler’s design gas velocity has to bereduced in order not to exceed the limitfor the permissible gas pressure loss.This is what has happened with themore efficient exhaust gas boiler designduring the past two decades becauseof the lower exhaust gas temperaturesof the diesel engines. In this context,Chapter II will show that a low gas velocityin particular will have a distinct influenceon the tendency towards soot deposits,a tendency which has becomeThis document, and more, is available for download at Martin's Marine Engineering Page - www.dieselduck.net9
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 10 and 11: oC1501401301201101000Fig. 11: Sulph
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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