Specified generators: dispersion modelling assessment

Guidance on how to do detailed air quality modelling for specified generators.

Applies to England and Wales

You may be required to do detailed dispersion modelling to assess the risk to the environment from your specified generator’s proposed emissions to air.

A suitably qualified consultant will need to do the dispersion modelling assessment using fit for purpose computer software.

Check if you need to do dispersion modelling

You need to read the guidance Specified generator: when you need a permit to make sure:

  • your generator is covered by the specified generator regulations
  • you know if your specified generator qualifies for transitional arrangements

You only need to do an air dispersion modelling assessment if your specified generator requires a complex bespoke permit. You must submit the report and your modelling input files with your application.

To find out if you need to apply for a specified generator complex bespoke permit you can:

Specified generators are classed as Tranche A or Tranche B generators. See the guidance Specified generator: when you need a permit which explains the difference between Tranche A and B. Both types of specified generator will require dispersion modelling where their emissions have a negative impact on air quality.

They could be:

  • Tranche A specified generator’s with high nitrogen oxides (NOx) emissions
  • Tranche A or Tranche B specified generators located very close to sensitive receptors or in or near an Air Quality Management Area (AQMA) declared for nitrogen dioxide (NO2)

This guidance explains the specific information you need to collect to do a dispersion modelling assessment for a specified generator. You must include this information in your report. Your report will need to demonstrate how you will safeguard air quality where there are significant impacts or potential breaches of an environmental standard.

Read the guidance Environmental permitting air dispersion modelling reports which explains what information you must include in your report.

Describe the site setting

Describe the site location and expected operational life.

State the number of generators on site and the thermal capacity for each.

You must state if there are one or more of the following within 1km of the site:

  • boundary of a neighbouring local authority
  • AQMA declared for NO2
  • any other specified generator site operating more than 50 hours per year, if known

Define the operating envelope

The Environment Agency will base your site permit conditions on the period(s) of operation.

Therefore, your modelling must represent how the site operates. You must describe the operating envelope – this is the way your site operates. The description must cover:

  • all periods of potential operations
  • hourly, daily, weekly or seasonal operations
  • planned operations at certain times of the year, such as to meet balancing market contract requirements
  • testing regimes
  • typical and maximum periods of continuous operation

Your assessment report must fully describe the number and capacities of generators and how they are used at these times.

You can use temporally varying emissions using an hourly emissions file where operations are scheduled at certain:

  • times of the day
  • days of the week
  • times of the year

Operational hours

The operational hours are the number of hours the specified generator will operate per year within the described operating envelope. Make sure you have accounted for ‘typical’ and ‘maximum’ operating hours per year.

If you operate under the ‘500 hour rule exemption’ you must account for the maximum number of operational hours allowed per 12 months over the rolling average period of:

  • 5 years for existing generators
  • 3 years for new generators

The operational hours for:

  • an annual assessment is the total number of hours per year
  • a short term assessment and statistical analysis is any hour or portion of an hour that the plant operates

Characterise the emissions

The emissions you will need to model depends on the fuel the specified generator uses, for example:

  • natural gas and gas oil fuelled specified generators, such as diesel will need NOx emission modelling
  • liquid or gaseous fuels, other than gas oil or natural gas are more likely to need sulphur dioxide (SO2) and particulate emission modelling
  • specified generators fitted with secondary abatement need to consider ammonia slip emissions

You must measure emissions concentrations to get the emission rates for Tranche A specified generators. The NOx measurements must be MCERTS approved and meet the standard BS EN 14792:2017.

To accurately measure stack primary NO2 to NOx ratio you need an inert gas sampling probe. The sample line must be made of a suitable corrosion resistant material, such as stainless steel, borosilicate glass or ceramic. It must not be made from copper or copper based alloys. At flue gas temperatures more than 250°C you need to use one of the following:

  • ceramic
  • glass
  • quartz
  • titanium

Polytetrafluoroethylene (PTFE) is only suitable for temperatures lower than 200°C.

You must quote emission concentrations at reference conditions of 273.15K, 101.3kPa, 0% moisture and 15% oxygen. In the modelling assessment report you must state the:

  • actual exhaust temperature
  • oxygen and moisture content
  • exhaust volumetric flow rates
  • measured or assumed primary NO2 to NOx ratios

When you model Tranche B specified generators that must comply with an emission limit value of 190mg/Nm3 within the time period of 10 to 20 minutes, you will need to account for potentially higher emissions during this period.

Model the effect of buildings and terrain

You must model the effect of buildings and terrain on dispersion, if relevant. A building could be an engine unit body or other structure which could influence dispersion.

You need to consider the effects of building downwash when:

  • the stack height is less than 2.5 times the height of a building
  • the distance of a building from the stack is less than 5L – where L is the lesser dimension of the building height and maximum projected width (such as the distance between 2 opposing corners of a roof)
Figure showing how to measure maximum projected width of a building to work out the effects of building downwash.

How to measure maximum projected width

You need to consider terrain when slope gradients are more than 1:10 (10%) in the modelling domain.

Explain the background concentration

You must explain how the ambient background concentration data you use to calculate the Predicted Environmental Concentrations (PECs) at individual receptor locations are representative.

You need local background data at receptor locations close to other sources, such as busy roads or major industry. Low resolution grid average background values are not likely to be suitable. This is important in or near AQMAs. You can ask the local authority for advice on representative background concentrations at these receptor locations.

You must not double count the background for existing sites.

Use UK Air to see a map of the Department of Environment, Food and Rural Affairs (Defra) UK monitoring network and the monitoring sites in England and Wales near your site.

Use environmental standards for air

You need to do your dispersion modelling assessment against the short and long term NO2 environmental standards. The NO2 environmental standards for human health receptors are:

  • 40µg/m3 (microgram per meter cubed) annual mean
  • 200µg/m3 hourly mean - must not be exceeded more than 18 times per calendar year

Impact on sensitive receptors

You need to assess against these standards at sensitive receptors. Sensitive receptors are where there is relevant public exposure, including where members of the public:

  • have access
  • are regularly present
  • can be exposed for a significant portion of the averaging time of the standard

These standards do not apply where health and safety at work provisions are in place and where members of the public do not have access.

Examples of relevant public exposure include:

  • annual mean – residential properties, schools, hospitals, care homes
  • hourly mean – residential properties, schools, hospitals, care homes, hotels, gardens, busy shopping streets, bus stations, railway stations (not fully enclosed), car parks where the public may spend an hour or more

You need to include the maximum hourly (100th percentile) NO2 PEC. This is to understand the potential health effects and the amount of risk to members of the public. There is no absolute hourly limit environmental standard for the acute exposure to NO2, but there can be effects on health over a certain threshold.

Impact on protected habitat sites

You may also need to assess the NOx impacts on:

  • Special Protection Areas (SPAs)
  • Special Areas of Conservation (SACs)
  • Ramsar sites
  • Sites of Special Scientific Interest (SSSIs)
  • Marine Conservation Zones (MCZs)

You can find out if your site requires this assessment by checking if there are any protected habitats within the distances given in the table in relation to the specified generator’s rated thermal input and main fuel type used.

Minimum screening distances to the habitat sites

Fuel type used Rated thermal input (MWth) of your SG Minimum distance from your SG to a Site of Special Scientific Interest or Marine Conservation Zone in metres Minimum distance from your SG to a Special Conservation Area, Special Protection Area or Ramsar wetland in metres
Natural gas, gas oil or woody solid biomass 1 to 2 750 750
Natural gas, gas oil or woody solid biomass 2 to 5 1,000 1,000
Natural gas, gas oil or woody solid biomass 5 to 10 1,500 1,500
Natural gas, gas oil or woody solid biomass 10 to 20 2,000 2,500
Natural gas, gas oil or woody solid biomass 20 to 50 2,000 5,000
Gas other than natural gas 1 to 2 1,000 1,000
Gas other than natural gas 2 to 5 1,500 1,500
Gas other than natural gas 5 to 10 2,000 4,000
Gas other than natural gas 10 to 20 2,000 5,000
Gas other than natural gas 20 to 50 2,000 10,000
Solid and liquid heavy fuel oil 1 to 2 2,000 2,000
Solid and liquid heavy fuel oil 2 to 5 2,000 4,000
Solid and liquid heavy fuel oil 5 to 10 2,000 8,000
Solid and liquid heavy fuel oil 10 to 50 2,000 10,000

You can ask for a free habitats (nature and conservation) screening assessment to check if you meet the criteria. Use our pre-application service and select the installations form.

Alternatively, if you use the specified generator screening tool this will provide a site specific screening distance for your site.

If it is, you need to assess impacts against site relevant:

  • critical levels
  • nutrient nitrogen critical loads
  • acid deposition critical loads

You can find this information on the air pollution information system.

Emissions modelling for other pollutants

Depending on the fuel your specified generator uses and if secondary abatement is fitted you may also need to do dispersion modelling assessment for SO2, particulates and ammonia.

Find more detailed information in the guidance environmental standards for air emissions.

NOx to NO2 conversion ratios to use

In your report you will need to state which NOx to NO2 conversion ratios you used or what photochemistry parameters and any assumptions you have made.

For primary NO2 to NOx ratios of 10% or less, you can use worst case NOx to NO2 conversion ratios of:

  • 35% for short term assessment
  • 70% for long term assessment

For high NOx emission specified generators you may calculate case specific ratios. You will need to use a suitable atmospheric chemical reaction method or model. The method or model must use valid input data, including:

  • measured or assumed primary NO2 to NOx ratios
  • representative background ozone (O3), NO and NO2
  • sunlight

Use UK Air to see a map of which Automatic and Rural Monitoring Network (AURN) sites in England and Wales measure O3, NO and NO2.

Results and impact assessment

You must compare your process contributions (PCs) and PECs against the environmental standards for air, and allow for modelling uncertainties.

Your impact assessment report must include:

  • breaches of air quality standards
  • relevant contour plots
  • results tables

Short term statistical analysis

You must carry out statistical analysis if short term predictions show that 19 or more hours exceed the environmental standard at a sensitive receptor over the modelled operating envelope.

You could use the:

  • hypergeometric probability distribution
  • Monte Carlo simulation

The base parameters are the:

  • number of hours where the PEC exceeds the standard
  • number of operating hours per year
  • defined operational envelope

You must multiply the calculated probability by a factor of 2.5 where both of these apply, the:

  • statistical method assumes independent and random operational hours, for example the hypergeometric distribution
  • defined operating envelope includes continuous operations of more than an hour

You need to:

  • justify your choice of the statistical analysis method
  • calculate and discuss probability and the likelihood of exceedance

Where the probability is:

  • 1% or less – exceedances are highly unlikely
  • less than 5% – exceedances are unlikely as long as the generator plant operational lifetime is no more than 20 years
  • more than or equal to 5% – there’s potential for exceedances and the regulator will consider if acceptable on a case by case basis

Annual mean assessment

If you do an annual mean assessment you can calculate annual mean PCs by scaling down long term predictions. To get the factor to scale down by, you need to divide the number of total operational hours per year by the total number of hours in your operating envelope.

For example, for 400 operational hours per year with a full year (8,760 hours) operating envelope, long term PCs can be scaled down by a factor of 400 divided by 8,760.

You can then calculate the PECs by adding representative annual mean background concentrations.

Hourly mean assessment

You must provide proposals to reduce the risk of the exceedance, where short term statistical analysis shows potential for the hourly mean standard to be exceeded by the PEC – that is where the probability is more than or equal to 5%.

For example, you could propose to restrict the number of operating hours.

Example calculation

Specified generator air dispersion assessment example calculation

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Updates to this page

Published 15 July 2019
Last updated 27 March 2023 + show all updates
  1. 'Air emissions risk assessment': clarified that the specified generator tool can also be used for Tranche A specified generators. 'ELV compliance verification': clarified this section. 'Impact on protected habitat sites': added Marine conservation zones also need considering for NOx impacts.

  2. Added table of minimum screening distances to protected habitats for simple bespoke permits, and guidance on where to get pre-application advice and a free habitats screening assessment in England.

  3. Corrected error under 'Use environmental standards for air'. Changed measurement to µg/m3 (microgram per meter cubed) from mg/m3 (milligram per meter cubed).

  4. First published.

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