Choice of emission factors

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Inventory compilers should choose default (Tier 1) or country-specific (Tier 2 and Tier 3) emission factors based on the application of the decision trees which consider the type and level of disaggregation of activity data available for their country.

CO2 EMISSIONS

CO2 emission factors are based on the carbon content of the fuel and should represent 100 percent oxidation of the fuel carbon. It is good practice to follow this approach using country-specific net-calorific values (NCV) and CO2 emission factor data if possible. Default NCV of fuels and CO2 emission factors (in Table 3.2.1 below) are presented in Tables 1.2 and 1.4, respectively, of the Introduction Chapter of this Volume and may be used when country-specific data are unavailable. Inventory compilers are encouraged to consult the IPCC Emission Factor Database (EFDB, see Volume 1) for applicable emission factors. It is good practice to ensure that default emission factors, if selected, are appropriate to local fuel quality and composition.

Table 3.2.1

Road transport default co2 emission factors and

UNCERTAINTY RANGES a

Table 3.2.1

Road transport default co2 emission factors and

UNCERTAINTY RANGES a

Fuel Type

Default (kg/TJ)

Lower

Upper

Motor Gasoline

69 300

67 500

73 000

Gas/ Diesel Oil

74 100

72 600

74 800

Liquefied Petroleum Gases

63 100

61 600

65 600

Kerosene

71 900

70 800

73 700

Lubricants b

73 300

71 900

75 200

Compressed Natural Gas

56 100

54 300

58 300

Liquefied Natural Gas

56 100

54 300

58 300

Source: Table 1.4 in the Introduction chapter of the Energy Volume. Notes:

a Values represent 100 percent oxidation of fuel carbon content. b See Box 3.2.4 Lubricants in Mobile Combustion for guidance for uses of lubricants.

Source: Table 1.4 in the Introduction chapter of the Energy Volume. Notes:

a Values represent 100 percent oxidation of fuel carbon content. b See Box 3.2.4 Lubricants in Mobile Combustion for guidance for uses of lubricants.

At Tier 1, the emission factors should assume that 100 percent of the carbon present in fuel is oxidized during or immediately following the combustion process (for all fuel types in all vehicles) irrespective of whether the CO2

has been emitted as CO2, CH4, CO or NMVOC or as particulate matter. At higher tiers the CO2 emission factors may be adjusted to take account of un-oxidised carbon or carbon emitted as a non-CO2 gas.

CO2 EMISSIONS FROM BIOFUELS

The use of liquid and gaseous biofuels has been observed in mobile combustion applications (see Box 3.2.1). To properly address the related emissions from biofuel combusted in road transportation, biofuel-specific emission factors should be used, when activity data on biofuel use are available. CO2 emissions from the combustion of the biogenic carbon of these fuels are treated in the AFOLU sector and should be reported separately as an information item. To avoid double counting, the inventory compiler should determine the proportions of fossil versus biogenic carbon in any fuel-mix which is deemed commercially relevant and therefore to be included in the inventory.

There are a number of different options for the use of liquid and gaseous biofuels in mobile combustion (see Table 1.1 of the Introduction chapter of this Volume for biofuel definitions). Some biofuels have found widespread commercial use in some countries driven by specific policies. Biofuels can either be used as pure fuel or as additives to regular commercial fossil fuels. The latter approach usually avoids the need for engine modifications or re-certification of existing engines for new fuels.

To avoid double counting, over or under-reporting of CO2 emissions, it is important to assess the biofuel origin so as to identify and separate fossil from biogenic feedstocks5. This is because CO2 emissions from biofuels will be reported separately as an information item to avoid double counting, since it is already treated in the AFOLU Volume. The share of biogenic carbon in the fuel can be acknowledged by either refining activity data (e.g. subtracting the amount of non-fossil inputs to the combusted biofuel or biofuel blend) or emission factors (e.g. multiplying the fossil emission factor by its fraction in the combusted biofuel or biofuel blend, to obtain a new emission factor), but not both simultaneously. If national consumption of these fuels is commercially significant, the biogenic and fossil carbon streams need to be accurately accounted for thus avoiding double counting with refinery and petrochemical processes or the waste sector (recognising the possibility of double counting or omission of, for example, landfill gas or waste cooking oil as biofuel). Double counting or omission of landfill gas or waste cooking oil as biofuel should be avoided.

CH4 AND N2O

CH4 and N2O emission rates depend largely upon the combustion and emission control technology present in the vehicles; therefore default fuel-based emission factors that do not specify vehicle technology are highly uncertain. Even if national data are unavailable on vehicle distances travelled by vehicle type, inventory compilers are encouraged to use higher tiered emission factors and calculate vehicle distance travelled data based on national road transportation fuel use data and an assumed fuel economy value (see 3.2.1.3 Choice of Activity Data) for related guidance.

If CH4 and N2O emissions from mobile sources are not a key category, default CH4 and N2O emission factors presented in Table 3.2.2 may be used when national data are unavailable. When using these default values, inventory compilers should note the assumed fuel economy values that were used for unit conversions and the representative vehicle categories that were used as the basis of the default factors (see table notes for specific assumptions).

It is good practice to ensure that default emission factors, if selected, best represent local fuel quality/composition and combustion or emission control technology. If biofuels are included in national road transportation fuel use estimates, biofuel-specific emission factors should be used and associated CH4 and N2O emissions should be included in national totals.

Because CH4 and N2O emission rates are largely dependent upon the combustion and emission control technology present, technology-specific emission factors should be used, if CH4 and N2O emissions from mobile sources are a key category. Tables 3.2.3 and 3.2.5 give potentially applicable Tier 2 and Tier 3 emission factors from US and European data respectively. In addition, the U.S. has developed emission factors for some alternative fuel vehicles (Table 3.2.4). The IPCC EFDB and scientific literature may also provide emission factors (or standard emission estimation models) which inventory compilers may use, if appropriate to national circumstances.

5 For example, biodiesel made from coal methanol with animal feedstocks has a non-zero fossil fuel fraction and is therefore not fully carbon neutral. Ethanol from the fermentation of agricultural products will generally be purely biogenic (carbon neutral), except in some cases, such as fossil-fuel derived methanol. Products which have undergone further chemical transformation may contain substantial amounts of fossil carbon ranging from about 5-10 percent in the fossil methanol used for biodiesel production upwards to 46 percent in ethyl-tertiary-butyl-ether (ETBE) from fossil isobutene (ADEME/DIREM, 2002). Some processes may generate biogenic by-products such as glycol or glycerine, which may then be used elsewhere.

Examples of biofuel use in road transportation Examples of biofuel use in road transportation include:

• Ethanol is typically produced through the fermentation of sugar cane, sugar beets, grain, corn or potatoes. It may be used neat (100 percent, Brazil) or blended with gasoline in varying volumes (512 percent in Europe and North America, 10 percent in India, while 25 percent is common in Brazil). The biogenic portion of pure ethanol is 100 percent.

• Biodiesel is a fuel made from the trans-esterification of vegetable oils (e.g., rape, soy, mustard, sun-flower), animal fats or recycled cooking oils. It is non-toxic, biodegradable and essentially sulphur-free and can be used in any diesel engine either in its pure form (B100 or neat Biodiesel) or in a blend with petroleum diesel (B2 and B20, which contain 2 and 20 per cent biodiesel by volume). B100 may contain 10 percent fossil carbon from the methanol (made from natural gas) used in the esterification process.

• Ethyl-tertiary-butyl-ether (ETBE) is used as a high octane blending component in gasoline (e.g., in France and Spain in blends of up to 15 percent content). The most common source is the etherification of ethanol from the fermentation of sugar beets, grain and potatoes with fossil isobutene.

• Gaseous Biomass (landfill gas, sludge gas, and other biogas) produced by the anaerobic digestion of organic matter is occasionally used in some European countries (e.g. Sweden and Switzerland). Landfill and sewage gas are common sources of gaseous biomass currently.

Other potential future commercial biofuels for use in mobile combustion include those derived from lignocellulosic biomass. Lignocellulosic feedstock materials include cereal straw, woody biomass, corn stover (dried leaves and stems), or similar energy crops. A range of varying extraction and transformation processes permit the production of additional biogenic fuels (e.g., methanol,dimethyl-ether (DME), and methyl-tetrahydrofuran (MTHF)).

It is good practice to select or develop an emission factor based on all the following criteria:

• Fuel type (gasoline, diesel, natural gas) considering, if possible, fuel composition (studies have shown that decreasing fuel sulphur level may lead to significant reductions in N2O emissions6)

• Vehicle type (i.e. passenger cars, light trucks, heavy trucks, motorcycles)

• Emission control technology considering the presence and performance (e.g., as function of age) of catalytic converters (e.g., typical catalysts convert nitrogen oxides to N2, and CH4 into CO2). Diaz et al (2001) reports catalyst conversion efficiency for total hydrocarbons (THCs), of which CH4 is a component, of 92 (+/- 6) percent in a 1993-1995 fleet. Considerable deterioration of catalysts with relatively high mileage accumulation; specifically, THC levels remained steady until approximately 60 000 kilometers, then increased by 33 percent to between 60 000 to 100 000 kilometres.

• The impact of operating conditions (e.g., speed, road conditions, and driving patterns, which all affect fuel economy and vehicle systems' performance)7,

• Consideration that any alternative fuel emission factor estimates tend to have a high degree of uncertainty, given the wide range of engine technologies and the small sample sizes associated with existing studies8.

The following section provides a method for developing CH4 emission factors from THC values. Well conducted and documented inspection and maintenance (I/M) programmes may provide a source of national data for emission factors by fuel, model, and year as well as annual mileage accumulation rates. Although some I/M programmes may only have available emission factors for new vehicles and local air pollutants, (sometimes called regulated pollutants, e.g. NOx, PM, NMVOCs, THCs), it may be possible to derive CH4 or N2O emission factors from these data. A CH4 emission factor may be calculated as the difference between emission factors for THCs and NMVOCs. In many countries, CH4 emissions from vehicles are not directly measured. They are a

UNFCCC (2004)

Lipman and Delucchi (2002) provide data and explanation of the impact of operating conditions on CH4 and N2O emissions.

Some useful references on bio fuels are available in Beer et al (2000), CONCAWE (2002).

fraction of THCs, which is more commonly obtained through laboratory measurements. USEPA (1997) and Borsari (2005) and CETESB (2004 & 2005) provide conversion factors for reporting hydrocarbon emissions in different forms. Based on these sources, the following ratios of CH4 to THC may be used to develop CH4 emission factors from country-specific THC data9:

• 4-stroke gasoline: 10-25 percent,

• natural gas vehicles: 88.0-95.2 percent,

• ethanol hydrated E100: 26.0-27.2 percent.

Some I/M programmes may collect data on evaporatives, which may be assumed to be equal to NMVOCs.10 Recent and ongoing research has investigated the relationship between N2O and NOx emissions. Useful data may become available from this work11.

Further refinements in the factors can be made if additional local data (e.g. on average driving speeds, climate, altitude, pollution control devices, or road conditions) are available, for example, by scaling emission factors to reflect the national circumstances by multiplying by an adjustment factor (e.g., traffic congestion or severe loading). Emission factors for both CH4 and N2O are established not just during a representative compliance driving test, but also specifically tested during running conditions and cold start conditions. Thus, data collected on the driving patterns in a country (based on the relationship of starts to running distances) can be used to adjust the emission factors for CH4 and N2O. Although ambient temperature has been shown to have impacts on local air pollutants, there is limited research on the effects of temperature on CH4 and N2O (USEPA 2004b). Please see Box 3.2.2 for information on refining emission factors for mobile sources in developing countries.

Gamas et. al. (1999) and Diaz, et.al (2001) report measured THC data for a range of vehicle vintage and fuel types.

11 For light motor vehicles and passenger cars, ratios N2O/NOx obtained in literature range around 0.10-0.25 (Lipmann and Delucchi, 2002 and Behrentz, 2003).

Refining emission factors for mobile sources in developing countries

In some developing countries, the estimated emission rates per kilometre travelled may need to be altered to accommodate national circumstances, which could include:

•Technology variations - In many cases due to tampering of emission control systems, fuel adulteration, or simply vehicle age, some vehicles may be operating without a functioning catalytic converter. Consequently, N2O emissions may be low and CH4 may be high when catalytic converters are not present or operating improperly. Diaz et al (2001) provides information on THC values for Mexico City and catalytic converter efficiency as a function of age and mileage, and this also chapter provides guidance on developing CH4 factors from THC data.

■ Engine loading - Due to traffic density or challenging topography, the number of accelerations and decelerations that a local vehicle encounters may be significantly greater than that for corresponding travel in countries where emission factors were developed. This happens when these countries have well established road and traffic control networks. Increased engine loading may correlate with higher CH4 and N2O emissions.

■ Fuel Composition - Poor fuel quality and high or varying sulphur content may adversely affect the performance of engines and conversion efficiency of post-combustion emission control devices such as catalytic converters. For example, N2O emission rates have been shown to increase with the sulphur content in fuels (UNFCCC, 2004). The effects of sulphur content on CH4 emissions are not known. Refinery data may indicate production quantities on a national scale.

Section 3.2.2 Uncertainty Assessment provides information on how to develop uncertainty estimates for emission factors for road transportation.

Further information on emission factors for developing countries is available from Mitra et al. (2004).

Table 3.2.2

Road transport n2o and ch, default emission factors and uncertainty ranges (a)

Table 3.2.2

Road transport n2o and ch, default emission factors and uncertainty ranges (a)

Fuel Type/Representative Vehicle Category

CH, ( kg /TJ)

(kg /TJ)

Default

Lower

Upper

Default

Lower

Upper

Motor Gasoline -Uncontrolled (b)

33

9.6

110

3.2

0.96

11

Motor Gasoline -Oxidation Catalyst (c)

25

7.5

86

8.0

2.6

24

Motor Gasoline -Low Mileage Light Duty Vehicle Vintage 1995 or Later (d)

3.8

1.1

13

5.7

1.9

17

Gas / Diesel Oil (e)

3.9

1.6

9.5

3.9

1.3

12

Natural Gas (f)

92

50

1 540

3

1

77

Liquified petroleum gas (g)

62

na

na

0.2

na

na

Ethanol, trucks, US (h)

260

77

880

41

13

123

Ethanol, cars, Brazil (i)

18

13

84

na

na

na

Sources: USEPA (2004b), EEA (2005a), TNO (2003) and Borsari (2005) CETESB (2004 & 2005) with assumptions given below. Uncertainty ranges were derived from data in Lipman and Delucchi (2002), except for ethanol in cars.

(a) Except for LPG and ethanol cars, default values are derived from the sources indicated using the NCV values reported in the Energy Volume Introduction chapter; density values reported by the U.S. Energy Information Administration; and the following assumed representative fuel consumption values: 10 km/l for motor gasoline vehicles; 5 km/l for diesel vehicles; 9 km/l for natural gas vehicles (assumed equivalent to gasoline vehicles); 9 km/l for ethanol vehicles. If actual representative fuel economy values are available, it is recommended that they be used with total fuel use data to estimate total distance travelled data, which should then be multiplied by Tier 2 emission factors for N2O and CH4.

(b) Motor gasoline uncontrolled default value is based on USEPA (2004b) value for a USA light duty gasoline vehicle (car) -uncontrolled, converted using values and assumptions described in table note (a). If motorcycles account for a significant share of the national vehicle population, inventory compilers should adjust the given default emission factor downwards.

(c) Motor gasoline - light duty vehicle oxidation catalyst default value is based on the USEPA (2004b) value for a USA Light Duty Gasoline Vehicle (Car) - Oxidation Catalyst, converted using values and assumptions described in table note (a). If motorcycles account for a significant share of the national vehicle population, inventory compilers should adjust the given default emission factor downwards.

(d) Motor gasoline - light duty vehicle vintage 1995 or later default value is based on the USEPA (2004b) value for a USA Light Duty Gasoline Vehicle (Car) - Tier 1, converted using values and assumptions described in table note (a). If motorcycles account for a significant share of the national vehicle population, inventory compilers should adjust the given default emission factor downwards.

(e) Diesel default value is based on the EEA (2005a) value for a European heavy duty diesel truck, converted using values and assumptions described in table note (a).

(f) Natural gas default and lower values were based on a study by TNO (2003), conducted using European vehicles and test cycles in the Netherlands. There is a lot of uncertainties for N2O. The USEPA (2004b) has a default value of 350 kg CH/TJ and 28 kg N2O/TJ for a USA CNG car, converted using values and assumptions described in table note (a). Upper and lower limits are also taken from USEPA (2004b)

(g) The default value for methane emissions from LPG, considering for 50 MJ/kg low heating value and 3.1 g CH4/kg LPG was obtained from TNO (2003). Uncertainty ranges have not been provided.

(h) Ethanol default value is based on the USEPA (2004b) value for a USA ethanol heavy duty truck, converted using values and assumptions described in table note (a).

(i) Data obtained in Brazilian vehicles by Borsari (2005) and CETESB (2004 & 2005). For new 2003 models, best case: 51.3 kg THC/TJ fuel and 26.0 percent CH4 in THC. For 5 years old vehicles: 67 kg THC/TJ fuel and 27.2 percent CH4 in THC. For 10 years old: 308 kg THC/TJ fuel and 27.2 percent CH4 in THC.

Table 3.2.3

N2O AND CH, EMISSION FACTORS FOR USA GASOLINE AND DIESEL VEHICLES

Vehicle Type

Emission Control Technology

n2o

CH,

Running (hot)

Cold Start

Running (hot)

Cold Start

mg/km

mg/start

mg/km

mg/start

Light Duty Gasoline Vehicle (Car)

Low Emission Vehicle (LEV)

0

90

6

32

Advanced Three-Way Catalyst

9

113

7

55

Early Three-Way Catalyst

26

92

39

34

Oxidation Catalyst

20

72

82

9

Non-oxidation Catalyst

8

28

96

59

Uncontrolled

8

28

101

62

Light Duty Diesel Vehicle (Car)

Advanced

1

0

1

-3

Moderate

1

0

1

-3

Uncontrolled

1

-1

1

-3

Light Duty Gasoline Truck

Low Emission Vehicle (LEV)

1

59

7

46

Advanced Three-Way Catalyst

25

200

14

82

Early Three-Way Catalyst

43

153

39

72

Oxidation Catalyst

26

93

81

99

Non-oxidation catalyst

9

32

109

67

Uncontrolled

9

32

116

71

Light Duty Diesel Truck

Advanced and moderate

1

-1

1

-4

Uncontrolled

1

-1

1

-4

Heavy Duty

Gasoline

Vehicle

Low Emission Vehicle (LEV)

1

120

14

94

Advanced Three-Way Catalyst

52

409

15

163

Early Three-Way Catalyst

88

313

121

183

Oxidation catalyst

55

194

111

215

Non-oxidation catalyst

20

70

239

147

Heavy Duty Gasoline Vehicle -Uncontrolled

21

74

263

162

Heavy Duty Diesel Vehicle

All -advanced, moderate, or uncontrolled

3

-2

4

-11

Motorcycles

Non-oxidation catalyst

3

12

40

24

Uncontrolled

4

15

53

33

Source: USEPA (2004b).

Notes:

a These data have been rounded to whole numbers.

b Negative emission factors indicate that a vehicle starting cold produces fewer emissions than a vehicle starting warm or running warming.

c A database of technology dependent emission factors based on European data is available in the COPERT tool at http ://vergina.eng. auth. gr/mechO/lat/copert/copert.htm.

d Because of the total-hydrocarbon limits in Europe, the CH4-emissions of European vehicles may be lower than the indicated values from USA (Heeb, et. al., 2003)

e These "cold starts" were measured at an ambient temperature of 68°F to 86°F (20°C to 30°C).

Table 3.2.4

Emission factors for alternative fuel vehicles (mg/km)

Vehicle Type

Vehicle Control Technology

N2O Emission Factor

CH4 Emission Factor

Light Duty Vehicles

Methanol

39

9

CNG

27 - 70

215 - 725

LPG

5

24

Ethanol

12 - 47

27 - 45

Heavy Duty Vehicles

Methanol

135

401

CNG

185

5 983

LNG

274

4 261

LPG

93

67

Ethanol

191

1227

Buses

Methanol

135

401

CNG

101

7 715

Ethanol

226

1 292

Sources: USEPA 2004c, and Borsari (2005) CETESB (2004 & 2005).

Table 3.2.5

Emission factors for european gasoline and diesel vehicles (mg/km), copert iv model

Vehicle Type

Technology/

Class

N2O Emission Factors (mg/km)

CH4 Emission Factors (mg/km)

Urban

Rural

Highway

Urban

Rural

Highway

Cold

Hot

Cold

Hot

Passenger Car

Gasoline

pre-Euro

10

10

6.5

6.5

201

131

86

41

Euro 1

38

22

17

8.0

45

26

16

14

Euro 2

24

11

4.5

2.5

94

17

13

11

Euro 3

12

3

2.0

1.5

83

3

2

4

Euro 4

6

2

0.8

0.7

57

2

2

0

Diesel

pre-Euro

0

0

0

0

22

28

12

8

Euro 1

0

2

4

4

18

11

9

3

Euro 2

3

4

6

6

6

7

3

2

Euro 3

15

9

4

4

7

3

0

0

Euro 4

15

9

4

4

0

0

0

0

LPG

pre-ECE

0

0

0

0

80

35

25

Euro 1

38

21

13

8

Euro 2

23

13

3

2

Euro 3 and later

9

5

2

1

Light Duty Vehicles

Gasoline

pre-Euro

10

10

6.5

6.5

201

131

86

41

Euro 1

122

52

52

52

45

26

16

14

Euro 2

62

22

22

22

94

17

13

11

Euro 3

36

5

5

5

83

3

2

4

Euro 4

16

2

2

2

57

2

2

0

Diesel

pre-Euro

0

0

0

0

22

28

12

8

Euro 1

0

2

4

4

18

11

9

3

Euro 2

3

4

6

6

6

7

3

2

Euro 3

15

9

4

4

7

3

0

0

Euro 4

15

9

4

4

0

0

0

0

Heavy Duty Truck & Bus

Gasoline

All Technologies

6

6

6

140

110

70

Diesel

GVW<16t

30

30

30

85

23

20

GVW>16t

30

30

30

175

80

70

Urban Busses & Coaches

30

30

30

175

80

70

CNG

pre-Euro 4

n.a.

5400

Euro 4 and later (incl. EEV)

900

Power Two Wheeler

Gasoline

<50 cm3

1

1

1

219

219

219

>50 cm3 2-stroke

2

2

2

150

150

150

>50 cm3 4-stroke

2

2

2

200

200

200

Notes:

1 Personal Communication: Ntziachristos, L., and Samaras, Z., (2005), LAT (2005) and TNO (2002).

2 The urban emission factor is distinguished into cold and hot for passenger cars and light duty trucks. The cold emission factor is relevant for trips which start with the engine at ambient temperature. A typical allocation of the annual mileage of a passenger car into the different driving conditions could be: 0.3/0.1/0.3/0.3 for urban cold, urban hot, rural and highway respectively.

3 Passenger car emission factors are also proposed for light duty vehicles when no more detailed information exists.

4 The sulphur content of gasoline has both a cumulative and an immediate effect on N2O emissions. The emission factors for gasoline passenger cars correspond to fuels at the period of registration of the different technologies and a vehicle fleet of ~50 000 km average mileage.

5 N2O and CH4 emission factors from heavy duty vehicles and power two wheelers are also expected to depend on vehicle technology. There is no adequate experimental information though to quantify this effect.

6 N2O emission factors from diesel and LPG passenger cars vehicles are proposed by TNO (2002). Increase in diesel N2O emissions as technology improves may be quite uncertain but is also consistent with the developments in the after treatment systems used in diesel engines (new catalysts, SCR-DeNOx).

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