803
1 INTRODUCTION
1.1 Economicaspectsofcoaltransportation
Shipping is the safest and most environmentally
benignformofcommercialtransport.Throughoutthe
last four decades, the shipping industry has
experiencedatrendofincreaseintotaltradevolume.
Supported by the world economic recovery in
2017, total volumes of the
global seaborne trade
expandedat4%andreached10.7billiontons.Nearly
half of the volume increase comprised of dry bulk
commodities.Adrybulkcargo(solidbulkcargo)isa
commodity which is shipped in large, unpackaged
amount.
These dry bulk commodities are usually divided
into two categories:
major bulks and minor bulks.
Some examples of major dry bulk commodities
include coal, ore and grain. Minor bulks include
steels, sugars and cements.Coal is the second
largestdrybulkcommodityintermsoftradevolume
(behindironore)thattransportedbysea,accounting
foraboutthe25%of
theworlddrybulktrade.
Coalisa mineralizedfossilfuel,minedextensively
throughouttheworldandwidelyutilizedasasource
ofdomesticandindustrialpower.Coalwhichisused
forpowergenerationaccountsformorethan75%of
thetotalcoaltransportedbysea.Coalislinked
tothe
energy market and its transport is affected by
seasonaldemand fluctuations.Coking coal,which is
usedformetallurgicalpurpose,amountstoabout25%
Sea Transportation of Coal Liable to Liquefaction
M.Popek
GdyniaMaritimeUniversity,Gdynia,Poland
ABSTRACT:Themarineindustryisavitallinkintheinternationaltrade,withvesselsrepresentingthemost
efficient,andoftentheonlymethodoftransportinglargevolumesofrawmaterials.Coalisamajorcargowith
hundredsofmillionsoftonsbeingshippedeveryyearfor
powerconsumptionandindustrialuses.Thevast
majorityofcoaltradedisdeliveredbysea.TheIMSBCCodespecifiesrequirementsrelatedtothesafestowage
and shipment of coal that may give rise to relevant on‐board risks, for example structural damage due to
impropercoaldistribution,chemicalreactionleading
tospontaneouscombustion,emissionofexplosivegases
andliquefaction.Ascoalisliabletoliquefaction,severalprecautionsshouldbetakenbeforeacceptingthecargo
forshipmentandproceduresforsafeloadingandcarriageshouldberespected.Accordingtotheanalysisofthe
data,theproportionoffinesinthecargoesshipped
worldwidehasbeenacceptedasanappropriatecriterionto
identifythepotentialofacoalcargoforliquefaction.
Themainpurpose ofthispaperis to investigatetheimpact of coal properties onthe ability to liquefy.The
relation between the degree of fragmentation and the value of the TML
was analyzed. In addition, the
possibilityofusingdifferentmethodfordeterminationoftheTMLwasdiscussed.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 13
Number 4
December 2019
DOI:10.12716/1001.13.04.13
804
of the total annual volume of coal. The increase in
seaborne transportation of coking coal has been
primarilydrivenbyanincreaseinsteelproduction.
As a seaborne commodity, it is nearly always
carried in bulk and is of considerable importance,
being shipped in large quantities from Indonesia,
United States’
East and Gulf Coasts, Canada’s West
Coast,Australia,SouthAfricaandRussia.Mostofthe
seaborne trade of coal is confined to large bulk
carriers (e.g. Panamax‐size and above) hence the
industry is relying on economies of scale on certain
well‐establishedtraderoutes.Coalmarketstodayare
very
dynamic and large variety of qualities are
traded.HigherimportdemandinChina,Republicof
Koreas and number of South‐East Asian countries
contributedtothevolumeincrease[UNCTAD2017].
Thestatistic inFigure 1representsthe volume of
coalthatwastransportedviaseabornetradebetween
2010and2017[UNCTAD
2018].
Figure1.Thevolumeofcoalseatransportation2010÷2017
Globally, 1.12 billion metric tons of coal were
transportedbysea in2012 whichshowedsignificant
increasefromthe930milliontonsofcoaltransported
in 2010. Global coal trade resumed growth in 2017,
increasing by 5.8% following a limited expansion in
2016 after a significant decline between 2014 and
2015.
Coalisanemission‐heavyfuelanditisthetarget
ofseveralenvironmentalregulationswhicharegoing
into effect nowadays. Furthermore, there is a
worldwide shift to renewable energy. A decrease in
theconsumptionofcoalwilldefinitelybeabighitto
the dry bulk shipping. Despite the
looming
environmental changes, coal still remains one of the
two major dry bulk commodities and there are no
signsofslowdowninitscurrenttradevolumesorits
short‐termprospects.
1.2 Coalasahazardouscargo
Despite the carriage of coal in bulk being a long‐
established trade provided with
a wealth of
experience,itremainsadifficultanddangerouscargo
totransportwithseveralmajorsafety considerations:
chemical reactions of cargo such as emission toxic
gases,spontaneouscombustion,cargoshiftingatsea–
loss or reduction of stability during a voyage,
liquefactionandcorrosionsofship’sholds.
TheInternational
MaritimeSolidBulkCargoCode
specifiestherequirementsrelatedtothesafeshipment
ofcoalthatmaygiverisetorelevanton‐boardrisks.
ThecurrentIMSBCCodeincludesregulationsforthe
shipment of coal as Group B “materials hazardous
onlyinbulk”(MHB)[IMO2017].
Coalemitsmethanewhichwhen
mixedwithairis
liabletoexplosionsifincontactwithanakedlight.In
certainconditions,anexplosionmightbeaugmented
byafollowingcoal‐dustexplosion.
Coals are subjected to heat which may lead to a
spontaneouscombustion.Thispossibilitywilldepend
on factors such as methods of
handling of the coal,
lengthoftimeon the ship, the ventilation provided,
weatherconditionsandambienttemperature.Ifcoal
undergoes a spontaneous oxidation it can result in
secondary hazard, which includes the production of
carbon monoxide as well as other toxic and
flammablegases.Carbonmonoxidehasnosmelland
isa “silentkiller” becauseit bindsto hemoglobinin
blood leading effectively to suffocation. In addition,
thecareshouldbetakenwhendealingwithcoalasit
isanoxygen‐depletingcargoascombustionconsumes
oxygen.Ifthecarbonmonoxidelevelincreasesyetthe
oxygendoesnotdecrease,then
thisindicatesthatthe
holdsarenotsealedeffectively.
Although, the cargo surface should be ventilated
to reduce the risk of gas explosion, such ventilation
may encourage spontaneous combustion.
Consequently, ventilation of coal must be very
carefullysupervisedanddirectedatthesurface area
only, in order to avoid air reaching
deep into the
cargo.
TheprovisionintheIMSBCCodestatesthat“This
cargo shall only be accepted for loading when the
temperature of the cargo is not higher than 55
o
C.” The
reasonforhavingamaximumcoaltemperaturelimit
isthefactthattheself‐heatingreactions arelikeany
chemical reaction hence the rate of reaction doubles
foreverytendegreerise intemperature.Even when
the cargo was loaded with temperature below 55
o
C,
the monitoring of the cargo during the voyage is
important because the issues with self‐heating may
occurduringthevoyage.
Theshipmentofcoalshouldbemonitoredforgas
variations during the voyage. Changes in the gas
concentrations will indicate whether self‐heating or
methaneemissionsaretakingpla ce.
Coals with a high Sulphur content (especially
when loaded wet) are liable to create a situation
wherebychemicalactioncancorrodesteelholdsides
and bulkheads.These problems may be worsened if
the coal temperature rises and the longer the cargo
remainsontheship.
Coal is liable to shifting
at sea thereby
endangeringthesafetyoftheshipconcerned.Cargoes
with an angle of repose greater than 35 degrees are
less prone to surface shift, but nevertheless need
trimmingtosufficientlycovertheentiretanktoparea
outtotheship’sside.
805
2 LIQUEFACTIONOFCOALCARGO
2.1 Liquefactioninmaritimetransport
Solidbulkcargoescanbecategorizedwithregardto
theirhazardsduringshipping.Liquefactionisoneof
the greatest safety risks in solid bulk shipping. The
most significant consequences for vessel resulting
from liquefaction include cargo shift which
progressively leads
to a loss of stability. The
consequence of the loss of stability can be such that
the vessel and the lives of those onboard are lost
[Andrei&Pazara2013].
Cargoliquefactionhasbecomeoneofthegreatest
concernsforsafecarriageofdrybulkoverthepastten
years.
Nine bulk carriers transporting ore
concentratesover10,000dwthavebeenlostfrom2008
to2017. 101crew membershave losttheir livesas a
consequence of ships capsizing [Intercargo 2018].
Fatality numbers are speculative, as it is usually
difficulttoestablishiftheliquefactionwasthecause
ofcapsizing.
Another
significantproblemduetoalossofship’s
stabilityisthepollutioncreatedbyharmfulproperties
of cargo discharged into the sea due to ship’s
capsizing [Cristian et. al 2015]. Cargoes at risk of
liquefactionincludeconcentratesandotherfine‐grain
materialsuchascoal.Thesecargoesusuallycontaina
portion of fine particles that are exhibiting low
permeabilitywhencompacted[Rose2014].
During the loading, these cargoes are usually
partially saturated. Forces applied by ship’s motion
and engine vibration cause particle rearrangement
and further compaction. In addition, moisture
migrationleadstoanincreaseinmoisturecontentin
partofthe
cargo.
Themainregulationforsolidbulkcargothatwas
developed by the International Maritime
Organization is the IMSBC Code which is key
instrument in mitigating the risks of cargo
liquefaction.The Code is mandatory under the
International Convention for Safety of Life at Sea
(SOLAS). The IMDG Code categorizes
solid bulk
cargoesbasedonhazardsinvolved.GroupAarethe
cargoeswhichmayliquefy.
Two important parameters, which should be
determined are the Flow Moisture Point (FMP) and
theTransportable Moisture Limit(TML). FMPis the
moisture content at which a sample of cargo will
begintoloseshearstrength.
Themoisturecontentofa
cargobeyondtheFMPmayliquefy[P&IAssociation
2012].TMLisdefinedas90%oftheTMP.
To control the risk of liquefaction, Group A
cargoes are tested at a minimum of semi‐annual
intervalstodeterminetheirTML.
The mandatory provision requires that
if a cargo
prone to liquefaction has a moisture content that
exceeds the Transportable Moisture Limit it should
notbeloaded.
ThreemethodsoftestingfortheFMPandTMLare
listed in Appendix 2 of the Code: Flow table test,
PenetrationTest andProctor‐Fagerberg test.As each
method has
its advantages, the selection of the test
methoddependsonthetypeofcargobeingtested.
The flow table is generally suitable for mineral
concentrates and other materials with a maximum
grainsizeof1mm.Penetrationtestisanalternativeto
theFlowtabletest.Itisgenerallysuitableformineral
concentratesuptoasizeof25mmandcoarsecargoes
suchascoal.
Proctor‐Fagerberg test is suitable for fine and
relativelycoarse–grainedmaterialsuptoatopsizeof
5mm.
2.2 Riskofcoalcargo
One of difficultieswithtransporting large quantities
of coal in
bulk is that it is a cargo capable of
liquefaction.Coalthatisatriskofliquefactionisthe
onecontaining atleastsome fineparticles andsome
moisture content. Although coal often looks dry in
appearance at the time of loading, the cargo may
containmoistureinbetweenthe
particles.According
to IMSBC Code regulation, coal is defined as a
dangerousgoodinsolidformin bulkGroup B(and
A) meaning that an individual coal product may
exhibiteitherGroupBpropertiesonly,orbothGroup
AandBproperties.Shippingthiscargosafelyisakey
concernfor
thedrybulkindustry.
At the time of loading, the coal is in solid state,
where the particles are in direct contact with each
other and there is physical strength of resistance to
shear strains. During the sea transportation, coal is
exposedtoenginevibration,ship’smotionsandwave
impact,
resulting in compaction of the cargo. This
leadstoareductionofspacesbetweentheparticles.If
compactionissuchthatthereismorewaterinsidethe
cargo than there are spaces between particles, the
waterpressureinsidethe cargocanrisesharply and
press the particles apart. This suddenly reduces
the
frictionbetweenparticles,andthustheshearstrength
ofthecargo[Jones&Bell2010].
The applicable provisions of the IMSBC in the
previousyearsincludedacriterionforacargobeing
declared as Group A in the Hazard section as “Can
liquefyif predominantly fine 75% less
than 5 mm coal”.
Furthermore, the requirement in soil mechanics
literatureisusuallyexpressedas0.003mm<D
10<0.3
mm,whereD
10representstheparticlesizeforwhich
only 10% of mass of the material is finer. When
expressedinaformthatwouldbemoreusualinthe
coal industry, the requirementis thatapproximately
15%ormoreofthematerialisfinerthan0.50mm(D
15) for liquefaction to be likely [Eckersley 1997]. The
proportionoffinesinthecargoesshippedworldwide
hastodatebeenacceptedasanappropriatecriterion
for estimation of the potential for liquefaction of a
coalcargo.
Australiancoalproducersandexportershavebeen
safely shipping millions of tonnes of coal
from
Australia for many decades using the appropriate
schedulecontainedinBCCodeandIMSBCCode.
Afewyearsago,Australianindustryhasinitiated
researchtostudywhatotherfactors,ifany,mayaffect
coal’s liability to liquefaction [IMO 2014]. The
researchhasbeendesignedtodevelopunderstanding
of coal’s
stability during shipping, including the
potentialforcargoliquefaction.
806
The TML methods in the IMSBC code provide
techniquestodeterminetheTMLforarangeofbulk
cargoes. The size distribution of the material being
tested is considered an important parameter when
selectingaTMLtest.
The project has investigated the behaviour of
minus50mmcoalcargoesbecause
thisisthematerial
size of a typical Australian black coals but current
TML tests are intended for products with smaller
particle sizes. The research was focused on
determination whether any coal is likely to liquefy
undershippingconditionandidentificationofasafe
methodtodeterminetheTMLfor
minus50mmcoals.
In2015,Australiaintroducedtheprocedureforthe
laboratory determination of TML for coals up to a
nominaltopsizeof50mm.Theprocedureisbasedon
the modification of Proctor/ Fagerber test for bulk
materials[IMO2015].
Currently, the Proctor/Fagerber test described in
IMSBCCode
hasbeenmodifiedto allowapplication
to coal with a top size of 50 mm. The research has
confirmedthattherearesomecoaltypesthatneedbe
declared as Group A and B products, and there are
some coals that may be declared as Group B only.
Criteriabased
onparticlesizedistributionhavebeen
establishedtoidentifyGroupBonlycoals.Coalshall
beclassifiedasGroupBonlybyatestdeterminedby
the appropriate authority or where it has the
followingparticlesizedistribution:
nomorethan10%byweightofparticleslessthan
1mm(D
10>1mm),
nomorethan50%byweightofparticleslessthan
10mm(D
50>10mm).
The main purpose of this paper is to investigate
theimpactofcoalpropertiesontheabilitytoliquefy.
Therelationbetweenthedegreeoffragmentationand
thevalue oftheTML wasanalysed. Inaddition, the
possibility of using different method for
determinationoftheTML
wasdiscussed.
3 EXPERIMENTAL
3.1 Materials
ThefollowingcoalcargosthatcamefromPolishcoal
mineswereusedinthetest:
sample A: an energetic coal (particle size
dimension0÷20mm);
sample B: coking coal (particle size dimension
0÷20mm);
sample C: an energetic coal (particle size
dimension0÷1mm);
sampleD:cokingcoal(particlesizedimension0÷1
mm).
sampleE:anenergeticcoal(blend:20%ofsample
Aand80%ofsampleC),
sampleF:cokingcoal(blend:20%ofsampleBand
80%ofsampleD).
3.2 Methods
Based on these rules, the ability of coal cargoes to
liquefywasassessedonthebasisofdeterminationof
essential parameters: grain size distribution and the
TMLoftestedcoals.
Thesampleshave
been testedin original state as
delivered.Thesieveanalysishasbeenperformedand
effective D
10 and D50 have been determined. For all
samples, the Transportable Moisture Limit has been
estimated by performing the Proctor/Fagerberg test
andtheFlowTabletest.Aseachoftheseprocedures
has a particular field of application, the TML of the
samples of coal has been tested in the original state
andafterdivision
bythesievingprocess.
3.3 Results
Grain size distribution was measured for each
sample.CoalAisacoarsercoal withlessthan30%
particles smaller than 1mm. Coal B is relatively fine
coalwithmorethan40%particlessmallerthen1mm.
The values of effective size D
10 and D50 are
presentedinTable1.
Table1. Value of effective size D10and D 50of tested
sample
_______________________________________________
Kindofmaterial EffectivesizeD10 EffectivesizeD50
[mm][mm]
_______________________________________________
SampleA0,432,00
SampleB0,251,50
SampleC<0,060,088
SampleD<0,060,088
_______________________________________________
Table2.EstimationofTML
_______________________________________________
Typeof TML
concentrate [weight%]
____________________________________
Proctor/Fagerberg FlowTable Test
Test
_______________________________________________
SampleA 15,1Attemptsof
determination
unsuccessful
SampleB 15,5Attemptsof
determination
unsuccessful
SampleC 16,223,1
SampleD 16,320,6
SampleE 16,117,5
SampleF 16,216,9
_______________________________________________
Criteria based on particle size distribution can
identifyGroupAcoals.
Based on the particle size criteria, coking and
energeticscoalcomingfrompolishcoalmineswould
be considered liquefiable since there are generally
morethan50% ofparticlesofthecoal finerthan10
mmandD
10isgenerallylessthan1mm.
Theresultsofestimationofcoal’sTMLdetermined
bythemethodrecommendedbytheIMSBCCodeare
presentedinTable2.
Theconductedresearchhasconfirmedthattested
types of coal need to be declared as Group A
products. The result confirms that successful
estimation
of the TML by the Flow Table test is not
possible whenD
10 valueislarger than 0.20 mm and
themaximumgrainsizeisgreaterthan1mm(samples
807
A and B). It may be concluded that the Flow Table
Test is not suitable for materials of the size
representingcoalcargo. Flow table testis applicable
tofinermaterialssuchascoalwithparticlesizebelow
1mmrepresentedbysamplesCandD.
Both the TML test
results and particle size
distribution confirm the functional relationship
betweenabilitytoliquefyand theirinherent particle
sizedistribution.Consideringtheresultsofestimation
of the TML for tested coals, it can be said that the
increaseof TML valuesis connected withincreasing
degree of concentrates grinding. Research confirms
the impact on the TML levels when different
componentcoalsweremixedtoformablendedcargo.
Significant differences between the TML values
obtained by the Flow Table test and the
Proctor/Fagerbergtestshowthatthemethodscannot
beusedalternatively.EachoftheTMLtestshasbeen
designed with the
intension of determining the
moisturecontent atwhichcargostrengthis likelyto
belostduetoliquefaction. However,theassumption
ofwhenthisoccursisdifferentforeachTMLtestand
they each give different results. For the Flow Table
test,itcorrespondstothemoisturecontentwhencone
strength is lost, and pla s tic deformation is first
observedon theflow table.Forthe Proctor/Fagerber
test, it corresponds to the moisture content at 70 %
saturation.
The results of investigation confirm the need for
evaluation of the modified test for estimation of the
TML including repeatability, reproducibility,
performance of blends
of coals and relationship
betweentestoutcomestocoalparticledistribution.
ThefindingsoftheAustralianresearchconfirmed
that the modified Proctor/Fagerberg method is
applicableforuseoncoalwhereparticlesizesareup
to50mm.Thetestwasadoptedforapplicationtocoal
cargoes transported in accordance with the
Coal
ScheduleinAppendix1oftheIMSBCCode.
4 CONCLUSION
Liquefaction is an aspect of solid bulk cargoes
behaviourthatoccursduringseatransportationandis
of considerable importance from both safety and
financialstandpoints.Itisclearthatpreventionofthe
risks linked to transportation of solid bulk cargoes
that may liquefy depend on correct measurementof
theTMLandmoisturecontentofthecargo.
Inaccurate declarations and certificates from
shippers appear to be a major problem with the
transport of coal liable to liquefaction, though it is
recognizedthattherearenumerouscomplications.
The familiarity with the IMSBC
Code remains
essential aswell as the awareness of contents of the
regulations by all parties involved in coal
transportation.
Forallcoals thatdo notmeetcriteria determined
only for Group B, the Transportable Moisture Limit
testingusingthemodifiedProctor/Fagerbergmethod
shouldbeundertaken.Itisrecommendedthatprior
to
utilization of this method, coals should be firstly
assessedforparticlesizedistribution.
The adoption of the Modified Proctor/Fagerberg
test to determine the TML and include screening
criteria test based on particle size distribution
guaranteesthesafetyshipmentofcoalscargo.
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