Biomass Heating and Boilers

Learn About Wood Fuels

Traditionally the most familiar type of fuel for a domestic heating system is log fuel. Even the user of a commercial system may begin by assuming the use of logs, but progress has been made to efficiently and conveniently use wood fuel beyond this.

Modern log burning boilers burn at around 90% efficiency and although they can be configured to need filling only two or three times a day, they are still relatively labour intensive and limited in size compared to domestic installations. Commercial biomass boilers needed to be capable of bigger outputs and automatic feed so most biomass boilers are designed for use with either wood chip or wood pellets.
Wood Chips
Wood chips are small pieces of wood that have been cut to between 5mm and 50mm (measured in the direction of the grain). Larger twigs and dust like particles are can be included but if too large they can bridge and block the feed system, too small and they can upset the combustion of the boiler. High bark content can have the same effect, so some care is needed in the quality of the wood chip, which is dictated by the raw material, chipper type and sharpness of blade, handling etc.
Wood Pellets 
Wood pellets are sawdust or fine shavings of wood extruded under pressure so that the wood resin binds the material into cylindrical shapes between 6mm and 10mm diameter and 10mm to 30mm long.. No artificial chemicals are added but starch or corn flour is sometimes used to improve stability: the result is a high density fuel with a low moisture content, which is less bulky to transport and store.

The choice between these two fuels is often an early decision to make when specifying a system.

Why Wood Chips?

  • Cheaper per kWh.
  • More locally obtained and can be from several sources (very often the reason for installing a wood chip boiler is availability of local, cheap fuel)
  • More likely to have a beneficial impact on local economy.

It is worth considering that all our wood chip boilers can be set up for running pellets, allowing future flexibility in supply but specific pellet boilers and their narrower feed augers are not suitable for conversion to wood chips.

Fuel stores and delivery methods are also an important consideration when deciding which fuel is to be used. Certainly the more restricted the space, the more likely the use of pellets as most users appreciate the potential to be able to run the heating system at full power for anything above a week between deliveries.

Time between deliveries is a major factor in designing a system. The higher the volume in one delivery the cheaper the fuel, yet wood chip is relatively bulky. Hence larger boilers use a large volume of fuel each day when working hard, so calculations are needed to balance the volume of space with the time between deliveries.

Because wood chip does not flow so cannot be "poured" like pellets, deliveries can be difficult, but options include delivery by tipping or scissor lift trailer, moving floor trailer, or a system of pre-filled container bins hook-lifted into place. Underground stores make the delivery easier but are expensive to build and lid types, sliding or lifting are a variable.

Pellets are normally delivered in bulk by a pressurised trailer, pumping the fuel through a pipe. This means the delivery vehicle can be up to 5 metres distance from the store and is another reason for selecting a pellet fuel system.

Why Wood Pellets?

  • Smaller storage space, benefit for delivery vehicle and storage at site.
  • Flow characteristics make them easier to deliver (they can be blown through pipes)
  • Less variation in moisture and physical form.
  • Can be bagged or delivered over longer distances
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