RENEWABLES BIOFUELS
With the burning of biofuels, a key issue is how to deal with the by-products of combustion
STANDARDS AND REGULATIONS
All the Approved Documents (ADs) for England and Wales are available from the Planning Portal (
http://www.planningportal.
gov.uk/buildingregulations/ approveddocuments). Of particular relevance are AD Part J – Combustion Appliances and Fuel Storage Systems and AD Part L – Combustion Appliances and Fuel Storage Systems.
The technical guidance for the Scottish Building Regulations is available. Of particular relevance are Section 3 - Environment (includes chimneys) and Section 6 (Energy) –
www.scotland.gov.uk/Topics/Built- Environment/Building/Building- standards/publications
when replacing existing plant, will require purpose designed flue systems due to the increased termination height requirements to be compliant with Local Air Quality Management (LAQM – see http://laqm.
defra.gov.uk). This will be site dependent as local air quality will vary, depending on the proximity to other polluting discharges and site location. The latest edition of the Institution of
Gas Engineers’ Installation of flued gas appliances in industrial and commercial premises, IGE UP/10 Edition 3, gives further advice on commercial systems operating in conjunction with gas-fired boiler plant, particularly focusing on natural and fan- assisted ventilation requirements.
Flue design Because of the very varied nature of the fuel, the components of the exhaust discharge will vary greatly. The focus needs to be on where the biomass discharges occur; the height at which the flue terminates; and the area in and around the discharge position. Some key considerations relating to
discharges are: l Biomass boilers have to comply with the Clean Air Act. This means that a submission has to be made to the local planning authority, which includes the exhaust gas volumes and constituents that can be established using the manufacturer’s technical literature. l In almost all cases, long horizontal
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flue runs are not appropriate. They can cause particular problems when the system is under part-load conditions. For example, hot water supply boilers have been regularly observed as being placed furthest away from the vertical rise of a flue, and during summer months the boiler is not able to effectively overcome the flue resistance and so will not function correctly.
l When using biomass, each boiler should have its own flue and not combine with other boilers. Wood-burning biomass appliances should be located as close as possible to the vertically rising stack.
l Unless recommended by the appliance or flue designer/manufacturer, fan-assisted flue systems on biomass systems are not considered as best design practice despite calculations that show a system may require a certain amount of assistance during some low load periods. The interaction between the fans within the boiler (and in particular the induced draught fan, usually situated on the outlet of the boiler) is carefully controlled by the boiler’s control system. Undue suction from external fan assistance may upset the carefully controlled combustion process.
l Despite the use of scrubbers/centrifugal cleaning of the exhaust gases, particular consideration must be given to cleaning and ease of maintenance access. To compare the requirements to meet the flue needs, three sample calculations, each
January 2012 CIBSE Journal 43
In Ireland, the technical booklets supporting the regulations (including the relevant amendments pamphlets) are Technical Booklet F2:2006 – Conservation of fuel and power in buildings other than dwellings and Technical Booklet L: 2006 – Combustion appliances and fuel storage systems, available at:
www.dfpni.gov.uk/index/buildings- energy-efficiency-buildings/ building-regulations/br-technical-
booklets.htm
The Biomass and Air Quality Guidance for Local Authorities gives some excellent guidance that is useful to both developers and installers, available at:
www.environmental-protection.org. uk/biomass
The requirements for chimneys are summarised as part of the HETAS Guide at:
http://guide.hetas.co.uk/ guide_chimneys.html
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