Guidance: Hydrogen combustion: comply with emission limit values

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Environment Agency

October 14

08:14 2024

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1. Who this guidance is for

This guidance on emerging techniques is for combustion plant within the scope of the:

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 ETN Position Paper, 2022. Figure 2 in the ETN Position Paper provides a range of correction values between 1.3 and 1.5, further calculations a range of 1.36 to 1.39, and the value of 1.37 as optimal. At 100% hydrogen substitution, 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 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

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