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Atmospheric Environment 42 (2008) 4632–4655

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New national emission inventory for navigation in Denmark

Morten Winther�

National Environmental Research Institute, Frederiksborgvej 399, 4000 Roskilde, Denmark

Received 24 January 2007; received in revised form 28 January 2008; accepted 28 January 2008

Abstract

This article explains the new emission inventory for navigation in Denmark, covering national sea transport, fisheries

and international sea transport. For national sea transport, the new Danish inventory distinguishes between regional

ferries, local ferries and other national sea transport. Detailed traffic and technical data lie behind the fleet activity-based

fuel consumption and emission calculations for regional ferries. For local ferries and other national sea transport, the new

inventory is partly fleet activity based; fuel consumption estimates are calculated for single years, and full fuel consumption

coverage is established in a time series by means of appropriate assumptions. For fisheries and international sea transport,

the new inventory remains fuel based, using fuel sales data from the Danish Energy Authority (DEA).

The new Danish inventory uses specific fuel consumption (sfc) and NOx emission factors as a function of engine type

and production year. These factors, which are used directly for regional ferries and, for the remaining navigation

categories, are derived by means of appropriate assumptions, serve as a major inventory improvement, necessary for

making proper emission trend assessments.

International sea transport is the most important fuel consumption and emission source for navigation, and the

contributions are large even compared with the overall Danish totals. If the contributions from international sea transport

were included in the Danish all-sector totals, the extra contributions in 2005 from fuel consumption (and CO2), NOx and

SO2 would be 5%, 34% and 167%, respectively. The 1990–2005 changes in fuel consumption as well as NOx and SO2

emissions for national sea transport (�45, �45, �81), fisheries (�18, 6, �18) and international sea transport (�14, 1, �14)

reflect changes in fleet activity/fuel consumption and emission factors. The 2006–2020 emission forecasts demonstrate a

need for stricter fuel quality and NOx emission standards for navigation, in order to gain emission improvements in line

with those achieved for other mobile sources.

The new fuel consumption estimates for national sea transport are regarded as much more accurate than the DEA fuel

sales data used previously, and the recommendation for DEA is to replace their current fuel sales figures by the new

estimates calculated in this project. Such updated fuel consumption time series for national sea transport will lead, in turn,

to changes in the energy statistics for fisheries (gas oil) and industry (heavy fuel oil), so the national energy balance can

remain unchanged. For international transport, fuel sales data as such are regarded as highly accurate for Denmark, since

they are compiled from audited information from the Danish oil suppliers, and the inventory approach follows good

practice for the United Nations Framework Convention of Climate Changes (UNFCCC) and United Nations Economic

Commission for Europe (UNECE) convention, when fleet activity data are missing. However, in order to make inventory

e front matter r 2008 Elsevier Ltd. All rights reserved.

mosenv.2008.01.065

630 1297; fax: +454630 1212.

ess: mwi@dmu.dk

ARTICLE IN PRESSM. Winther / Atmospheric Environment 42 (2008) 4632–4655 4633

upgrades and to support Danish policymakers in bunker emissions allocation, fleet activity-based estimates calculated in a

further project would be useful.

r 2008 Elsevier Ltd. All rights reserved.

Keywords: Sea transport; Heavy fuel oil; Gas oil; NOx; SO2

1. Introduction

Emissions from ship engines are harmful to theenvironment on both a regional and global scale.Apart from the emission of the greenhouse gas CO2,ship engines contribute significantly to anthropo-genic emissions of NOx, SOx and PM. In atmo-spheric chemistry it is a well-known fact that NOx

and VOC are precursors of ground-level ozone, andthat NOx and SOx emissions contribute to theformation of secondary particles in the atmosphere.Well-known health effects associated with PM,ozone and NOx comprise respiratory diseases andpremature death from heart and pulmonary dis-eases, see e.g. Corbett et al. (2007), Cofala et al.(2007) and Hammingh et al. (2007). Moreover, animportant environmental effect of the NOx and SOx

emission from ship engines is contribution toacidification of the environment. In addition, NOx

contributes to eutrophication of the terrestrial andaquatic environments, and ground-level ozone isresponsible for damage to vegetation (e.g. Cofala etal., 2007; Hammingh et al., 2007).

Navigation is moreover an important source ofemissions in terms of national emission totals. Fornational navigation (national sea transport, fisheriesand recreational craft) in Denmark, the largestemission shares are noted for SO2 and NOx. Theemission shares are 91% and 17%, respectively, inrelation to the total for mobile sources in the Danish2005 inventory. For international sea transport, theemission contributions are large even comparedwith the overall Danish totals. If the contributionsfrom international sea transport were included inthe Danish national totals, in 2005 the fuelconsumption (and CO2) percentage addition wouldbe 5%, and the corresponding NOx and SO2

percentage additions would amount to 34% and167%, respectively.

Essentially there are two methods that can beused to produce fuel consumption and emissionestimates for navigation. The fleet activity (FA)method uses detailed information on ship move-ments and ship classes (e.g. vessel type and size,

engine type and age, and fuel type) as well as thecorresponding fuel consumption figures and emis-sion factors. Examples of global inventories usingfleet activity data are Eyring et al. (2005), Corbettand Kohler (2003), Endresen et al. (2003) andSkjølsvik et al. (2000). On a regional level, Whallet al. (2002) set up inventories for the North Sea/Baltic Sea and the EMEP area, and Lloyd’s Registerof Shipping (1995, 1998a, b) makes inventories forthe North East Atlantic, Baltic Sea and Mediterra-nean/Black Sea. The fuel-based method, on theother hand, uses fuel sales data in combination withfuel-related emission factors. Examples of globalfuel-based emission inventories are Endresen et al.(2005), Olivier and Berdowski (2001) and Corbettet al. (1999).

In the context of national emissions reporting forthe United Nations Framework Convention ofClimate Changes (UNFCCC) and the UnitedNations Economic Commission for Europe Con-vention of Long Range Transboundary Air Pollu-tants (UNECE LRTAP), national sea transportincludes ships sailing between two national ports,regardless of flag. Fisheries include national fishingvessels, and international sea transport includesships regardless of flag sailing from a national portwith a foreign destination (IPCC, 1997; EMEP/CORINAIR, 2006). International sea transport isexcluded from the national emission totals reportedto the conventions.

According to the guidelines for the UNFCCCand UNECE conventions, it is good practice to usefuel sales data to support the emission calculationswhen fleet activity-based fuel consumption estimatesare missing (IPCC, 1997; EMEP/CORINAIR,2006).

In practice, all countries use the official fuel salesreported for national and international sea trans-port in their national inventories. However, prior toinput data usage, modifications of the fuel salesfigures are made individually by many countries aspart of their inventory method. The adjustments aremade in situations when fuel sales figures seemunrealistic compared with actual fleet activity, either

ARTICLE IN PRESSM. Winther / Atmospheric Environment 42 (2008) 4632–46554634

in navigation subcategories or for navigation as awhole. However, some form of further adjustment isalways necessary between different fuel consump-tion sectors in the country fuel sales statistics, inorder to maintain the grand national energybalance.

The following examples explain how differentcountries use fuel sales data in combination withfleet activity-based estimates.

In Sweden, Greece, Norway and the UnitedKingdom, the national inventories rely entirely onnational fuel sales split into domestic and interna-tional sea transport. Sweden (2007) and Greece(2006) make fuel-based calculations, and Norway(2006) and the United Kingdom (2007) disaggregatethe annual results for domestic sea transportaccording to specific fleet activity calculations.

In France (2006) and Spain (2007), total fuel salesunderpins the national inventories, and a divisionbetween national and international fuel consumptionis made based on expert judgment and results fromsingle studies carried out in these two countries.

In The Netherlands (2007), fleet activity calcula-tions are made for international sea transport andfuel consumption for national sea transport iscalculated as the difference between total fuel salesand the estimated fuel consumption for interna-tional sea transport. Conversely, the Italian inven-tory (Italy, 2007) is produced by means of a bottom-up estimate for national sea transport, and theremainder of fuel sales from the national statistics isassumed to be used by international sea transport.Belgium (2007) also produces a fleet activity-basedestimate for domestic navigation, and relies onreported fuel sales for international navigation.

Finland (2007) produces detailed fleet activityestimates for domestic and international sea trans-port. The domestic estimates are regarded asaccurate, and the difference between estimated andstatistical fuel consumption is balanced out byadjustments made to other consumption sectors inthe fuel statistics. For international navigation, fuelsales are used to balance the level of the bottom-upinventory.

Until recently, the Danish inventory for naviga-tion prepared by the National EnvironmentalResearch Institute (NERI) of Denmark used thefuel sales figures for national sea transport andinternational sea transport reported by the DanishEnergy Authority (DEA), directly. On the basis ofthese data, a simple inventory was set up, enablingthe production of national emission reports as

required by the UNFCCC and UNECE convention.However, in a new project funded by the DanishEnvironmental Protection Agency in 2006, NERIhas established an improved methodology fornavigation in Denmark, covering national seatransport, fisheries and international sea transport.For national sea transport, the inventory distin-guishes between regional ferries, local ferries (smallferries) and other national sea transport. Forregional ferries the new methodology performsdetailed calculations of fuel consumption andemissions based on fleet activity data and ferry-specific technical information. For local ferries andother national sea transport, the new inventory ispartly bottom-up based; fuel consumption estimatesare obtained from detailed calculations made forsingle years and combined with fuel-related emis-sion factors. For fisheries and international seatransport, the new inventory remains fuel based.However, for fisheries the fuel consumption figureschange as a result of transferral between estimatedresults for national sea transport and official salesdata for national sea transport and fisheries.

This article explains the new national inventorymethodology for navigation in Denmark, and themost important fuel consumption as well as NOx

and SO2 emission results. Cross-sector comparisonswith other mobile sources are also made for the2006–2020 forecast period. An important task hasbeen to gather data to support improved estimatesfor national sea transport. This involves sailingstatistics and technical data for regional ferries, fuelconsumption totals for local ferries and othernational sea transport, and the establishment ofnew specific fuel consumption (sfc) figures andemission factors for ship engines in general. Thearticle also explains the more simplified fuel-basedcalculations for fisheries and international seatransport.

The entire model covers the historical period1990–2005 and a forecast until 2030, and comprisesalso the emission components of NMVOC, CH4,CO, CO2, N2O and particulates. Full documenta-tion of the model is given in Winther (2008). Forrecreational craft, please refer to Winther andNielsen (2006) for more details.

2. Methodology to calculate fuel consumption and

emissions

For national sea transport, the new methodologyuses fuel consumption figures based on fleet activity

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Table 1

Overview of the EU directives and IMO MARPOL Annex VI

limit values in relation to marine fuel quality

Legislation Heavy fuel oil Gas oil

S % Impl. date S % Impl. date

EU Directive 93/12 None 0.2a 1.10.1994

EU Directive 1999/32 None 0.2 1.1.2000

EU Directive 2005/33

SECA—Baltic Sea 1.5 11.08.2006 0.1 1.1.2008

SECA—North Sea 1.5 11.08.2007 0.1 1.1.2008

Outside SECAs None 0.1 1.1.2008

MARPOL Annex VI

SECA—Baltic Sea 1.5 19.05.2006

SECA—North Sea 1.5 21.11.2007

Outside SECAs 4.5 19.05.2006

aSulphur content limit for fuel sold inside EU.

M. Winther / Atmospheric Environment 42 (2008) 4632–4655 4635

estimates for regional ferries, local ferries and othernational sea transport. Detailed traffic and technicaldata lie behind the fuel consumption and emissioncalculations for regional ferries, as explained inSection 2.1. For local ferries, a bottom-up estimateof fuel consumption for one single year (1996) isused to establish the fuel consumption for allyears in the inventory period, according to thedevelopment in local ferry traffic. For the remainderof the traffic between two Danish ports, newbottom-up estimates are calculated for 2 years(1995 and 1999), based on a database set up forDenmark in an earlier Danish project. Fuelconsumption for other years in the inventory periodis based on the 1995 and 1999 figures by usingappropriate assumptions. For local ferries and othernational sea transport, further explanation isprovided in Section 2.2.

The new fuel consumption estimates for nationalsea transport replace the previous fuel consumptionestimate time series, which was based directly on thefuel sales statistics from the DEA. Also for fisheries,the previous inventory was based entirely on sector-specific fuel sales. However, the fuel consumptionchanges for national sea transport introducechanges into the fuel consumption data for fisheriesand, as a consequence, the gas oil consumptionfor fisheries in the new inventory differs fromthe fuel sales figures reported by the DEA.A further description of estimated vs. actual fuelsales differences is given in Section 4.3. Forinternational sea transport the fuel sales data areused directly as input data for fuel-based calcula-tions; see Section 2.2.

Standard curves for sfc figures and NOx emissionfactors in g kWh�1 are used for regional ferries, as afunction of engine production per year. Fromthese factors, fuel-related NOx emission factors(gGJ�1) are derived for the remaining categories:local ferries, other national sea transport, fisheriesand international sea transport. In general, theSO2 emission factors for gas oil rely on thesulphur limits in EU legislation. For heavy fueloil, weighted SO2 emission factors are based on fuelend-use information from the Danish energystatistics, by assuming the sulphur content fortwo different heavy fuel qualities used in nationalsea transport and international sea transport.When the ‘SOx Emission Control Areas’ (SECA)enter into force, a fuel sulphur content of 1.5% willbe used for the less clean heavy fuel oil quality(Table 1).

For domestic ferries, fuel consumption andemissions are calculated for each ferry service/ferrycombination as the product of the number of roundtrips, sailing time per round trip (h), engine size(kW), engine load factor and fuel consumption andemission factor (g kWh�1). Individual data for fueltype, engine type and engine year are used for eachregional ferry, as explained in Section 2.1. For localferries, other national sea transport, fisheries andinternational sea transport, the emissions arecalculated as the product of total fuel consumptionand average fuel-related emission factors (seeSection 2.2). The latter take into account enginetype and average engine lifetime, which are assumedfor each category.

2.1. Regional ferries

The number of round trips per regional ferryroute is obtained from Statistics Denmark (2006).Table 2 lists the number of return trips for the mostimportant domestic regional ferry routes in Den-mark in the period 1990–2005. For these ferryroutes the following detailed traffic and technicaldata: ferry name, year of service, engine size(MCR), engine type, fuel type, average load factor,auxiliary engine size and sailing time (single trip),have been provided by the Danish Ferry HistoricalSociety (DFS, 2006). In total, data for 75 differentferries are present in the dataset.

The number of round trips for the local ferries isderived as the round trip totals from Statistics

ARTICLE IN PRESS

Table

2

Number

ofreturn

tripsforeach

ferryroute

comprisedin

thepresentproject

Ferry

service

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

Halsskov–Knudshoved

10,601

10,582

11,701

11,767

12,420

12,970

13,539

13,612

5732

––

––

––

–

Korsør–Nyborg,DSB

9305

9167

9237

8959

8813

8789

8746

3258

––

––

––

––

Tars–Spodsbjerg

7656

8835

9488

9535

9402

9562

9000

9129

7052

6442

6477

6498

6468

6516

6497

6494

Korsør–Nyborg,Vognmandsruten

7512

7363

7468

7496

7502

7828

7917

8302

3576

––

––

––

–

SjællandsOdde–Ebeltoft

3908

3978

4008

3988

4325

4569

5712

8153

7851

7720

4775

4226

3597

3191

2906

2889

Kalundborg–Arhus

1907

2400

3162

2921

2913

3540

4962

4888

4483

1454

1870

1804

2037

1800

1750

1725

Hundested–Grenaa

1026

1025

1032

1030

718

602

67

––

––

––

––

–

Kalundborg–Samsø

873

873

860

881

826

811

813

823

824

850

828

817

833

831

841

867

København–Rønne

558

545

484

412

427

426

437

465

458

506

491

430

413

397

293

0

Kalundborg–Juelsm

inde

–1326

1733

1542

1541

1508

856

––

––

––

––

–

Køge–Rønne

––

––

––

––

––

––

––

154

488

SjællandsOdde–Arhus

––

––

––

––

–2339

1799

1817

1825

2359

2863

2795

Localferries

176,891

179,850

181,834

178,419

202,445

209,129

182,750

197,489

200,027

202,054

201,833

200,130

208,396

208,501

206,297

205,564

M. Winther / Atmospheric Environment 42 (2008) 4632–46554636

Denmark minus the round trip totals for theregional ferry routes listed in Table 2. No trafficforecast is available for the years later than 2005,and in this case the data for 2005 have been used tosupport the fuel consumption and emission calcula-tions. For regional ferries, an average enginelifetime of 30 years has been assumed in order tocontrol for the de-commissioning of engines in theforecast part of the model calculations.

It can be seen from Table 2 that several of theregional ferry routes were closed in the period from1996 to 1998, mainly due to the opening of theGreat Belt Bridge (connecting Zealand and Funen)in 1997. Hundested–Grenaa and Kalundborg–Juels-minde were closed in 1996, Korsør–Nyborg (DSB)was closed in 1997, and Halsskov–Knudshoved andKorsør–Nyborg (Vognmandsruten) were closed in1998. The ferry route Copenhagen–Rønne wasreplaced by Køge–Rønne in 2004, and from 1999a new ferry connection was opened between Sjæl-lands Odde and Arhus.

For regional ferries, the calculations are fleetactivity based and for each ferry service/ferrycombination, the fuel consumption and emissionsin year X are found as the product of the number ofround trips, sailing time per round trip (h), enginesize (kW), engine load factor and fuel consumption/emission factor (g kWh�1):

EðX Þ ¼X

i

NiTiSi;jPiLFjEFk;l;y (1)

where E is the fuel consumption/emissions, N thenumber of round trips, T the sailing time per roundtrip in h, S the ferry share of ferry service roundtrips, P the engine size in kW, LF the engine loadfactor, EF the fuel consumption/emission factor ing kWh�1, i the ferry service, j the ferry, k the fueltype, l the engine type and y the engine year.

The 1990–2005 aggregated figures of averageengine size, P in kW, and load factor, LF in %,are shown in Table 3 for main engines, auxiliaryengines and as totals, together with aggregatedg kWh�1-based factors of NOx, SO2 and sfc (also inMJkWh�1). The weighted figures are derivedfrom the model calculations and take into accountthe annual hours of operation for each singleferry present in the database. Table 3 also liststhe total annual sailing times in hours in thesame time period. In Section 3, the sfc and emissionfactor basis behind the model is explained in moredetail.

ARTIC

LEIN

PRES

S

Table 3

Weighted average engine size (kW) and load factors (%MCR), total annual sailing times (h), and sfc (g kWh�1 and MJkWh�1), NOx and SO2 emission factors (g kWh�1) for regional

ferries

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Pmain (kW), weighted main

engine size

10,391 9908 9332 9366 9427 10,012 10,618 11,214 12,356 13,552 12,673 12,434 12,109 12,303 12,553 12,058

LF (%MCR), weighted

main engine load factor

70 69 70 70 70 71 74 77 79 77 74 74 75 75 74 78

Paux (kW), weighted aux.

engine size

2069 1851 1791 1757 1803 1834 1815 1543 1469 1566 1558 1536 1569 1518 1516 1388

LF (%MCR), weighted

aux. engine load factor

50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50

Ptotal (kW), weighted total

engine size

12,460 11,759 11,122 11,123 11,230 11,846 12,433 12,757 13,825 15,118 14,231 13,970 13,678 13,821 14,069 13,446

LF (%MCR), weighted

total engine load factor

66 66 67 67 67 68 70 74 76 74 71 72 72 72 71 75

Time (h), total sailing time 91,996 95,108 100,747 98,242 98,324 97,945 105,278 92,850 64,709 46,558 40,659 38,660 38,413 37,577 38,460 38,322

sfc (g kWh�1), weighted 223 223 222 222 222 221 217 217 216 216 213 212 210 211 212 210

sfc (MJkWh�1), weighted 9.3 9.3 9.4 9.4 9.4 9.3 9.2 9.2 9.2 9.2 9.1 9.0 8.9 9.0 9.0 8.9

EF NOx (g kWh�1),

weighted

12.5 11.7 10.9 11.0 11.0 11.1 11.3 10.6 10.4 10.2 10.9 11.1 11.6 11.2 11.0 11.1

EF SO2 (g kWh�1),

weighted

6.3 4.9 3.1 3.9 4.5 4.8 3.0 2.2 1.5 1.5 1.5 1.4 1.3 1.4 1.6 1.6

M.

Win

ther

/A

tmo

sph

ericE

nviro

nm

ent

42

(2

00

8)

46

32

–4

65

54637

ARTICLE IN PRESSM. Winther / Atmospheric Environment 42 (2008) 4632–46554638

2.2. Local ferries, other national sea transport,

fisheries and international sea transport

Fuel consumption figures for local ferries andother national sea transport used in the new Danishmethodology are based on fleet activity estimatesfrom a previous Danish study (local ferries) and newcalculations based on already prepared databasesfrom the same Danish study (other national seatransport).

For the local ferries, a bottom-up estimate of fuelconsumption for 1996 has been taken from theDanish work in Wismann (2001). The latter projectcalculated fuel consumption and emissions for allsea transport in Danish waters in 1995/1996 and1999/2000. In order to cover the entire 1990–2005inventory period, the fuel figure for 1996 has beenadjusted according to the developments in localferry route traffic shown in Table 2.

For the remaining part of the traffic between twoDanish ports, other national sea transport, newbottom-up estimates for fuel consumption havebeen calculated for the years 1995 and 1999 byWismann (2007). The calculations use the databaseset up for Denmark in the Wismann (2001)study, with actual traffic data from the Lloyd’sLMIS database (not including ferries). The databasewas split into three vessel types: bulk carriers,container ships and general cargo ships, and fivesize classes: 0–1000, 1000–3000, 3000–10,000,10,000–20,000 and 420,000 DTW. The calculationsassume that bulk carriers and container shipsuse heavy fuel oil, and that general cargo ships usegas oil.

Wismann (2007) estimates consumption of heavyfuel and gas oil in 1995 to be 0.38 and 0.46 PJ,respectively. In 1999, consumption of heavy fueland gas oil calculated was lower: 0.36 and 0.39 PJ,respectively. The 11% lower fuel use in 1999compared with 1995 corresponds with a decreasein vessel kilometres of 18%, and this supportsthe general fact that traffic between Danish portshas decreased in the last three or four decades(T. Wismann, 2007, pers. comm.). It is moreoverlikely that the opening of the Great Belt Bridgeconnection in 1997 has played a role in the decline inthe consumption of fuel from 1995 to 1999. Due tolack of data for years other than 1995 and 1999, adecision has been made to use the 1995 and 1999figures for 1990–1995 and 1999 onwards, respec-tively, and to find the 1996–1998 fuel consumptionfigures from interpolation.

For fisheries, the new methodology remains fuelbased. However, the input fuel data differ from thefuel sales figures previously used for this category.The changes are the result of further data processingof the reported gas oil sales for national seatransport and fisheries (DEA, 2006a), prior toinventory input. For years when the fleet activityestimates of fuel consumption for national seatransport are smaller than reported fuel sold, fuelis added to fisheries in the inventory. Conversely,lower fuel sales in relation to bottom-up estimatesfor national sea transport means that fuel is beingsubtracted from the original fisheries fuel salesfigure in order to make up the final fuel consump-tion input for fisheries.

For international sea transport, reported fuelsales data are used for Denmark directly, due tolack of fleet activity data to underpin the establish-ment of detailed fuel consumption estimates in atime series. Many countries use navigation fuel salesfigures from national statistics in their inventorycalculations (see Section 1), either directly, due tolack of fleet activity data, or by means of adjust-ments made to other subsectors in the salesstatistics, in order to maintain the national energybalance. In the case of international sea transport,the fuel sales data as such are regarded as veryaccurate for Denmark, and consist of fuel sold toships (regardless of flag) in Danish ports with aforeign destination. The fuel sales data compriseaudited information from the oil suppliers’ monthlyreports, which are used to monitor the legal fuelreserve kept by the oil suppliers in each case (PeterDal, DEA, 2007, pers. comm.).

Based on expert judgement, rough assumptionshave been made as regards engine type for the usageof heavy fuel and gas oil in the different navigationcategories. These data are listed in Table 4. Data forlocal ferries, other national sea transport andinternational sea transport are from Kristensen(2006), and data for fishing vessels are from TheDanish Fishermen’s Association (H. Amdissen,2006, pers. comm.). The same sources also indicateengine lifetimes of around 20 years for the localferries, 30 years for other national sea transport/international sea transport, and 20 and 10 years,respectively, for medium-speed and high-speedengines installed in fishing vessels.

For the remaining navigation categories, the fuel-based emission estimates are obtained as theproduct of total fuel consumption and averagefuel-related emission factors based on engine type

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Table 4

Fuel consumption share (engine type) for heavy fuel and gas oil

Category Heavy fuel oil Gas oil

Slow

speed (%)

Medium

speed (%)

High

speed (%)

Slow

speed (%)

Medium

speed (%)

High

speed (%)

Local ferries – – – 50 50

Other national sea transport 75 25 – 25 75 –

Fisheries – – – – 50 50

International sea transport 75 25 – 25 75 –

M. Winther / Atmospheric Environment 42 (2008) 4632–4655 4639

and average engine lifetime:

EðX Þ ¼X

i

ECi;kEFk;l;y (2)

where E is the fuel consumption/emissions, EC theenergy consumption, EF the fuel consumption/emission factor in gGJ�1 fuel, i the category (localferries, other national sea transport, fisheries,international sea transport), k the fuel type, l theengine type and y the average engine year.

The emission factor inserted in Eq. (2) is found asan average of the emission factors representing theengine ages, which, for a given calculation year, X,are comprised by the average lifetime:

EFk;l;y ¼

Pyear¼X�LTyear¼X EFk;l

LTk;l(3)

3. Legislation, fuel consumption and emission factors

3.1. Fuel and emission legislation

The engines used in navigation have to complywith NOx emission limits agreed by the Interna-tional Marine Organization (IMO) MARPOL 73/78Annex VI. In terms of sulphur, EU directives givestrict fuel quality standards for maritime fuels andprohibit the use of high sulphur fuels in SECAs inthe Baltic Sea and the North Sea. The SECAs arealso agreed by IMO in MARPOL 73/78 Annex VI.

3.1.1. IMO emission limits for NOx

For NOx, the emission legislation is relevant fordiesel engines with a power output 4130 kWinstalled on a ship constructed on or after 1 January2000, and diesel engines with a power output4130 kW, which have undergone major conversionon or after 1 January 2000.

For engine-type approval, the NOx emissions aremeasured using a test cycle (ISO 8178), which

consists of several steady-state modes with differentweighting factors. The NOx emission limits for shipengines relate to their rated engine speed (n) given inrevolutions per minute (RPM). The limits are asfollows:

�

17 g kWh�1, no130RPM � 45� n�0.2 g kWh�1, 130pno2000RPM � 9.8 g kWh�1, nX2000RPM.

3.1.2. Sulphur content in marine fuels

Table 1 gives an overview of the EU directivesand IMO MARPOL Annex VI limit values forcontent of sulphur in marine fuels.

According to Directive 93/12, from 1 October1994, it is not legal in the EU to sell marine gas oilwith a sulphur content exceeding 0.2%. Marine gasoil with higher sulphur content can, however, bebrought into the EU and be used for navigationalpurposes. From 1 January 2000, Directive 1999/32prohibits any usage of marine gas oil in the EU witha sulphur content exceeding 0.2%, and from 1January 2008 this sulphur limit is strengthened to0.1% in Directive 2005/33.

From 19 May 2006, IMO MARPOL Annex VIprohibits the use of heavy fuel oil with sulphur levelsexceeding 1.5% in the Baltic Sea, and the date thatEU Directive 2005/33 enters into force is 11 August2006. The latter directive sets the same sulphur limitfor the North Sea SECA by 11 August 2007(MARPOL Annex VI date is 21 November 2007).

3.2. Fuel consumption and emission factors

Generally, fuel consumption and emission factorsare classified according to engine type and fuel type.In the case of regional ferries, fuel consumption andemission factors in g kWh�1 are used, since for these

ARTICLE IN PRESSM. Winther / Atmospheric Environment 42 (2008) 4632–46554640

vessels detailed traffic, ferry engine and operationaldata are present to support a detailed inventoryapproach.

For the remaining navigation sectors, the onlyactivity data present are figures for total fuelconsumption. To provide emission data for theseparts of the emission inventory, fuel-related emis-sion factors are derived for each engine/fuel type,from the ratio between the g kWh�1-based emissionfactors and sfcs.

3.2.1. Specific fuel consumption

The standard curves for sfc (g kWh�1) are shownin Fig. 1 for slow-, medium- and high-speed engines,as a function of engine production year. For gasturbines, a mean fuel consumption figure of240 g kWh�1 is used. All fuel consumption datacome from the Danish TEMA2000 emission model(Ministry of Transport, 2000).

The fuel consumption trend graph was producedin the late 1990s for the Danish TEMA 2000 model,and because the regression curve is supported byactual fuel consumption factors for engines pro-duced up until the mid 1990s, the graph is regardedas being most accurate in relation to engines builtduring that period. For newer engines, the fuelconsumption trend is established based on expertjudgement. The graph is, however, still regarded asvalid in relation to its use in estimating emissionestimates for engines in situations that prevail today(Kristensen, 2006).

For regional ferries, sfc figures have beenobtained from the Danish Ferry Historical Society

0

50

100

150

200

250

300

1949

1953

1957

1961

1965

1996

919

7319

Engine pr

sfc

(g/k

Wh)

Medium spe

High speed (

Fig. 1. Specific fuel consumption for ship engin

(DFS, 2006). These sfc values are in line with the sfcvalues shown in Fig. 1.

3.2.2. NOx emission factors

The NOx emission factors (g kWh�1) for slow-and medium-speed engines come from Kjemtrup(2006). The data are shown in Fig. 2, together withNOx emission factors, for high-speed engines. Forgas turbines, a mean NOx emission factor of4 g kWh�1 is used. The emission information forhigh-speed engines and gas turbines comes from theDanish TEMA2000 emission model (Ministry ofTransport, 2000).

The increase in fuel efficiency up to 2000 causedthe NOx emission factors to increase. However, atthe beginning of the 1990s (slow-speed engines) andby the end of the 1990s (medium-speed engines),NOx emission performance is improved, mainly dueto improved engine design. The emission improve-ments are of a sufficient size to enable the IMO NOx

emission requirements in 2000 to be met.The NOx emission factors in g kg�1 fuel shown in

Fig. 3 are derived as the ratio between sfc and theNOx emission factors from Figs. 1 and 2, respec-tively. In order to obtain emission factors in gGJ�1,the emission factors from Fig. 3 are multiplied bythe lower heating values (LHVs in gMJ�1; 40.9 forheavy fuel oil and 42.7 for gas oil).

3.2.3. SO2 emission factors

Fig. 4 shows the sulphur percentage figures forheavy fuel and gas oil used in the Danish inventory.The marine gas oil sulphur percentage is the same asthe sulphur content limit given in EU Directive

7719

8119

8519

8919

9319

9720

0120

05

oduction year

ed (4-stroke) Slow speed (2-stroke)

4-stroke)

es per engine production year (g kWh�1).

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0

5

10

15

20

25

1949

1953

1957

1961

1965

1969

1973

1977

1981

1985

1989

1993

1997

2001

2005

Engine production year

NO

x (g/

kWh)

Medium speed (4-stroke) Slow speed (2-stroke)

High speed (4-stroke)

Fig. 2. NOx emission factors for ship engines per engine production year (g kWh�1).

0

20

40

60

80

100

120

1949

1953

1957

1961

1965

1969

1973

1977

1981

1985

1989

1993

1997

2001

2005

Engine production year

NO

x (g/

kg fu

el)

Medium speed (4-stroke) Slow speed (2-stroke)

High speed (4-stroke)

Fig. 3. NOx emission factors for ship engines per engine production year (g kg�1 fuel).

0

0.5

1

1.5

2

2.5

3

3.5

4

1990

1992

1994

1996

1998

2000

2002

2004

2006

2008

2010

Inventory year

S-%

Heavy fuel: Nat. navigation Diesel

Heavy fuel: Int. navigation

Fig. 4. S % for heavy fuel and marine gas oil per inventory year.

M. Winther / Atmospheric Environment 42 (2008) 4632–4655 4641

ARTICLE IN PRESS

Table 5

Fuel consumption, and NOx and SO2 emission results for Danish navigation in 2005

Fuel type Category Subcategory FC (PJ) SO2 (tonnes) NOx (tonnes)

Gas oil National sea transport Regional ferries 3.13 294 3871

Gas oil National sea transport Local ferries 0.61 57 796

Gas oil National sea transport Other national sea 0.39 37 585

Gas oil Total national sea transport 4.14 387 5252

Gas oil Fisheries 6.56 615 8837

Gas oil International sea transport 13.92 1304 20,881

Gas oil Grand total 24.61 2306 34,969

Heavy fuel National sea transport Regional ferries 0.32 312 436

Heavy fuel National sea transport Other national sea 0.36 352 733

Heavy fuel National sea transport Total national sea 0.68 664 1169

Heavy fuel International sea transport 20.59 35,241 41,944

Heavy fuel Grand total 21.27 35,905 43,113

Overall total 45.91 38,211 78,108

M. Winther / Atmospheric Environment 42 (2008) 4632–46554642

93/12 (starting: 1 October 1994), 1999/32 (starting: 1January 2000) and 2005/33 (starting: 1 January2008). Prior to 1994, the sulphur level is assumed,based on information from Statoil (C. Thomsen,2005, pers. comm.). From 2008 onwards, the gas oilsulphur content is from EU Directive 2005/33.

For heavy fuel oil, the sulphur percentages havebeen calculated based on fuel end-use informationfrom the DEA (2006b). A weighted average iscalculated from two different heavy fuel qualities(1% and 3% for national sea transport and 1% and3.5% for international sea transport). From 2006/2007,1 when the SECAs enter into force, a fuelsulphur content of 1.5% is used for the less cleanheavy fuel oil quality in the weighted sulphurpercentage. The estimated 2007 fuel sulphur contentfor heavy fuel oil is used also for the years 2008onwards.

In order to obtain emission factors in gGJ�1, thesulphur percentages from Fig. 4 are inserted in thefollowing expression together with the LHV:

EFðSO2Þ ¼2� 104S%

LHV(4)

where EF is the emission factor in gGJ�1,S% ¼ sulphur percentage and LHV ¼ 40.9MJ g�1

fuel.Eq. (4) uses 2.0 kg SO2 kg S�1, the chemical

relation between burned sulphur and generated SO2

provided in EMEP/CORINAIR (2006).

12007 is used as the effective year in the inventory.

4. Results

The fuel consumption and NOx and SO2 emissionresults for national sea transport, fisheries andinternational sea transport are shown in Table 5for 2005. International sea transport dominatestotal fuel consumption as well as total NOx and SO2

emissions, with shares of 75%, 80% and 96%,respectively, in 2005. This is mainly due to a highuse of heavy fuel oil, characterised by a high contentof sulphur, which is predominantly used by slow-speed engines with relatively higher NOx emissionfactors. The total fuel consumption and NOx

emission shares for national sea transport are 10%and 8%, respectively, and the corresponding sharesfor fisheries are 14% and 11%. For SO2, theemission share for fisheries is only 1.6%, since noheavy fuel oil is being used by the fishing vessels.

4.1. National sea transport

The 1990–2005 fuel consumption, NOx and SO2

emission results for regional ferries are shown inFigs. 5–7.

Fuel consumption for the regional ferries de-creases by 53% from 1990 to 2005 and, in generalterms, the development in the ferry fuel consump-tion corresponds to trends in ferry traffic. The mainreason for the fuel consumption reduction is theopening of the Great Belt Bridge connection in1997, which resulted in a reduction in ferry traffic(as explained in Section 2.2).

Total NOx emissions reduce by 56% from 1990 to2005. A sudden emission increase is seen for the

ARTICLE IN PRESS

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

Fuel

con

sum

ptio

n (P

J)

Halsskov-Knudshoved

Korsør-Nyborg, DSB

Kalundborg-Århus

Sjællands Odde-Ebeltoft

Hundested-Grenaa

København-RønneKorsør-Nyborg,VognmandsrutenKøge-RønneSjællands Odde-Århus

Fig. 5. Fuel consumption for Danish ferries, 1990–2005.

0

500

1000

1500

2000

2500

3000

3500

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

NO

x (to

ns)

Halsskov-KnudshovedKorsør-Nyborg, DSBKalundborg-ÅrhusSjællands Odde-Ebeltoft

København-Rønne

Hundested-GrenaaKorsør-Nyborg,VognmandsrutenKøge-RønneSjællands Odde-Århus

Fig. 6. NOx emissions for Danish ferries, 1990–2005.

0

200

400

600

800

1000

1200

1400

1600

1800

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

SO

2 (to

ns)

Halsskov-KnudshovedKorsør-Nyborg, DSB

Kalundborg-Århus

Sjællands Odde-Ebeltoft

Hundested-Grenaa

København-RønneKorsør-Nyborg,VognmandsrutenKøge-RønneSjællands Odde-Århus

Fig. 7. SO2 emissions for Danish ferries, 1990–2005.

M. Winther / Atmospheric Environment 42 (2008) 4632–4655 4643

Kalundborg–Arhus ferry route in 2000, whereferries using slow-speed engines enter into service.Conversely, a significant NOx emission reduction isregistered for the ferry route Sjællands Odde–Ebeltoft

in 2000. From this year, only new ferries equippedwith gas turbine engines have been in use.

For SO2, a significant emission reduction in theorder of 87% is calculated for 1990–2005. For theferry route Kalundborg–Arhus there is a substantialreduction in heavy fuel oil use from 1992, and from1996 only gas oil is used. A shift away from the useof heavy fuel oil also explains the SO2 emissionreductions in 1996 and 1997 for the SjællandsOdde–Ebeltoft ferry route. From 1998 onwards,the ferry route between Zealand and Bornholm(København–Rønne, replaced by Køge–Rønne in2004) is the largest source of SO2 emissions, due tothe use of heavy fuel oil as the sole fuel type.

The 1990–2005 fuel consumption and NOx andSO2 emission results for the three national seatransport subcategories are shown in Fig. 8.

The fuel consumption trend for local ferriesfollows the trend in traffic levels, and an increaseof 16% is calculated for 1990–2005. SO2 emissionsfor local ferries are insignificant; however, theemission increase is the same as for fuel consump-tion, due to constant SO2 emission factors for gasoil throughout the period. The NOx emissionincrease is higher (49%), due to the developmentin average emission factors based on assumedengine lifetime.

ARTICLE IN PRESS

0

1

2

3

4

5

6

7

8

9

10

1990

Fuel

con

sum

ptio

n (P

J)

Ferries Ferryboats Other national sea

0

2000

4000

6000

8000

10000

12000

NO

x (to

ns)

Ferries Ferryboats Other national sea

0

1000

2000

3000

4000

5000

6000

SO

2 (to

ns)

Ferries Ferryboats Other national sea

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Fig. 8. 1990–2005 time series of fuel consumption and NOx and SO2 emissions for national sea transport.

M. Winther / Atmospheric Environment 42 (2008) 4632–46554644

For other national sea transport, the fuel consump-tion and emission changes calculatedare less certain than the ferry estimates, due to the

assumptions made for years other than 1995and 1999. Results show a fuel consumption decreaseof 11% from 1990 to 2005, because of a lower

ARTICLE IN PRESSM. Winther / Atmospheric Environment 42 (2008) 4632–4655 4645

level of traffic in 1995 compared with 1999. Con-versely, the NOx emissions rise by 10% inthe 1990–2005 period, the explanation for thisemission increase being the same as that forthe local ferries. The SO2 emission decreasesby 27% due to a gradually diminishing share ofheavy fuel oil with a high content of sulphur beingsold.

0

10

20

30

40

50

60

70

1990

Fuel

con

sum

ptio

n (P

J)

National sea Fisheries Inte

0

20000

40000

60000

80000

100000

120000

NO

x (to

ns)

National sea Fisheries

0

10000

20000

30000

40000

50000

60000

70000

SO

2 (to

ns)

National sea Fisheries

1991 1992 1993 1994 1995 1996 19

1990 1991 1992 1993 1994 1995 1996 19

1990 1991 1992 1993 1994 1995 1996 19

Fig. 9. 1990–2005 time series of fuel consumptio

4.2. All navigation categories

The 1990–2005 fuel consumption and NOx andSO2 emission results for national sea transport,fisheries and international sea transport are shownin Fig. 9.

For national sea transport, fuel consumption andthe emissions of NOx and SO2 decrease by 45%,

rnational sea

International sea

International sea

97 1998 1999 2000 2001 2002 2003 2004 2005

97 1998 1999 2000 2001 2002 2003 2004 2005

97 1998 1999 2000 2001 2002 2003 2004 2005

n and emissions (all navigation categories).

ARTICLE IN PRESSM. Winther / Atmospheric Environment 42 (2008) 4632–46554646

45% and 81%, respectively, from 1990 to 2005.Over the same period, the fisheries reductions are18% for fuel consumption and SO2, whereas NOx

emissions increase by 6%. For international seatransport, fuel consumption and SO2 emission bothdecline by 14%, whereas NOx emissions rise by 1%.For national sea transport, the SO2 emissionsdecrease more than total fuel consumption, due toa decrease in the share of high sulphur fuel beingsold.

International sea transport is the most dominantsource of fuel consumption and emissions in theperiod from 1990 to 2005. The highest fuelconsumption and NOx emission shares are calcu-lated for 1994–2001. In this period, the internationalfuel consumption share is around 80%, and theNOx emission shares lie between 83% and 86%.From 1993 onwards the SO2 emission share forinternational sea transport is 490%. A maximumemission share of 97% is calculated for the years1999, 2000 and 2001.

Gas oil and HFO consumption fo

0123456789

1990

Fuel

con

sum

ptio

n (P

J)

Total fuel consumption for national sea

0

2

4

6

8

10

12

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

Fuel

con

sum

ptio

n (P

J)

Estimated

Sales

1991 1991 1992 1993 1994 1995 1996 1

Fig. 10. Calculated fuel consumption vs. sta

4.3. Discrepancy between fuel sales and calculated

fuel consumption for national sea transport

Fig. 10 shows the fuel consumption calculatedand the statistical fuel sales for 1990–2005, per fueltype and as totals for national sea transport.

For gas oil, the surplus of calculated fuelconsumption is very large for the years until 1992,and from 1997 to 1999; only for 2005 the calculatedgas oil fuel total becomes lower than statistical sales.For heavy fuel oil, only the years 1994–1997 show asurplus of calculated fuel, and from 1998 onwardsthe calculated fuel consumption is significantly lowerthan statistical sales. In terms of totals, calculatedfuel consumption lies well above fuel sales up until1999, and the greatest difference is noted for 1998(42%). Small variations between calculated con-sumption and sales are found for 2000, 2001 and2004; the differences are 4%, 2% and �6%,respectively. In 2002, 2003 and 2005 calculated fuelconsumption is almost 20% below the sales total.

r national sea: estimated and sales

Gas oil (estimated) Gas oil (sales)

HFO (estimated) HFO (sales)

: estimated and sales

2000

2001

2002

2003

2004

2005

997 1998 1999 2000 2001 2002 2003 2004 2005

tistical sales for national sea transport.

ARTICLE IN PRESSM. Winther / Atmospheric Environment 42 (2008) 4632–4655 4647

There are various potential reasons for thediscrepancies between the fuel consumption andfuel sales figures. From the fuel suppliers’ side,errors such as sector misallocations or incorrectfuel-type descriptions may disturb the fuel balance,and general calculation uncertainties may bring acertain degree of inaccuracy to the fuel consumptionfigures arrived at. However, since the new bottom-up fuel consumption estimates for national seatransport are fleet activity based, these new esti-mates are regarded as more accurate than the fuelsales reported by the DEA.

According to the DEA, the most likely reason forthe discrepancies between estimated consumptionand sales figures is inaccurate specification of thecustomer made by the oil suppliers. This inaccuracycan be caused by a sector misallocation in the salesstatistics between national sea transport and fish-eries for gas oil, and between national sea transportand industry for heavy fuel oil (Peter Dal, DEA,2007, pers. comm.).

As a consequence, in the new inventory a decisionwas made to change the time series for gas oilconsumption for fisheries, which was previouslybased on the direct fuel sales reported in the Danishstatistics for this sector. Compared with the earlierinventory, fuel consumption for fisheries reducedfrom 1990 to 2004 and increased in 2005, accordingto the differences between estimated gas oil con-sumption and reported sales for national seatransport; cf. Fig. 10.

4.4. Differences between the new and the previous

inventory for Danish navigation

Fig. 11 displays the differences between the newand the previous inventory results for fuel con-sumption, NOx and SO2, shown as ratios for theyears 1990–2005. The previous inventory was basedon direct fuel sales reported in DEA statistics, andconstant NOx emission factors from the EMEP/CORINAIR guidebook (EMEP/CORINAIR,2006), whose factors are taken directly from Lloyd’sRegister of Shipping (1995). The SO2 emissionfactors relied on older end-use sales data from theDEA for different heavy fuel oil qualities, whichhave been updated in the present project, resultingin minor emission factor changes.

For national sea transport, fuel consumptionchanges significantly in the new inventory comparedwith the fuel consumption basis in the previousinventory, as explained in Section 4.3. In terms of

NOx, the inventory differences are due to updatedfuel consumption figures and NOx emission factorsin the new model, the emission impact from theformer being the greatest. The development of thenew NOx emission factors from MAN DIESEL isexplained in Section 3.2.2. The largest and smallestNOx emission changes occurred in 1990 (28%) and1993 (�1%).

For SO2, the emission trend is very sensitive tochanges in heavy fuel consumption. This means thatin 1994–1997, the new SO2 estimates are lower thanthe previous estimates, due to smaller heavy fuelconsumption in the new model compared with theprevious figures based on sales; cf. Fig. 10. Theopposite is the case for 1990–1993, and from 1998onwards. The largest and smallest emission changesfor SO2 are noted for 1995 (62%) and 1993 (�8%).

For fisheries, the consumption of gas oil is lowerin the new inventory compared with the inventoryfigures used previously, for all years except 2005.The fuel consumption differences are due to theadjustments made between fisheries and nationalsea transport in the new inventory for gas oil, alsoexplained in Section 4.2. SO2 emissions change inthe same way as fuel consumption, due to constantemission factors. The largest and smallest changesbetween the new and the earlier results are noted for1998 (�33%) and 2004 (4%). For NOx, the largestand smallest changes, respectively, appear in 1992(�41%) and 2005 (5%).

For international sea transport, fuel consumptionremains unchanged in the new inventory; hence theemission reductions for NOx and SO2 are driven byemission factor changes. For NOx, the emissionreductions are quite significant in the 1990–2005time period; the largest and smallest reductionsobtained with the new inventory are noted for 1992(27%) and 2005 (13%). For SO2, the updates of thesulphur content for heavy fuel oil made in newinventory explain the emission changes (most visiblefor 1990, 1991 and 2003).

4.5. Input parameters from the new Danish inventory

compared with other studies and the previous Danish

inventory

The NOx emission factors from the new Danishinventory correspond well with the factors fromWhall et al. (2002) and Endresen et al. (2003) forslow-speed engines (Table 6). For the remainingengine types, the factors from the new Danishinventory are around 10–20% lower. For sfc fairly

ARTICLE IN PRESS

Fuel consumption: Ratio between new and previous inventory

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1990

New

: Pre

viou

s

National sea transport Fisheries International sea transport

NOx emissions: Ratio between new and previous inventory

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

New

: Pre

viou

s

National sea transport Fisheries International sea transport

SO2 emissions: Ratio between new and previous inventory

0.000.200.400.600.801.001.201.401.601.80

New

: Pre

viou

s

National sea transport Fisheries International sea transport

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Fig. 11. Ratios between new and old inventory results for fuel consumption, NOx and SO2.

M. Winther / Atmospheric Environment 42 (2008) 4632–46554648

good agreement is evident between the new Danishfactors and the factors from Whall et al. (2002) andEndresen et al. (2003), for slow- and medium-speedengines. The largest sfc differences are noted forhigh-speed engines (Endresen et al., 2003) and gasturbines (Whall et al., 2002), where sfc factors arearound 20% higher than the figures used in the newDanish inventory.

It is normal to expect differences between inputdata for different inventories. However, the aggre-gated sfc and NOx emission factors derived from thenew Danish inventory are based on precise informa-tion of engine production year for regional ferries,and engine type and lifetime assumptions for theremaining navigation categories. In this way, thefinal fuel consumption figures and emission factors

ARTICLE IN PRESS

Table 8

Sulphur percentage of heavy fuel oil and marine gas oil used in

different studies

Study Year Heavy fuel Gas oil

Endresen et al. (2005) (Europe) 2002 2.41 0.54

Endresen et al. (2003)a 2000 2.7 0.5

Whall et al. (2002) 2000 2.7 0.5

Danish inventory (old) 2000 3.5 0.2

Danish inventory (new) 2000 3.4 0.2

aValues from EMEP/CORINAIR (1999), repeated in the 2006

version of the guidebook.

Table 6

NOx (g kg�1) and sfc (g kWh�1) per engine type from different studies

Study Year Unit Slow speed Medium speed High speed Gas turbines

Whall et al. (2002) 2000 NOx g kg�1 92a/93b 65a/66b 59a/60b 20

Endresen et al. (2003) 2000 NOx g kg�1 87 57 57 –

Danish inventory (prev.) 2000 NOx g kg�1 87 57 57 16c

Danish inventory (new) 2000 NOx g kg�1 89 51 53 17

Whall et al. (2002) 2000 sfc g kWh�1 185a/195b 203a/213b 203a/213b 290a/305b

Endresen et al. 2003 2000 sfc g kWh�1 195 215 230 –

Danish inventory (new) 2000 sfc g kWh�1 200 219 211 240

aHeavy fuel oil.bMarine gas oil.cGas turbine engines are not considered in the previous NERI method.

Table 7

Aggregated factors for NOx (g kg�1), sfc (g kWh�1) and fuel

sulphur content used in different studies

Study Year NOx

(g kg�1)

sfc

(g kWh�1)

S %

Eyring et al. (2005) 2001 76.4 212a 2.1

Corbett and Kohler

(2003)

2001 78.2 210 2.2

Endresen et al.

(2003)

2000 75.9 201b 2.2

Danish inventory

(old)

2000 81 – 1.8

Danish inventory

(new)

2000 69 210 1.8

aRough estimate from Endresen (pers. comm., 2007).bAggregated figure (based on an energy demand of 72% for

transport vessels (sfc ¼ 206 g kWh�1) and 28% for non transport

vessels (sfc ¼ 221 g kWh�1).

M. Winther / Atmospheric Environment 42 (2008) 4632–4655 4649

take into account both engine type and age for allnavigation categories in the Danish inventory.Consequently, the present approach is consideredto be more accurate than the use of constant factorsby other inventories.

However, it will lead too far to conclude that theDanish inventory results are more accurate than theresults obtained by other fleet activity-based in-ventories. This is not least considering the fact thatonly for national sea transport, the estimates arefleet activity based, whereas for fisheries andinternational sea transport the Danish inventoryremains purely fuel based. Fleet activity-basedinventories like Corbett and Kohler (2003), End-resen et al. (2003) and Eyring et al. (2005) have allshown to be more accurate than fuel-based ones.A general note is, however, that if not alreadyincluded, fleet activity inventories will improve theirprecision, if an engine production year-specificresolution of the fleet, together with correspondingsfc and emission factors, makes up the calculationbasis.

The total sfc factor from the new Danish model isvery similar to the sfc factors derived from the otherstudies, whereas for NOx the Danish total emissionfactor is around 10% lower (Table 7). The latterdifference is due to the inventory variations regard-ing specific emission factors and fuel consumptionshares per engine type. The main reason for theNOx emission factor difference between the new andthe earlier Danish inventory is due to a different fuelconsumption weighting for slow-speed and medium-speed engines, and the fact that fuel combustion ingas turbines is not taken into consideration in theearlier Danish inventory.

The sulphur content for all fuel is between 10%and 20% lower in the Danish inventories compared

with the figures derived from the other studies. Thereasons for these discrepancies are the specificfuel-type mix for Danish navigation and differentsulphur content per fuel type, as shown in Table 8.For heavy fuel oil, the sulphur content from the

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0

10000

20000

30000

40000

50000

60000

2006

2007

2008

2009

2010

2011

2012

2013

2014

2015

2016

2017

2018

2019

2020

2006

2007

2008

2009

2010

2011

2012

2013

2014

2015

2016

2017

2018

2019

2020

NO

x (to

ns)

Other

Total national sea

Working machinery

Road

0200400600800

100012001400160018002000

SO

2 (to

ns)

Total national sea

Other

Fig. 12. Emission projections of NOx and SO2 for Danish mobile

sources 2006–2020.

M. Winther / Atmospheric Environment 42 (2008) 4632–46554650

Danish inventory is based on end-use fuel sales datafor two different fuel qualities, and sulphur contentsare assumed in each case. The weighted sulphurcontent is around 40% higher than the figure forEuropean fuel sales based on specific fuel samples,found by Endresen et al. (2005). Conversely, thelatter source finds a sulphur content for marine gasoil that is around 170% higher than the sulphurcontent used in the Danish inventory. To determinehow well the Danish data fit with the real nationalpicture in terms of fuel sulphur content, it isdesirable to obtain data from Danish bunkersamples, in a similar way as carried out by Endresenet al. (2005).

As regards engine loads, the weighted annualaverages for %MCR used for regional ferries in thenew Danish inventory depends on the actual ferriesbeing used. In 2000, the weighted load factor is 0.71;see Table 3. For main engines in the same year, theaggregated load factor is 0.74, and this latter factortakes into account each trip’s port manoeuvring andat-sea operations. However, the latter mode ofoperation accounts for by far most of the total fuelconsumption for the individual ferry. Withouttrying to quantify explicitly the load factors forport manoeuvring and in open sea, one mayconclude that the at-sea factor in 2000 is slightlyhigher than 0.74.

The Danish figure for main engine load factorsaligns reasonably well with the average engine loadsassumed in other studies. Whall et al. (2002) use80% for main engines at sea and 20% for in-portand manoeuvring conditions. EMEP/CORINAIRsuggests 80% at sea, 20% for in-port and 40% formanoeuvring conditions. Endresen et al. 2003 use aweighted factor of 70%, and Eyring et al. (2005) usebetween 65% and 75% for non-cargo vessels.

4.6. Forecast results for navigation and other mobile

sources

Fig. 12 shows total Danish NOx and SO2

emissions from mobile sources in the forecast period2006–2020. The fuel consumption forecast from theDEA (not shown) remains at a constant level fornational sea transport and fisheries (total nationalsea), and the 1% emission reduction for NOx from2006 to 2020 is caused solely by small changes in theNOx emission factors. For other mobile sources,such as road transport and non-road workingmachinery (agriculture and industry), the expectedNOx emissions become significantly smaller. The

emission reductions (59% for non-road and 67%for road) are due to the gradually strengtheningemission standards for the engines used.

Already in 2011 the non-road working machinerygroup becomes a less significant source of NOx

emissions than total national sea transport, and in2020 the NOx emission from road transport isalmost the same as that for total national seatransport. A strengthening of the NOx emissionstandards in 2010 and 2015 is currently underdiscussion in the IMO, and the present forecastresult shows that more strict emission standards willhave to be decided for future engines if emissionimprovements are to be achieved in line with thosecalculated for other mobile sources.

For SO2, total emissions from Danish mobilesources are largely dominated by the emissioncontributions from total national sea transport.Most of these emissions are due to a certain use ofheavy fuel oil by vessels in the category ‘othernational sea transport’. The trend in SO2 emission

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factors is explained in more detail in Section 3.2.3.The calculated emissions from total national seareduce substantially by the time that the definedSECAs enter into force, and the maximum sulphurpercentage of heavy fuel oil is set to 1.5%. Still, SO2

emissions are at a level that indicates that naviga-tion is the sector to address if the emissions frommobile sources are to be reduced even further infuture.

5. Summary and discussion

This article explains the new emission inventoryfor navigation in Denmark, covering national seatransport, fisheries and international sea transportin the period from 1990 to 2005. The major fuelconsumption as well as NOx and SO2 emissionresults are shown, and explanations are given for thedifferences between statistical fuel sales and the fuelconsumption data calculated by the inventory. Thedifference in results between the new and theprevious Danish inventory are also explained, andthe main inventory calculation parameters for thenew inventory are compared with those used inother studies. Finally, a NOx and SO2 emissionprojection is presented for the 2006–2020 period,including domestic navigation as well as otherDanish mobile sources.

For national sea transport, the new Danishinventory distinguishes between regional ferries,local ferries and other national sea transport.Detailed traffic and technical data lie behind thefuel consumption and emission calculations forregional ferries. For local ferries and other nationalsea transport, the new inventory is partly bottom-upbased; fuel consumption estimates are calculated forsingle years using a database set up for Denmark inan earlier Danish project, and fuel consumption forother years in the inventory period is established byusing appropriate assumptions. For fisheries andinternational sea transport, the new inventoryremains fuel based. For international sea transport,fuel sales data are used directly, whereas for fisheriesthe fuel consumption figures are sales data adjustedby transferral with the calculated fuel differencesbetween estimates and official sales data fornational sea transport.

Standard curves for sfc and NOx emission factorsin g kWh�1 are used for regional ferries, as afunction of engine production year. Fuel-relatedNOx emission factors (gGJ�1) per inventory yearare derived for the remaining categories: local

ferries, other national sea transport, fisheries andinternational sea transport. The SO2 emissionfactors rely on (1) the sulphur limits determined inEU legislation for gas oil and for heavy fuel oil inSECAs and (2) on fuel end-use information fromthe Danish energy statistics for two different heavyfuel qualities prior to the SECA starting dates.

For regional ferries the fuel consumption andemissions are calculated for each ferry service/ferrycombination as the product of number of roundtrips, sailing time per round trip (h), engine size(kW), engine load factor and fuel consumption/emission factor (g kWh�1). For local ferries, othernational sea transport, fisheries and internationalsea transport, the emissions are calculated as theproduct of total fuel consumption and average fuel-related emission factors based on engine type andaverage engine lifetime.

International sea transport is the most importantfuel consumption and emission source for naviga-tion, and the contributions are large even comparedwith the overall Danish totals, especially for NOx

and SO2. For 2005 the fuel consumption (and CO2)percentage addition is 5%, and the correspondingNOx and SO2 percentage additions amount to 34%and 167%, respectively, when international seatransport is seen in relation to the Danish nationaltotals. For international sea transport, the decline infuel consumption and SO2 emission calculated is14% from 1990 to 2005, whereas the NOx emissionsrise by 1%.

The 1990–2005 results for national sea transportshow a fuel consumption and NOx emissiondecrease of 45%, and for SO2 the emission declineis 81%. For national sea transport, the largestcontributions come from ferries, and for thesevessels, the calculated fuel consumption, NOx andSO2 emission decreases are 53%, 56% and 87%,respectively. The reduction calculated for fisheries is18% both for fuel consumption and for SO2,whereas with regard to the NOx emission, anincrease of 6% is calculated.

Forecast results indicate that already in 2011, thenon-road working machinery group becomes asmaller source than national sea transport in termsof NOx emissions, and in 2020 the NOx emissionfrom road transport is almost the same as thatfor total national sea transport. For SO2, most ofthe emissions from Danish mobile sources stemfrom a certain use of heavy fuel oil by vessels inthe category ‘other national sea transport’. Eventhough the introduction of SECAs reduces the SO2

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emissions substantially, they are still at a level thatindicates that navigation is the sector to address ifthe SO2 emissions from mobile sources are to bereduced even further in future.

The 2006–2020 emission forecasts show a needfor more strict fuel quality and NOx emissionstandards for navigation, in order to gain emissionimprovements in line with those achieved for othermobile sources.

For national sea transport in Denmark, the fuelconsumption estimates obtained with the newmodel are regarded as much more accurate thanthe DEA fuel sales data used in the previous modelversion. The large fluctuations in reported fuel salescannot be explained by the actual development inthe traffic between different national ports. Also,countries like Italy, Belgium and Finland rely onestimated fuel figures instead of sales figures (seeSection 1). The general uncertainties associated withfuel sales statistics are also highlighted by Peters andOlivier (1999), who summarise fuel consumptionand CO2 emissions globally.

There are different potential reasons for thedifferences between estimated fuel consumptionand reported sales for national sea transport inDenmark. According to the DEA, the latter fueldifferences are most likely explained by inaccuratecostumer specifications made by the oil suppliers.This inaccuracy can be caused by a sector mis-allocation in the sales statistics between national seatransport and fisheries for gas oil, and betweennational sea transport and industry for heavy fueloil (Peter Dal, DEA, 2007, pers. comm.). It wouldtherefore be desirable for the Organization of OilSuppliers in Denmark (www.ofr.dk) to review theirprocedures of sales registrations, and make anynecessary changes.

As a side remark, fuel investigations for fisheriesmade prior to the initiation of the present projecthave actually pointed out a certain area ofinaccuracy in the DEA statistics. No enginesinstalled in fishing vessels use heavy fuel oil, eventhough a certain amount of heavy fuel oil is listed inthe DEA numbers for some statistical years (H.Amdissen, Danish Fishermen’s Association, 2006,pers. comm.).

Moreover, two situations may occur that wouldlead to discrepancies in DEA fuel sales figuresversus a perfect fuel base for bottom-up inventorycalculations. Firstly, the fuelling pattern would beaffected in a situation where a vessel makes ajourney where the first part is a domestic trip,

without fuelling from the start, and where thesecond part is international. Another situation mayoccur if a vessel is fully loaded with fuel prior to atrip, for which the first leg is domestic and thefollowing leg is international.

In terms of fuel consumption calculation, someuncertainty also exists in the model calculationparameters. Bearing in mind the model assumptionsmade, the fuel consumption trend calculated fornational sea transport reflects the traffic pattern,and for 1999 the fuel consumption uncertainty fornational sea transport as a whole is estimated to be11% in 1999 (see Winther, 2008), based on themethod proposed by the IPCC (2000). This marginof uncertainty only leaves room for a minordisplacement of the curve for estimated fuelconsumption, and cannot explain the large fluctua-tions in reported fuel sold.

It is recommended to replace the current DEAtime series of fuel sales for national sea transport bythe new bottom-up fuel consumption estimatescalculated in this project. If this decision was made,a collaboration between NERI and the DEA couldbe established like the one that already existsbetween NERI and the DEA for aviation. Here,NERI calculates the jet fuel split for domesticand international flights (Winther, 2001). Anupdated time series for fuel consumption fornational sea transport will introduce changes tothe energy statistics for fisheries and industry,since the revealed differences between the salesfigures and bottom-up estimates for national seatransport are balanced out by adjusting the salesfigures for fisheries (for gas oil) and industry (heavyfuel oil).

For international transport, fuel sales data assuch are regarded as highly accurate for Denmark,since they are compiled from audited informationfrom the Danish oil suppliers. The fuel sales dataare input directly into the inventory calculations,and this methodology approach follows goodpractice for the UNFCCC and UNECE conven-tions, when fleet activity data are missing.

However, instead of fuel-based estimates it wouldbe very useful to implement a new project in whichthe fuel consumption and emissions for interna-tional sea transport in Denmark are calculatedbased on actual vessel movements, as has alreadybeen carried out for domestic ferries. Followinggood practice outlined in the UNFCCC andUNECE conventions, the movements should in-clude only the first international leg for vessels

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starting from Danish ports, i.e. trips made by vesselsstarting from Danish ports with foreign ports astheir first destination. Such project results wouldstrongly support the work made by Danish policy-makers dealing with the issue of bunker emissionsallocation; fuel sales for Danish international seatransport only give what has been filled into thevessel fuel tanks, and not what has actually beenused on the above-mentioned trips.

Several fleet activity-based inventories on a globalscale, despite the significant degree of disparitybetween their research results, point out uncertain-ties in international fuel statistics. Analyses carriedout by Corbett and Kohler (2003) and Eyring et al.(2005) calculate much more fuel for internationaltransport than reported by the International EnergyAgency (IEA) and the Energy Information Admin-istration (EIA). The authors question the accuracyof the sales reported and indicate problems withregard to the fuel definitions applied in the statistics.Endresen et al. (2003) calculate a total fuelconsumption for international sea transport whichis much closer to international fuel statistics.However, in later work Endresen et al. (2005)examine fuel reports from the EIA and IEA andfind that fuel consumption for navigation may beunder-reported in the statistics.

The differences in fuel results obtained bydifferent research teams clearly demonstrate thedifficulties and the complexity involved in makingdetailed fleet activity-based inventories. In recogni-tion of this, the researchers active in this field arehere encouraged to carry out more studies in orderto reach better agreement in terms of input data andmethodological assumptions. Development of har-monised calculation models would be a very helpfultool in support of the global policy work carried outby the IMO and UNFCCC in allocating bunkeremissions internationally. The development of aconsolidated global inventory baseline is alsorecommended by the International Council onClean Transportation in their status report on airpollution and greenhouse gas emissions from ocean-going ships (Friedrich et al., 2007).

This project uses new information on trends inNOx emissions from different types of ship engines,starting with the production year 1949 and proceed-ing up to the engines of today (2005). The emissiondata have been provided by the ship enginemanufacturer, MAN DIESEL, which has a 75%world market share in relation to ship engineproduction. Emission data from this source ensure

a good representation of the emission factors usedfor the emission calculations.

Introduction of fuel consumption and emissionfactors as a function of engine production year is amajor inventory improvement. This kind of disag-gregated input data are used in many other parts ofthe inventories, e.g. road transport (COPERT IIImodel: Ntziachristos and Samaras, 2000) and non-road machinery (Winther and Nielsen, 2006), andthis level of detail is necessary in order to makeproper assessments of the emission trends.

This project shows that if engine production yeardata exist for the vessels for which traffic andtechnical data are available, fuel consumption andemission calculations become a straightforwardpractice for inventory makers. This is also true incases where assumptions have to be made in orderto account for missing data, e.g. for engine type andlifetime; and also here accuracy of results is beingimproved.

However, a real improvement in the emissioncalculations in the present project would be the useof experimentally determined transient factors inthe model calculations, to account for the variationsin engine loads that occur during the normal rangeof ship operation. Another area of further work isto investigate sfc for the most modern engines and,if necessary, make appropriate updates to fuelconsumption data, which currently are based onexpert judgement for these engines.

Acknowledgements

Many thanks should be given to Hans OttoKristensen, the Technical University of Denmark;Niels Kjemtrup and Sven Hemmingsen, MANDIESEL; Tom Wismann and Jacob Geertinger,FORCE Technology; and Henrik Amdissen, Dan-ish Fishermen’s Association (Hanstholm), for dataused in the project calculations. Also, many thanksto Øyvind Endresen, Norske Veritas, for discussionsduring the writing of the paper.

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