2 Screening tool for domestic coal burning 2.1 INTRODUCTION The use of solid fuels for domestic heating has largely been replaced by alternative fuels throughout the United Kingdom. However, there are a few areas where there is still significant solid fuel burning, particularly where alternative fuels are not readily available. The adoption of Smoke Control Areas and the resulting use of solid smokeless fuels (SSF) in most urban areas has reduced the potential for exceedences of the proposed new PM10objectives. Nonetheless, there remain a few areas where the possibility of exceedence as the result of coal burning remains. This work was intended to help identify potential areas of exceedence.
2.2 DISPERSION MODELLING The dispersion model, ADMS Version 2.2, was used to estimate ground level primary PM10concentrations in this study. It is an up-to-date model in which the boundary layer structure is characterised by the height of the boundary layer and the Monin-Obukhov length, a length scale dependent on the friction velocity and heat flux at the surface. Concentration distributions are assumed to be Gaussian in stable and neutral conditions, but the vertical distribution is non-Gaussian in convective conditions to take account of the skewed structure of the vertical component of turbulence. It contains a number of complex modules that can allow for the effects of plume rise, complex terrain and buildings. The model is described in a User Manual [CERC, 1995].
The model was used to predict annual average ground level PM10concentrations resulting from 1 g/s primary emissions from a 1 km square area source. Surface roughness was assumed to be 1 m, typical of urban areas. An emission height of 10 m was used to represent emissions at roof level. Average meteorological data for Wyton was used in the analysis. The model was used to predict ground level concentrations on a 31 x 31, 1 km grid. The ground level concentrations resulting from larger area emissions (more than one 1 km grid square) were calculated by addition. Table 1 shows the calculated maximum ground level concentrations for a 1 g s-1 km-2 emission for a range of source sizes.
Table 1: Ground level concentrations for a 1 g s-1 km-2 emission
Source size, km x km
Ground level concentration, mg m-3
1 x 1
9.4
2 x 2
11.0
3 x 3
12.6
4 x 4
13.5
5 x 5
14.3
10 x 10
17.0
Exceedences of the proposed 24-hour mean PM10 objective (as an equivalent annual mean concentration of 28 mgm-3) are predicted to occur when there are more than N people per square kilometre in coal burning households, where N is calculated from:
N =
(28 - b)
3600 x 24 x 365
c
1000 F U
where b is the annual average background concentration, mg m-3;
c is the ground level concentration for unit emission, mg m-3, taken from Table 1;
F is the PM10 emission factor for domestic coal burning, 10.4 kg/t (Salway et al,
1996);
U is the per capita coal consumption in coal burning houses, 1.15 t/a (Abbott, 1996)
Figure 1 shows N plotted against background concentration, b, for a range of area sizes. Figure 2 shows a similar plot for SSF burning areas based on an emission factor of 2.75 kg/t and a per capita consumption of 0.76 t/a.
2.3 REVIEW AND ASSESSMENT PROCEDURE The risk of exceedence of the proposed objectives in coal-burning areas may be estimated as follows.
Determine the area, A, under consideration. Three representative 'area types' have been considered in the assistance, i.e.
a small village (approx. 1 km2 area)
a small town (approx. 16 km2 area)
a large town (approx. 100 km2 area)
Users should select the area most appropriate to their situation. Where there is doubt, the larger area should be chosen e.g. a large village would be represented as a 'small town'.
Determine the population, p, in the most populated square kilometre.
Determine the proportion of land, L, occupied by open space or farmland i.e. excluding gardens and residential roads.
Determine the proportion of households, C, burning coal. Assume 10% of houses burning solid fuel in Smoke Control Areas are burning coal.
Estimate the maximum density of people in households burning coal:
D =
p C
( 1 - L)
Determine background concentration for 2004 from the national maps. The scoping assessment will not be affected greatly by the element of double-counting introduced resulting from the inclusion of domestic emissions in the background maps.
Read off the maximum density of people in coal burning households for the appropriate background emission and area size from Figure 1. If this maximum density exceeds the value of D given above, then the risk of exceeding the objective as a result of domestic coal burning is small.
The risk of exceedence due to SSF use is thought to be less than the risk due to coal use and a graph for SSF has not been included in the report on technical information. The method has also been simplified by considering three sizes of urban areas only.
2.4 EXAMPLES
A small coal burning village.
Assume :
a population of 3000 in a 1 km square ;
60% parkland or open space in the square kilometre;
50 % of households burn coal.
then the maximum density of people in households burning coal per square kilometre is :
3000 x 0.5/(1-0.6) =3750.
In this example, assume a background annual average PM10concentration of 21 mg m-3 , gravimetric (from, for example, the national maps discussed in section 4). Then Figure 1 shows that for a small area of less than 1 km2, the method suggests a risk of exceedence if the number of people in coal burning households is greater than 2000. It is concluded that this screening model indicates that a more detailed assessment is necessary.
A town with significant coal burning
Assume :
an area of approximately 4 km x 4 km ;
a population in the most densely populated 1 km square of 8000;
30% parkland or open space in the most populated square km;
20 % of households burn coal.
then the maximum density of people in households burning coal per square kilometre is :
8000 x 0.2/(1-0.3) =2285.
Assume, in this example, also a background annual average PM10concentration of 23 mg m-3, gravimetric. Then Figure 1 shows that for an area of approximately 16 km2, the method suggests a risk of exceedence if the number of people in coal burning households is greater than 1000. It is concluded that this screening model indicates that a more detailed assessment is necessary.
A city
Assume :
an area of approximately 10 km x 10 km ;
a population of 8000 in the most densely populated 1 km square;
30% parkland or open space in the most populated square km;
4 % of households burning coal.
then the maximum density of people in households burning coal per square kilometre is :
8000 x 0.04/(1-0.3) =460.
Assume, in this example, a background annual average PM10concentration of 22 mg m-3, gravimetric. Then Figure 1 shows that for an area of approximately 100 km2, the method suggests a risk of exceedence if the number of people in coal burning households is greater than 930. It is concluded that this screening model indicates that a more detailed assessment is not necessary.
Section 1 (Introduction) Section 3 (Industrial Stacks) 