Hydrogen combustion: comply with emission limit values
How to comply with emission limit values (ELVs) for hydrogen combustion plant greater than 1 megawatt thermal input (MWth).
1. Who this guidance is for
This guidance on emerging techniques is for combustion plant within the scope of the:
- Industrial Emissions Directive (IED)
- best available technique (BAT) reference (BREF) documents
- Medium Combustion Plant Directive (MCPD)
The environmental regulators developed this guidance as there are no ELVs for hydrogen combustion in the legislation. The guidance is meant to set out what the regulators expect until there is enough uptake for them to understand what is achievable for air pollutant emissions. When appropriate, they will develop BAT documents and associated emission levels (AELs) which will guide the permitting process in the legislation.
Unlike combustion of conventional fuels, combustion of hydrogen produces no direct emissions of carbon dioxide. Therefore, it is a credible avenue for low-carbon combustion processes. However, it is important not to compromise local air quality in the transition to low-carbon fuels.
This guidance sets out ELVs for oxides of nitrogen (NOx) and carbon monoxide (CO), as appropriate, for combustion of hydrogen within the scope of:
- plant covered by the MCPD, referred to as medium combustion plant (MCP)
- plant covered within the scope of Chapter III of the IED, referred to as large combustion plant (LCP)
The combustion of hydrogen results in a change in the flue gas volume. Therefore, you need to use a correction factor to achieve equivalence with ELVs defined for natural gas on a mass per unit volume basis, as described in the ETN Position Paper, 2023. Figure 1 in the ETN Position Paper provides a range of ELV correction values related to the hydrogen content of a natural gas/hydrogen blended fuel, up to a maximum of 1.37 at 100% hydrogen. This correction factor of 1.37 is applied to ELVs for combustion of natural gas, as set out in the MCPD and IED, to get ELVs for the combustion of hydrogen.
2. Definitions of existing, new and new-new plant
In this guidance, the following definitions apply to hydrogen combustion plant.
Existing
For LCP, ‘existing plant’ means plant with a permit for combustion of a conventional fuel that was put into operation before 8 January 2014. This is specified in IED Article 30(2).
For MCP, ‘existing plant’ means plant put into operation before 20 December 2018. This is specified in MCPD Article 3(6).
New
For LCP, ‘new plant’ means plant with a permit for combustion of a conventional fuel that was put into operation on or after 8 January 2014 (as specified in IED Article 30(2)), but before 1 January 2025.
For MCP, ‘new plant’ means plant put into operation after 20 December 2018 (as specified in MCPD Article 3(6)) but before 1 January 2025.
New-new
‘New-new’ means plant with a permit for combustion of hydrogen or a hydrogen and natural gas blend put into operation after 1 January 2025.
Changing from one definition to another
In certain circumstances, such as substantial refurbishment, existing plant will be redefined as new-new. It may be necessary for plant to go through a lot of retrofitting to transition to hydrogen firing. For guidance on refurbishment, contact your regulator.
3. Minimum ELVs for hydrogen as a single fuel
Table 1 (for existing plant) and Table 2 (for new plant) give the NOx ELVs you must meet for combustion of hydrogen by plant and thermal input.
In this guidance, for all plant the reference conditions for ELVs are:
- 273.15K
- 101.3kPa
- dry
- 3% oxygen for boilers
- 15% oxygen for gas turbines and engines
Table 1. ELVs for existing plant using 100% hydrogen fuel
| Plant type and thermal input | NOx ELV (mg/Nm3) |
|---|---|
| Gas turbines: greater than or equal to 50MWth | 68.5 |
| Boilers: greater than or equal to 50MWth | 137 |
| Gas engines: greater than or equal to 50MWth | 137 |
| Gas turbines: greater than 5MWth and less than 50MWth | 205.5 |
| Boilers: greater than 5MWth and less than 50MWth | 274 |
| Gas engines: greater than 5MWth and less than 50MWth | 260.3 |
| Gas turbines: greater than or equal to 1MWth and less than or equal to 5MWth | 205.5 |
| Boilers: greater than or equal to 1MWth and less than or equal to 5MWth | 342.5 |
| Gas engines: greater than or equal to 1MWth and less than or equal to 5MWth | 260.3 |
Table 2. ELVs for new plant using 100% hydrogen fuel
| Plant type and thermal input | NOx ELV (mg/Nm3) |
|---|---|
| Gas turbines: greater than or equal to 50MWth | 68.5 |
| Boilers: greater than or equal to 50MWth | 137 |
| Gas engines: greater than or equal to 50MWth | 102.8 |
| Gas turbines: greater than 5MWth and less than 50MWth | 68.5 |
| Boilers: greater than 5MWth and less than 50MWth | 137 |
| Gas engines: greater than 5MWth and less than 50MWth | 130.2 |
| Gas turbines: greater than or equal to 1MWth and less than or equal to 5MWth | 68.5 |
| Boilers: greater than or equal to 1MWth and less than or equal to 5MWth | 137 |
| Gas engines: greater than or equal to 1MWth and less than or equal to 5MWth | 130.2 |
Where permitted ELVs are lower than those outlined in the applicable directive because of air quality constraints, you must apply the appropriate factor from Table 2 or Table 3 to the permitted ELVs to get ELVs for combustion of hydrogen.
4. Minimum ELVs for hydrogen as a blended fuel
The relationship between composition of hydrogen in fuel (by volume) and flue gas volume per net thermal energy input is not linear. Therefore, you must use a banding system for ELVs from fuel blends.
Table 3 gives the ELVs you must meet. You must achieve an ELV for both NOx and CO for combustion of blended fuels. Table 3 sets out ELVs for both pollutants.
Table 3. ELVs for fuel blends of hydrogen with natural gas
| Substitution of hydrogen (% v/v) | Pollutant ELV expressed as a percentage of the analogous natural gas ELV |
|---|---|
| 0 to 20% | 100% |
| Greater than 20% to 50% | 107% |
| Greater than 50% to 75% | 115% |
| Greater than 75% to 90% | 125% |
| Greater than 90% to 95% | 130% |
| Greater than 95% to 100% | 137% |
Here is an example to help you calculate ELVs based on the information set out in Table 3.
Example
A 100 MWth gas turbine which would previously have been subject to a NOx ELV of 50 mg/Nm3 and a CO ELV of 100 mg/Nm3 for natural gas is proposed to switch to a fuel blend of 85% hydrogen with 15% natural gas. The ELVs which now must be complied with are 62.5 mg/Nm3 for NOx and 125 mg/Nm3 for CO.
Where permitted ELVs are lower than those outlined in the applicable directive because of air quality constraints, you must apply the appropriate factor from Table 3 to the permitted ELVs to get ELVs for combustion of hydrogen and natural gas as a blended fuel. This is as specified in the previous section for use of hydrogen as a single fuel.
When you use a blend of natural gas from the transmission and distribution system and hydrogen, you must sum the volumetric component of hydrogen in the natural gas with the volume of hydrogen which is added. This total volumetric component of hydrogen in the fuel will inform the ELV which you must meet.
5. Provisions for setting ELVs
You must determine provisions for setting ELVs for combustion of hydrogen, based on Annex V of the IED and Annex II of the MCPD, in line with either:
- the relevant agency guidance
- the guidance IED Compliance Protocol for Utility Boilers and Gas Turbines, 2022
6. Compliance with ELVs
You must meet the requirements for monitoring as set out in Annex V, Part 3 of the IED or Annex III, Part 1 of the MCPD, as applicable.
For continuous measurements, you will have complied with the ELVs set out in this guidance if the evaluation of the measurement results indicates, for operating hours within a calendar year, that you have met all the following conditions.
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No validated monthly average value exceeds the relevant ELV. Where a fuel blend is used, the relevant ELV is determined based on the corresponding monthly average volumetric component of hydrogen.
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No validated daily average value exceeds 110% of the relevant ELV. Where a fuel blend is used, the relevant ELV is determined based on the corresponding daily average volumetric component of hydrogen.
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95% of validated hourly average values over the year do not exceed 200% of the relevant ELV. Where a fuel blend is used, the relevant ELV is determined based on the ELV corresponding to the highest volumetric component of hydrogen in any operating hour of the year.
The validated average values are determined as set out in point 10 of IED Annex V, Part 3 and relevant section of Part 4.
Where continuous measurements are not required, you will have complied with the ELVs set out in this guidance if you do not exceed ELVs in either the:
- results of each of the series of measurements
- other procedures defined and determined according to the rules laid down by the competent authorities
7. Cost-benefit analysis where plant cannot meet these ELVs
You may not be able to achieve the ELVs set out in this guidance on plant put into operation before 1 January 2025 due to technical or economic reasons that you can demonstrate. If so, the regulator will set ELVs after considering your cost-benefit analysis, using a process that is consistent with that described in IED Article 15(4). This will be based on the following.
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The cost of additional harm of NOx and any ammonia emissions over a period of 25 years, compared to the costs associated with secondary abatement techniques (unless such techniques are demonstrably technically unsuitable). You must consider the impact on plant efficiency associated with secondary abatement techniques. A shorter period will be permissible if you can provide evidence of a committed closure date.
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The additional costs of harm must be based on the additional NOx and any ammonia emissions associated with the proposed achievable NOx ELV case, compared to the relevant ELVs set out in this guidance.
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You must not attribute costs of benefits to any changes in CO2 emissions based on the UK government valuation of greenhouse gas emissions, as this is not based on cost of harm. In the case of point 1 in this section, the operator must propose lowest technically feasible emission limit. They must review this limit every 4 years using these principles, until BAT-AELs for hydrogen combustion are set up.
All plant put into operation after 1 January 2025 (new-new plant) must meet the ELVs for new plant.
8. Contact your regulator
The guidance on medium combustion plant (MCP): comply with emission limit values gives more information about monitoring, reporting, emission limits and the standards which apply across the UK.
If you have any questions, contact your regulator.
England
Contact the Environment Agency.
General enquiries
National Customer Contact Centre
PO Box 544
Rotherham
S60 1BY
Email enquiries@environment-agency.gov.uk
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.
Wales
Contact Natural Resources Wales.
Scotland
Contact the Scottish Environment Protection Agency.
Northern Ireland
If you have a question about combustion plant policy, contact the Chemicals and Industrial Pollution Policy (CIPP) Branch.
If you have a question about permitting for a part A or B installation, contact the Industrial Pollution and Radiochemical Inspectorate (IPRI).
If you have a question about a part C permit or installation, contact the relevant district council.
Updates to this page
Last updated 21 October 2024 + show all updates
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Updated section 1 to refer to the ETN Position Paper from March 2023.
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First published.