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Soot Deposits and Fires in Exhaust Gas Boilers

Introduction

The demand tor the highest possible overall fuel efficiency is reflected in developments over the last two decades in the propulsion market for oceangoing ships. Today, this market is dominated by highly efficient two-stroke low speed diesel engines which run on low quality fuels and utilise (recover) the exhaust gas heat by means of an exhaust gas boiler.
In the same period, reduced specific • fuel oil consumption, i.e. the increased thermal efficiency of the diesel engine, has resulted in lower exhaust gas temperatures. Based on ISO ambient reference conditions (25 °C air and seawater) and with the present nominal ratings of the MC engines, the exhaust gas temperature after the turbocharger is about 250-270 °C, but may be lower for derated engines

Rise in soot fire Incidents

As a consequence of the lower exhaust gas temperatures and the remaining steam consumption requirements, the exhaust gas boiler has been designed to become more and more efficient. This involves the use of a large heat transfer surface and thus a boiler design with a low internal gas velocity as well as tubes with "extended" surfaces.

Furthermore, the quality of the fuels has decreased significantly during the same period. Whereas the average fuel quality may not have deteriorated as much as predicted, single deliveries have shown exceedings of the normal data, as a result of a more efficient refinery process. The residual fuel oils available on the market today contain considerably higher quantities of asphalt, carbon and sulphur that contaminate the exhaust gas and thereby increase the risk of soot deposits on the exhaust gas boiler tubes.

In recent years, and possibly as a consequence of both the deteriorated fuel and the above highly efficient and perhaps "overstretched" design, it also seems that the tendency to fouling, i.e. soot deposits on the exhaust gas boiler tubes, has increased and, in some cases, has resulted in soot fires. In extreme cases, the soot fire has developed into a high temperature iron fire in which the boiler itself bums. The above-mentioned tendency is confirmed by DnV's statistics, which reveal a rise in soot fire incidents since 1988, see Fig. 1 [9], a rise, which may also have been caused by slow steaming of ships due to the low freight rates in recent years.

It is evident that the high fuel efficiency target must be met without jeopardizing the reliability of the ship. It is therefore important to know the main reasons for the occurrence of soot deposits and fires so we can take the proper countermeasures against them with a correct exhaust gas boiler/system design, etc.

Warning triangle - risk of soot fire

When soot fires occur, the diesel engine will normally be blamed since the soot particles in fact originate from the engine's fuel combustion. As, in principle, particles in the exhaust gases are unavoidable from a modem diesel engine running on heavy fuel [9], the causes of soot deposits/fires may be approached by asking a different question; What makes the soot particles deposit and/or what causes the ignition of the soot deposits?

This question may be illustrated by the "warning triangle" in Fig. 2 showing the three items which are all needed for a soot fire: soot deposits, ignition and oxygen. As the exhaust gas smoke from a diesel engine, due to its high air excess ratio, contains about 14% oxygen, the soot deposits and ignition items in particular may be of interest as it would be difficult to remove the oxygen.

Scope of this paper

This paper is divided into two chapters which, in principle, may be considered as two separate papers.

The intention of Chapter I is to give a quick introduction to the most commonly used exhaust gas boiler types, steam systems and relevant parameters. Reading this chapter will form a good introduction before proceeding to the issues of principle discussed in Chapter II.

Chapter II deals with the essential conditions causing soot deposits and fires in exhaust gas boilers. The reasons for soot deposits and their ignition are identified on the basis of, among other things, statistical material. In this context, recommendations are given which are relevant to the design and operation of exhaust gas systems and boilers.

Chapter I

Basic Information and Boiler Definitions Particulate emissions from diesel engines

Low speed diesels have been leading the way with regard to the acceptance of low-grade fuels, low fuel consumptions and high reliability. In this process, the presence of particulates in the exhaust gas, from an operational point of view, always has been, and no doubt always will be, unavoidable.

The typical exhaust gas emission values for the most commonly discussed pollutants, NOx, SOx, CO, HC, and particulates, are shown in Fig. 3. In the context of this paper, only the particulate/soot emissions, and to some degree the hydrocarbons (HC), are of interest and will be described In the following.

Sources of particulate emissions Particulates in the exhaust gas may originate from a number of sources: [1]
• Agglomeration of very small particles of partly burned fuel
• Ash content of fuel oil and cylinder lube oil
• Partly burnt lube oil
• Peeling-off of combustion chamber/exhaust system deposits.

Typical form and rate of particulate emissions [1]
Once fuel is atomised in the combustion chamber of a diesel engine, the combustion process takes place from small droplets of fuel which evaporate, ignite, and are subsequent^ burnt. During this process, a minute part of the oil, comprising mainly carbon, will be left as a "nucleus".

Particulate emissions will vary substantially with the fuel oil composition and lube oil type and dosage. It is therefore

difficult to state general emission rates for particulates, but when the engine is operating on heavy fuel oil. values of the order of 120-150 mg/Nm3, corresponding to some 0.8-1.0 g/kWh, may be considered typical.

In general, the particles are small and, when the engine operates on heavy fuel oil, it may be expected that over 90% of them will be less than 1 micron in size, excluding flakes of deposits, and peeling-off from the combustion chamber or exhaust system walls.

The particulates also include some of the ash content of the oil, i.e. the trace metals. The above-mentioned contribution from the lubricating oil consists mainly of calcium compounds, viz. sulphates and carbonates, as calcium is the main carrier of alkalinity in lube oil to neutralise sulphuric acid.

A test of the soot deposits in a boiler with gilled tubes has shown that about 70% of the soot is combustible [10].

Hydrocarbons [1]
During the combustion process, a very small part of the hydrocarbons will leave the engine unburnt, and others will be formed. These are referred to as unbumt hydrocarbons, and they are normally stated in terms of equivalent CH4 content.

The content in the exhaust gas from large diesel engines can be up to 300 ppm, but depends, among other factors, very much on the maintenance condition of the fuel injection system and, to some extent, on the type of fuel and the cylinder oil dosage.

The figure to some extent overlaps the figure for particulates, as these consist partly of hydrocarbons.

Sticky effect of paniculate emissions If the right - or rather the wrong - conditions prevail, the soot particulates may deposit in the exhaust gas boiler.

Furthermore, the lower the exhaust gas and heating surface temperatures become, the faster the soot is deposited and the harder it becomes to remove it [3]. The explanation is that under such conditions the soot may be "wet" with oil and/or other gas condensates like hydrocarbons, and this may have an increasing effect on the tendency of soot to deposit, as the soot may be more sticky.

Soot fires in exhaust gas boilers [2]

A fire in the exhaust gas boiler may develop in two or three stages: An ignition of soot which normally develops into and remains as a small and limited fire, but which under extreme condrtions may develop into a high temperature fire.

Ignition of soot
Ignition of soot may arise in the presence of sufficient oxygen when the deposits of combustible materials have a sufficiently high temperature (higher than the flash point) at which they will liberate sufficient vapour, which may be ignited by a spark or a flame.

The main constituent of the soot deposit is particulates but, in addition, some unbumt residues of fuel and lubricating oils may be deposited in the boiler because of faulty combustion equipment and, in particular, in connection with starting and low speed running of the engine.

The potential ignition temperature of the soot layer is normally in the region of 300-400 °C, but the presence of unburnt oil may lower the ignition temperature to approximately 150 °C, and under extreme conditions even down to 120 °C.

This means that ignition may also take place after shutdown of the main engine as a result of glowing particles (sparks) remaining on the boiler tubes.

Small soot fires
Small soot fires in the boiler are most likely to occur during manoeuvring with the main engine in low load operations, These fires do not cause damage to the boiler, or damage is very limited, but the fires should be carefully monitored.

Heat from the fire is mainly conducted away with the circulation water and steam and with the combustion gases.

High temperature fires
Under certain conditions, a small soot fire may develop into a high temperature fire. The reactions involved here are:
a) Hydrogen fire
This occurs because dissociation of water into hydrogen and oxygen or, in connection with carbon to carbon monoxide and hydrogen, may occur under certain condrtions. A hydrogen fire may start if the temperature is above 1000 °C
b) Iron fire
An iron fire means that the oxidation of iron at high temperatures occurs at a rate sufficiently high to make the amount of heat release from the reactions sustain the process. These reactions may take place at a temperature in excess of 1100 °C.

Boiler types

The types of exhaust gas boilers utilizing the diesel engine exhaust gas heat may, in principle, be divided into two main groups;

• Water tube boilers
• Smoke tube boilers

Water tube boilers

This is the boiler type most frequently used - often in connection with high exhaust gas heat utilisation. The exhaust gas is passed across the outside of the boiler tubes, with the water flowing inside. see Fig. 4. In order to make the boiler as efficient and as compact as possible, the heat transfer area on the gas side of the tubes may often be expanded with, for example, narrowly spaced, gilled (finned) or pinned tubes.

The water tube boiler type will normally not be equipped with a steam space (also called steam collector or steam drum), but will sometimes be operated in connection with a separate steam drum or, more often, with the steam drum of the oil-fired boiler.

This type of boiler will, in most cases, be fitted with a soot-blowing arrangement in order to remove any soot deposits. The soot depositing tendency has been on the increase due to the low gas velocity and temperature normally used today.

In several cases the increased occurrence of soot deposits on this type of boiler has been followed by a soot fire.

In extreme cases - as mentioned earlier -the high temperature caused by the soot fire has resulted in a so-called iron fire in which the boiler itself bums. This may have occurred due to leakage of water from the boiler because of the high temperature. The iron fire may also have occurred because the crew tried to put out the fire by activating the soot blowers for the injection of steam or water. The high temperature would thus cause dissocation of steam into oxygen and hydrogen. The oxygen may then have caused oxidation of the iron, i.e. an iron fire.

Gilled and pinned tubes are more vulnerable to soot fires than plain tubes. because the highest metal temperatures will occur on the edge of the gills, which will thus be the most likely starting point for an iron fire [8].

Smoke tube boilers

In the smoke tube boilers, the gas is conducted through a bundle of tubes with small internal diameters (of the magnitude 30-80 mm) and surrounded on the outside by water, see Fig.5 The smoke tube boiler type is often chosen in specific cases where it is desirabte to operate the exhaust gas boiler independently of the oil-fired boiler. This is possible as the smoke tube boiler may be frtted with its own separate steam drum.

In general, a high gas velocity in the boiler tubes is desirable, in order to make the heat transfer as high as possible and the soot deposit as low as possible [7].

As a cleaning system is very difficult to install, this boiler type is therefore designed to have a self-cleaning effect, which may be obtained by using a relatively high design mean gas velocity, exceeding some 20 m/s, through the tubes [7].

In some cases soot has blocked some of the boiler tubes, with a consequent increase in pressure loss and reduction in boiler efficiency. The solution may be to clean the tubes manually at regular intervals, although this may be expensive.

On the other hand, the soot deposits have very seldom led to damage caused by soot fire, because the boiler tubes are surrounded/cooled by water and the heat surface has a limited area.

Boiler steam systems

Exhaust gas boiler steam systems may be designed in many different versions, with one or two pressure levels, with or without preheater sections, etc.

As examples the most commonly used steam systems - both simple and advanced -are described overleaf.

Normal exhaust gas boiler system

The exhaust gas boiler system normally used for the production of saturated steam needed for heating sen/ices is shown in Fig. 6.

This is a simple, single-pressure steam system in which the exhaust gas boiler consists solely of an evaporator section. The feed water is pumped directly to the oil-fired boiler which is used as a common steam drum for the oil-fired boiler and the exhaust gas boiler.

Separate steam drums may also be used, so that one boiler can be run if the other should malfunction.

Because of its simplicity and low capital cost, the above system is widely used and is often entirely adequate when the steam production is viewed as a means of meeting the steam demand for heating services on the ship.

Special exhaust gas boiler system with turbogenerator

When a turbogenerator, i.e. a steam turbine driven electrical generator, is installed (utilising the steam available after deduction of steam for heating services), the exhaust gas boiler system will be more advanced.

An example of such a system is shown in Fig. 7. The boiler is, apart from the evaporator, also fitted with an economiser (preheater) and superheater. In this system too, the steam drum of the oil-fired boiler is normally used as a common steam drum.

The influence of a boiler's pinch point

A boiler's pinch point is a parameter that can tell us a lot about the boiler's design and potential behaviour in operation. It will therefore be defined below, and its influence on some important boiler parameters will be discussed in this section.
A boiler's T/Q diagram and definition of pinch point
A temperature/heat transfer diagram, a so-called T/Q diagram, illustrates the characteristic temperature course through the exhaust gas boiler. As an example valid for the special exhaust gas boiler system shown in Fig. 7, a T/Q diagram is shown in Fig. 8.

The utilisation efficiency of an exhaust gas boiler is characterised by its pinch point. The pinch point is the lowest temperature difference between the exhaust gas and the saturated steam, i.e. the temperature deference between the exhaust gas leaving the evaporator section and the saturated steam, see the T/Q diagram in Fig. 8.

Normally, the steam pressure will be 7 bar abs. or above, corresponding to a minimum evaporation temperature of 165 °C. According to the T/Q diagram the gas outlet temperature, even for a boiler wrth feed water preheater section, will therefore not be lower than about 160 °C, when 15 °C or above is used as the pinch point.

A boiler's steam production and heat transfer surface

The influence of the pinch point on the exhaust gas boiler design will be evident from the following exampte.
The graphs in Fig. 9 show the influence of the pinch point on the boiler's heat transfer surface and steam production [4]. By way of example, the graphs in Fig. 9 indicate that an exhaust gas boiler with a pinch point of 5 °C, compared with one with a pinch point of 15 °C, will produce 10% more steam, but at the expense of having a heat transfer surface about 2.3 times that of the original boiler surface, and the gas velocity through the boiler may be correspondingly reduced, as otherwise the pressure loss across the boiler may be too high.

A boiler's pressure loss and gas velocity

In principle, the pinch point may be considered a measure of how extensive and how efficient the heat utilisation of the exhaust gas boiler is.

The lower the pinch point, the larger the heat transfer surfaces and the more efficient is the exhaust gas boiler, and the higher is the gas pressure loss across the boiler. As the maximum permissible gas pressure loss has a certain limitation, the boiler's design gas velocity has to be reduced in order not to exceed the limit for the permissible gas pressure loss.

This is what has happened with the more efficient exhaust gas boiler design during the past decade because of the lower exhaust gas temperatures of the diesel engines. In this context, Chapter II will show that a low gas velocity in particular will have a distinct influence on the tendency towards soot deposits, a tendency which has become worse due to the low quality residual fuels on the market today.

Low pinch point and soot deposits

The pinch point is therefore a parameter that may influence the occurrence of soot deposits when the pinch point and thus the gas velocity is low. Conversely, a boiler designed with a high pinch point need not be a boiler with a high gas velocity. Such a boiler can, in principle, also be designed with a low gas velocity i.e. a low gas pressure loss across the boiler.

Permissible exhaust gas pressure loss

The permissible gas pressure loss across the exhaust gas boiler, as previously mentioned, has an important influence on the gas velocity through the boiler. Thus, if a high pressure loss is acceptable, it is possible to design the boiler wrth a high gas velocity, but if only a small pressure loss is permissible, the gas velocity will be low.

The permissible pressure loss across the boiler depends on the pressure losses of the total exhaust gas system after the diesel engine's turbocharger(s).

Permissible back-pressure of exhaust gas system for MC engines

At the specified MCR of the engine, the total back-pressure in the exhaust gas system after the turbocharger - indicated by the static pressure measured as the wall pressure in the circular pipe after the turbocharger - must not exceed 350 mm WC (0.035 bar).

In order to have a back-pressure margin for the final system, it is recommended at the design stage that about 300 mm WC (0.030 bar) at specified MCR is used initially.

The back-pressure in the exhaust gas system depends on the gas velocity, i.e. it is proportional to the square of the exhaust gas velocity, and hence to the pipe diameter to the 4th power. It is recommended not to exceed 50 m/s in the exhaust gas pipes at specified MCR. It has by now become normal practice, in order to avoid too much pressure loss, to have an exhaust gas velocity in the pipes of about 35 m/sec at speci-
fied MCR.

As long as the total back-pressure of the exhaust gas system – incorporating all resistance losses from pipes and components - complies with the above-mentioned requirements, the pressure losses across each component, such as the exhaust gas boiler and silencer, may be chosen independently.

Permissible pressure loss across boiler

At specified MCR, the maximum recommended pressure toss across the exhaust gas boiler is normally 150 mm WC.

This pressure loss depends on the pressure loss in the rest of the system, as mentioned above. Therefore, if an exhaust gas silencer/spark arrester is not installed, the acceptable pressure loss across the boiler may be somewhat higher than the maximum of 150mm ' WC, whereas, if an exhaust gas silencer/spark arrester is installed, it may be necessary to reduce the maximum pressure loss.

It should be noted that the above-mentioned pressure loss across the boiler also incorporates the pressure losses from the inlet and outlet transition boxes.

Chapter II

Boiler Experience and New Design Criteria

Statistical analyses of soot fires

Soot fires in exhaust gas boilers were very unusual some years ago but, during the last decade, and especial^ the last five years, soot deposits and soot fires have occurred more often.

Analyses of soot fires indicate that, in most cases, they occur in connection with manoeuvring, often following a stay in harbour.

On the basis of a sample of 82 ships, most of which are equipped with twostroke main engines and water tube type boilers, the NK "Guide to Prevention of Soot Fire on Exhaust Gas Economizers 1992" [5] presents a statistical parameter survey of soot fires. The survey covers 53 ships with trouble (soot fire and damage) and, for comparison purposes, also 29 NK ships with no trouble. The engines are in the power range of about 5,000-40.000 bhp, and about 10% of the boilers are of the large capacity
types, including dual pressure type boilers.

It should be noted that ships with trouble were extracted from a representative sample of all NK ships, while ships with no trouble were limited to cases in which NK received answers from shipyards or boiler makers.

The parameters stated below have (where known) been obtained from the shipyards and boiler makers in question. The parameters have been studied with regard to any distinct influence on boiler troubles and, if any such influence was found, it is indicated in Table 1.

Trouble/no trouble comparisons for some of the most interesting parameters have been made in graphical form and are shown in Figs. 10, 11, 12, 13 and 14, Even though the ships included in the examination have been freely selected, for which reason simple comparisons may not be made, the results of the comparisons may be considered as being very indicative.

Influence of main engine type

It is rather interesting, but not surprising, to see that, as shown in Fig. 10, the make and type of main engine had no distinct influence on the risk of soot fire. Thus, the ships equipped with, for example, MAN B&W, Sulzer or Mitsubishi two-stroke main engines, all seem to have had the same relative number of cases with and without soot-fire troubles. Furthermore, statistics show that the occurrence of soot fires is also largely independent of whether it is a short or a long stroke engine.

There is no information regarding the type of fuel oil, but, as we are dealing with two-stroke engines, heavy fuel oil is probably used. Operating the engine on heavy residual fuels of low quality probably has an increasing effect on the tendency towards soot deposits. As low quality heavy residual fuels are cheap, this tendency may be considered as an unavoidable parameter now and in the future (unless, for example, special fuel additives are used, as indicated in recent information). Special features regarding "Operation on Heavy Residual Fuels" have earlier been described in an
MAN B&W Paper [6].

Influence of extended tube surface

*Fig. 11 shows, somewhat surprisingly, that the shape of the water tube elements used in exhaust gas boilers of the water tube type had no distinct influence on the tendency towards soot fires.

in fact, the type of boiler fitted with plain tube elements had almost the same relative number of soot fire problems as boilers fitted with tube elements with an extended surface. On the other hand, the severe cases of soot fire, with buming down of the tube elements themselves, may be more of a risk for boilers with extended tube surface than for those with plain tubes, as the potential area, or should we say reservoir for soot deposits, is bigger.

Influence of exhaust gas temperature It has often been claimed that the latest development of diesel engines, involving lower exhaust gas temperatures, is causing the soot deposits in the exhaust gas boilers.

On the other hand, when we only consider the influence of the exhaust gas temperature itself, the statistical analyses show rather clearly that this is not correct, see Fig. 12.

Fig. 12 shows that neither the inlet nor outlet temperature of the exhaust gas boiler has any distinct influence on the occurrence of soot fires. Even at inlet temperatures as high as 325-350 °C. and outlet temperatures as high as 225-250 °C, soot fires occur, and even at outlet temperatures as low as 100-150 °C, many boilers had no such trouble.

The lower exhaust gas temperature can only be blamed for its possible negative influence on other boiler parameters like larger heat transfer area and lower gas velocity, whch can influence the occurrence of soot fires, see next section.

Fig. 12 tells us nothing about the potential influence of the low gas temperature in the boundary layer on the cold boiler tubes. This type of low gas temperature may, despite the above results, still have an increasing effect on the tendency towards soot deposrts. as the soot on the tube surfaces may be made wet and sticky by gas condensates.

Influence of low gas velocity

The statistical analyses of soot fires show, as indicated in the above table, that one of the parameters that has a distinct influence is the gas velocity in the boiler, see Fig. 13.

All exhaust gas boilers based on a design gas velocity lower than 10 m/s had soot fire trouble, whereas relatively few boilers based on a design gas velocity higher than 20 m/s had such trouble.

One of the dominant parameters influencing the occurrence of soot fires, since it increases the tendency towards soot deposits is therefore – according to the statistical material - the LOW GAS VELOCITY in the BOILER, see also the lower side of the warning triangle in Fig. 2.

Stickiness of the soot

The low gas velocity seems to be an important factor. On the other hand, the low gas velocity limit is probably a "floating" limit which may also depend on the actual Stickiness of the soot in the exhaust gas smoke, which again may depend on the actual residual fuel used (containing asphalt, carbon and sulphur).

Thus the stickier the soot, the more easily it will stick to the boiler tubes. One could claim that the Stickiness of the soot is the dominant factor for the occurrence of soot deposits.

On the other hand, looking at Fig. 13 we can see that this could mean that only the exhaust gas boiler with a low design mean gas velocity included in the statistics had exhaust gas smoke containing sticky soot, and this seems improbable.

Regarding the Stickiness of the soot. the latest information received has revealed that, due to a chemical reaction with the hydrocarbons, the use of a fuel additive containing iron oxide may involve that the soot will be less sticky and more dry. The exact chemical background for this observatton is not cteariy understood.

The result will be a reduction in the tendency towards soot deposits because the soot is less sticky and the gas velocity limit for soot deposits will, in turn, be reduced, i.e. the soot deposits will be less sensitive to the low gas velocity.

Such a fuel additive may therefore be useful in cases where the exhaust gas botes have suffered from soot deposits.

Influence of low water inlet velocity and low circulation water flow ratio

The diagrams in Fig. 14 show the influence of the water inlet velocity to the boiler and the circulation water flow ratio (circulation water and steam production mass flow ratio), and also indicate an important influence on the occurrence of soot fires.

Thus, the lower the water inlet velocity to the boiler, and the lower the circulation water ratio, the higher the likelyhood of soot fire problems.

A sufficient circulation water flow rate is therefore important for avoiding critical damage to exhaust gas boilers.

This is because a low circulation water flow rate means a high gas temperature on the tube surfaces, which in turn increases the risk of ignition of the soot deposits. See the upper left side of the warning triangle in Fig. 2.

When the boiler has been designed in such a way that soot deposits do not occur, there is, of course, no soot to ignite.

This may explain the trouble-free cases in Fig. 14 with boilers with a low circulation water tow rate, even though the ignition potential exists.

The impact of low gas velocities

The tendency in the statistical material seems quite clear: when the actual gas velocity in the boiler is lower than a certain value, soot particles in the exhaust gas will deposit on the tubes whereas, if the gas velocity is higher, the soot particles will be blown away, i.e. the boiler itself will have a self-cleaning effect. Compare the smoke tube boilers.

According to some boiler makers, the gas velocity limit for soot deposits is about 12 m/s, but may depend on the gas constituents, as discussed above,

Part load running of main engine

It is important to distinguish between a boiler's design mean gas velocity and the actual gas velocity in the boiler.

When, for example, a ship is sailing with reduced speed or is manoeuvring, the diesel engine's power output, and thereby also the amount of exhaust gas, will be reduced. This means that under specific operating conditions, the actual mean gas velocity in the boiler can be lower than 50% of the boiler's design mean gas velocity.

This could explain why soot fire problems had occurred on a few boilers with a design mean gas velocity higher than 20 m/s, ref. Fig. 13, as the actual gas velocity at part load was lower than 12 m/s. A second explanation could be, as mentioned above, that the actual gas velocity limit tor soot deposits was relatively high (wet soot) in the cases in question. A third explanation could, of course, be that the ignition potential did not exist (hgh circulatton water flow rate).

Inlet piping to boiler

Another factor that can reduce the actual gas velocity in a specffic part of the boiler is the design of the inlet piping to the boiler. It is thus not only the actual mean gas velocity through the boiler that is the decisive factor tor soot deposits. It is in fact the boiler's lowest gas velocity that is decisive, as illustrated by the example below.

In one case, a smoke tube boiler suffered from soot clogging caused by a nonuniform gas tow due to a 90° bend just before the inlet to the boiler. Clogging with dry, hard and consistent soot only occurred in that comer of the boiler, with the bw gas velocity. No problems were experienced on sister ships with the same main engine and boiler types, but with a long straight inlet pipe to the boiler.

In the case of this boiler, the gas velocities of the boiler's inlet flow were found using an advanced flow calculation model. Fig. 15 shows the original gas flow distribution of the inlet piping, and Fig. 16 shows the gas flow distribution of a revised, recommended inlet piping which, among other things, incorporates a diffusor.

The calculated flow distribution of the original system, see Fig. 15, shows rather high gas recirculation caused by, and occurring close to, the bend before the inlet to the boiler itself. Where the gas velocity is low. this non-uniform tow distribution may result in deposits of soot particles on the boiler tubes.

The calculation for the revised inlet piping, see Fig. 16, shows an inlet flow to the boiler which is more uniformly distributed, and the cause of the clogging of the boiler tubes, i.e. the tow gas velocities, may therefore be expected to be eliminated. A revised inlet piping with baffle plates could also be used. If possible, of course, a long straight inlet pipe to the boiler, without bends, would be preferable, but was not practicable.

Summary of main reasons for soot flres

Given the points discussed in this paper, and with due consideration for the statistical material and the warning triangle for soot fires (Fig. 2). a general and fairly simple explanation of the main reasons for soot fires may now be gwen by using the analogies below.

Analogy with snow (soot deposits)

In a snowstorm at below-zero temperature, the snowflakes (dry soot partteulates) will not easily deposit on the ground unless the wind (gas) velocity is reduced, as it is for example behind a fence. The low wind velocity will cause the snowflakes to deposit and form a snowdrift, and if the wind direction changes (higher velocity), part of the snowdrift may move. This means that at a certain low wind (gas) velocrty, the snowflakes (dry soot partteulates) will deposit.

In a thaw, for example, when the snowflakes are wet (wet soot), the snowflakes will deposit more easily, and a change in the wind direction (higher velocity) will make only a small part of the snowdrift move. Thus, the wet snowflakes (wet soot) will deposit, but will do so already at a wind velocity (gas velocity) which is higher than the wind (gas) velocity for the above-mentioned frozen snowflakes (dry soot). In general, therefore, high wind (gas) velocities and frozen snowflakes (dry soot) will reduce the tendency towards deposits.

Analogy with coal briquettes (ignition)

Igniting a coal briquette (dry soot) for a grill is quite difficult, as its ignition temperature is rather high. On the other hand, if the briquettes have been wetted with oil (wet soot), the ignition temperature will be lower and it will be easier to ignite the briquettes (wet soot). The higher the temperature of the wetted briquettes (wet soot), the easier they are to set on fire.

So in general, the drier the briquettes (soot), and the lower the temperature, the more difficult they will be to ignite.

Analogy with putting-out a fondue fire (oxygen)

If the oil in a fondue pot has become too hot and has been set on fire. The easiest way to extinguish the fire is to put a cover over the fire and stop the supply of oxygen. When a soot fire occurs in an exhaust gas boiler, similar action has to be taken. In this case the oxygen supply is stopped by stopping the diesel engine, as the engine's exhaust gas still contains about 14% oxygen.

Present boiler design criteria

As an introduction to the next section discussing the recommended new boiler design criteria, this section will briefly describe the present main characteristics of the water tube and the smoke tube boilers.

Water tube boiler

In present-day two-stroke engine plants, the design mean gas velocity for water tube boiler types is normally about 10-11 m/s, which may explain the high frequency of soot fire problems h this type of boiler, ref. Fig. 13.

These boilers are normally fitted with steam or - most often used today – high pressure air soot blowers. MAN B&W recommends the use of these four times a day. MAN B&W also recommends manual cleaning (normally water washing) at regular intervals. On the other hand. experience has shown that in boilers designed tor high velocities, e.g. above 25-30 m/sec, soot deposits are small and do not constitute a problem. So in these boilers, soot blowers are not usually necessary [8].

Smoke tube boiler

Today the most common smoke tube boilers have a relatively high design mean gas velocity, higher than 20 m/s, giving a setf-cteaning effect in the tubes, as a soot cleaning arrangement would be very expensive on this type of boiler. The upper limit for gas velocity depends on the given design gas pressure loss across the boiler [7).

The point of the high design gas velocity is that the smoke tube boiler type will rarely be exposed to soot deposits and soot fires. On the other hand, this type of boiler has a high pinch point, so its utilisation efficiency is normally lower than that of the water tube boiler type.

Recommended new boiler design criteria

Gwen the points discussed in this paper, the risk of soot deposits and ignition followed by soot fires may be minimized by respecting the following four main parameters - valid for both water and smoke tube boilers:
• The gas velocity in the boiler must not be too low, - this reduces the main risk factor
for soot deposits
• The gas temperature on the boiler surfaces must not be too low, - this reduces the additional risk of soot deposits due to the formation of wet soot
• The engine smoke emission should not be allowed to deteriorate, - since this would increase the tendency towards soot deposits
• The circulation water flow velocity and ratio in the boiler must not be too low, this keeps the gas temperature at the boundary layer of the boiler tubes below the ignition temperature of the soot.

The first three of these parameters relate to the soot deposits, whereas the number four parameter relates to the risk of ignition of the soot.

The new boiler design criteria that can be recommended on the basis of the above four parameters, with due consideration for the influence of parameter 1, see points a) and b) below, are thus as follows:
a) The design mean gas velocity of the boiler should be higher than about 15 m/s, but the limit may. in fact, depend on how dry and sticky the soot is (fuel type/fuel additive)
b) The pinch point temperature of the boiler should be higher than 15 °C or, even better, higher than 20 °C
c) The boiler's exhaust gas outlet temperature should not be lower than about 155 °C as otherwise condensation of sulphuric acid in the exhaust gas could make the soot sticky
e) The inlet piping to the boiler should be designed so the gas flow velocity distribution is as uniform as possible, in order to avoid local points wtih a particularly low gas velocity
e) The exhaust gas design back-pressure of the boiler should be as high as possible - increasing the gas velocity in the boiler. This means that the pressure losses in the remaining parts of the exhaust gas system should be dimensioned as low as possible (large pipe diameters)
f) A dumping condenser should be installed to control steam production/ consumption. A gas by-pass valve installed to control the steam production would reduce the gas velocity in the boiler
- and consequent^ increase the risk of soot deposits
-and cannot, therefore, be recommended
g) Trie feed water circulation temperature at the boiler inlet, for boilers with an economiser (preheater) section, should be higher than 120-130 °C as otherwise too low temperatures could cause some of the gas constituents. such as fuel and lub. dl vapour, to condense on the cold boiler tube surfaces, and this could increase the tendency towards soot deposits. Another advantage of this is that the temperature of the preheater tube surfaces can then be higher than the dew point of the sulphuric acid in the gas, thus minimising the risk of sulphuric acid corrosion
h) The circulation water How velocity and ratio at the boiler inlet should be as high as possible in order to keep the gas temperature at the boiler tube surface as tow as possible (in contrast to point g). This should reduce the risk of ignition of possible soot deposits, which can happen at temperatures above some 150 °C and, under extreme conditions, even as low as 120°C. It is therefore also very important to ensure the best suction conditions so that cavitation does not occur in the circulating pumps under any
working conditions, as otherwise the circulating water flow could be reduced or even stopped.

The supplementary recommendations below apply only to boilers of the water tube type:
i) a by-pass duct wrth an automatically operated on/off valve (open/closed at 50% MGR) may in certain operating conditions be recommended for water tube boilers. If, for example, the ship is often slow steaming, i.e. the diesel engine operates at low toad, such an installation will prevent soot deposits on the boiler tubes by by-passing all the gas and thereby avoiding low gas velocities and the associated risk of soot deposits in
the boiler
j) Automatic soot blowers for cleaning four times a day should be installed in water tube boilers in order to clean the tubes of soot. The pressure of the soot blowing medium should be as high as possible during the entire soot blowing sequence
k) Fixed waterwashing system and/or manual cleaning at regular intervals Waterwashing is performed in order to clean the boiler comptetely of soot which has not been cleaned away by the soot blowers. The exhaust gas piping between engine and boiler should be so arranged that the boiler can be cleaned more thoroughly from time to time when the engine is stopped in harbour without the risk of flooding the engine/turbochargers with cleaning fluid. Water washing should preferably be undertaken while the tubes are still hot [8],

A temperature monitoring system mounted above the boiler may be recommended as a means of detecting a fire in the boiler as soon as it starts.

Recommended operating conditions

In view of the damage that can be caused by an extensive soot fire in the exhaust gas boiler, it is recommended, during the operation of the ship, to give due consideration to the following:

Normal operating conditions
a) Soot-blowing
If soot-blowing equipment is installed, we recommend checking its efficiency and adjusting the number of daily soot-blowings accordingly

b) Preheated feed water during start-up
In order to avoid the condensation of some of the gas constituents, preheated feed water should always be used (temperature about 120-130 °C) during start-up and during low load operation [8), especially if the boiler is not fitted with a by-pass duct/valve which can be activated in these running conditions

c) Water circulation, correct functioning
It should be ascertained that the boiler's water circulation system and its control system are functioning property

d) Water circulation after engine stop
After the engine is stopped, the boiler's water circulating pump should be kept running until the boiler temperature has fallen below 120 °C, because wet oily soot may catch fire as low as this temperature. On the other hand, it is recommended not to stop the circulating pump in harbour unless the boiler has been checked and is clean

e) Heavy smoke from engine
If excessive smoke is observed, either constantly or during acceleration, this is an indication of a worsening of the situation. The cause should be identified and remedied.
Excessive smoke could be caused by defective fuel valves, a jiggling governor, incorrect adjustment of the governor fuel limiter, or the malfunctioning of one (of two) auxiliary blowers, etc.
The boiler should be checked and cleaned if necessary.

Recommendation in soot fire situations

On the other hand, if a soot fire does start after all, we recommend either of the following two types of measures, depending on the level of fire:

Fire level 1, where an initial soot fire has just been discovered:

a) Stop the main engine, and thereby the oxygen supply to the fire

b) Continue operating the water circulating pump

c) Never use soot btowers for fire fighting, as air will feed the fre with oxygen, and steam will involve a risk of high temperature fire

d) Stop the air circulation through the engine, and thereby the air supply to the fire. i.e. keep air pressure on the diesel engine's exhaust valve closing mechanism (closed valves)

e) Water washing, if fitted, may be used to extinguish the fire. This is normally connected to the ship's fire fighting water system.

In a well-run plant any fire that starts will be small, and if the above emergency action is taken immediately, the fire will be damped down quickly, and water circulated by the pump will help keep the tubes cool and reduce any heat damage caused by the fire [2],

If the soot fire has turned into an iron fire, this can be indicated by a loss of water, for example, if the feed water consumption increases very much and/or if a low level alarm in the steam drum is activated. A temperature sensor (normally maximum 400 °C) will not normally be able to measure the high temperatures.

Fire level 2, where boiler tubes have melted down:

a) Stop the main engine, if it is not stopped already

b)Stop the circulating water pump

c) Ctose va/ves on the water circulation line

d) Discharge the (remaining) water from the exhaust gas boiler sections

e) Coo/ down with plenty of splash water directly on the heart of the fire

DnV warns that. if a soot fire has turned into a high-temperature fire (hydrogen/iron fire), care should be taken when using water for extinguishing: the fire may become worse unless large amounts of water are applied directly to the heart of the fire. The main aim,
when one discovers an initial small fire, is to prevent it turning into a high-temperature fire.

DnV is preparing new fire-prevention requirements for Rules covering the boiter/economiser arrangements of newbuildings where soot deposits may constitute a fire hazard [9].

Closing Remarks

In principle, the most efficient exhaust gas waste heat recovery system will contribute to the best overall economy on the ship provided, of course, that the recovered heat, for example in the form of steam, is needed on board the ship.

Normally, the exhaust gas boiler design will be based on a steam production requirement related to the rather high steam consumption needed in extreme winter conditions.

On the other hand, when the ship operates worldwide in normal trades, this situation may occur only a few days a year. The choice of a smaller boiler, with lower design steam production, may therefore mean few disadvantages, provided the steam requirement for normal sea service can be met.

One advantage of this will be that the design gas velocity through the smaller boiler will be higher and, as explained in this paper, this will reduce the risk of soot deposits and fires.

As an additional advantage, the exhaust gas boiler will be cheaper.

A boiler and system design based on the correct criteria will reduce the risk of soot deposits and fires in exhaust gas boilers. The use of such criteria is therefore very important and could probably be introduced to advantage into the recommendations of the Classification Societies. This would also allow boiler makers to offer boilers on equal competitive conditions.

The use of special fuel additives with iron oxide seems to reduce the stickiness of the soot and may be useful in cases where the exhaust gas boilers are vulnerable to soot deposits (for example large capacity boilers).

References

[1 ] Emission Control of Two-Stroke Low Speed Diesel Engines
MAN B&W Diesel A/S
Copenhagen, Denmark, 1991

[2] Aalborg Boilers
Instruction K. 7400.2 Water Tube Boilers with Gilled Tubes for Exhaust Gas
Type AV-6N Operation and Maintenance

[3] Dtesel Engine Waste Heat Recovery
Jens Peter Hansen
Aalborg Ciserv International A/S
Svensk Sjofarts Tidning 4, 1991

[4] Sunrod Exhaust Gas Economizers (brochure)

[5] Guide to Prevention of Soot Fire on Exhaust Gas Economizers
(in Japanese) 1992
Nippon Kaiji Kyokai, Tokyo

[6] Operation on Heavy Residual Fuels
MAN B&W Diesel A/S
Copenhagen, Denmark, 1991

[7] Konstruktion von Abgaskessein
Dipl. Ing. P. Mester
AG Weser, Bremen
TUHH Thema G, 1981

[8] Technical Information from Department for Ships in Service
Fires in Exhaust Gas Boilers
V. No.: 87-V056, Nov. 1987
Det norske Veritas

[9] A Motorship Supplement
Ship Repair, March 1993
News from Det norske Veritas (DnV)

[10] Betrieb von Abgaskessein
Dipl. Ing. A. Norgaard
Aalborg Vaerft A/S
Aalborg, Denmark
TUHH Thema H, 1981