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.
IPPC is the new permitting regime introduced in the UK to satisfy the requirements
of the EU Pollution Prevention and Control (PPC) Directive. It replaces the
old Integrated Pollution and Control (IPC) regime for the environmental regulation
of industrial installations. As part of the IPPC Permit application, operators
of an installation must quantify all point source and fugitive emissions to
air, water and land arising from the installation activities, and then assess
the potential impact of these emissions on the receiving environment.
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,
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