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 assessment

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.

Dispersion factors

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.

Habitats assessment

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

concentration modelling.

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