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

In general, the particles are small and,when the engine operates on heavy fueloil, it may be expected that over 90%of them will be less than 1 micron in size,excluding flakes of deposits, and peeling-offfrom the combustion chamber orexhaust system walls.The particulates also include some ofthe ash content of the oil, i.e. the tracemetals. The above-mentioned contributionfrom the lubricating oil consistsmainly of calcium compounds, viz.sulphates and carbonates, as calciumis the main carrier of alkalinity in lube oilto neutralise sulphuric acid.A test of the soot deposits in a boilerwith gilled tubes has shown that about70% of the soot is combustible.HydrocarbonsDuring the combustion process, a verysmall part of the hydrocarbons will leavethe engine unburnt, and others will beformed. These are referred to as unburnthydrocarbons, and they are normallystated in terms of equivalent CH 4content.The content of unburnt hydrocarbons inthe exhaust gas from large diesel enginescan be up to 300 ppm, but depends,among other factors, very much on themaintenance condition of the fuel injectionsystem and, to some extent, on thetype of fuel and the cylinder oil dosage.The hydrocarbon figure to some extentoverlaps the figure for particulates, asthese consist partly of hydrocarbons.Sticky effect of particulate emissionsIf the right – or rather the wrong – conditionsprevail, the soot particulatesmay deposit in the exhaust gas boiler.Furthermore, the lower the exhaust gasand heating surface temperatures become,the faster the soot is depositedand the harder it becomes to remove it.The explanation is that under such conditionsthe soot may be “wet” with oiland/or other gas condensates like hydrocarbons,and this may have an increasingeffect on the tendency of soot todeposit, as the soot may be more sticky.Soot fires in exhaust gas boilersA fire in the exhaust gas boiler may developin two or three stages, see Fig.15 and Ref. [2]. The ignition of soot normallydevelops into a small and limitedfire, but under extreme conditions itmay develop into a high-temperature fire.Ignition of sootIgnition of soot may arise in the presenceof sufficient oxygen when the depositsof combustible materials have a sufficientlyhigh temperature (higher thanthe flash point) at which they will liberatesufficient vapour, which may be ignitedby a spark or a flame.The main constituent of the soot depositis particulates but, in addition, someunburnt residues of fuel and lubricatingoils may be deposited in the boiler becauseof faulty combustion equipmentand, in particular, in connection withstarting and low speed running of theengine.The potential ignition temperature of thesoot layer is normally in the region of300-400 o C, but the presence of unburntoil may lower the ignition temperatureto approx. 150 o C, and under extremeconditions even down to 120 o C.This means that ignition may also takeplace after stop of the main engine asa result of glowing particles (sparks)remaining on the boiler tubes.Small soot firesSmall soot fires in the boiler are mostlikely to occur during manoeuvring withthe main engine in low load operation.These fires do not cause damage tothe boiler, or damage is very limited, butthe fires should be carefully monitored.Heat from the fire is mainly conductedaway with the circulation water andsteam and with the combustion gases.High-temperature firesUnder certain conditions, a small sootfire may develop into a high-temperaturefire. The photo in Fig. 16 shows anexample of an exhaust gas boiler whichhas had a high-temperature fire, wherethe boiler tubes have burned and melted.The reactions involved here are (see alsostage 3 in Fig.15).a. Hydrogen fireThis occurs because dissociation ofwater into hydrogen and oxygen or, inconnection with carbon, into carbonmonoxide and hydrogen, may occurunder certain conditions. A hydrogenfire may start if the temperature isabove 1000 o C.b. Iron fireAn iron fire means that the oxidationof iron at high temperatures occurs ata rate sufficiently high to make theamount of heat release from the reactionssustain the process. Thesereactions may take place at a temperaturein excess of 1100 o C.In this connection, it is important torealise that also water (H 2O) may goin chemical reaction with iron (Fe),i.e. the use of the steam based sootblower will feed the fire.This document, and more, is available for download at Martin's Marine Engineering Page - www.dieselduck.net13