Biomass and combustion plant emissions in an application

Biomass and other combustion plant raise two separate consent questions on a development, and conflating them is the usual reason a scheme stalls. Planning deals with the effect of emissions on local air quality and on sensitive receptors. Environmental permitting deals with the plant itself once it passes a thermal input threshold. A boiler can need a permit and still have its stack height determined for planning purposes, and the two processes do not talk to each other automatically.

The practical sequence is to fix the fuel, size the plant, determine the stack height, and only then draw the roof.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

Emissions of concern and the receptors at risk

For biomass the dominant concerns are oxides of nitrogen and particulate matter, both PM10 and the finer PM2.5 fraction. Gas-fired plant produces nitrogen dioxide with negligible particulate. Oil-fired plant adds sulphur dioxide. Combined heat and power engines produce more nitrogen dioxide per unit of heat than a boiler, which is why they often drive the assessment on a mixed use scheme.

The receptors that matter are the ones the authority's air quality officer will list: existing residential windows, schools and nurseries, hospitals and care homes, and the development's own residents where the plant serves the building they live in. Designated habitats within the relevant screening distance bring Natural England into the picture.

Where the site sits inside an Air Quality Management Area, the starting position is harder. The authority has already accepted that an objective is being exceeded, and a new source of the pollutant in question needs a correspondingly stronger case.

Stack height determination methods

Stack height is determined, not chosen, and the method depends on the plant.

For boilers burning fuels containing sulphur, the traditional route is the chimney height memorandum method, which takes the fuel sulphur content, the thermal input and the nature of the surrounding buildings and returns a minimum height. It is quick, it is conservative, and for a modest boiler in an uncomplicated setting it is often all the authority wants.

For larger or more complex cases, the dispersion modelling route derives the height needed to meet air quality objectives at the relevant receptors. That is the method to use where the memorandum result is implausible, where there are tall buildings nearby, or where the plant is in an Air Quality Management Area.

Building downwash is the factor most often underestimated. A stack terminating below the roofline of an adjacent building, or within the recirculation zone of its own building, performs far worse than its height suggests. The common rule of thumb is that the terminal should clear the roof of the building it serves and any building within a defined radius, with the margin established by the assessment rather than assumed.

Flue termination also has a design consequence that architects should be told about early. A 12 metre stack on a three storey building is a visible element, and retro-fitting height after the elevations are approved means a non-material amendment at best.

Where a permit is required alongside planning

Thermal input is the trigger. Medium combustion plant with a rated thermal input of 1 megawatt or more and less than 50 megawatts falls within the medium combustion plant regime under the Environmental Permitting (England and Wales) Regulations 2016, with permit or registration requirements and emission limit values that differ by fuel, plant size and whether the plant is new or existing. Larger plant moves into the main permitting regime.

Three points worth knowing before the design is fixed:

• Aggregation rules can bring several smaller units into scope where they share a stack or would normally do so, so four 300 kilowatt boilers are not automatically outside the regime.
• Burning waste wood rather than virgin biomass changes the regime entirely, bringing the plant within waste incineration or co-incineration controls with much tighter limits and monitoring.
• Deadlines for existing plant to meet the requirements are phased by size, so an existing boiler being retained as part of a redevelopment may acquire obligations the client has not budgeted for.

Permit emission limits and planning assessment criteria are not the same thing. Meeting the permit limit does not demonstrate that the local air quality objectives are met at a window 15 metres away, and an Air Quality Assessment has to address the latter.

Fuel specification and its effect

Fuel choice changes the numbers more than almost any mitigation.

Wood pellet of a consistent specification burns more cleanly and more predictably than wood chip, which varies in moisture content and produces higher particulate emissions when wet. Virgin wood is cleaner than recovered wood, which may carry contaminants. Moisture content drives both efficiency and emissions, and a fuel specification without a moisture limit is not a specification.

So the application should state the fuel, the specification standard, the moisture range and the supply arrangements, and the assessment should model that fuel rather than a generic one. Authorities increasingly condition fuel type, because it is the one variable an operator can change after commissioning that would invalidate the assessment.

Fuel delivery and storage come with their own planning consequences: vehicle movements, the size and position of the fuel store, and the dust and odour implications of handling. Those belong in the transport and amenity sections of the application, not as an afterthought.

Smoke control area implications

Large parts of urban England are smoke control areas under the Clean Air Act 1993. Within them, emitting smoke from a chimney is an offence unless an authorised fuel or an exempt appliance is used.

For biomass this is decisive. A boiler intended to burn wood chip in a smoke control area must be an exempt appliance for that fuel, and the exemption is specific to appliance and fuel combination rather than general. Check the current exemption lists before specifying, because a client who has already ordered the plant has a problem that planning cannot solve.

Enforcement in this area moved to a civil penalty regime under later amendments, which made action easier for authorities rather than harder. Verify the current position with the local authority's environmental health team, since smoke control area boundaries are revised and not always well mapped.

Dispersion modelling scope

Where modelling is needed, the scope should be agreed with the authority before it is run, and the submission should record what was agreed.

Operating assumptions deserve particular care. Modelling a boiler at continuous full load when it will modulate seasonally overstates the impact, while modelling at average load understates the peak. The defensible approach is to present both and discuss the result, which also answers the reviewer's first question before it is asked.

Report the impact against the objectives using recognised significance descriptors rather than inventing a scale, and state the percentage of the objective represented by the process contribution at each receptor.

Conditions on fuel, operation and monitoring

The conditions these schemes attract are predictable, and offering draft wording with the application usually produces better versions than waiting.

1. Fuel specification, naming the fuel type, the standard it must meet and the maximum moisture content.
2. Stack height and termination point by reference to a drawing, with a requirement to retain it.
3. Abatement equipment specified by performance rather than product, with a requirement that it be maintained.
4. Commissioning emissions testing, with results submitted before the plant enters regular service.
5. Periodic monitoring at a stated frequency, with a reporting route to the authority.
6. Hours of operation where the plant is near residential receptors and noise is also an issue.

That last point is a reminder that combustion plant rarely arrives alone as an issue. The flue, the fans and the fuel deliveries all generate noise, and a Noise Impact Assessment assessed against the same receptors usually has to run in parallel. Where a kitchen extract shares the same roof, the Ventilation and Extraction Statement needs to show both terminations and the separation between them. For the specific chimney height question, the related note on chimney heights, biomass boilers and the Clean Air Act goes further into the determination method.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.