Medium combustion plant (MCP): comply with emission limit values
Source: gov.uk (2025-07-09)

When the MCP is exempt from meeting emission limit values (ELVs), which ELVs apply and the deadlines for meeting them.

If you need a permit, before you apply you must check what ELV your MCP must meet.

This guidance explains:
- the ELV the MCP must meet, based on its type, such as if it’s new or existing, the fuel it burns and technology it uses
- if the MCP is exempt from meeting ELVs
It also explains when the MCP may need to meet a stricter ELV, such as when the MCP, including a mobile MCP deployment, is located within:
- a local authority Air Quality Management Area (AQMA) air quality zone
- the minimum distance to a habitats site
If your MCP is also a specified generator see the ELVs and controls that apply to specified generators (https://www.gov.uk/guidance/specified-generator-comply-with-permit-conditions).

If you have a MCP that’s already permitted as part of a Part A1 Industrial Emissions Directive (IED) activity (https://www.gov.uk/guidance/medium-combustion-plant-and-specified-generators-environmental-permits#ied-chapter-2-permits-affected-by-the-regulations) you will need to check which ELVs you must meet by what date.

You will need to do monitoring to demonstrate you can meet the ELVs set in your permit conditions. See the:
- deadlines for meeting ELVs (https://www.gov.uk/guidance/medium-combustion-plant-when-you-need-a-permit#permitting-and-compliance-dates)
- guidance on how to comply with your MCPD permit (https://www.gov.uk/guidance/medium-combustion-plant-and-specified-generator-permits-how-to-comply)
Minimum ELVs your MCP must comply with

A new MCP must meet ELVs from 20 December 2018 or before you commission your new MCP.

MCPD sets the minimum ELVs your MCP must comply with. See the MCPD Annex 2 tables (https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32015L2193#d1e32-15-1). For:
- a new MCP that is not an engine or gas turbine, the ELVs are listed in part 2, table 1
- a new MCP that is an engine or gas turbine, the ELVs are listed in part 2, table 2
- an existing MCP that is not an engine or gas turbine, between more than 5 and less than 50MWth the ELVs are listed in part 1, table 2 – it must meet these by 1 January 2025
- an existing MCP that is not an engine or gas turbine, between greater than or equal to 1 and less than 5MWth the ELVs are listed in part 1, table 1 – it must meet these by 1 January 2030
- an existing MCP between 1 and 50MWth which is an engine or gas turbine, the ELVS are listed in part 1, table 3 – it will depend on its capacity as to what deadline applies
You must refer to the notes with the tables.

ELV to use for MCP using 2 or more fuels

How to calculate ELVs for this type of MCP.

Co-fired and multi-fuelled MCPs

To calculate the ELV for one MCP which uses 2 or more fuels at the same time, you will first need to determine the:
- MCP’s total rated input (net calorific value) using Regulatory Guidance Note (RGN) 2 (https://www.gov.uk/government/publications/rgn-2-understanding-the-meaning-of-regulated-facility)
- fuel consumption for the monitoring period
You then need to determine the fuel weighted ELV based on the:
- Annex 2 ELV for each fuel
- thermal input delivered by each fuel
For fuel 1:
- multiply the relevant Annex 2 ELV, corrected to the same common oxygen concentration where necessary, by the fuel’s thermal input (find the oxygen correction formula in Part 7 of the Industrial Emissions Directive)
- divide the sum by the MCP’s total rated thermal input
Do this calculation for each fuel. Then add together each fuel’s contribution to get the MCP’s aggregated ELV.

If one of the fuels is a gas or liquid, there may not be an ELV for some pollutants. For example, there will be negligible SO2 and particulate emissions from natural gas, therefore the ELV contribution from that fuel is zero.

You normally need to apply for a bespoke permit for this type of MCP.

Dual fuel MCPs

Two different ELVs apply where different fuels are fired on the same MCP separately. For example, where liquid fuel is used as a backup fuel if the natural gas supply is interrupted.

You normally need to apply for a bespoke permit for this type of MCP.

Gas turbine with supplementary firing

Two different ELVs apply where a gas turbine has a supplementary fired waste heat recovery boiler (WHRB) – one for the gas turbine and one for the boiler.

Where both the gas turbine and boiler are being fired, or the gas turbine alone, the gas turbine ELV applies at 15% O2.

Where the WHRB is being fired alone (auxiliary mode), the boiler ELV applies at 3% O2.

ELV to use for new fuels

ELVs for MCP where fuels are not natural gas, gas oil or solid biomass will be set in line with the New Fuels Direction (https://www.gov.uk/government/publications/the-environmental-protection-england-medium-combustion-plant-new-fuel-direction-2024). You can use these ELVs for the following fuels.

Landfill gas engines

MCPD does not include landfill gas as a fuel in a combustion plant. The Environment Agency and Natural Resources Wales will accept the SO2 ELV for biogas.

Hydrogen

Use guidance on Hydrogen Combustion: How to comply with emission limit values (https://www.gov.uk/guidance/hydrogen-combustion-comply-with-emission-limit-values).

Hydrotreated vegetable oil (HVO)

HVO is a drop-in fuel replacement for gas oil. You can use gas oil ELVs. You do not need to vary your environmental permit to use HVO instead of gas oil where you meet the requirements of Using drop-in fuels in combustion plant instead of gas oil: RPS 337 (https://www.gov.uk/government/publications/using-drop-in-fuels-in-combustion-plant-instead-of-gas-oil-rps-337).

For the combustion of new fuels not listed you should Get advice before you apply for an environmental permit (https://www.gov.uk/guidance/get-advice-before-you-apply-for-an-environmental-permit).

ELV exemptions

Check if your MCP is:
- exempt from meeting ELVs
- required to meet relaxed ELVs or there’s a delay to meeting them
Less than 500 operating hours per year exemption

Existing MCPs operating less than 500 hours per year as a 5 year rolling average are exempt from meeting MCPD ELVs.

New MCPs operating less than 500 hours per year as a 3 year rolling average are exempt from meeting MCPD ELVs.

If a dual fuel MCP uses its backup fuel in an emergency, or for less than 50 hours testing in a year, the MCP is exempt from meeting the ELVs that apply to the backup fuel.

A MCP that is also a Part B activity must comply with MCPD ELVs even if the MCP operates for less than 500 hours per year on a:
- 5 year rolling average – existing
- 3 year rolling average – new
However, you do not need to monitor every year. Instead, you must monitor every:
- 1,500 operating hours for MCP 1 to 20MWth
- 500 operating hours for MCP greater than 20MWth
Or every 5 years, whichever comes first.

Extension to 1,000 hours

You can extend the 500 operating hours per year to 1,000 operating hours per year when you use an emergency or standby MCP:
- for backup power generation in islands when the power supply is interrupted
- to supply heat in exceptionally cold weather – the Department for Environment, Food and Rural Affairs (Defra) or Welsh Government will advise the Environment Agency or Natural Resources Wales when they class cold weather as exceptional
For dust the ELV of 200mg/Nm3 applies for plants firing solid fuel.

How to calculate operating hours

Operating hours start from when the MCP is operating and discharging emissions to air. The clock starts ticking at the end of start up and stops at the beginning of shut down.

The regulators have adopted the IED rule on start up and shut down. This defines a minimum start up load and a minimum shut down load for stable generation. Below this the combustion unit cannot deliver:
- its output to the National Grid safely and reliably
- useful heat or electricity to an industrial or commercial site
See the detail in the Joint Environmental Programme (JEP) guide: IED compliance protocol for utility boilers and gas turbines (https://www.energy-uk.org.uk/publications/jep-ied-compliance-protocol-lcp-bref-update-2022/).

For each MCP you must record its start up and shut down value. This means one of the following:
- the start up and shut down thresholds as a fixed percentage of rated output
- another separate criterion, for example a physical state of the MCP such as a valve opening which indicates stable operation
You must keep start up and shut down periods as short as possible.

Manufacturers usually recommend a minimum percent load for their engines depending on the fuel. For example, typically 30% for diesel engines. The regulators will accept this where the value is not determined.

If the MCP has multiple flues, the start time begins once the first flue passes its start up threshold.

For gas turbines, ELVs apply above 70% full load, so the start up and shut down value is 70% full load.

The first 12 month period starts on the day the permit is issued. You need to add up the operating hours of the first 5 or 3 year period and take the average. Thereafter the following 12 month period is added and the first 12 month period dropped off and you calculate a new average.

If your MCP qualifies for a 500 hour exemption it can run for more than 500 hours per 12 month period but it must not exceed:
- 2,500 over 5 years
- 1,500 over 3 years
The maximum number of operational hours allowed in a single 12 month period is 750:
- before the MCP’s 5 or 3 year rolling operational period starts
- if the MCP closes before the 5 or 3 year operational period ends
You must keep records of each 12 month period of operational hours for each MCP for 6 years.

You must tell the Environment Agency or Natural Resources Wales if any MCP exceeds the 500 hour annual operational limit in one 12 month period. You must prove that you have not exceeded the 500 hours rolling average over 5 or 3 years.

Gas compressor stations

Gas turbines more than 5MWth used to drive gas compressor stations on the gas national transmission system are exempt from meeting ELVs until 1 January 2030.

The compressor must be for natural gas on the National Grid and their operation must be essential to the safety and security of national gas supply.

Fuel supply interruption

When there’s a shortage of fuel supply or it’s stopped or interrupted, the regulator can allow a temporary derogation from obligations to comply with your emission limit value. If you normally operate your MCP with:
- low sulphur fuel and cannot because of an interruption to fuel supply – you can get a temporary derogation for up to 6 months
- natural gas and you need to fire on standby liquid fuels (which would need secondary abatement to meet MCPD ELVs) – you can get a temporary derogation for up to 10 days – this may be longer but only where there is a demonstrable overriding need to maintain energy supplies
You must contact the Environment Agency (#contact), or contact Natural Resources Wales by emailing xxx@xxx.xx (mailto:xxx@xxx.xx), as soon as possible and within 10 days of using the alternative fuel. You must:
- tell them why you need (or needed) to use an alternative fuel and how long you are likely to do this for
- provide evidence of fuel interruption and why you must maintain energy supplies
The Environment Agency or Natural Resources Wales:
- will confirm acceptance in writing
- must immediately inform Defra or Welsh Government
MCP located near a habitats site

A habitats site can be a:
- special area of conservation
- special protection area
- Ramsar site
- site of special scientific interest
- marine conservation zone
If your MCP is of low risk to air quality and can meet the rules in a standard rule permit, it sets the minimum requirements for the distance from the habitats site.

Natural Resource Wales do not issue standard rule permits anymore – only bespoke.

The limits that apply to your MCP will depend on the fuel it uses and its technology.

If your MCP is of higher risk to air quality you will need to apply for a bespoke permit and may need to do an air emissions risk assessment of impacts to nature conservation sites. Use the guidance Medium combustion plant: apply for an environmental permit (https://www.gov.uk/guidance/medium-combustion-plant-apply-for-an-environmental-permit).

In Wales, use this guidance on air emissions risk assessment (https://naturalresources.wales/permits-and-permissions/medium-combustion-plants-and-specified-generators).

Dark smoke

The MCP must not persistently emit dark smoke. See the guidance on how to identify dark smoke (https://www.gov.uk/preventing-air-pollution/dark-smoke).

Stack arrangements

Vertical stacks that are not obstructed by caps and cowls allow for the greatest dispersion of air pollutants.

You should use this design for:
- new MCP as required by the available standard rules permits
- existing MCP if their stack arrangements can be redesigned
You may need to comply with other permissions such as planning requirements.

Monitoring requirements

For a new MCP you must start monitoring its emissions within 4 months of the permit being issued or the start of operation, whichever is the latest.

For an existing MCP the Environment Agency and Natural Resources Wales will accept previous monitoring results as evidence of compliance with the relevant ELVs provided:
- you can prove which MCP was monitored
- the monitoring was taken within 2 years before the permit application date
- the monitoring was done to the relevant standard
For other existing MCP you must start monitoring:
- within 4 months of the permit being issued for standard rules permits
- before the compliance deadline for bespoke permits
You must have your permit issued by:
- 1 January 2024 – for an existing MCP between 5MWth and 50MWth
- 1 January 2029 – for an existing MCP between 1MWth and less than 5MWth
You are required to do periodic monitoring for new and existing MCP at least every:
- 3 years for a MCP less than or equal to 20MWth
- year for a MCP greater than 20MWth
- 1,500 hours of operation for limited operating hours MCP less than 20MWth with a minimum frequency of once every 5 years
- 500 hours of operation for limited operating hours MCP greater than 20MWth with a minimum frequency of once every 5 years
In England there are 2 monitoring standards available for MCP:
- monitoring stack emissions; low risk MCPs and specified generators (https://www.gov.uk/government/publications/monitoring-stack-emissions-low-risk-mcps-and-specified-generators/monitoring-stack-emissions-low-risk-mcps-and-specified-generators) (formerly called TGN M5)
- MCERTS (https://www.gov.uk/government/collections/monitoring-emissions-to-air-land-and-water-mcerts)
Your environmental permit will tell you which standard to use.

You can use the monitoring stack emissions: low risk MCPs and specified generators (https://www.gov.uk/government/publications/monitoring-stack-emissions-low-risk-mcps-and-specified-generators/monitoring-stack-emissions-low-risk-mcps-and-specified-generators) standard for:
- specified generators and MCPs permitted under simple bespoke or standard rule permits
- individual MCPs with a rated thermal input of less than or equal to 20MWth, that aggregate to less than 50MWth, which use natural gas or gas oil (light fuel oil), providing your MCP is not located in an AQMA
- MCPs operating for less than 500 hours per year, without a specified ELV
- diesel back-up generator MCPs and specified generators that are operated for less than 50 hours per year for testing, without a specified ELV
- specified generators which use natural gas and have a rated thermal input of less than 5MWth
In all other cases you will need to use the MCERTS standard.

Where continuous emission monitoring systems are used on MCPs and specified generators you can use the MCERTS standard for low risk monitors (https://www.gov.uk/government/publications/mcerts-performance-standards-for-low-risk-plant-monitors).

Apply for a MCPD environmental permit

You need to:
- select the correct permit to apply for
- complete and submit the correct form, the fee and required information
See the guidance Medium combustion plant: apply for an environmental permit (https://www.gov.uk/guidance/medium-combustion-plant-apply-for-an-environmental-permit).

Contact your regulator

England

Contact the Environment Agency.

General enquiries

National Customer Contact Centre PO Box 544RotherhamS60 1BY

Email xxx@xxx.xx (mailto:xxx@xxx.xx)

Telephone 03708 506 506

Telephone from outside the UK (Monday to Friday, 8am to 6pm GMT) xx-xxxx-xxxx

Monday to Friday, 8am to 6pm.

Wales

Contact Natural Resources Wales (https://naturalresources.wales/about-us/contact-us/general-enquiries).