RENEWABLES BIOFUELS
Gas Fired Plant: 1956 Clean Air Act Memorandum Input
Uncorrected chimney
kW gross 500 5U
Building heights Height above roof
Height - U (m) Radius (m) Within 5U – H (m) 0.6 x U (min 1.0m)
0.9 4.49
Oil Fired Plant: 1956 Clean Air Act Memorandum Input
District Uncorrected Chimney
kW gross 500 5U
Type Height U (m) Radius (m)
A 1.25 6.25 7.0 1.0
Height above Ground Level 8.0
Building Heights Height above roof Height above
Within 5U - H (m) 0.6 x U (min 1.0m) Ground Level
7.0 1.0 Districts A,B,C,D & E range from an 'undeveloped area' to 'heavy urban and industrial development'
Biomass Plant (up to 1000kW) –
LAQM.TG(09) Attached building* Height (m) 4.0
Initial height U 6.0 8.0
5U 30.0
Building heights 7
Preliminary height
above roof (m) (m) Radius (m) Within 5U (m) Above Ground (m) 2.0
9.0 *This is the structure that the flue is connected to, or rises through
With flue design, the focus needs to be on where the biomass discharges occur
for 500 kW boilers with ‘adjacent’ 7m high buildings, are given below, one for gas, one for oil and one for biomass. For gas and oil boilers, it is determined in accordance with the Chimney heights – third edition of the 1956 Clean Air Act memorandum, 1981 and for biomass boilers the Local Air Quality Management – Technical Guidance LAQM. TG(09). In terms of the ‘memorandum’, U is
the uncorrected chimney height that is determined with respect to gross kW, fuel type and sulphur content. The radius is 5U, within which the height
To determine the correct flue arrangements for biomass boilers is not only more demanding, but also less definitive than for fossil fuels
of any building should be taken into account in establishing the final height of the flue. For biomass installations,
LAQM.TG(09) determines U as the ‘attached building height + initial height above roof’ where the ‘initial height above roof’ is 2m for boilers under 1,000 kW and 3m for larger installations. The ‘preliminary height’ is the ‘height of any building within 5U’ + ‘initial height above roof’, but is also subject to any requirement by the local authority officer for a ‘dispersal study’. The key difference results from the
biomass requirements needing to include the height of the ‘attached building’ in the assessment of the surrounding buildings’ ‘radius of influence’. So, in this case, this could be any building within a radius of 30m of the flue. This compares with just 7m for the gas and oil boilers. And the resulting calculation produces only a preliminary height that must be checked with the local authority before proceeding with the construction.
44 CIBSE Journal January 2012
Plant-room ventilation One of the most important areas of consideration within the plant room is correct ventilation. Where a biomass appliance is installed
in the same space as a gas boiler, the additional natural ventilation provisions for the biomass-fuelled appliance shall be at least 6 sq cm per kW heat input at low level, and 3 sq cm per kW heat input at high level (approximately 8 sq cm per kW at low level and 4 sq cm per kW at high level, based on heat output). Where a biomass appliance is installed
in the same space as a gas-fired boiler, the additional mechanical ventilation provisions for the biomass-fuelled appliance should be as required for the gas-fired boiler and as detailed in IGE/UP/10 Edition 3. This requires a minimum inlet air supply rate (combustion plus ventilation) of 0.75 cu m/second per 1,000 kW boiler input power, with a maximum ventilation system removing 0.4 cu m/second per 1,000 kW. To determine the correct flue arrangements
for biomass boilers is not only more demanding but also less definitive than for fossil fuels. Increasing concerns over particle sizes and air quality are likely to make the requirements more demanding in the future but, in any case, the success of an installation will depend not only on appropriate knowledge and appropriate calculations being undertaken, but by also having a constructive relationship with the local authority from the earliest possible stage. CJ
l RichaRd PaRkhouse is technical director of A1 Sheet Metal Flues.
www.a1flues.co.uk
www.cibsejournal.com
Richard Parkhouse
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