Emission Factor of Sulphur Dioxide Content
Part 1 in this series dealt with the emissions of carbon dioxide: many of the factors that influence the emissions of carbon dioxide also apply to sulphur dioxide. As with carbon dioxide in Part 1, the ‘emission factors’ for sulphur dioxide have been derived by direct calculation involving fuel usage and composition.
The AEA Atmospheric Emission Inventory Annual Report provided a major source of data for this paper. It also provided an ‘estimated precision value’ associated with calculating sulphur-dioxide mass emissions: it is of the order of ±10% and is largely dependent on the accuracy of the monitoring of the consumption, fuel analysis and estimations of the sulphur retained in the ash.
The ‘emission factors’ established for the various types of fuels, coupled with the historical trends of changing patterns of energy utilisation in the UK, have been used to establish the corresponding changing pattern of sulphur-dioxide mass emissions over the 1970–1994 period. The changing demands for primary energy have been discussed in Part 1 of this series.
These data show the overwhelming dominance of the energy sector as the major source of sulphur dioxide and clearly demonstrate that, over this period, 99% of sulphur dioxide is generated by the energy sector. It also demonstrates that solid fuels were responsible for 75% of these emissions and oil products the remainder.
The DOE report has shown that the annual mass emission of sulphur dioxide, arising from fossil fuels, decreased from 6.37 Mtonnes in 1970 to 2.7 Mtonnes in 1994, and this paper aims at providing a detailed analysis of this decline.
The calculated theoretical SO2 emissions in grammes of SO2/kg for each of the fuel types outlined in the following table are based on the fuel consumption, heat content and on an estimate of the retained sulphur in the coal ash estimated to be 20% in the domestic sector and 10% in the industrial and commercial sectors.
Natural gas consumed in the UK contains a very low sulphur content and, in this analysis, is regarded as having an emission factor of zero.
It can be seen from Table 2 and Table 3 that the emission factors for coal are spread over the wide band from 640 to 1424 grammes/GJ corresponding to 1–1.7% sulphur content, whereas the range of emission factors for fuel oil is much narrower, i.e. 1149–1305 grammes/GJ with sulphur contents of 2.5?2.8%. The distillates are at the lower end of the emission factor spectrum of 18–178 grammes/GJ with sulphur contents ranging from <0.05 to 0.4%.
Solid fuel was the major contributor to the mass emission of sulphur dioxide in 1970 and retained its number-one position in 1994, while natural gas was a zero contributor to sulphur dioxide in 1970 and 1994.
Clearly there has been a major overall downward trend in the level of SO2 emissions from the fossil fuels. The replacement of coal and petroleum by natural gas over this period has been the major reason for the decline in emissions by all the users with the exception of transport, where there has been no fuel substitution and the demand for energy in this sector has expanded rapidly.
The corresponding percentage contribution of sulphur dioxide generated compared with the changes in fossil-fuel consumption for the years 1970 and 1994 are shown in Table 5.
The downward trend in the demand for solid fuel is reflected in the overall decline in the mass emissions of SO2 arising from this fuel.
The decline in sulphur-dioxide emissions arising from petroleum are attributed to the reduced demand for this product coupled with the reduced consumption of high sulphur-bearing fuel-oil by the manufacturing and power-generating industries and the dramatic increase in the demand for the light low-sulphur bearing fuels used in the transport sector.
Globally, SO2 emissions from fossil-fuel combustion have been seen to increase in the 20th century: this trend has been most evident in industrially developing countries during the 1970 to 1994 period. The annual SO2 emissions in the UK, as shown in Fig. 1, declined from 6.4 to 2.7 million tonnes, i.e. a 58% reduction, which is equivalent to a 2.4% annual fall. The emissions peaked in the early 1970s, but have shown a significant downward trend ever since. It can be seen from Fig. 1 that the emissions arising from solid fuels remained relatively stable up to 1987, but declined thereafter. However, the emissions from petroleum steadily declined from 1970 to 1984 and were stable thereafter, which can be explained in both cases by the change in the energy-mix employed largely in favour of natural gas. Fig. 2 shows the decline in the SO2 emission/energy unit (emission factor) indexed to 1990=100: it amounted to a 53% fall. The relationship between annual SO2 emissions to GDP calculated relative to the value at 1990 is demonstrated in Fig. 2 in that, during 1970 the SO2 emission was 18.4 tonnes per £ million of GDP at 1990 price, but by 1994 this had declined very dramatically to 5.5 tonnes per £ million of GDP. This could be accounted for by the reduced energy intensity in the manufacturing industry and the change in fuel mix to low-polluting fuels.
Fig. 3 demonstrates the changing pattern of the estimated SO2 emission-factors from 1975 to 1993. The factor for coal remains relatively stable over this period; however, there is a marked decline in the petroleum emission-factor over the 1975 to 1990 period. This is largely due to the dramatic reduction in the demand for the high-sulphur fuel-oil consumed by manufacturing industry and the increase in the demand for road-vehicle fuel, which has a low sulphur-content compared with industrial fuel-oils.
Fig. 4 outlines the estimated mass emission of sulphur dioxide arising from the individual energy-consuming sectors based on the product of the calculated or empirically-derived emission factors and annual fossil-fuel consumptions. It identifies the changing pattern in the emissions that have occurred over the period 1970 to 1994 with the major energy-consuming sectors. It can also be seen that in the power, domestic/service and industrial sectors, sulphur-dioxide emissions have declined.
Four major pieces of environmental legislation have been enacted to control sulphur dioxide during the 1970–1994 period.
Firstly, the Control of Pollution Act of 1974 included regulations under section 76 to limit the sulphur content of oil fuel (defined as any liquid-petroleum product produced in a refinery); the later EC Directive 87/219 EEC set a limit of 0.3% on the sulphur content of gas oil used as a domestic fuel.
Secondly, in 1980, a European Council Directive 80/779/EEC was enacted and came into force in 1983: it prescribed mandatory maximum values for ground-level concentrations in ?g/m3 of smoke and sulphur dioxide.
Thirdly, an EEC Directive has been enacted imposing legislative limits on the sulphur content of petrol and diesel fuels. In 1990, the standards were 0.1% for petrol and 0.3% for diesel fuel. In 1994, the legislation reduced the permissible sulphur content for diesel fuel from 0.3% to 0.2% and by 1996 both petrol and diesel fuel had a maximum of 0.05% sulphur. The road-transport sector only contributes 4% of the total UK sulphur: however such emissions are generated at ground level and not dispersed via tall chimneys and so is very relevant to public health.
Fourthly, in 1990, a European Council Directive was enacted to limit the sulphur dioxide emissions into the air from large combustion plant with thermal inputs equal to or greater than 50 MW. In response to this Directive, the UK prepared a national action plan for the reduction of SO2 emissions from these plants for the period 1991?2003. The plan was designed to limit the overall emissions from these plants, from a base line in 1980, by 15% in 1987, 21% in 1993, 45% in 1998 and 63% by 2003.
Tags: sulphur dioxide
