The Simple Calculation of Atmospheric Impact Limits (SCAIL) Combustion screening tool is held on the UK Centre for Ecology and Hydrology (CEH) website. You can use it to model the effects of oxides of nitrogen (NOₓ) and sulphur dioxide (SO₂) emissions from small and medium sized combustion sources on sensitive habitat sites.

The tool:

• uses source and stack parameters to estimate NOₓ, SO₂, nutrient nitrogen deposition and acid deposition on the habitat sites and their features

• provides a database of Special Areas of Conservation (SACs), Special Protection Areas (SPAs) and Sites of Special Scientific Interest (SSSIs) and their sensitive features.

When to use the tool

You need to read the guidance Medium combustion plant (MCP): apply for an environmental permit to check if your MCP:

• requires a bespoke permit
• does not ‘screen out’ using the ‘stage 1’ minimum screening distance
• is not also a specified generator – for specified generators follow the guidance Specified generator: apply for an environmental permit

How to use the tool

To use SCAIL Combustion you need knowledge on air quality assessment and dispersion modelling. You may need to use a consultant.

This guidance explains how to use the tool for MCP bespoke permit screening. SCAIL Combustion also provides a general user guide on the CEH website.

Run mode

SCAIL Combustion has 3 run modes:

• Hybrid Mode – calculates the highest point on an 80 degree sector centred on the nearest part of each habitat site
• Conservative Mode – assumes the nearest part of each habitat site is in the prevailing wind direction
• Realistic Mode – uses the actual position of the nearest part of each habitat site to make predictions

You should only use ‘Hybrid Mode’ or ‘Conservative Mode’ for MCP bespoke permit screening.

Run mode for very complex terrain

SCAIL Combustion assumes flat terrain and therefore terrain effects are not accounted for in the screening model.

You must use ‘Conservative Mode’ if your MCP is in an area of very complex terrain, for example in a hilly or mountainous area with slopes exceeding 30 degrees

‘Hybrid Mode’ is a reasonable worst-case screen which is suitable for most other situations.

Emission parameters

To run SCAIL Combustion you need the:

• MCP stack location
• stack height
• stack exit diameter
• stack exit temperature
• stack emissions exit velocity
• pollutant emission rates
• operating hours

You also need to select whether your MCP is a new or existing MCP, it’s:

• new – if it’s put into operation on or after 20 December 2018
• existing – if it was put into operation before 20 December 2018

Find more detail in the guidance Medium combustion plant: when you need a permit.

Screening distance and search radius

SCAIL Combustion automatically suggests the ‘Search Radius’ once you have selected ‘Plant Capacity’ and ‘Sulphurous’ or ‘non-Sulphurous’ fuel. Do not use the suggested values.

Instead manually enter the appropriate minimum screening distance in kilometres. Find these in the guidance Medium combustion plant: apply for an environmental permit.

SCAIL Combustion has 2 location fields ‘Installation Location’ and ‘Stack Grid Reference’.

‘Installation Location’ is the location used to search for SACs, SPAs and SSSIs within a specified search radius in the ‘Designated Site details’ section of the tool. You must make sure the ‘Installation Location’ you enter does not exclude any SACs, SPAs and SSSIs that you need to assess within the relevant minimum screening distance for your MCPs.

Stack height and how to treat buildings

You should measure stack height from the ground. SCAIL Combustion does not model the effect of buildings on stack emissions. You may need to calculate a lower effective height of release instead of using the actual stack height.

You should use an effective height of release instead of the actual stack height in SCAIL Combustion where both of the following apply:

• there is a SAC, SPA or SSSI within 500 metres
• the actual stack height is less than 2.5 times the height of nearby buildings

Follow these steps to estimate the effective height of release to use in SCAIL Combustion:

1. Take the actual stack height in metres.
2. Take the height of the nearest large building structure to the stack in metres.
3. Subtract the height of the building from the actual stack height to get the clearance.
4. If the clearance from step 3 is less than or equal to 1 metre, the effective height of release is 1.66 metres.
5. If the clearance from step 3 is greater than 1, multiply the clearance by 1.66 to get the effective height of release in metres.

Stack exit diameter and velocity

For vertical and unimpeded stacks you do not need to alter the diameter and velocity from the actual values.

You should avoid non-vertical or impeded stacks. If your MCP stack is horizontal or impeded you need to artificially reduce the exit velocity and estimate an equivalent diameter. Follow these steps:

1. Divide the exit velocity in metres per second by 1,000 to get the reduced velocity.
2. Divide the actual volumetric flow rate in metres cubed per second by the reduced velocity from step 1.
3. Multiply the value from step 2 by 1.273.
4. Calculate the square root of the value calculated in step 3 to get the equivalent diameter.

Emission rates

You should calculate the NOₓ and SO₂ emission rates from the emission limit values (ELVs) relevant to your MCP. The ELVs are set at reference conditions. You need to make sure your emission rates are derived from volumetric flow rates normalised to the same reference conditions. To do this you will need the actual:

• temperature
• oxygen content percentage
• moisture content percentage

Once the ELV and normalised volumetric flow rate are at the same reference conditions you can calculate the emission rate in grams per second. You will need to multiply the ELV in milligrams per cubic metre by the normalised volumetric flow rate in cubic metres per second and divide by 1,000.

Operating hours

SCAIL Combustion can scale long-term predictions by the number of operating hours in a year.

For continuous MCP emissions you should set this to 8,760 hours.

For discontinuous, intermittent MCPs you can set the operating hours to the maximum number of hours needed for all operational scenarios. Your permit will be restricted to the number of operating hours you choose.

For MCPs which are back-up generators only, you should set the operating hours to match your testing regime, to a maximum of 50 hours. You do not need to include emergency use hours related to power outages in the assessment. Your permit will be restricted to the number of testing hours you choose.

Assess if your process contribution is significant

Significant contribution criteria

We have set specific long-term screening criteria for MCP bespoke screening to decide where MCPs could make a significant contribution to a likely significant effect on habitat sites. These are:

• 4% for new MCP applications
• 8% for existing MCP applications

These criteria are different to our more conservative long-term 1% insignificant contribution criteria – see the guidance Air emissions risk assessment for your environmental permit. The MCP specific criteria considers the:

• pessimism and predictions biases in the screening tool
• inherent model uncertainties
• differences in context and risk from new and existing emissions

Assess whether your application screens out

For annual NOₓ, annual SO₂, nutrient nitrogen deposition and acid deposition, SCAIL Combustion will calculate the process contribution (PC) and the predicted environmental concentration (PEC) as a percentage of the relevant standard.

An MCP application will ‘screen out’ where SAC, SPA and SSSI are within the minimum screening distance and for annual NOₓ, annual SO₂, nutrient nitrogen deposition and acid deposition, either:

• the PC as a percentage of relevant standard is less than the significant contribution criteria
• the PC is greater than the significant contribution criteria, the PEC is less than 70% of the relevant standard

If it screens out you can apply for a low risk simple bespoke permit.

You will need a high risk complex bespoke permit if a SAC, SPA or SSSI within the minimum screening distance does not ‘screen out’. You will need to submit a detailed air dispersion modelling report. You must provide information on any actions you are taking to reduce air emissions impacts to prevent harm to the habitats that ‘screen in’. Detailed modelling requires specialist knowledge. You should find a consultant to do it for you.

Present your results

Low risk simple bespoke application

You should present the results in a table. Each habitat site assessed should have the following information for annual NOₓ, annual SO₂, nutrient nitrogen deposition and acid deposition assessment:

• PC
• background concentration or deposition rate
• PEC
• critical levels for NOₓ and SO₂
• critical loads for nutrient nitrogen and acid deposition
• PC as a percentage of the relevant standard
• PEC as a percentage of the relevant standard
• whether the result screens in or out

You will get most of this information from the SCAIL Combustion results. It also provides a saveable results file in comma-separated values (CSV) format. The CSV results file does not list all the information needed. Therefore you will need to extract data from it and add it to your results table.

High risk complex bespoke application

Your consultant should follow the guidance Environmental permitting: air dispersion modelling reports to do the detailed modelling required.

What to provide with your application

Low risk simple bespoke application

We need to confirm the screening outcome and check that SCAIL Combustion has been run correctly.

You need to provide the following information with your results for each MCP on the permit:

• rated thermal input in megawatts
• grid reference
• actual stack height
• actual exit diameter
• actual exit temperature
• actual exit velocity
• pollutant emission rates
• ELVs
• volumetric flow rate normalised to the ELV reference conditions
• actual oxygen content
• actual moisture content
• how the MCP operates and their maximum operating hours
• location, height and other dimensions of nearby building structures

For back-up generators, you will need to:

• provide details of the testing schedule, including the typical and maximum number of hours for each test
• show how likely a power outage is – include any historical data on outages (such as how often and how long they lasted) and note any planned infrastructure upgrades or maintenance that could affect reliability

High risk complex bespoke application

The guidance Environmental permitting: air dispersion modelling reports explains what information you need to provide in your detailed modelling report.

Contact the Environment Agency