Block 8: Environmental Issues and Public Health - Air Pollution Chapter 4: The Principal Sources Of The Common Air Pollutants; Methods Of Estimating Emission Rates (Continued)

Methods of estimating industrial point source emissions:

The process of identifying all significant emission sources and estimating or measuring the emission rates (units of g/s are frequently used) of pollutants from each point source are essential prerequisites to developing a point source emission inventory, a far from trivial task.

Whilst continuous stack emission monitoring would yield a record of emission rates for each stack, this method is generally only used for large point sources, in countries where continuous emission monitoring (CEM) is mandatory (compulsory). In the absence of CEM, periodic stack measurements may be used to obtain an estimate of emission rates. However, these periodic emission rates should be used with caution - a measurement taken over a period of a few hours annually may not be representative of average operating conditions, and accurate measurements require considerable care.

Figure 4.3: Stack emissions from Consol Glass, Sacks Circle, Bellville

A widely used alternative is the use of emission factors to estimate emission rates.

The general formula for estimating emission rates is:

Emission rate (g/s) = Activity rate (kg/s or l/s) x Emission factor (g/kg or g/l).

(Other consistent units may be used.)

The US EPA’s Compilation Of Air Pollutant Emission Factors (AP-42) is a widely used reference both of emission factors and methodology. Needless to say, due care should be taken to check that the AP-42 emission factors are applicable to the specific local source, for example by checking if the fuel used, the technology installed and the stack gas cleanup systems are sufficiently similar for the AP-42 emission factors to be applicable.

In general, the use of fossil fuels (coal, fuel oil or gas (LPG or LNG)) is a major source of pollutant emissions, and emissions of the greenhouse gas carbon dioxide (CO₂). However, the pollutant profile of each of these fuel sources is markedly different, with gas being by far the least polluting.

Fuels may be compared for pollution potential on the basis of their emissions per kg or per litre, as illustrated in the Table 4.3.

Of the above fuels, LPG (liquefied petroleum gas) is the "cleanest" - it produces far lower emissions of SO₂, PM10 and VOCs compared with the other fuels, and NOx levels are similar.

Note that these emission factors are to some extent dependent on the individual fuel composition. For example, the SO₂ emissions from coal and fuel oil are a direct function of the sulphur content of these fuels. Good design and the installation of emission control devices such as precipitators or baghouses (to reduce PM10 emissions), or stack gas scrubbers (for SO₂ emissions) may reduce emissions of these pollutants by 70 to 90%.

The above emission factors may be compared to the US EPA AP-42 values.

A point source emission inventory is a database (usually electronic) of all point source emission rates (for each pollutant), together with relevant data on the location of each source, annual production rate or fuel consumption rate used as the basis for estimating the emission rate (or measured stack flow rates and concentrations) in a defined area. Questions of the completeness of the database (whether all significant sources have been included), the accuracy of the measured or estimated emission rates, and the activity pattern (whether the plant operates continuously or periodically) should be addressed in compiling the database.

An example of part of the South Durban emissions database is shown in Table 4.4 below.

Table 4.4: Part of the South Durban Emission Inventory (1998 - 2000 data)