Comparative environmental assessment of Athens urban buses—Diesel,CNG and biofuel powered

E.A. Nanaki a,n, C.J. Koroneos b, G.A. Xydis c, D. Rovas b

a University of Western Macedonia, Department of Mechanical Engineering, Bakola and Sialvera, 50100 Kozani, Greeceb Laboratory of Heat Transfer and Environmental Engineering, Aristotle University of Thessaloniki, PO Box 483, 54124 Thessaloniki, Greecec Centre for Research and Technology Hellas, Institute for Research & Technology of Thessaly Technology Park of Thessaly, 1st Industrial Area,38500 Volos, Greece

a r t i c l e i n f o

Keywords:Climate changeCity public transportationEnvironmental assessmentAthens

a b s t r a c t

Greenhouse gases (GHGs) emitted by road transport vehicles as a direct result of fossil fuel combustionand other environmental pollutants released throughout the life cycle of petroleum based fuels,encourage a shift towards alternative transport fuels. Within this frame, an environmental assessmentwas performed so as to evaluate the environmental implications of alternative fuels (natural gas andbiofuels) penetration in the city buses of the city of Athens. The results are evaluated in terms of CO2, CO,HC, PM and NOx emissions. The findings show that CO2 emissions are significantly reduced in CNG busescompared to diesel powered buses. CO2 emissions can also be reduced by 7.85% in B10 blends and 78.45%in B100 blends, compared to diesel. The environmental assessment can be considered as a basis so as toinvestigate the viability of replacement of petroleum- based diesel with natural gas and biofuels in citytransport buses. Concepts for sustainable bus transportation can be incorporated using the methodologydefined in this study, in order to promote a sustainable transportation system and mitigate the climatechange.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Energy-related CO2 emissions, resulting from burning fossilfuels, represent the major part of recent human-made greenhousegas emissions (GHG). Emissions from the transport sector are animportant source of CO2 in many countries. Public transportationsystems providing mobility and access to most activities play acrucial role to cities; nevertheless the shift to a low carbon publictransportation system is challenging. In this context, cities have akey role to play in the global agenda for addressing the challengeof CO2 emissions mitigation. Especially in the case of Europeancities that face problems caused by transport and traffic, thechallenge is to enhance mobility while at the same time reducingcongestion and air pollution Despite the fact that improvementshave been made in the energy efficiency of various transport modesand non-fossil fuels have been introduced, increased transportdemand is outweighing these benefits (European EnvironmentEnergy (EEA), 2007).

In 2007, the transport sector accounted for 18% of total 2007GHG, reaching a total of 24 million tones of CO2 equivalents (anincrease of 33% on 1997 levels) (⟨http://www.epp.eurostat.ec.europa.eu⟩). Road transport is a significant source of air pollutionin Greece, particularly within urban areas. To be more specific,road transport contributed to approximately 80% of these CO2

emissions highlighting the very real challenge in restructuring asector which is intensive in energy demand, environmentalimpacts and continues to grow. The split between road transportoil in Greece from 1978 onwards is approximately 37% diesel and63% gasoline, with the majority of the gasoline demand being forprivate car use (Papagiannaki and Diakoulaki, 2009). Based onour previous study (Koroneos and Nanaki, 2007) it is shown thata number of elements influenced the increased demand forprivate transportation. The increasing GDP, and therefore incomeof households, allowed householders to travel in more “luxury”benefiting from faster and more accessible private transportationas well as being a symbol of wealth. Additionally, distancestravelled to work, shopping and leisure activities increasedensuring that total distance travelled by private cars measuredin 1000 mio pkm also continued to increase. Nevertheless,Greece's economic crisis has lead an increasing number ofmotorists in urban areas such as Athens to resort to the capital'svarious forms of public transport as higher gasoline prices and

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http://dx.doi.org/10.1016/j.tranpol.2014.04.0010967-070X/& 2014 Elsevier Ltd. All rights reserved.

n Corresponding author.E-mail addresses: evananaki@gmail.com (E.A. Nanaki),

koroneos@aix.meng.auth.gr (C.J. Koroneos),gxydis@mail.ireteth.certh.gr (G.A. Xydis), drovas@aix.meng.auth.gr (D. Rovas).

Please cite this article as: Nanaki, E.A., et al., Comparative environmental assessment of Athens urban buses—Diesel, CNG andbiofuel powered. Transport Policy (2014), http://dx.doi.org/10.1016/j.tranpol.2014.04.001i

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the unprecedented economic crisis dent the attraction of privatevehicle use (⟨http://archive.ekathimerini.com/4dcgi/_w_articles_politics_0_28/05/2010_117332⟩). Public transportation plays avital role in the transportation system of an area and it also helpsto minimize traffic congestion and other traffic-related external-ities. Transportation problems are among the most pressingstrategic development problems in many cities and are oftenconsidered as a major constraint for long-term urban develop-ment. In addition, transportation problems are very closely relatedto land development, economic structure, energy policies, andenvironmental quality. Since all citizens are either enjoying thetransportation system or, and often at the same time, sufferingfrom it, it is an important element of the urban quality of life.

The public transportation system of Athens consists of buses,trolley buses, a subway system (metro) as well as tram. On thedemand side, the daily traffic is of 2650,000 passengers. Theaverage bus speed in the city centre is 7 km/h; whereas in areasoutside the city centre the average speed increases to 13 km/h.The use of public transport in the whole metropolitan area ofAthens in 2004 came up to 31.7%, whereas 68.3% representedprivate vehicles. As far as the main city is concerned 45%represented the use of public transport, whereas 55% representedthe use of private vehicles (Website of European MetropolitanTransport Authorities, 2007). The strong gap between modal sharein the main city and in the whole metropolitan area, where publictransport accounts, in average, for 30% of motorized trips, illus-trates one of the main challenges facing public transport autho-rities: developing public transport in the suburbs and the lessdense parts of the metropolitan areas.

The main objective of this study is to evaluate and analyse theenvironmental impacts from public transportation vehicles inurban areas and especially in the city of Athens and recognizepossible routes for achieving the ambitious EU targets of 20–20–20 on energy and climate. Furthermore, this study aims to deter-mine the CO2 emissions reduction that could be achieved due topenetration of alternative transport fuels in urban bus fleet. Thedata used for this analysis were taken from the databases of AthensUrban Transport Organization.

2. Methodology

The emission estimation methodology covers the exhaustemissions of CO, NOx, CO2, PM and HC for each vehicle technologyof Athens's bus fleet. PM mass emissions in vehicle exhaust mainlyfall in the PM2.5 size range. Therefore, all PM mass emission factorsare assumed to correspond to PM2.5. The methodology usedconsiders the fuel used by different vehicle categories and theiremission standards. In this respect, the vehicle category of busesincludes urban CNG buses and urban diesel buses according toemission-control legislation categories (EURO I, II, III, IV, V, VI).

The technical data used take into account national variations.The variations may include parameters such as the fuel consump-tion, the composition of vehicle park, vehicle age, driving patterns,some fuel parameters and climatic conditions. Other variationswhich may exist, e.g. variations in vehicle maintenance, are notaccounted for, because there is not enough data available to do so.The calculation is based on the following main types of inputparameters: total fuel consumption, vehicle technology, vehiclepark, driving condition, emission factors.

The emission factors are stated in units of grammars pervehicle-kilometer for each vehicle technology. These averageEuropean emission factors are determined using typical valuesfor driving speeds, ambient temperatures, highway-rural-urbanmode mix, trip length, etc. (Ntziachristos and Samaras, 2009).Based on the above and in order to calculate the emissions of each

vehicle technology, the following equation was used:

Ei; j¼ΣkðNj; k�Mj; k� EFi; j; kÞ ð1Þwhere, j are the vehicle categories of diesel and CNG buses, k is thetechnology of each category (i.e. EURO I, EURO II, etc.), Nj, k is thenumber of vehicles in the city's under study bus fleet of category jand technology k, Mj, k represents the average annual distancedriven per vehicle of category j and technology k, EFi, j, krepresents the technology-specific emission factor of pollutant ifor vehicle category j and technology k.

From the above mentioned it is obvious that it is necessary tohave data regarding the number of vehicles and the annualnumber of vehicle-km per technology. These vehicle-km dataare then multiplied by the emission factors. Data concerningAthens's bus fleet (number of vehicles, categories, engine type aswell as annual distance driven) were obtained from Athens UrbanTransport Organization. The emissions factors used, are obtainedfrom the studies of Ntziachristos and Samaras (2009), Nylund et al.(2004), Beer et al. (2000); Table 1 presents the emissions factorsused for the diesel bus fleet; whereas Table 2 presents theemissions factors used for the CNG bus fleet.

3. Athens public transportation system

Athens metropolitan area is the most populous area in Greecewith 4.0 million people. The region covers an area of 1450 km2

encompassing 83 local authorities (municipalities) in 3 counties.Athens belongs to the Attica region and covers 35% of its surfacearea, with the Athens urban administrative area covering a total of544 km2. Athens in terms of both surface area and population isdensely populated (5882 people per square kilometre). The AthensMetropolitan area is surrounded by mountains from West to Eastand by the Aegean Sea from the South. Within the central urbanarea, the existence of several hills has an influence upon thetransport in the city, causing local roads to have steep gradients.The Athens urban area has spread rapidly in recent decades andcontinues to expand, mainly to the East and the North.

The transport infrastructure in Athens consists of a road net-work with a total length of 8000 km. The main road networkcovers 1826 km. The center of the city is the area bounded by theinner ring road (an area of 9.2 km2). There is also a (middle) ringroad system surrounding an area of 111 km2. The road traffic inAthens—both in private and public modes- involves significanttraffic delays and low traffic speeds.

OASA is the shareholder in the Public Transport Operators thatare members of the OASA Group: ETHEL S.A., (Thermal Buses),

Table 1Emissions factors for diesel bus fleet.

Engine type CO2 (g/km) CO (grkm) HC (g/km) PM (g/km) NOx (g/km)

EURO I 1.397 1.50 0.3 0.45 16EURO II 1.386 1.35 0.2 0.2 14EURO III 1.351 1.00 0.15 0.18 9EURO IV 1.343 0.95 0.09 0.06 6.38EURO V 1.330 0.74 0.06 0.01 3.83

Table 2Emissions factors for CNG bus fleet.

Engine type CO2 (g/km) CO (g/km) HC (g/km) PM (g/km) NOx (g/km)

EURO II 1.100 2.70 4.7 0.01 15EURO III 1.250 1.00 1.33 0.01 10

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Please cite this article as: Nanaki, E.A., et al., Comparative environmental assessment of Athens urban buses—Diesel, CNG andbiofuel powered. Transport Policy (2014), http://dx.doi.org/10.1016/j.tranpol.2014.04.001i

ILPAP S.A. (Electric Buses) and ISAP S.A. (Athens—Piraeus ElectricRailways). Public transportation is also provided by Public Trans-port Operators (Attiko Metro Operation Company (AMEL), TRAMand TRAINOSE—General Division of Suburban Transport based oncontracts entered into between these operators and OASA. ILPAPoperates an electric bus network of 22 lines that serve primarilythe Athens and Piraeus city centres. The company owns andoperates a fleet of 315 single trolley buses 12 m long and 51articulated trolley buses 18 m long. ISAP S.A. operates the ElectricRailway line that runs between Piraeus and Kifisia (metro line 1),serving 24 stations; whereas the total length of the network is25.6 km. Attiko Metro Operation Company S.A. (ΑΜΕL S.A.) hasbeen running Lines 2 and 3 of the Athens Metro since 2000,commissioning year of these two lines. The entire network is51.1 km long and consists of 32 stations (including four that are incombined use with the Suburban Railway). Athens InternationalAirport and the area of Messogia are also directly connected withthe city center through 4 Metro Stations within a 22.1 km-longsection of the Suburban Railway. Table 3 summarizes the annualpassenger traffic during the period of 2005–2008 per publictransport mode Athens Urban Transport Organization (2010).

ETHEL S.A. operates the buses that serve the capital. Itstransport network comprises of 319 bus lines and operatesapproximately 16.000 routes daily covering the capital in itsentirety. ETHEL S.A's transportation network has a length of8500 km. The fleet of buses consists of 2.151 vehicles of averageage of 9 years, out of which 414 are powered by natural gas and1.737 by diesel. Table 4 summarizes the characteristics of ETHELS.A.’s bus fleet. In 2010, the buses in operation carried out 12,815routes per day. In addition, in 2010, ETHEL S.A. served 419 milliontransports for covering the needs of AMEL and ISAP. The bus linenetwork is composed of the following lines:

� 40 core lines that connect the Athens and Piraeus city centerswith the centers of the peripheral municipalities. The radiallines connect the Athens and Piraeus city centers with thecenters of the neighboring municipalities.

� 20 inter-municipal lines that connect the municipalities of theAttica basin without crossing the Athens and Piraeus citycenters.

� 123 local lines that operate within the limits of one or a groupof neighboring municipalities and act as suppliers to thecore lines.

� 19 express lines� 7 school lines.

In 2009 ETHEL S.A. withdrew 255 urban diesel buses withobsolete engine technology (EURO I) and received 320 new dieselbuses with new engine technology (EURO IV and EURO V) and SCR(selective catalytic reference) catalysts, in order to modernize ourfleet and protect the environment.

4. Environmental assessment of Athens urban buses

The main anthropogenic sources of air pollution in the metro-politan area of Athens can be attributed to industry (E40% of thetotal Greek industrial activity), transportation (E50% of the totalautomobile traffic) and heating. A large body of literature con-cerning the air pollution in Athens has been carried out. It includesinter alia the studies of Flassak and Moussiopoulos (1988),Asimakopoulos et al. (1992), Kallos et al. (1993), Pilinis et al.(1993), Ziomas (1998), Sotiropoulou et al. (2004, 2006). Diesel-fuelled buses can be an important source of air pollutants. Theycan emit significant amounts of carbon dioxide (CO2), nitrogenoxides (NOx), particulate matter (PM2.5 and PM10), sulphur dioxide(SO2), sulphates (SO4), carbon monoxide (CO), and volatile organiccompounds (VOCs). These air pollutants can contribute to theformation of smog and have been linked to a variety of acute andchronic health outcomes, including respiratory illnesses, heartdisease, and premature death (Crump et al., 1991; Crump, 1999;Wichmann et al., 2000; Environmental Protection Agency, 2002).

Natural gas is used widely as a combustion energy source,including power generation and industrial cogeneration; for theseapplications it is typically delivered at low to moderate pressurevia utility pipeline. The use of natural gas, either as CNG (Com-pressed Natural Gas) or LNG (Liquefied Natural Gas) has been inthe field of urban public transport the first alternative to dieseltechnology which has been implemented from the first half of the90 s. Most natural gas vehicles utilize fuel cylinders containingnatural gas that has been compressed at high pressure (200–220 bar), reducing its volume by 99% compared to standardatmospheric conditions; this allows significantly greater drivingrange between fueling events. Natural gas is composed primarilyof methane (typical composition: 87–96% methane, 1.5–5.1%ethane and 0.1–1.5% propane); it is commercially produced fromoil fields or from natural gas fields.

Table 3Annual passenger traffic per public transport mode.

2005 2006 2007 2008

ETHEL 393,611,762 413,292,350 423,509,574 421,080,000ILPAP 84,231,072 88,442,626 91,815,297 92,200,000ISAP 124,038,181 135,821,809 148,725,181 149,050,000AMEL 172,197,626 177,363,555 185,719,257 193,100,000TRAM 12,922,259 14,488,146 18,729,130 19,800,000TRAINOSE 2,905,457 2,741,216 3,345,974 3,500,000Total 789,906,358 832,149,702 871,844,413 878,730,000

Table 4Types of buses and their engine technology operating in Athens.

Types of buses

Fuel type diesel EURO I EURO II EURO III EURO IV EURO V Grand total

MIDI 32 195 22012 m 366 206 28118 m 337 100Total 398 738 281 220 100 1737

Fuel type CNG EURO I EURO II EURO III EURO IV EURO V Grand total

MIDI12 m 294 12018 mTotal 294 120 414

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Please cite this article as: Nanaki, E.A., et al., Comparative environmental assessment of Athens urban buses—Diesel, CNG andbiofuel powered. Transport Policy (2014), http://dx.doi.org/10.1016/j.tranpol.2014.04.001i

While diesel engines use a lean combustion mixture andcompression ignition, natural gas engines are spark-ignited andcan use either a lean-burn or stoichiometric combustion mixture.The first generation of heavy-duty natural gas engines introducedin both developed and developing countries were lean burnengines based on diesel engine designs, with fuel injectionsystems borrowed from stationary natural gas engine technology.The earliest engines also employed simple open-loop fuel controlsystems and did not employ any after-treatment. The engine-outPM and NOx emission levels from a lean-burn CNG engine withoutany after-treatment system are low enough to outperform aconventional diesel engine (Euro III), or an advanced diesel enginewith electronic control of fuel injection but without EGR or aftertreatment (Euro IV).

Fig. 1 describes the development of emission regulations forEuropean heavy-duty vehicles. The transition from Euro III to EuroIV class implies a remarkable tightening of emission values. Thelimit for particulate emissions (PM) decreases by 80% while thelimit for nitrogen oxides (NOx) decreases more moderately, by 30%.The limit for particulate emissions is the same for Euro IV and V,resulting to a reduction of NOx by an additional 40%. The next

phase, Euro VI is already being discussed, and this class will enterinto effect during the next decade. It is noted, that Euro I standardswere introduced in 1992, followed by the introduction of Euro IIregulations in 1996. In 1999, Directive 1999/96/EC introduced EuroIII standards (2000), as well as Euro IV/V standards (2005/2008).In December 2007, the Commission published a proposal for EuroVI emission standards [COM(2007) 851].

Fig. 2 presents the kilometers driven and the fuel consumptionof the diesel-powered bus fleet. The fuel consumption of EURO IIengine is on average 2 times greater than this of EURO I enginetechnology and 7 times greater than this of EURO V. The 1737diesel powered buses in the city of Athens, in 2009, carried out91,302,660 km and consumed 51,271,350 l of diesel oil and emitted125,467 t of CO2.

In 2009 diesel buses in Athens with EURO II engine technologyemitted 53,765 t of CO2 whereas EURO V engine technology busesemitted 6991 t of CO2 (Fig. 3). As the engine technology improves(EURO I to EURO V) the emitted CO2 per kilometer driven isreduced. It is obvious that as the engine technology improves(EURO Ι to EURO V) the emissions of NOx, PM, CO and HC resultingfrom the 1737 diesel buses are significantly reduced (Fig. 4).Looking at diesel vehicles, the variation of emissions is greaterfor Euro II vehicles than for the other engine technologies.

The 414 CNG buses in Athens, in 2009, carried out 21,698 kmand emitted 24,811 t of CO2. In 2009 CNG buses in Athens withEURO II engine technology emitted 16,950 t of CO2 whereas EUROIII engine technology buses emitted 7991 t of CO2 (Fig. 5). CNGbuses with EURO II engine technology emitted 72,422 t of HCwhereas EURO III engine technology buses emitted 8.36 t of HC(Fig. 6). The high HC emissions for EURO II engine technology isattributed to the lean-burn technology. However, it should benoted that the HC emissions from CNG buses are more than 95%methane, which is neither toxic nor reactive. Nevertheless,methane is a strong greenhouse gas, and should be taken intoaccount when calculating total greenhouse gas emissions. The PM

EURO VI

EURO V EURO IV

EURO III

EURO II

EURO I

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0

1

2

3

4

5

6

7

8

9NOx g/kWhPM g/kWh

Fig. 1. Trends in European Emission Standards for heavy duty vehicles.

Fig. 2. Kilometers driven and fuel consumption from diesel bus fleet.

Fig. 3. CO2 emissions per diesel engine type in 2009.

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emissions vary from 0154 t (Euro II) to practically zero (Euro III).All CNG vehicles perform well in these emissions, as do the dieselvehicles equipped with a properly functioning CRT filter. It isobvious that as the engine technology improves (EURO ΙI to EUROIII) the emissions of CO, HC, PM and NOx resulting from the 414CNG buses are significantly reduced (Fig. 6).

CNGs show similar to higher CO emissions compared with theDiesel emissions. The use of CNG leads to an important decrease inNOx emissions compared to diesel buses. The mass of particlesemitted by CNG engines is approximately 10 times lower than fordiesel engines (EURO II). The highest CO values are found amongEuro II vehicles, both diesel and CNG. Both NOx and PM emissionshave a clear downward trend along with newer Euro emissionstandards, although certain bus models do not follow the generaltrend. Finally, as far as the CO2 emissions are concerned, it isnoticed that CNGs emissions are lower compared with dieselpowered buses (Fig. 7).

5. Biodiesel use in Athens urban buses

Various alternative transport fuels such as biodiesel, ethanol,bioethanol, are already in use. Given that, in order to meet the

targets set within the Directive 2009/28/EC regarding the promo-tion of the use of energy from Renewable Energy Sources (RES) theelaboration of policies and measures targeting at the fulfillment ofthe “20–20–20” obligations and the acceleration of the Greekeconomy through “green” development and enhanced competi-tiveness of the private sector is required. The term biodieselcommonly refers to fatty acid methyl or ethyl esters made fromvegetable oils or animal fats, whose properties are good enough tobe used in diesel engines. The regulations limiting such propertiesare EN-14214 in Europe (UNE EN-14214, 2003) and (ASTM D)-6751-03 in USA (ASTM D), although ethyl esters are not yetacknowledged as biodiesel in Europe (Council Directive, 2003).

In this context, five alternative biodiesel blends that can beused in the urban buses of Athens are examined, so as to perform acomparative diesel and biodiesel analysis of air pollutants emis-sions. Emission calculations are based on 2009 data. The biodieselblends, under study, consist of:Β10 (10% biodiesel and 90% diesel),B30 (30% biodiesel and 70% diesel), B50 (50% biodiesel and 50%diesel), B80 (80% biodiesel and 20% diesel) and B100 (100%biodiesel).

The use of biodiesel blends in urban transport has bothadvantages and disadvantages. Figs. 8–10 are indicative not onlyof the environmental reductions that might be achieved with the

Fig. 4. Annual pollutants emissions per diesel engine type in 2009.

0

4.000

8.000

12.000

16.000

20.000

EURO II EURO III

tn/y

ear

CO2

Fig. 5. CO2 emissions per CNG engine type in 2009.

Fig. 6. Annual pollutants emissions per CNG engine type in 2009.

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Please cite this article as: Nanaki, E.A., et al., Comparative environmental assessment of Athens urban buses—Diesel, CNG andbiofuel powered. Transport Policy (2014), http://dx.doi.org/10.1016/j.tranpol.2014.04.001i

use of different biodiesel mixes in the urban buses, but also of theenvironmental burdens. To be more specific, the HC, CO andPM emissions are reduced as the percentages of biodiesel in thebiodiesel mixes are increased. For instance, HC emissions stem-ming from B100 are 67, 36% lower than the equivalent dieselemissions; whereas CO emissions stemming from B100 are 48, 11%lower than the equivalent diesel emissions and PM emissionscoming from Β100 are 47, 19% lower compared with dieselemissions. On the other hand, NOx emissions are increased asthe percentages of biodiesel in the biodiesel mixes are increased(10.29% increase in B100 blend compared to equivalent dieselemissions). Finally, as far as the CO2 emissions are concerned, it isnoticed that these are reduced as the percentages of biodiesel inthe biodiesel mixes are increased. CO2 emissions coming from

Β100 blend is 78.45% lower compared to equivalent diesel emis-sions; whereas CO2 emissions coming from B10 blend are 7.85%lower compared to equivalent diesel emissions.

The abovementioned results of this study are in accordancewith our previous study (Nanaki and Koroneos, 2009) where itwas shown that biodiesel is beneficial with respect to the saving offossil energy and to the greenhouse effect; nevertheless is detri-mental regarding acidification, and euthrophication.

6. Discussion/conclusions

The promotion of environmentally friendly and energy efficientvehicles, with the use of new technologies and cleaner fuels

Fig. 7. Annual pollutants emissions per CNG engine type in 2009.

Fig. 8. Annual pollutants emissions per different biodiesel blend and diesel.

Fig. 9. Annual CO2 emissions per each biodiesel blend and diesel.

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constitutes a major policy component of the EU. EU directives,related to air emissions limits from internal combustion enginesfor vehicles from 1998 till now (“Euro” standards), have beentransposed into the Greek legislation. In this context and given theambitious EU targets of 20–20–20, the identification and investi-gation of climate change impact from the use of public transporta-tion is of great significance. In this context, in the present study anenvironmental assessment of urban diesel and CNG bus fleet forthe city of Athens, in 2009 was performed. Additionally, theenvironmental repercussions of the use of five different biodieselblends in the bus fleet under study were also examined. Theintense traffic problem that exists in the capital of Athens inconjunction with the targets set by EU necessitates the applicationof techniques that contribute to efficient traffic management andto the implementation of measures that upgrade the publictransport services, improve the quality of life of citizens andincrease the use of alternative transport fuels.

The study indicates that penetration of CNG as well as biodieselto the public transportation system of Athens could result in lowerCO2 emissions. Lower emissions are of great significance for asustainable urban transportation system. In order, to obtain loweremissions in CNG buses it is necessary to lean towards advancedengine technology by favoring injection systems. In addition, thereplacement of CNG technology by CBG (biomethane) is consid-ered to be sustainable low carbon solution that could be furtherinvestigated. Despite the environmental benefits from the use ofCNG technology in the bus fleet of Athens, major barriers are theinfrastructure issues. The infrastructure of CNG fuelling stations,gas upgrading plants and gas injection exists or is expanding in afew countries like Austria, Germany, Italy, Netherlands, Swedenand Switzerland. In other countries, refueling infrastructure isrudimentary and has to be extended or still created. The develop-ment of methane vehicles is strongly hampered by very highinvestment costs that are required for the build-up of the neededmethane refueling infrastructure. The main driver behind invest-ments in the methane refueling infrastructure has mostly been thenatural gas industry, especially when it comes to the promotionand construction of public refueling opportunities for passengercars and vans. The expansion of private refueling facilities forcommercial fleets of light and heavy duty and public transportcompanies of urban buses and trucks mainly results from localinitiatives between public authorities and industry. The disparitiesin the level of development for using methane in transport inEurope are due to specific national investment strategies and to acertain extent also to the availability of economic resources. Inaddition to this, investments for the development of infrastructuretake time, which is even more evident in the case of CNG stations

where investments are at least five times higher than for conven-tional liquid fuels. More established NGV (Natural Gas Vehicle)countries needed more than 15 years to develop the infrastructureof today. It is therefore clear that countries like Greece, that is nowstarting the construction of methane refueling stations, willrequire time, at least until 2025 or beyond, to guarantee adequaterefueling. Political support and binding targets, incentives andsubsidy schemes would certainly speed up the build-up of infra-structure. A coherent public policy will also be crucial (Report ofthe European Expert Group on Future Transport Fuels, 2011). To bemore specific, improvement of the legislative framework andestablishment of CNG standards in conjunction with training oftechnicians (for converting of vehicles) as well as with thecooperation of the whole chain of the fuel sector in Greece forthe promotion of CNG (Government/Vehicle Importers/VehicleService Stations/Existing Fuel Station Grids) are necessary stepsfor CNG deployment (Website of DEPA).

Furthermore, the use of adapted after- treatment can decreaseexhaust emission pollutants level, produced by diesel-poweredbus. In that case, some trap technologies can be associated withadapted Diesel—fuel formulations, constant filter maintenance,etc. As it has been shown in previous studies, the most effectiveway to reduce regulated emissions is to replace old vehicles withnew ones; whereas the most effective way to cut GHG emissions isto switch from fossil fuels to efficient biofuels (Nylund et al., 2012).

The use of biodiesel in the public transportation system ofAthens can be beneficial; nevertheless the use of most advancedliquid biofuel HVO (Hydro treated Vegetable Oil) can reducesignificantly NOx emissions. It is noted that a very cost-effectiveand rapid way to reduce emissions is to use HVO in Euro I and EuroII engines since in those the reduction of emissions in g/km is thehighest and it can take place in all Euro I and Euro II vehiclesimmediately. HVO can be used also in the most modern dieselengines and after treatment systems where high FAME concentra-tions may cause technical difficulties (Nylund et al., 2011). Biodie-sel has the potential to curb greenhouse gas emissions in thetransportation sector as long as it satisfies all sustainability criteriaas established by the EU legislation in force, including thebiodiesel blends present in the market as well as future advancedtypes that might be developed. Higher blends of biodiesel mightbe needed to fulfil the climate change targets. Only comparativelymarginal alterations to the distribution system are needed. It isnoted that no adaptation in consumer behavior is needed (drivingrange, refueling habits, look and feel of the car).

The results of the present study can be used as an input to thestrategic decision- making process for future transport energypolicy and also to identify key areas of interest for further

Fig. 10. Air pollutant emissions per each biodiesel blend and diesel depicted in logarithmic scale.

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Please cite this article as: Nanaki, E.A., et al., Comparative environmental assessment of Athens urban buses—Diesel, CNG andbiofuel powered. Transport Policy (2014), http://dx.doi.org/10.1016/j.tranpol.2014.04.001i

technology research and development of the public transportsystem in Athens. A transportation public system, which fostersa positive attitude towards alternative fuels and new power trainconcepts using captive fleets as forerunners, is a significantparameter for a sustainable transportation system, which aims atthe improvement of public transportation energy efficiency,improvement of the quality of the offered public transport servicesmaking it by this way more attractive to passengers and toenvironmental sustainability.

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Please cite this article as: Nanaki, E.A., et al., Comparative environmental assessment of Athens urban buses—Diesel, CNG andbiofuel powered. Transport Policy (2014), http://dx.doi.org/10.1016/j.tranpol.2014.04.001i