A break from the old regime
With regards to emissions to atmosphere, the impact assessment which forms part of the Integrated Pollution Prevention and Control (IPPC) regime Permit application can often prove difficult for operators to undertake in-house. Step forward Dr Richard Lowe of consultant, URS, who explains the procedures involved in assessing the impact from an installation's activities of emissions to air.
Modelling was undertaken using the UK model ADMS3. It was important to incorporate all sources that release the same pollutant into one modelling assessment. For the proposed gas-fired power station, the main emissions points were from the main gas turbine, a derivative gas turbine and an auxiliary boiler, all venting through a multi-flue stack. Screening modelling was undertaken for stack height determination, followed by detailed dispersion modelling of the emissions from several different potential process operating scenarios, including the occasional use of process gas in addition to mains gas.
Factors that affect dispersion from point sources include the presence of buildings and elevated terrain in the vicinity of the sources. In this assessment, only complex terrain was required, since the height of the stack was significantly greater than that of the surrounding process buildings. While the site lies close to the coast, coastline effects were not included in the modelling assessment, as this module of the model has not been fully validated. For dispersion modelling including the effect of buildings, there is a balance between accurately representing the site (and including every potentially significant building) and keeping the model manageable in terms of run time and complexity.
Even without the incorporation of building effects, the inclusion of three sources and complex terrain meant run-times in excess of six hours per year of hourly sequential meteorological data. The use of sequential meteorological data is preferred by the Environment Agency (EA) over statistical data as the latter tends to smooth out extreme conditions and therefore does not produce the worst case results for a particular site. A minimum of four years of meteorological data must be modelled to allow for variations from year to year.
The model predicts ground level concentrations of pollutants arising from on-site emissions on a user-defined domain around and beyond the installation boundary. This is known as the Process Contribution (PC). This concentration must then be added to representative estimates of the background Ambient Concentration (AC) for each pollutant, to give the Predicted Environmental Concentration (PEC). It is the PEC that must be compared with appropriate air quality standards, such as the National Air Quality Strategy Objectives for criteria pollutants (e.g. NO2, SO2, PM10). Different pollutants have standards based on different averaging times and the modelling allows for a direct comparison with each. It is important to stress that the maximum short-term average PC (e.g. the one-hour average) should not be added to the maximum short-term average AC. Generally the two maxima do not occur simultaneously. Typically a multiple of the annual average AC is used, and this is detailed in DEFRA Guidance for Local Authorities, for example.
From the power station, the worst case concentrations were found to be for NO2 under certain release scenarios. However, the PEC was predicted to be significantly below the air quality standard, so no further review of stack heights or abatement systems was considered necessary. In addition, it could be demonstrated that the worst case concentration significantly improved with continued operation of the plant, due to changes in operating regime.
As part of the IPPC impact assessment, an examination of the potential installation effects on Special Areas of Conservation (SACs) or Habitats sites has to be undertaken. For the client's plant, a candidate SAC was identified adjacent to the installation boundary. Consequently, an assessment of the rate of deposition of NOx and SO2 from the site air emissions on the SAC was carried out. This yielded predicted deposition rates, which could then be compared with the critical loads for the species concerned to demonstrate that the loads were not going to be exceeded on the basis of emissions from the installation.
Detailed air dispersion modelling is frequently required to quantify the potential impacts of point source releases to atmosphere from an installation undergoing permitting. Such modelling can be quite complex, requiring a detailed understanding of the emissions sources and their operating conditions, as well as local conditions such as the weather, terrain and presence of buildings that could affect pollutant dispersion. It is important that the modelling results are compared appropriately with relevant air quality standards or objectives and that an assessment of the errors, approximations and limitations of the modelling study is included.
Finally, dispersion modelling may be required to assess the impacts of installation
emissions on sensitive Habitats sites, which requires deposition as well as