465
1
INTRODUCTION
The Baltic Sea has been a sulphur emission control
area (SECA) defined by the IMO (International
MaritimeOrganization)forseveralyears.Since2015,
themaximumsulphurcontentoffuelhasbeensetat
0.1 %, which is still stricter than international non‐
SECA standards (0.5 % from 2020) [1]. In
practice,
shipowners have two options when operating in a
sulphuremissioncontrolarea:usealow‐sulphurfuel
(suchaslowsulphurmarinegasordieseloils,LNG,
etc.)orinstallsulphurscrubbersontheship[2,3].In
2020,morethan400scrubbershipswereoperatingin
the Baltic Sea
and more than 3000 worldwide [4].
Many shipping companies chose to use low‐sulphur
fuel, but some invested in scrubbers to use cheaper,
high‐sulphur heavy fuel oil in their fleet. The
popularityofscrubberinvestmentsstronglydepends
on the type of vessels and the shipowner’s business
model.In2017,almost
50%ofRORO(roll‐on,roll‐off)
vessels operating in the Baltic Sea region had
scrubbers installed. This is mainly related to
investmentdecisions,whicharebasedonthefactthat
RORO/ROPAX vessels typically have permanent
routes and fixed timetables. During normal
operations, they do not leave the SECA
area of the
Baltic Sea like some other ship types, such as bulk
carriers,tankers,orcontainerships[5].
Chemically, the operating principle of sulphur
scrubbersisquitesimple[6].Thesulphurcompounds
inthefluegasesreacteasilywithwater;thesulphur
binds as various compounds in the wash water
and
does not migrate into the atmosphere, causing acid
rain and environmental damage [2,7]. The technical
structures of sulphur scrubbers can be divided into
two main categories [8,9]: In open‐loop systems, the
seawater used in the wash cycle is discharged
untreated back to the sea afterwards. Closed‐loop
systems
have a separate wash cycle, where the
sulphur compounds react with sodium hydroxide.
These systems do not depend on the properties of
Open-loop Scrubbers and Restricted Waterways: A Case
Study Investigation of Travemünde Port and Increased
Sulphur Emissions Immediately After the Scrubbers are
Turned Off
E.Altarriba,S.Rahiala&T.Tanhuanpää
1
South‐EasternFinlandUniversityofAppliedSciences,Kotka,Finland
ABSTRACT:Open‐loopsulphurscrubbersmustbeswitchedoff,andthefuelmustbechangedtolow‐sulphur
fuelbeforeenteringGermaninlandwaters.Immediatelyafterthescrubbersareturnedoff,warmexhaustgases
causetheresidueleftinthescrubber
tovaporise,leadingtotheincreasedsulphurcontentoftheexhaustgas.
The momentaryincreasein sulphuremissions immediatelyafter theopen‐loop scrubbers areturned off has
receivedlittleattentioninresearch.Thispaperpresentstheonboardmeasurementresultsofexhaustgasesand
examinestheeffectsofsulphurcompoundsreleased
intotheair.Inthiscase,theobservedsulphuremission
peakisproblematicduetothegeographicallocation.Theshipsailstotheriverport,passingthecoastaltownof
Travemünde, where the exhaust gases are released. Due to this, the emissions are more harmful when
comparedtoemissionsgenerated
intheopenseas.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 17
Number 2
June 2023
DOI:10.12716/1001.17.02.24
466
seawater (salinity, alkalinity, temperature) and thus
workinlow‐salinitybrackishorfreshwaterareas.The
amount of wash water discharged intothe sea from
theclosed‐loopscrubbersisminimal,andthewateris
purified. In addition, there are hybrid scrubber
modelsinwhichitispossibletochangethe
operating
modefrom openloop toclosed loopandvice versa.
Globally,themostcommonscrubbertypeisanopen‐
loopscrubber,butintheBalticSearegion,closed‐loop
orhybridscrubbersarecommon[4].
Open‐loopscrubbershavesparkeda debateabout
thepossibleadverseeffectsofdischarging
washwater
into the sea [10‐12]. The sulphur compounds in the
wash water are not an actual problem, but heavy
metal concentrations and PAH compounds are of
particularconcern[4,13].Thelimitvaluessetforwash
water are presented in tables 1 and 2 [14]. There is
significant discussion, especially
in environmentally
sensitive areas, concerning the adequacy of these
valuesintheboundaryconditions[15,16].
Table1.Washwaterlimitvaluesforacidityandturbidity
________________________________________________
ConditionLimit
________________________________________________
Differenceinturbidity Below25FNU(15minaverage)
DifferenceinpH Below2pH(difference)
SeawaterpHpHatleast6.5measuredat
4metersfromthedischargeoutlet
________________________________________________
Table2.WashwaterlimitvaluesforPAHconcentration
________________________________________________
Washwaterflow(t/MWh) PAHconcentration(μg/l)
________________________________________________
<12250
1‐2.5.0900
2.5‐5.0450
5.0‐11.25200
11.25‐22.50100
22.50‐45.0050
45.00‐95.0025
________________________________________________
Despitethese boundaryconditions (tables1& 2),
thedischargeofwashwaterisprohibitedonGerman
internal waterways, because there is insufficient
information on the environmental effects of wash
water[17].Forthisreason,manyothercountries,such
asFinland,Sweden,theUnitedKingdom,France,the
UnitedStatesandChina,
havealsobannedtheuseof
open‐loopscrubbersin certainport areas[17,18].On
shipsequippedwithopen‐loopscrubbers,thismeans
thatbeforeenteringinternalwaterways,thefuelmust
be changed to a low‐sulphur variant, and the
scrubbers must be switched off [18,19]. The fuel
changeprocess
iscarriedoutinadvancesothatitis
certain that there is no more sulphurous fuel in the
fuel lines before arriving at the restricted area. The
actual switch‐off threshold depends on the technical
structure of the scrubber. If there is no flue gas
bypass,thescrubberisoften
keptrunningaslongasit
is allowed to reduce the thermal stress inside the
scrubber. Thus, the total environmental impact of
scrubbers is seen as essential to reducing the
environmentalimpactofships.
Theobjectiveofthispaperistoexploretheimpact
of switching off open‐loop scrubbers
in a localised
area. Taking the port of Travemünde as a case
example, we examine the sulphur dioxideemissions
when a ship arrives at port. The actual operational
situationistheobjectofinterestbecauseitcanreveal
phenomena that do not appear in standard
measurementsormethodsbasedonemission
factors.
In addition, studying the research problem with
simulationwouldrequirequiteadvancedmodels.The
studied vessel is a ROPAX‐type (roll on/roll off
passenger) ship equipped with open‐loop scrubbers.
Ships of the same type arrive at the port of
Travemündeeveryday.The fuel is changedtolow‐
sulphur marine gas oil before arrival, and the
scrubbersareswitchedoffwhentherestrictedareais
reached.Afterthis,thehotexhaustgasesvaporisethe
sulphur‐containing material left in the scrubber
column, causing a momentary increase in sulphur
emission levels. Our measurement results show that
sulphur emissions increase when
the ship passes
through the channel in Travemünde. Although the
sulphur emissions are not large in principle, the
settlementʹs proximity makes them more harmful
than the emissions generated in the open sea.
Consideringtheconcernaboutthedischargeofwash
waters, this peak in sulphur emissions has received
surprisingly
littleattentionintheresearchfield.
2
METHODS
The studied vessel is a ROPAX shipwith fourmain
engines (9L46D Wärtsilä, power 10,395 kW/engine).
Four open‐loop scrubbers (ECO‐DeSOx, Ecospray
technologies)areinstalledinthestacksofeachmain
engine.Thescrubberswereretroactivelyinstalledon
the ship at the end of 2014 before the stricter SECA
restrictions came into force. The scrubbers are in
constantanddaily use,exceptinthe restrictedareas
ofGermany.Thescrubbersoperateforapproximately
60enginehoursbeforeenteringtherestrictedarea.
The ship operates on a regular route between
Finland and Germany. Measurements were
conducted in both cases when
the ship was in the
open sea and approaching port. This paper reports
only the measurements done when approaching the
port. During the open sea cruise, the scrubbers
worked efficiently without problems, and the
emissions were within the permitted limits. Upon
arrival at the port, the ship operated fully in
accordance
with the requirements set by legislation
and regulations. Two different fuel types are used:
ultra‐lowsulphurmarinegasoil(ULSMGO)isused
whentheshiparrivesattheportofTravemünde,but
heavy fuel oil (RMG 380) is used for cruises in the
opensea.
The destination port
of the vessel during the
measurement trip was Travemünde, located in the
estuaryoftheTraveRiver(figures1and2).Itisoneof
the largestferry ports in the Baltic Searegion; there
are usually around 20‐40 port calls per day
(arrivals/departures). RORO/ROPAX vessels
(passenger or cargo) cover
35 % of the vessels
regularlycallingattheport.[20]
The fuel is changed to ultra‐low sulphur marine
gas oil about 90 minutes before arriving at the
restrictedarea.Changingthefueltakesapproximately
40 minutes and thus ensures that the fuel has been
changedbeforetherestricted
area.Thescrubbersare
switchedoffwhenreachingtherestrictedareastarting
infrontofthebreakwateroftheMolenfeuerbeacon.
Since the scrubbers do not have exhaust gas bypass
467
channels, the scrubbers are kept running as long as
possible, even though the fuel has already been
changed.Thisis donetoreducethethermalloadon
thescrubbers.
Figure1.Theportentrancechannel(©EliasAltarriba)
Figure2. Travemünde port (Source: OpenStreetMap
Deutschland)
Thisstudyisbasedontheemissionmeasurements
conducted on the ship during normal operations
whentheshiparrivesatport.Thisapproachhasbeen
chosenbecausetherearerelativelyfewstudiesbased
on shipboard emission measurements, and actual
emissions may differ from the measurements made
during the ship classification process.
Operational
situations, such as this case, cannot be adequately
modelledwithemissionfactorsorcoefficients.
The portableanalyser HoribaPG350 wasusedas
themeasurementdevice.Thedevicewaslocatedafter
the scrubber in the stack. The measured emission
components, measurement methods and related
standards are listed in table 3
[21‐25]. The
temperature, humidity, and pressure of the
combustion air of the engines have been estimated
consideringtheprevailingconditions.
Table3.Emissioncomponents,measurementmethodsand
standards
________________________________________________
Component MethodStandard
________________________________________________
NOX Chemiluminescence [21]SFS‐EN14792/2017
SO
2 IR‐absorption[22]CEN/TS17021/2017
CO IR‐absorption[23]SFS‐EN15058/2017
CO2 IR‐absorption[24]ISO12039/2019
O
2 Paramagnetic[25]SFS‐EN14789/2017
________________________________________________
The shipʹs voyage data has been recorded on the
L3 Valmarine APIS terminal so that the results of
emissionmeasurementscanbecomparedwithengine
load and fuel consumption. In addition, the data
recorded by the control system of the scrubbers has
beenapplied toverifythemomentwhen the
system
was turned off. Moreover, the analysis utilises the
exhaustgastemperaturedataprovidedbythesystem.
Thedataobtainedfrombothsources,aswellasfrom
emission measurements, have been recorded at one‐
minute intervals. The sources of voyage data are
shownintable4.
Table4.Variablesanddatasources
________________________________________________
VariableSource
________________________________________________
Fuelconsumption L3ValmarineAPIS
EnginepowerL3ValmarineAPIS
Fuelrackposition Ecospraycontrolsystem
Temperature(inletdeSOx) Ecospraycontrolsystem
Temperature(stack)Ecospraycontrolsystem
WaterflowEcospraycontrolsystem
Scrubberon/offEcospraycontrolsystem
________________________________________________
3 RESULTS
Thischapterpresentsthemeasurementresultswhen
the ship approaches the port of Travemünde on 13
September 2022. The measurement results are
presented for a period of 70 minutes. Ultra‐low
sulphurmarinegasoilisusedasfuelthroughoutthe
entireperiod.Atthetimestamp00:23,thescrubbers
are turned off. About a few minutes after this
procedure,theshippassestheMolenfeuerbreakwater
andtravelsalongthechannelthroughthetown.The
shipstarts manoeuvring totheSkandinavienkai pier
(approximately 00:50). Upon arrival at the port and
channelarea(after00:20),allfourmainengines(ME1‐
4)
are running. Still, in practice, ME2 and ME3 are
almostidle,whileME1andME4producemostofthe
requiredthrust.Thepowercurvesoftheenginesare
illustratedinfigure3.
Figure3.Mainenginepowercurves.
468
The sulphur dioxide concentrations measured in
thefluegasofME4areshowninfigure4.Thecurve
showsarelativelyrapidriseintheconcentrationlevel
afterthescrubbersareswitchedoff.Whenusingultra‐
low sulphur fuel, the flue gases should not contain
many sulphur compounds because the
sulphur
contentofthefuelisabout6mg/kg,accordingtothe
bunker delivery note. The concentration of about 2
ppmproducedbythemeasuringdeviceintheperiod
of 00:00‐00:27; could contain a measurement error
(figure 4). The Horiba PG350 applies the IR‐
absorption method to measure carbon
monoxide,
carbon dioxide, and sulphur dioxide emissions, and
especially the increase in CO concentration (in this
case about 50‐60 ppm) when the engine operates
under partial load, often produces a slight
measurementerrorinSO
2concentrations[22‐24].This
same problem has been observed when measuring
LNG ships, where there should be no sulphur
emissionsatall(butCOconcentrationscanbe100‐300
ppm,inwhichcasetheSO
2concentrationgivenbythe
measuringdevicecanbe5‐10ppm).
Figure4.SO2emissionlevel(ME4)
The sulphur scrubbers are switched off at 00:23,
but the concentrations increase a few minutes later.
Thereisno informationavailableabouthowquickly
thescrubbersturnoff. The waterflowsare900‐1000
m
3
/h,whichleadstoacertaindelayinthesystem.The
temperature data recorded by the scrubber system
gives an indication of this. Unfortunately, the
temperature sensors of the ME4 scrubber gave
incorrect values. However, a similarly loaded ME1
scrubbercan beused asa referenceforhow quickly
thetemperature
levelsriseafterturningoffthewater
circulation.Asteadilyrisingcurve indicatesthat the
scrubber binds part of the thermalenergy contained
inthefluegaseswhentheenginerunsatpartialload.
Thisperiodissurprisinglylong,about20minutes.No
exactinformationisavailableonthecondition
ofthe
temperaturesensors.Itiswellknownthatthecooled,
moist exhaust gas coming from the scrubber
effectively smears all surfaces. If the stack
temperaturesensor is dirty,thevalues givenbythis
sensormaybetoolow.Inanycase,whencomparing
theincreaseoftheconcentrationcurve(figure
4)with
thetemperaturelevelcurve(figure5),astrongrisein
concentrationsistimedtotheinitialstage,whichlasts
about10‐15minutes.
Figure5.Exhaustgastemperaturelevels(ME1)
The measured sulphur dioxide emissions in
relationtothefuelconsumedbytheengineareshown
in figure 6. Relative to fuel consumption, a more
reliable understanding of the development of
emission levels is obtained. The concentration alone
can vary considerably depending on operating and
load conditions. The maximum increase was
found
between 00:32 and 00:48. After this, both
concentrations(figure4)andthevaluesinrelationto
the consumed fuel (figure 6) settled down and
approachedaconcentrationlevelof10ppm(figure4).
Asaruleofthumb,whentheSO
2concentrationdrops
by around 10 ppm and the CO concentrations are
high (125‐168 ppm during the last 10 minutes), the
SO
2 values should be approached with caution.
However, sulphur compound emissionsmay still be
at a higher level, even though most of the sulphur‐
containing material left in the scrubbers has already
beenvapourised.
Figure6.SO2emissionlevel(ME4)
The emission measurement results presented in
this paper concern only one main engine (ME4). As
showninfigure3,theloadconditionsoftheengines
varyuponarrivalattheport.Eachisequippedwitha
separate scrubber, and the amount of exhaust gases
affects the heating time and the vaporisation
of
sulphur‐containing material. This issue still requires
further measurements, but another measurement
session performed on 3 May 2022 in the same ship
and port will provide us with more data.
469
Unfortunately, due to technical problems, no engine
dataisavailablefromthatperiod,butfigure7shows
the SO
2 concentrations (ppm) of main engines ME1
and ME2. The manoeuvring method has been quite
similar, whereby ME1 is more heavily loaded while
ME2isidling.Figure7showshowSO
2concentrations
rise quickly when the scrubber is switched off. The
shape of the curve is very similar compared to the
curvesinfigures4and6.Otherwise,whentheME2is
idling,lessexhaustthermalenergyisavailabletoheat
the scrubber, so concentration levels increase more
slowly. On the
other hand, vaporisation continues
longer. The temperature of the exhaust gas, the
chemical compositionof the deposit, andtherate of
heatincreasecouldinfluencethephenomenon.
Figure7.SO2emissionlevel(ME1andME2)
The measurements presented in this study are
insufficienttoaccuratelycalculatethetotalamountof
sulphurdioxidereleasedintotheair.Atthattime,all
fourenginesshouldbemeasuredsimultaneously,for
whichourmeasuringdeviceisinsufficient.However,
the amount can be estimated as follows. When the
cumulative sulphur dioxide
emission of the ME4
(after the time stamp 00:23) until the end of the
measurement period is calculated in relation to the
fuel consumption, the total amount of released
sulphurdioxideis862g.TheME1hasbeenrunwitha
similarloadprofile,sotheamountofsulphurdioxide
emissions can be estimated at the same level. It is
more difficult to estimate the emission levels of the
idling ME2 and ME3 without complete
measurements. As shown in figure 7, the idling
engineheatsupthescrubberslowly,sotheemission
peakisnotassharpbutlastslonger.In
addition,this
peakisnotdirectlycausedbytheenginerunningbut
bythevaporisationofthesulphurcompoundsleftin
thescrubber.Sinceallmainenginesandscrubbersare
operationalwhen theship isat sea,the scrubbersof
idling engines can also be estimated to be equally
dirty
compared to the scrubbers of ME1 and ME4.
Suppose it is estimated that scrubbers of ME2 and
ME3 engines release approximately 50 % of the
sulphur emissions compared to the level caused by
ME1 and ME4. In that case, a total of 2500 g of
sulphur dioxide emissions were produced by
a ship
arriving at the port. If more sulphur dioxide is
releasedfromthescrubbersofidlingengines,thetotal
amountwillnaturallyincrease.Theassessmentofthe
harmful effects of this is significantly influenced by
the immediate proximity of the settlement, as in the
open sea, these numbers would
not be a matter of
concern.
4
DISCUSSIONANDCONCLUSIONS
The Baltic Sea is a SECA area where the maximum
sulphur content of the fuel is set very low (0.1 %).
When the restrictions came into force in 2015, there
wasmuchdiscussionaboutthemosteconomicaland
environmentally friendly option to implement the
new restrictions; low‐sulphur
fuel or different
scrubbers[26,27].Later,concernalsoaroseaboutthe
effects of discharge water from the open‐loop
scrubber on the marine environment [11,12].
However, this was hardly discussed when scrubber
investmentsweretopical,norhasmuchthoughtbeen
giventothesulphuremissionpeakspresentedinthis
article.
Inthisstudy,withthehelpofexperimentscarried
out onboard, it has been possible to monitor the
vesselʹs air emissions, particularly sulphur dioxide,
whenreachingtheopen‐loopscrubberbanarea.This
paperpresentsthemomentaryincreaseinthesulphur
contentofexhaustgaseswhenthesulphurscrubbers
are
switchedoff.Ourmeasurementsshowedthatthe
SO
2 emission levels were very low apart from the
observedpeaks.Themomentaryincreaseislikelydue
tothevaporisationofsulphur‐containingmaterialleft
in the scrubber under the influence of hot exhaust
gases. The same phenomenon could be detected in
two different measurement sessions, and there are
also previous
observations of the problem [15]. This
evaporation is a short‐term phenomenon. When on
theopen sea,it ishardly significant,consideringthe
total emission levels of the ship. For example,
according to Jalkanen et al. [16], in 2020, the total
amountofSOxemissionsfromshipsoperatinginthe
Baltic
Seawas8.3thousandtons.
However,inthiscase,theshipdoesnotvoyagein
the open sea but in the immediate vicinity of a
settlement. This makes assessing the harmfulness of
emissions more challenging. It is generally known
that SO
2 emissions cause adverse environmental
effects, such as acidification. SO
2 can also affect
particle matter (PM) emission; PM emission can be
created by secondary formation from precursor
emissions, such as SO
2. The health risks of air
pollution are largely caused by particles, such as
immunological and toxic effects in the lungs,
increased cardiovascular system diseases and
mortality. For example, Ytreberg et al. [28] have
evaluatedthatthedamagecostsforSO
2are4,000–19,
000€2010/t
emissionintheBalticSearegion.Accordingto
MattheyandBünger[29],theaveragehealthdamage
costofSO
2duetoemissionsfromanunknownsource
is 13,600 in €2016/t
emission in Germany. The
correspondingvaluesforPM2.5emissionsare6,000–
30,000€2010/t
emission[28]and58,400€2016/temission[29].
However,our measurementsdidnotincludePM,so
we do not know if the phenomenon affected, for
example, the chemical composition of the particles.
Noristheremuchinformationintheliteratureabout
the phenomenonʹs effects on PM measurements.
Teinilä et al. [15] found that the increase in
470
temperature in the port operation of the scrubber
increasedtheorganicmatterofPM.Inthiscase,this
researchproblemisfurthercomplicatedbythechosen
scale.Willtheissuebeconsideredatagenerallevelor
fromthepointofviewofthehealthofresidentsofthe
town?
Orshouldthisresearchproblembegeneralised
toallportcitiesinasimilarposition?
Whenassessingadverseeffects,theamountofship
traffic,whichproducessulphuremissionpeakswhen
arrivingattheport,mustbeconsidered.Inthisstudy,
abroaderreviewhasnotbeencarriedout,butsimilar
shipsareknown toarriveat theport every day. On
the other hand, it is known that pure open‐loop
scrubbersareintheminorityintheBalticSearegion:
AccordingtoYtrebergetal.[4],83%oftheBalticSea
scrubberfleetcanswitchtoaclosed‐loop
mode.The
closed‐loopscrubberscanbeusedinarestrictedarea,
eliminating the sulphur peak problem. The issue
wouldalsoberesolvediftheopen‐loopscrubberhad
anexhaustgasbypasschannel.
Abanonopen‐loopscrubbershasbeenproposed,
butin theBalticSearegion, this
has only appliedto
internal waterways in Germany. In Sweden, a
discussion has started about banning open‐loop
scrubbers in internal waterways after the Swedish
Transport Agency and the Swedish Agency for
Marine and Water Management proposed it [18,30].
Thus, it is good to consider whether the internal
waterway ban is
sufficient or whether it should be
implemented more widely. Often in environmental
protection,abancanleadto newproblems,andthe
effects must be evaluated extensively. For example,
this peak of sulphur dioxide emissions immediately
uponarrivalatportisduetotherestrictionofopen‐
loop scrubbers. If
a similar regulation is introduced
more widely, but only for internal waterways, will
suchunnecessarysulphurdioxidepeaksalsoincrease
in other ports? On the other hand, if open‐loop
scrubbers are completely banned, the problem is
solved.
ACKNOWLEDGEMENTS
Theresearchdiscussedinthispaperhasbeencarriedoutas
partoftheresearchproject“Benchmarkingstudyofmarine
emission reduction technologies (MEPTEK, A75791)”,
funded by the European Regional Development Fund
(ERDF).Acknowledgementsareduetotheregionalcouncils
(Regional Council of Kymenlaakso and Helsinki‐Uusimaa
RegionalCouncil)fororganising
thefunding.Wethankthe
accredited emission measurement laboratory of Kymilabs
(T197/SFS‐EN ISO/IEC 17025, laboratory NB2450) for
performing the emission measurements. Special thanks to
theshipowners(Tallinkand Finnlines), partners and other
entities that collaborated on the project (Kotka Maritime
Research Centre, FinnishShipowners’ Association, Finnish
Transport Infrastructure Agency,
Finnish Transport and
Communications Agency, FinnishMeteorological Institute,
FinnishEnvironmentInstitute,PortofHaminaKotka).
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