The Power Stations category reports as near as is possible the emissions from electricity generation by companies whose main business is producing electricity (Major Power Producers) and hence excludes autogenerators. The fuel consumption entries from DUKES are chosen to obtain the best match with this definition. The coal and natural gas entries used are very close to this definition but the fuel oil entry does contain a small contribution from transport undertakings and groups of factories. Electricity generation from the combustion of waste is reported under power stations.

In the NAEI, the Autogenerators category reports emissions from electricity generation by companies primarily for their own consumption. The inventory currently makes no distinction between electricity generation and combined heat and power or heat plants. Hence CHP systems where electricity is fed into the public system would be classified as Power Stations and CHP systems where the electricity is used by the autogenerator would be classified as Autogenerators.

The emission factors used for Power Stations are shown in Table A7. The NO_x emissions from coal and oil stations are based on estimates for individual power stations released by the Environment Agency (1998). The Environment Agency emissions are reported on a power station basis so that those from coal fired plant will include emissions from the fuel oil used to light up the boilers. A correction has been applied to the data so that the coal emissions reported in the NAEI pertain only to the coal burnt, and the oil emissions apply only to the oil burnt.

Table A7 Emission Factors for Power Stations

d Brain (1994)

f USEPA (1977)

g IPCC(1997)

p EMEP/CORINAIR(1996)

s Stewart et al (1996)

u Stewart (1997)

v IPCC(1995)

z+ Environment Agency (1999)

In 1997 two UK power stations burnt Orimulsion which is an emulsion of bitumen and water. DTI (1998) gives the UK consumption of Orimulsion. This fuel is only used by the ESI so this data was used in the category Power Stations. The carbon content of the fuel was taken from the manufacturer's specification (BITOR, 1995). The emissions of NO_x and SO₂ were taken from Environment Agency (1998) but emission factors for NMVOC, methane, black smoke and N₂O were derived from those of heavy fuel oil but adjusted on the basis of the gross calorific value. The CO emission factor is based on measured data.

Emission estimates of PM₁₀ are based on total particulate emissions submitted to the Environment Agency and the Scottish Environmental Protection Agency by the operators and take into account the operation of electro-static precipitators and installation of FGD. Total particulate emitted are assumed to comprise of 88% and 67% PM₁₀ for oil and coal respectively. Where reported data are incomplete, emission rates for the appropriate fuel are derived and applied to the remaining power stations.

Electricity has been generated from the incineration of municipal waste for some years now, though generation capacity has recently increased markedly owing to construction and upgrading of incinerators to meet new regulations by the end of 1996. Data has become available (DTI, 1998) on the amount of waste used in electricity generation and the emissions from the incinerators (Environment Agency, 1999). In previous inventories, these emissions were reported as waste disposal, but it is now possible to report the electricity generation component separately under Power Stations.

In addition to MSW combustion, the inventory now reports emissions from the combustion of scrap tyres and poultry litter to generate electricity. The tyre emissions are based on the reported plant capacity (Poll, 1998) and a carbon emission factor based on the carbon content of tyres (Ogilvie, 1995). IPCC default factors based on oil are used. In the case of poultry litter, the carbon emission factor is zero and those for other pollutants are based on Environment Agency (1999) data and IPCC(1997) defaults for biomass. The consumption of poultry litter is based on the operator's estimates of plant capacity.

Emission estimates were made from the generation of electricity from landfill gas and sewage gas (DTI, 1998). It was assumed that the electricity from this source was fed into the public supply or sold into non-waste sectors and hence classified as public power generation. The gases are normally used to power reciprocating gas (or duel-fuel engines) which may be part of combined heat and power schemes. The emission factors used were those of a 2-stroke lean burn reciprocating engine (USEPA, 1997). These engines are normally part of CHP schemes with the heat produced being used locally. DTI (1998) reports the energy for electricity production and for heat production separately. The emissions for electricity generation are categorised under public power whilst those for heat production are reported under miscellaneous for landfill gas and public services for sewage gas.

Some of the emission factors in Table A7 refer to coke, SSF, OPG, LPG and town gas which were used in small quantities in the past.

The emission factors used are shown in Table A3. Included in this category is an emission from the combustion of petroleum coke. This emission arises from the operation of fluidized bed catalytic crackers. During the cracking processes coke is deposited on the catalyst degrading its performance. The catalyst must be continuously regenerated by burning off the coke. The hot flue gases from the regeneration stage are used as a source of heat for the process. Since the combustion provides useful energy and the estimated amount of coke consumed is reported (DTI, 1998), the emissions are reported as energy emissions rather than as a fugitive emissions. Emission factors are either based on operators' data (UKPIA, 1998) or IPCC (1997) defaults for oil.