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

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as predicted, single deliveries haveshown exceedings of the normal data,as a result of a more efficient refineryprocess. The residual fuel oils availableon the market today contain considerablyhigher quantities of asphalt, carbonand sulphur that contaminate the exhaustgas and thereby increase the riskof soot deposits on the exhaust gasboiler tubes.In recent years, and possibly as a consequenceof both the deteriorated fueland the above highly efficient and perhaps“overstretched” design, it also seemsthat the tendency to fouling, i.e. sootdeposits on the exhaust gas boiler tubes,has increased and, in some cases, hasresulted in soot fires. In extreme cases,the soot fire has developed into a hightemperature iron fire in which the boileritself burns. The above-mentioned tendencyis confirmed by DnV’s statistics,which reveal a sudden rise in soot fireincidents since 1985, see Fig. 1 andRef. [1], a rise, which may also havebeen caused by slow steaming of shipsdue to the low freight rates in recentyears.Since 1998, we have again seen a fallin the number of incidents, probablycaused by the effect of the new recommendedexhaust gas boiler design criteriaintroduced about 10 years ago, anddescribed in this paper.It is evident that the high fuel efficiencytarget must be met without jeopardisingthe reliability of the ship. It is thereforeimportant to know the main reasons forthe occurrence of soot deposits andfires so we can take the proper countermeasuresagainst them with a correctexhaust gas boiler/system design, etc.Warning triangle –risk of soot fireIgnition(of the soot)Fig. 2: Warning triangle - risk of soot fireWhen soot fires occur, the diesel enginewill normally be blamed since the sootparticles in fact originate from the engine’sfuel combustion. As, in principle, particlesin the exhaust gases are unavoidablefrom a modern diesel engine running onheavy fuel Ref. [1], the causes of soot deposits/firesmay be approached by askinga different question: What makes thesoot particles deposit and/or whatcauses the ignition of the soot deposits?This question may be illustrated by the“warning triangle” in Fig. 2 showing thethree factors which are all needed for asoot fire: soot deposits, oxygen and ignition.As the exhaust gas smoke from adiesel engine, due to its high air excessratio, contains about 14% oxygen, thesoot deposits and ignition items are ofparticular interest, as the oxygen cannotbe removed.Oxygen O 2(in exhaust gas smoke)Soot deposits(on boiler tubes)Scope of this paperThis paper is divided into two chapterswhich, in principle, may be consideredas two separate papers.The intention with Chapter I is to give aquick introduction to the most commonly-usedexhaust gas boiler types,steam systems and relevant parameters.Reading this chapter will form agood introduction before proceeding tothe issues of principle discussed inChapter II.Chapter II deals with the essential conditionscausing soot deposits and firesin exhaust gas boilers. The reasons forsoot deposits and their ignition are identifiedon the basis of statistical materialetc. In this context, recommendationsare given which are relevant to the designand operation of exhaust gas systemsand boilers.4This document, and more, is available for download at Martin's Marine Engineering Page - www.dieselduck.net
Chapter IBasic Information andBoiler DefinitionsHeat balance of a main engineWhen considering a heat balance diagramwhich, by way of example, is shown inFig. 3 for a nominally rated highly efficientengine version 6S60MC-C (or 6S60ME-C),operating on 80% SMCR (80% of specifiedmaximum continuous rating), the mostattractive waste heat source is the exhaustgas heat. Approximately one fourth ofthe fuel energy comes out as exhaustgas heat.Even though the exhaust gas temperaturethe last 25 years has decreased about130°C, from approx. 375°C to approx.245°C (ISO), as a result of the obtainedhigher efficiency of diesel engines, exhaustgas boilers are installed on almost allmerchant ships of today. However, thisdevelopment has been accompaniedby more trouble, as mentioned before.Permissible exhaust gasback-pressureThe permissible gas pressure loss acrossthe exhaust gas boiler has an importantinfluence on the gas velocity through theboiler. Thus, if a high pressure loss isacceptable, it is possible to design theboiler with a high gas velocity, but if onlya small pressure loss is permissible, thegas velocity will be low.The permissible pressure loss across theboiler depends on the pressure lossesof the total exhaust gas system after thediesel engine’s turbocharger(s).Permissible back-pressure of exhaustgas system for MC/MC-C andME/ME-C enginesAt the specified MCR of the engine, thetotal back-pressure in the exhaust gassystem after the turbocharger, indicatedby the static pressure measuredas the wall pressure in the circular pipeafter the turbocharger, must not exceed350 mm WC (0.035 bar), see Fig. 4.In order to have a back-pressure marginfor the final system, it is recommendedat the design stage that about 300 mmWC (0.030 bar) at specified MCR isused initially.The back-pressure in the exhaust gassystem depends on the gas velocity,i.e. it is proportional to the square of theexhaust gas velocity, and hence to the pipe6S60MC-CSMCR: 13,560 kW and 105.0 r/minService point: 80% SMCRFuel100%Shaft poweroutput 50.5%Lubricatingoil cooler3.3%Jacket watercooler5.8%Exhaust gas25.0%Air cooler14.6%Heat radiation0.8%∆∆∆p system∆p 1p 2p 3Max p 350 mm W.C.Design p 300 mm W.C.SparkarresterExhaustgassilencer∆∆system
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Page 8 and 9: OC Temperature TExhaust gas boilerP
Page 10 and 11: oC1501401301201101000Fig. 11: Sulph
Page 12 and 13: 21% O 279% N 297% HC3% S97% HC2.5%
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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-

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

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