Guidance: Specified generators: dispersion modelling assessment

Natural Resources Wales

March 27
08:00 2023

You may be required to do detailed dispersion modelling to assess the risk to the environment from your specified generators 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 generators 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 250C you need to use one of the following:

  • ceramic
  • glass
  • quartz
  • titanium

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

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.

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