Biomass

 

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In general biomass in the domestic sector almost always refers to wood fuel, which is only sustainable if it comes from renewable forest sources. Biomass systems can have high levels of efficiency, typically 60-80% in ranges, pellet stoves, log stoves and log boilers.

Biomass fuel usually takes the form of wood pellets or wood chips. The burning of wood is considered to be a carbon neutral process since the CO2 released when energy is generated, is balanced by that absorbed during the fuel's production (i.e. replacement tree growth). It is most cost effective when a local fuel source is used, which also helps to reduce transport pollution which might otherwise be associated with the solid fuel.

The major drawback is the physical size and installation cost of the fuel storage and delivery system. A 20kW thermal boiler typically consumes 0.6m3 of wood chip daily in winter, and the volume of one tonne of dried wood is about 6m3.


Biomass Boiler Wood Storage Volume

To estimate the required storage capacity of wood chips or wood pellets for a ´domestic´ sized boiler, follow this simple rule of thumb. It assumes a 8 week supply§, equivalent to 4 weeks at full operation and 4 weeks at 50% load.

  • 8 hours operation x 5 days a week x 4 weeks x peak load (of boiler) x boiler efficiency = Energy used (kWh)
  •   --  plus  --
  • 8 hours operation x 5 days a week x 4 weeks x half load (of boiler) x boiler efficiency = Energy used (kWh)
  •   --  adjust the hours and days according to use  --
  • next divide the energy used by the heat content (CV) of the wood per kg taking account of the boiler efficiency
  • so, energy used in period kWh /CV (kWh/kg) x boiler efficiency (say 0.9) = wood mass volume (kg)
  • divide this figure by the bulk volume to get storage volume
  • wood mass volume (kg)/ bulk volume (kg/m3) = minimum storage volume (m3)

Example - the storage for a 60kW boiler using pellets would be as follows:

  • (8 x 5 x 4 x 60 x 0.9)+ (8 x 5 x 4 x 30 x 0.9) = 8,640 + 4,320 = 12,960 kWh
  • so, 12,960 / 4.4 = 2,945 kg of pellets
  • therefore the minimum volume for pellet storage is 2,945 / 650 = 4.5 m3
  • volume for wood chip storage is(12,960 / 3.1) / 230 = 18.1 m3

Where wood has the following properties:


CV
Bulk Density
Wood Chip
3.1 kWh/kg
230 kg/m3
Pellets
4.4 kWh/kg
650 kg/m3

§Note a 8 week supply with 3 winter fills and 1 summer fill should cover the whole heating season - 32 weeks supply which is approximately equivalent to the annual heating period

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Thermal Store Vessel

A buffer tank (thermal store or accumulator) stores heat to provide a buffer between load variations in order that biomass boiler appliance efficiency can be optimised, thus alleviating slow response issues.

Tanks are sized between 10 and 20 litres/kWth plant capacity (CTG012); a lower value may be used where loads do not not fall to zero or for pellet fuelled boilers which are more responsive.



Related Sites & Information
-- links to info on biomass, biofuels, wood chip, wood pellets, etc. --
Visit the Biomass Energy Centre This link opens a new window for information on biomass derived solid, liquid and gaseous fuels and associated conversion technologies for the UK
Carbon Trust's page on biomass heating Carbon Trust This link opens a new window
Wood fuelled stoves and boilers advice from the Carbon Trust This link opens a new window
Wood Energy Scotland This link opens a new window is a very informative site for domestic and commercial properties
Information on woodfuel in England at the Forestry Commission This link opens a new window 

  These links open in a new window



 

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