National Atmospheric Emissions Inventory

Appendix 1

The Methodology of the National Atmospheric Emissions Inventory


2.3 Power Stations

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 NOx emissions from coal and oil stations are based on estimates for individual power stations released by the Environment Agency (1997). The EA emissions are reported on a power station basis so 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 apply only to the coal burnt, and the oil emissions apply only to the oil burnt.

Table A7 Emission Factors for Power Stations

  Unit C1 CH4 N2O NOx NMVOC CO SO2 PM10 BS
Coal kg/t 588.23n 0.0137d 0.073h 6.93z 0.0307z 2.37z 21.6z 0.513z+ 0.25 t
Fuel Oil kg/t 850a 0.0054k 0.0243g 9.82z 0.0146k 2.6e 43.7z 2.89z+ 1.0 t
Orimulsion kg/t 597.5c 0.0163j 0.017j 8.69z 0.0437j 26e 66.2z 0.544z+ 0.832j
Gas Oil kg/t 857a 0.0432b 0.026g 2.75z 0.0865b 0.473z 3.25z 5.5 0.75t
Natural Gas g/th 1501r 0.579g 0.351u 4.21z 0.510m+ 1.55z 0 0.034 NE
Sour Gas g/th 1916s 0.0228p 0.0095g 10.4z 0.0912s 12s 3.85z NE NE
Coke kg/t 795.4 n 0.0534b NE 4.8b 0.0801b 6.6f 19w 0.288 0.225t
SSF kg/t 766.3n 0.0528b NE 8.64b 0.0792b 6.6f 19w 0.23 0.225t
MSW kg/t 75l 0.0008o 0.148p 1.96z 0.018z 0.245z 0.50z 0.209z+ 3.29
LPG g/th 1874 a 0.0971m 0.0095g 9.5b 0.194m 0.25i 0 0.295 NE
OPG g/th 1627a 0.324b NE 13.6b 0.647b 0.25i 0 0.295 NE
Landfill/ Sewage Gas g/th NE 64.9m NE 94.6m 4.99m 17.4m 0 NE NE
Town Gas g/th 1599 y 0.38b NE 9.5m 0.380b 0.25i 0 NE NE

1 Emission factor as kg carbon/ t fuel
a UKPIA (1989)
b CORINAIR (1992)
ccBITOR(1996)
dStewart et al. (1996) estimated from total VOC factor assuming one third is methane after CORINAIR (1992)
e Stewart et al (1996)
fUSEPA (1977)
gIPCC(1997)
hFynes et al. (1994)
i Walker et al. (1985)
j As fuel oil but adjusted on basis of gross calorific value
k Stewart et al. (1996) estimated from total VOC factor assuming 27.2% is methane after USEPA(1995)
l Royal Commission on Environmental Pollution (1993)
mmUSEPA(1995)
m+ USEPA(1995) estimated from total VOC factor using methane factor given
n British Coal (1989)
oEstimated from THC data in CRI(Environment Agency, 1997a) assuming 3.3% methane split given in EMEP/CORINAIR(1996)
pEMEP/CORINAIR(1996)
gStewart et al. (1996a) estimated from total VOC factor assuming 20% is methane after CORINAIR(1992)
r British Gas (1992)
sStewart et al (1996)
t Keddie et al. (1978); Timmis et al. (1988)
uStewart (1997)
wMunday (1990)
yBritish Gas (1988)
z Based on reported emissions data from CRI and Station Operators (1997)
z+Reported particulate emissions and assumed PM10 fractions (1995)

Currently two UK power stations burn Orimulsion which is an emulsion of bitumen and water. DTI (1997) 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 NOx and SO2 were taken from Environment Agency (1997) 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. Data has become available (DTI, 1997) on the amount of waste used in electricity generation and the emissions from the incinerators (Environment Agency, 1997a). 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.

Emission estimates were made from the generation of electricity from landfill gas and sewage gas (DTI, 1997). 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, 1995).

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.

2.4 Solid Fuel Transformation

 2.4.1 Introduction

The fuel consumptions for these categories are taken from DTI (1997). The emissions from simple combustion sources in these industries are calculated as in the base combustion module. However, sources involving fuel transformation require a more complex treatment. The problem in estimating emissions from these sources is that fuels are manufactured from other fuels producing process emissions and that subsequently the fuel is burnt elsewhere producing further emissions. Hence it is necessary to define a method of allocating the emissions to the various possible sources without double counting. The approach used by the NAEI is based on a carbon balance over coke production, SSF production, blast furnaces and town gas production. The emission of nitrous oxide was not estimated from these sources.

The processes involved are:

Coke Production

  coal ® coke + coke oven gas + carbon emission

SSF Production

  coal ® SSF + carbon emission

Town Gas Production

  coal ® town gas + carbon emission

Iron and Steel Blast Furnaces

  coke ® blast furnace gas + carbon emission + carbon in steel

Hence by estimating the carbon content of the coal or coke consumed in these processes and the carbon content of the coke, coke-oven gas, blast furnace gas, steel, town gas and SSF produced, the carbon emission from each process can be calculated. This approach is in accord with IPCC Guidelines (IPCC, 1997).

A similar procedure could be applied to sulphur dioxide emissions, however this is complicated by the cleaning of the blast-furnace gas and coke-oven gas produced and possible retention of the sulphur by blast furnace slag. Hence in practice, the treatment of sulphur dioxide depends on the information available and is unique to each process.

2.4.2 Coke Production

Applying the same nomenclature as that used in Section 1.3.2, the emission of carbon from coke production is:

  E(car,coke prod,c) = A(coke prod,c) × e(car,coke prod,c)
      - cout1
where
  cout1   = A(coke made,ck) × e(car,coke made,ck)
        + E(car,coke prod,cog)
        + E(car,I&S, cog)
        + E(car,collieries,cog)
        + E(car,other industry,cog)
        + E(car,TG prod,cog)
and
  A(coke made,ck) = Mass of coke made (kg)
  e(car,coke made,ck) = Carbon content of coke made (kg/kg)
  car = Carbon
  ck = Coke
  cog = Coke oven gas
  c = Coal

The carbon contents of coking coal and coke made are 710 kg/t and 820 kg/t respectively.

For emissions of other pollutants, it is assumed that emissions arise only from the combustion of the blast furnace gas and coke oven gas produced. The SO2 emission factors used, are derived from CRI data (Environment Agency, 1997a) and will reflect gas cleaning. The emission factors are given in Table A6. Process emissions of NMVOC are estimated on the basis of 0.424 kt/Mt of coke produced based on CRI data (Environment Agency, 1997a).

The inventory also includes emissions from the flaring of coke oven gas. These are based on data on coke oven gas losses given in DTI(1997) and combustion emission factors given in Table A6.

2.4.3 SSF Production

The emission of carbon from SSF production is:

  E(car,ssf made,c) = A(ssf prod,c) × e(car,ssf prod,c)
      - A(ssf made,ssf) × e(car,ssf made,ssf)
where

  A(ssf made,ssf) = Mass of SSF made (kg)
  e(car,ssf made,ssf) = Carbon content of SSF made (kg/kg)
  ssf = Solid smokeless fuel

For sulphur a similar mass balance as that for carbon is applied. The same equations are used but substituting SO2 emission factors for carbon ones.

Other pollutants are dealt with using a factor reflecting the ratio of the total carbon input to the process to the amount of carbon emitted:

  cin = A(ssf prod,c) ´ e(car, ssf prod,c)
      + A(ssf prod,ck) ´ e(car, ssf prod,ck)
      + A(ssf prod,ng) ´ e(car, ssf prod,ng)
  factor2 = 1 - A(ssf made,ssf) ´ e(car,ssf made,ssf)
      cin
where

  ng = Natural gas

The emission is then calculated as

  E(p,ssf prod,c) = A(p,ssf prod,c) ´ e(p,ssf prod,c) ´ factor2

Here e(p, ssf prod,c) is the SSF production emission factor and is based on a default value for coal combustion and is given in Table A5.

2.4.4 Town Gas Production

The procedure for town gas is different from the other fuel transformation industries because it is clear from the level of consumption of LPG, natural gas and coke oven gas relative to the production of town gas that much of these fuels were added to the supply of town gas made from coal and oil. The procedure adopted was to perform a carbon balance on the fuels consumed by town gas manufacture and the town gas produced. The carbon emissions were then allocated proportionately to the town gas production fuel consumption categories. The computational scheme is:

  E(p, TG Prod, f) = A(TG Prod, f) ´ e(p, TG Prod, f) ´ factor3

where
  factor3 = 1 - cout
      cin

and        
  cin = å A( TG Prod, f) ´ e(car, TG Prod, f)
    f
  cout = å A(s, tgas) ´ e(car, s, tgas)
      s

where

  A(TG Prod, f) = Usage of fuel f in town gas production (kg)
  e(p, TG Prod, f) = Emission factor of pollutant p from fuel f in town gas production (kg/kg)
  e(car, TG Prod, f) = Carbon content of fuel f in town gas production (kg/kg)
  A(s, tgas) = Usage of town gas in source s (therm)
  e(car, s, tgas) = Emission factor of carbon from town gas in source s (kg/therm)
  tgas = Town gas

Emissions of sulphur dioxide are calculated using the simpler scheme:

  E(SO2, TG Prod, f) = A(TG Prod, f) × e(SO2, TG Prod, f)

2.4.5 Iron and Steel Blast Furnaces

The carbon emission from blast furnaces is calculated as:

  E(car,I&S BF,ck) = A(I&S BF,ck) × e(car,I&S BF,ck)
      - E(car,coke prod,bfg)
      - E(car,I&S,bfg)
      - A(I&S BF,steel made) × e(car,I&S BF, steel made)

where

  A(I&S B F,ck) = Blast furnaces coke consumption (kg)
  A(I&S BF,steel made) = Crude steel production (kg)
  e(car,I&S BF, steel made) = Carbon content of steel made (kg/kg)
  bfg = Blast Furnace Gas

The final term in the equation is the carbon sequestrated in the steel produced. It is assumed that the carbon content of steel is around 1.7 kt C/Mt crude steel. This is a very approximate estimate but the carbon sequestrated is very small compared with the other terms.

For other pollutants the emissions are estimated based on the methodology described in IPCC(1997) for blast furnace charging and pig iron tapping. The emission factors are expressed in terms of the emission per Mt of steel produced and are given in Table A8. Data on steel production is reported in ISSB(1997).

Table A8 Emission Factors for Blast Furnaces (kt/Mt Steel)

Pollutant Blast Furnace Charging Pig Iron Tapping Total
NOx - 0.076 0.076
CO 1.33 0.112 1.442
NMVOC 0.1 0.02 0.12
SO2 - 0.03 0.03