This
Is a Headline
Soot Deposits
and Fires in
This is a subheadline
Exhaust gas Boilers
Content
Introduction...................................................................................................... 5
Rise in soot fire incidents................................................................................... 5
Warning triangle – risk of soot fire................................................................. 6
Scope of this paper..................................................................................... 6
Basic Information and Boiler Definitions............................................................. 7
Heat balance of a main engine..................................................................... 7
Boiler types................................................................................................. 8
Boiler steam systems................................................................................. 10
The influence of a boiler’s pinch point......................................................... 11
Sulphuric acid corrosion............................................................................ 12
Steam production – influence of ambient temperatures............................... 13
Particulate emissions from diesel engines................................................... 15
Soot fires in exhaust gas boilers................................................................. 15
Boiler Experience and Design Criteria.............................................................. 17
Statistical analyses of soot fires....................................................................... 17
The impact of low gas velocities................................................................. 20
Summary of main reasons for soot fires...................................................... 21
Recommended boiler design criteria.......................................................... 22
Recommended operating conditions.......................................................... 23
Closing Remarks............................................................................................. 25
Reference....................................................................................................... 25
Soot Deposits and Fires in Exhaust gas Boilers
3
4
Soot Deposits and Fires in Exhaust gas Boilers
Soot Deposits and Fires in Exhaust Gas Boilers
Introduction
temperature after the turbocharger for
transfer surface and thus a boiler de
The demand for the highest possible
standard engines without waste heat
sign with a low internal gas velocity as
overall fuel efficiency is reflected in de
recovery system, is about 240270°C,
well as tubes with “extended” surfaces.
velopments in the propulsion market for
but may be lower for derated engines.
oceangoing ships. Today, this market is
Furthermore, the quality of the fuels
dominated by highly efficient twostroke
The name “exhaust gas economiser”
has decreased significantly over time.
low speed diesel engines which run on
is often used for an exhaust gas boiler
Whereas the average fuel quality may
low quality fuels and utilise (recover) the
which is not able to operate separately,
not have deteriorated as much as pre
exhaust gas heat by means of an ex
i.e. without its own steam drum. In this
dicted, single deliveries have shown ex
haust gas boiler/economiser.
paper, the name “exhaust gas boiler”
ceedings of the normal data as a result
will be used in general, also in cases
of a more efficient refinery process. The
The development of high efficiency en
where “exhaust gas economiser”, in
residual fuel oils available on the mar
gines has resulted in reduced specific
principle, should have been used.
ket today contain considerably higher
fuel oil consumption, i.e. increased ther
quantities of asphalt, carbon and sul
mal efficiency of the diesel engine, and
Rise in soot fire incidents
phur that contaminate the exhaust gas
thereby lower exhaust gas tempera
As a consequence of the lower exhaust
and thereby increase the risk of soot
tures. Based on ISO ambient reference
gas temperatures and the remaining
deposits on the exhaust gas boiler
conditions (25°C air and 25°C cooling
steam consumption requirements, the
tubes.
water), and with the present nomi
exhaust gas boiler has been designed
nal ratings of the MC/MCC and ME/
to become more and more efficient.
Some years ago, and possibly as a con
MEC/ME-B engines, the exhaust gas
This involves the use of a large heat
sequence of both the deteriorated fuel
Number of soot fire/overheating incidents per year
70
60
50
40
30
20
10
0
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
Year
Fig. 1: Number of soot firedamaged exhaust gas boilers in DNVclassed vessels (ref. about 5,000 vessels)
Soot Deposits and Fires in Exhaust gas Boilers
5
quality and the above highly efficient
engine, the fuel injection pressure is
contains about 14% oxygen, the soot
and perhaps “overstretched” design,
more or less independent of the load,
deposits and ignition items are of par
it also seemed that the tendency to
i.e. high at all loads. Hence, a bet
ticular interest, as the oxygen cannot
fouling, i.e. soot deposits on the ex
ter combustion, especially at lower
be removed.
haust gas boiler tubes, had increased
loads, and correspondingly less soot
and, in some cases, had resulted in
will be the result for ME engine types.
soot fires. In extreme cases, the soot
Scope of this paper
This paper is divided into two chap
fire had developed into a high tem
Futhermore, the introduction of the
ters which, in principle, may be con
perature iron fire in which the boiler
slide fuel valves of the main engines
sidered as two separate papers.
itself burned. The abovementioned
may have had a positive influence on
tendency was confirmed by DNV’s
the reduction of fire incidents, togeth
The intention with Chapter I is to give
statistics, which reveal a sudden
er with the introduction of the Alpha
a quick introduction to the most com
rise in soot fire incidents since 1985,
lubricator.
monly used exhaust gas boiler types,
see Fig. 1 and Ref. [1], a rise, which
steam systems and relevant param
may also have been caused by slow
It is evident that the high fuel efficien
eters. This chapter will form a good
steaming of ships due to the low
cy target must be met without jeop
introduction before proceeding to the
freight rates at that time.
ardising the reliability of the ship. It is
issues of principle discussed in Chap
therefore important to know the main
ter II.
Since 1998, we have again seen a fall
reasons for the occurrence of soot
in the number of incidents, probably
deposits and fires in order to take
Chapter II deals with the essential
caused by the effect of the new rec
the proper countermeasures against
conditions causing soot deposits and
ommended exhaust gas boiler design
them with a correct exhaust gas boil
fires in exhaust gas boilers. The rea
criteria introduced in the 1990s, and
er/system design, etc.
sons for soot deposits and their igni
described in this paper.
tion are identified on the basis of sta
tistical material, etc. In this context,
Because of underreporting, statistics
When soot fires occur, the diesel en
recommendations are given which are
on exhaust gas boiler fire/overheating
gine will normally be blamed since the
relevant to the design and operation
incidents after 2003 have not been
soot particles in fact originate from the
of exhaust gas systems and boilers.
worked out. Thus, whilst insurance
engine’s fuel combustion. As, in prin
statistics for a certain period showed
ciple, particles in the exhaust gases
0.58 damage per ship per year, a
are unavoidable from a modern diesel
DNV statistic for the same period re
engine running on heavy fuel, Ref. [1],
vealed only 0.15 damage per ship per
the causes of soot deposits/fires may
year. The lower number of damage
be approached by asking a different
reported to DNV is probably caused
question: What makes the soot par
by more insurances with hull and ma
ticles deposit and/or what causes the
chinery deductibles.
ignition of the soot deposits?
illustrated by the “warning triangle” in
had a positive influence on the reduc
Fig. 2 showing the three determining
tion of the fire incidents. Thus, on a
factors to be met for a soot fire: soot
mechanically controlled MC engine
deposits, oxygen and ignition. As the
the fuel injection pressure follows
exhaust gas smoke from a diesel en
the engine load, whereas for an ME
gine, due to its high air excess ratio,
6
Soot Deposits and Fires in Exhaust gas Boilers
(of Ignit
the ion
so
ot)
ke)
2
n O smo
yge as
Ox st g
au
The answer to this question may be
controlled ME engine may also have
exh
The introduction of the electronically
(in
Warning triangle – risk of soot fire
Soot deposits
(on boiler tubes)
Fig. 2: Warning triangle - risk of soot fire
Basic Information and Boiler
Definitions
Permissible exhaust gas backpressure
Heat balance of a main engine
The permissible gas pressure loss
Studying a heat balance diagram
across the exhaust gas boiler has an
An increased back-pressure may in
which, by way of example, is shown in
important influence on the gas velocity
volve a too high thermal loading of the
Fig. 3 for a nominally rated highly ef
through the boiler. Thus, if a high pres
engine components together with a
ficient engine version 6S60MCC7 (or
sure loss is acceptable, it is possible to
“too sensitive” turbocharger. Further
6S60MEC7), operating on 80% SMCR
design the boiler with a high gas veloc
more, the IMO NOx compliances may
(specified maximum continuous rating),
ity, but if only a small pressure loss is
be difficult to meet.
the most attractive waste heat source
permissible, the gas velocity will be low.
the turbocharger, must not exceed 350
mm WC (0.035 bar), see Fig. 4.
is the exhaust gas heat. Approximately
In order to have a backpressure mar
one fourth of the fuel energy comes out
The permissible pressure loss across
gin for the final system, it is recom
as exhaust gas heat.
the boiler depends on the pressure
mended at the design stage that about
losses of the total exhaust gas system
300 mm WC (0.030 bar) at SMCR is
after the diesel engine’s turbocharger(s).
used initially.
130°C, from approx. 375°C to ap
Permissible back-pressure of exhaust gas
The backpressure in the exhaust gas
prox. 245°C (ISO), as a result of the
system for MC/MC-C and ME/ME-C/ME-B
system depends on the gas velocity,
obtained higher efficiency of diesel en
engines
i.e. it is proportional to the square of
gines, exhaust gas boilers are installed
At the SMCR of the engine, the total
the exhaust gas velocity, and hence to
on almost all merchant ships of today.
backpressure in the exhaust gas sys
the pipe diameter to the 4th power. It is
However, this development has been
tem after the turbocharger, indicated
recommended not to exceed 50 m/s in
accompanied by more trouble, as men
by the static pressure measured as the
the exhaust gas pipes at SMCR.
tioned before.
wall pressure in the circular pipe after
Even though since 1980 the exhaust
gas temperature has decreased about
6S60MCC7
Max ∆ psystem < 350 mm W.C.
Design ∆ psystem < 300 mm W.C.
SMCR: 13,560 kW and 105.0 r/min
Service point: 80% SMCR
∆ p1
Spark
arrester
∆p2
Exhaust
gas
silencer
Shaft power
output 50.5%
Lubricating
oil cooler
3.3%
Jacket water
cooler
5.8%
∆ psystem
∆ p3
Exhaust gas
25.0%
Exhaust
gas
boiler
Air cooler
14.6%
Fuel
100%
Heat radiation
0.8%
Fig. 3: Heat balance of main engine 6S60MC-C7 IMO Tier I at 80% SMCR
T/C
Fig. 4: Permissible exhaust gas backpressure at 100% SMCR
Soot Deposits and Fires in Exhaust gas Boilers
7
It has become normal practice, in or
Different types of water tube elements:
der to avoid too much pressure loss,
to have an exhaust gas velocity in the
pipes of about 35 m/sec at SMCR.
As long as the total backpressure of
Gilled tubes
Pinned tubes
the exhaust gas system, incorporat
ing all resistance losses from pipes
and components, complies with the
abovementioned
requirements,
the
pressure losses across each compo
nent, such as the exhaust gas boiler
Plain tubes
and silencer, may be chosen indepen
dently.
Spiral tubes
Water tubes
Permissible pressure loss across boiler
At SMCR, the maximum recommended
Exhaust gas
pressure loss across the exhaust gas
boiler is normally 150 mm WC.
Fig. 5: Exhaust gas boiler – water tube type. The boiler shown is the vertical type without steam drum
This pressure loss depends on the
pressure loss in the rest of the system,
as mentioned above. Therefore, if an
„„
Water tube boilers
exhaust gas silencer/spark arrester is
„„
Smoke tube boilers
Steam drum
(Space)
not installed, the acceptable pressure
loss across the boiler may be some
Water tube boilers
what higher than the maximum of 150
This is the most frequently used boiler
mm WC, whereas, if an exhaust gas si
type – often in connection with high
lencer/spark arrester is installed, it may
exhaust gas heat utilisation. The ex
be necessary to reduce the maximum
haust gas passes across the outside
pressure loss.
of the boiler tubes, with the water flow
Water
Smoke
tubes
ing inside, see Fig. 5. In order to make
It should be noted that the above­
the boiler as efficient and as compact
mentioned pressure loss across the
as possible, the heat transfer area on
boiler also incorporates the pressure
the gas side of the tubes may often be
losses from the inlet and outlet transi
expanded with, for example, narrowly
tion boxes.
spaced, gilled (finned) or pinned tubes.
Boiler types
The clearance between the gilltype fins
The types of exhaust gas boilers utilis
(face to face) is in general 1013 mm,
Fig. 6: Exhaust gas boiler – smoke tube type.
ing the diesel engine exhaust gas heat
and the thickness of the gills is about
may, in principle, be divided into two
23 mm.
The boiler shown is the vertical type with steam
drum
Exhaust gas
main groups:
8
Soot Deposits and Fires in Exhaust gas Boilers
The water tube boiler type will normally
(also called steam collector or steam
not be equipped with a steam space
drum), but will sometimes be operated
in connection with a separate steam
tude of 30100 mm) and surrounded on
mean gas velocity, exceeding some 20
drum or, more often, with the steam
the outside by water, see Fig. 6. The
m/s, through the tubes.
drum of the oilfired boiler.
smoke tube boiler type is often chosen
in specific cases where it is desirable
In some cases soot has blocked some
The soot deposits on the upper side
to operate the exhaust gas boiler inde
of the boiler tubes, with a consequent
of the boiler tubes, and this type of
pendently of the oilfired boiler. This is
increase in pressure loss and reduction
boiler will most often be fitted with a
possible as the smoke tube boiler may
in boiler efficiency. The solution may be
sootblowing arrangement in order to
be fitted with its own separate steam
to clean the tubes manually at regular
remove any soot deposits. The soot
drum.
intervals, although this may be expen
deposit tendency in the 1990s was on
sive.
the increase due to the low gas veloc
In general, a high gas velocity in the boil
ity and temperature often used at that
er tubes is desirable in order to achieve
On the other hand, soot deposits have
time.
the highest possible heat transfer and
very seldom led to damage caused by
the lowest possible soot deposits.
soot fire, because the boiler tubes are
In several cases, the increased occur
surrounded/cooled by water and the
rence of soot deposits on this type of
As a cleaning system is very difficult to
boiler was followed by a soot fire.
install, this boiler type is designed to
heat surface has a limited area.
have a selfcleaning effect, which is ob
In extreme cases – as mentioned later
tained by using a relatively high design
– the high temperature caused by the
soot fire resulted in a socalled iron fire
in which the boiler itself burned. This
may have occurred due to leakage of
water from the boiler because of the
high temperature. The iron fire could
also have occurred because the crew
tried to put out the fire by activating the
Exhaust
gas boiler
evaporator
soot blowers for the injection of steam
or water. The high temperature would
thus cause dissociation 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 vul
nerable to soot fires than plain tubes,
because the highest metal tempera
tures will occur on the edge of the gills,
Saturated
steam for
heating services
Exhaust gas
Oil-fired
boiler with
steam drum
Atmospheric
surplus
condenser
Feedwater
pumps
which will thus be the most likely start
ing point for an iron fire.
Circulating
pumps
Hot well
Smoke tube boilers
In smoke tube boilers, the gas is con
ducted through a bundle of tubes with
Fig. 7: Normal exhaust gas boiler system for steam production.
Single pressure steam system with evaporator section only
small internal diameters (of the magni
Soot Deposits and Fires in Exhaust gas Boilers
9
°C Temperature T
300
Exhaust gas boiler
Preheater
Saturated
steam for
heating
services
Oil-fired
boiler with
steam drum
Heat
exc.
Evaporator
Superheater
Exhaust gas
Turbogenerator
(steam turbine)
Condenser
Condensate pumps
Atmospheric
surplus
condenser
250
Superheated
steam
Saturated
steam
150
Fig. 8: Special exhaust gas boiler system with turbo generator for electricity
production. Single pressure steam system with preheater, evaporator and
superheater sections
Pinch
point
Steam/water 7 bar abs
100
Feed
water
pump
Hot well
Exhaust gas
Feedwater
50
0
0%
Superheater
Heat transmission Q
20% 40% 60% 80% 100%
Evaporator
Preheater
Fig. 9: T/Q diagram for an exhaust gas boiler
Boiler steam systems
to the oilfired boiler which is used as
Special exhaust gas boiler system with tur-
Exhaust gas boiler steam systems can
a common steam drum for the oilfired
bogenerator
be designed in many different versions,
boiler and the exhaust gas boiler.
For engines with a waste heat recovery
with one or two pressure levels, with or
without a preheater section, etc.
The most commonly used steam sys
systems where a turbogenerator, i.e. a
Separate steam drums may also be
steam turbine driven electrical genera
used, so that one boiler can be run if
tor, is installed (utilising the steam avail
the other should malfunction.
able after deduction of steam for heat
tems – both simple and advanced – are
described below.
ing services), the exhaust gas boiler
Because of its simplicity and low capital
system will be more advanced.
cost, the above system is widely used
Normal exhaust gas boiler system
and is often entirely adequate when the
A general but old example of such a
The exhaust gas boiler system normally
steam production is viewed as a means
system is shown in Fig. 8. The boiler
used for the production of saturated
of meeting the steam demand for heat
is, apart from the evaporator, also fit
steam needed for heating services is
ing services on the ship.
ted with a preheater and a superheater.
shown in Fig. 7.
In this system too, the steam drum of
The loss of water from the exhaust gas
the oilfired boiler is normally used as a
This is a simple, singlepressure steam
boiler is in the magnitude of about 1%
common steam drum.
system in which the exhaust gas boiler
of the steam production.
consists solely of an evaporator sec
tion. The feed water is pumped directly
10
Soot Deposits and Fires in Exhaust gas Boilers
The influence of a boiler’s pinch point
The utilisation efficiency of an exhaust
lower than about 165°C, when 20°C or
A boiler’s pinch point is a parameter
gas boiler is characterised by its pinch
above is used as the pinch point.
that can tell us a lot about the boiler’s
point. The pinch point is the lowest
design and potential behaviour in oper
temperature difference between the
A boiler’s steam production and heat trans-
ation. It will therefore be defined below,
exhaust gas and the saturated steam,
fer surface
and its influence on some important
i.e. the temperature difference between
The influence of the pinch point on the
boiler parameters will be discussed in
the exhaust gas leaving the evaporator
exhaust gas boiler design will be evi
this section.
section and the saturated steam, see
dent in the following example.
the T/Q diagram in Fig. 9.
A boiler’s T/Q diagram and definition of
The graphs in Fig. 10 show the influ
pinch point
Normally, the steam pressure will
ence of the pinch point on the boiler’s
A temperature/heat transfer diagram,
be above 7 bar abs. (6 barg) and of
heat transfer surface and steam pro
a socalled T/Q diagram, illustrates
ten equal to 8 bar abs. (7 barg), cor
duction, Ref. [3]. By way of example,
the characteristic temperature course
responding to a minimum evaporation
the graphs in Fig. 10 indicate that an
through the exhaust gas boiler. As an
temperature of 165°C. According to the
exhaust gas boiler with a pinch point of
example valid for the special exhaust
T/Q diagram, the gas outlet tempera
5°C, compared with one with a pinch
gas boiler system shown in Fig. 8, a
ture, even for a boiler with feed water
point of 15°C, will produce 10% more
T/Q diagram is shown in Fig. 9.
preheater section, will therefore not be
steam, but at the expense of having a
heat transfer surface about 2.3 times
sur
fac
e
t tr
an
sfe
r
%
125
Temperature
°C
110
105
100
230
75
220
190
180
25
200
St
ea
m
pro
du
175
n
c t io
150
141
125
100
tle
tg
as
tem
pera
ture
Ou
210
200
50
232
225
He
a
Relative steam
production
Relative heat
transfer surface
Example:
132% larger exhaust gas
boiler required to produce
10% more steam at 5 °C
pinch point compared
with 15 °C
170
160
Steam temperature
80 60 50 40
30
75
50
25
0
20
15
10
5
3
Pinch point ºC
Fig. 10: Influence of a boiler´s pinch point, relative to 15oC. The graph shows the relative influence of the pinch point on an exhaust gas boiler´s heat transfer
surface (size and investment) and steam production [3]
Soot Deposits and Fires in Exhaust gas Boilers
11
The lower the pinch point, the larger the
ºC Approximated sulphuric acid dew point
150
heat transfer surfaces and the more ef
ficient the exhaust gas boiler is, and the
higher the gas pressure loss across the
boiler is. As the maximum permissible
140
gas pressure loss has a certain limita
tion, the boiler’s design gas velocity has
to be reduced in order not to exceed
130
the limit for the permissible gas pres
sure loss.
120
This is what happened with the more
efficient exhaust gas boiler design dur
110
ing the 1980s and 1990s because of
the lower exhaust gas temperatures of
diesel engines. In this context, Chapter
100
0
1
2
3
4
5 wt %
Sulphur (S) content in fuel
Fig. 11: Sulphuric acid dew point of exhaust gas shown as a function of the sulphur content in the fuel
II will show that a low gas velocity in
particular will have a distinct influence
on the tendency towards soot depos
its, a tendency which has become
worse due to the low quality residual
°C Exhaust gas temperature after T/C -/+ 15 °C
350
fuels on the market today.
Low pinch point and soot deposits
The pinch point is therefore a param
eter that may influence the occurrence
300
Tropical (45 °C air)
of soot deposits when the pinch point
and thus the gas velocity is low.
250
ISO (25 °C air)
Winter (10 °C air)
200
30
40
50
60
70
80
90
100 110 % SMCR
Engine shaft power
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 pres
sure loss across the boiler.
Fig. 12: Influence of ambient air temperature on the exhaust gas temperature after turbocharger for a
6S60MCC7
Sulphuric acid corrosion
A high degree of utilisation of the ex
that of the original boiler surface. The
A boiler´s pressure loss and gas velocity
haust gas heat requires the lowest
gas velocity through the boiler may be
In principle, the pinch point may be
possible exhaust gas boiler outlet tem
correspondingly reduced, as otherwise
considered a measure of how extensive
perature which, if the required steam
the pressure loss across the boiler
and how efficient the heat utilisation of
pressure and thereby the evaporation
might be too high.
the exhaust gas boiler is.
temperature is sufficiently low, is limited
mainly by the risk of corrosion of the ex
12
Soot Deposits and Fires in Exhaust gas Boilers
haust gas boiler heating surfaces due
in the exhaust gas, but is rather difficult
rather slowly and is catalysed by soot
to sulphuric acid condensation.
to establish.
deposits, etc., on the heating surfaces.
Corrosion starts when the temperature
The chemical reactions are as follows:
Valid for the exhaust gas after turbo
of the boiler tube surfaces is equal to,
a. at fuel combustion:
charger from MC/MCC or ME/MEC/
or lower than, the dew point tempera
S + O2 → SO2
ME-B main engines, Fig. 11 shows, as
ture of the sulphuric acid. Furthermore,
a guide, the sulphuric acid dew point
the temperature of the boiler tube sur
b. at cooling of exhaust gas in the
as a function of the sulphur content in
faces (gas side) is almost equal to the
the fuel. With an average 2.9% sulphur
water temperature in the boiler, due
2SO2 + O2 → 2SO3
temperature range of 560°  200°C:
to the fact that the heat transfer coef
content in the fuel, the dew point of
sulphuric acid in the exhaust gas from
ficient on the gas side is extremely low
c. at reaction with water:
the main engine can be expected to be
compared to that on the water side.
SO3 + H2O → H2SO4
about 135°C, which means that in this
case the temperature of boiler circulat
The sulphuric acid dew point tempera
The chemical reaction b., in particular,
ing water or feed water at the boiler in
ture depends especially on the content
is rather difficult to establish, the rea
let should be kept higher than 135°C.
of sulphur in the fuel oil and of oxygen
son being that the reaction takes place
Steam production – influence of ambient temperatures
Steam production
kg/h
ISO ambient conditions (25 °C)
Total steam
production
3,000
During normal operation of the ship, the
ambient air and seawater temperatures
will change, and this will have an influ
ence on the exhaust gas temperature.
2,000
Thus, the exhaust temperature af
Surplus steam
ter turbochargers will decrease about
1,000
1.6°C for each 1.0°C reduction of the
turbocharger intake air temperature,
Steam consumption
and vice versa.
0
40
50
60
70
80
90
100 %SMCR
As an example, valid for a 6S60MCC7
engine, Fig. 12 shows the influence of
Steam production
kg/h
Winter ambient conditions (10 °C)
2,000
the turbocharger air intake tempera
Total steam
production
Extra steam needed
valid for ISO reference conditions (25°C
air/25°C c.w.), tropical air temperature
of 45°C and a winter air temperature of
10°C, respectively.
1,000
Steam consumption
0
ture on the exhaust gas temperature,
40
50
60
70
80
A similar example (see Fig. 13) val
90 100 %SMCR
Engine shaft power
Fig. 13: Influence of ambient air temperarure on the steam production of an exhaust gas boiler installed
on an Aframax tanker with main engine 6S60MCC7
id for an Aframax tanker having a
6S60MCC7 main engine installed,
shows the corresponding steam pro
duction of an exhaust gas boiler with
an evaporator section only, and based
Soot Deposits and Fires in Exhaust gas Boilers
13
upper graph for the ISO (25°C air)
Heat
21% O2
79% N2
14.0%
O2
based boiler design shows that too
76.2%
N2
much steam will be produced, and the
4.5% CO2
Heat
Heat
97% HC
Heat
21% O
O2
21%
O2 2
3%21%
S79%
N2
79% N
79% N2 2 9.0 kg/kWh
97% HC
97% HC
HC Air
97%
97% HC
Engine
Exhaust
2.5% CA
3% SS 170.0 kg/kWh
3%
Process
gas
3% S
Fuel oil
0.5% S97% HC
97% HC
97% HC
2.5% CA
CA 0.7 kg/kWh
2.5%
2.5% CA Lube oil
Work
0.5% SS
0.5%
Fig. 14: Typical emissions
0.5% S from an MC/ME type low speed diesel engine
14.0% O
O2
14.0%
5.1%
14.0%
OH22O 2
76.2% N
N22
76.2%
1500
ppm
76.2%
N NOx
4.5% 2CO
CO22
4.5%
4.5%CO
600
ppm
2 SOx
5.1% H
H2O
O
5.1%
5.1%
H2O 2 CO
60 ppm
1500
ppm
NOxx
1500 ppm NO
1,500ppm
ppm NO
180
HC
x
600 ppm
ppm SO
SO
600
xx
600mg/Nm
ppm SO3x part
120
60
ppm
CO
60 ppm CO
60 ppm CO
180 ppm
ppm HC
HC
180
180 ppm HC 3
3
part
120
mg/Nm
120 mg/Nm part
120 mg/Nm3 part
Fig. 14:
14: Typical
Typical emissions
emissions from
from an
an MC/ME
MC/ME type
type low
low speed
speed diesel
diesel engine
engine
Fig.
Fig. 14: Typical emissions from an MC/ME type low speed diesel engine
Potential ignition temperature
Stage 11 Ignition
Ignition of
of soot
soot
Stage
Dry soot
300400°C
Type
of
soot
Potential ignition
ignition temperature
temperature
Potential
Type of soot
Wet (oily)
Dry soot
soot
Dry
Wet (oily)
(oily)
Wet
and 20°C
point,
together
with
based
boilerpinch
design
shows
that too
too
based
boiler
design
shows
that
means
of
the atmospheric
surplus
con
the needed
steam
consumption
for
much
steam will
will
be produced,
produced,
and the
the
much
steam
be
and
heating
services.
The
upper
graph
for
surplus
steam
has
to
be
dumped
by
surplus steam has to be dumped by
denser. However, in winter time (10°C
air) with a lower exhaust gas tempera
the ISO
(25°C
air) based surplus
boiler design
means
of the
the atmospheric
atmospheric
surplus
con
means
of
con
ture,
the However,
steam
production
will
be(10°C
lower,
shows that
too much
steamtime
will be
pro
denser.
in winter
denser. However, in winter time (10°C
whereas
the
steam
consumption
in
duced,
the surplus
steam
haswill
to be
air)
with and
a lower
exhaust
gas tempera
air) with a lower exhaust gas tempera
crease,
thatofthe
oil
fired
boiler
dumped
by means
thewill
atmospheric
ture,
themeaning
steam
production
will
be lower,
lower,
ture,
the
steam
production
be
may
occasionally
have
to start
up
surplus
condenser.
However,
inwill
winter
whereas
the
steam consumption
consumption
will
into
whereas
the
steam
in
supplement
the with
steam
production.
time (10°C
air)
lower
gas
crease,
meaning
thatathe
the
oil exhaust
fired boiler
boiler
crease,
meaning
that
oil
fired
temperature,
the have
steam
will
may
occasionally
have to
toproduction
start up
up to
to
may
occasionally
start
be
lower,
whereas
the
steam
consump
supplement
the
steam
production.
supplement the steam production.
tion will increase, meaning that the oil
fired boiler may occasionally have to
Stage 1 Ignition of soot
Type of soot
on thegraph
steamfor
pressure
of 8
bar abs.,
upper
graph
the ISO
(25°C
air)
upper
(25°C
air)
surplus
steamforhastheto ISO
be dumped
by
start up to supplement the steam pro
duction.
150°C (120°C)
Stage 2 Small soot fires
300400°C
300400°C
150°C (120°C)
(120°C)
150°C
Small
soot
fires
are most likely to occur during
Stage
Small soot
soot fires
fires
Stage
22 Small
manoevring/low
engine
limited
Small soot fires
are load
mostwith
likelyno
to or
occur
during
Small soot fires are most likely to occur during
boiler manoevring/low
damage
engine load with no or limited
manoevring/low engine load with no or limited
boiler damage
damage
boiler
Stage 3 High-temperature fires
Stage
High-temperature
fires
A small
soot fire may fires
develop into a
Stage
33 High-temperature
small soot
sootfire
fire with
may develop
develop
into aa
hightemperature
the
following
AA small
fire
may
into
hightemperature
fire with
with the
the following
following
reactions
involved:
hightemperature
fire
reactions involved:
involved:
reactions
a.
Hydrogen fire, temperature > 1,000°C
a.
Hydrogen fire, temperature > 1,000°C
a. Hydrogen
temperature
> 1,000°C
Dissociation
of fire,
water
into hydrogen
and oxygen:
Dissociation of
of water
water into
into hydrogen
hydrogen and
and oxygen:
oxygen:
Dissociation
2H2O  2H2 + O2
O
 2H
2H22 ++ O
O22
2H22O
2H
H2O + C  H2 + CO
}H2 and CO are combustible
O ++ C
C
H
H22 ++ CO
CO
H22O
H
b.
and CO
CO are
are combustible
combustible
}H22 and
}H
Iron
> 1,100°C
b. fire,
Irontemperature
fire, temperature
temperature
1,100°C
b.
Iron
fire,
>> 1,100°C
Examples
of
reaction
with
iron:
Examples of
of reaction
reaction with
with iron:
iron:
Examples
heat heat
2Fe +2Fe
O2 +
 2FeO+
2Fe
+O
O2FeO+
22  2FeO+ heat
}The
boiler
tubes
are
burning
}The}The
boiler
tubes
areare
burning
boiler
tubes
burning
O
 FeO+
FeO+
+ heat
Fe
H22O
FeO+
H2 +HHheat
Fe + H
Fe
++ 
H

2O
22 + heat
Fig.15:
15:Development
Developmentof
sootfire
firein
anexhaust
exhaustgas
gasboiler
boiler
Fig.
15:
Development
ofofaaasoot
soot
fire
ininan
an
exhaust
gas
boiler
Fig. 15: Fig.
Development
of a soot
fire
in an
exhaust
gas boiler
Soot
Deposits
and
Fires
in Exhaust
Exhaust
Gas
Boilers
14
Soot
Deposits
and
Fires
in
Exhaust
gas
Boilers
14 Soot
Deposits
and
Fires
in
Gas
Boilers
14
14 Soot Deposits and Fires in Exhaust Gas Boilers
Fig.
Fig.16:
16:High
Hightemperature
temperaturefire
fireofof
ofaaagas
gasfired
firedwater
watertube
tubetype
typeboiler
boiler
Fig.
16:
High
temperature
fire
fired
boiler
Fig.
16:
High
temperature
fire
of
a gas
gas
firedwater
watertube
tubetype
type
boiler
Particulate emissions from diesel
During this process, a minute part of
The content of unburnt hydrocarbons
engines
the oil, comprising mainly carbon, will
in the exhaust gas from large diesel en
Low speed diesels have been leading
be left as a “nucleus”.
gines can be up to 300 ppm, but de
the way with regard to the acceptance
pends, among other factors, very much
of lowgrade fuels, low fuel consump
Particulate emissions will vary substan
on the maintenance condition of the
tion and high reliability. In this process,
tially with the fuel oil composition and
fuel injection system and, to some ex
the presence of particulates in the ex
lube oil type and dosage. It is therefore
tent, on the type of fuel and the cylinder
haust gas, from an operational point of
difficult to state general emission rates
oil dosage.
view, always has been and, no doubt,
for particulates, but when the engine is
always will be unavoidable.
operating on heavy fuel oil, values of
The hydrocarbon figure overlaps the
the order of 120150 mg/Nm3, corre
figure for particulates to some extent,
The typical exhaust gas emission val
sponding to some 0.81.0 g/kWh, may
as these consist partly of hydrocar
ues for the most commonly discussed
be considered typical.
bons.
particulates, are shown in Fig. 14. In
In general, the particles are small and,
Sticky effect of particulate emissions
the context of this paper, only the par
when the engine operates on heavy
If the right – or rather the wrong – con
ticulate/soot emissions, and to some
fuel oil, it may be expected that over
ditions prevail, the soot particulates
degree the hydrocarbons (HC), are of
90% will be less than 1 micron in
may deposit in the exhaust gas boiler.
interest and will be described in the fol
size, excluding flakes of deposits, and
lowing.
peelingoff from the combustion cham
Furthermore, the lower the exhaust gas
ber or exhaust system walls.
and heating surface temperatures be
pollutants, NOx, SOx, CO, HC, and
Sources of particulate emissions
come, the faster the soot is deposited
Particulates in the exhaust gas may
The particulates also include some of
and the harder it becomes to remove
originate from a number of sources:
the ash content of the oil, i.e. the trace
it. The explanation is that under such
metals. The abovementioned contri
conditions the soot may be “wet” with
Agglomeration of very small particles
bution from the lubricating oil consists
oil and/or other gas condensates like
of partly burnt fuel
mainly of calcium compounds, viz. sul
hydrocarbons, and this may have an in
phates and carbonates, as calcium is
creasing effect on the tendency of soot
Ash content of fuel oil and cylinder
the main carrier of alkalinity in lube oil
to deposit, as the soot may be more
lube oil
to neutralise sulphuric acid.
sticky.
„„
Partly burnt lube oil
A test of the soot deposits in a boiler
Soot fires in exhaust gas boilers
with gilled tubes has shown that about
A fire in the exhaust gas boiler may de
„„
Peelingoff of combustion chamber/
70% of the soot is combustible.
velop in two or three stages, see Fig.
„„
„„
exhaust system deposits.
15 and Ref. [2]. The ignition of soot
Hydrocarbons
normally develops into a small and lim
Typical form and rate of particulate emis-
During the combustion process, a very
ited fire, but under extreme conditions
sions
small part of the hydrocarbons will
it may develop into a hightemperature
Once fuel is atomised in the combus
leave the engine unburnt, and others
fire.
tion chamber of a diesel engine, the
will be formed. These are referred to
combustion process takes place from
as unburnt hydrocarbons, and they are
Ignition of soot
small droplets of fuel which evaporate,
normally stated in terms of equivalent
Ignition of soot may arise in the pres
ignite, and are subsequently burnt.
methane (CH4) content.
ence of sufficient oxygen when the de
posits of combustible materials have
a sufficiently high temperature (higher
Soot Deposits and Fires in Exhaust gas Boilers
15
than the flash point) at which they will
ed. The reactions involved here are (see
liberate sufficient vapour, which may be
also stage 3 in Fig.15):
ignited by a spark or a flame.
A.Hydrogen fire: This occurs because
The main constituent of the soot de
dissociation of water into hydrogen
posit is particulates but, in addition,
and oxygen or, in connection with
some unburnt residues of fuel and lu
carbon, into carbon monoxide and
bricating oils may be deposited in the
hydrogen, may occur under certain
boiler because of faulty combustion
conditions. A hydrogen fire may start
equipment and, in particular, in con
if the temperature is above 1000oC.
nection with starting and low speed
running of the engine.
B.Iron fire: An iron fire means that the oxi
The potential ignition temperature of
dation of iron at high temperatures
the soot layer is normally in the region
occurs at a rate sufficiently high to
of 300400oC, but the presence of un
make the amount of heat release from
burnt oil may lower the ignition temper
the reactions sustain the process.
ature to approx. 150oC, and under ex
These reactions may take place at
o
treme conditions even down to 120 C.
a temperature in excess of 1100oC.
In this connection, it is important to
This means that ignition may also take
realise that also water (H2O) may go
place after stop of the main engine as
in chemical reaction with iron (Fe),
i.e. the use of the steam based soot
a result of glowing particles (sparks) re
maining 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 opera
tion. These fires cause no or only lim
ited damage to the boiler, 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. Fig. 16 is an example of an exhaust
gas boiler hightemperature fire, where
the boiler tubes have burned and melt
16
Soot Deposits and Fires in Exhaust gas Boilers
blower will feed the fire.
Boiler Experience and Design Criteria
Statistical analyses of soot fires
Parameter
Soot fires in exhaust gas boilers were
Ship type
no
very unusual before 1986, but during
Main engine type
no
the period 1986-2002, soot deposits
Main engine(s) MCR power
no
Any distinct influence?
Fig. 17
and soot fires occurred more often.
Boiler sections (evaporator, preheater, etc.)
no
Analyses of soot fires indicate that, in
Type of boiler tubes (plain, gilled, etc.)
no
Fig.18
most cases, they occur in connection
Exhaust gas inlet/outlet temperatures
no
Fig. 19
Design mean gas velocity in exhaust gas boiler
yes
Fig. 20
Water inlet velocity to boiler
yes
Fig. 21
Circulation water flow ratio
yes
Fig. 21
with manoeuvring, often following a
stay in harbour.
On the basis of a sample of 82
ships, most of which equipped with
twostroke main engines and water
tube type boilers, the NK “Guide to
Table 1: Statistical parameter survey of soot fires in exhaust gas boilers Ref.: Nippon Kaiji Kyokai, Tokyo
(NK), 1992
Prevention of Soot Fire on Exhaust Gas
Economizers 1992”, Ref. [4], presented
a statistical parameter survey of soot
Number of cases
12
fires/overheating incidents. The survey
covered 53 ships with trouble (soot
Trouble
10
No trouble
fire and damage) and, for comparison
purposes, also 29 NK ships with no
trouble. The engines were in the power
8
range of about 4,00030,000 kW, and
about 10% of the boilers were the large
6
capacity types, including dual pressure
type boilers.
It should be noted that the ships with
trouble were extracted from a repre
4
2
sentative sample of all NK ships, while
the ships with no trouble were limited
to cases in which NK received answers
0
K
L-G
L-MC
K-MC
RND
S-MC
RD
RL
RTA
D E
H
L KZ
V
D
D
from shipyards or boiler makers.
MAN B&W
Sulzer
Mitsubishi UE
The parameters stated in table 1 have
MAN
Pielstick
Main engine types
been obtained from the shipyards and
boiler makers in question. The param
Fig. 17: Boiler trouble – influence of main engine type
eters have been studied with regard to
any distinct influence on boiler trouble
which can be seen in Table 1.
Trouble/no trouble comparisons for
and are shown in Figs. 17, 18, 19, 20
some of the most interesting param-
and 21.
eters have been made in graphical form
Soot Deposits and Fires in Exhaust gas Boilers
17
Even though the ships included in the
examination have been freely selected,
Number of cases
20
for which reason simple comparisons
cannot be made, the results of the
Trouble
15
No trouble
comparisons may be considered very
indicative.
10
Influence of main engine type
It is rather interesting, but not surpris
ing, to see that the make and type of
main engine had no distinct influence
5
0
100
150
200
250
300
350 °C
Inlet gas temperature
200
250
300
350 °C
Outlet gas temperature
on the risk of soot fire, as shown in Fig.
17. Thus, the ships equipped with, for
example, MAN B&W, Sulzer (Wärtsilä)
Number of cases
15
or Mitsubishi twostroke main engines,
all seem to have had the same rela
10
tive number of cases with and without
soot fire trouble. Furthermore, statistics
show that the occurrence of soot fires
is also largely independent of whether it
is a short or a long-stroke engine.
5
0
100
150
There is no information regarding the
type of fuel oil, but, as we are dealing
Fig. 19: Boiler trouble – influence of exhaust gas inlet and outlet temperature
with twostroke engines, heavy fuel oil
has probably been used. Operating the
engine on heavy residual fuels of low
quality probably has an increasing effect
on the tendency towards soot depos
its. As low quality heavy residual fuels
are cheap, this tendency may be con
Number of cases
60
sidered as an unavoidable parameter
now and in the future (unless, for exam
50
Trouble
No trouble
40
ple, special fuel additives are used, as
indicated in recent information). Special
featuresregarding “Operation on Heavy
30
Residual Fuels” have been described in
an MAN Diesel paper, Ref. [5].
20
Influence of extended tube surface
10
0
Fig. 18 shows, somewhat surprisingly,
Spiral
Square gilled
Gilled
Pinned
Plain
Type of boiler tubes
Fig. 18: Boiler trouble – influence of type of boiler tubes.
One exhaust gas boiler may count more than once as preheater sections, evaporator sections, etc. are
considered as separate cases
18
Soot Deposits and Fires in Exhaust gas Boilers
that the shape of the water tube ele
ments used in exhaust gas boilers of
the water tube type had no distinct in
fluence on the tendency towards soot
fires.
boilers based on a design gas velocity
Number of cases
15
Trouble
No trouble
10
higher than 20 m/s had such trouble.
One of the dominant parameters influ
encing the occurrence of soot fires, as
it increases the tendency towards soot
5
deposits, is therefore – according to the
statistical material – the low gas veloc
0
0
5
10
15
20
25
30
m/s
Design mean gas velocity
Fig. 20: Boiler trouble – influence of design mean gas velocity in exhaust gas boiler
ity 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
In fact, the type of boiler fitted with plain
low as 100150°C, many boilers had no
important factor. On the other hand,
tube elements almost had the same
such trouble.
the low gas velocity limit is probably a
relative number of soot fire problem as
“floating” limit which may also depend
boilers fitted with tube elements with an
The lower exhaust gas temperature can
on the actual stickiness of the soot in
extended surface. On the other hand,
only be blamed for its possible negative
the exhaust gas smoke, which again
the severe cases of soot fire, with burn
influence by requiring other boiler pa
may depend on the actual residual fuel
ing down of the actual tube elements,
rameters like larger heat transfer area
used (containing asphalt, carbon and
may be more of a risk for boilers with an
and lower gas velocity, which can influ
sulphur).
extended tube surface than for those
ence the occurrence of soot fires, see
with plain tubes, because the potential
the next section.
area is bigger, or should we say forms a
reservoir for soot deposits.
Thus, the stickier the soot, the more
easily it will stick to the boiler tubes.
Fig. 19 tells us nothing about the po
One could claim that the stickiness of
tential influence of the low gas tem
the soot is the dominant factor for the
Influence of exhaust gas temperature
perature in the boundary layer on the
occurrence of soot deposits.
It has often been claimed that the de
cold boiler tubes. This type of low gas
velopment of high efficiency diesel en
temperature may, despite the above re
On the other hand, Fig. 20 then shows
gines involving lower exhaust gas tem
sults, still have an increasing effect on
that only the exhaust gas boiler with a
peratures causes soot deposits in the
the tendency towards soot deposits, as
low design mean gas velocity included
exhaust gas boilers. However, when we
the soot on the tube surfaces may be
in the statistics had exhaust gas smoke
only consider the influence of the actual
made wet and sticky by gas conden
containing sticky soot, and this seems
exhaust gas temperature, statistical
sates.
improbable.
Influence of low gas velocity
Regarding the stickiness of the soot,
analyses show rather clearly that this is
not correct, see Fig. 19.
The statistical analyses of soot fires
information has revealed that, due to
Fig. 19 shows that neither the inlet
show, as indicated in table I, that one
a chemical reaction with the hydrocar
nor the outlet temperature of the ex
of the parameters that has a distinct in
bons, the use of a fuel additive con
haust gas boiler has any distinct influ
fluence is the gas velocity in the boiler,
taining iron oxide may involve that the
ence on the occurrence of soot fires.
see Fig. 20.
soot will be less sticky and more dry.
Even at inlet temperatures as high as
The exact chemical background for this
325350°C, and outlet temperatures
All exhaust gas boilers based on a de
as high as 225250°C, soot fires oc
sign gas velocity lower than 10 m/s had
cur, and even at outlet temperatures as
soot fire trouble, whereas relatively few
observation is not clearly understood.
Soot Deposits and Fires in Exhaust gas Boilers
19
deposits. See the upper left side of the
Number of cases
warning triangle in Fig. 2.
20
Trouble
15
When the boiler has been designed in
No trouble
such a way that soot deposits do not
occur, there is, of course, no soot to
10
ignite.
5
This may explain the troublefree cases
in Fig. 21 for boilers with a low circu
0
0
1
2
3
4
5
6
7
8
Water inlet velocity to boiler
9
lation water flow rate, even though the
m/s
Number of cases
ignition potential exists.
The impact of low gas velocities
15
The statistical material shows a clear
tendency: when the actual gas velocity
in the boiler is lower than a certain val
10
ue, the soot particles in the exhaust gas
will deposit on the tubes whereas, if the
5
gas velocity is higher, the soot particles
will be blown away, i.e. the boiler itself
0
0
2
4
6
8
10
12
Circulation water/steam flow ratio (normal service)
Fig. 21: Boiler trouble – influence of water circulation in water tubes
will have a selfcleaning effect. Com
pare the smoke tube boilers.
According to some boiler makers, the
gas velocity limit for soot deposits is
The result will be a reduction in the ten
cate an important influence on the oc
about 12 m/s, but may depend on the
dency towards soot deposits because
currence of soot fires.
gas constituents, as discussed above.
the soot is less sticky and the gas ve
Part load running of main engine
locity limit for soot deposits will, in turn,
Thus, the lower the water inlet velocity
be reduced, i.e. the soot deposits will
to the boiler, and the lower the circu
It is important to distinguish between a
be less sensitive to the low gas velocity
lation water ratio, the higher the likeli
boiler’s design mean gas velocity and
hood of soot fire problems is.
the actual gas velocity in the boiler.
useful in cases where the exhaust gas
A sufficient circulation water flow rate
When, for example, a ship is sailing
boilers have suffered from soot deposits
is therefore important to avoid critical
with reduced speed or is manoeuvring,
damage to exhaust gas boilers.
the diesel engine’s power output, and
Such a fuel additive may therefore be
Influence of low water inlet velocity
thereby also the amount of exhaust
and low circulation water flow ratio
This is because a low circulation water
gas, will be reduced. This means that
The diagrams in Fig. 21 show the influ
flow rate increases the risk of creating
under specific operating conditions, the
ence of the water inlet velocity to the
spots of steam bubbles which again in
actual mean gas velocity in the boiler
boiler and the circulation water flow
volves spots with a high gas tempera
can be lower than 50% of the boiler’s
ratio (circulation water and steam pro
ture on the tube surfaces, which in turn
design mean gas velocity.
duction mass flow ratio), and also indi
increases the risk of ignition of the soot
20
Soot Deposits and Fires in Exhaust gas Boilers
This could explain why soot fire prob
90° bend just before the inlet to the
particulates) will not easily deposit on
lems had occurred on a few boilers with
boiler, see Fig. 22, left. Clogging with
the ground unless the wind (gas) ve
a design mean gas velocity higher than
dry, hard and consistent soot only
locity is reduced, as it is for example
20 m/s, ref. Fig. 20, as the actual gas
occurred in that corner of the boiler,
behind a fence. The low wind velocity
velocity at part load was lower than 12
with the low gas velocity. However, no
will cause the snowflakes to deposit
m/s. A second explanation could be, as
problems were experienced on sister
and form a snowdrift, and if the wind
mentioned above, that the actual gas ve
ships with the same main engine and
direction changes (higher velocity),
locity limit for soot deposits was relatively
boiler types with a long straight inlet
part of the snowdrift may move. This
high (wet soot) in the cases in question
pipe to the boiler, see Fig. 22, right.
means that at a certain low wind (gas)
Inlet piping to boiler
Summary of main reasons for soot
Another factor that can reduce the
fires
actual gas velocity in a specific part
Given the points discussed in this pa
In a thaw, for example, when the
of the boiler is the design of the inlet
per, and with due consideration for
snowflakes are wet (wet soot), the
piping to the boiler. It is thus not only
the statistical material and the warn
snowflakes will deposit more easily,
the actual mean gas velocity through
ing triangle for soot fires (Fig. 2), a
and a change in the wind direction
the boiler that is the decisive factor for
general and fairly simple explanation
(higher velocity) will make only a small
soot deposits. It is in fact the boiler’s
of the main reasons for soot fires may
part of the snowdrift move. Thus, the
lowest gas velocity that is decisive, as
now be given by using the analogies
wet snowflakes (wet soot) will de
illustrated by the following example.
below.
posit, but will do so already at a wind
In one case, a smoke tube boiler
Analogy with snow (soot deposits)
than the wind (gas) velocity for the
suffered from soot clogging caused
In a snowstorm at belowzero tem
abovementioned frozen snowflakes
by a nonuniform gas flow due to a
perature, the snowflakes (dry soot
(dry soot).
velocity, the snowflakes (dry soot par
ticulates) will deposit.
velocity (gas velocity) which is higher
Wrong Correct
In general, therefore, high wind (gas)
velocities and frozen snowflakes (dry
soot) will reduce the tendency to
wards deposits.
Soot
deposits
Boiler
tube
section
Boiler
tube
section
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
Inlet
piping
Recirculation
of gas
temperature will be lower and it will
Inlet
piping
Exhaust
gas
Fig. 22: Exhaust gas boiler – influence of inlet pipings
Uniform
gas
velocity
Exhaust
gas
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 are to ignite.
Soot Deposits and Fires in Exhaust gas Boilers
21
Analogy with puttingout a fondue fire (oxygen)
should be dimensioned as low as
If the oil in a fondue pot has become
The first three of these parameters re
too hot and has been set on fire, the
late to the soot deposits, whereas the
easiest way to extinguish the fire is to
fourth parameter relates to the risk of
put a cover over the fire and stop the
ignition of the soot.
supply of oxygen.
possible (large pipe diameters).
F. A dumping condenser should be in
stalled to control steam production/
consumption. A gas bypass valve
Recommended boiler design criteria
installed to control the steam pro
When a soot fire occurs in an exhaust
The boiler design criteria that can be
duction will reduce the gas velocity
gas boiler, similar action has to be
recommended on the basis of the four
in the boiler – and consequently in
taken. In this case the oxygen sup
main parameters mentioned in the
crease the risk of soot deposits – and
ply is stopped by stopping the diesel
above section, with due consideration
therefore cannot be recommended.
engine,
for the influence of the low gas veloc
The supplementary recommenda
ity, are thus as follows:
tions below apply only to boilers of
as the engine’s exhaust gas still con
tains about 14% oxygen.
the water tube type:
Referring to soot deposits:
A.The design mean gas velocity of the
G.A bypass duct with an automatically
General four main parameters
boiler should be higher than 20 m/s,
operated on/off valve (open/closed
Given the points discussed above, the
but the limit may, in fact, depend on
at approx. 40% SMCR) may in cer
risk of soot deposits and ignition fol
how dry and sticky the soot is (fuel
tain operating conditions be recom
lowed by soot fires may be minimised
type/fuel additive)
mended, especially for water tube
by respecting the following four main
parameters – valid for both water and
smoke tube boilers:
„„
is often slow steaming, i.e. the diesel
boiler should be higher than 15°C or,
engine operates at low load, such an
even better, 20°C.
installation will prevent soot deposits
The gas velocity in the boiler must
not be too low – this reduces the
main risk factor for soot deposits.
„„
type boilers. If, for example, the ship
B.The pinch point temperature of the
on the boiler tubes by bypassing all
C.The boiler’s exhaust gas outlet tem
the gas and thereby avoiding low gas
perature should not be lower than
velocities and the associated risk of
165°C as otherwise condensation
soot deposits in the boiler.
The gas temperature on the boiler
of sulphuric acid in the exhaust gas
tube surfaces must not be too low
could make the soot sticky
– this reduces the additional risk of
soot deposits due to the formation of
cleaning should be installed in wa
D.The inlet piping to the boiler should
ter tube boilers in order to clean the
be designed so that the gas flow
tubes of soot. The pressure of the
velocity distribution is as uniform as
soot blowing medium should be as
The engine smoke emission should
possible, in order to avoid local points
high as possible during the entire
not be allowed to deteriorate – as
with a particularly low gas velocity.
soot blowing sequence. As the pos
wet soot.
„„
this will increase the tendency to
sible steam pressure used is only
wards soot deposits.
about 7 barg, and in some cases 6
E.The exhaust gas design pressure
„„
barg, the use of highpressure air will
The circulation water flow velocity
loss across the boiler should be as
and ratio in the boiler must not be
high as possible – increasing the gas
too low – this keeps the gas tem
velocity in the boiler. This means that
I. Fixed water washing system and/
perature at the boundary layer of the
the pressure losses in the remain
or manual cleaning at regular inter
boiler tubes below the ignition tem
ing parts of the exhaust gas system
vals. Water washing is performed
perature of the soot.
22
H.Automatic soot blowers for frequent
Soot Deposits and Fires in Exhaust gas Boilers
be better.
in order to clean the boiler com
pletely of soot which has not been
Referring to ignition
cleaned away by the soot blowers.
K.The circulation water flow velocity
In order to avoid the condensation
The exhaust gas piping between
and ratio at the boiler inlet should
of some of the gas constituents,
the engine and the boiler should be
be as high as possible in order to
preheated feed water should always
so arranged that the boiler can be
keep the gas temperature at the
be used (temperature higher than
cleaned more thoroughly from time
boiler tube surface as low as pos
140°C) during startup and dur
to time when the engine is stopped
sible (in contrast to point J). The
ing low load operation, especially if
in harbour without the risk of flood
water flow ratio (water flow/steam
the boiler is not fitted with an on/off
ing the engine/turbochargers with
production ratio) is recommended
bypass duct/valve which can be ac
cleaning fluid. Water washing should
to be equal to or higher than 6.
tivated in these running conditions.
preferably be undertaken while the
This should reduce the risk of igni
tubes are still hot, making it easier
tion of possible soot deposits, which
C.Water circulation, correct function
to remove the soot as it will “crack”.
can happen at temperatures above
ing. It should be ascertained that the
some 150°C and, under extreme
boiler’s water circulation system and
conditions, even as low as 120°C.
its control system are functioning
If the abovementioned on/off ex
B.Preheated feed water during startup
haust bypass is installed, the boiler
properly.
can be bypassed. Water washing
It is therefore also very important to
should then also be carried out dur
ensure the best suction conditions
D.Water circulation after engine stop.
ing sea service as often as possible
so that cavitation does not occur
After the engine is stopped, the boil
(when the exhaust pressure loss in
in the circulating pumps under any
er’s water circulating pump should
creases), and not only during stops
working conditions, as otherwise the
be kept running until the boiler tem
in harbour. After water washing, it
circulating water flow could be re
perature has fallen below 120°C,
should be checked that no soot is
duced or even stopped.
because wet oily soot may catch
left, as remaining wet soot may in
fire at temperatures as low as this.
crease the risk of soot deposits
A
system
On the other hand, it is recommend
when continuing operation
mounted above the boiler might be
ed not to stop the circulating pump
recommendable as a means of de
in harbour unless the boiler has been
tecting a fire in the boiler as soon as
checked and is clean.
J.The water circulation temperature at
the boiler inlet, for boilers with a pre
temperature
monitoring
it starts.
heater section, should be higher than
F. Heavy smoke from engine. If exces
140°C as otherwise too low temper
Recommended operating conditions
sive smoke is observed, either con
atures could cause some of the gas
In view of the damage that can be
stantly or during acceleration, this
constituents, such as fuel and lube
caused by an extensive soot fire in the
is an indication of a worsening of
oil vapour, to condense on the cold
exhaust gas boiler, it is recommended,
the situation. The cause should be
boiler tube surfaces, and this could
during the operation of the ship, to give
identified and remedied. Excessive
increase the tendency towards soot
due consideration to the following:
smoke could be caused by defective
deposits. An advantage of this is that
fuel valves, a jiggling governor, incor
the temperature of the preheater
rect adjustment of the governor fuel
tube surfaces can then be higher
Normal operating conditions
limiter, or malfunctioning of one (of
than the dew point of the sulphuric
A.Sootblowing. If sootblowing equip
two) auxiliary blowers, etc. The boiler
acid in the gas, thus minimising the
ment is installed, we recommend check
should be checked and cleaned if
risk of sulphuric acid corrosion.
ing its efficiency and adjusting the num
necessary.
ber of daily sootblowings accordingly.
Soot Deposits and Fires in Exhaust gas Boilers
23
Operating conditions in water leakage
the case of emergency and with a
diesel engine’s exhaust valve closing
situations
clean boiler. In addition, they empha
mechanism (closed valves).
DNV has described a case where a
sise that every possible precaution
water leakage was discovered from a
must be observed to prevent soot
water tube type exhaust gas boiler, Ref.
fire.
[6]. In order to get to port, the water
E.Use water washing, if fitted, to extin
guish the fire. This is normally con
nected to the ship’s fire fighting wa
circulation was shut off. When arriving
Actions to be taken during dry running:
at anchorage, the exhaust gas boiler
E.Increase the frequency of soot blow
overheated, and the crew found that
ing considerably, and perform soot
In a wellrun plant any fire that starts
a high temperature soot fire had oc
blowing several times prior to ma
will be small, and if the above emer
curred.
noeuvring
gency action is taken immediately,
ter system.
the fire will be damped down quickly,
The above case shows how important
F. Inspect the boiler frequently and, if
and water circulated by the pump will
it is to cool the tubes to avoid ignition
any soot is present, then water wash
help keep the tubes cool and reduce
of the soot, i.e. the water circulation
the boiler and increase the soot
any heat damage caused by the fire,
through the tubes must always function
blowing frequency
Ref. [2].
correctly.
G.The boiler instruction manual must
If the soot fire has turned into an iron
In this case, the water circulation could
be read carefully and its instructions
fire, this can be indicated by a loss
not continue because of the water leak
are always to be followed.
of water, for example, if the feed wa
age. Therefore, in such a situation the
below actions are recommended:
ter consumption increases very much
Operating in soot fire situations
and/or if a low level alarm in the steam
If a soot fire does start after all, one of
drum is activated. A temperature sen
Actions to be taken prior to dry running:
the following two types of measures,
sor (normally max. 400°C) will not
A.When shutting off the water cir
depending on the level of fire, is rec
normally be able to measure the high
ommended:
temperatures.
haust gas boiler can cool down and
Fire level 1: an initial soot fire has just
Fire level 2: boiler tubes have melted
any smouldering of soot depos
been discovered:
down:
A.Stop the main engine, and thereby
A.Stop the main engine, if it is not
culation, the main engine should
also be shut down so that the ex
its on the boiler tubes can die out.
the oxygen supply to the fire.
stopped already.
B.The heating surface should be in
spected carefully for soot depos
its, and water washing performed,
both
for
cleaning
and
B.Continue operating the water circu
B. Stop the circulating water pump.
lating pump.
cooling.
C. Close valves on the water circulation
C.Never use soot blowers for firefight
line.
ing, as air will feed the fire with oxy
C.Make every effort to reestablish the
water circulation to the boiler, there
gen, and steam will involve a risk of
high temperature fire.
D.Discharge the (remaining) water from
the exhaust gas boiler sections.
by reducing the dry running period to
a minimum.
D.Stop the air circulation through the
engine, and thereby the air supply to
D.Boiler manufacturers allow dry run
ning of exhaust gas boilers only in
24
Soot Deposits and Fires in Exhaust gas Boilers
the fire, i.e. keep air pressure on the
E.Cool down with plenty of splash wa
ter directly on the heart of the fire.
DNV warns that, if a soot fire has
A boiler and system design based on
Reference
turned into a hightemperature fire
the correct criteria will reduce the risk
[1]News from Det Norske Veritas
(hydrogen/iron fire), care should be
of soot deposits and fires in exhaust
(DNV), March 1993 and 2004
taken when using water for extin
gas boilers. The use of such criteria
guishing. The fire may become worse
is therefore very important and could
unless large amounts of water are ap
probably be introduced with advan
K.7400.2, Water Tube Boilers with
plied directly to the heart of the fire.
tage into the recommendations of the
Gilled Tubes for Exhaust Gas, Type
The main objective when
Classification Societies. This would
AV6N, Operation and Mainte
also allow boiler makers to offer boil
nance
[2]
Aalborg
Boilers,
Instruction
discovering an initial small fire is
ers on equally competitive conditions
to prevent it from turning into a
(by, for example, specifying automatic
hightemperature fire.
soot blowers in water tube boilers).
Closing Remarks
The use of special fuel additives with
[4] Guide to Prevention of Soot Fire on
In principle, the most efficient exhaust
iron oxide seems to reduce the sticki
Exhaust Gas Economizers 1992,
gas waste heat recovery system will
ness of the soot and may be useful in
Nippon Kaiji Kyokai, Tokyo
contribute to the best overall econo
cases where the exhaust gas boilers
my on the ship provided, of course,
are vulnerable to soot deposits (for
that the recovered heat, for example
example large capacity boilers).
in the form of steam, is needed on
board the ship.
[3] Sunrod Exhaust Gas Economizers
(brochure, 1992)
[5]Guidelines for Fuels and Lubes
Purchasing and Operation on Hea
vy
Residual Fuels, MAN Diesel ,
The statistical material from DNV (Fig.
Copenhagen Denmark, February
1) shows a considerable reduction in
2009
Normally, the exhaust gas boiler de
soot fire cases from 1998 to 2003,
sign will be based on a steam produc
but also indicates that great atten
tionrequirement related to the rather
tion to installation and operation of
high steam consumption needed in
exhaust gas boilers is still needed.
extreme winter conditions.
Thus, great attention to operation is
[6] Casualty Information
needed when sailing at reduced ship
However, when a ship operates world
speeds, i.e. under low load operation
wide in normal trades, these winter
of the main engine.
conditions may occur only a few days
a year. The choice of a smaller boiler
with lower design steam production
may therefore mean few disadvantag
es, provided the steam requirement
fornormal sea service can be met.
One advantage of this will be that the
design gas velocity through the small
er boiler will be higher and, as ex
plained in this paper, this will reduce
the risk of soot deposits and fires.
As an additional advantage, the ex
haust gas boiler will be cheaper.
Soot Deposits and Fires in Exhaust gas Boilers
25
26
Soot Deposits and Fires in Exhaust gas Boilers
Soot Deposits and Fires in Exhaust gas Boilers
27
All data provided in this document is non-binding. This data serves informational
purposes only and is especially not guaranteed in any way. Depending on the
subsequent specific individual projects, the relevant data may be subject to
changes and will be assessed and determined individually for each project. This
will depend on the particular characteristics of each individual project, especially
specific site and operational conditions. Copyright © MAN Diesel & Turbo.
5510-0065-02ppr Sep 2014 Printed in Denmark
MAN Diesel & Turbo
Teglholmsgade 41
2450 Copenhagen SV, Denmark
Phone +45 33 85 11 00
Fax +45 33 85 10 30
[email protected]
www.marine.man.eu
MAN Diesel & Turbo – a member of the MAN Group