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

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