Guidance: Emergency backup diesel engines on installations: best available techniques (BAT)

Environment Agency

August 21
12:32 2023

This guidance applies to emergency backup diesel engines that are:

  • on combustion installations Part A 1.1 activity greater than 50 megawatt thermal (MWth)
  • part of an Industrial Emissions Directive (IED) installation as a directly associated activity
  • both static and mobile this guidance does not apply if the mobile plant is temporary

This guidance gives:

  • build standards for diesel engines which minimise emissions of oxides of nitrogen (NOx)
  • ways of operating diesel engines that minimise impacts on air quality

Examples of use

Diesel engines are used as emergency backup generation on IED installations. This can be:

  • for black start restoring power without relying on the external electric power transmission network
  • as emergency backup power supply at data centres
  • for emergency backup power supply on manufacturing sites
  • in the electricity balancing market occasionally, when NOx is abated
  • for power on large construction sites

Unabated diesel engines emit high levels of NOx. They can breach short term air quality standards within a few hours of operation, especially where significant capacity is installed.

Regulatory requirements

Where appropriate, as a minimum you must meet the requirements of:

  • Schedule 25A of the medium combustion plant directive
  • Schedule 25B of the specified generators regulation

To do this, check the guidance on medium combustion plant and specified generators: environmental permits.

Build standards

Engines must be optimised to reduce emissions (emissions optimised). Engines that are optimised to reduce fuel (fuel optimised) have greater emissions and will not meet BAT unless they have secondary abatement.

Combustion plant specification sheets that keep to one or more of the former 2g TA Luft and United States Environment Protection Agency (EPA) Tier 2 (or equivalent) standards are acceptable proof of BAT plant. These do not need on-site exhaust emission monitoring.

If you can show your engine achieves the following guidance level (which is not an ELV compliance requirement), it can be considered emissions optimised.

Approximately 750mg per m3 NOx (as NO2) at 15% O2 standard temperature and pressure, dry, 273K and 101.3kPa (equivalent to 2,000mg per m3 at 5% O2 commonly termed 2g) at a typical emergency load (usually greater than 67% of standby power rating).

You should send copies of your engine specification sheets when you apply for your permit, as these will provide evidence of these requirements and proof of BAT. The Environment Agency may consider greater than 2g diesel engines for technical reasons. For example, standby diesels on nuclear installations that are critical for safety. But they should be the best available.

Your stack design should ensure good flue gas dispersion. Stacks should be vertical and emissions should not be obstructed by caps or cowls.

Operational controls

Minimise how much you test diesel engines. You must test for less than 50 hours a year.

Avoid testing engines when the air quality is poor.

Do not test more than one engine at a time.

When using backup diesel generators, you must manage the impacts on air quality to minimise harm to human health and the environment.

Contact the Environment Agency

You can request more guidance on operating practices for data centres.

General enquiries

National Customer Contact Centre
PO Box 544
S60 1BY


Telephone 03708 506 506

Telephone from outside the UK (Monday to Friday, 8am to 6pm GMT) +44 (0) 114 282 5312

Monday to Friday, 8am to 6pm.

Published 21 August 2023

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