National Atmospheric Emissions Inventory

Appendix 1

The Methodology of the National Atmospheric Emissions Inventory

2.6 Emissions From the Extraction and Distribution of Fossil Fuels

 2.6.1 Coal Mining

The NAEI reports emissions of methane from coal mining in the categories Deep Mined Coal and Open Cast Coal. Emissions are calculated from coal production statistics reported in DTI (1996). Data on the shallower licensed mines are not published and were supplied to us by Barty (1995) up to 1994. Following privatisation the distinction between licensed mines and deep mines no longer exists. For 1995, data from 1994 was used but since then distinction has been abandoned. The emission factors used are shown in Table A12 and are taken from Bennet et al. (1995).

Table A12 Methane Emission Factors for Coal Mining (kg/t coal)

  pre-1991 1991 1992 post-1992
Deep Mined Coal1 11.3 11.5 12.3 10.3
Open Cast Coal 0.34 0.34 0.34 0.34
Licensed Mines 1.36 1.36 1.36 1.36
1 Includes an emission of 1.16 kg CH4 /t coal for storage and transit emissions
The deep mine factors are aggregate factors for all British Coal mines and reflect the methane utilization measures employed in the pits operating. For 1990-1993 the deep mined emission factor varies because the number of pits operating declined over the period. Pre-1991 and post-1992, emission factors are used for 1990 and 1993 respectively. Emissions from solid fuel transformation plants are negligible (Williams, 1993)

2.6.2 Oil and Natural Gas

The methodology used to estimate emissions from the offshore oil and gas industry has been completely revised for the 1996 Inventory. The new estimates are based on the UKOOA sponsored SCOPEC Inventory (SCOPEC, 1997). This is a detailed inventory based on returns from the operating companies for the year 1995. For years prior to 1995 the estimates are based on the UKOOA study for 1991 (UKOOA, 1993). This is the source that was used for previous inventories but has now been retrospectively revised by UKOOA to incorporate new data on flaring and loading emissions. (Russell, 1997). The emissions reported in these studies have been integrated into the NAEI and extrapolated to other years to produce a time series. The diesel oil consumption by offshore installations is not reported separately but is included under coastal shipping. In order to avoid double counts the UKOOA estimates have been corrected to remove diesel oil emissions.

In the NAEI, emissions from venting, well testing, fugitives and loading are combined under the category offshore oil & gas. The other categories used are offshore flaring and offshore own gas use.
The offshore emissions are estimated in the following categories, each with its own methodology.

2.6.2.1 Offshore Flaring
This includes flaring from offshore platforms and onshore terminals. Flaring emission data for CO2 , SO2, NOx , CO, NMVOC and methane were taken from the SCOPEC(1997) and UKOOA(1993) studies for 1995 and 1991respectively. These emissions were extrapolated to other years using the data on flaring volumes reported in DTI(1997). This involved calculating aggregate emission factors (kt pollutant /Mm3 gas flared). The SCOPEC estimate was used for 1995 and 1996 and the UKOOA estimate for 1970-1994. The aggregate emission factors are given in Table A13.

Table A13 Aggregate Emission Factors for Flaring, Fuel Gas Consumption and Well Testing.

    C SO2 NOx CO NMVOC CH4
Flaring kt/Mm3 1995- 1970-94 0.721 0.716 0.000091 0.000119 0.0252 0.0393 0.00902 0.00952 0.0102 0.0107 0.01184 0.00744
Fuel Gas kt/Mtherm 1995- 1970-94 2.09 1.93 0.00039 0.000371 0.0252 0.0393 0.0096 0.00699 0.000199 0.00156 0.00135 0.000972
Well Tests kt/Well Explored 1995- 1970-94 2.69 1.46 0.1 0.1 0.0047 0.0087 0.024 0.0445 0.063 0.117 0.027 0.05

2.6.2.2 Offshore Own Gas Use
This refers to the use of unrefined natural gas on offshore platforms and onshore terminals as a fuel in heaters, boilers, turbines and reciprocating engines. Gas combustion emission data for CO2 , SO2, NOx , CO, NMVOC and methane were taken from the SCOPEC(1997) and UKOOA(1993) studies for 1995 and 1991 respectively. In the case of the UKOOA(1993) data it was necessary to apply a correction to remove the contribution from diesel oil consumption. This correction was based on the more detailed breakdown of emissions in the SCOPEC Study and was necessary to avoid double counting with fuel oil consumption reported elsewhere in the inventory. These emissions were extrapolated to other years using the data on 'producers own use' reported in DTI(1997). The DUKES category 'producers own use' is mostly if not wholly natural gas used on offshore installations for running turbines, engines, heaters and boilers. This involved calculating aggregate emission factors (kt pollutant /Mtherm). The SCOPEC estimate was used for 1995 and 1996 and the UKOOA estimate for 1970-1994. The aggregate emission factors are given in Table A13.

2.6.2.3 Well Testing
The 1996 Inventory reports emissions from well testing for the first time. This activity involves the combustion of crude oil and crude gas that are not included in UK Energy Statistics. Combustion emission data for CO2 , SO2, NOx , CO, NMVOC and methane were taken from the SCOPEC(1997) and UKOOA(1993) studies for 1995 and 1991 respectively. The estimates of the amounts of crude oil and gas burnt are fairly speculative. A correction was applied to the 1991 data to remove the contribution from diesel oil consumption in drilling in order to prevent double counting. Data from 1995 and 1991 were extrapolated to other years using data on the numbers of wells explored, reported in DTI(1997a). Unfortunately the number of wells tested is only a small proportion of the number of wells explored and that proportion may vary from year to year. Also the number of wells explored varies considerably from year to year. Hence this method of extrapolation can only give a reflection of the well testing activity and is used because actual data on well testing is not collected. This variation accounts for the discrepency in the aggregate emission factors (kt pollutant /well explored) calculated for 1991 and 1995. The SCOPEC estimate was used for 1995 and 1996 and the UKOOA estimate for 1970-1994. The aggregate emission factors are given in Table A13.

2.6.2.4 Other Emissions from Platforms and Terminals
These include emissions of methane and NMVOC from platforms and terminals arisng from cold venting and fugitive emissions and also NMVOC emissions from storage of crude oil at terminals. The approach adopted was determined by the data available. This was to extrapolate the SCOPEC(1997) and UKOOA(1993) estimates of total emissions (i.e. flaring, gas combustion, venting, fugitives and storage) using data on the numbers and type of platform or terminal. This involved calculating aggregate emission factors (kt pollutant/platform). The types of installation considered were oil platforms, gas platforms, oil/gas platforms, oil terminals and gas terminals. The platform data was taken from DTI(1997a) and Oilfield Publications (1996). The SCOPEC estimate was used for 1995 and 1996 and the UKOOA estimate for 1970-1992. The SCOPEC data was less detailed so the extrapolation was made on the total number of platforms rather than the various types. The aggregate emission factors are given in Table A14.

The resulting emission estimates include flaring and gas consumption which have been estimated separately. In order to avoid double counting the flaring and gas consumption components were subtracted. The emissions for the years 1993 and 1994 were then estimated from interpolation.

Table A14 Aggregate Emission Factors used for Emissions from Platforms, Terminals, Loading and Unloading

  Period Units CH4 NMVOC
Gas Platforms Oil Platforms Oil/Gas Platforms 1970-92 1970-92 1970-92 kt/installation kt/installation kt/installation 0.589 0.327 0.763 0.0754 0.393 0.666
All Platforms 1995-96 kt/installation 0.551 0.414
Gas Terminals Oil Terminals 1970-92 1970-92 kt/installation kt/installation 3.0 0.076 0.425 0.315
Gas Terminals Oil Terminals 1995-96 1995-96 kt/installation kt/installation 2.75 1.29 0.337 1.27
Offshore Loading 1970-96 t/kt oil 0.018 2
Onshore Loading 1970-94 t/kt oil 0.018 1
Onshore Loading 1995-96 t/kt oil 0.018 0.85
Unloading 1970-96 t/kt oil 0.018 0.1

2.6.2.5 Loading Emissions
Emissions from tanker loading and unloading have been revised in the 1996 Inventory. They are based on the tonnage of oil shipped rather than the number of platforms where loading occurs. This data is taken from DTI (1997, 1997a) and covers the amount of crude oil shipped by tanker from :

It is assumed that no emission occurs from the amounts of crude oil transported by pipeline. The emission factors are taken from UKOOA (1995) and Russell (1997) and are shown in Table A14.

Leakage from the Gas Transmission System.
The NAEI category Gas Leakage covers emissions of methane and NMVOC from the UK Gas Transmission System. The estimates are derived from specific leak rates measured on the various types of gas mains and services together with data on the total lengths and types of the gas mains and services and their upgrading schedule (British Gas, 1993). The composition of natural gas is 91.6% w/w methane and 4.5% w/w NMVOC (British Gas,1994) with the remainder being nitrogen and carbon dioxide.

2.6.2.7 Gasoline Distribution
The NAEI category gasoline distribution covers evaporative emissions from the distribution and sale of petrol. The total default emission factor is 5.5 t/kt petrol sold (Richards et al., 1984) with the following breakdown:

Tankers 0.6 t NMVOC/kt fuel
Depot Stocks 1.4 t NMVOC/kt fuel
Service Stations 1.7 t NMVOC/kt fuel
Filling Vehicles 1.8 t NMVOC/kt fuel

The default factor is used up to 1988 but for more recent years a correction is applied to account for the decrease in volatility in petrol. For example the factor for 1995 was 91% of the default. (Passant et al, 1998)

2.7 Solvent Use

 The NAEI category Solvents is based on the work of the Emission Monitoring and Abatement Department at NETCEN. The total is based on contributions from the following sectors:


The estimates are based on data on solvent use in the sectors supplied by trade associations and manufacturers. The time series is then estimated on the basis of production statistics of the various products. The methodology is described in Passant et al (1998). The breakdown into speciated categories is based on Rudd (1995).