The NAEI reports emissions of methane from coal mining in the categories Deep Mined Coal; Coal Storage and Transport; Open Cast Coal. Emissions are calculated from saleable coal production statistics reported in DTI, (2000). Data on the shallower licensed mines are not published and were supplied to us by Barty (1995) up to 1994. Licensed mines referred to privately owned mines and were generally smaller and shallower than the nationalised mines. The distinction was sufficiently marked to allow the use of a separate emission factor. Following privatisation, the distinction between licensed mines and deep mines no longer exists. For 1995, data from 1994 were used but in subsequent years the distinction has been abandoned. The emission factors used are shown in Table A16.

a Bennet et al (1995)

b Value for 1998 assumed

c Estimated factor based on RJB (2000a) data.

d Williams (1993)

The licensed and open cast factors are taken from Williams (1993). The deep mined factors for 1990 -1992 and the coal storage factor are taken from Bennet et al (1995). This was a study on deep mines which produced estimates of emissions for the period 1990-93. This was a period over which mines were being closed, hence the variation in emission factors. The emission factors for 1998-99 are based on operator's measurements of the methane extracted by the mine ventilation systems. The mines surveyed cover around 90% of deep mined production. No time dependent data are available for 1993-97, so the 1998 factor was used. Methane extracted is either emitted to atmosphere or utilized for energy production. Methane is not flared for safety reasons. The factors reported in Table 1 refer to emissions and exclude the methane utilised. The coal storage and transport factor is only applied to deep mined coal production.

These estimates only include emissions from working mines and exclude closed mines. It was previously believed that emissions became negligible once a coal mine closed, particularly if the mine flooded (Williams, 1993). However, a recent review sponsored by DETR, suggests that these emissions may be significant. (Sage, 2001). A number of estimates have been made ranging from 20 to 300 kt methane in 1999. More research is required to obtain a reliable estimate.

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 emission factors are used for 1990 and post-1992 emission factors for 1993 onwards. In earlier inventories, emissions from the storage and transport of coal were aggregated with those from deep mined coal but now they are split. The coal storage and transport factor is only applied to deep mined coal production. Emissions from solid fuel transformation plants are negligible (Williams, 1993)

The emissions estimates for the offshore industry are based on the UKOOA sponsored SCOPEC Inventory (SCOPEC, 2000). This is a detailed inventory based on returns from the operating companies for the years 1995-1999. For years prior to 1995 the estimates are based on the UKOOA study for 1991 (UKOOA, 1993). This source was used for previous inventories but was retrospectively revised by UKOOA to incorporate new data on flaring and loading emissions. (Russell, 1997). The emissions reported in these studies pertain to the offshore platforms and onshore terminals on the UK Continental Shelf Area and represented by UKOOA.

UKOOA have produced guidelines for estimating and reporting emissions. (E&P Forum, (1994), UKOOA (1995)). The operators are required to report flaring and venting volumes and compositions. Methodologies have been developed to estimate combustion emissions from turbines, boilers and heaters and fugitive emissions. These can be calculated on a plant item by item basis. Where detailed data are not provided estimates are made on the basis of oil and gas throughput. However, the proportion of emissions estimated on throughput has fallen and in the 1999 inventory, no estimates were made in this way.

The data reported in the SCOPEC database must be reconciled with the UK Energy Statistics and integrated into the NAEI without double counting emissions. The data available only cover 1995-99 and 1991 and must be extrapolated to other years to produce a time series. The diesel oil consumption by offshore installations is not reported separately in the UK Energy Statistics 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, offshore emissions are estimated in the following categories each with its own methodology:

• Offshore flaring
• Offshore Oil & Gas (well testing)
• Offshore Oil & Gas (venting)
• Offshore Oil & Gas
• Offshore Loading
• Onshore loading
• Storage
• Offshore own gas use
• Gas Separation Plant

This includes flaring from offshore platforms and onshore terminals. Flaring emission data for CO_2, SO₂, NO_x, CO, NMVOC, N₂O and methane were taken from the SCOPEC (2000) and UKOOA (1993) studies for 1995-1999 and 1991 respectively. These emissions were extrapolated to other years using the data on flaring volumes reported in DTI (2000). This involved calculating aggregate emission factors (kt pollutant /Mm³ gas flared). The SCOPEC estimates were used for 1995-1999 and the UKOOA estimate for 1970-1994. The aggregate emission factors are given in Table A17a. Emissions of nitrous oxide were estimated using a factor of 0.000088 kt/Mm³ based on SCOPEC (2000).

The UKOOA/SCOPEC data do not include flaring on onshore oil production fields. These emissions are added based on data collected by DTI (2000a) and the offshore flaring factors. The contribution is very small.

In addition, an estimate of NMVOC emissions from refinery flares is based on estimates supplied by UKPIA (2000), see Section 2.6.15

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 CO_2, SO₂, NO_x, CO, NMVOC, N₂O and methane were taken from the SCOPEC (2000) and UKOOA (1993) studies for 1995-1999 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 fuel gas consumption by the offshore industry reported in DTI (2000). The DUKES category ‘producers own use’ is mostly if not wholly natural gas used on offshore installations for running turbines, engines, heaters and boilers. The DUKES category oil and gas extraction refers to ethane, propane and butane used in gas separation plants at oil terminals. It is now clear that the fuel gas emissions reported by SCOPEC include methane and natural gas liquids, hence it correct to match these emissions with the natural gas, ethane, propane and butane data reported in DTI (2000). This involved calculating aggregate emission factors (kt pollutant /Mtherm) for fuel gas defined as natural gas, LPG (butane and propane) and OPG (ethane). The SCOPEC estimates were used for 1995-1999 and the UKOOA estimate for 1970-1994. The aggregate emission factors are given in Table A17b. The SCOPEC inventory does not consider PM₁₀ , however ann estimate of PM10 from gas combustion is made using a USEPA(1997) factor for gas turbines.

Emissions of nitrous oxide were estimated using a factor of 0.000516 kt/Mtherm fuel gas based on SCOPEC (2000). These emission factors apply to the mixture of methane, ethane, propane and butane used. In the NAEI database they are applied to the categories:

• Offshore own gas use: natural gas
• Gas separation plant: LPG
• Gas separation plant: OPG

In previous inventories, it was assumed that the propane, butane and ethane consumption pertained to refineries rather than terminals. This assumption resulted in a double count, which has now been removed.

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, NO_x, CO, NMVOC and methane were taken from the SCOPEC (2000) and UKOOA (1993) studies for 1995-1998 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 1995-1999 and 1991 were extrapolated to other years using data on the numbers of wells explored, reported in DTI (2000b). 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 are not collected. The SCOPEC estimate was used for 1995-1999 with the 1995 factor used for all previous years. The aggregate emission factors are given in Table A17c. Well testing is reported under 1B2a Oil Production since many of the wells produce oil and gas.

These include emissions of methane and NMVOC from platforms and terminals arising from cold venting and fugitive emissions and also from storage of crude oil at terminals. The approach adopted was determined by the data available. For the years 1995-1999, detailed estimates from the SCOPEC (2000) survey were used.

For the years 1970-1992 estimates were extrapolated. This involved extrapolating the 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 were taken from DTI (1997) and Oilfield Publications (1996). 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 estimated in Sections 2.7.2.1 and 2.7.2.2 were subtracted. The emissions for the years 1993 and 1994 were then estimated by interpolation.

Other emissions from platforms and terminals are reported in the NAEI category offshore oil & gas. It is not possible to split oil and gas production emissions since oil and gas are frequently produced on the same platform. From 1995 onwards, the offshore oil & gas category contains emissions of fugitives and processes only. Venting and storage are reported in their respective categories.

Table A17d Aggregate Emission Factors used for Emissions from Platforms and Terminals

Emissions from tanker loading and unloading were taken from the SCOPEC (2000) study for 1995-99. They are based on the tonnage of oil shipped and site specific emission factors, which account for recently applied abatement measures. However data for the North Tees terminal has been replaced by alternative, more accurate, numbers provided by the operator. Data for previous years were estimated in a similar manner using oil shipment data taken from DTI (2000, 2000b) and previous issues. These cover the amount of crude oil shipped by tanker from:

• production sites to UK users and export
• onshore terminals to UK users and export

It is assumed that no emission occurs from the amounts of crude oil transported by pipeline. Default emission factors were used for earlier years and are taken from UKOOA (1995) and Russell (1997) and are shown in Table A18. The emission factors for 1999 are aggregate factors for all installations and include abatement measures and are based on SCOPEC(2000).

The NAEI category Gas Leakage covers emissions of methane and NMVOC from the UK Gas Transmission and Distribution 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 (British Gas, 1993, Transco, 2000). The emissions for 1997-99 are based on the results of a new model (Transco 2000) which reflects recent conditions in the gas industry more accurately and supercedes the earlier model. The emissions from 1990-96 are based on the British Gas model which provided historical data for 1991-94 but projected estimates for 1995-96. The estimates of the new model were used to replace the old model estimates for 1997-98 and are slightly higher. The composition of natural gas assumed for 1990-96 is 91.6% w/w methane (British Gas, 1994) and that assumed for 1997-99 is 88.18% w/w methane, Transco (2000). An extra emission of 40 kt methane has been included to account for high-pressure mains which were not included in the British Gas and Transco models. (DETR, 1999)

Emissions during the storage, distribution, and sale of petrol are covered by seven NAEI source categories, each of which is further divided into emissions of leaded and unleaded petrol. The source categories are:

Emissions would also occur from storage tanks at refineries. This source is included together with emissions from the storage of crude oil and other volatile materials in the NAEI source category ‘refinery (tankage)’ – see section 2.6.16

The estimate for road and rail tanker loading at refineries is supplied by UKPIA. The remaining estimates are based on methodologies published by the Institute of Petroleum (reference) or, in the case of petrol terminal storage, based on methods given by CONCAWE (1985). The calculations require information on petrol density, given in DUKES, and petrol Reid Vapour Pressure, data for which have been obtained from a series of surveys carried out by the Associated Octel company between 1970 and 1994. The methodology also requires assumptions to be made on the level of vapour recovery in place at terminals and petrol stations. These assumptions have been made taking into account a survey carried out by Petroleum Review (2000) which included questions on petrol station controls, and the timescales recommended in Secretary of State’s Guidance for petrol terminals (PG 1/13 (97).