Review of the potential impact on air quality from increased wood fuelled biomass use in london
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Review of the Potential Impact on Air Quality from Increased Wood Fuelled Biomass Use in London. Peter Coleman. Acknowledgements. Ruth Calderwood from City of London and Jareed Boow from London Councils

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Review of the Potential Impact on Air Quality from Increased Wood Fuelled Biomass Use in London

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Review of the Potential Impact on Air Quality from Increased Wood Fuelled Biomass Use in London

Peter Coleman


  • Ruth Calderwood from City of London and Jareed Boow from London Councils

  • Colleagues who did all the work; Robert Stewart, John Abbott, Alan Leonard, Alan Collings, Pat Howes and Nick Barker.

Great Smog December 1952

Desired Air Quality

Scope of London Report

  • commissioned by London Councils via the City of London

  • Drivers for increasing use of biomass

  • Legislation and potential controls on biomass plant

  • Sustainability and transport impacts

  • Combustion technologies

  • Fuels

  • Air quality impacts of renewable scenarios

  • A toolkit for the assessment of planning applications for biomass heating

Pollutants Associated with Biomass

  • Nitrogen oxides

  • Polyaromatic hydrocarbons (PAHs) formed from incomplete combustion

  • Metals when treated wood is burned As, Cu, Cr(VI)

  • Dioxins where waste or treated wood (PCP/ lindane) is burned

  • Particles (PM10 / PM2.5)

    • Coarse material entrained chars, fine alkali metal salts

    • Poor combustion increases coarse particle emissions

    • Modern plant emissions lower and TSP ≈ PM10 ≈ PM2.5

    • Abatement therefore difficult

Pollution Control Regulation for Biomass

  • IPPC A(1)

  • >50MW combustion plant burning any fuel

  • 3-50MW combustion plant burning fuels containing or derived from waste

  • >1te/hour incineration of waste

  • LA-PPC A(2)

  • Co-incineration of non-hazardous waste associated with any A(2) process less than 50MW

  • Incineration of non-hazardous waste in an incineration plant with a capacity <1te/h

  • Co-incineration of non-hazardous waste in a co-incineration plant which is not otherwise an A(1) or A(2) process

  • LA-PPC Part B

  • Installations in which any Part B activity is carried out but no A activity including

  • 20-50MWth input non-waste wood

  • Combustion of fuels containing or derived from waste 0.4-3MW exempt from WID

  • Incineration of non-hazardous waste in a plant exempt from WID but between 50 and 1 te/hour

  • Otherwise Clean Air Act 1993

Modelling Study

  • National Biomass Strategy suggested 7 – 10% of energy demand from biomass, perhaps 2% from wood fuel combustion

  • London Energy Partnership looked at stretch targets for carbon reduction by 2026 investigated impacts of 5 scenarios of biomass

  • Concern at cumulative impact on air quality of large numbers of biomass heat/power developments in London

  • What could the impact on air quality be ?


Emission Factors Used

Modelling Assumptions

  • No geographic information on where development likely.

  • Followed LAEI residential and industrial/commercial energy use.

  • Wide range of biomass to energy conversion technologies available including AD, pyrolysis, CHP and other advanced technologies

  • Technologies in use by 2026 likely to be different from now; however the initial wave is wood fuelled combustion.

  • Assumed all take up by wood fuelled combustion

  • Emissions assumed not to be “just compliant” nor “best available” but typical of modern appliances – used UNECE Guidebook emission factors

  • Date mismatch on background air quality is 2020 emissions 2026

PM10 Emissions (te/yr)

NOx Emissions (te/yr)

Modelled 2003 PM10

Modelled PM10 2020 BAU

Modelled PM10 2020 Scenario 1

Modelled PM10 2020 Scenario 3

Key Points from Scenario modelling

  • PM10

    • BAU below annual and 24 hourly average objectives in 2010 and 2020

    • Scenario 1 may be difficult to achieve 24 hourly objective near busy roads as adds 4 to a background of 24 ug/m3

    • Scenario 3 both objectives difficult – central London an. av. of 37 ug/m3

  • PM2.5

    • BAU below objective in 2010 and 2020

    • Scenario 1 may be difficult to achieve near busy roads 24 hourly objective as adds 4 on to a background of 17 ug/m3

    • Scenario 3 widespread breaking of cap concentrations approx 30 ug/m3

    • Exposure reduction hard to achieve without further measures on other sources

  • NO2

    • BAU exceeds objective in 2010 and 2020

    • 3 to 10 ug/m3 increase makes alternative measures more necessary

  • Uncertainty in the Scenarios

  • Potential Impact on Air Quality non-trivial – what can be done ?

Measures available to EHOs

  • For larger appliances permits

  • Smaller appliances Clean Air Act

  • Planning permission where required

  • Worst case nuisance

  • Voluntary agreements on emission quality

Clean Air Act Powers 1

  • Requires all new furnaces other than domestic furnaces to be capable of operating smokelessly and to be notified to the local authority (s4).

  • Allows the Secretary of State to prescribe emission limits on grit and dust from furnaces other than domestic furnaces (s5).

  • Prohibits the use of a furnace other than a domestic furnace in a building or outdoors which burns pulverised fuel, solid fuel at 45.4 kg/h or more or liquid and gas fuels at 366.4 kW or more unless it has grit and dust arrestment plant fitted which have been agreed by the local authority or unless the Local Authority has been satisfied that the emissions will not be prejudicial to health or a nuisance (s6).

  • The limit of 45.4 kg/h for solid wood fuels implies @ 10MJ/kg 126 kW, @ 20 MJ/kg 252kW. Hence, pellet appliances would be caught by the arrestment plant and chimney heights provisions at larger sizes than wood chip or green logs.

Clean Air Act Powers 2

  • Where a furnace is burning pulverised fuel, solid fuel at 45.4 kg/h or more of liquid and gas fuels at 366.4 kW or more the Local Authority may direct that measurements of the dust emissions are made (s10). However, if the furnace is burning solid matter at less than 1.02 te/h or liquid or gas at 8.21 MW or less then the Local Authority can be required to carry out the measurements (s11).

  • Allows the local authority to request the occupier of a building to provide such information as may be reasonably required on the furnaces in the building and the fuels or wastes burnt on them (s12).

  • Prohibits the use of furnace with a chimney which burns pulverised fuel, solid fuel at 45.4 kg/h or more or liquid and gas fuels at 366.4 kW or more unless the chimney height has been approved by the Local Authority following the provision of relevant information by the applicant, unless application was made and the Local Authority did not respond within 8 weeks or a longer time mutually agreed (s14, s15).


  • Where planning permission is required then LAs have an opportunity to influence the impact on air quality of biomass.

  • Air quality is a material planning consideration particularly in Air Quality Management Areas (PPS 23 Pollution Control).

  • PPS 22 Renewable energy states “Small scale renewable energy schemes utilising technologies such as ….Biomass heating, …can be incorporated both into new developments and some existing buildings. Local planning authorities should specifically encourage such schemes through positively expressed policies in local development documents.”

  • Planning and air quality can achieve joint objectives of sustainability.

  • Possible use of S106 agreements under the Town and Country Planning Act 1990 to work with developers.


  • EHOs approve chimney heights

  • 3rd ed Chimney Heights Memorandum not suitable for biomass

  • Existing LAQM Technical Guidance (LAQM TG(03)) inappropriate undergoing revision

  • Toolkit designed as a simple process to assist in the range 50kW to 2MW with chimney height determination

  • Based on approach used for Technical Guidance

  • NOTE uses emission rate so combination of plant size and pollutant concentration

  • Also provided a tool for LAQM to estimate when domestic solid fuel use unlikely to cause exceedances for annual mean objective for PM2.5

24 hour mean PM

Conclusions 1

  • Biomass offers many advantages as a source of energy.

  • Inappropriate biomass use may impact significantly on air quality particularly cumulative impacts.

  • The impact on air quality of a development is influenced by the energy demand, the appliance selected, the fuel quality the abatement used and the chimney height installed.

  • range of emissions especially particles from appliances of same output is significant and influenced by appliance design and fuel quality.

  • Regulation currently does not require high quality appliances except through planning system.

Conclusions 2

  • A toolkit of approaches has been developed to assess the air quality impact of wood combustion both plant by plant and cumulatively.

  • A fuel certification scheme could greatly reduce uncertainty in future plant performance.

  • Industry needs clear guidance to enable appropriate stalled projects to proceed.

  • Industry needs predictability of projects not variations between LAs in treatment.

  • “Significance” and “cumulative impact” need addressing.

  • Developers and local authorities can achieve acceptable gains in sustainability without degrading air quality significantly.


Possible Future Actions

  • Fuel certification scheme for pellet, chip and log to reassure purchasers, regulators and supply chain - in early stages

  • Guidance to developers on chimney heights published in appropriate fora to access relevant users

  • Publication of emission factors so LAs know range of performance

  • Reminder to specify emission performance in planning permission

  • Tools to convert between units

  • Revision of Chimney Heights Memorandum to provide clarity

  • Emission Limits for CAA appliance size range fit for purpose



  • London Biomass Report

Recent Happenings

  • Edinburgh 7 schools biomass project stopped

  • Dundee rejected ‘high quality’ biomass plant as will increase air pollution even though no exceedences predicted with 5 fold conservative modelling and no increase ‘significant’ at receptors.

  • Concern about lifetime emissions in absence of powers to ensure good operating practice for un-permitted plant

Drivers for Increased Biomass Use

  • Issues

    • Climate change; biomass energy is approaching carbon neutrality (excluding limited supply chain impacts)

    • Sustainable

    • Energy security

  • Policy Reponses

    • EU Biomass Action Plan

    • UK Energy White Paper

    • Biomass Task Force

    • Biomass Strategy

    • Merton rule 10-20% of energy demand of developments from renewables

    • Zero-carbon new homes by 2016

Air Quality Strategy Environment Act 1995

  • PM10

    • 50 ug/m3 24h mean not to be exceeded >35 times/year

    • 40 ug/m3 annual mean

  • PM2.5

    • <25 ug/m3 (12.5 Scotland)

    • 15%; reduction between 2010 and 2020

  • NO2

    • 200 ug/m3 1h mean not to be exceeded >18 times/year

    • 40 ug/m3 annual mean

  • PAHs

    • 0.25 ng BaP /m3 by 2020, EU target 1 ng BaP/m3 by2012

Size Distribution (Erlich et al. 2007)

Clean Air Act Emission Limits

Comparison of EU Measurement Methods

Emission Limits elsewhere


  • Anywhere

    • Is the appliance permitted?

    • Is it a <16.12kW boiler for domestic purposes

    • Is it capable of smokeless operation?

    • Burning at >45.4 kg/h (approx 120 – 200kW)?

    • If so, then agreed abatement equipment must be used; and

    • Chimney height must be determined unless AQ impact shown to be so far as practicable not prejudicial to health.

  • If within a smoke control area then an exempt appliance


  • ADMS used to predict ground level concentrations for a unit emission rate from stacks between 10.6 and 40m and diameters from 0.1-1m

  • Stack at centre of 10m cube building

  • Discharge temperature 100oC

  • Discharge velocity sufficient to overcome pressure drop

  • Heathrow 2005 metrological data

  • 1m roughness

  • 1km x1km 10m grid receptors

  • 24 model scenarios

  • Annual mean, 90th %ile 24h mean, 99.8th %ile hourly mean

  • Emission rate for 1 ug/m3 estimated

Requirements from Developer

  • OS grid coordinates of stack

  • Height of stack above ground

  • Diameter of stack

  • Dimensions of buildings within 5 stack heights

  • Description of the combustion appliances

  • Description of abatement equipment

  • Maximum rates of emission of particles and NOx (not necessarily at capacity

  • If size fractionated PM available use if not all PM2.5.

Annual mean NO2 /PM 1 ug/m3

99.8th %ile hourly NO2 of 40 ug/m3

Modelled NOx 2020 Scenario 1 Increment

Modelled NOx 2020 Scenario 3 Increment

Process – PM10

  • Calculate a background adjusted emission rate EA

  • EA = E/(32-G)

  • E = emission rate; G annual average background concentration

  • If existing concentration >32 then

  • EA= E/delta C

  • Where delta C is maximum allowed increment

  • Use Nomagraph to estimate effective stack height

  • If this is <2.5 times building height or buildings closer than 5 stack heights then use CHM to calculated corrected stack height

  • In all cases;

  • >3m above any adjacent area to which there is general access,

  • > calculated effective stack height,

  • > any building within 5 stack heights

Process – PM2.5 /NOx

  • Calculate a background adjusted emission rate EA

  • EA = E/(25-G)

  • As before

  • NOx

  • EA = E/(40-G) annual mean

  • EA = E/(200-G) hourly average

  • Minimum chimney height is the tallest of the three calculated


  • A 500kw wood boiler in a building 30m high by 20m square. Stack diameter 0.5m

  • Particle emission rate from CAA 0.048 g/s

  • NOx from Corinair gives 0.075 g/s

90th %ile 24 hour mean PM10 1 ug/m3

Annual mean NO2 /PM 1 ug/m3



99.8th %ile hourly NO2

Example (cont.)

  • Critical effective stack height less than 2.5 x building height

  • From CHM

  • Building width B =root (202+202) = 28.3 m

  • Lesser of building height (30m) and width K = 28.3 m

  • T = 30 + 1.5 x 28.3 = 72.5m

  • T > corrected stack height so

  • Corrected chimney height C = 30 +9.5 x (1-(30/72.5)) = 35.6 m

  • Required stack height 35.6m (5.6m) if no taller buildings within 47.5m of stack

Screening Assessment for Domestic Biomass

  • Seeks to identify maximum density of domestic solid fuel use before exceeding PM2.5 objective

  • Estimate number of solid fuel using houses in 500m x 500m square

  • Weight houses

  • Ceq = C + 0.36 anthracite + 0.56 SSF + 0.79 wood

  • Estimate proportion of open space L

  • Deq = Ceq/(1-L)

  • If Deq < D then PM2.5 objective unlikely to be exceeded

Nomograph for Risk of Exceeding for PM2.5

Validation with 2003 NOx Measurements

Modelled NOx/NO2 2020 BAU

Modelled 2003 NOx

Modelled NOx/NO2 2010 BAU

Modelled NOx/NO2 2020 Scenario 1

Modelled PM10 2010 BAU

Modelled NOx/NO2 2020 Scenario 3

Modelled PM10 2020 Scenario 1 Increment

Modelled NOx 2020 Scenario 3 Increment

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